Variability of the climatic oceanic frontal zones and its connection with the large-scale atmospheric forcing

Abstract

Global satellite sea surface temperature (SST) measurements and NCEP/NCAR reanalysis wind data for the period of 1982–2009 have been used to study the seasonal and interannual variability of the main climatic oceanic frontal zones (OFZ; subpolar, subtropical and equatorial) associated with the large-scale atmospheric forcing. The seasonal variability of the OFZ is manifested in variations of their intensity (magnitude of the meridional gradient of SST) and latitudinal position of the cores (defined as areas of maximum SST gradient). The maximum intensity of the subpolar OFZ is observed in summer of the corresponding hemisphere, while subtropical OFZ are intensified synchronously in both hemispheres during boreal winter. Subtropical OFZ cores in both hemispheres shift synchronously to the south/north during the winter/summer of the Northern hemisphere, which is caused by the seasonal meridional migration of the areas of the maximum convergence of Ekman transport. All subpolar and subtropical OFZ reveal a pronounced quasi-decadal (7–10years) variability, manifested in the variations of their intensity and latitudinal position of the zones’ cores. Strengthening of the SST gradient is accompanied by a displacement of the zones’ cores to the north in both hemispheres for subpolar OFZ, while subtropical OFZ cores migrate to the poles in this situation. Positive correlations between the maximum magnitude of the meridional gradient of zonally averaged SST and meridional shear of zonal wind (which is an estimate of the Ekman convergence intensity) were found for all subpolar and subtropical OFZ. Variability of the latitudinal position of subpolar OFZ cores is associated with the Ekman advection variability due to zonal wind variations (strengthening of zonal wind results in a shift of subpolar OFZ cores to the south/north in the Northern/Southern hemispheres). A period of the variability of the North Pacific equatorial OFZ is 4–5years and is determined by the variability of the ENSO system. During the El Niño events, the sharp decrease of the intensity of the equatorial OFZ take place.