Seasonal Dependence of Coupling between Storm Tracks and Sea Surface Temperature in the Southern Hemisphere Midlatitudes: A Statistical Assessment

Abstract

Two-way coupling between sea surface temperature (SST) variations in the midlatitude southern oceans and changes of synoptic-scale (2–8 day) eddy activities in the lower and upper troposphere throughout the year is investigated based on lagged maximum covariance analysis using reanalysis datasets from 1951 to 2000. Results show a strong seasonal dependence of the coupling, as characterized by the most prominent one in austral midsummer (January). On one hand, SST variations in austral late spring (primarily October) are likely to influence storm tracks in the following January. That is, significant warm SST anomalies in the western midlatitude areas of South Atlantic and south Indian Ocean could result in the systematic strengthening of the low-level and upper-level eddy activities, which is presumably attributed to the coherent intensification of the SST front and the lower-tropospheric baroclinicity. Particularly, interannual variability (a spectral peak at 4 yr) of SST in the western midlatitude South Atlantic in October could play a predominant role in driving the corresponding variability of the Southern Hemisphere storm tracks three months later. The timing of the discernible response of storm tracks is also discussed based on the preliminary results of sensitivity experiments. On the other hand, the strengthened eddy activities in January continue to induce the dipolelike patterns of SST anomalies in the midlatitude southern oceans. Those SST response patterns are, to the first order, determined by changes of the net surface heat flux. The anomalous Ekman advections in part driven by the storm-track changes also contribute to SST anomalies in the southern subtropical South Atlantic and the western midlatitude South Pacific.

The ‘Day Zero’ Cape Town drought and the poleward migration of moisture corridors

“…We demonstrate how a poleward shift of the large-scale synoptic features that dominate the Southern Hemisphere midlatitude meteorological variability (in particular the jet-stream and stormtracks) imposed a dramatic rainfall shortage in SW South Africa. Although the very strong 2015-2016 El Niño event occurred during the ongoing drought in the Cape Town area, and may therefore have contributed to the exceptional increase in global and regional temperatures during these three years, the feedback between SSTs and atmospheric dynamics (particularly ENSO) on winter rainfall in the region is far less clear than for the summer rainfall region of South Africa (Reason et al 2002, Philippon et al 2012, Zhang et al 2018. Results here show a strong SAM influence together with an expansion of the semi-permanent subtropical anticyclones in the South Atlantic and South Indian Oceans as key factors for a poleward deviation of moisture corridors, and consequently for the regional drying trend which culminated in the 'Day Zero' water crisis.…”

Remote Influence of the Midlatitude South Atlantic Variability in Spring on Antarctic Summer Sea Ice

“…We first examined whether the coupling between storm-track activity and SSTAs (refer to Z20) can be reproduced in the ERA-Interim (1979-2018. The lagged MCA is applied as a function of 850-hPa v′T′ and SSTAs in the SH midlatitudes (30°-60°S, 0°-360°E) (hereinafter, the SH-MCA).…”

“…Particularly, synoptic‐eddy vorticity fluxes, closely linked to storm‐track activity, act to convert the response of atmospheric circulation from baroclinic to equivalent barotropic and amplify it downstream (Deser et al., 2007). Our previous studies have suggested that SST variation in the western SA frontal zone has predominant influence on the hemispheric storm‐track activity in austral summer (Zhang et al., 2018, 2020, hereinafter Z20). Hence, we hypothesize that the SA SST variation can lead to the planetary‐scale atmospheric response through synoptic‐eddy feedback on the time–mean flow, and in turn imprint on Antarctic summer sea ice.…”

Influence of the Kuroshio Interannual Variability on the Summertime Precipitation over the East China Sea and Adjacent Area

“…To quantify influences of the Kuroshio on the marginal seas, we conduct the lagged maximum covariance analysis (MCA), which has been widely used in the previous studies regarding the midlatitude ocean-atmosphere interaction (e.g., Frankignoul 1999, 2002;Wu 2013, 2015;Zhang et al 2018). The MCA isolates pairs of spatial patterns that maximize their covariance and the associated time coefficients by performing a singular value decomposition of the covariance matrix between two fields.…”