ARTICLE IN PRESS Deep-Sea Research II 53 (2006) 853–865 www.elsevier.com/locate/dsr2 Spatial and temporal variation in shallow seawater temperatures around Antarctica David K.A. Barnesa,Ã, Veronica Fuentesb, Andrew Clarkea, Irene R. Schlossc, Margaret I. Wallacea aBritish Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK bAlfred-Wegner-Institut, Columbustrasse, D-2750 Bremerhaven, Germany cInstituto Anta´rtico Argentino, Cerrito 1248 (C1010AAZ) Buenos Aires, Argentina, and CONICET, Argentina Received 5 June 2005; accepted 3 March 2006 Abstract The variability of Southern Ocean sea-surface temperatures (SST) are important to understanding coastal biology yet are poorly known amongst biologists. We compare sea temperatures at a constant depth (10–20 m) at coastal localities both sides of the Polar Front (PF), around the Scotia-Arc, the West Antarctic Peninsula and at high oceanic latitudes (around the margins of East Antarctica). We assess the wider context of these values by investigating minimum and maximum temperatures and ranges throughout the Southern Ocean using remotely sensed SST data. Data to date show weekly, daily and hourly variation in shallow sea temperature can be one-third of total annual variability (in the summer) but can be very constant (in winter). From comparison across scales in time and space, the strong seasonal signal is the most striking feature of Antarctic shallow sea temperatures even at highest oceanic latitude sites. The winter sea temperatures at localities within the PF are similar (near freezing), but upper temperatures and thus the annual range vary predictably with latitude in a cline. This amounts to 0.2 1C annual range/100 km of latitude between 541S and 671S. The annual range in sea temperatures is little different at SubAntarctic islands, whether they are north or south of the PF. r 2006 Elsevier Ltd. All rights reserved. Keywords: Southern Ocean; South Georgia; King George Island 1. Introduction Antarctica. As a result, our current understanding of patterns of temporal and spatial variability in Although oceanographic observations have been seawater temperature in the shallows is limited, as made widely around Antarctica, there are still evidenced by the citation of data from very few relatively few direct subsurface measurements from locations around Antarctica. Even definitions of coastal or nearshore locations south of 681Sinwest which water comprises the Southern Ocean can Antarctica, on the east side of the Antarctic differ: typically marine biologists refer to it as inside Peninsula, or around most of East Antarctica; these the Polar Front (PF), whilst oceanographers define it gaps comprise the majority of the coastline of as inside the northern edge of the Antarctic Circumpolar Current (ACC). It is known that the ÃCorresponding author. Southern Ocean was warm during the late Mesozoic, 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.03.008 ARTICLE IN PRESS 854 D.K.A. Barnes et al. / Deep-Sea Research II 53 (2006) 853–865 but has cooled steadily through the Cenozoic. It tivity, Temperature and Depth measuring device) probably reached the current situation of polar have been made at a number of fixed nearshore conditions a few million years ago (Clarke and localities around Antarctica (see Fig. 1). These Crame, 1989; Lear et al., 2000; Zachos et al., 2001). include East Cumberland Bay, South Georgia; In winter, shallow seawater temperatures around Borge Bay, Signy Island; and various points along Antarctica reach freezing point. Despite little spatial the Western Antarctic Peninsula (WAP) such as the variability in absolute minimum values, there is a fjords of southern King George Island and Anvers strong seasonal signal driven by the brief summer rise Island, in the Palmer Archipelago. The patchiness in in temperature, the magnitude of which can vary space (most direct CTD measurements have been both spatially and between years at any one location made in the NW Antarctic Peninsula and the Scotia (Clarke, 1988). In 1965 Littlepage demonstrated that arc) was mirrored until recently by a similar in shallow waters, the seasonal variability of sea patchiness in time. Now an increase in the frequency temperature at 25 m depth in southern McMurdo of CTD measurements and in situ temperature- Sound, Ross Sea, was from À1.99 to À1.41 1C, with logger deployments have enabled examination of a standard deviation of only 0.11 K, suggesting it to temperature variability over much smaller temporal be one of the most thermally constant near-surface scales. For this study data were collected from the environments anywhere on earth (although it should primary literature, concentrating on data where sea be noted that Littlepage was unable to sample during temperature was measured at 10–20 m depth at open-water conditions from late February to early localities around East and West Antarctica and April 1961). Clarke’s (1988) comparison of this islands immediately to the north of the PF. oceanic high-latitude pattern with that at one of the The sources of sea-temperature data are listed in outlying archipelagos (Signy Island, South Orkney Table 1. Islands) has long been taken as representing the We used remotely sensed sea-surface temperature extremes within the Southern Ocean. Since this study (SST) data produced by NOAA and obtained from was published, a number of other coastal sites the Climate Diagnostics Center (http://www.cdc. around Antarctica have taken measurements of noaa.gov/cdc/). Specific data used were derived nearshoreseawatertemperature,manyofthemas from the NOAA Optimum Interpolation (OI) SST part of the Ecology of the Antarctic Sea Ice Zone v2 analysis, which is produced weekly on a 11 grid (EASIZ) programme of Scientific Committee for using in situ and satellite SSTs, plus SSTs simulated Antarctic research (SCAR). by ice cover. OI.v2 SST monthly fields are derived In the current study we investigate how seawater by a linear interpolation of the weekly OI.v2 fields temperature at shallow depth varies around Ant- then the daily values are averaged over a month. arctica, both temporally and spatially. Specifically The spatial resolution of the monthly fields is the we ask whether: same as that of the weekly fields (11 square). We processed OI.v2 monthly mean SST data covering (a) the seawater temperatures reported for McMur- the period December 1981–April 2005. Long-term do Sound by Littlepage (1965) are typical for monthly means (LTMs) were calculated by aver- this location, and whether they are representa- aging the monthly fields over the entire time series, tive of all high oceanic latitudes, producing a sequence of 12 spatial fields of average (b) the seasonal pattern reported for Signy Island monthly SST. The maximum and minimum SST by Clarke and Crame (1989) and Clarke and during this sequence of 12 monthly fields were Leakey (1996) is typical for other lower-latitude examined, and the range between them determined Antarctic coastal locations, and along with the month at which the maximum and (c) there is a cline in shallow-water seawater tempera- minimum temperatures were reached. ture along the Antarctic Peninsula to the Scotia arc, as suggested by Wiencke and Dieck (1989). 2.1. The oceanographic setting The continental shelf around Antarctica is 2. Materials and methods unusually deep (for summary statistics see Clarke and Johnston, 2003). As a result most of the shelf is In the last three decades regular measurements well below the winter mixed-layer depth, which is using reversing thermometers and CTDs (Conduc- typically 100 m (Meredith et al., 2004). This is a ARTICLE IN PRESS D.K.A. Barnes et al. / Deep-Sea Research II 53 (2006) 853–865 855 Fig. 1. Map showing the locations of scientific stations recording shallow sea temperatures used in the current study. Table 1 Sources of shallow sea temperature data for various high-latitude southern locations Location Sources Marion Branch et al. (1993) Falkland/Malvinas Is. Arkhipkin et al. (2004) Tierra del Fuego Balestrini et al. (1998) South Georgia Is. North (1980), unpublished BAS marine assistant records Signy Is. Clarke (1988), Clarke and Leakey (1996) King George Is. Rakusa-Suszcewski (1996), Schloss et al. (1997, 2002) Anvers Is. Krebs (1975), Baker et al. (1996, 1997), LTER database Adelaide Is. Rothera Time Series (RaTS) database, B.A.S. Crozet Is., Heard Is., Balleny Is. Wiencke and Dieck (1989) Macquarie Is., Budd coast, Mawson coast Lewis (2001) Ellis Fjord Gallagher and Burton (1988) McMurdo Sound Littlepage (1965) different situation from that on most shelves else- 2002). The marine source water for coastal waters where, where the seabed is frequently within or close around Antarctica is derived from Circumpolar to the depth of the winter mixed layer. The thermal Deep Water (CDW), the warm saline water of the characteristics of the surface waters of the Southern ACC. CDW is characterised by potential tempera- Ocean are dominated by exchange with the atmo- tures between +1 and +2 1C, salinities between sphere, sea-ice dynamics and interaction with the 34.6 and 34.7, relatively high nutrient contents, and deeper water (Klinck, 1998; Smith and Klinck, a low oxygen content (Hofmann et al., 1996; ARTICLE IN PRESS 856 D.K.A. Barnes et al. / Deep-Sea Research II 53 (2006) 853–865 Meredith et al., 2004). Upper CDW (UCDW) is The first is the depth range of the seabed that is defined by the temperature maximum, while Lower bathed in AASW. Here organisms will be subject to CDW (LCDW) is defined by the salinity maximum. thermal variability over a range of temporal scales Typical UCDW modified by interaction with other (Clarke, 2001) and a strong seasonality at many sites. water masses over the WAP shelf has a potential At depths below this, the seabed will be thermally temperature in the range +1.0–+1.45 1C and a more stable, though subject to changes associated salinity greater than 34.5 (Klinck, 1998).