On the Predominant Nonlinear Response of the Extratropical Atmosphere to Meridional Shifts of the Gulf Stream

Abstract

The North Atlantic atmospheric circulation response to the meridional shifts of the Gulf Stream (GS) path is examined using a large ensemble of high-resolution hemispheric-scale Weather Research and Forecasting Model simulations. The model is forced with a broad range of wintertime sea surface temperature (SST) anomalies derived from a lag regression on a GS index. The primary result of the model experiments, supported in part by an independent analysis of a reanalysis dataset, is that the large-scale quasi-steady North Atlantic circulation response is remarkably nonlinear about the sign and amplitude of the SST anomaly chosen over a wide range of GS shift scenarios. The nonlinear response prevails over the weak linear response and resembles the negative North Atlantic Oscillation (NAO), the leading intrinsic mode of variability in the model and the observations. Further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the shift of the North Atlantic eddy-driven jet, which is reinforced, with nearly equal importance, by the high-frequency transient eddy feedback and the low-frequency wave-breaking events. Additional sensitivity simulations confirm that the nonlinearity of the circulation response is a robust feature found over the broad parameter space encompassing not only the varied SST but also the absence/presence of tropical influence, the varying lateral boundary conditions, and the initialization scheme. The result highlights the fundamental importance of the intrinsically nonlinear transient eddy dynamics and the eddy-mean flow interactions in generating the nonlinear downstream response to the meridional shifts in the Gulf Stream.

Simulating the Midlatitude Atmospheric Circulation: What Might We Gain From High-Resolution Modeling of Air-Sea Interactions?

“…There is some uncertainty in the estimated magnitudes, a point to which we return below, as they depend on an assumed 2-month response time to SST forcing (for monthly averages to reach full amplitude). This choice is roughly consistent with the 20-30 days needed to reach maximum amplitude in the observational analysis of [94,95] and in highresolution simulations [14,64]. It also agrees with the 2month response time for monthly averages in AGCM studies (e.g., [14,96]), and the 2 to 3-month lag seen in the maximum co-variability between SST and large-scale atmospheric anomalies.…”

“…Thus, this paradigm might need to be revisited. This is further underscored by the strong non-linearity seen in recent experiments (e.g., [64,65]), while the QG paradigm in [62,63] relies on linear storm track dynamics.…”

A New Framework for Near‐Surface Wind Convergence Over the Kuroshio Extension and Gulf Stream in Wintertime: The Role of Atmospheric Fronts

“…Over the past few decades, there has been a steadily growing body of evidence suggesting that ocean mesoscale and frontal-scale features in the Kuroshio Extension (KE) and Gulf Stream (GS) regions are forcing the atmosphere (e.g., Chelton et al, 2004;Kuwano-Yoshida & Minobe, 2017;Ma et al, 2016;Piazza et al, 2016;Seo et al, 2017;Small et al, 2014;Xie, 2004). In particular, a positive correlation between sea surface temperature (SST) and near-surface wind speed over western boundary currents (WBCs) suggest an ocean-toatmosphere forcing through turbulent heat fluxes (Nonaka & Xie, 2003), whose variability on monthly and longer timescales is largely driven by internal ocean processes (Bishop et al, 2017).…”

Meridional Gulf Stream Shifts Can Influence Wintertime Variability in the North Atlantic Storm Track and Greenland Blocking

“…However, we also suggest that there may be a ocean-driven effect upon the atmosphere near zero lag, which we want to explore in the present study. Recent work by Seo et al (2017) also examined GS shifts leading atmosphere circulation changes by 1 year, again most relevant to the synoptic meridional temperature advection signal in the eastern North Atlantic. The oceanic-forced atmospheric variability could redden the atmosphere as it reinforces the wind-driven forcing, thus creating low-frequency NAO signals, which have been noted in the long-term atmospheric record (e.g., Da Costa & Verdiere, 2002;Deser & Blackmon, 1993;Woollings et al, 2015).…”

“…This differs from our approach since previous findings (Kwon & Joyce, 2013) have shown storm track shifts in the eastern Atlantic follow meridional GS shifts by at least 1 year, in contrast to what we will see here. Recent work by Seo et al (2017) also examined GS shifts leading atmosphere circulation changes by 1 year, again most relevant to the synoptic meridional temperature advection signal in the eastern North Atlantic. Our focus here is on the region near Greenland.…”