Rapid climate change in the ocean west Rapid climate change in the ocean west of the Antarctic Peninsula during the of the Antarctic Peninsula during the

second half of the 20second half of the 20thth century century

Mike MeredithMike MeredithJohn KingJohn King

Background: atmospheric changeBackground: atmospheric change

Well-known that air temperatures Well-known that air temperatures are rising more rapidly at the are rising more rapidly at the western Antarctic Peninsula than western Antarctic Peninsula than anywhere else in the Southern anywhere else in the Southern Hemisphere… (nearly 3˚C since Hemisphere… (nearly 3˚C since 1955).1955).

(e.g. Harangozo and King, 1998)(e.g. Harangozo and King, 1998)

Faraday air temp (solid) and 70ºW ice extent (dashed) (from King and

Harangozo, 1998)M2 = atmospheric pressure difference across the Peninsula.

Background: atmospheric changeBackground: atmospheric change

• Atmospheric circulation has become more cyclonic over the Peninsula Atmospheric circulation has become more cyclonic over the Peninsula (with more warm, northerly winds).(with more warm, northerly winds).

• Air temperature at the Peninsula shows a strong dependence on ice Air temperature at the Peninsula shows a strong dependence on ice extent.extent.

Background: sea ice changesBackground: sea ice changes

• Length of sea ice season has shortened over past couple of Length of sea ice season has shortened over past couple of decadesdecades

• Sea ice in Bellingshausen Sea has retreated since 1950s Sea ice in Bellingshausen Sea has retreated since 1950s (based on rather sparse ship observations; King and (based on rather sparse ship observations; King and Harangozo, 1998)Harangozo, 1998)

(Parkinson, 2002)

And the ocean?And the ocean?

• Changes in atmosphere and sea ice fields appear Changes in atmosphere and sea ice fields appear strongly coupled in WAP region.strongly coupled in WAP region.

• Role of ocean in these changes has been widely Role of ocean in these changes has been widely speculated on, but remains unclear.speculated on, but remains unclear.

• Here, we use Here, we use in situ in situ data from 1955 to 1994 from data from 1955 to 1994 from NODC archives (e.g. Levitus et al., 2005; Boyer et NODC archives (e.g. Levitus et al., 2005; Boyer et al., 2005)al., 2005)

• >1400 CTD casts in 60-75>1400 CTD casts in 60-75ººS, 60-100S, 60-100ººW.W.

• Concentrate here on upper-ocean changes, rather Concentrate here on upper-ocean changes, rather than e.g. Circumpolar Deep Water changes.than e.g. Circumpolar Deep Water changes.

• Strong negative Strong negative anomaly in anomaly in Bellingshausen Bellingshausen existed during existed during 1955-1964, 1955-1964, coincident with coincident with extensive sea extensive sea ice.ice.

• Progressive Progressive warming, warming, coinciding with coinciding with strong strong atmospheric atmospheric warming.warming.

(Surface ocean, summertime only).

Decadal temperature anomalies, 1955-1994Decadal temperature anomalies, 1955-1994

• Much greater Much greater than rate of than rate of warming of warming of global ocean.global ocean.

• Strongly Strongly surface-surface-intensified.intensified.

• Decays to Decays to around zero by around zero by 100m depth.100m depth.

Temperature trends, 1955-1998Temperature trends, 1955-1998

• Patchy Patchy coverage, but coverage, but strong increase strong increase in salinity at in salinity at WAP.WAP.

• Also strongly Also strongly surface-surface-intensifiedintensified

• But shows an But shows an increase increase in in salinity, not a salinity, not a decrease …decrease …

Salinity trends, 1955-1998Salinity trends, 1955-1998

• What is causing these changes in the ocean?What is causing these changes in the ocean?

• Increased upwelling of warm, salty CDW from below?Increased upwelling of warm, salty CDW from below?– unlikely, trends are surface-intensified.unlikely, trends are surface-intensified.

• Some combination of atmospheric warming and Some combination of atmospheric warming and decreased precipitation?decreased precipitation?– possible, but evidence suggests an possible, but evidence suggests an increaseincrease in precipitation at in precipitation at

WAP, consistent with more cyclonic atmospheric circulationWAP, consistent with more cyclonic atmospheric circulation

• Something to do with the sea ice?Something to do with the sea ice?– hmmm…hmmm…

So….So….

• Test impact of decreased sea ice production Test impact of decreased sea ice production using adapted Price-Weller-Pinkel mixed layer using adapted Price-Weller-Pinkel mixed layer model coupled to Renfrew ice production model.model coupled to Renfrew ice production model.

• This shows (slight) decrease in salinity in winter, This shows (slight) decrease in salinity in winter, due to reduced brine rejection.due to reduced brine rejection.

• But shows large increase in salinity in summer, But shows large increase in salinity in summer, due to reduction in freshening as ice melts.due to reduction in freshening as ice melts.

• Anomalies are spread over depth of mixed layer, Anomalies are spread over depth of mixed layer, which is much deeper in winter – hence which is much deeper in winter – hence asymmetrical.asymmetrical.

• All ocean data are from the summer!All ocean data are from the summer!

• Effect decreases with depth.Effect decreases with depth. S = summer; W = winterS = summer; W = winter

Reduced ice production simulated Reduced ice production simulated as shift from dashed lines to solid as shift from dashed lines to solid lines.lines.

Why does this matter?Why does this matter?

• Physical impacts:-Physical impacts:-

• Higher ocean temperature and salinity are caused by reduced Higher ocean temperature and salinity are caused by reduced ice production BUT are also positive feedbacks – will act to ice production BUT are also positive feedbacks – will act to reduce future sea ice productionreduce future sea ice production

• Changes are very significant - temperature change is Changes are very significant - temperature change is equivalent to energy required to form around 0.3m of sea ice; equivalent to energy required to form around 0.3m of sea ice; c.f. typical sea ice thicknesses in Bellingshausen of 0.5-1m c.f. typical sea ice thicknesses in Bellingshausen of 0.5-1m (Timmerman et al., 2004)(Timmerman et al., 2004)

• Will contribute significantly to future Peninsula warming (air Will contribute significantly to future Peninsula warming (air temperature at Peninsula strongly correlated with ice extent) temperature at Peninsula strongly correlated with ice extent)

Temperature (°C)

0 1 2 3 4 5 6 7

0

20

40

60

80

100

% r

igh

tin

g in

24

h

2001 data2002 data

Proportions of limpets righting, bivalves burrowing or Proportions of limpets righting, bivalves burrowing or scallops swimming decline rapidly with temperature scallops swimming decline rapidly with temperature (Peck et al., 2004)(Peck et al., 2004)

N = 200

N = 300

Why does this matter?Why does this matter?

• Marine species at Antarctic Peninsula have Marine species at Antarctic Peninsula have evolved to deal with low temperatures, and a evolved to deal with low temperatures, and a low seasonal range in temperature.low seasonal range in temperature.

• Changing the environment they live in could Changing the environment they live in could have serious consequences.have serious consequences.

• A 2A 2°C change in ocean temperature could °C change in ocean temperature could lead to “population or species level losses” lead to “population or species level losses” (Peck et al., 2004)(Peck et al., 2004)

• Populations can’t move very much further Populations can’t move very much further south to keep cold.south to keep cold.

• Krill stocks in SW Atlantic (which are Krill stocks in SW Atlantic (which are sourced, at least partially, from WAP) sourced, at least partially, from WAP) are in declineare in decline

• This couldThis could be due to decrease in sea be due to decrease in sea ice (and hence algae)ice (and hence algae)

• But krill are known to favour cold But krill are known to favour cold water also …water also …

19

76

19

78

19

80

19

82

19

84

19

86

19

88

19

90

19

92

19

94

19

96

19

98

20

00

20

02

Year

De

ns

ity

(n

o.

m-2

)

1

10

100

1000

0 100 200 300 400

Winter ice duration (days)

Kril

l den

sity

(no

.m-2

)

1920s and 1930s

post 1976 era

1000

100

10

1

0.1

(Atkinson et al., Nature, 2004)

SummarySummary• Long-term changes at the Western Peninsula include a very Long-term changes at the Western Peninsula include a very

strong warming and a salinification of the upper ocean.strong warming and a salinification of the upper ocean.

• Both of these are driven at the surface by atmosphere-ice-ocean Both of these are driven at the surface by atmosphere-ice-ocean interaction.interaction.

• There are significant ecological consequences for these long-term There are significant ecological consequences for these long-term changes, possibly extending throughout the regional food web.changes, possibly extending throughout the regional food web.

• Temperature and salinity changes are positive feedbacks, acting Temperature and salinity changes are positive feedbacks, acting to sustain and enhance the WAP warming, and further reduce ice to sustain and enhance the WAP warming, and further reduce ice production, making continued impacts on the physical system and production, making continued impacts on the physical system and ecosystem even more likely.ecosystem even more likely.

• Meredith, M.P. and J.C. King. “Rapid climate change in the ocean west of the Meredith, M.P. and J.C. King. “Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20Antarctic Peninsula during the second half of the 20thth century”. century”. Geophysical Geophysical Research Letters, Research Letters, vol. 32vol. 32, , 2005.2005.