Currents, Energy and Climate (3) CCmTY0PKGDs  

Figure 1.  Animation of ocean surface currents by NASA/Goddard Space Flight Center Scientific Visualization Studio

Since oceans cover 71% of the Earth's surface, they are the major receptor and distributor of solar energy.  With currents constantly in motion, the oceans are also continually  exchanging energy and moisture with the atmosphere.  The NASA visualization above shows ocean surface currents, based on actual sea current data collected between June 2005 and December 2007, and demonstrates the dynamic nature of the air/sea interface.   It is the combination of air and ocean currents that distribute solar energy (heat) and determine both short-term weather patterns and long-term climate.

The tropics receive the majority of solar radiation striking the Earth, as it is only between the Tropic of Cancer (23.5o N) and the Tropic of Capricorn (23.5o S) that the sun is directly overhead at least one day of the year. Within this region both vertical and horizontal movements of energy between the large ocean surfaces and the atmosphere drive the climatic patterns of the planet. The totality and interactions of these extraordinarily complex multi-dimensional processes are poorly understood; and may never be completely, and precisely, known.The warm topical waters evaporate moisture into the overlying air, which as it is warmed becomes less dense and rises until directed either north or south, depending upon the hemisphere, by the troposphere. As this moisture laden air rises it also cools, with the water vapor forming clouds and eventually precipitation, which releases heat into the surrounding atmosphere. At about 30o North, or South, the cooled air sinks toward the surface and eventually flows back toward the equator, closing the loop. This flow of equator-bound air currents are called the "trade winds".

In addition to the vertical flux of energy into the atmosphere, energy is carried from the tropics by ocean currents, the most notable of which is the ocean conveyor or the "great ocean conveyor belt". The ocean conveyor (generalized in Figure 1.) is a vast, slow moving system of ocean currents, moving energy and nutrients throughout the planets oceans, and having a major impact upon world climate. More technically known as a density current, or a thermo-haline system for heat and salt, the two factors which predominate in determining the density of sea water. Warm water is lighter, or less dense than cool water, and salt water heavier than fresh water. Other factors being equal, warm water will float on top of cool water and fresh water will float on salt water. In oceans, water density is determined by both temperature and salinity and if the difference in density is great enough, distinct layers may form in the water column, similar to the temperature stratification that may occur in freshwater bodies. Thus in ocean systems, such as the conveyor, less dense surface waters may be circulating in one direction, while denser, deeper waters are traveling in a different direction.


Figure 2. The Great Ocean Conveyor Belt. (Intergovernmental Panel on Climate Change)


The conveyor system also depends upon the contours, or geography, of the ocean floor. In the Atlantic, warm water flows along the surface in a northerly direction, until cooled by evaporation, and polar winds in the Norwegian Sea. This cool water sinks into a deep ocean basin, and flows southward through deep ocean trenches between Labrador, Greenland, Iceland and the British Isles. As the cooling water sinks, it releases large amounts of energy into atmosphere. As shown in the figure, two such locations (determined by the contour of the ocean floor) occur in the North Atlantic. The heat released in these locations is carried by the prevailing winds over Scandinavia, the British Isles and Western Europe, providing a much more moderate climate than other locations at a similar latitude. The third major sink is off the coast of Antarctica.

In contrast to the deep cooler currents, which are largely confined in ocean trenches, the warm surface currents are widespread and diffuse and thus very slow and difficult to detect. Estimates vary, but most fall within a range of 1,000 to 2000 years for a given parcel of water to make a complete round-trip journey.

Interacting with both other ocean currents and with the atmosphere, the conveyor plays a critical role in the distribution of energy and in the distribution of ocean nutrients. The conveyor is critical for the health of our ocean fisheries, which sustain millions of people, and for the agricultural climate of vast regions.

Ocean and air currents transfer heat energy from warm tropics toward the poles, with ocean currents playing a major role. This movement is responsible for the observation that warming of global climate change is more acute in the polar regions than in the temperate zones. Regional, or local, climate is usually determined by air currents, such as the "jet stream" which transfer heat over the land masses, in a general west to easterly direction. As increasing greenhouse gases re-radiate more energy back to the earths surface (global warming) the patterns are air and sea current energy transfer will be altered, resulting in changing regional climates. Thus global warming will be observed a variation in local climates (warmer or cooler, wetter or drier), not necessarily a uniform global temperature increase. Drastic changes in regional climates are presently being experienced around the world.

There is considerable speculation that one of the consequences of global climate change could be a slowdown, or even shutdown of the conveyor system, which would have a drastic impact upon the climate of the British Isles, Western Europe and eastern North America. A probable cause for such an event could be the rapid melting of the Arctic Ice cap and the Greenland Ice Sheet, which are currently pouring vast amounts of fresh water into the North Atlantic. The resulting influx of lower density fresh-water could slow, or even halt, the density driven Great Conveyor System. The loss of the conveyor heat source, and the subsequent inflow of polar air would return the British Isles and Western Europe to the conditions experienced in previous Ice Ages

It is widely believed that a sudden release of melt water from the Canadian Laurentian Shield, and through the Great Lakes, at the end of the Pleistocene caused a shutdown of conveyor consequently a short Ice Age (the Younger Dryas) in the British Isles and Western Europe. A slowdown in the conveyor system also occurred about 500 years ago, with a 300 year period in which substantially cooler temperatures were recorded in Colonial America as well as Europe. Such a climatic change is termed an "Abrupt Climate Change" as it could take place in a period of just a few years.