Rapid warming of the ocean around South Georgia, Southern Ocean, during the 20th century: Forcings, characteristics and implications for lower trophic levels

Abstract

The Southern Ocean is known to have warmed considerably during the second half of the 20th century but there are few locations with data before the 1950s. In addition, assessments of change in this region are hampered by the strong seasonal bias in sampling, with the vast majority of data collected during the austral summer. However, oceanographic measurements near South Georgia span most of the last century, and we here consider almost year-round data from this location over an 81-year period (1925–2006). We observe significant warming between the early and late 20th century, with differential warming between summer and winter months and an indication that late 20th century summer temperatures peaked ∼6 days earlier. To quantify the long-term warming trend in this highly variable data, a mixed model utilising a Residual Maximum Likelihood (REML) method was used. Over the 81-year period, a mean increase of ∼0.9 °C in January and ∼2.3 °C in August was evident in the top 100 m of the water column. Warming diminished below 100 m and approached 0 at 200 m. Thus the long-term warming around South Georgia is substantial—more so than documented previously for the circumpolar warming of the Southern Ocean. We examine potential causal effects of this trend, including local atmospheric and cryospheric change, the influence of upstream waters and the role of coupled modes of climate variability such as El Niño/Southern Oscillation and the Southern Annular Mode (SAM). It is likely that all of these play a part in the observed temperature increase. However, the role of the SAM is strongly indicated, via its likely role in the circumpolar warming trend in the Southern Ocean, and also by the atypical response of the South Georgia region to changes in heat fluxes associated with the SAM. Furthermore, the combination of a regional decline in ice extent and strong upstream warming likely explains a significant part of the strong seasonal variation apparent in the warming trend. In addition, we consider the implications that long-term warming has for South Georgia's lower trophic levels. For Euphausia superba, at their northern limit, we find a significant negative relationship between summer South Georgia water temperatures and mean summer density of E. superba across the southwest Atlantic sector of the Southern Ocean. Simple abundance and growth rate relationships with our long-term temperature data appear to show declining habitat suitability for E. superba. In contrast, the warming trend is likely to favour other macro- and mesozooplankton species that occupy the more northerly parts of the Antarctic Circumpolar Current, and it is likely to promote phytoplankton growth.