WEBVTT 00:01.733 --> 00:04.833 RONALD SMITH: The announcements are 00:04.833 --> 00:07.533 there's a problem set due today, and 00:07.533 --> 00:11.803 you'll be starting a new lab next week. 00:11.800 --> 00:17.700 Be sure to bring your laptop to lab on Monday and Tuesday. 00:17.700 --> 00:22.800 This is a mostly a computer-based data analysis 00:22.800 --> 00:27.130 type lab, and you're going to need your laptop from the very 00:27.133 --> 00:29.103 beginning of that exercise. 00:32.767 --> 00:33.897 Any questions before we start? 00:33.900 --> 00:36.700 I'm going to go over the exam first thing here. 00:40.333 --> 00:46.703 Ok, so problem one, I show you a top view 00:46.700 --> 00:51.370 of a Focault pendulum. 00:51.367 --> 00:54.727 So imagine you're looking down at this thing. 00:54.733 --> 00:59.233 And the bob is swinging back and forth on this line. 00:59.233 --> 01:02.903 So just kind of looking down like this. 01:02.900 --> 01:06.530 I asked you to sketch the forces on the bob and explain 01:06.533 --> 01:09.603 how the track will rotate if the pendulum is in the 01:09.600 --> 01:10.600 Southern Hemisphere. 01:10.600 --> 01:14.530 So in the Southern Hemisphere, Coriolis force acts to the 01:14.533 --> 01:17.003 left of the motion vector. 01:17.000 --> 01:24.270 So when that bob is moving upwards on this blackboard, 01:24.267 --> 01:28.327 the Coriolis force is going to be in that direction. 01:28.333 --> 01:32.203 And when it's moving in the other direction, the Coriolis 01:32.200 --> 01:35.070 force is going to be like that. 01:35.067 --> 01:38.827 So the Coriolis force shifts depending on the direction the 01:38.833 --> 01:39.533 bob is moving. 01:39.533 --> 01:42.473 So what is that going to do to the plane in 01:42.467 --> 01:43.967 which the bob is swinging? 01:43.967 --> 01:47.127 Well this part when it's swinging that way it's going 01:47.133 --> 01:51.633 to deflect it a little bit like that. 01:51.633 --> 01:54.333 And then when it's swinging back and this force acts on 01:54.333 --> 01:58.503 it, it's going to deflect it a little bit like that. 01:58.500 --> 02:01.700 And then that's going to repeat that way and then that 02:01.700 --> 02:02.970 way and then that way. 02:02.967 --> 02:08.867 So basically the plane in which the bob is rotating is 02:08.867 --> 02:16.567 going to rotate in the counterclockwise direction. 02:16.567 --> 02:19.897 So if you came back several hours later and it started out 02:19.900 --> 02:24.370 here, now it's going to be oscillating like that. 02:24.367 --> 02:27.167 And, of course, if we were in the Northern Hemisphere, it 02:27.167 --> 02:28.697 would be the opposite. 02:28.700 --> 02:34.000 It would rotate in the clockwise direction. 02:34.000 --> 02:36.800 Questions on that? 02:36.800 --> 02:37.230 Yes. 02:37.233 --> 02:40.433 STUDENT: What does CCW mean? 02:40.433 --> 02:41.703 PROFESSOR: Counterclockwise. 02:47.733 --> 02:49.633 Is that something the TA put on there? 02:49.633 --> 02:50.333 STUDENT: Yeah. 02:50.333 --> 02:50.633 PROFESSOR: Yeah. 02:50.633 --> 02:53.903 STUDENT: I said it was to the left. 02:53.900 --> 02:54.370 PROFESSOR: Yeah. 02:54.367 --> 02:58.427 See left is a little bit ambiguous because what does 02:58.433 --> 02:59.473 left mean here? 02:59.467 --> 03:01.597 I don't know. 03:01.600 --> 03:03.800 OK, question two. 03:03.800 --> 03:06.000 Explain how cool winds and a gust wind are created by a 03:06.000 --> 03:06.570 thunderstorm. 03:06.567 --> 03:14.527 So the basic idea there is you get rain coming out of the 03:14.533 --> 03:16.373 base of the cloud. 03:16.367 --> 03:18.697 You know that below the cloud, the relative humidity 03:18.700 --> 03:21.100 is less than 100%. 03:21.100 --> 03:24.830 So as soon as that rain falls below cloud base, it's going 03:24.833 --> 03:28.233 to begin to evaporate. 03:28.233 --> 03:31.573 Now maybe only a small fraction will evaporate, maybe 03:31.567 --> 03:32.597 all of it will evaporate. 03:32.600 --> 03:36.830 I remember when I used to live in Colorado, you'd see--in the 03:36.833 --> 03:40.703 summertime you'd see thunderstorms, deep convective 03:40.700 --> 03:44.400 storms, with the rain coming out of the base, and it would 03:44.400 --> 03:45.270 all evaporate. 03:45.267 --> 03:47.627 None of it reached the surface of the Earth. 03:47.633 --> 03:50.603 When you see that, by the way, when it doesn't reach the 03:50.600 --> 03:53.800 surface here, it's called virga. 03:53.800 --> 03:56.430 But in any case, whether it all evaporates or just some of 03:56.433 --> 04:02.873 it, it takes heat to evaporate water, and so--and that heat 04:02.867 --> 04:03.927 comes out of the air. 04:03.933 --> 04:07.433 So there's a cooling that takes place, an evaporative 04:07.433 --> 04:12.973 cooling that takes place just below the cloud base, and that 04:12.967 --> 04:18.127 takes air and makes it cooler and more dense. 04:18.133 --> 04:22.703 So it then falls out of the sky until it hits the surface, 04:22.700 --> 04:25.370 and then spreads out. 04:25.367 --> 04:28.597 And the front of that is called the gust front. 04:28.600 --> 04:35.030 So the basic idea here is that evaporation of raindrops below 04:35.033 --> 04:39.873 the cloud base cools the air, makes it sink and spread out. 04:39.867 --> 04:43.367 So that's the idea behind if you've experienced this in the 04:43.367 --> 04:46.167 summertime, you hear a thunderstorm in the distance, 04:46.167 --> 04:49.667 and then for a few minutes later the wind 04:49.667 --> 04:51.667 begins to pick up. 04:51.667 --> 04:54.527 It'll be a cool wind, typically, and blowing from 04:54.533 --> 04:56.573 the direction of the thunderstorm. 04:56.567 --> 04:57.267 That's this. 04:57.267 --> 04:59.697 That's that cool air coming down from 04:59.700 --> 05:02.070 the base of the cloud. 05:02.067 --> 05:03.297 Questions on that? 05:05.900 --> 05:06.730 Question three. 05:06.733 --> 05:10.873 I give you 45 degrees north. 05:10.867 --> 05:15.367 There's a pressure gradient, 0.002 Pascals per meter, with 05:15.367 --> 05:18.667 pressure increasing towards the west. So on a map view, 05:18.667 --> 05:24.267 north, south, east, west, the question states that the 05:24.267 --> 05:27.167 pressure is greater towards the west because it's high 05:27.167 --> 05:30.697 pressure over here relative to low pressure over there. 05:30.700 --> 05:34.370 The lines of constant pressure, if you drew them in, 05:34.367 --> 05:41.367 the isobars would be oriented north-south like that. 05:41.367 --> 05:45.027 An air parcel sitting in there is going to feel a pressure 05:45.033 --> 05:47.633 gradient acting from high to low. 05:47.633 --> 05:51.533 So that's the pressure gradient force. 05:51.533 --> 05:54.833 And if we can assume that the air is in geostrophic balance, 05:54.833 --> 05:59.903 the Coriolis force must be equal and opposite to that. 05:59.900 --> 06:02.770 We're in the Northern Hemisphere, 45 06:02.767 --> 06:05.227 degrees north latitude. 06:05.233 --> 06:08.473 So if the Coriolis force is like that, the wind must be 06:08.467 --> 06:13.027 such that the Coriolis force is at right angles to the 06:13.033 --> 06:16.203 wind, and to the right of the wind. 06:16.200 --> 06:19.930 So there's only one possible solution for the wind, and 06:19.933 --> 06:22.433 that would be that vector. 06:22.433 --> 06:24.673 So that the Coriolis force is to the 06:24.667 --> 06:28.827 right of the wind motion. 06:28.833 --> 06:32.803 So, well I'm getting ahead of the story, this is part B. But 06:32.800 --> 06:35.900 basically, the velocity direction would 06:35.900 --> 06:38.000 be towards the south. 06:38.000 --> 06:41.800 You could state that as a northerly wind if you wanted, 06:41.800 --> 06:44.230 but the direction will be towards the south. 06:44.233 --> 06:48.133 Now, the magnitude of the geostrophic wind is given by 06:48.133 --> 06:52.473 the pressure gradient divided by 2 rho 06:52.467 --> 06:55.297 omega sine of the latitude. 06:55.300 --> 06:56.870 So let's put in some numbers on that. 06:56.867 --> 07:03.297 It's 0.002, 2 the density of air at sea level is about 1.2 07:03.300 --> 07:05.130 kilograms per cubic meter. 07:05.133 --> 07:09.103 The rotation rate of the Earth is 7.27 times 10 07:09.100 --> 07:10.870 to the minus 7. 07:10.867 --> 07:16.767 And the sine of 45 degrees is about 0.71. 07:16.767 --> 07:20.897 So that turns out to be 16.2 meters per second. 07:20.900 --> 07:24.300 I would recommend you notice I left off the units, but I 07:24.300 --> 07:25.930 don't recommend that. 07:25.933 --> 07:29.333 If you put the units for everything, Pascals per meter, 07:29.333 --> 07:33.633 kilograms per cubic meter, seconds to the minus 1, you 07:33.633 --> 07:36.373 can work it out and you should get units of 07:36.367 --> 07:39.197 speed meters per second. 07:39.200 --> 07:42.130 And if you don't, maybe you've left out something. 07:42.133 --> 07:45.073 Maybe you've got the units wrong or you've left out a 07:45.067 --> 07:46.467 factor or something like that. 07:46.467 --> 07:50.397 So it's always good to check the units on that. 07:50.400 --> 07:56.470 For part C, again, what you've assumed is geostrophic 07:56.467 --> 08:00.067 balance, which says that the Coriolis force and the 08:00.067 --> 08:04.367 pressure gradient force are equal and opposite. 08:04.367 --> 08:07.627 That is the definition of geostrophic balance. 08:07.633 --> 08:08.133 Yeah. 08:08.133 --> 08:08.533 STUDENT: Is the rotation rate 10 to the minus 7 or 10 08:08.533 --> 08:09.803 to the minus 5? 08:11.500 --> 08:13.870 PROFESSOR: 10 to the negative sorry, this is wrong. 08:13.867 --> 08:15.667 10 to the minus 5. 08:15.667 --> 08:16.927 Thank you. 08:20.900 --> 08:26.200 If you ever I mean I gave you that constant on the equation 08:26.200 --> 08:29.800 sheet, but if you ever need it and can't remember it, just 08:29.800 --> 08:35.030 take 2 pi, which is the number of radians in a circle, and 08:35.033 --> 08:41.433 divided it by the length of a day expressed in seconds. 08:41.433 --> 08:46.503 And you'll get that number, 7.27 times 10 to the so it's 08:46.500 --> 08:49.830 just the rotation rate of the Earth basically, expressed in 08:49.833 --> 08:51.073 radians per second. 08:55.567 --> 08:55.927 Four. 08:55.933 --> 08:59.973 Explain why the sky appears blue, but a cloud appears 08:59.967 --> 09:03.027 white under similar illumination from the Sun. 09:03.033 --> 09:05.033 So the idea is we have our observer. 09:07.733 --> 09:09.133 We have a cloud here. 09:09.133 --> 09:16.303 And the Sun is illuminating the cloud, but is also seeing 09:16.300 --> 09:19.530 radiation that's been scattered I don't have to draw 09:19.533 --> 09:20.773 another beam there. 09:25.400 --> 09:28.400 Some light is scattered out of this beam coming directly to 09:28.400 --> 09:32.700 the observer, and that's blue light. 09:32.700 --> 09:35.500 But this is white light. 09:38.000 --> 09:41.100 So the difference has to do with the size of the particles 09:41.100 --> 09:43.370 that are doing the scattering. 09:43.367 --> 09:48.797 In the case of the sky, the particles are molecules. 09:52.333 --> 09:56.133 And the condition that the wavelength is much, much 09:56.133 --> 10:00.603 greater than the diameter of the particle is met, and that 10:00.600 --> 10:02.230 puts us into the Rayleigh scattering regime. 10:06.133 --> 10:09.273 In that case, short wavelengths are scattered much 10:09.267 --> 10:12.927 more strongly than longer wavelengths. 10:12.933 --> 10:15.573 Remember, the part of the spectrum we can see with our 10:15.567 --> 10:20.327 eye, the visible part of the spectrum has 10:20.333 --> 10:23.203 red, green and blue. 10:23.200 --> 10:25.100 Blue being the shorter. 10:25.100 --> 10:26.830 Red being the longer wavelength. 10:26.833 --> 10:29.803 So in Rayleigh scattering where short wavelengths are 10:29.800 --> 10:33.700 scattered more strongly, that'll be the blue light 10:33.700 --> 10:35.600 that's scattered more than the green and the red. 10:35.600 --> 10:39.800 So this light scattered out of the Sun's beam to your eye by 10:39.800 --> 10:44.900 molecules is going to be dominated by the blue light. 10:44.900 --> 10:47.570 For the cloud itself, the particles are much larger. 10:47.567 --> 10:48.827 They're the cloud droplets. 10:53.600 --> 10:58.270 They fall into the category that the wavelength is the 10:58.267 --> 11:05.197 same order of magnitude as the cloud droplet. 11:05.200 --> 11:08.700 Which puts us into the Mie scattering range where all 11:08.700 --> 11:11.070 wavelengths are scattered equally. 11:11.067 --> 11:14.667 So whatever color was illuminating this, you'd have 11:14.667 --> 11:16.067 the same color coming out. 11:16.067 --> 11:19.727 So if it's white light illuminating the cloud, and 11:19.733 --> 11:24.303 the Sun's radiation is usually composed of roughly equal 11:24.300 --> 11:26.500 mixtures of blue, green and red, so that would appear 11:26.500 --> 11:28.530 white to our eye. 11:28.533 --> 11:30.333 Then the light scattered by the cloud is 11:30.333 --> 11:33.533 also going to be white. 11:33.533 --> 11:38.403 By the way, if you have a setting Sun illuminating the 11:38.400 --> 11:41.770 cloud where some of the blue light has already been 11:41.767 --> 11:45.397 scattered out, it may be that the cloud is 11:45.400 --> 11:48.370 illuminated by red light. 11:48.367 --> 11:52.227 In that case, the cloud would appear red as well. 11:52.233 --> 11:54.703 But the point here is that under Mie scattering, it 11:54.700 --> 11:58.070 scatters equally whatever is illuminating the cloud. 11:58.067 --> 12:02.367 So it'll keep the color the same as it scatters radiation 12:02.367 --> 12:05.367 from the cloud. 12:05.367 --> 12:07.867 Questions there? 12:07.867 --> 12:08.567 Question five. 12:08.567 --> 12:13.467 Why are hurricanes not found over the sea near the equator? 12:13.467 --> 12:16.397 Well right near the equator within four or five degrees of 12:16.400 --> 12:20.070 the equator, there's not sufficient Coriolis force. 12:20.067 --> 12:24.127 In order to form a hurricane, as the air moves in and begins 12:24.133 --> 12:27.073 to lift upwards, there's got to be a Coriolis force to give 12:27.067 --> 12:29.667 it a sense of spin. 12:29.667 --> 12:32.527 And if you don't have a sufficient Coriolis force, 12:32.533 --> 12:37.973 then you can't really form a hurricane. 12:37.967 --> 12:40.727 Part B, in the tropical south Atlantic. 12:40.733 --> 12:44.803 So let's assume you're south of the Equator enough to have 12:44.800 --> 12:48.730 a Coriolis force, but it turns out in the Atlantic Ocean, 12:48.733 --> 12:53.273 there's a cold current that comes up from the southern 12:53.267 --> 12:57.727 ocean that drops the ocean temperature below that 12:57.733 --> 13:02.633 threshold value it's 27, 28 degrees Celsius, and 13:02.633 --> 13:04.173 therefore, you don't have the ocean 13:04.167 --> 13:08.227 warmth to create a hurricane. 13:10.967 --> 13:12.197 Questions there? 13:14.633 --> 13:17.773 Question six I thought was straightforward. 13:17.767 --> 13:22.427 Basically is why water drops form on the outside of a cool 13:22.433 --> 13:23.703 glass of water? 13:26.333 --> 13:30.933 So you've got cold water or cool water in there at some 13:30.933 --> 13:32.333 temperature. 13:32.333 --> 13:36.633 And you want to know when beads of water would form on 13:36.633 --> 13:37.103 the outside. 13:37.100 --> 13:39.330 Well, of course the water is not coming from inside the 13:39.333 --> 13:42.573 glass through the glass, that water is coming from the 13:42.567 --> 13:44.067 atmosphere around. 13:44.067 --> 13:47.697 The only role of the water in the glass is to control the 13:47.700 --> 13:52.100 temperature of the outer surface of the glass. 13:52.100 --> 13:56.730 And the key condition is that the temperature of that glass, 13:56.733 --> 14:03.003 if it's less than the dew point of the atmosphere around 14:03.000 --> 14:07.530 it, then you will get condensation. 14:07.533 --> 14:11.933 The idea is that the glass will remove heat from the air 14:11.933 --> 14:15.973 adjacent to it, drop the saturation vapor pressure 14:15.967 --> 14:22.467 down, and if it can drop it low enough, it can drop the 14:22.467 --> 14:27.597 temperature down below the dew point, then you'll bring water 14:27.600 --> 14:30.330 out of the vapor state and condense it on the 14:30.333 --> 14:31.573 outside of the glass. 14:31.567 --> 14:34.997 So I was looking for a clear explanation of that. 14:39.933 --> 14:42.203 Question seven. 14:42.200 --> 14:46.670 How do the raindrops form that we find falling from a tall 14:46.667 --> 14:47.897 cumulonimbus cloud? 14:54.133 --> 14:59.733 In the summertime you get tall cumulus clouds with heavy rain 14:59.733 --> 15:01.833 out the bottom. 15:01.833 --> 15:04.733 The fact that they're tall means that the top part of it 15:04.733 --> 15:09.833 is certainly going to be at a temperature lower than zero 15:09.833 --> 15:10.903 degrees Celsius. 15:10.900 --> 15:14.530 So there's going to be super cooled water up here. 15:19.733 --> 15:23.303 And that's going to be the key for generating precipitation. 15:23.300 --> 15:27.470 So using the ice phase mechanism then you can convert 15:27.467 --> 15:29.497 these to snowflakes. 15:29.500 --> 15:33.800 They'll fall, when they fall below the zero degree line, 15:33.800 --> 15:36.500 they'll melt and form raindrops and then they'll 15:36.500 --> 15:37.230 fall to Earth. 15:37.233 --> 15:40.273 So what you had to say there was to mention, at least 15:40.267 --> 15:44.067 mention, the ice phase mechanism because that's how 15:44.067 --> 15:46.527 the hydrometer would be formed. 15:46.533 --> 15:50.133 And then you had to describe how it would melt on the way 15:50.133 --> 15:52.973 down to form a raindrop. 16:01.100 --> 16:03.100 Question 8. 16:03.100 --> 16:11.130 I imagine on the Earth some air moving northward with a 16:11.133 --> 16:15.073 temperature of 20 degrees C, and some air moving southward 16:15.067 --> 16:19.527 with a degree of--with a temperature of 10 degrees C. 16:19.533 --> 16:23.073 And I asked you to compute how much heat is 16:23.067 --> 16:25.197 being transported northward. 16:25.200 --> 16:32.100 The idea there is that the heat transported is given by 16:32.100 --> 16:34.830 the rate I'll put a dot over it to indicate it's a rate the 16:34.833 --> 16:37.533 rate at which you're moving mass forward times and heat 16:37.533 --> 16:44.373 capacity of the air times well, if it's a two stream 16:44.367 --> 16:47.067 difference and you want to compute the net amount of 16:47.067 --> 16:48.827 heat, then you'd use the temperature differences 16:48.833 --> 16:50.403 between the two streams. That's the 16:50.400 --> 16:51.400 way I did the problem. 16:51.400 --> 16:59.330 So my answer for this was 10 to the eleventh kilograms per 16:59.333 --> 17:05.603 second for the mass flow rate, and that was given. 17:05.600 --> 17:10.500 The heat capacity of air is 1,004, and that has units of 17:10.500 --> 17:15.600 Joules per kilogram per degree Kelvin. 17:15.600 --> 17:17.270 And then the temperature difference between the two 17:17.267 --> 17:21.097 streams, the delta-T was 10 degrees. 17:21.100 --> 17:23.100 And look how the units are going to work there. 17:23.100 --> 17:26.170 This is going to be in Kelvin or Celsius, in either case 17:26.167 --> 17:27.727 it'll cancel. 17:27.733 --> 17:32.473 The kilograms will cancel, and you get something with Joules 17:32.467 --> 17:35.897 per second, which is a watt, by the way. 17:35.900 --> 17:44.870 So I ended up with what did I get 1.5 times 10 to the 17:44.867 --> 17:49.867 seventeenth oh sorry. 17:49.867 --> 18:07.297 Let's see, about 1.004 times 10 to the fifteenth watts, 18:07.300 --> 18:13.600 which could be expressed as about 1.004 petawatts. 18:13.600 --> 18:15.770 Peta meaning the shortcut for 10 to the fifteenth. 18:15.767 --> 18:19.097 So it's approximately 1 times 10 to the fifteenth watts. 18:19.100 --> 18:23.570 Now, there was some confusion because while I described 18:23.567 --> 18:27.167 these two air masses moving northward and southward, in 18:27.167 --> 18:31.367 the end I said how much heat is transported northward. 18:31.367 --> 18:33.897 What I meant to say, what I hoped you would interpret that 18:33.900 --> 18:37.970 is the net amount transported northwards. 18:37.967 --> 18:39.667 And that's what I've computed here. 18:39.667 --> 18:42.797 It may be, though, that some of you computed just the 18:42.800 --> 18:45.830 amount of heat transported northward by the northward 18:45.833 --> 18:48.533 moving current. 18:48.533 --> 18:50.573 Maybe that could be supported given by the 18:50.567 --> 18:51.427 way I wrote the question. 18:51.433 --> 18:55.333 But what I intended was the net heat given the fact that 18:55.333 --> 19:00.973 there's both a northward and a southward block of air moving. 19:00.967 --> 19:06.427 So look at the grading and see what the TAs did about that. 19:06.433 --> 19:07.673 Questions there? 19:12.733 --> 19:15.603 We're getting close to the end here. 19:15.600 --> 19:21.930 Question nine says that on a rainy day a centimeter of rain 19:21.933 --> 19:27.773 falls on a 10,000 square kilometer area. 19:27.767 --> 19:29.197 Estimate the total latent heat. 19:29.200 --> 19:35.270 So you imagine some big cloud system raining out, putting a 19:35.267 --> 19:38.967 layer of water on the surface of the Earth. 19:38.967 --> 19:43.197 You just have to compute the volume of that layer. 19:43.200 --> 19:45.800 Then from the volume multiplied by the density of 19:45.800 --> 19:50.330 fresh water to get the mass of that layer. 19:50.333 --> 19:53.603 And then knowing the latent heat of condensation, you know 19:53.600 --> 19:57.830 how much heat had to be released in order to condense 19:57.833 --> 20:01.273 that much vapor to a liquid. 20:01.267 --> 20:04.267 So from the mass you just multiply that by the latent 20:04.267 --> 20:07.527 heat of condensation. 20:07.533 --> 20:12.073 So the dimensions of this give the volume, the mass--the 20:12.067 --> 20:15.627 density of water, which is 1,000 kilograms per cubic 20:15.633 --> 20:17.233 meter gives you the mass. 20:17.233 --> 20:19.303 And then the latent heat of condensation gives you the 20:19.300 --> 20:24.970 rest. And the answer is about 1.5 times 10 to the 20:24.967 --> 20:26.727 seventeenth joules. 20:30.400 --> 20:32.730 STUDENT: 2.5. 20:32.733 --> 20:33.803 PROFESSOR: Sorry, 2.5. 20:33.800 --> 20:35.600 Thank you. 20:35.600 --> 20:37.670 That 2.5 comes from the latent heat. 20:37.667 --> 20:38.897 Thank you. 20:41.567 --> 20:44.867 The last problem then was to explain the reason for the 20:44.867 --> 20:47.567 rainy season in these two cities. 20:55.667 --> 20:59.427 Jerusalem lies here. 20:59.433 --> 21:05.473 And Asuncion, Paraguay, except it shifted in longitude occurs 21:05.467 --> 21:06.727 somewhere here. 21:09.333 --> 21:13.333 Now, Jerusalem has a--it's in the Northern Hemisphere. 21:13.333 --> 21:16.373 Wettest month is January, and notice from the temperatures, 21:16.367 --> 21:18.867 that's also the cooler month. 21:18.867 --> 21:24.067 So Jerusalem gets a wintertime rainy season, and that is 21:24.067 --> 21:28.527 clearly due to the southward shift of the polar front. 21:32.000 --> 21:36.570 So what's happened is that during the winter season, this 21:36.567 --> 21:39.097 polar front has moved southward and is giving you 21:39.100 --> 21:42.670 frontal cyclones that come through that latitude and give 21:42.667 --> 21:44.067 you the rain. 21:44.067 --> 21:48.127 In Asuncion, Paraguay, Southern Hemisphere, the 21:48.133 --> 21:50.803 wettest month is December. 21:50.800 --> 21:53.600 In the Southern Hemisphere that's summertime. 21:53.600 --> 21:56.200 Notice also from the temperature that that's the 21:56.200 --> 21:57.830 warmer month. 21:57.833 --> 22:01.003 So you know immediately that is a summertime 22:01.000 --> 22:02.870 precipitation maximum. 22:02.867 --> 22:07.367 And that guarantees that it's going to be a southward shift 22:07.367 --> 22:12.797 of the ITCZ, giving you precipitation there. 22:12.800 --> 22:16.200 So the ITCZ in the Southern Hemisphere summer moves across 22:16.200 --> 22:19.300 the Equator into the Southern Hemisphere and brings 22:19.300 --> 22:22.800 precipitation to Asuncion. 22:22.800 --> 22:26.000 So that's going to be convective rain, not frontal 22:26.000 --> 22:32.830 rain, and it's going to be in the summer wet season. 22:32.833 --> 22:34.073 Any questions on that? 22:36.867 --> 22:39.497 The average grade on that exam was a bit lower 22:39.500 --> 22:40.870 than the first one. 22:40.867 --> 22:43.497 It was 74%. 22:43.500 --> 22:48.170 And if you've got maybe 10 degrees--10 degrees--10 points 22:48.167 --> 22:51.027 lower than that, maybe drop me a note, come see me, we'll 22:51.033 --> 22:53.573 talk about how to improve things in the future. 22:53.567 --> 22:56.367 But for some reason, the grades were a little bit lower 22:56.367 --> 22:57.867 on this than the other one. 22:57.867 --> 23:02.097 And take that into account when you're don't be overly 23:02.100 --> 23:05.430 critical of yourself, because apparently your classmates 23:05.433 --> 23:07.403 found that to be a tough exam as well. 23:23.867 --> 23:28.097 We're going to continue where we left off. 23:28.100 --> 23:28.970 Question, yes? 23:28.967 --> 23:29.797 STUDENT: I was wondering what the average score corresponds 23:29.800 --> 23:31.030 to in grade? 23:35.167 --> 23:36.797 PROFESSOR: I don't do that calculation until the 23:36.800 --> 23:37.570 end of the course. 23:37.567 --> 23:39.997 I keep them just as numerical scores until 23:40.000 --> 23:41.100 the end of the course. 23:41.100 --> 23:43.270 You can judge that for yourself, perhaps, based on 23:43.267 --> 23:44.397 the average grade. 23:44.400 --> 23:47.870 But that's not a calculation that I do until I get all the 23:47.867 --> 23:49.097 scores at the end. 23:54.567 --> 23:57.727 So we're talking about atmospheric forcing of the 23:57.733 --> 24:01.073 ocean, and I showed this slide last time. 24:01.067 --> 24:02.467 I'll just go back over it. 24:02.467 --> 24:06.527 Basically the ocean is driven by the atmosphere above it in 24:06.533 --> 24:08.803 these three ways. 24:08.800 --> 24:12.930 Heat added, and taken away, fresh water added and taken 24:12.933 --> 24:14.503 away, and the wind stress. 24:18.667 --> 24:26.567 We can quantify this, and recent investigators have 24:26.567 --> 24:31.497 tried to do this by producing maps, in this case, of the 24:31.500 --> 24:38.470 heat flux in and out of the ocean, or the water flux in or 24:38.467 --> 24:44.167 out of the ocean, or the wind stress applied 24:44.167 --> 24:45.227 to the ocean surface. 24:45.233 --> 24:49.033 So we know a bit about this forcing. 24:49.033 --> 24:54.333 And last time I derived some formulas for this, how a layer 24:54.333 --> 24:58.303 of depth D, which will feel the direct effect of this 24:58.300 --> 25:01.900 forcing, would respond to fresh water, 25:01.900 --> 25:04.300 heat, or wind stress. 25:04.300 --> 25:07.970 I won't go back over those derivations, but for the wind 25:07.967 --> 25:11.097 stress, which is I think the most difficult of the three, 25:11.100 --> 25:13.270 the concept involved this Ekman layer. 25:13.267 --> 25:16.467 When the wind blows over the ocean and puts a frictional 25:16.467 --> 25:20.767 stress on it, the first few tens of meters or hundreds of 25:20.767 --> 25:25.067 meters responds to that directly, but in an odd way. 25:25.067 --> 25:28.567 It moves off well, it has a spiral with depth. 25:28.567 --> 25:29.667 I'm not going to talk about that. 25:29.667 --> 25:34.327 But if you average that out, the net of all that motion is 25:34.333 --> 25:36.473 directly to the right of the wind stress. 25:39.333 --> 25:40.903 That's this big arrow here. 25:40.900 --> 25:44.000 That's the direction of Ekman transport, exactly at right 25:44.000 --> 25:49.200 angles to the wind stress that's being applied. 25:49.200 --> 25:54.070 So if you had a wind, say, from the southwest to the 25:54.067 --> 25:56.927 northeast, giving you wind stress in that direction 25:56.933 --> 26:01.603 that's the black arrow and you gave it a few hours to come 26:01.600 --> 26:04.870 into this new state of balance called the Ekman force 26:04.867 --> 26:08.697 balance, that force would be balanced by the Coriolis 26:08.700 --> 26:12.730 force, and if that's going to be the Coriolis force, then 26:12.733 --> 26:14.773 the Ekman transport must be in that direction. 26:14.767 --> 26:21.997 So the CF is at right angles to the net Ekman transport. 26:22.000 --> 26:25.830 Now, this is not identical with the geostrophic balance. 26:25.833 --> 26:29.103 Remember, geostrophic balance was a balance between Coriolis 26:29.100 --> 26:32.400 force and pressure gradient force. 26:32.400 --> 26:35.770 I have no pressure gradient force in this problem. 26:35.767 --> 26:40.127 This instead is a balance between a frictional stress 26:40.133 --> 26:43.033 being applied to the ocean, and the Coriolis force. 26:45.567 --> 26:46.727 But it's very real. 26:46.733 --> 26:50.333 I mean you can measure this easily in the ocean. 26:50.333 --> 26:53.303 If the wind is blowing briskly from one direction you can 26:53.300 --> 26:55.370 measure the surface water moving off 26:55.367 --> 26:56.697 in the right direction. 26:56.700 --> 26:58.330 I'll mention this in a week or so. 26:58.333 --> 27:00.303 The same thing applies to icebergs. 27:00.300 --> 27:04.430 When the wind blows on an iceberg, it moves at right 27:04.433 --> 27:07.173 angles to that force. 27:07.167 --> 27:08.927 The old sea captains would make notes of 27:08.933 --> 27:10.033 that in their log. 27:10.033 --> 27:12.533 Winds are from the south today, but the icebergs are 27:12.533 --> 27:14.603 moving off to the east. What's going on there? 27:14.600 --> 27:19.070 Well, you guys know, it has to do with this kind of a balance 27:19.067 --> 27:20.197 including the Coriolis force. 27:20.200 --> 27:23.830 So again, not the same as geostrophic balance, but they 27:23.833 --> 27:27.533 both involve the Coriolis force in an important way. 27:30.067 --> 27:33.227 These are the formulas that I derived last time. 27:33.233 --> 27:37.003 If you added an amount of heat per unit area Q over A, you 27:37.000 --> 27:39.630 can compute how the temperature of some depth D 27:39.633 --> 27:42.203 will change. 27:42.200 --> 27:46.670 If you add a layer of fresh water to the ocean of 27:46.667 --> 27:52.497 thickness little d, and then you mix that in to some layer 27:52.500 --> 27:56.670 of depth D, there's how the salinity will change. 27:56.667 --> 28:00.727 If you put a wind stress tau on the ocean, and that is 28:00.733 --> 28:04.673 mixed down to depth d, that's what the speed will be of that 28:04.667 --> 28:06.967 Ekman flow that goes off at right angles. 28:06.967 --> 28:07.897 Yeah. 28:07.900 --> 28:13.130 STUDENT: In the Ekman wind stress, is the rho-- 28:13.133 --> 28:14.603 in the denominator is that the density of water or air? 28:14.600 --> 28:15.470 PROFESSOR: Correct. 28:15.467 --> 28:15.867 Correct. 28:15.867 --> 28:18.297 Water. 28:18.300 --> 28:20.830 I left off the subscript, but that should be water. 28:20.833 --> 28:22.873 when you're computing tau, however, I gave you 28:22.867 --> 28:24.427 a formula for tau. 28:24.433 --> 28:26.103 It was rho u squared. 28:26.100 --> 28:30.670 That was the density of air, and the speed of the wind. 28:30.667 --> 28:33.727 So in the formula for tau that I gave you, it's the density 28:33.733 --> 28:35.233 of air that appears there. 28:35.233 --> 28:37.633 But down to the bottom here it's the density of sea water. 28:37.633 --> 28:39.103 STUDENT: So they don't divide out? 28:39.100 --> 28:40.270 PROFESSOR: No because there a factor of a 28:40.267 --> 28:41.197 thousand different. 28:41.200 --> 28:42.900 STUDENT: OK. 28:42.900 --> 28:46.570 PROFESSOR: The density of air at sea level is about 1.2 28:46.567 --> 28:47.627 kilograms per cubic meter. 28:47.633 --> 28:50.633 The density of sea water's about 1,000 kilograms per 28:50.633 --> 28:51.503 cubic meter. 28:51.500 --> 28:51.730 Yeah. 28:51.733 --> 28:52.433 STUDENT: Just real quick again, the little d versus big 28:52.433 --> 28:53.703 D, the big D is the depth and-- 28:57.167 --> 29:00.527 PROFESSOR: D--in all cases, D is this layer that is 29:00.533 --> 29:02.733 feeling the direct influence of these forcings. 29:05.933 --> 29:08.203 And it's hard to predict exactly what that'll be. 29:08.200 --> 29:10.730 It'll depend on how much turbulence there is, how deep 29:10.733 --> 29:12.773 things are going to get mixed. 29:12.767 --> 29:14.827 But it's something very small compared to 29:14.833 --> 29:16.173 the total ocean depth. 29:16.167 --> 29:20.497 It may be only a few tens of meters if the winds are weak, 29:20.500 --> 29:21.770 and there's not much turbulence. 29:21.767 --> 29:24.597 It could be 100 meters or 200 meters if the winds are very 29:24.600 --> 29:28.030 strong and you're mixing the ocean very vigorously. 29:28.033 --> 29:31.833 So I can't give you a fixed value for d, it'll depend on 29:31.833 --> 29:34.603 conditions. 29:34.600 --> 29:35.870 Good questions there. 29:38.733 --> 29:43.073 So if those are our three forcings, then what do they do 29:43.067 --> 29:43.797 to the ocean? 29:43.800 --> 29:49.730 Well the first two, the heat and the fresh water input, 29:49.733 --> 29:55.703 will modify the temperature and the salinity. 29:55.700 --> 29:58.630 The temperature and salinity will in turn change the 29:58.633 --> 30:01.073 density of the seawater. 30:01.067 --> 30:06.967 And then gravity will act on those density differences to 30:06.967 --> 30:11.197 produce currents down in the ocean. 30:11.200 --> 30:15.730 And those currents are called thermohaline currents. 30:15.733 --> 30:19.933 The name reminds you is they have to do with heat and salt. 30:19.933 --> 30:23.233 But they're acting through the role of heat and salt in 30:23.233 --> 30:26.003 controlling seawater density. 30:26.000 --> 30:28.770 Typically these currents are slow and deep. 30:31.767 --> 30:34.267 As I will show you in the next few minutes, they are 30:34.267 --> 30:38.127 responsible for the water mass distributions in the ocean, 30:38.133 --> 30:41.373 for the conveyor belt circulation, and for example, 30:41.367 --> 30:43.927 also for estuary circulation. 30:43.933 --> 30:47.103 So these are very important circulations in the ocean, 30:47.100 --> 30:51.900 even though they tend to be rather slow in the speed of 30:51.900 --> 30:52.930 their motions. 30:52.933 --> 30:54.703 These are the thermohaline currents. 30:54.700 --> 30:59.400 So let's take a look at the Atlantic Ocean along this 30:59.400 --> 31:03.630 section of 25 west longitude. 31:03.633 --> 31:09.073 Here is a temperature section given where this goes north to 31:09.067 --> 31:14.527 south, sea level down to 6,000 meters. 31:14.533 --> 31:17.603 And the black is the terrain. 31:17.600 --> 31:20.600 Now, the first thing that strikes you about this image 31:20.600 --> 31:24.900 perhaps is the steepness of this terrain. 31:24.900 --> 31:28.000 Oh my goodness, are there really these sharp spikes and 31:28.000 --> 31:30.400 cliffs in the ocean? 31:30.400 --> 31:33.830 And the answer is please remember that when you make a 31:33.833 --> 31:37.903 diagram like this, there is a lot of vertical exaggeration. 31:37.900 --> 31:42.770 For example, that distance is 5 kilometers or 6 kilometers, 31:42.767 --> 31:46.627 whereas this distance is something like 15,000 31:46.633 --> 31:47.973 kilometers. 31:47.967 --> 31:51.567 So there is really an enormous vertical exaggeration. 31:51.567 --> 31:55.227 Where I could squish this down or stretch this out, these 31:55.233 --> 31:59.873 supposedly steep slopes would become very, very gradual. 31:59.867 --> 32:03.067 Probably only 1% or 2% slopes. 32:03.067 --> 32:05.597 So don't be fooled by this. 32:05.600 --> 32:08.600 It's only the vertical exaggeration that gives you 32:08.600 --> 32:11.770 that apparent steepness. 32:11.767 --> 32:15.267 Now, for the temperature itself, you can't read the 32:15.267 --> 32:19.597 labels very well, but some of these are slightly negative, 32:19.600 --> 32:26.200 temperature of minus 0.02--sorry 0.2 or 0.4. 32:26.200 --> 32:31.700 If I've got a temperature of minus 0.4 Celsius, is that 32:31.700 --> 32:33.900 super cooled water? 32:33.900 --> 32:35.870 Is that super cooled water? 32:40.000 --> 32:40.830 How many think yes? 32:40.833 --> 32:42.333 Show of hands. 32:42.333 --> 32:43.733 How many think no? 32:43.733 --> 32:46.273 The danger of putting no up, I'm going to ask you why. 32:46.267 --> 32:49.497 Why don't you think that's super cooled water? 32:49.500 --> 32:51.170 STUDENT: Is it because it has salt in it? 32:51.167 --> 32:51.867 PROFESSOR: Yes. 32:51.867 --> 32:55.397 So it's not super cooled water because the salt actually 32:55.400 --> 33:00.170 depresses the sea level by a degree or two. 33:00.167 --> 33:03.267 So in fact, you can have a temperature down to maybe 33:03.267 --> 33:08.327 minus 1 or 1 1/2 degrees Celsius, and still be above 33:08.333 --> 33:12.603 the freezing point because the salt has suppressed the 33:12.600 --> 33:13.670 freezing point slightly. 33:13.667 --> 33:15.867 So this is not super cooled water. 33:15.867 --> 33:19.727 It's not about to freeze if you introduce some kind of a 33:19.733 --> 33:22.803 freezing nucleus to it or something. 33:22.800 --> 33:26.900 I want you to notice this band of blue coming down here. 33:26.900 --> 33:29.970 We're going to identify that as Antarctic Bottom Water. 33:29.967 --> 33:34.227 It's very cold water that's formed where the ice sheets 33:34.233 --> 33:38.273 from Antarctica float out over the ocean and 33:38.267 --> 33:40.397 cool the ocean down. 33:40.400 --> 33:44.100 And that water becomes so cold and so dense that it falls to 33:44.100 --> 33:47.700 the bottom of the ocean and then spreads northward. 33:47.700 --> 33:50.970 In this diagram it reaches the Equator. 33:50.967 --> 33:52.797 Depending--in some other sections, it actually gets a 33:52.800 --> 33:54.900 little bit north of the Equator. 33:54.900 --> 33:55.670 This is amazing. 33:55.667 --> 34:00.597 So a water mass formed under the ice shelves of Antarctica 34:00.600 --> 34:02.970 falls to the bottom of the ocean, and then under its own 34:02.967 --> 34:09.227 weight slowly moves, sinking and spreading, actually in 34:09.233 --> 34:11.833 some cases getting north of the Equator. 34:11.833 --> 34:15.903 That's Antarctic Bottom Water, AABW. 34:15.900 --> 34:19.270 One of the important water masses in the Atlantic Ocean 34:19.267 --> 34:20.497 is the AABW. 34:23.967 --> 34:28.397 There is a layer of warm water floating right on the top in 34:28.400 --> 34:32.100 the tropical and equatorial regions. 34:32.100 --> 34:35.330 And there are some other water masses up in here that will be 34:35.333 --> 34:38.273 a little bit easier to identify when we look at the 34:38.267 --> 34:39.867 salinity field. 34:39.867 --> 34:42.267 So let me look at the salinity field here. 34:42.267 --> 34:53.797 Atlantic salinity, and 34.7, 34.9, and so on. 34:53.800 --> 34:57.000 The Antarctic Bottom Water you can maybe see it there, but 34:57.000 --> 35:00.270 it's not as easily seen in the temperature field. 35:00.267 --> 35:03.527 But look at this thing. 35:03.533 --> 35:04.733 There's a tongue. 35:04.733 --> 35:08.373 Apparently, a water mass is being formed here in the 35:08.367 --> 35:15.267 Antarctic Ocean so sorry, in the Southern Ocean. 35:15.267 --> 35:17.267 Not close to the shores of Antarctica, but in the 35:17.267 --> 35:18.067 southern Ocean. 35:18.067 --> 35:24.267 It sinks and then it spreads northwards as a tongue, again, 35:24.267 --> 35:25.727 reaching the Equator. 35:25.733 --> 35:28.233 Actually going a little bit north of the Equator. 35:28.233 --> 35:31.003 That's called the Antarctic Intermediate Water, AAIW. 35:36.600 --> 35:38.530 There's also a water mass here. 35:38.533 --> 35:40.103 It's formed in the North Atlantic. 35:40.100 --> 35:44.230 It sinks and it spreads southward getting 35:44.233 --> 35:47.273 south of the Equator. 35:47.267 --> 35:51.067 That's the North Atlantic Deep Water, NADW, North Atlantic 35:51.067 --> 35:53.397 Deep Water. 35:53.400 --> 35:55.970 So what's going on in all of these cases? 35:55.967 --> 35:58.897 At the surface of the ocean where the atmosphere and the 35:58.900 --> 36:05.030 ocean touch, the atmosphere is imprinting a certain 36:05.033 --> 36:09.873 temperature salinity characteristic to the water. 36:09.867 --> 36:12.497 That determines the water's density. 36:12.500 --> 36:15.700 That water will then sink down to its appropriate density 36:15.700 --> 36:18.500 level and spread out. 36:18.500 --> 36:21.230 That was a case for the Antarctic Bottom Water, the 36:21.233 --> 36:23.573 Antarctic Intermediate Water, and the north 36:23.567 --> 36:25.027 Atlantic Deep Water. 36:25.033 --> 36:29.273 Notice, their roots all go back to the surface of the 36:29.267 --> 36:32.767 ocean, because that's where they can get their properties. 36:32.767 --> 36:35.067 They get the temperature from the heat fluxes, they get 36:35.067 --> 36:38.227 their salinity from the fresh water fluxes, and then they 36:38.233 --> 36:40.973 sink and they spread out and they're found thousands of 36:40.967 --> 36:43.967 kilometers away from where they were formed, and moving 36:43.967 --> 36:48.397 very slowly there under the direction of these currents. 36:52.267 --> 36:55.597 Oxygen doesn't play any role in how these water masses 36:55.600 --> 36:58.130 move, but they provide a good tracer. 36:58.133 --> 37:02.373 Here's dissolved oxygen, and you find the 37:02.367 --> 37:04.867 lowest values here. 37:04.867 --> 37:07.367 And notice this curious structure. 37:07.367 --> 37:11.597 Right at the ocean surface, of course, you find the highest 37:11.600 --> 37:12.630 values of oxygen. 37:12.633 --> 37:17.173 Well, the oxygen is being taken in from the atmosphere, 37:17.167 --> 37:21.227 mixed down a couple of hundred meters, and then following 37:21.233 --> 37:24.273 that water mass here it is following the north Atlantic 37:24.267 --> 37:28.927 Deep Water, here it is following the Antarctic 37:28.933 --> 37:31.533 Intermediate Water right there. 37:31.533 --> 37:39.833 But surprisingly, the lowest oxygen is in the tropics, just 37:39.833 --> 37:41.833 below the--not touching the atmosphere, but just below it. 37:41.833 --> 37:45.303 So there's not a very good communication or mixing 37:45.300 --> 37:48.470 between the surface of the ocean and this water or it 37:48.467 --> 37:51.367 wouldn't have that depleted oxygen. 37:51.367 --> 37:54.397 The reason why there is not strong communication there is 37:54.400 --> 37:55.630 because it's very stable. 37:55.633 --> 37:58.533 There's a thermocline there, just like an inversion in the 37:58.533 --> 38:02.103 atmosphere prevents vertical mixing. 38:02.100 --> 38:06.770 So you get this strongly depleted oxygen water here, so 38:06.767 --> 38:11.967 close to its oxygen source, but yet cut off by this stable 38:11.967 --> 38:13.997 inversion preventing vertical mixing. 38:14.000 --> 38:15.230 So we can-- 38:18.133 --> 38:18.673 Yeah, question. 38:18.667 --> 38:21.667 STUDENT: Is the oxygen depleted there because 38:21.667 --> 38:23.527 organisms use it up? 38:23.533 --> 38:26.603 PROFESSOR: Organisms use as soon as the water 38:26.600 --> 38:31.030 leaves I should have said this, thanks Julia as soon as 38:31.033 --> 38:34.403 the water leaves the surface it begins to lose oxygen 38:34.400 --> 38:39.700 because animals use it up, and respiration takes place to use 38:39.700 --> 38:40.270 up the oxygen. 38:40.267 --> 38:44.067 So the older the water is, the less oxygen it has in it. 38:44.067 --> 38:47.167 STUDENT: And so the fact that the oxygen can mix more deeply 38:47.167 --> 38:49.567 in other regions of the world, does that mean that it won't 38:49.567 --> 38:52.127 be used up as-- 38:52.133 --> 38:53.603 PROFESSOR: No, it might mean that, but it might 38:53.600 --> 38:57.070 just mean that that water has been down there a longer 38:57.067 --> 38:58.327 period of time. 39:02.033 --> 39:05.703 So the cartoon then of the Atlantic Ocean, the same 39:05.700 --> 39:08.900 north-south section I've been telling you about, south to 39:08.900 --> 39:10.370 north, looks like this. 39:10.367 --> 39:15.297 You've got this warm surface water heated by the Sun kind 39:15.300 --> 39:18.600 of floating, it's quite shallow. 39:18.600 --> 39:21.830 You've got the Antarctic Intermediate Water coming in 39:21.833 --> 39:24.173 here, you've got the North Atlantic Deep Water, and then 39:24.167 --> 39:26.967 you've got the Antarctic Bottom Water. 39:26.967 --> 39:29.227 Eventually they lose their characteristics. 39:29.233 --> 39:32.773 They mix in with their environments, but for at least 39:32.767 --> 39:35.597 hundreds, maybe thousands of kilometers, during their 39:35.600 --> 39:39.830 transport, they retain enough of their property that we can 39:39.833 --> 39:43.933 identify them by where their source region was. 39:43.933 --> 39:46.233 So this is the concept of the water mass. 39:46.233 --> 39:50.403 In oceanography we have this concept of the water mass. 39:50.400 --> 39:53.700 It's a mass of water obtaining its temperature, salinity 39:53.700 --> 39:58.270 properties at the air-sea boundary, and then sinking and 39:58.267 --> 40:03.967 moving slowly, and for years retaining that property until 40:03.967 --> 40:08.997 eventually it mixes in with its environment. 40:09.000 --> 40:12.200 And these motions are driven by these slow 40:12.200 --> 40:16.470 thermohaline-driven currents. 40:16.467 --> 40:17.827 Questions on this? 40:17.833 --> 40:22.773 So this is the concept of the water mass in oceanography. 40:22.767 --> 40:26.567 It's a bit like the concept of the air mass in meteorology 40:26.567 --> 40:31.367 where on a day like this, cool, brisk, I can say maybe 40:31.367 --> 40:33.527 this is a Canadian air mass. 40:33.533 --> 40:38.073 It got its dryness, its cool temperatures in Canada, now 40:38.067 --> 40:41.867 it's moved down to me in New England, but I know from its 40:41.867 --> 40:44.227 properties where it came from. 40:44.233 --> 40:47.833 Whereas a couple days from now if it's really moist and humid 40:47.833 --> 40:52.173 and warm, likely that's Caribbean air coming up from 40:52.167 --> 40:56.267 the Caribbean transported to my location, retaining some of 40:56.267 --> 40:58.997 the properties it acquired when it was down 40:59.000 --> 41:01.030 in the lower latitudes. 41:01.033 --> 41:05.603 So the concepts are roughly similar to each other. 41:05.600 --> 41:11.600 Now, I wanted to show you a famous thermohaline current 41:11.600 --> 41:14.670 called the conveyor belt circulation. 41:14.667 --> 41:17.027 I wanted to show this diagram again you've seen it before to 41:17.033 --> 41:23.603 remind you that the Atlantic Ocean is slightly saltier than 41:23.600 --> 41:29.170 the Pacific, and slightly cooler than the Pacific, 41:29.167 --> 41:30.527 mostly just saltier. 41:30.533 --> 41:34.203 But as a result, the Atlantic Ocean--look at the density 41:34.200 --> 41:37.170 numbers--The Atlantic Ocean water is on average slightly 41:37.167 --> 41:39.667 denser than the Pacific Ocean. 41:39.667 --> 41:42.297 So imagine you've got these two oceans, OK, they're 41:42.300 --> 41:46.770 separated by North and South America, but they have 41:46.767 --> 41:49.497 different densities. 41:49.500 --> 41:52.970 If there's any leak between the two of them, that's going 41:52.967 --> 41:56.697 to try to equilibrate by setting up a circulation 41:56.700 --> 42:00.430 between them, and that exactly is what we think happens to 42:00.433 --> 42:03.673 give this large scale conveyor belt circulation. 42:03.667 --> 42:06.527 Notice that in the Pacific Ocean, the 42:06.533 --> 42:07.633 water is less dense. 42:07.633 --> 42:14.133 It seems to rise and then flow into the Atlantic. 42:14.133 --> 42:17.373 The Atlantic Ocean being somewhat denser, this water 42:17.367 --> 42:22.127 sinks, and then flows on the bottom of the ocean back into 42:22.133 --> 42:22.903 the Pacific. 42:22.900 --> 42:26.430 So there's kind of an overturning circulation. 42:26.433 --> 42:29.873 Now OK, it has to go through a very circuitous route because 42:29.867 --> 42:31.197 they're not directly connected. 42:31.200 --> 42:34.370 They can't make their way across this barrier or across 42:34.367 --> 42:35.167 this barrier. 42:35.167 --> 42:39.497 But they do share the southern Ocean in common. 42:39.500 --> 42:43.630 So they can communicate and close this circulation by 42:43.633 --> 42:46.733 running the currents down through the Southern Ocean. 42:46.733 --> 42:49.873 That's the conveyor belt circulation. 42:53.567 --> 42:58.167 Another example of a slow thermohaline circulation. 43:01.000 --> 43:03.670 A third example is the estuary circulation. 43:03.667 --> 43:08.797 An estuary is defined as a semi-enclosed body of water 43:08.800 --> 43:12.230 with some connection to the ocean, and 43:12.233 --> 43:14.633 with fresh water inputs. 43:14.633 --> 43:17.433 Long Island Sound would be a good example of that. 43:17.433 --> 43:21.503 There are fresh water inputs the Connecticut River, the 43:21.500 --> 43:25.870 Quinnipiac River that we studied, the Housatonic River. 43:25.867 --> 43:29.027 And there is a connection to the ocean that goes out 43:29.033 --> 43:32.473 through the race here so that you can mix ocean 43:32.467 --> 43:34.027 water in and out. 43:34.033 --> 43:37.173 What we find is that while the ocean salinity is typically 35 43:37.167 --> 43:40.297 parts per thousand, the salinity in Long Island 43:40.300 --> 43:42.870 Sound's only about 20 parts per thousand. 43:42.867 --> 43:46.567 I think in your field trip you found 18, but generally that's 43:46.567 --> 43:48.627 pretty close to this. 43:48.633 --> 43:51.703 So what's happening is it's a place where mixing occurs 43:51.700 --> 43:55.270 between fresh water coming off the land and ocean water 43:55.267 --> 43:58.427 mixing in from the ocean. 43:58.433 --> 44:03.403 So remember, salty water is denser. 44:03.400 --> 44:05.330 Fresher water is less dense. 44:05.333 --> 44:12.603 So what happens is that the denser, fully salt ocean water 44:12.600 --> 44:19.330 slides in along the bottom, has fresh water added to it, 44:19.333 --> 44:23.533 making it less dense so it rises, and then it floats out 44:23.533 --> 44:24.373 on the top. 44:24.367 --> 44:26.897 So right through here there's going to be a two-layer 44:26.900 --> 44:28.270 circulation. 44:28.267 --> 44:33.797 It's illustrated up here with the fresh water being added, 44:33.800 --> 44:37.470 and that slightly reduced salinity water flowing out of 44:37.467 --> 44:39.967 the top, and the full ocean water 44:39.967 --> 44:41.097 flowing in at the bottom. 44:41.100 --> 44:43.730 So you get a two-layer circulation right at the 44:43.733 --> 44:47.733 entrance region to an estuary, driven by this salinity 44:47.733 --> 44:49.273 difference. 44:49.267 --> 44:53.597 That would be called a thermohaline circulation. 44:53.600 --> 44:54.900 Does everybody get that? 44:54.900 --> 44:55.770 What's driving it? 44:55.767 --> 45:00.567 So if I turned off these rivers in some magical way, 45:00.567 --> 45:03.997 and let the mixing continue, after a few months, this would 45:04.000 --> 45:06.070 come up to 35 parts per thousand. 45:06.067 --> 45:08.297 It's the fresh water inputs that's keeping this 45:08.300 --> 45:10.770 lower than the 35. 45:10.767 --> 45:13.527 And it's that two-layer circulation that's providing 45:13.533 --> 45:17.503 the mixing between the estuary and the open ocean. 45:20.967 --> 45:21.727 Questions there? 45:21.733 --> 45:22.203 Yeah. 45:22.200 --> 45:24.170 STUDENT: So it's still called a thermohaline, even though in 45:24.167 --> 45:27.367 this case it's just driven by salinity? 45:27.367 --> 45:28.067 PROFESSOR: True. 45:28.067 --> 45:30.967 Because you still got the haline part of it. 45:30.967 --> 45:32.227 So it would still be called thermohaline. 45:35.133 --> 45:39.573 The Mediterranean Sea has one of these, but it works just in 45:39.567 --> 45:40.827 the opposite direction. 45:40.833 --> 45:44.373 Remember, the Mediterranean Sea is, especially in 45:44.367 --> 45:46.667 summertime, a very dry climate. 45:46.667 --> 45:49.297 It doesn't rain there, and there's an excess of 45:49.300 --> 45:52.530 evaporation over precipitation. 45:52.533 --> 45:55.703 Even when you include the rivers that flow into it, 45:55.700 --> 45:58.370 there's an excess of evaporation over 45:58.367 --> 46:00.427 precipitation. 46:00.433 --> 46:04.773 So it becomes saltier than the ocean itself. 46:04.767 --> 46:06.227 And there is a connection. 46:06.233 --> 46:08.273 It's the Straits of Gibraltar. 46:08.267 --> 46:12.397 That little narrow connection between Spain and Morocco that 46:12.400 --> 46:15.200 allows some exchange to occur. 46:15.200 --> 46:20.470 Just remember now, in this case, the Mediterranean water, 46:20.467 --> 46:23.397 while it's fairly warm, the salt outweighs that. 46:23.400 --> 46:25.930 It's a denser water mass. 46:25.933 --> 46:27.833 So here's a cross section through the Straits of 46:27.833 --> 46:30.033 Gibraltar, and here's the 46:30.033 --> 46:31.433 Mediterranean Sea in the ocean. 46:31.433 --> 46:36.133 So that dense, salty water flows out on the bottom, 46:36.133 --> 46:40.133 because it's denser, and the ocean water at 35 parts per 46:40.133 --> 46:42.873 thousand flows in on the top. 46:42.867 --> 46:49.227 A two-layer circulation driven by the salt difference. 46:49.233 --> 46:54.873 Just remember, it's opposite in a sense to the estuary that 46:54.867 --> 46:57.297 I showed you just before this. 46:57.300 --> 47:00.300 This has a place in the history books. 47:00.300 --> 47:05.770 In both World Wars, the German U-boat captains knew about 47:05.767 --> 47:10.467 this, and the British had set up a sonar listening station 47:10.467 --> 47:16.267 at Gibraltar to listen for the engines of U-boats trying to 47:16.267 --> 47:19.667 get in and out of the Mediterranean Sea. 47:19.667 --> 47:24.767 But the U-boats captains realized they could come out 47:24.767 --> 47:28.127 to this place, drop down about 100 meters, turn off their 47:28.133 --> 47:34.873 engines, and then just drift silently past the British into 47:34.867 --> 47:35.967 the Mediterranean. 47:35.967 --> 47:39.397 Then they could start their engines, sink a few ships, and 47:39.400 --> 47:41.930 when they wanted to come back home, they would come over 47:41.933 --> 47:45.903 here, sink a bit deeper, turn off their engines, and this 47:45.900 --> 47:50.170 current would take them back out into the Atlantic past the 47:50.167 --> 47:51.697 British sonar station. 47:51.700 --> 47:54.730 So this was known at the time, and it was used 47:54.733 --> 47:56.403 in a military fashion. 47:59.133 --> 48:00.473 Questions on that? 48:03.200 --> 48:04.670 Now, we've got to get to the wind driven currents. 48:04.667 --> 48:08.067 We won't get very far on it today, but I wanted to alert 48:08.067 --> 48:11.927 you to the complexity of this subject. 48:11.933 --> 48:16.373 We've got to think of a chain of events that occurs as to 48:16.367 --> 48:19.097 how the wind generates the major ocean currents. 48:21.933 --> 48:24.533 And there are four bullets to this. 48:24.533 --> 48:27.673 The wind stress acts on the ocean as 48:27.667 --> 48:30.027 the frictional stress. 48:30.033 --> 48:33.703 That generates an Ekman layer, which moves water off at right 48:33.700 --> 48:36.300 angles to the wind stress. 48:36.300 --> 48:41.300 That Ekman layer air flow then piles up water in certain 48:41.300 --> 48:43.530 parts of the world ocean, giving a little bit of 48:43.533 --> 48:45.673 elevation variation. 48:45.667 --> 48:48.267 We call that ocean topography. 48:48.267 --> 48:51.497 It's not large, it's only a few centimeters or tens of 48:51.500 --> 48:54.570 centimeters, but it's very important. 48:54.567 --> 48:57.627 And then that ocean topography, through the 48:57.633 --> 49:01.573 hydrostatic law, sets up horizontal pressure gradients 49:01.567 --> 49:03.367 in the ocean. 49:03.367 --> 49:06.467 If the water's heaped up here and a little lower here, 49:06.467 --> 49:08.497 you're going to have higher pressure under the heaped up 49:08.500 --> 49:11.500 water, lower pressure under the lower water. 49:11.500 --> 49:13.830 So you're going to have a horizontal pressure gradient. 49:13.833 --> 49:15.833 If you have a horizontal pressure gradient, I think you 49:15.833 --> 49:17.133 know what's going to happen. 49:17.133 --> 49:20.473 You're going to set up a geostrophic current. 49:20.467 --> 49:22.227 A few hours later, you're going to have a 49:22.233 --> 49:24.533 geostrophically balanced current, and that's going to 49:24.533 --> 49:27.203 be an ocean current this time. 49:27.200 --> 49:30.600 So that's what we're going to be looking for. 49:30.600 --> 49:34.530 So the winds over the ocean, for example, in the Atlantic 49:34.533 --> 49:38.673 Ocean, the winds in the tropical regions are from east 49:38.667 --> 49:43.027 to west. The Ekman flux will be towards the north. 49:43.033 --> 49:45.073 Up in mid-latitudes, the winds are westerlies. 49:45.067 --> 49:48.627 The Ekman flux is toward the south. 49:48.633 --> 49:51.933 The Ekman flow is going to pile up water in the middle of 49:51.933 --> 49:53.403 the Atlantic. 49:53.400 --> 49:56.330 And then with that high pressure, you're going to 49:56.333 --> 50:00.503 generate an anti-cyclone in the way of ocean currents 50:00.500 --> 50:03.300 around that going in this same direction. 50:07.467 --> 50:10.867 And for years and years, the theoretical oceanographers 50:10.867 --> 50:13.067 knew that that little variation in sea 50:13.067 --> 50:15.967 level had to be there. 50:15.967 --> 50:22.097 But finally about 15 years ago well, more than that now we 50:22.100 --> 50:25.430 were able to measure this for the first time by flying a 50:25.433 --> 50:30.773 satellite in orbit and having a laser point down from the 50:30.767 --> 50:34.667 satellite, bounce it off the ocean surface, time how long 50:34.667 --> 50:36.267 it takes to get that signal down and 50:36.267 --> 50:37.527 back up to the satellite. 50:40.333 --> 50:42.873 We were able to measure ocean height for the first time 50:42.867 --> 50:46.927 within a few centimeters of accuracy. 50:46.933 --> 50:50.473 Then for the first time, we were able to actually make a 50:50.467 --> 50:53.567 map of this tiny little elevation 50:53.567 --> 50:54.427 in the Earth's surface. 50:54.433 --> 50:55.703 And here it is. 50:55.700 --> 50:58.130 It's called Ocean Dynamic Topography. 50:58.133 --> 51:01.373 It's given in units of centimeters. 51:01.367 --> 51:05.427 And for example, when you see red here, the water is piled 51:05.433 --> 51:09.173 up about that much. 51:09.167 --> 51:11.197 So you wouldn't ever notice this if you were sailing 51:11.200 --> 51:12.130 across the ocean. 51:12.133 --> 51:13.933 Remember, the ocean's five kilometers deep. 51:13.933 --> 51:15.803 What does 40 centimeters mean? 51:15.800 --> 51:21.300 But this is relative to the geoid, relative to a constant 51:21.300 --> 51:23.500 level surface on the ocean. 51:23.500 --> 51:26.600 And it's that piling up then that gives rise to a pressure 51:26.600 --> 51:31.070 gradient that then causes the major ocean 51:31.067 --> 51:32.697 currents in the ocean. 51:32.700 --> 51:35.370 We're out of time, but this will be the theme on Monday. 51:35.367 --> 51:37.197 We'll talk about how these currents get moving.