Decadal Variability of the Eighteen Degree Water derived from the
Northwest Atlantic Regional Climatology
Abstract
Ocean heat analyses of the North Atlantic Ocean based on the new
high-resolution Northwest Atlantic (NWA) Regional Climatology (RC)
developed at the NOAA’s National Centers for Environmental Information
(NCEI) revealed decadal variability of the Eighteen Degree Water (EDW)
depth that may be instrumental for understanding the localized heat
accumulation in the NWA. The EDW is an important element of the
Northwest Atlantic heat balance and an indicator of the ocean-atmosphere
interaction in this region. The EDW deepening, or “heaving”, on
decadal timescales are most likely caused by increasing Ekman pumping
due to changes in the wind stress curl pattern over the NWA. The NCEI’s
NWARC has also revealed that the highest rates of heat gain occur in the
Sargasso Sea, southeast of the Gulf Stream path in the region occupied
by the EDW. The volume of EDW depends on many factors, of which the most
important are: Ekman pumping, heat fluxes at the air-sea surface, and
heat advection within the Gulf Stream and the subtropical recirculation
gyre. However, heat accumulation in several “pockets” southeast of the
Gulf Stream and its extension seem to be most closely connected to EDW
heaving. The depths of EDW for two independent ~30-year
periods and their differences were computed and analyzed in conjunction
with the changes in the curl of wind stress. As the comparison between
the EDW depths mapped on three different spatial grids with 1°x1°,
1/4°x1/4°, and 1/10°x1/10° resolutions illustrate, the grid resolution
does matter for mapping EDW on decadal timescales. The 30-year climate
shift of the EDW depths between 1985-2010 and 1955-1984 compares quite
well with the climatic shift in Ekman vertical velocities derived from
the changes in the wind stress curl over the same time period. Comparing
the eddy-permitting EDW heaving inferred from the NCEI’s NWARC and the
~30-year shift of the curl of wind stress, and
consequently Ekman pumping, confirms a strong resemblance of the
eddy-permitting and eddy-resolving EDW heaving patterns with two tightly
localized pockets of heat accumulation southeast of the Gulf Stream and
its extension.