Role of the Gulf Stream and Kuroshio–Oyashio Systems in Large-Scale Atmosphere–Ocean Interaction: A Review

Bo Qiu,Claude Frankignoul,Hisashi Nakamura,Lu Anne Thompson,Michael A Alexander,Nicholas A Bond,Young-Oh Kwon

doi:10.1175/2010jcli3343.1

Abstract

Abstract Ocean–atmosphere interaction over the Northern Hemisphere western boundary current (WBC) regions (i.e., the Gulf Stream, Kuroshio, Oyashio, and their extensions) is reviewed with an emphasis on their role in basin-scale climate variability. SST anomalies exhibit considerable variance on interannual to decadal time scales in these regions. Low-frequency SST variability is primarily driven by basin-scale wind stress curl variability via the oceanic Rossby wave adjustment of the gyre-scale circulation that modulates the latitude and strength of the WBC-related oceanic fronts. Rectification of the variability by mesoscale eddies, reemergence of the anomalies from the preceding winter, and tropical remote forcing also play important roles in driving and maintaining the low-frequency variability in these regions. In the Gulf Stream region, interaction with the deep western boundary current also likely influences the low-frequency variability. Surface heat fluxes damp the low-frequency SST anomalies over the WBC regions; thus, heat fluxes originate with heat anomalies in the ocean and have the potential to drive the overlying atmospheric circulation. While recent observational studies demonstrate a local atmospheric boundary layer response to WBC changes, the latter’s influence on the large-scale atmospheric circulation is still unclear. Nevertheless, heat and moisture fluxes from the WBCs into the atmosphere influence the mean state of the atmospheric circulation, including anchoring the latitude of the storm tracks to the WBCs. Furthermore, many climate models suggest that the large-scale atmospheric response to SST anomalies driven by ocean dynamics in WBC regions can be important in generating decadal climate variability. As a step toward bridging climate model results and observations, the degree of realism of the WBC in current climate model simulations is assessed. Finally, outstanding issues concerning ocean–atmosphere interaction in WBC regions and its impact on climate variability are discussed.

Highlights

IntroductionAtmosphere–ocean interactions are exceptionally strong over western boundary currents and their eastward extensions (hereafter collectively WBCs): for example, the largest mean and variance at interannual and longer time scales of the net surface heat flux (Qnet) over the global ocean occurs in WBC regions
Atmosphere–ocean interactions are exceptionally strong over western boundary currents and their eastward extensions: for example, the largest mean and variance at interannual and longer time scales of the net surface heat flux (Qnet) over the global ocean occurs in WBC regions (Wallace and Hobbs2006)
Over most of the midlatitude ocean, Qnet variability is primarily controlled by the atmospheric variability: Qnet forcing on sea surface temperature anomalies (SSTAs) dominates the negative Qnet feedback that tends to damp them after they are generated

Summary

Introduction

Atmosphere–ocean interactions are exceptionally strong over western boundary currents and their eastward extensions (hereafter collectively WBCs): for example, the largest mean and variance at interannual and longer time scales of the net surface heat flux (Qnet) over the global ocean occurs in WBC regions A similar mechanism was identified by Wu et al (2005), using a ‘‘model surgery’’ approach, which allows or deactivates atmosphere–ocean coupling in a selected region, and by Qiu et al (2007), who used a linear Rossby wave model and statistical models for the influence of SSH on SST and for the $ 3 t response to KOE SSTAs. In addition to the ;20-yr decadal variability, some model simulations exhibit significant 40–50-yr multidecadal peaks in the basinwide North Pacific SST with high correlations to the tropical Pacific SST (Kwon and Deser 2007; Zhong et al 2008), which is suggested from observations (Nakamura et al 1997; Minobe 1997; Deser et al 2004).

Performance of climate models on simulation of WBC variability

Outstanding issues

Findings

Summary