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 Ϫ1 ) with a 43yr mean of about 60 MT s Ϫ1 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. * Woods Hole Oceanographic Institution Contribution Number 10260.

The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation

“…Its time evolution (BTSF PC1) is discussed in Hátún et al (2005a), MHS, and DF08. It tends to lag the NAO by 2 years, and is broadly consistent with the transport indices of Curry and McCartney (2001) and the altimetry data of Häkkinen and Rhines (2004). BTSF PC1 is well correlated with MOC PC1 (all correlations are given for linearly detrended data, even though the trend is not removed when displaying the time series), such that the strengthening of the Atlantic MOC after a positive NAO phase is accompanied by an intensification of the subpolar and subtropical gyres (Fig.…”

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

“…Ezer (2013) and Ezer et al (2013) suggested that the weakening Gulf Stream and AMOC, which affect the hotspot SLR acceleration during recent decades, are linked to the Atlantic Multidecadal Oscillation (AMO) but are also affected by the SLP and winds associated with the NAO. The effects of NAO-related wind stress curl and surface heat flux in the basin interior on interannual and decadal variability of the Gulf Stream have also been suggested by other studies (e.g., Curry and McCartney 2001;Di Nezio et al 2009;Chaudhuri et al 2011). This effect is shown to be important from 1986 to 1998 compared to the periods before and after (Meinen et al 2010).…”

Coral Climate History of the Subtropical North Atlantic (CorClim)

“…1), at a lag of almost one year. This lag is similar to the response time of the Subtropical Gyre and Gulf Stream circulations to NAO forcing (e.g., Curry and McCartney, 2001), which hints at potential changes in the local water mass composition driving the Sr/Ca signal. The coral record shows a subdecadal variability (7.5-year cycle) that compares well with that of the NAO index, while the long-term trend (e.g.…”