The Ocean Currents Climate and Biosphere

The Role of the Ocean Currents in the Climate System

Modern ocean circulation represents a complex three-dimensional phenomenon which is determined by the Earth's geography and spatial patterns of surface wind, and surface heat and freshwater fluxes. Surface ocean currents are directly driven by wind and the existence of large-scale oceanic gyres (Figure 1) is explained by prevailing westerlies in the mid-latitudes and trade winds in the tropics. The divergence of surface wind-driven currents creates upward vertical movement of water (upwelling), which plays an important role in nutrients supply to the upper ocean layer. Apart from that, winds and tidal energy are the primary sources of vertical mixing in the ocean interior. Without vertical mixing provided by wind and tides, the deep ocean would be essentially stagnant. Surface fluxes of heat and freshwater, although do not represent a direct energy source for the ocean currents, play an important role in controlling the ocean circulation by changing sea water temperature and salinity. The latter determine horizontal density gradient which drives the currents in the ocean interior.

The balance between surface heat and freshwater fluxes also determines the areas where the deep ocean water masses are formed. Currently, these deep water masses are formed in several isolated locations: in the Nordic Seas and the Labrador Sea in the North Atlantic, and around Antarctica. Although the areas of deep water formation occupy only a small fraction of the ocean, they play a fundamental role in driving of the meridional overturning circulation, also known as the ocean thermohaline circulation or 'the ocean conveyor belt'. The upper branch of the ocean conveyor is represented by the northward transport of warm water masses along the surface currents of which Gulf Stream is the most prominent one (Figure 1). When reaching high latitudes of the North Atlantic, surface water is cooled down by losing energy into the atmosphere and eventually reaches the high density which allows surface water to sink to the bottom of the ocean. This water then slowly moves southward along the American continental slope and reaches the Southern Ocean, where it mixes with the deep water masses formed around Antarctica. It is believed that most of deep water eventually rises to the surface in the Southern Ocean in the areas of wind-driven upwelling, thus closing the conveyor loop. The existence of the thermohaline circulation is closely related to the existence of deep water formations areas. At present, there is no deep water formation in

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Major Ocean Currents

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Figure 1 A simplified cartoon of the surface (red) and deep ocean currents (blue). The major areas of deep water formation are shown by ovals. Yellow dots indicate the upper branch and light blue dots indicate the lower branch of the Atlantic thermohaline circulation.

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Figure 1 A simplified cartoon of the surface (red) and deep ocean currents (blue). The major areas of deep water formation are shown by ovals. Yellow dots indicate the upper branch and light blue dots indicate the lower branch of the Atlantic thermohaline circulation.

the Pacific Ocean, and, as a result, there is no the thermohaline circulation in this ocean. The later explains very different climate conditions in the high latitudes of the Atlantic and Pacific oceans.

Although an average velocity associated with the meridional overturning circulation is rather small compared to typical velocities of surface ocean currents, the meridional overturning circulation is responsible for a large portion of the ocean meridional heat transport. Currently, about 1 PW of energy (1 PW = 1015W) is transported northward in the North Atlantic, that is about one-fifth of the total energy transport in the atmosphere-ocean system in the Northern Hemisphere. The influence of the ocean currents on climate is illustrated by Figure 2 a. It shows deviations of local annual surface air temperature from its zonally averaged values. It is seen that annual air temperature over the northern North Atlantic, and, especially over the Nordic Seas, is much higher than average temperature for the same latitudes. Thus the main reason for mild climate conditions over most of Europe is the existence of vigorous Atlantic thermohaline circulation. Since the release of heat transported by the oceanic currents into the atmosphere occurs primary during winter, this prevents forming of the sea ice in the high latitudes, and results in a considerable reduction of the amplitude of seasonal temperature variations. As shown in Figure 2b, the difference between summer and winter temperatures over the Western Europe is much smaller than for the same latitudes in Asia and North America. All these factors, in combination with a stable moisture transport from warm North Atlantic, allow the existence of extended temperate and broad-leaf forests over most of Europe.

Ocean Currents and Climate Change

The importance of the ocean currents for climate and climate change has been demonstrated in a number of modeling studies, which showed that the Atlantic thermohaline circulation may change rapidly in response to change in climatological conditions, such as increased freshwater flux into the North Atlantic due to massive iceberg discharge from surrounding ice sheets, as it happened many times during the glacial age, or due to intensification of atmospheric hydrological cycle and melting of the Greenland ice sheets, that may happen in the future as a result of global warming. Changes in the thermohaline circulation, in turn, lead to dramatic changes in the ocean heat transport and global climate.

Numerical experiments performed with climate models demonstrate that a complete shutdown of the Atlantic thermohaline circulation under present-day climate conditions will cause surface air temperature cooling by more than 10 °C over the Nordic Seas and northwestern Europe. The cooling is caused by cessation of the northward oceanic heat transport into high latitudes and amplified by a southward expansion of sea ice margin. The cooling is most pronounced in winter when it is almost twice stronger than in annual mean. Changes in the ocean currents not only affect local temperature but via several oceanic and atmospheric teleconnection mechanisms spread the climate change over the globe. In particular, the cooling caused by the shutdown of the thermohaline circulation is simulated over most of the Northern Hemisphere, although the magnitude of cooling in other areas is smaller than that over the northern North Atlantic. At the same time, a decrease of interhemispheric oceanic heat transport causes a warming in the Southern Hemisphere, most pronounced in the Southern Atlantic and around Antarctica. The

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Heat Transport The Biosphere

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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