Variations in formation and circulation of ocean water may cause some of the thousands of years to decadal scale variations in climate. Cold water forms in the Arctic and Weddell seas. This cold, salty water is denser than other water in the ocean, so it sinks to the bottom and gets ponded behind seafloor topographic ridges, periodically spilling over into other parts of the oceans. The formation and redistribution of North Atlantic cold bottom water accounts for about 30 percent of the solar energy budget input to the Arctic ocean every year. Eventually this cold bottom water works its way to the Indian and Pacific oceans, where it upwells, gets heated, and returns to the North Atlantic. Thermohaline circulation is the vertical mixing of seawater driven by density differences caused by variations in temperature and salinity. Variations in temperature and salinity are found in waters that occupy different ocean basins and those found at different levels in the water column. When the density of water at one level is greater than or equal to that below that level, the water column becomes unstable and the denser water sinks, displacing the deeper, less-dense waters below. When the dense water reaches the level at which it is stable, it tends to spread out laterally and form a thin sheet, causing intricately stratified ocean waters. Thermohaline circulation is the main mechanism responsible for the movement of water out of cold polar regions and exerts a strong influence on global climate. The upward movement of water in other regions balances the sinking of dense cold water, and these upwelling regions typically bring deep water, rich in nutrients, to the surface. Thus regions of intense biological activity are often associated with upwelling regions.
The coldest water on the planet is formed in the polar regions, with large quantities of cold water originating off the coast of Greenland and in the Weddell sea of Antarctica. The planet's saltiest ocean water is found in the Atlantic ocean, and this is moved northward by the Gulf stream. As this water moves near Greenland it is cooled, then sinks to flow as a deep cold current along the bottom of the western North Atlantic. The cold water of the Weddell sea is the densest on the planet, where surface waters are cooled to -35.4°F (-1.9°C), then sink to form a cold current that moves around Antarctica. some of this deep cold water moves northward into all three major ocean basins, mixing with other waters and warming slightly. most of these deep ocean currents move at a few to ten centimeters per second.
Presently, the age of bottom water in the equatorial Pacific is 1,600 years, and in the Atlantic it is 350 years. Glacial stages in the North Atlantic correlate with the presence of older cold bottom waters, approximately twice the age of the water today. This suggests that the thermohaline circulation system was only half as effective at recycling water during recent glacial stages, with less cold bottom water being produced during the glacial periods. These changes in production of cold bottom water may in turn be driven by changes in the North American ice sheet, perhaps itself driven by 23,000-year orbital (milankovitch) cycles. such a growth in the ice sheet would cause the polar front to shift southward, decreasing the inflow of cold saline surface water into the system required for efficient thermohaline circulation. several periods of glaciation in the past 14,500 years (known as the Dryas) are thought to have been caused by sudden, even catastrophic injections of glacial meltwater into the North Atlantic, which would decrease the salinity and hence density of the surface water. This in turn would prohibit the surface water from sinking to the deep ocean, inducing another glacial interval.
shorter-term decadal variations in climate in the past million years is indicated by so-called Heinrich Events, defined as specific intervals in the sedimentary record showing ice-rafted debris in the North Atlantic. These periods of exceptionally large iceberg discharges reflect that decadal-scale sea surface and atmospheric cooling are related to thickening of the North American ice sheet followed by ice-stream surges associated with the discharge of the icebergs. These events flood the surface waters with low-salinity freshwater, leading to a decrease in flux in the cold-bottom waters, and hence a short-period global cooling.
Changes in the thermohaline circulation rigor have also been related to other global climate changes. Droughts in the Sahel and elsewhere are correlated with periods of ineffective or reduced thermohaline circulation, because this reduces the amount of water drawn into the North Atlantic, in turn cooling surface waters and reducing the amount of evaporation. Reduced thermohaline circulation also reduces the amount of water that upwells in the equatorial regions, in turn decreasing the amount of moisture transferred to the atmosphere and reducing precipitation at high latitudes.
Atmospheric levels of greenhouse gases such as CO2 and atmospheric temperatures show a correlation to variations in the thermohaline circulation patterns and production of cold-bottom waters. CO2 is dissolved in warm surface water and transported to cold-surface water, which acts as a sink for the Co2. During times of decreased flow from cold, high-latitude surface water to the deep ocean reservoir, Co2 can build up in the cold polar waters, removing itself from the atmosphere and decreasing global temperatures. In contrast, when the thermohaline circulation is vigorous, cold oxygen-rich surface waters down-well, dissolving buried Co2 and even carbonates, releasing this Co2 into the atmosphere and increasing global temperatures.
The present-day ice sheet in Antarctica grew in the Middle Miocene, related to active thermohaline circulation that caused prolific upwelling of warm water that put more moisture in the atmosphere, falling as snow on the cold southern continent. The growth of the southern ice sheet increased the global atmospheric temperature gradients, which in turn increased the desertification of midlatitude continental regions. The increased temperature gradient also induced stronger oceanic circulation, including upwelling, removal of Co2 from the atmosphere, lowering global temperatures, and bringing on late Neogene glaciations.
ocean-bottom topography exerts a strong influence on dense bottom currents. Ridges deflect currents from one part of a basin to another and may restrict access to other regions, whereas trenches and deeps may focus flow from one region to another.
Was this article helpful?