The Influence of Local and Remote Processes
on Air-Sea Interaction over Western Boundary Current Regions
- by
Michael Alexander
Mike is interested in the atmospheric
and oceanic processes that lead to SST variability in WBC regions
and how the SST anomalies feedback on the atmosphere. Local processes that impact
SST variability in the Kuroshio and Gulf Stream regions include surface
fluxes and the “reemergence mechanism” (e.g. Alexander and
Deser 1995). In the latter, temperature anomalies created by surface
fluxes are spread by turbulence over the deep winter mixed layer.
After the mixed layer abruptly shoals in spring, the anomalies are stored
in the seasonal thermocline during summer, when surface fluxes damp
the concurrent SST anomalies. The thermal anomalies are then re-entrained
into the mixed layer when it again deepens in the following fall
and winter. The reemergence process enhances the winter-to-winter persistence
of SST anomalies (Deser et al. 2002; Park et al. 2006) and can impact
the atmospheric circulation when the thermal anomalies return to
the surface in the following winter (Cassou et al. 2007 and Liu et al.
2007).
Variability in the Kuroshio and Gulf Stream regions are
influenced by remote air-sea interactions in other parts of the extratropics.
Fluctuations in wind stress curl over the central North Pacific and
North Atlantic generate oceanic Rossby waves that impact thermocline
depth and the strength of the WBCs 3-7 years later. The resulting
changes in the strength and position of the WBCs affect the SSTs, alter
the surface heat fluxes, and thereby feedback on the atmospheric circulation.
This coupled atmosphere-ocean interaction appears to generate decadal
oscillations over the North Pacific in the NCAR coupled climate model
(Alexander et al. 2006, Kwon and Deser 2007).
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Anomalous SST (shading) and SLP (contours) as constructed
by subtracting the composite of 10 El Niño events from
the composite of 10 La Niña events in the 1950-2003
period, for a) August-September [AS(0)] and b)January-February
[JF(1)], where 0 indicates the ENSO year and 1 the following
year. The shading interval is 0.25ºC and the contour interval
is 1mb. The values are obtained from the National Center for
Environmental Prediction (NCEP) reanalysis. |
Air-sea interaction in the western
North Pacific and North Atlantic are also influenced by atmospheric
signals originating in the tropics. ENSO and lower-frequency changes
in the Indo-Pacific basin, alter the wind stress, surface fluxes and
precipitation over the North Pacific, which can then impact the underlying
ocean. The ENSO-induced changes are not limited to winter, as SST
anomalies in the Kuroshio region are larger than their winter counterparts
(See Fig. 1 and Alexander et al. 2004). Similarly, changes in
the tropical Atlantic influence the North Atlantic over the course
of the seasonal cycle (Peng et al. 2005, 2006).
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