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.