We are now on the west coast of Greenland, in a small town called Qeqertarsuaq. Here, the fjord behind me is always filled with large amounts of icebergs. This is because the Greenland ice sheet is continuously shedding ice into the ocean. However, we are at about the same latitude as Norway, where there is no ice in the fjords, and hardly any ice on land. Why is this so? And how is the Sun’s energy redistributed on the surface of the Earth? These are the topics of this week’s lecture.
As you learnt in previous lectures, the Sun is the main driver of climate. However, the amount of energy received by the Earth at the top of the atmosphere, at a given time, varies with latitude. Incoming solar radiation is strongest close to the equator, where the Sun’s rays strike nearly directly overhead. However, close to the poles, such as here in Norway, the Sun’s rays strike at an angle, and the incoming radiation is spread over a larger area of the atmosphere. As a consequence, the tropics receive a larger amount of energy per unit area compared to the high latitudes. To balance the visible incoming short-wave solar radiation, the Earth’s atmosphere emits infrared long-wave thermal radiation.
The amount of outgoing long-wave radiation depends on the temperature of the emitting substance. In the case of the atmosphere, the cold poles and cold cloud tops emit the least, whereas the warm, dry, cloudless areas of the tropics emit the most. The thermal radiation emitted to space by the atmosphere does not decrease as rapidly with latitude as the absorption of incoming solar radiation. In the tropics, incoming solar radiation exceeds the outgoing thermal radiation, giving a net surplus of heat retained by the atmosphere. Poleward of about 40 degrees, there is a net deficit of incoming solar radiation compared to the outgoing thermal radiation. As a result, the atmosphere loses energy to space at high latitudes.
To summarize - the annual mean radiation balance at the top of the atmosphere is negative poleward of about 40 degrees of latitude, and positive equatorward of 40 degrees. This latitudinal gradient in annual mean radiation must be balanced by a poleward flux of energy by the climate system. By integrating the top of the atmosphere net radiative flux, we can calculate the amount of poleward energy transport required in each hemisphere. This inferred poleward energy transport peaks at about 5 pettawats in the mid-latitudes, and is the result of atmosphere and ocean heat transport. If there was no such poleward heat transport, the tropics would be warmer and the polar regions would be much colder.
In other words, the poleward transport of heat by the atmosphere and the ocean greatly reduces the temperature contrast between the high and the low latitudes. How much of the poleward transport is driven by the atmosphere and how much is driven by the ocean? This question is hard to answer, as total heat transport is very difficult to measure. However, meteorological measurements and satellite observations combined with models give an estimate of the atmospheric heat transport. Now, but subtracting the atmospheric contribution from the total transport we can estimate the ocean heat transport as a residual. At about 30 degrees of latitude, the contributions by the atmosphere and the ocean are approximately equal. Poleward of this latitude, the atmosphere dominates the heat transport.
The ocean heat transport on the other hand peaks at about 20 degrees of latitude, and contributes very little to the transport of heat at high latitudes.
Atmospheric circulation is a key component of the climate system. Not only does it give us the winter storms we are so used to here in Bergen, it also carries large quantities of heat and moisture from the tropics to high latitudes. In the atmosphere the annual mean meridional, or north-south, circulation is described by warm air rising close to the equator. Aloft, this warm air flows towards the poles, cools, and sinks in the subtropics, forming what is called the Hadley cells. In the mid-latitudes weaker cells, referred to as the Ferrell cells, circulate in the opposite direction. In effect, the Ferrell cells are transporting heat from high to low latitudes.
However, the Ferrell cells and the mean meridional circulation contribute very little to the total atmospheric heat transport. It turns out, that the main contribution to transport of heat to high latitudes is governed by atmospheric eddies. These same atmospheric eddies, or cyclones and anticyclones, are also responsible for everyday weather in the mid-latitudes. This is a phenomenon people on the west coast of Norway are very much familiar with. We frequently experience weather systems coming up from the tropics and hitting the coast with large changes in winds, temperature and precipitation. The cyclones which dominate the mid-latitudes have a peak in heat transport at about 50 degrees of latitude.
Many of them originate in the tropics, and in addition to heat, they carry with them large quantities of moisture. This moisture can be seen in the figure as streaks of white, and you will notice that the cyclones, or storm systems, track across the Atlantic in a north-easterly direction. Precipitation events are coloured in pink, and in particular during early winter, you will see large precipitation events along the west coast of Norway. Several of these events are caused by cyclones originating in the tropics.
About 70% of the Earth’s surface is covered by ocean. The ocean receives more than half of the energy entering the climate system, and has an enormous capacity to store heat. The absorption of solar energy is balanced by evaporation of water at the ocean surface, providing moisture and heat to the atmosphere. The atmosphere, in part, drives the circulation of the ocean through the stress exerted by the winds on the surface. The wind driven circulation is dominated by the large gyres in each ocean basin, as well as the strong Antarctic circumpolar current. Along the western boundaries of the major ocean basins you will pick out strong poleward flowing currents. In particular, the Kuroshio off Japan and the Gulf Stream off North America.
These warm, fast flowing currents are capable of carrying significant amounts of heat poleward. This poleward heat transport, by the western boundary currents, is supported by a slow flow of cold water in the interior of the ocean basins. Note that the western boundary currents, such as the Gulf Stream, produce eddies. These eddies are equivalent to the atmospheric cyclones and anticyclones. However, the heat transport by the ocean eddies is much less. Now, what is the imprint of atmospheric and oceanic circulation on climate and weather in the high latitudes? As we have shown, the main contribution is a net transport of heat and moisture to the polar regions, compensating for the loss of heat at the top of the atmosphere.
However, there are large regional differences in climate. As an example, the climate in western Europe is relatively mild compared to the east coast of Canada, although they are at the same latitude. As we have learnt in this lecture, the redistribution of heat by the ocean and atmosphere is key to setting the climate at any point on the Earth’s surface. This is particularly true at high latitudes, such as here in Greenland, where the input of energy from the Sun is very small. In the winter months, the Sun is hardly above the horizon, and disappears for nearly two months. To summarize, the mild winters in Norway and in western Europe, is due to the combined effects of ocean and atmospheric circulation.
In particular, the Gulf Stream and it’s extension into the Nordic seas combined with persistent westerly winds, is key to the warm temperatures in Norway. Had it not been for these combined effects, the climate in Norway would have been very similar to here in Greenland.