On the interannual variability of ocean temperatures around South Georgia, Southern Ocean: forcing by El Niño/Southern Oscillation and the Southern Annular Mode
The ocean around South Georgia, in the southwest Atlantic sector of the Southern Ocean, is highly productive, with large stocks of Antarctic krill supporting extensive colonies of marine and land-based predators. The operation of this ecosystem is strongly influenced by physical forcings, and the role of the El Niño/Southern Oscillation (ENSO) phenomenon has been highlighted previously. Here we examine in detail the transmission of ENSO signals to South Georgia, and investigate other sources of interannual variability. ENSO variability generates anomalies in sea surface temperature (SST) across the South Pacific via atmospheric teleconnections. These anomalies are advected toward South Georgia within the Antarctic Circumpolar Current (ACC), and previous studies have focussed on long-period advection (order of 2-3 years) from the southwest Pacific. We observe here, however, that the region close to the Antarctic Peninsula in the southeast Pacific is especially susceptible to ENSO forcing via anomalous meridional winds; this induces SST anomalies that are advected to South Georgia on a much more rapid timescale (order 5-6 months). The phasing of these teleconnections is such that anomalies that reach the southeast Pacific from farther west tend to be reinforced here by air-sea-ice interaction. We also find an important role for the Southern Annular Mode (SAM) in determining SST variability at South Georgia. This is a circumpolar mode of climate variability, and thus can readily influence local SST at South Georgia directly. The SAM is, however, not perfectly zonally symmetric, and (like ENSO) has a particular impact on meridional winds in the southeast Pacific. The average timescale for SAM influence on South Georgia SST is shorter than that of ENSO, since it includes a stronger component of direct local forcing. The South Georgia ecosystem is not self-sustaining, with import of krill from breeding and nursery grounds upstream in the ACC being important. We speculate here that these varying meridional winds close to the Antarctic Peninsula play a direct role in promoting/restricting the injection of shelf waters (and the krill therein) into the ACC, following which anomalies in krill density would be advected toward South Georgia. This offers a dynamical mechanism that might contribute to interannual changes in biological communities at South Georgia, in addition to existing theories. Both SAM and ENSO have shown long-period changes in recent decades, with ENSO exhibiting a higher preponderance of El Niño events compared with La Niña events, and the SAM showing a marked trend toward a higher index state. Such long-period behaviour is likely to induce changes in the South Georgia ecosystem via their impacts on advection and SST, for which an understanding of the physical mechanisms elucidated here will be key to unravelling.