WEBVTT 00:02.067 --> 00:03.997 RONALD SMITH: Well we're going to talk about El 00:04.000 --> 00:05.970 Nino today. 00:05.967 --> 00:09.727 I've got a PowerPoint presentation, but before I get 00:09.733 --> 00:11.633 into that, I wanted to give a bunch of definitions. 00:11.633 --> 00:21.303 So El Nino is a naturally occurring oscillation in the 00:21.300 --> 00:27.330 Pacific Ocean between two rather distinct states. 00:31.133 --> 00:37.873 El Nino was defined about 30 or 40 years ago as a state 00:37.867 --> 00:39.427 that the ocean went into occasionally. 00:42.533 --> 00:45.333 Originally it was defined as a condition of having warmer 00:45.333 --> 00:50.903 than usual waters in the Eastern Tropical Pacific, and 00:50.900 --> 00:54.430 low biological productivity. 00:54.433 --> 00:57.503 Those two things were the original definition. 00:57.500 --> 01:00.200 As we began to study it, we realized that there are many 01:00.200 --> 01:04.330 other things going on involving trade wind strength, 01:04.333 --> 01:07.503 Walker circulation, precipitation patterns. 01:07.500 --> 01:13.000 So now it is-- definition has broadened to include a number 01:13.000 --> 01:19.130 of related symptoms, and I'll be discussing those in the 01:19.133 --> 01:20.373 PowerPoint presentation. 01:20.367 --> 01:21.897 Let me go through these two diagrams to 01:21.900 --> 01:23.130 kind of get it started. 01:26.067 --> 01:29.067 Now, one thing that's most curious about this is that 01:29.067 --> 01:34.527 these two states can exist without any external forcing. 01:34.533 --> 01:36.803 It's like one day you wake up in the morning and you feel 01:36.800 --> 01:38.630 great, another morning you wake up in the morning you 01:38.633 --> 01:42.303 feel crummy, and there's nothing external necessarily 01:42.300 --> 01:43.130 that's driving this. 01:43.133 --> 01:46.403 It's just kind of the way it is. 01:46.400 --> 01:48.530 Well El Nino's not exactly like that. 01:48.533 --> 01:50.433 There's a lot of physics involved in it. 01:50.433 --> 01:53.733 But it doesn't seem to be driven by anything external. 01:53.733 --> 01:58.073 It's not the Sun, it's not the tilt of the Earth, it's not 01:58.067 --> 02:02.267 some kind of change occurring by human-induced changes. 02:02.267 --> 02:07.097 It just seems to have periods of several months when it does 02:07.100 --> 02:09.570 one thing and periods of several months when it does 02:09.567 --> 02:11.297 something else. 02:11.300 --> 02:13.830 And this will kind of get you started on the discussion. 02:13.833 --> 02:17.903 So I've got two identical cross-sections here. 02:17.900 --> 02:24.370 They run West to East through the Equatorial Pacific Ocean. 02:24.367 --> 02:29.467 So there's Asia on this side, the Andes in South 02:29.467 --> 02:31.227 America on this side. 02:31.233 --> 02:34.133 The International Date Line would be somewhere in the 02:34.133 --> 02:38.633 middle that'd be 180 degrees West or East longitude. 02:41.233 --> 02:45.273 And most of the time you've got a situation that looks 02:45.267 --> 02:46.267 something like this. 02:46.267 --> 02:53.697 You've got very warm water in the Western Pacific, warm air 02:53.700 --> 02:57.470 as well, and because of that you've got a lot of convective 02:57.467 --> 02:59.397 precipitation. 02:59.400 --> 03:03.100 Sometimes this part of the ocean is called the warm pool. 03:03.100 --> 03:06.800 If you say a warm pool to an oceanographer or to an 03:06.800 --> 03:10.230 atmospheric scientist, they know that you're referring, or 03:10.233 --> 03:13.173 they suspect you're referring to the Western Pacific because 03:13.167 --> 03:17.167 there we get the warmest ocean temperatures anywhere on the 03:17.167 --> 03:20.667 globe, at least during this phase of the El 03:20.667 --> 03:21.927 Nino, La Nina cycle. 03:21.933 --> 03:26.433 So this is a warm pool in the Western Equatorial and 03:26.433 --> 03:28.273 Tropical Pacific. 03:28.267 --> 03:32.567 Lower atmospheric pressure because the air is warmer, and 03:32.567 --> 03:35.197 because the air is warm it's less dense, so the pressure 03:35.200 --> 03:40.170 underneath, warmer air is usually lower pressure. 03:40.167 --> 03:43.497 During those periods of time you have strong Easterly trade 03:43.500 --> 03:48.630 winds, so I've drawn them this way. 03:48.633 --> 03:53.503 The trade winds return a loft in what's called the Walker 03:53.500 --> 03:55.070 circulation. 03:55.067 --> 03:59.527 So remember, the Hadley cell you would see if I sliced it 03:59.533 --> 04:02.333 this way, across the Equator, here I'm slicing 04:02.333 --> 04:03.733 it along the Equator. 04:03.733 --> 04:06.403 So we don't see the Hadley circulation, but we do see 04:06.400 --> 04:09.770 this Walker circulation. 04:09.767 --> 04:13.397 It returns the low level Easterlies in upper level 04:13.400 --> 04:14.530 Westerlies. 04:14.533 --> 04:19.403 In the Eastern Tropical Pacific you have generally 04:19.400 --> 04:23.230 higher air pressure. 04:23.233 --> 04:28.673 Cooler ocean waters, and the cooler waters makes it, at the 04:28.667 --> 04:34.227 surface, makes it easier for nutrient rich water from below 04:34.233 --> 04:37.933 to mix up to the surface, in part due to coastal upwelling 04:37.933 --> 04:41.133 that we spoke about last time. 04:41.133 --> 04:43.233 And as a result of that, you have generally a high 04:43.233 --> 04:50.173 biological productivity during this phase of the cycle. 04:50.167 --> 04:55.227 So let's say then we go into an El Nino situation. 04:55.233 --> 04:59.103 Now the word El Nino comes from the fact that when this 04:59.100 --> 05:06.530 switchover occurs, it usually occurs in November, December 05:06.533 --> 05:11.603 or January, not too far from the Christmas season. 05:11.600 --> 05:14.170 The countries along the West Coast of South America are 05:14.167 --> 05:19.327 generally Catholic in their religion, and the Christ child 05:19.333 --> 05:20.533 is worshipped. 05:20.533 --> 05:23.773 And the fact that this switchover, when it occurs, 05:23.767 --> 05:27.467 usually occurred within a few weeks of Christmastime meant 05:27.467 --> 05:29.397 that it was given the name El Nino, the 05:29.400 --> 05:31.100 child, the Christ child. 05:31.100 --> 05:33.630 That's where the term comes from. 05:33.633 --> 05:35.603 So it has kind of everything reversed. 05:35.600 --> 05:40.270 You've got weaker trade winds, cooler than usual ocean 05:40.267 --> 05:44.027 temperatures and air temperatures, higher air 05:44.033 --> 05:48.433 pressures, the Walker circulation is weakened, I 05:48.433 --> 05:52.003 haven't drawn it here lower pressure than usual in the 05:52.000 --> 05:54.900 East side of the Pacific. 05:54.900 --> 05:59.770 Along with precipitation occurring along the coast of 05:59.767 --> 06:02.267 South American near the Equator. 06:02.267 --> 06:05.527 With the warm water here you have strongly 06:05.533 --> 06:09.103 stabilized the ocean. 06:09.100 --> 06:11.630 Warm water floating above cold water. 06:11.633 --> 06:14.503 The nutrients are still down there below, but they can't 06:14.500 --> 06:20.530 mix to the surface, so you get a low biological productivity 06:20.533 --> 06:25.503 in that part of the cycle. 06:25.500 --> 06:26.300 Any questions on that? 06:26.300 --> 06:26.730 Yes. 06:26.733 --> 06:29.073 STUDENT: Why does the Walker Circulation weaken? 06:29.067 --> 06:31.327 PROFESSOR: Well because it's part of this-- 06:31.333 --> 06:36.133 the air that goes Westward in the trade wind has to move 06:36.133 --> 06:37.073 back Eastward. 06:37.067 --> 06:41.067 So that whole circulation is called the Walker circ-- 06:41.067 --> 06:45.597 it's just closing the cycle on the strong 06:45.600 --> 06:47.270 Easterly trade winds. 06:47.267 --> 06:51.027 But remember, when air in the Walker circulation reaches 06:51.033 --> 06:56.303 this point and then tries to descend to match up with this, 06:56.300 --> 06:59.630 that descending air is going to prevent clouds from forming 06:59.633 --> 07:02.903 here, and that's consistent with what I've drawn. 07:02.900 --> 07:06.830 And here, when this air has to ascend to then form the rest 07:06.833 --> 07:09.673 of the Walker circulation, the ascent occurs in these 07:09.667 --> 07:10.767 convective clouds. 07:10.767 --> 07:12.567 So that's consistent as well. 07:12.567 --> 07:14.927 Then when I've weakened the Walker circulation, that's 07:14.933 --> 07:16.403 consistent with the way I've drawn the 07:16.400 --> 07:18.230 clouds there as well. 07:18.233 --> 07:20.633 So there are physical laws connecting all of 07:20.633 --> 07:22.103 these things together. 07:22.100 --> 07:25.230 But on this diagram, and in fact, in most of what I'll be 07:25.233 --> 07:28.073 saying today, there's going to be very little explanation 07:28.067 --> 07:32.197 about why the system would switch back and forth. 07:32.200 --> 07:36.100 So I'm mostly going to be talking about what we observe 07:36.100 --> 07:40.630 the atmosphere ocean to do in this part of the world, how 07:40.633 --> 07:45.003 the various symptoms tie together physically. 07:45.000 --> 07:47.970 But the scientific community does not have a clear 07:47.967 --> 07:51.067 understanding of what would make this system suddenly 07:51.067 --> 07:54.467 switch from one phase to the other. 07:54.467 --> 07:57.397 So I'm not going to spend a lot of time dealing with that. 07:57.400 --> 08:01.130 It's kind of one of the remaining unknowns. 08:01.133 --> 08:02.873 It's not that there hasn't been work on it, it's not 08:02.867 --> 08:06.397 there hasn't been theories proposed, but I would say it's 08:06.400 --> 08:09.730 not a solved problem in terms of what causes the system to 08:09.733 --> 08:11.503 cycle back and forth like this. 08:14.067 --> 08:21.397 So I'll get in now to the way some of these things look. 08:30.067 --> 08:31.327 And stop me if you have questions. 08:38.633 --> 08:42.633 So it's a natural oscillation in the air-sea state of the 08:42.633 --> 08:46.173 Tropical Pacific Ocean. 08:46.167 --> 08:49.167 I'm going to start out with a lot of attention 08:49.167 --> 08:51.167 paid to this region. 08:51.167 --> 08:55.067 We described it last time as a region of coastal upwelling. 08:55.067 --> 08:58.767 And that, indeed, is where the studies of El Nino began. 08:58.767 --> 09:01.127 That's where it was first identified, and that's where a 09:01.133 --> 09:03.703 number of the early definitions come from is the 09:03.700 --> 09:07.700 variability along the Peruvian Coast there. 09:11.967 --> 09:14.567 And to remind you, a typical well, this is not necessarily 09:14.567 --> 09:19.427 typical, this is a temperature anomaly from a year ago, just 09:19.433 --> 09:23.233 about a year ago today these are what the sea surface 09:23.233 --> 09:27.273 temperature anomalies look like. 09:27.267 --> 09:30.867 The blue means cooler, so you've got a cooler water than 09:30.867 --> 09:34.927 usual in the Eastern Tropical Pacific a year ago. 09:34.933 --> 09:37.603 And which side of the cycle? 09:37.600 --> 09:40.700 Is that El Nino or La Nina? 09:40.700 --> 09:41.600 STUDENT: La Nina. 09:41.600 --> 09:42.570 PROFESSOR: La Nina, right. 09:42.567 --> 09:49.297 So a year ago, there was a La Nina existing, because you can 09:49.300 --> 09:52.400 identify that immediately because of the colder than 09:52.400 --> 09:55.930 usual conditions in the sea surface temperature in the 09:55.933 --> 09:58.403 Eastern Tropical Pacific. 09:58.400 --> 10:01.200 So a year ago there was La Nina, and near the end of the 10:01.200 --> 10:03.700 presentation I'll bring that up to date, we'll see what 10:03.700 --> 10:08.330 state the Tropical Pacific has today. 10:08.333 --> 10:11.673 So here's what you need to know for terminology. 10:11.667 --> 10:14.327 I wasn't able to get the umlauts over the n's there. 10:14.333 --> 10:18.873 But El Nino and La Nina, which are the phases of this 10:18.867 --> 10:23.267 oscillation, the Southern Oscillation Index is defined 10:23.267 --> 10:27.967 as the pressure difference between Darwin in Australia, 10:27.967 --> 10:35.327 and Tahiti in the Eastern Central Pacific Ocean, and 10:35.333 --> 10:39.203 that is a measure of this pressure oscillation that I 10:39.200 --> 10:39.930 told you about. 10:39.933 --> 10:43.203 So on occasion you'll see plots of the Southern 10:43.200 --> 10:47.270 Oscillation Index, and that's the pressure 10:47.267 --> 10:50.367 symptom of this cycle. 10:50.367 --> 10:52.097 The Walker circulation I've defined. 10:52.100 --> 10:56.100 It's a circulation that occurs more or less in the plane of 10:56.100 --> 11:00.900 the Equator, taking some of the trade wind flow and 11:00.900 --> 11:04.600 returning it back to the East aloft in the upper 11:04.600 --> 11:06.670 troposphere. 11:06.667 --> 11:10.967 In a few minutes I'll show you the TOA array, which we now 11:10.967 --> 11:14.227 use to monitor El Nino. 11:14.233 --> 11:17.233 It's quite a remarkable set of instrumentation. 11:17.233 --> 11:20.133 Sea surface temperature we talked about before, and we've 11:20.133 --> 11:24.233 already defined thermocline and primary productivity. 11:24.233 --> 11:27.133 I'll be using those terms freely as we go through the 11:27.133 --> 11:28.573 discussion today. 11:28.567 --> 11:33.197 So be sure you're clear on all of those terminologies. 11:33.200 --> 11:35.330 So here's a list of some of the symptoms then. 11:35.333 --> 11:38.373 So during El Nino you've got reduced biological 11:38.367 --> 11:41.667 productivity in the Eastern Pacific. 11:41.667 --> 11:44.167 Warm water in the Eastern Pacific. 11:44.167 --> 11:45.797 Weak or reversed trade winds. 11:45.800 --> 11:48.530 Lower pressure in the Eastern Pacific. 11:48.533 --> 11:52.773 Rain in the East, and as I'll show you later on, there are 11:52.767 --> 11:56.867 some distant climate anomalies that are connected with El 11:56.867 --> 12:00.727 Nino, but occur in other parts of the world as well. 12:00.733 --> 12:05.333 I'll get to that later in the lecture. 12:05.333 --> 12:08.173 So here's kind of where it began. 12:08.167 --> 12:10.367 As you saw from the chlorophyll map, it's a very 12:10.367 --> 12:15.127 highly productive region along the Peruvian Coast because of 12:15.133 --> 12:19.703 the cold water being brought up in the Humboldt current or 12:19.700 --> 12:25.270 the Peru current, generally destabilizing that ocean. 12:25.267 --> 12:30.027 And then the winds, the trade winds, with the Ekman layer 12:30.033 --> 12:34.273 pumping water offshore causing coastal upwelling bringing 12:34.267 --> 12:36.897 nutrient rich waters to the surface. 12:36.900 --> 12:40.300 Well given that then, and here's an example of a 12:40.300 --> 12:44.430 Peruvian fishing boat catching anchovies. 12:44.433 --> 12:46.873 Given that understanding, what would cause such a phenomenon 12:46.867 --> 12:51.027 where the anchovy catch increased from the '50s up to 12:51.033 --> 12:54.403 the '60s, and then in the early '70s, especially in 12:54.400 --> 12:56.300 1973, it crashed. 12:56.300 --> 12:58.970 Well this is a buildup of the fishing fleet. 12:58.967 --> 13:06.827 More and more--more and more fish, and people, and ships 13:06.833 --> 13:08.403 involved in fishing. 13:08.400 --> 13:10.130 What would cause a sudden drop? 13:10.133 --> 13:11.133 Two possibilities. 13:11.133 --> 13:13.733 It could be overfishing. 13:13.733 --> 13:18.133 But it turns out later on this rebounds again. 13:18.133 --> 13:19.803 So it turns out it's not overfishing. 13:19.800 --> 13:23.100 It was some change in the natural condition of the 13:23.100 --> 13:26.730 Eastern Tropical Pacific, and that was the onset of one of 13:26.733 --> 13:29.173 the strongest El Nino situations. 13:29.167 --> 13:33.027 So it was first defined as this drop in biological 13:33.033 --> 13:36.773 productivity, which we know has something to do with 13:36.767 --> 13:40.827 coastal upwelling, the stability of the ocean 13:40.833 --> 13:45.473 thermocline, and we'll follow that train of cause and 13:45.467 --> 13:48.297 effect as we go. 13:48.300 --> 13:51.370 Here's the way it's defined today. 13:51.367 --> 13:54.067 Largely in terms of the surface temperature of the 13:54.067 --> 13:57.897 Pacific Ocean, and the depth of the thermocline. 13:57.900 --> 14:04.530 So during La Nina, you have generally a steeply inclined 14:04.533 --> 14:10.503 thermocline, deeper in the Western Pacific with lots of 14:10.500 --> 14:12.130 warm water then. 14:12.133 --> 14:15.603 But shallower in the Eastern Pacific, which means the cold 14:15.600 --> 14:17.700 water is brought up very close to the 14:17.700 --> 14:21.600 surface, even to the surface. 14:21.600 --> 14:24.130 The normal condition is halfway in between, and then 14:24.133 --> 14:28.373 the El Nino situation is when the thermocline is flatter 14:28.367 --> 14:29.727 across the Pacific Ocean. 14:32.267 --> 14:35.667 Remember the Date Line is in here, so this is 120 East and 14:35.667 --> 14:38.527 this is 80 degrees West. The Date Line is 14:38.533 --> 14:40.673 somewhere in there. 14:40.667 --> 14:43.897 But this means you've got a big deep layer of warm water 14:43.900 --> 14:45.170 near the Eastern-- 14:47.600 --> 14:51.600 in the Eastern part of the Tropical Pacific preventing 14:51.600 --> 14:55.600 efficient mixing of nutrient rich waters to the surface. 14:55.600 --> 15:00.130 So that's the way we envision this today after 30 or 40 15:00.133 --> 15:02.503 years of study. 15:02.500 --> 15:06.500 And the Walker circulation looks something like this. 15:06.500 --> 15:11.330 Now, in the normal circulation you get strong trade winds, 15:11.333 --> 15:15.633 rising motion over the warm pool, and then a return of air 15:15.633 --> 15:18.333 aloft towards the East and then sinking. 15:18.333 --> 15:21.803 During the Walker circulation that is weaker and breaks up 15:21.800 --> 15:25.330 and you get rising motion in the East. And this may 15:25.333 --> 15:26.073 exaggerate it. 15:26.067 --> 15:29.827 This shows an actual reverse trade winds. 15:29.833 --> 15:31.133 That isn't always the case. 15:31.133 --> 15:35.533 But at least they are weaker than they would be during the 15:35.533 --> 15:36.933 typical situation. 15:36.933 --> 15:41.833 So weakened trade winds, and low pressure and rising motion 15:41.833 --> 15:46.273 in the Eastern Tropical Pacific. 15:46.267 --> 15:48.697 The ocean surface temperatures differ dramatically. 15:48.700 --> 15:53.770 Here's an example of a map derived by satellite using 15:53.767 --> 15:56.567 infrared radiation emitted from the sea surface to 15:56.567 --> 15:57.897 determine its temperature. 15:57.900 --> 16:02.370 During El Nino, and indeed, these are anomalies I believe, 16:02.367 --> 16:04.997 rather than absolute temperatures. 16:05.000 --> 16:08.400 Yeah, but they show this warm anomaly as much as three or 16:08.400 --> 16:11.330 four degrees warmer than normal for sea surface 16:11.333 --> 16:15.333 temperature in this coastal region, and then extending out 16:15.333 --> 16:20.733 along the Equator in the Eastern Tropical Pacific. 16:20.733 --> 16:23.533 So the temperature differences are really substantial. 16:23.533 --> 16:23.903 Yes? 16:23.900 --> 16:24.700 STUDENT: What's the difference between an anomaly and a 16:24.700 --> 16:27.200 normal temperature map? 16:27.200 --> 16:27.500 PROFESSOR: Yeah. 16:27.500 --> 16:29.170 So what they do is subtract-- 16:29.167 --> 16:36.027 for every pixel, they subtract off a long-term average sea 16:36.033 --> 16:37.433 surface temperature. 16:37.433 --> 16:40.503 So what you end up with is of course, when you plot, then, 16:40.500 --> 16:44.970 normal conditions, it's a very small variation because you 16:44.967 --> 16:48.097 subtracted off most of the normal considerations, most of 16:48.100 --> 16:49.330 the normal pattern. 16:49.333 --> 16:52.403 So what's left allows you to amplify these changes. 16:52.400 --> 16:55.170 So you just subtract the normal I think some of the 16:55.167 --> 16:58.997 plots you're working with in lab this week, some of that 16:59.000 --> 17:01.730 data was given to you in terms of anomaly, rather than the 17:01.733 --> 17:02.733 actual temperature. 17:02.733 --> 17:07.033 So it's a common trick played by climatologists to bring out 17:07.033 --> 17:09.203 the differences between the current state 17:09.200 --> 17:12.430 and the normal state. 17:12.433 --> 17:16.403 It doesn't mean, for example, that these are the warmest 17:16.400 --> 17:17.670 temperatures in the ocean. 17:17.667 --> 17:21.067 It might still be slightly warmer over here, but because 17:21.067 --> 17:24.097 that's normally warm that doesn't show up in this 17:24.100 --> 17:25.370 anomaly map. 17:29.633 --> 17:32.533 I mentioned global impacts. 17:32.533 --> 17:35.473 This is not completely understood either, but people 17:35.467 --> 17:39.797 have once they understood this cycle of changing conditions 17:39.800 --> 17:43.870 in the Tropical Pacific, they tried to then correlate it to 17:43.867 --> 17:46.427 climate in other parts of the world. 17:46.433 --> 17:50.303 There's been a few rather well-known papers been 17:50.300 --> 17:54.330 published on this the last 5 or 10 years. 17:54.333 --> 17:57.673 For example, here's what we think the El Nino weather 17:57.667 --> 18:01.197 patterns are for the winter season. 18:01.200 --> 18:04.830 If you have an El Nino in the particular year in the winter 18:04.833 --> 18:09.033 season, Northern Hemisphere winter, December, you should 18:09.033 --> 18:18.203 expect to get generally, drier conditions in through here, 18:18.200 --> 18:19.330 warmer conditions there. 18:19.333 --> 18:20.733 Well that's part of the definition, I 18:20.733 --> 18:21.833 have that in my diagram. 18:21.833 --> 18:24.233 But elsewhere you can get anomalies as well. 18:24.233 --> 18:28.033 For example, even here in New England, you might have a 18:28.033 --> 18:31.733 warmer than typical winter because of some kind of remote 18:31.733 --> 18:35.273 effect of this El Nino cycle. 18:35.267 --> 18:40.497 In the Pacific Northwest you'd have warmer conditions, in the 18:40.500 --> 18:44.670 Southern part of the US you'd have drier conditions. 18:44.667 --> 18:47.227 In the summertime the relationships work a little 18:47.233 --> 18:49.273 bit different, but they're shown here. 18:54.867 --> 18:59.867 One of the ways we monitor El Nino is to keep track of the 18:59.867 --> 19:05.567 ocean surface temperatures in these four kind of commonly 19:05.567 --> 19:06.697 defined areas. 19:06.700 --> 19:12.770 All the scientists who work on El Nino agree to monitor these 19:12.767 --> 19:13.967 four temperatures. 19:13.967 --> 19:15.867 Sea surface temperature in area one, 19:15.867 --> 19:17.467 two, three, and four. 19:17.467 --> 19:21.167 So you'll find plot, for example, of Nino four or Nino 19:21.167 --> 19:24.827 three, a temperature plotted as a function of time. 19:24.833 --> 19:26.333 It's right on the Equator. 19:26.333 --> 19:30.133 Usually it goes 5 degrees North to 5 degrees South, and 19:30.133 --> 19:32.733 that's kind of a commonly agreed upon way 19:32.733 --> 19:35.403 to monitor El Nino. 19:35.400 --> 19:36.500 And here's how it's done. 19:36.500 --> 19:43.000 So there's a rather remarkable array of buoys that was put in 19:43.000 --> 19:47.830 a decade or so ago across the Equator in the Pacific. 19:47.833 --> 19:51.603 Many of these have thermistor chains that go down into the 19:51.600 --> 19:55.300 ocean a couple hundred meters, so you can monitor not only 19:55.300 --> 19:57.630 conditions at the surface, but subsurface 19:57.633 --> 19:59.403 temperatures as well. 19:59.400 --> 20:02.030 That's where that diagram about the depth of the 20:02.033 --> 20:08.073 thermocline came from, looking at the thermistor chain data 20:08.067 --> 20:10.897 dangling beneath these buoys. 20:10.900 --> 20:13.600 But you can log in, you can just go to their website at 20:13.600 --> 20:17.570 anytime day or night and get current data for what the 20:17.567 --> 20:19.827 ocean is doing in this part of the world. 20:19.833 --> 20:22.133 So it makes it a very if you've got a new theory, if 20:22.133 --> 20:25.103 you've got a better theory than anyone else, you can test 20:25.100 --> 20:28.070 it immediately because this data is available, not only 20:28.067 --> 20:31.867 historically going back 10 years or so, but it's 20:31.867 --> 20:35.197 available day by day, so you can test new theories and 20:35.200 --> 20:37.470 maybe make predictions. 20:37.467 --> 20:41.197 So it's become a very kind of open-ended egalitarian subject 20:41.200 --> 20:44.100 where anybody with a better idea can put it forward 20:44.100 --> 20:48.600 because the data is there for anybody to work on. 20:52.300 --> 20:55.770 So here's a couple of plots then. 20:55.767 --> 21:00.327 One is the ocean temperature departures for the combination 21:00.333 --> 21:04.003 of Nino three and four. 21:04.000 --> 21:07.470 And then below that is the SOI, the Southern Oscillation 21:07.467 --> 21:10.667 Index, which is defined as the pressure difference, it's 21:10.667 --> 21:13.627 Tahiti minus Darwin. 21:13.633 --> 21:17.073 Right, Tahiti is the Eastward-most station of the 21:17.067 --> 21:21.767 two, and Darwin is, of course, is in Northern Australia. 21:21.767 --> 21:24.367 So be sure you get the sign of this right. 21:29.133 --> 21:31.803 So, you know, you wouldn't expect to find such a tight 21:31.800 --> 21:36.270 relationship between these two rather different quantities. 21:36.267 --> 21:38.197 One is a sea surface temperature, one is an 21:38.200 --> 21:40.200 atmospheric pressure. 21:40.200 --> 21:42.270 However there is a relationship between the two, 21:42.267 --> 21:44.197 and I think let's see which way it works. 21:44.200 --> 21:51.900 So when the Eastern Tropical Pacific is warmer, that means 21:51.900 --> 21:56.270 Tahiti has a let's see, which way does that work. 21:56.267 --> 22:02.727 By the way, this is plotted not in pressure units, but in 22:02.733 --> 22:06.333 units of standard deviations. 22:06.333 --> 22:08.703 So there's a normal fluctuation to this pressure 22:08.700 --> 22:10.200 difference. 22:10.200 --> 22:13.000 You compute the standard deviation and then you mark 22:13.000 --> 22:16.630 whether you are close to normal relative to one 22:16.633 --> 22:20.503 standard deviation, or one or two or three standard 22:20.500 --> 22:22.770 deviations away from normal. 22:22.767 --> 22:24.767 So that's the way that is. 22:24.767 --> 22:26.827 That standard deviation, again, I think is the quantity 22:26.833 --> 22:30.833 you're dealing with in lab this week. 22:30.833 --> 22:38.973 So generally, when you have warmer conditions you have 22:38.967 --> 22:44.467 lower pressure in Tahiti relative to Darwin. 22:44.467 --> 22:48.427 That would make sense because if there's warmer conditions 22:48.433 --> 22:57.173 in the East, which would be this condition, with warmer 22:57.167 --> 23:00.997 air aloft hydrostatically, that would mean lower 23:01.000 --> 23:03.000 atmospheric pressure beneath it. 23:03.000 --> 23:05.300 So there is a physical law connecting these two, but 23:05.300 --> 23:08.530 still, it's a bit interesting that it comes out to be such a 23:08.533 --> 23:09.703 nice relationship. 23:09.700 --> 23:13.470 So where are the El Nino periods then? 23:13.467 --> 23:18.467 Well one of the most dramatic ones was in 1973, and that was 23:18.467 --> 23:20.727 the drop I showed you in the ocean 23:20.733 --> 23:23.033 productivity in that region. 23:23.033 --> 23:26.733 There have been some other since then, especially 1983, 23:26.733 --> 23:32.873 '84, '87, '88, '92, and then one in '90 I guess 23:32.867 --> 23:34.467 that would be '98. 23:34.467 --> 23:37.727 And you can spot them either from the ocean temperature 23:37.733 --> 23:40.773 departures or from that pressure difference, 23:40.767 --> 23:44.667 East-West, across the Pacific Ocean. 23:47.767 --> 23:50.467 Questions on this? 23:50.467 --> 23:52.967 So how often does it occur? 23:52.967 --> 23:54.627 Well it's not periodic. 23:54.633 --> 23:58.673 It's not as if you can find an equal spacing. 23:58.667 --> 24:01.597 If you had to make a guess, maybe you would say that it's 24:01.600 --> 24:05.130 about every, what, five to seven years, 24:05.133 --> 24:06.573 something like that. 24:06.567 --> 24:11.527 But it's not periodic, it's not predictable, and, you know 24:11.533 --> 24:14.003 the first person that comes up with a really accurate way to 24:14.000 --> 24:18.430 predict El Nino will be rich and famous, because it's an 24:18.433 --> 24:22.703 unsolved geophysical problem, but it also has big 24:22.700 --> 24:25.930 implications for the way people live, for agriculture, 24:25.933 --> 24:27.633 for fishing, and so on. 24:27.633 --> 24:33.103 So it's a big question as to what causes these changes and 24:33.100 --> 24:36.570 how to predict them in the future. 24:36.567 --> 24:37.827 No questions on that? 24:40.967 --> 24:42.867 So where are we today? 24:45.400 --> 24:50.970 This is from a week or so ago in 2011. 24:50.967 --> 24:54.167 Here's a sea surface temperature map, and a sea 24:54.167 --> 24:55.667 surface temperature anomaly. 24:55.667 --> 24:57.597 So we'll get to address that question 24:57.600 --> 24:58.970 of map versus anomaly. 24:58.967 --> 25:03.827 So there's the warm pool, and the temperature is given here 25:03.833 --> 25:07.103 so there's actually temperatures higher than 28 25:07.100 --> 25:10.530 degrees Celsius in the warm pool. 25:10.533 --> 25:13.473 Generally, we've got pretty cold conditions it looks like 25:13.467 --> 25:14.967 here, but what's the anomaly? 25:14.967 --> 25:17.927 Well the anomaly is cold too. 25:17.933 --> 25:23.433 So we are in another La Nina situation. 25:23.433 --> 25:26.573 Has it been a La Nina ever since last year at this time? 25:26.567 --> 25:31.597 Well here's a plot of Nino one, two, three, four, and 25:31.600 --> 25:34.770 then just Nino four, and look what's happened. 25:34.767 --> 25:38.067 We were in a La Nina a year ago. 25:38.067 --> 25:41.367 We came out of it during the summer. 25:41.367 --> 25:45.727 These are anomalies plotted, SST anomalies for these 25:45.733 --> 25:47.933 regions we defined in here. 25:47.933 --> 25:49.533 And now we've slid back into it. 25:49.533 --> 25:52.573 So it never was really an El Nino I would say. 25:52.567 --> 25:56.527 It never got strong enough or persisted long enough to have 25:56.533 --> 25:57.133 an El Nino. 25:57.133 --> 26:03.203 So it wouldn't--19, sorry, 2010, 2011 would not show up 26:03.200 --> 26:05.800 as a El Nino period. 26:05.800 --> 26:11.700 And now we're solidly back into a La Nina situation with 26:11.700 --> 26:14.330 cold water in the Eastern Pacific, high 26:14.333 --> 26:17.633 productivity, and so on. 26:17.633 --> 26:21.633 Questions on how to read these diagrams? 26:21.633 --> 26:24.133 So I want you to be able to look at a map like this and 26:24.133 --> 26:27.973 tell what is the state of the Pacific Ocean. 26:27.967 --> 26:28.467 Yeah, Jordan. 26:28.467 --> 26:29.367 STUDENT: Is the one and two referring to, like, where the 26:29.367 --> 26:30.597 temperature is--? 26:33.433 --> 26:35.673 PROFESSOR: They just sum the two together. 26:35.667 --> 26:38.127 They take the let's go back to that. 26:38.133 --> 26:42.533 They often don't want to distinguish between the two, 26:42.533 --> 26:45.973 so they'll just sum these two together. 26:45.967 --> 26:48.867 And sometimes they do that with three and four, they sum 26:48.867 --> 26:49.867 the two together. 26:49.867 --> 26:54.397 It all depends what kind of detail you want to have in 26:54.400 --> 26:55.730 your SST description. 26:55.733 --> 26:59.703 STUDENT: I'm sorry, was that 3.4 that, like dotted red and 26:59.700 --> 27:01.430 green thing? 27:01.433 --> 27:02.533 PROFESSOR: Where am I looking here? 27:02.533 --> 27:05.333 STUDENT: On the previous slide that you were just on. 27:05.333 --> 27:11.073 Is that 3.4, like the red and green overlap area? 27:11.067 --> 27:11.897 PROFESSOR: Yeah...maybe that's right. 27:11.900 --> 27:13.600 Maybe that's 3. 27:13.600 --> 27:14.330 you're quite right. 27:14.333 --> 27:15.903 That's not the average of the two. 27:15.900 --> 27:19.630 That's actually four plus that little bit of three. 27:19.633 --> 27:20.433 That's correct. 27:20.433 --> 27:22.233 Thanks for spotting that. 27:22.233 --> 27:23.633 I was wrong on that. 27:23.633 --> 27:24.203 Melanie, is that right? 27:24.200 --> 27:25.430 STUDENT: It's half of four and half of three. 27:27.400 --> 27:28.430 PROFESSOR: Oh, it's half of the two. 27:28.433 --> 27:28.733 Sorry. 27:28.733 --> 27:33.233 So 3.4 is that part of four and that part of three. 27:33.233 --> 27:35.833 Melanie's our El Nino expert. 27:35.833 --> 27:39.573 So she'll tell me afterwards all the things 27:39.567 --> 27:40.827 I said wrong today. 27:44.267 --> 27:47.267 Let's see. 27:47.267 --> 27:52.927 OK now, here's another way to look at it. 27:52.933 --> 27:55.333 I think I've shown you diagram like this before. 27:55.333 --> 27:58.633 It's called a Hovmuller diagram. 27:58.633 --> 28:02.573 You take data along the Equator, and this is a 28:02.567 --> 28:07.797 longitude scale where the Date Line is right there. 28:07.800 --> 28:11.230 And then you plot sea surface temperature well this is wind 28:11.233 --> 28:18.373 in this case trade winds, as a function of time. 28:18.367 --> 28:22.767 So you get a time-distance diagram. 28:22.767 --> 28:27.197 So at each time you can see what the strength of the trade 28:27.200 --> 28:32.100 winds were across the Pacific Ocean. 28:32.100 --> 28:37.800 So these are the trade wind strengths between 28:37.800 --> 28:40.330 5 North and 5 South. 28:40.333 --> 28:44.403 They are negative because a positive velocity would be 28:44.400 --> 28:48.370 towards the East, and as you know, trade winds blow towards 28:48.367 --> 28:53.067 the West. So generally you've got trade winds all through 28:53.067 --> 28:56.327 this part of the Central Pacific, and these are the U 28:56.333 --> 29:01.573 anomalies where you subtract off the normal. 29:01.567 --> 29:05.897 And indeed, what we're finding there is that we actually have 29:05.900 --> 29:12.830 a bit of a reverse anomaly in the Eastern Pacific during the 29:12.833 --> 29:14.633 last year or so. 29:18.767 --> 29:21.267 And that is--that's connected with the La Nina situations 29:21.267 --> 29:24.197 you look at my diagram over there. 29:24.200 --> 29:28.030 So it's not that the trade winds reversed, but they're 29:28.033 --> 29:32.433 weaker than normal, at least in the Eastern Tropical 29:32.433 --> 29:35.503 Pacific, and that gave rise to the or that was consistent 29:35.500 --> 29:37.670 with the La Nina situation [Correction: The diagram shows 29:37.667 --> 29:37.797 stronger trades near the dateline and weaker trades in 29:37.800 --> 29:37.830 the eastern Pacific. 29:37.833 --> 29:39.333 La Nina is consistent with the former anomaly.]. 29:39.333 --> 29:41.703 Now what's going on beneath the surface in 29:41.700 --> 29:44.170 this month of 2011? 29:44.167 --> 29:47.867 Again, trying to understand how to understand these maps. 29:51.067 --> 29:54.727 Here is the temperature anomaly in degrees Celsius for 29:54.733 --> 29:56.733 a few days ago. 29:56.733 --> 29:59.203 From the TAO array. 29:59.200 --> 30:03.900 And there is warmer than usual ocean water beneath the 30:03.900 --> 30:07.270 surface, but not much of an anomaly at the surface in the 30:07.267 --> 30:09.027 Western Tropical Pacific. 30:09.033 --> 30:12.433 However, in the Eastern Tropical Pacific, cold beneath 30:12.433 --> 30:14.673 the surface, but also at the surface. 30:14.667 --> 30:17.327 And we see it here expressed in a vertical section where 30:17.333 --> 30:21.103 the thermocline is there allowing that cold water to 30:21.100 --> 30:22.930 come up to the surface. 30:22.933 --> 30:27.833 And that, again, is consistent with the La Nina situation 30:27.833 --> 30:32.233 that we saw here with colder conditions in the Eastern 30:32.233 --> 30:34.473 Tropical Pacific. 30:34.467 --> 30:40.567 So I think that is everything about El Nino. 30:40.567 --> 30:43.727 And we can stop and discuss this for a few minutes. 30:43.733 --> 30:45.303 Are there some questions about El Nino? 30:45.300 --> 30:45.570 Yeah. 30:45.567 --> 30:49.167 STUDENT: So when you say it's an El Nino time, you're not 30:49.167 --> 30:52.127 talking about the whole year, you're have to be talking 30:52.133 --> 30:53.633 about-it would be kind of a specific period of time? 30:53.633 --> 30:54.603 Does it change during--? 30:54.600 --> 30:55.870 PROFESSOR: Yeah, let me be clear on that. 30:55.867 --> 30:59.767 So usually when you start an El Nino it starts around 30:59.767 --> 31:04.797 Christmastime, but then it could last a few months or 31:04.800 --> 31:06.970 even a year. 31:06.967 --> 31:09.327 So we would refer to that full period of time, as long as it 31:09.333 --> 31:12.673 persists, as the El Nino period. 31:12.667 --> 31:17.227 I think I showed you a case where it lasted longer than-- 31:17.233 --> 31:24.473 for example, well that one there was really, it extended 31:24.467 --> 31:27.327 about a year and a half. 31:27.333 --> 31:33.033 Whereas that one was really gone in less than one year. 31:33.033 --> 31:36.133 And this one you had kind of a weak one, but it persisted 31:36.133 --> 31:40.333 actually for several years. 31:40.333 --> 31:44.333 So this comes back to the question of predictability. 31:44.333 --> 31:45.803 It's kind of random. 31:45.800 --> 31:50.630 And when you go into a El Nino, you're not sure if 31:50.633 --> 31:53.033 you're going to come right back out of it. 31:53.033 --> 31:56.003 It could kind of diddle along for a while in a weak El Nino 31:56.000 --> 31:58.130 and last a bit longer. 31:58.133 --> 32:05.133 But I'd say more times than not it's over in about a year. 32:05.133 --> 32:07.033 You come out of that thing about 12 months after 32:07.033 --> 32:09.473 you went into it. 32:09.467 --> 32:11.427 Does that help, Rachel? 32:11.433 --> 32:12.003 Other questions? 32:12.000 --> 32:12.800 Yes, Victoria. 32:12.800 --> 32:15.230 STUDENT: If it's not an El Nino, is it always an El 32:15.233 --> 32:15.903 Nino--La Nina? 32:15.900 --> 32:17.470 PROFESSOR: Well so this is confusing. 32:17.467 --> 32:19.727 The terminology has changed a bit on this. 32:19.733 --> 32:24.403 Originally, there was normal and there was El Nino. 32:24.400 --> 32:26.400 Those were the two things we heard 32:26.400 --> 32:29.030 scientists talking about. 32:29.033 --> 32:32.573 Some clever individual decided that they should think of this 32:32.567 --> 32:36.527 as really a yin/yang situation where it was one thing or 32:36.533 --> 32:37.673 another, rather than normal. 32:37.667 --> 32:40.027 So if you're going to come up with something that's the 32:40.033 --> 32:44.503 opposite of El Nino, well it's going to be La Nina. 32:44.500 --> 32:47.670 I don't like it myself, but now it's already embedded in 32:47.667 --> 32:50.327 the literature as being kind of the opposite extreme with a 32:50.333 --> 32:51.733 range of normal in between. 32:51.733 --> 32:55.933 So current terminology would be El Nino-- 32:55.933 --> 32:56.873 well, it's here-- 32:56.867 --> 33:00.297 El Nino, normal if you're, say, within one standard 33:00.300 --> 33:02.500 deviation or so of this. 33:02.500 --> 33:07.570 And then La Nina you're down in the other extreme. 33:07.567 --> 33:10.597 So if you read the literature over the years, you see that 33:10.600 --> 33:12.470 terminology shift a little bit. 33:12.467 --> 33:15.197 But don't be confused about it. 33:15.200 --> 33:18.130 In the old days, everything like this was normal and that 33:18.133 --> 33:19.473 was El Nino. 33:19.467 --> 33:24.527 Today it's El Nino, a range of normal, and then La Nina out 33:24.533 --> 33:27.103 the other side. 33:27.100 --> 33:27.470 Yes. 33:27.467 --> 33:29.897 STUDENT: When you showed us the trade winds diagram and 33:29.900 --> 33:34.230 the anomalies, didn't you say that when they had strong 33:34.233 --> 33:36.703 winds it was a El Nino? 33:36.700 --> 33:38.670 But what you drew is--? 33:38.667 --> 33:39.967 PROFESSOR: Let's go back and check that because I 33:39.967 --> 33:41.827 may have misspoken. 33:41.833 --> 33:48.703 So what I'm claiming over here is that the La Nina, this 33:48.700 --> 33:51.870 normal situation with colder water, has a 33:51.867 --> 33:55.297 stronger trade winds. 33:55.300 --> 33:56.470 And--yeah, maybe that was opposite-- 33:56.467 --> 33:57.997 let's go back and look at that diagram and see 33:58.000 --> 34:00.970 if I have that right. 34:00.967 --> 34:02.827 Thanks for pointing that out. 34:02.833 --> 34:04.433 Let's see, that diagram was-- 34:07.767 --> 34:09.027 yeah. 34:11.733 --> 34:13.573 My computer has locked here. 34:26.700 --> 34:28.270 Here, was it? 34:28.267 --> 34:30.897 Yeah. 34:30.900 --> 34:35.000 Yeah, so it may be I put the emphasis on the Eastern part, 34:35.000 --> 34:37.570 maybe I shouldn't have done that. 34:37.567 --> 34:41.327 We expect that during a La Nina situation we have strong 34:41.333 --> 34:43.933 trade winds. 34:43.933 --> 34:45.933 And that would probably correspond to this. 34:45.933 --> 34:47.933 This is the anomaly. 34:47.933 --> 34:50.433 So over the Central Pacific not the Eastern Pacific over 34:50.433 --> 34:52.373 the Central Pacific we had stronger 34:52.367 --> 34:53.297 than usual trade winds. 34:53.300 --> 34:54.530 STUDENT: So that's stronger, the negative? 34:57.367 --> 34:59.127 PROFESSOR: Negative would be remember, trade winds 34:59.133 --> 35:05.333 are from East to West. So when you see a negative anomaly in 35:05.333 --> 35:09.503 a region which is normally negative, that means it's a 35:09.500 --> 35:13.830 stronger Easterly wind. 35:13.833 --> 35:15.773 STUDENT: Is it measuring wind speek, or--? 35:15.767 --> 35:17.967 PROFESSOR: No, it's not--it's U, it's the 35:17.967 --> 35:19.867 East-West components of the wind. 35:19.867 --> 35:22.997 Remember when you did the balloon lab and you plotted up 35:23.000 --> 35:29.600 the data from your pilot balloon, you plotted U, which 35:29.600 --> 35:33.070 was positive towards, with the wind toward the East, negative 35:33.067 --> 35:34.867 towards the West. It's that convention 35:34.867 --> 35:37.127 that we're using here. 35:37.133 --> 35:39.833 You can be sure of that because this is the normal 35:39.833 --> 35:43.273 situation and those numbers are negative, indicating that 35:43.267 --> 35:47.067 that's an Easterly wind blowing towards the West. 35:47.067 --> 35:51.467 That's the negative sense of this quantity U that I'm 35:51.467 --> 35:52.027 discussing. 35:52.033 --> 35:52.373 Yes. 35:52.367 --> 35:54.867 STUDENT: Where are the hectoPascals coming from? 35:54.867 --> 35:56.367 Is that what hPa stands for? 36:03.433 --> 36:06.273 PROFESSOR: Yeah, that's a pressure unit. 36:06.267 --> 36:09.627 They're referring to the level at which this is measured. 36:09.633 --> 36:13.633 So this is at the--this is not at the sea surface. 36:13.633 --> 36:18.703 This is the 850 hectoPascal level, which is about a 36:18.700 --> 36:22.830 kilometer and a half above the ocean surface. 36:22.833 --> 36:25.673 Remember again, when you're doing the balloon lab and you 36:25.667 --> 36:28.927 plotted things with altitude, very often atmospheric 36:28.933 --> 36:32.673 scientists use pressure as their indication of how high 36:32.667 --> 36:34.927 you are in the atmosphere. 36:34.933 --> 36:39.403 So this is data not at the sea surface, but about 1,500 36:39.400 --> 36:46.370 meters above the sea surface, at the 850 hectoPascal level. 36:46.367 --> 36:46.897 Good question. 36:46.900 --> 36:47.170 Yeah. 36:47.167 --> 36:49.467 STUDENT: What is this map called again? 36:49.467 --> 36:51.367 PROFESSOR: Well it's called a Hovmuller diagram. 36:51.367 --> 36:56.767 H-O-V-M-U-L-L-E-R. Hovmuller diagram. 36:56.767 --> 36:58.167 And it's used commonly in 36:58.167 --> 37:00.597 atmospheric and oceanic sciences. 37:00.600 --> 37:03.700 Whenever you want to see how data along a 37:03.700 --> 37:07.700 line varies with time. 37:07.700 --> 37:10.800 So it wouldn't always have to be longitude. 37:10.800 --> 37:12.630 You could construct some other line. 37:12.633 --> 37:16.173 But in this case, they've chosen East-West as their 37:16.167 --> 37:19.727 linear dimension, and then time going up 37:19.733 --> 37:22.003 as their time dimension. 37:26.000 --> 37:27.800 Other questions on this? 37:27.800 --> 37:28.170 Yes. 37:28.167 --> 37:31.167 STUDENT: Would you go over why it's negative? 37:31.167 --> 37:35.297 PROFESSOR: Why these numbers are negative? 37:35.300 --> 37:40.870 The convention is for this quantity U, it's positive when 37:40.867 --> 37:44.327 the wind blows toward the East, and negative when it 37:44.333 --> 37:52.303 blows towards the West. You do see a little bit of positive 37:52.300 --> 37:55.670 wind near the edges. 37:55.667 --> 38:00.527 But over the vast majority of the Central Tropical Pacific 38:00.533 --> 38:05.573 you have the trade winds, and that is a wind that blows from 38:05.567 --> 38:08.767 East to West. We call them Easterlies because of where 38:08.767 --> 38:12.327 they're blowing from, and with this U convention that's a 38:12.333 --> 38:15.273 negative on the sign convention. 38:19.967 --> 38:22.667 I'm glad you asked that because this is confusing with 38:22.667 --> 38:25.827 those signs there. 38:25.833 --> 38:28.903 Other questions on these diagrams? 38:28.900 --> 38:32.730 STUDENT: And the one to the right is during an El Nino? 38:32.733 --> 38:33.533 PROFESSOR: Well the one on the 38:33.533 --> 38:36.033 right is the anomaly. 38:36.033 --> 38:39.803 So what they've done is taken this data and subtracted off 38:39.800 --> 38:45.730 the normal, the long-term average, to get an anomaly. 38:45.733 --> 38:51.503 So when you find a positive anomaly, that would be a 38:51.500 --> 38:55.530 weaker trade wind. 38:55.533 --> 38:57.573 It's only two meters per second where 38:57.567 --> 38:58.767 this was six and eight. 38:58.767 --> 39:02.267 So what that's going to do is basically drop the strength of 39:02.267 --> 39:05.227 the trade winds by a couple of meters per second, but keep 39:05.233 --> 39:08.203 them going towards the West. 39:08.200 --> 39:11.670 STUDENT: So those values are the actual speed or are they 39:11.667 --> 39:13.167 the difference from the average? 39:13.167 --> 39:15.097 PROFESSOR: They are the difference, they are the 39:15.100 --> 39:18.730 actual minus the average. 39:18.733 --> 39:21.803 I haven't shown you the average here, but these 39:21.800 --> 39:27.970 numbers are this number minus whatever is the normal wind at 39:27.967 --> 39:29.827 that--at each location. 39:38.267 --> 39:39.527 Anything else on El Nino? 39:45.533 --> 39:47.003 So I think we have a few minutes to 39:47.000 --> 39:49.130 start the next subject. 39:49.133 --> 39:50.373 Let me find this-- 40:02.033 --> 40:03.433 We won't get very far into this. 40:06.033 --> 40:14.273 The subject which comes next in the course is ice. 40:14.267 --> 40:16.797 So it's a little odd because we're jumping from El Nino, 40:16.800 --> 40:21.000 which is primarily a Tropical phenomenon, to the study of 40:21.000 --> 40:23.900 ice in the climate system, which is primarily a high 40:23.900 --> 40:25.330 latitude subject. 40:25.333 --> 40:28.303 But they're both very important in the way the 40:28.300 --> 40:32.100 atmosphere and the ocean work to form our climate. 40:32.100 --> 40:36.930 I wanted to first just run through a set of definitions 40:36.933 --> 40:40.973 for the different types of ice that we'll be interested in. 40:40.967 --> 40:47.427 Sea ice is frozen seawater, where you take seawater, you 40:47.433 --> 40:51.903 blow cold wind over it, you draw the heat out of it, you 40:51.900 --> 40:54.130 bring it down to the freezing point, and then 40:54.133 --> 40:55.673 eventually you freeze it. 40:55.667 --> 40:58.697 That is sea ice. 40:58.700 --> 41:03.930 Ice sheets, like Greenland and Antarctica, are large plateaus 41:03.933 --> 41:08.603 of ice formed from compacted snow. 41:08.600 --> 41:11.800 Snow that fell month after month, year after year, piled 41:11.800 --> 41:14.700 up, squeezed, compressed to form ice. 41:14.700 --> 41:16.870 So the origins of these two things are 41:16.867 --> 41:18.067 almost completely different. 41:18.067 --> 41:21.527 In one case you freeze seawater, the other case you 41:21.533 --> 41:24.133 just compact snow. 41:24.133 --> 41:30.433 Glaciers are streams of moving ice. 41:30.433 --> 41:32.533 They're moving under the influence of gravity. 41:32.533 --> 41:36.633 So normally if you've got a snow fall on the top of a 41:36.633 --> 41:40.533 mountain that builds up, after a while it gets deep enough, 41:40.533 --> 41:43.703 it begins to be pulled gravitational down the slopes 41:43.700 --> 41:46.030 of the mountain. 41:46.033 --> 41:52.233 Ice shelves are fixed, floating ice sheets. 41:52.233 --> 41:56.373 They're basically ice that's come off an ice sheet, flowed 41:56.367 --> 41:59.227 down to sea level in a glacier, and then has spread 41:59.233 --> 42:05.773 out over the ocean surface still fixed to the land. 42:05.767 --> 42:08.767 I'll show you diagrams of all of these. 42:08.767 --> 42:10.597 Icebergs, I'm sure you know that one. 42:10.600 --> 42:16.130 Those are chunks of drifting, floating ice broken off from 42:16.133 --> 42:18.103 glaciers or ice shelves. 42:18.100 --> 42:20.330 And then, of course, on land, you can have permanent or 42:20.333 --> 42:23.373 seasonal snow fields, just areas that have received 42:23.367 --> 42:26.267 either just snow for that winter and it's going to melt 42:26.267 --> 42:30.297 off the next summer, or snow that is able to persist over 42:30.300 --> 42:33.930 the summer season and still be there the next year would be 42:33.933 --> 42:36.833 called a permanent snow field. 42:39.600 --> 42:44.300 So these are important terms in the descriptions I'll be 42:44.300 --> 42:48.270 going through mostly next time. 42:48.267 --> 42:49.467 These are equally important. 42:49.467 --> 42:53.567 These are the physical properties of ice that we'll 42:53.567 --> 42:57.527 need to know in order to understand the way ice works 42:57.533 --> 42:59.373 in the climate system. 42:59.367 --> 43:03.397 First of all, there's this latent heat 43:03.400 --> 43:07.630 of melting or freezing. 43:07.633 --> 43:09.473 You can look it up, you can Google it. 43:09.467 --> 43:15.297 The value is 334,000 Joules per kilogram. 43:15.300 --> 43:20.500 Every time you take water and freeze it, you have to remove 43:20.500 --> 43:22.970 that much heat. 43:22.967 --> 43:27.227 And every time you take ice and melt it, you have to put 43:27.233 --> 43:30.233 that same amount of heat back in. 43:30.233 --> 43:32.303 So it's a reversible process. 43:32.300 --> 43:36.770 That number applies to either melting or freezing. 43:39.300 --> 43:40.830 And you can do that in a variety of ways. 43:40.833 --> 43:44.173 You can radiate that heat away, you can have cold winds 43:44.167 --> 43:47.797 or warm winds, provide that heat, or remove that heat. 43:47.800 --> 43:50.200 But in any case, whenever you're melting or creating 43:50.200 --> 43:53.670 ice, you've got to deal with that amount of heat. 43:53.667 --> 43:54.767 It's a large number. 43:54.767 --> 43:58.527 It's not as large as the latent heat of condensation. 43:58.533 --> 44:01.033 Remember, the latent heat of condensation was many times 44:01.033 --> 44:02.773 that number. 44:02.767 --> 44:05.827 You might want to look back and see what that number was. 44:05.833 --> 44:09.173 This is still big, but it's not as big as a number needed 44:09.167 --> 44:14.067 to condense water or to evaporate water. 44:14.067 --> 44:16.827 I want to remind you also that while the freezing point of 44:16.833 --> 44:21.403 fresh water is zero degrees Celsius in fact, that's the 44:21.400 --> 44:27.100 basis on which the Celsius scale is defined for salt 44:27.100 --> 44:30.400 water with a salinity of approximately 35 parts per 44:30.400 --> 44:34.800 thousand, that freezing point is depressed to about minus 44:34.800 --> 44:36.600 two Celsius. 44:36.600 --> 44:40.730 You'd have to cool the water a bit colder if it's sea water 44:40.733 --> 44:41.973 before it'll start to freeze. 44:45.400 --> 44:48.230 This third bullet, I'm sure you know this, but I want to 44:48.233 --> 44:51.973 impress on you how remarkable it is. 44:51.967 --> 44:56.967 Most objects, most materials, when you freeze them from the 44:56.967 --> 45:01.867 liquid they reduce their volume. 45:01.867 --> 45:04.927 That is, they become more dense. 45:04.933 --> 45:08.533 Almost every subject known to man does that. 45:08.533 --> 45:11.333 Water is one of the very few exceptions that when you 45:11.333 --> 45:14.733 freeze it it actually expands. 45:14.733 --> 45:16.633 I think the only other one that I know that 45:16.633 --> 45:19.703 does this is bismuth. 45:19.700 --> 45:24.070 Bismuth has that same property of water as when you condense 45:24.067 --> 45:27.497 it from the liquid to the solid it actually expands. 45:27.500 --> 45:33.070 Now you know this, because if you leave a bottle of water 45:33.067 --> 45:36.667 out on a cold night and it freezes, it'll expand and 45:36.667 --> 45:37.997 break the jar. 45:38.000 --> 45:39.270 So you know it's expanding. 45:39.267 --> 45:42.997 You also know it because when you drop an ice cube into a 45:43.000 --> 45:45.730 glass of water, it doesn't sink to the bottom, it floats 45:45.733 --> 45:46.473 on the top. 45:46.467 --> 45:48.697 You're so accustomed to seeing that, but I want you to kind 45:48.700 --> 45:51.800 of marvel at that the next time you see it. 45:51.800 --> 45:56.870 That the solid is actually less dense then the liquid. 45:56.867 --> 45:59.767 It's a very rare circumstance, it's a very rare situation. 45:59.767 --> 46:03.527 But it's very controlling in the way ice works in the 46:03.533 --> 46:04.233 climate system. 46:04.233 --> 46:07.533 The fact that it floats on top of the ocean, for example, 46:07.533 --> 46:09.133 rather than sinking to the bottom. 46:09.133 --> 46:11.103 It changes everything in terms of the way the 46:11.100 --> 46:13.800 climate system works. 46:13.800 --> 46:19.570 So a typical density for ice is about 917 kilograms per 46:19.567 --> 46:22.297 cubic meter. 46:22.300 --> 46:25.700 Typical density for fresh water is about 1,000. 46:25.700 --> 46:29.430 Typical density for sea water's about 1,025. 46:29.433 --> 46:34.073 So this is less dense than both of those, and would float 46:34.067 --> 46:38.927 either in fresh water or in seawater. 46:38.933 --> 46:40.173 Any questions on that? 46:42.500 --> 46:43.470 This is an interesting one. 46:43.467 --> 46:48.227 If you've got a bucket of seawater and you freeze it, or 46:48.233 --> 46:54.373 start to freeze it, as the water freezes, it'll expel 46:54.367 --> 46:56.967 most of the salt. 46:56.967 --> 47:00.897 So that if you take a chunk of that ice that you formed and 47:00.900 --> 47:05.370 melt it, the salinity is going to be far less than the salt 47:05.367 --> 47:06.467 water it was made from. 47:06.467 --> 47:07.627 I'll give you an example. 47:07.633 --> 47:10.133 If you start with a bucket of ocean water with a salinity of 47:10.133 --> 47:16.003 say, 35 parts per thousand, and you froze it very slowly, 47:16.000 --> 47:19.100 you could actually generate ice that when melted had a 47:19.100 --> 47:22.930 salinity of perhaps only 5 parts per thousand. 47:22.933 --> 47:25.533 It really does expel most of the salt when 47:25.533 --> 47:28.333 you freeze that water. 47:28.333 --> 47:31.573 And that plays a big role, as you might imagine, in the way 47:31.567 --> 47:33.067 the Arctic Ocean works. 47:33.067 --> 47:39.127 When you form sea ice, the water beneath it gets saltier 47:39.133 --> 47:44.473 because you've expelled that salt into the ocean. 47:44.467 --> 47:46.827 The Inuit people know this. 47:46.833 --> 47:55.003 When they're out travelling over the sea ice hunting, if 47:55.000 --> 47:58.370 they need water, well, you could dig a hole through the 47:58.367 --> 48:02.627 ice and get sea water to drink, but humans cannot live 48:02.633 --> 48:04.933 on seawater, it's too salty. 48:04.933 --> 48:07.803 But they could knock off a chunk of-- 48:07.800 --> 48:09.800 of course, the best thing would be if there's snow. 48:09.800 --> 48:12.030 If there's snow on top of the ice, gather 48:12.033 --> 48:12.973 that up and melt it. 48:12.967 --> 48:14.827 That'll be fresh water. 48:14.833 --> 48:18.603 But if there's no snow on the ice, take a chunk of the sea 48:18.600 --> 48:22.000 ice and melt it down and you could live on that. 48:22.000 --> 48:24.530 It's not very tasty, it's brackish water, but you could 48:24.533 --> 48:29.233 live on that because it's enough fresher than the ocean 48:29.233 --> 48:33.773 water was, that the human can exist on that particular kind 48:33.767 --> 48:35.197 of brackish water. 48:35.200 --> 48:37.230 So this is important. 48:37.233 --> 48:41.403 For climate purposes it's important that ice and snow is 48:41.400 --> 48:45.370 highly reflective of sunlight. 48:45.367 --> 48:50.197 The typical albedo for ocean water is 48:50.200 --> 48:53.330 something less than 0.1. 48:53.333 --> 48:58.073 It absorbs more than 90% of the light that hits it. 48:58.067 --> 49:04.697 On land, a more typical albedo is something like .2, 0.2. 49:04.700 --> 49:08.230 But yet for ice and snow, the albedo is more like 0.8. 49:08.233 --> 49:12.103 In other words, something like 80% of the radiation that hits 49:12.100 --> 49:14.230 it is reflected back to space. 49:14.233 --> 49:16.933 And that, as you can imagine, albedos playing such an 49:16.933 --> 49:19.673 important role in climate, that's an important role for 49:19.667 --> 49:23.997 ice controlling the albedo of the surface. 49:24.000 --> 49:26.600 And the last one I want to mention is that ice, while we 49:26.600 --> 49:29.930 think of it as something brittle, when you take a chunk 49:29.933 --> 49:33.203 of ice and hit it with a hammer, it will shatter. 49:33.200 --> 49:36.230 When it's under high enough pressure it'll flow like a 49:36.233 --> 49:38.603 liquid, like a viscous liquid. 49:38.600 --> 49:41.400 And we'll talk about that when it comes to how glaciers and 49:41.400 --> 49:42.330 ice sheets move. 49:42.333 --> 49:46.033 So review these and we'll start on the description of 49:46.033 --> 49:48.403 ice and the climate system next time.