Convection in the central Irminger Sea; insights into variability and
the roles of surface forcing and stratification from 19 years of high
resolution mooring data
Abstract
Transformation of light to dense waters by atmospheric cooling is key to
the Atlantic Meridional Overturning in the Subpolar Gyre. Convection in
the center of the Irminger Gyre determines the transformation of the
densest waters east of Greenland. We present a 19-year (2002-2020)
weekly time series of hydrography and convection in the central Irminger
Sea based on (bi-)daily mooring profiles supplemented with Argo
profiles. A 70-year annual time series of shipboard hydrography shows
that this mooring period is representative of longer term variability.
The depth of convection varies strongly from winter to winter (288-1500
dbar), with a mean March climatogical mixed layer depth of 470 dbar and
a mean maximum density reached of 27.70 ± 0.05 kg m-3. The densification
of the water column by local convection directly impacts the sea surface
height in the center of the Irminger Gyre and thus large-scale
circulation patterns. Both the observations and a Price-Weller-Pinkel
(PWP) mixed layer model analysis show that the main cause of interannual
variability in mixed layer depth is the strength of the winter
atmospheric surface forcing. Its role is three times as important as
that of the strength of the maximum stratification in the preceeding
summer. Strong stratification as a result of a fresh surface anomaly
similar to the one observed in 2010 can weaken convection by
approximately 170 m on average, but changes in surface forcing will need
to be taken into account as well when considering the evolution of
Irminger Sea convection under climate change.