An atmospheric origin of themulti‐decadal bipolar seesaw

Zhaomin Wang1, Xiangdong Zhang2, Zhaoyong Guan1, Bo Sun3, Xin Yang4 & Chengyan Liu1

1Polar Climate System and Global Change Lab, NUSIT2International Arctic Research Center, University of Alaska Fairbanks, USA

3Polar Research Institute of China4British Antarctic Survey, UK

Recent sea ice changes

Multi‐decadal bipolar seesaw in polar SATs?

(Chylek et al., 2010, GRL) 

Arctic

Antarctic

Multi-decadal ocean temperature changes in the Southern Hemisphere Oceans (90oS-EQ)

(Gille 2008)

• Ocean temperature data from ship-based hydrographical surveys and from autonomous floats

• Large warming since the 1930s

• Little changes since the 1970s

Linear trends in SST over 1950-1979 and over 1980-2009

ERSSTV3b HadISST1950-1979 1950-1979

1980-2009 1980-2009

Does the seasewpattern also exist in the early time?

• Very low spatial and temporal coverage in the SO before the 1920s

• Much less observations during the World Wars

• The early time seesaw pattern cannot be confirmed by the SST data.

Simulated annual regression of the SST on the SAM (Hall and Visbeck 2002)

Simulated annual regression of the SST on the SAM (Sen Gupta and England 2006)

The effects of the SAM on the SO SST: Model results

Hogg et al. (2008) Screen et al. (2009)

1/12o 1/4o 1o

Results from eddy-resolving ocean models suggest a potential warming effect of increased eddy-induced poleward heat transport in response to positive SAM (Hogg et al. 2008; Screen et al. 2009).

The poleward migration of ACC

• Some studies suggest a poleward migration of the ACC in response to poleward shifted and intensified westerlies.

• It was argued that this migration contributes to the SO warming.

• BUT, the large warming occurred between the 1930s and 1970s, when the SAM index was going down, and little changes or even cooling afterwards, when the SAM index was going up.

SAM index

Enhanced sea salt aerosol release induced by intensified surface windscan make the cloud brighter and hence lead to more reflection of solar radiation back to space.

Observed SAM-SO SST relationship on multi-decadal scales

•The annual mean SSTs are regressed on a smoothed (11-year running mean) Visbeck SAM index (from 1887-2005;Visbeck 2009).

• Broad surface cooling in the SO corresponds to an increasing SAM index on multi-decadal scales.

ERSSTV3b

HadISST

What causes multi-decadal variability in the SAM?(NH forcing)

• Coherent multi-decadal variability between the SAM, the AMO, the NH land SATs, and NH mean surface temperature.

• Two interpretations or hypothesis:

• Greenhouse gases and sulfate aerosols

• Multi-decadal oscillations of the AMOC

• Recent land surface warming led the N. Atl. warming.

What causes multi-decadal variability in the SAM?(tropical forcing)

• SAM varies coherently with tropical SST.

• Tropical surafce temperature change can lead to amplified change in the upper troposphere.

• Equator-to-pole temperature gradient will hence be greatly altered.

The dynamic link between the SAM and the NH thermal state

• Associated with cooling or warming of the NH, there exist significant changes in the tropical upper troposphere temperature.

• Corresponding to the recent NH warming, there is warming in the middle troposphere and cooling below and above this layer at SH mid-latitudes.

• Two important mechanisms for the SAM variability:

•The meridional temperature gradient in the upper layer (Chen et al. 2008)• Static stability at SH mid-latitudes (Lu et al. 2008)

1958-1971

1979-2001

Concluding Remarks

• There exists a multi-decadal bipolar seesaw pattern.

• No evidence to support the dominant role of the AMOC in this seesaw pattern.

• This seesaw pattern is attributable to the weakening or strengthening of the SH westerlies induced by NH and tropical tropospheric cooling or warming .

• NH and tropical warming provided another necessary condition for the formation of Antacrtci ozone hole.

• These finding poses a new and great challenge to the current generation of climate models.

Many thanks!