The Influence of Sea Surface Temperatures on the Northern Winter Stratosphere: Ensemble Simulations with the MAECHAM5 Model

Abstract

The role of interannual variations in sea surface temperatures (SSTs) on the Northern Hemisphere winter polar stratospheric circulation is addressed by means of an ensemble of nine simulations performed with the middle atmosphere configuration of the ECHAM5 model forced with observed SSTs during the 20-yr period from 1980 to 1999. Results are compared to the 40-yr ECMWF Re-Analysis (ERA-40). Three aspects have been considered: the influence of the interannual SST variations on the climatological mean state, the response to El Niño-Southern Oscillation (ENSO) events, and the influence on systematic temperature changes. The strongest influence of SST variations has been found for the warm ENSO events considered. Namely, it has been found that the large-scale pattern associated with the extratropical tropospheric response to the ENSO phenomenon during northern winter enhances the forcing and the vertical propagation into the stratosphere of the quasi-stationary planetary waves emerging from the troposphere. This enhanced planetary wave disturbance thereafter results in a polar warming of a few degrees in the lower stratosphere in late winter and early spring. Consequently, the polar vortex is weakened, and the warm ENSO influence clearly emerges also in the zonal-mean flow. In contrast, the cold ENSO events considered do not appear to have an influence distinguishable from that of internal variability. It is also not straightforward to deduce the influence of the SSTs on the climatological mean state from the simulations performed, because the simulated internal variability of the stratosphere is large, a realistic feature. Moreover, the results of the ensemble of simulations provide weak to negligible evidence for the possibility that SST variations during the two decades considered are substantially contributing to changes in the polar temperature in the winter lower stratosphere.

The Stratospheric Pathway of La Niña

“…The pioneer modeling works of Sassi et al (2004) and Manzini et al (2006) showed a negligible response to La Nina in the polar stratosphere, which was not statistically different from neutral winters. Nonetheless, more recent modeling studies reported a significant stratospheric cooling during La Nina (Calvo et al 2010) and a robust strong vortex related to suppressed anomalous upward propagation (Li and Lau 2013).…”

“…Several indices have been used in the literature, considering different oceanic regions, such as Nino-3 (N3) (5°N-5°S, 150°-90°W) (Hoerlinget al 1997;Manzini et al 2006) or Nino-4 (N4) (5°N-5°S, 160°E-150°W) (Kug and Ham 2011;Zhang et al 2015). However, most recent studies used the Nino-3.4 (N3.4) (5°N-5°S, 170°-120°W) index from the National Centers for Environmental Prediction/Climate Prediction Center (NCEP/CPC) (e.g., Free and Seidel 2009;Butler and Polvani 2011;Garfinkel et al 2012;Butler et al 2014;Barriopedro and Calvo 2014;Domeisen et al 2015).…”

“…The stratospheric signal during the warm ENSO phase (El Nino) has been extensively documented (Garcia-Herrera et al 2006;Manzini et al 2006;Fletcher and Kushner 2011); during El Nino winters, upward wave activity toward the stratosphere is enhanced through constructive interference between the El Nino anomalies and the climatological eddies, resulting in a warmer and a weaker polar vortex. Related zonal mean zonal wind anomalies in the stratosphere propagate downward into the troposphere, having an impact on the North Atlantic and European (NAE) region (Cagnazzo and Manzini 2009;Ineson and Scaife 2009;Bell et al 2009).…”

Variability of the Northern Hemisphere polar stratospheric cloud potential: the role of North Pacific disturbances

“…Manzini et al, 2006;Orsolini et al, 2008, and references therein), it is surprising that the PNA impact on the coldness of the polar stratosphere is not as important as the one exerted by the West-Pacific pattern. This may arise from the fact that the PNA pattern is associated with flux anomalies that are not located as poleward, nor extending as high into the stratosphere as the West-Pacific pattern (Figure 7).…”

Effects of meridional sea surface temperature changes on stratospheric temperature and circulation

“…However, most climate model simulations are performed by prescribing globally uniform SST increases (e.g., Shu et al, 2011), and the impacts of SST gradient changes on stratospheric circulation and temperatures are rarely discussed. Both modeling and observational studies have shown that the SST changes associated with ENSO events have significant effects not only on tropospheric circulation (e.g., Seager et al, 2003;Feng et al, 2013) but also on stratospheric circulation (e.g., Manzini et al, 2006;Xie et al, 2011Xie et al, , 2012. The BDC, for instance, is strengthened during warm phases of ENSO events via enhancement of the vertical propagation of waves (e.g., Sassi et al, 2004;Calvo et al, 2010).…”