Observations of Large-Scale Ocean–Atmosphere Interaction in the Southern Hemisphere

Abstract

The authors provide a detailed examination of observed ocean-atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño-Southern Oscillation (ENSO). Results are examined separately for the warm (November-April) and cold (May-October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM-and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ϳ1 week, and that the e-folding time scale of the SAMrelated SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer.

Accelerated warming of the Southern Ocean and its impacts on the hydrological cycle and sea ice

“…The spatial pattern of SST anomalies associated with the high index polarity of the AAO (warm SST anomalies in the middle latitudes and cold SST anomalies in the high latitudes) bear strong resemblance to the structure of the first SST EOF mode. This indicates that the AAO is the dominant mechanism responsible for the dominant SST variability in the Southern Ocean in the unforced control experiment, which is also consistent with previous observational and modeling studies (14)(15)(16). CCSM3 provides two ensemble members for 20C3M, that is forced with the observed anthropogenic and natural forcings.…”

State of the Climate in 2008

“…The 5.5% of the data area that is inconsistent in the observed case is close to the median of this range of area values, indicating the differences are not field significant. Inconsistent trends in the midlatitude Southern Hemisphere strongly resemble the surface temperature pattern of the negative phase of the SAM (Ciasto and Thompson 2008), which did indeed show a negative trend in the last decade.…”

Seasonal Relationships between Large-Scale Climate Variability and Antarctic Sea Ice Concentration

“…3). The cold season SIC dipole is also observed to be considerably stronger in magnitude than its warm season counterpart, a difference that may be related to the enhancement of air-sea heat fluxes during the cold season months (Ciasto and Thompson 2008). These anomalous heat fluxes will, in turn, enhance the perturbations of an SST field already near the freezing point, and thus already primed for sea ice growth.…”

“…For example, evaluation of the relationships between SIC and the SAM/ENSO have generally focused on annual time scales or particular seasons of sea ice growth and decline. Given that the SH atmospheric circulation and mixed layer ocean associated with the SAM and ENSO vary as a function of season (Ciasto and Thompson 2008), it is of interest to examine how interannual associations with SIC evolve over a full seasonal cycle. Moreover, the asymmetry of sea ice responses to positive and negative phases of the SAM/ ENSO remains little explored, and further questions remain in regard to the leading patterns of SIC variability and their relation to the leading patterns of climate variability.…”