Seasonal and interannual variability of an index of deep atmospheric convection over western boundary currents

Abstract

The seasonal and interannual variability of an index measuring the potential for deep (surface-to-tropopause) convection over the extratropical oceans is studied using reanalysis data. It is found that most of the conditional instability is concentrated over the world's western boundary currents during winter, but shifts equatorward of the currents in summer. Conditional instability is only detected over the Gulf Stream and the East Australian Current in their respective summer season.The coupled ocean-atmosphere mechanisms controlling the variability of the convective index are then studied. It is found that the convective index displays a large interannual variability, which is primarily controlled by the erratic displacements of the storm tracks. Only weak negative feedback from the oceans is singled out on short (intraseasonal) time-scales, reflecting the stabilization of the troposphere through the development of cold sea-surface temperature anomalies. A larger role for warm oceanic advection in destabilizing the troposphere is, however, suggested on longer (interannual and decadal) time-scales. Citation: L. Sheldon, A. Czaja. 2014. Seasonal and interannual variability of an index of deep atmospheric convection over western boundary currents. Q.

Influence of the Decadal Variability of the Kuroshio Extension on the Atmospheric Circulation in the Cold Season

“…Indeed, cooling from below is an inherently stabilizing process while heating from below is a destabilizing one, conducive to convective overturning and deeper vertical penetration. Similarly, Sheldon and Czaja (2014) have shown that during winter, convective instabilities are very frequent over the KOE region, and a lower SST would lead to fewer occurrences, while a warmer SST would increase it. As shown in Fig.…”

“…While a strong impact on the large-scale atmospheric circulation is found during the KOE warming (positive phase), there is little large-scale response during the negative phase. This asymmetry may result from the difference between positive and negative SST impact on the overlying atmosphere, as a positive temperature anomaly leads to more convective instabilities and deeper vertical penetration in the atmosphere (Deser et al 2004;Sheldon and Czaja 2014). Indeed, the air temperature anomaly has a deep vertical structure and amplifies with height in the positive case, but not in the negative case.…”

A Stokes drift approximation based on the Phillips spectrum

“…Although more work is needed to quantify the impact of these processes on ocean-only and coupled models, it appears clear that the impact on the sea surface temperature (SST) may be on the order of 0.5 K (Fan and Griffies, 2014;Janssen et al, 2013;Breivik et al, 2015). As the coupled atmosphere-ocean system is sensitive to such biases, for instance through the triggering of atmospheric deep convection, see Sheldon and Czaja (2014), wave-induced mixing could play an important role in improving the performance of coupled climate and forecast models. 1.04 0.240 0.920 Table 1: Statistics of the two Stokes drift velocity profiles for three parametric unimodal spectra and two bimodal spectra.…”

Investigating the Local Atmospheric Response to a Realistic Shift in the Oyashio Sea Surface Temperature Front

“…In the upper troposphere the circulation forms two cells with northward (southward) flow north (south) of the front in HR, whereas the flow is southward at all latitudes in LR. The larger low-level u E anomaly in HR over the warm SST, reflecting both warmer temperatures and enhanced low-level moisture, also reduces the low-level stability (not shown), in a region that is frequently convectively unstable (Czaja and Blunt 2011;Sheldon and Czaja 2014). The zonal wind response is weak in both simulations (not shown): in the LR, zonal wind changes are less than about 1.5 m s 21 and are consistent with the cyclonic circulation to the east, while the HR zonal wind changes are of opposite sign and even smaller.…”