Wednesday 19 April, 10:00 - EARTH SYSTEM PHYSICS SEMINAR

[Earth System Physics Section]

EARTH SYSTEM PHYSICS SEMINAR
Wednesday 19 April - 10:00 hrs
Central area, 2nd floor, ex-SISSA bldg.

ROLE OF TRADE WIND VARIATIONS IN PACIFIC AND GLOBAL DECADAL CLIMATE VARIABILITY

Matthew England
Climate Change Research Centre
University of New South Wales
Australia

The role of observed trade wind variations associated with the
Interdecadal Pacific Oscillation (IPO) in driving global climate is
investigated using a high-resolution ocean model forced by observed wind
and surface flux trends over the Pacific. In the main perturbation
experiment the surface momentum, heat and freshwater flux trends as
observed in the Pacific region (1992-2011) are applied regionally, while
the rest of the globe is forced by CORE normal year surface fluxes.  This
period is characterized by a marked Walker circulation and trade
acceleration, and a trend toward a strongly negative phase of the IPO with
significant east Pacific cooling.  This experiment is compared to a
control CORE normal year experiment.  An idealized “future” trend
experiment is also investigated wherein the applied atmospheric trends
during 1992-2011 are reversed back to normal-year conditions over the
period 2012-2031.

We find a strengthening of the Equatorial Undercurrent (EUC) in response
to strengthened winds, which brings cool water to the surface of the
eastern Pacific, and an increase in the Pacific shallow overturning cells,
which in turn drives additional heat into the subsurface western Pacific.
The wind acceleration also results in an increase in the strength and
subsequent heat transport of the Indonesian throughflow, which transports
some of the additional heat from the western Pacific into the Indian
Ocean. This results in a warm subsurface western Pacific, a cool upper
eastern Pacific and a warm subsurface Indian Ocean, with an overall
increase in Indo-Pacific heat content.

Experiments applying a symmetric reversal of the atmospheric fields mimic
a return to the positive phase of the IPO, characterized by weaker Pacific
trade winds.  In response we find a slowdown of the (EUC) and Pacific
shallow overturning cells, and a resulting return to climatological SST
conditions in the western and eastern Pacific.  The ITF also slows back
down to its original values.  However, the temperature, heat content and
ITF responses are not quite symmetric due to an overall increase in the
surface heat flux into the ocean associated with the cooler surface of the
Pacific, and also due to irreversible heat transfer from the Pacific into
the Indian Ocean via the ITF.  There is also irreversible heat transport
across the thermocline via diapycnal mixing, further contributing to this
asymmetry. Here we find an Indo-Pacific subsurface ocean that remains
warmer than it was in its initial state.   This could have implications
for long-term heat content changes in the ocean interior.

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