Numerical
modeling, paleoclimate and observational studies indicate that
both the wind-driven and thermohaline circulation can play an
important role in longer-term (greater than decadal) climate
variability. The U.S. National Oceanic and Atmospheric Administration
addresses both components to satisfy its missions of detecting,
attributing and forecasting long-term climate change. We contribute
to NOAA's mission by developing and providing observational
benchmarks (i.e., indices) for various components of the wind-driven
circulation (hereinafter WDC) and MOC in the Atlantic Ocean.
Many
early NOAA programs (e.g. STACS, ACCP) were searching for indices
of critical North Atlantic WDC and MOC features to monitor.
Although not originally NOAA programs, other studies have considered
the contribution of southern hemisphere features to the MOC.
For continuity of the upper layer limb of the MOC, exchanges
are required: from the Indian Ocean to the South Atlantic; across
the South Atlantic; and across the equator. The inter-ocean
exchange takes place through the Benguela/Agulhas system, south
of South Africa. The Agulhas Current at its retroflection sheds
energetic rings that carry salt and warm water into the South
Atlantic. Satellite altimetric measurements have been calibrated
to provide estimates of the transport of the Agulhas Current
and the separated rings. The extension of the Benguela Current
brings the Indian Ocean waters to the central South Atlantic
as it flow northwestward in the South Atlantic subtropical gyre.
The
pathways of the upper limb MOC transport are then complicated
by the wind-driven circulation features along the western boundary
and the interior tropical Atlantic (i.e., equatorial upwelling,
off-equatorial down welling, zonal currents), that provide obstacles
for this limb to move from the South Atlantic to the North Atlantic.
Currently, there is insufficient understanding and data to identify
precisely these pathways. However numerical models do provide
some initial guidance. Using an eddy-resolving numerical circulation
model, (Fratantoni et al., 2000) concluded that 14 Sv of upper
limb MOC flow is partitioned among three pathways connecting
the equatorial and tropical wind-driven gyre: a frictional western
boundary current accounting for 6.8 Sv; a diapycnal pathway
involving wind-forced equatorial upwelling and interior Ekman
transport, 4.2 Sv; and North Brazil Current (NBC) rings shed
at the NBC retroflection, 3 Sv. The results of an AOML, university
observational program indicate that previous estimates both
in the numbers of rings per year and in their contribution to
hemispheric exchanges were low. Based on the results of this
work, a monitoring strategy is being developed to monitor ring
formation and propagation.
Both
the intensity of the subtropical gyre and a component of the
warm upper level poleward flow in the North Atlantic are being
monitored by submarine cable observations in the Straits of
Florida. Similarly, the characteristics of the cold deep return
flow are being tracked by research vessel transects across the
DWBC east of the Bahamas. In the North Atlantic Ocean, time-series
of both the upper layer temperature structure within the subtropical
gyre and total water column changes across the basin are being
maintained.
The
recent history of these and other components of the MOC and
WDC motions are characterized by data collected over the past
10 to 50 years. These benchmarks are designed to serve several
purposes. Independently these benchmarks serve as indices for
(1) the intensity of various components of the MOC and WDC,
thereby providing alerts for dramatic changes in these features
and (2) verification of the ability of GCM's to simulate the
ocean's role in climate variability. Collectively, when assimilated
into GCM's they will provide global benchmarks for detection
and attribution of climate change. All the benchmarks presently
available are shown in Figure 1. We will now describe a few
of these indices, when sufficient data are available we provide
a description of the characteristics of various scales of variability.
