The Gulf Stream Convergence Zone in the Time-Mean Winds

Abstract

The distribution of surface divergence in the northwest Atlantic is investigated using 10 years of satellite wind observations from QuikSCAT and a 1-yr simulation from the COAMPS atmospheric model. A band of time-mean surface convergence overlies the Gulf Stream [called here the Gulf Stream convergence zone (GSCZ)] and has been attributed previously to a local boundary layer response to Gulf Stream SST gradients. However, this analysis shows that the GSCZ results mainly from the aggregate impacts of strong convergence anomalies associated with storms propagating along the storm track, which approximately overlies the Gulf Stream. Storm surface convergence anomalies are one to two orders of magnitude greater than the time-mean convergence and produce a highly asymmetric divergence distribution skewed toward convergent winds. The sensitivity of the sign and magnitude of the time-mean divergence to extreme weather events is demonstrated through analysis using an extreme-value filter, conditional sampling based on rain occurrence, and comparison to its median and mode. Vertical velocity and surface pressure are likewise affected by strong storms, which are characterized by upward velocity and low surface pressure. Storms are thus an important process in shaping the mean state of the atmosphere in the northwest Atlantic. These results are difficult to reconcile with the prevailing view that SST “anchors” surface convergence, upward vertical velocity, and increased rain over the Gulf Stream through a local boundary layer adjustment mechanism.

Atmospheric Signature of the Agulhas Current

“…Diagnosis of the pressure adjustment mechanism identifies a very similar relation to that for the Gulf Stream (Minobe et al, 2008). Recent studies, however, indicate that the anchoring of precipitation over the Gulf Stream front is mainly associated with the atmospheric frontal precipitation associated with synoptic scale extratropical cyclones (O'Neill et al, 2017;, Sheldon et al, 2017Vannière et al, 2017). A similar mechanism might acts over the Agulhas Current, as a large part of the rainfall there is also related to atmospheric fronts (Catto et al, 2012).…”

“…Earlier studies implicated the pressure adjustment mechanism in producing the observed pattern of wind convergence and divergence over major SST frontal regions (Minobe et al, 2008;Shimada & Minobe, 2011). Recent studies suggest that the low-level convergence and associated rainband over the Gulf Stream is instead the result of the interaction of synoptic-scale atmospheric variability with the sharp SST front (O'Neill et al, 2017;Sheldon et al, 2017).…”

“…Recent studies indicate that a model horizontal resolution of at least 0.25°is required to resolve ocean-atmosphere interaction over such currents Smirnov et al, 2015). Several mechanisms explaining the role of the Gulf Stream and the Kuroshio on precipitation are proposed (Minobe et al, 2008(Minobe et al, , 2010O'Neill et al, 2017;Vannière et al, 2017;Xu et al, 2011;Sasaki et al, 2012). However, the influence of the Agulhas Current on the weather and climate of Southern Africa is not well known compared to the impact of the Gulf Stream and the Kuroshio on weather and climate.…”

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

“…Despite the noticeable imprint of both the KE and the GS SST fronts on the time-mean NSWC, however, as pointed out in Parfitt andO'Neill et al (2017), the time-mean atmospheric state in these regions is heavily skewed toward the contribution from extreme tail end values an order or two of magnitude larger than observed in the mean; specifically, the contribution from synoptic weather systems. Consequently, on any individual day, these patterns are rarely recovered.…”

JRA-55CHS: An Atmospheric Reanalysis Produced with High-Resolution SST

“…These time-mean atmospheric features have been interpreted as a manifestation of local boundary-layer responses to steep sea-surface temperature (SST) gradients (e.g., Small et al 2008;Wallace et al 1989;Hayes et al 1989;Lindzen and Nigam 1987). Recent studies have argued, however, that they can be a consequence of frequent passage of synoptic-scale atmospheric disturbances since SST fronts act to anchor stormtracks (Nakamura et al 2008;O'Neill et al 2017). Furthermore, it has been pointed out that SST fronts can modify individual synoptic-scale disturbances (Vannière et al 2017;Sheldon et al 2017) and detection frequency of atmospheric fronts tends to increase near the oceanic fronts (Parfitt et al , 2017.…”