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Atlantic Studies: 
Studying the seasonal to interannual variability in upper ocean thermal energy content, transport, and property fluxes of heat, fresh water, and carbon is important in order to monitor and understand how the ocean influences climate fluctuations, and to improve our ability to predict important climatic signals such as the North Atlantic Oscillation (NAO) and tropical Atlantic variability. The thermohaline circulation is known to occur at long time scales in equal importance or even more importance than the wind-driven circulation because it couples the full volume of the global ocean to the atmosphere, forming a global circulation network of mass and heat transports. The classical picture of the "conveyor belt" indicates that the North Atlantic exports cold deep water and imports warm upper ocean water from the South Atlantic. This thermohaline overturning cell is composed of northward transports of warm surface- and intermediate-layer waters in the upper 1000 m, southward transport of North Atlantic Deep Water (NADW), and at the bottom northward flowing Antarctic Bottom Water. The net balance is to the north and, as a result, the Atlantic is a peculiar ocean because it is the only ocean that transfers heat northward across the equator.

Some intriguing and yet unanswered questions are: How much heat is transported into the North Atlantic and from where does it come? What are the main passages from the South to the North Atlantic? How is the upper limb of the "conveyor belt" circulation supplied? How do changes in the strength of the lower limb affect the atmosphere away from the poles? And what is the ocean's role in modifying the atmospheric circulation on interannual and decadal time scales?

Key questions in the North Atlantic focus on the forcing of decadal signals in the NAO through heat content/flux variability and changes in the overturning circulation. Through observational programs, AOML has been conducting studies into the variability of the ocean circulation in the center of the Atlantic subtropical gyre. Some of the key findings have been: (1) decadal signals in subsurface temperatures and transport that are correlated with atmospheric patterns such as the NAO (Figure 11); (2) seasonal variability in poleward heat flux, suggesting the importance of high frequency variability on important climate forcing agents; (3) long-term changes in deep water mass characteristics such as Labrador Sea Water (Figure 12); and (4) a major reduction in Southern Ocean deep water production during the 20th century that may be explained by chemical tracers analysis. Contributing observational programs that have resulted in advancing our understanding of the ocean's role in forcing climate fluctuations include: (1) long term and repeated measurements of the deep water properties off the coast of Florida (Abaco Island, Bahamas) and transatlantic sections along 24°N; (2) high frequency sampling of the upper ocean temperature through the Volunteer Observing Ship (VOS) program using expendable bathythermographs (XBTs) in both high horizontal resolution and low resolution mode; and (3) sustained transport observations of the Florida Current using low cost voltage measurement supplied from undersea telephone cables (Figure 13). A new project for FY-00 is the analysis of historical hydrological data sets for the Black Sea with the goal of studying the response of an enclosed sea to changes in the Europe-North Atlantic climate system.

In the tropical and South Atlantic, key questions center on the pathways of the upper limb of the overturning circulation: How much warm and salty upper layer water enters the Atlantic from the Indian Ocean? How much is colder and fresher water originating out of the Drake Passage? What are the main pathways of the two competing sources and the mechanisms that originate the transfers? Studies at AOML have already shown: (1) the important role of the Benguela Current and Agulhas rings shed at the retroflection in supplying the tropical Atlantic with warm, near-surface waters as part of the upper limb of the overturning circulation (Figure 14); (2) that boundary current variability of the North Brazil Current creates between five to eight eddies a year that transport water from the Southern Hemisphere northward (Figure 15); (3) that near the equator and in the interior, the circulation pathways follow complicated patterns and contain substantial seasonal variability (Figure 16); and (4) that the upper limb of the overturning circulation once across the equator enters the Caribbean Sea through the southernmost passages of the Caribbean Island chain (Figure 17). Observational programs that have answered some of these questions include: (1) the PALACE floats experiment, to study the pathways of the intermediate water in the equatorial region and to measure the upper ocean thermal field in the tropical Atlantic; (2) the Benguela Current Experiment, to understand the interocean exchanges of heat and mass and to follow the path of the intermediate water from the Indian to the Atlantic Ocean; (3) the North Brazil Current Rings Experiment, to determine and quantify the role of the rings shed at the retroflection of the North Brazil Current on the transfer of heat and mass from the South Atlantic to the North Atlantic; and (4) The Windward Island Passage Monitoring Experiment to study the partition and variability of upper ocean transport along the Caribbean Island Chain.

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