This cold, saline water sinks because of its greater density. It then flows southward deep along the ocean floor of the Atlantic Ocean through the Indian Ocean, eventually mixing with the surface currents in the Pacific Ocean. The surface current flows back throughout the Indian Ocean into the Atlantic Ocean, returning surface water to the North Atlantic Ocean and driving the Gulf Stream The Gulf Stream flows along the east coast of the United States and across the northern region of the Atlantic Ocean, causing Great Britain and Europe to have fairly moderate temperatures.
Scientists do not completely understand this flow of water, but they think that the influx of freshwater into the North Atlantic Ocean causes a disruption to the flow. Scientists think that if the conveyor slows or stops, the warmer surface water would not be propelled back toward the north Atlantic through the Gulf Stream. Cold water, in general, is denser than warm water. Likewise, water with a high salinity is denser than water that contains less salt. Surface ocean currents are primarily driven by winds.
Deep ocean currents, on the other hand, are mainly a result of density differences. The thermohaline circulation, often referred to as the ocean's "conveyor belt", links major surface and deep water currents in the Atlantic, Indian, Pacific, and Southern Oceans. Multiple mechanisms conspire to increase the density of surface waters at high latitudes.
Cold winds blowing over the oceans chill the waters beneath them. These winds also increase evaporation rates, further removing heat from the water. These chilled waters have increased densities, and thus tend to sink.
Formation of sea ice also helps to increase the density of water near Earth's poles. Thermohaline and wind-driven currents cannot therefore be separated by oceanographic measurements. There are thus two distinct forcing mechanisms, but not two separate circulations. Change the wind stress, and the thermohaline circulation will change; alter thermohaline forcing, and the wind-driven currents will also change. It is because of thermohaline forcing that wind-driven currents are relegated to the upper ocean — in unstratified water they would extend to the bottom.
In these models, different surface forcing fields can be prescribed, and by designating the surface wind-stress as zero, a purely thermohaline circulation can be computed. The required turbulent mixing can be varied independently from the surface wind stress, as they appear in different terms of the hydrodynamic equations.
The resulting zonally integrated circulation is essentially similar to the circulation obtained with wind-stress forcing, but lacks the wind-driven cells known as Ekman cells which consist of surface water that is pushed along by the wind and returns within the upper few hundred metres of the ocean.
On the other hand, when wind stress remains constant, the vertically integrated circulation looks similar with or without thermohaline forcing, with the exception of the Antarctic Circumpolar Current.
Wind-driven and thermohaline circulations can thus be disentangled to some extent by models, which helps us understand what aspects of the circulation are linked to what surface boundary conditions. This is useful in analysing the effect of a change in forcing, such as a freshwater influx, on currents — a typical problem in palaeoclimate studies, in which sediment data suggest that freshwater influx has caused major changes in currents in the past.
It is also highly relevant when considering the ocean's response to global warming, because evaporation, precipitation and runoff are expected to increase in a warmer world. How strongly might changes in thermohaline circulation affect climate? To what extent do Europe's mild winters depend on the transport of heat by the Gulf Stream and North Atlantic Current? This heat transport warms the climate on both sides of the Atlantic, and is therefore not the main reason that Europe is warmer than Newfoundland — this phenomenon is mainly due to the prevailing winds in the two regions.
But ocean currents do make the northern Atlantic much warmer than at comparable latitudes in the northern Pacific. Schlesinger and his team simulated the potential effects with an uncoupled ocean general circulation model and with it coupled to an atmosphere general circulation model.
They found that the thermohaline circulation shut down irreversibly in the uncoupled model simulation, but reversibly in the coupled model simulation. Doing nothing to abate global warming would be foolhardy if the thermohaline circulation shutdown is irreversible. Coauthors are U.
0コメント