Ocean Gyre Circulation Changes Associated with the North Atlantic Oscillation

Abstract

Observational evidence is presented for interannual to interdecadal variability in the intensity of the North Atlantic gyre circulation related to the atmospheric North Atlantic Oscillation (NAO) patterns. A two-point baroclinic pressure difference between the subtropical and subpolar gyre centers?an oceanic analogue to the much-used sea level pressure (SLP)-based atmospheric NAO indices?is constructed from time series of potential energy anomaly (PEA) derived from hydrographic measurements in the Labrador Basin and at Station S near Bermuda. Representing the upper 2000-db eastward baroclinic mass transport between the two centers, the transport index indicates a Gulf Stream and North Atlantic Current that gradually weakened during the low NAO period of the 1960s and then intensified in the subsequent 25 years of persistently high NAO to a record peak in the 1990s. The peak-to-peak amplitude difference was 15?20 megatons per second (MT s?1) with a 43-yr mean of about 60 MT s?1 a change of 25%?33% occurring between 1970 and 1995. The timing of the ocean fluctuation is organized around the same temporal structure as the NAO index. The two are not directly covariant, but to first order, the ocean signal reflects a time integration, through mixed layer ?memory? and Rossby wave propagation, of the atmospheric forcing. To some degree, the gyre PEA histories are fluctuating in antiphase reflecting latitudinal shifts of the surface westerlies across the North Atlantic. Differences in forcing mechanisms and baroclinic responses in each gyre, however, are reflected by divergences in the details of their PEA histories. The subpolar PEA changes are primarily thermally driven through diabatic mixing and surface buoyancy fluxes associated with water mass transformation. Salinity changes, stemming from the occasional passages of low-salinity surface lids (?Great Salinity Anomalies?) through the region, contribute relatively little to the Labrador Basin PEA variability. The interior subtropical gyre PEA history is dominated by quasi-adiabatic vertical displacements of the main pycnocline, and supplemented by changes in the locally formed subtropical mode water as well as by changes in middepth density structure related to advective?diffusive import of Labrador Sea Water. Multiyear composite fields of North Atlantic potential energy centered in time on the extreme high and low transport periods provide a broad geographic context for the transport index. Basin-scale shifts of oceanic baroclinic pressure gradients between the extreme phases reinforce the sense and amplitude of changes reflected in the Bermuda?Labrador Basin transport index.