Mike Meredith John King

1. Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century Mike Meredith John King

2. Background: atmospheric change Well-known that air temperatures are rising more rapidly at the western Antarctic Peninsula than anywhere else in the Southern Hemisphere… (nearly 3˚C since 1955). (e.g. Harangozo and King, 1998)

3. Background: atmospheric change Faraday air temp (solid) and 70ºW ice extent (dashed) (from King and Harangozo, 1998) M2 = atmospheric pressure difference across the Peninsula. • Atmospheric circulation has become more cyclonic over the Peninsula (with more warm, northerly winds). • Air temperature at the Peninsula shows a strong dependence on ice extent.

4. Background: sea ice changes • Length of sea ice season has shortened over past couple of decades • Sea ice in Bellingshausen Sea has retreated since 1950s (based on rather sparse ship observations; King and Harangozo, 1998) (Parkinson, 2002)

5. And the ocean? • Changes in atmosphere and sea ice fields appear strongly coupled in WAP region. • Role of ocean in these changes has been widely speculated on, but remains unclear. • Here, we use in situ data from 1955 to 1994 from NODC archives (e.g. Levitus et al., 2005; Boyer et al., 2005) • >1400 CTD casts in 60-75ºS, 60-100ºW. • Concentrate here on upper-ocean changes, rather than e.g. Circumpolar Deep Water changes.

6. Decadal temperature anomalies, 1955-1994 • Strong negative anomaly in Bellingshausen existed during 1955-1964, coincident with extensive sea ice. • Progressive warming, coinciding with strong atmospheric warming. (Surface ocean, summertime only).

7. Temperature trends, 1955-1998 • Much greater than rate of warming of global ocean. • Strongly surface-intensified. • Decays to around zero by 100m depth.

8. Salinity trends, 1955-1998 • Patchy coverage, but strong increase in salinity at WAP. • Also strongly surface-intensified • But shows an increase in salinity, not a decrease …

9. So…. • What is causing these changes in the ocean? • Increased upwelling of warm, salty CDW from below? • unlikely, trends are surface-intensified. • Some combination of atmospheric warming and decreased precipitation? • possible, but evidence suggests an increase in precipitation at WAP, consistent with more cyclonic atmospheric circulation • Something to do with the sea ice? • hmmm…

10. Test impact of decreased sea ice production using adapted Price-Weller-Pinkel mixed layer model coupled to Renfrew ice production model. • This shows (slight) decrease in salinity in winter, due to reduced brine rejection. • But shows large increase in salinity in summer, due to reduction in freshening as ice melts. • Anomalies are spread over depth of mixed layer, which is much deeper in winter – hence asymmetrical. • All ocean data are from the summer! • Effect decreases with depth. S = summer; W = winter Reduced ice production simulated as shift from dashed lines to solid lines.

11. Why does this matter? • Physical impacts:- • Higher ocean temperature and salinity are caused by reduced ice production BUT are also positive feedbacks – will act to reduce future sea ice production • Changes are very significant - temperature change is equivalent to energy required to form around 0.3m of sea ice; c.f. typical sea ice thicknesses in Bellingshausen of 0.5-1m (Timmerman et al., 2004) • Will contribute significantly to future Peninsula warming (air temperature at Peninsula strongly correlated with ice extent)

12. Why does this matter? Proportions of limpets righting, bivalves burrowing or scallops swimming decline rapidly with temperature (Peck et al., 2004) • Marine species at Antarctic Peninsula have evolved to deal with low temperatures, and a low seasonal range in temperature. • Changing the environment they live in could have serious consequences. • A 2°C change in ocean temperature could lead to “population or species level losses” (Peck et al., 2004) • Populations can’t move very much further south to keep cold. N = 300 N = 200

13. Krill stocks in SW Atlantic (which are sourced, at least partially, from WAP) are in decline • This couldbe due to decrease in sea ice (and hence algae) • But krill are known to favour cold water also … (Atkinson et al., Nature, 2004)

14. Summary • Long-term changes at the Western Peninsula include a very strong warming and a salinification of the upper ocean. • Both of these are driven at the surface by atmosphere-ice-ocean interaction. • There are significant ecological consequences for these long-term changes, possibly extending throughout the regional food web. • Temperature and salinity changes are positive feedbacks, acting to sustain and enhance the WAP warming, and further reduce ice production, making continued impacts on the physical system and ecosystem even more likely. • Meredith, M.P. and J.C. King. “Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century”. Geophysical Research Letters, vol. 32, 2005.