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

Abstract

[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(0.3 Sv) 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.

Mean Circulation and Variability of the Tropical Atlantic during 1952–2001 in the GECCO Assimilation Fields

“…STC divergences/convergences show small interannual variations, less than Ϯ2 Sv, which were also found in simulation model studies by Hüttl and Böning (2006) and Kröger et al (2005). The correlation between poleward-diverging Ekman transports, the EUC at 23°W and the STC layer convergence suggests a lagged response of the STC loop to the wind-forced Ekman transport at 10°S-N on decadal time scales.…”

“…The correlation between poleward-diverging Ekman transports, the EUC at 23°W and the STC layer convergence suggests a lagged response of the STC loop to the wind-forced Ekman transport at 10°S-N on decadal time scales. Hüttl and Böning (2006) also showed such covariability in a purely thermohaline-forced model run and a strong decrease in correlation toward the eastern parts of the EUC. This is in contrast to GECCO, where the high correlation of STC divergences/convergences with eastern equatorial heat content suggests that there is a strong influence of the long-term STC loop variability on the upper eastern equatorial Atlantic.…”

“…involving a remote forcing mechanism by the wind stress curl farther away from the equator have been suggested in model studies by Lee and Fukumori (2003) and Capotondi et al (2005) for the Pacific and Hüttl and Böning (2006) for the Atlantic. However, we could not clearly detect such a mechanism in GECCO when comparing variability of the wind stress curl field in the tropical and subtropical Atlantic and transports at 10°S and 10°N.…”

“…Other Atlantic model studies have shown a strong influence on the STC strength and pathways by the meridional overturning circulation (MOC; see Fratantoni et al 2000;Malanotte-Rizzoli et al 2000;Jochum and Malanotte-Rizzoli 2001;Hazeleger and Drijfhout 2006) and the type of wind forcing used Inui et al 2002). High-resolution model experiments by Hüttl and Böning (2006) have further shown a remote influence by changes in the North Atlantic MOC.…”

On the propagation and decay of North Brazil Current rings

“…[14] The model has been used previously for several studies in the Tropical Atlantic, e.g., in the works of Hüttl and Böning [2006] and Kirchner et al [2009]; a variant of the model was also used by Brandt and Eden [2005]. The study of Kirchner et al [2008] included a detailed model data comparison, indicating a realistic model behavior with respect to observed mean transports and its variability.…”