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Stratosphere-Troposphere Dynamical Coupling

As outlined in a recent review of stratosphere-troposphere coupling (STC) (Thompson et al., 2011). the robustness of the STC following Antarctic spring ozone depletion via the Southern Annular Mode (SAM) has been confirmed in a number of model studies. Polvani et al. (2011) indicated that ozone depletion is the dominant driver. An interactive feedback to the ocean seems to be of minor importance for the SAM response to ozone depletion, as shown in one CCM study with coupled ocean (Sigmond et al., 2010). Antarctic surface climate changes related to the SAM include a poleward shift of the extratropical jets in SH summer (Archer and Caldeira, 2008), an associated shift in precipitation in CMIP3 simulations (Son et al., 2009), and a warming of the west Antarctic region and a summer season cooling (e.g. Steig et al., 2009). The dynamical mechanisms of STC remain poorly understood (Kushner, 2010). Recent studies suggested wave interference associated with Siberian snow cover (Smith et al., 2010) and wave driving of the stratosphere and the NAM by tropical sea surface temperature (SST) anomalies (Fletcher and Kushner, 2011) as triggers for NAM generation. Gerber et al. (2010) also found an unexplained delayed stratospheric peak in persistence, which might have implications for seasonal prediction.

Recently, decadal variations in stratospheric H2O were suggested to have contributed to an enhanced surface warming in the 1990s, as well to the reduced global surface warming after the decline in H2O concentrations in the 2000s (Solomon et al., 2010). Grise et al. (2009) attributed the observed upper tropospheric cooling in the Antarctic to a decrease in downwelling longwave radiation associated with stratospheric ozone depletion. The future radiaitve forcing by stratospheric ozone will largely depend on a number of factors, like the growing impact of N2O (Ravishankara et al., 2009). Waugh et al. (2009) emphasised the importance of zonal asymmetries in ozone for the cooling of the Antarctic high latitudes. Stratospheric change can further influence tropospheric composition by the direct mass exchange from the stratosphere into the (e.g., Isaksen et al. 2009). Hsu and Prather (2009) found reductions in stratosphere-troposphere exchange (STE) of at most 10% in the NH from 1979 to 2004. For the 21st century Hegglin and Shepherd (2009) found in a stratosphere resolving CCM for a moderate GHG scenario an increase of 23% in the ozone transport into the troposphere.

STC also seems to be influenced by anomalies in tropical SSTs associated with El Niño-Southern Oscillation (ENSO) events (Cagnazzo et al., 2009; Ineson and Scaife, 2009; Orsolini et al., 2008). On the other hand, there are indications of a downward influence of the stratosphere on ocean variability (e.g., Sen-Gupta and England, 2006). In Antarctica, Lefebvre and Goosse (2008) found that the observed increase in SAM is associated with an increase of sea-ice extent in the Amundsen-Ross, Pacific and Indian sectors of the Southern Ocean and a decrease close to the Antarctic Peninsula, while in the Arctic, the positive phase of the NAM has been associated with a decrease in Arctic sea ice (Rigor et al., 2002). However, while the observed NAM returned from a high phase period back to more neutral conditions in the last decade (Francis et al., 2009), the decrease in Arctic sea-ice has accelerated more than expected from model projections (e.g., Stroeve et al., 2007).