On the interannual variability of ocean temperatures around South Georgia, Southern Ocean: Forcing by El Niño/Southern Oscillation and the Southern Annular Mode

Abstract

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 3 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.

Analysis of trends and sudden changes in long-term environmental data from King George Island (Antarctica): relationships between global climatic oscillations and local system response

“…The reason why similar effects could not be shown during previous strong El Niño events, e.g. 1991/92 and 1997/98, may be attributed to the increasing quality of the data set, with a more constant sampling scheme after 2000, as well as the fact that all El Niño events are different (Meredith et al 2008). Extrapolating the short-term effect to the natural conditions, this freshening may abate plankton biomass in the water column corroborated by slightly decreasing salinity and Chl a content from 1993 onwards.…”

“…Both SAM and ENSO have shown long-period changes in frequency, intensity and duration (Meredith et al 2008), leading to a dominance of positive SAM and/or La Niña conditions from the late 1980 onwards (Stammerjohn et al 2012). Therefore, changes in the coastal ecosystems of the WAP region are to be expected, particularly since the climate cycles appear to have a great influence on SST and other parameters.…”

Changing distributions of sea ice melt and meteoric water west of the Antarctic Peninsula

“…Whilst SAM variability is intrinsically extra-tropical, ENSO variability is sourced in the equatorial Pacific region. However, this mode of climate variability has a marked impact on the southeast Pacific sector of the Southern Ocean via both atmospheric and oceanic teleconnections (Meredith et al, 2008b;Turner, 2004;Yuan, 2004 Niño and/or negative SAM conditions result in the greater local preponderance of colder, drier air masses. The coincident phasing of these two atmospheric modes can thus amplify or dampen the atmospheric circulation changes and climatic anomalies at the WAP (Clem and Fogt, 2013;Fogt and Bromwich, 2006;Stammerjohn et al, 2008b).…”

“…The existence of cyclonic, semi-enclosed gyres separating the northeastward flow at the shelf break and the southwestward flow of the APCC has been postulated, though the structural persistence of such features has not been resolved in detail. Further information is presented in Beardsley et al (2004); Klinck et al (2004); Savidge and Amft (2009) etc., though in general the temporal variability of ocean circulation at the WAP is relatively poorly constrained On interannual timescales, the major drivers of atmospheric variability at the WAP are the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO) phenomenon (Marshall et al, 2006;Meredith et al, 2008b;Stammerjohn et al, 2008b). SAM is characterized by meridional movements of atmospheric mass between a node over Antarctica and an annulus encircling the lower-latitude Southern Ocean .…”

On the phytoplankton bloom in coastal waters of southern King George Island (Antarctica) in January 2010: An exceptional feature?

“…Significant interannual climate variability in sea ice extension and ocean surface temperatures along the WAP has been associated with the Southern Annular Mode (SAM) and the El Niñ o Southern Oscillation (ENSO; Meredith et al 2008;Schloss et al 2012). Climate warming is supposed to induce important changes in polar ecosystems, from microbial communities (Piquet et al 2011) to apex predators' levels (Trathan et al 2007).…”