Professor Freedman introduces the major themes of the course: the crisis of the Roman Empire, the rise of Christianity, the threats from barbarian invasions, and the continuity of the Byzantine Empire.  At the beginning of the period covered in this course, the Roman Empire was centered politically, logistically, and culturally on the Mediterranean Sea. Remarkable for its size and longevity, the Empire was further marked by its tolerance.

Stability in the ocean is based on the density of the water. Density must increase with depth in order for the ocean to be stable. Density is a function of both temperature and salinity, with cold salty water having a higher density than warm fresh water. Temperature and salinity in the ocean can be affected by the atmosphere. Heat can be added to or removed from the ocean, and precipitation and evaporation change the salinity of the ocean. Surface winds also act as a forcing mechanism on the ocean by creating a wind stress forcing which pushes surface waters.

Plate tectonics and ocean bathymetry are discussed. Bathymetry is the study of ocean depth, which is affected in some regions by plate tectonics and mantle dynamics. Mid-ocean ridges are formed at plate boundaries where mantle material is rising to the ocean crust and solidifying as it cools to form new ocean crust material. Seamounts are volcanoes that have formed from molten mantle material pushing up through the ocean crust, but these volcanoes lie below sea level. These features are measured using acoustic depth profiling.

The seasonal cycle on Earth causes shifts in the bands of precipitation in the northern and southern hemispheres. The polar front shifts between high and mid-latitudes which causes a latitudinal shift in the occurrence of frontal cyclones. The Intertropical Convergence Zone also shifts across the equator bringing bands of precipitation to different tropical regions throughout the year. Regional climates on Earth have been classified based on temperature and precipitation values. Areas affected by seasonal shifts in the ITCZ and polar front are included in this classification scheme.

There are several factors that impact climate on Earth. Different areas on Earth have different climates depending on factors such as their latitude and surrounding terrain. Maps of annual average precipitation illustrate these variations in climate. Continentality also affects climate based on the ability to change temperatures on land versus in the oceans and also the imbalance of land mass between the northern and southern hemispheres. Seasonality is a dominant factor in climate.

Mid-latitude frontal cyclones gain energy from temperature gradients rather than latent heat release as is the case with convective storms. They form in the belt of westerly winds and therefore generally move west to east in both the northern and southern hemispheres. A mid-latitude frontal cyclone develops from a kink in the polar front, and eventually warm and cold fronts develop around a low pressure center to form the storm. An example of this type of storm is a nor’easter, which commonly occurs in New England and is named for the northeasterly winds that precede the storm’s arrival.

There are three main types of convective storms: airmass thunderstorms, severe thunderstorms and hurricanes. These storms are all driven by the release of latent heat into the atmosphere during condensation of water vapor. Severe thunderstorms include both squall line thunderstorms and tornados. They acquire energy from water vapor in the atmosphere over land and therefore typically require warm air temperatures and high humidity. Hurricanes gain energy from water vapor evaporated from the ocean surface. This requires warm ocean temperatures, and is the reason hurricanes weaken over land.

Large scale air motion in the atmosphere occurring sufficiently above the surface is in geostrophic balance. Areas of high and low pressure anomalies in the atmosphere are surrounded by rotating flow caused by the balance between the pressure gradient and Coriolis forces. The direction of rotation around these pressure anomalies reverses between the northern and southern hemispheres due to the reversal in sign of the Coriolis force across the equator.

The circulation in the atmosphere is composed of three circulation cells in the northern and southern hemispheres. These cells are caused by the rotation of the Earth which creates the Coriolis force. The Coriolis force deflects northern hemisphere motion to the right and southern hemisphere motion to the left. The majority of large-scale motion in the atmosphere is in geostrophic balance, meaning the Coriolis force acting on the motion is balanced by a pressure gradient force.

There is a latitudinal gradient of heat on the Earth caused by the tilt of the Earth’s axis with respect to the sun. This tilt produces seasonal fluctuations in heat input from the sun, as well as an excess of heat received on average annually near the equator. Heat is transferred poleward by both the ocean and atmosphere in an attempt to balance the Earth’s energy budget. The circulation of the Earth also causes a separation of the atmospheric circulation into three main circulation cells, each transporting heat towards the poles.