Mechanisms of decadal variability in the shallow subtropical-tropical circulation of the Atlantic Ocean:: A model study

[ 1] A suite of basin-scale models of the thermohaline and wind-driven circulation in the Atlantic Ocean is used to study the mechanisms of decadal variability in the shallow subtropical-tropical cells (STCs). The emphasis is on the spatial patterns of the transport anomalies in the tropical thermocline, particularly their manifestation in the equatorial current system and on the relative role of changes in the deep meridional overturning cell (MOC) associated with variations in the formation of Labrador Sea Water (LSW) in the subpolar North Atlantic. Using wind stress and heat flux variations based on NCEP/ NCAR-reanalysis products, the variability of the zonally integrated STC transports is similar to that obtained in a recent regional model study, corroborating the role of both the southern and northern STC in supporting wind-driven transport anomalies of O(2 Sv) near the equator. Sensitivity experiments indicate that changes in subarctic MOC transports associated with the strong variability in LSW formation during the last decades contributed a signal of O( 0.3Sv) to the upper-layer equatorial transports. Whereas the local wind-driven variability clearly dominates on interannual-decadal timescales and is confined to depths down to 150 m, the weak MOC-related signal is primarily reflected in an interdecadal modulation of the STC transports. While a strong part in the STC's transport anomalies is associated with the western boundary current (NBC), there is an important contribution also by weaker, interior ocean flow anomalies which tend to counteract the variability of the NBC.