The Relation between Decadal Variability of Subtropical Mode Water and the North Atlantic Oscillation*

Abstract

The Bermuda station ''S'' time series has been used to define the variability of subtropical mode water (STMW) from 1954 to 1995. This record, which shows decadal variability at a nominal period of about 12-14 yr, has been used as a baseline for seeking correlation with large-scale atmospheric forcing and with decadal north-south excursions of the Gulf Stream position defined by the subsurface temperature at 200-m depth. A common time period of 1954-89 inclusive, defined by the data sources, shows a high degree of correlation among the STMW potential vorticity (PV), Gulf Stream position, and large-scale atmospheric forcing (buoyancy flux, SST, and sea level pressure). Two pentads with anomalously small and large STMW PV were further studied and composites were made to define a revised North Atlantic Oscillation (NAO) index associated with the decadal forcing. During years of low PV at Bermuda, the NAO index is low, the Gulf Stream is in a southerly position, and the zero wind stress curl latitude is shifted south as are the composite extratropical winter storm tracks, in comparison to the period of high PV at Bermuda. Because the NAO, Gulf Stream separation latitude, and STMW PV variations are in phase with maximum annually averaged correlation at zero year time lag, the authors hypothesize that all must be either coupled with one another or with some other phenomenon that determines the covariability. A mechanism is proposed that could link all of the above together. It relies on the fact that during periods of high STMW PV, associated with a northerly Gulf Stream and a high NAO, one finds enhanced production of mode water in the subpolar gyre and Labrador Sea. Export of the enhanced Labrador Sea Water (LSW) component into the North Atlantic via the Deep Western Boundary Current can influence the separation point of the Gulf Stream in the upper ocean once the signal propagates from the source region to the crossover point with the Gulf Stream. If the SST signal produced by the 100-km shift of the Gulf Stream along a substantial (1000 km) length of its path as it leaves the coast can influence the NAO, a negative feedback oscillation may develop with a timescale proportional to the time delay between the change of phase of the airsea forcing in the Labrador Basin and the LSW transient at the crossover point. Both a simple mechanistic model as well as a three-layer numerical model are used to examine this feedback, which could produce decadal oscillations given a moderately strong coupling.

Coherent surface‐subsurface fingerprint of the Atlantic meridional overturning circulation

“…[5] In the control simulation, the above dipole pattern is due to variations of the strength of the subpolar gyre, the northern recirculation gyre (NRG) north of the Gulf Stream, and the shift of the Gulf Stream path, all linked to AMOC variations ( Figure 2e). The stronger AMOC is associated with a stronger deep western boundary current (DWBC), which leads to the strengthening of the cyclonic NRG and a southward shift of the Gulf Stream path ( Figure S3), consistent with previous modeling and observational studies [Thompson and Schmitz, 1989;Joyce et al, 2000;Zhang and Vallis, 2006Vallis, , 2007Peña-Molino and Joyce, 2008]. The strengthening of the cyclonic NRG and southward shift of the Gulf Stream lead to an upper level divergence of oceanic advected mass and heat flux, thus cooling in the subsurface and decreasing in the SSH there.…”

The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability

“…Indeed, the subtropical-subpolar gyres of the North Atlantic and the Gulf Stream-recirculation system strongly respond to NAO variability [Taylor and Stephens, 1998;Joyce et al, 2000;Curry and McCartney, 2001;Frankignoul et al, 2001b;Visbeck et al, this volume], but also show significant intrinsic variability of their own on interannual and longer timescales [e.g., Jiang et al, 1995;Meacham, 2000;Cessi and Primeau, 2001;Cessi and Paparella, 2001;Dewar, 2001]. Both intrinsic and forced dynamics lead to changes in heat transport across the mean path of the separated Gulf Stream through expansion -contraction of the gyres, or equivalently, through large-scale anomalous currents acting on mean temperature gradients.…”

“…The forced response of the thermocline-ocean mixed layer to large-scale wind forcing orchestrated by the atmosphere and its subsequent feedback on the atmospheric flow has been studied in a hierarchy of idealized coupled models [e.g., Jin, 1997;Münnich et al, 1998;Neelin and Weng, 1999;Cessi, 2000;Primeau and Cessi, 2001;Marshall et al, 2001a], in the lines of the scenario put forward by Latif and Barnett [1994; for the North Pacific. Similarly, the buoyancy forcing of the ocean driven by the NAO through its impact on Labrador Sea convection, and its possible subsequent feedback on the NAO through changes in the path of the separated Gulf Stream has been studied in observations and a simple model by Joyce et al [2000]. These studies can generally be understood in a forced delayed oscillator framework, as widely used for ENSO, with the ocean circulation providing the delay (typically set by the propagation time of long Rossby waves across the North Atlantic -from 5 to 10 years, depending on latitude -or set by advection, again typically 5 to 10 years for subpolar-subtropical gyre exchange), and intrinsic NAO variability the source of stochastic forcing (Figure 12).…”

Two centuries of limited variability in subtropical North Atlantic thermocline ventilation

“…The rate of mode water formation appears closely related to the phase of the North Atlantic Oscillation (NAO) 2,[7][8][9] , the dominant mode of atmospheric variability in the region that is generally defined as the winter (December-March) atmospheric pressure difference between Iceland and the Azores 10 . In a negative phase of the NAO, the pressure difference weakens, slowing the speed of the atmospheric jet stream across the North Atlantic and increasing storminess off eastern North America.…”