Transcript ppt - dcs.ex.ac.uk - University of Exeter

SAPRISE annual meeting, University of
Exeter, 24-26 June 2013
Systematic winter SST biases in
the northern Arabian Sea in
HiGEM and the CMIP3
Deepthi Marathayil, Andy Tuner, Len
Shaffrey & Richard Levine
Motivation
Arabian Sea an important source of moisture for the
Indian monsoon (Gimeno et al., 2010).
CMIP models typically contain dry biases over India
and wet biases in the WEIO.
Cold biases in the Arabian Sea in winter/spring are
known to be detrimental to subsequent summer
monsoon precipitation (in HadGEM2: Levine & Turner,
2012).
This study was motivated by examination of the very
weak monsoon in HiGEM and cold biases in the
northern Arabian Sea during winter.
Winter SST versus winter monsoon
relationship
DJF northern Arabian Sea relationship with meridional
wind over the northern AS & coast.
Substantial cold biases common among CMIP3.
Strong correlation with excessive winds (cc=0.72; too
strong winter monsoon).
Winter SST versus low-level specific
humidity relationship
Winter SST biases also related to dry biases in lowlevel specific humidity over northern Arabian Sea.
Cold dry air is advected across the northern coast of
the Arabian Sea (cc=0.66).
Winter SST versus LH flux bias at the
surface
Negative correlation with LH flux over the northern
Arabian Sea (cc=-0.59) suggests atmosphere is leading
to the cold bias, consistent with cold dry air advection.
Winter SST versus 1.5m temperature
over Pakistan and NW India
Surface air temperatures during winter to the north of
Arabian Sea and in upstream position of winter
monsoon winds are much too cold (cc=0.68).
Cold dry air is advected across Arabian Sea, leading to
excessive LH flux from surface and cold SSTs.
SST, v925, sh925, LH flux & SAT
relationships
Composite of cold minus warm winters
in Arabian Sea
Composite
difference of
seven coldest
minus seven
warmest CMIP3
models in winter.
Enhanced
meridional
temperature
gradient,
excessive
monsoon winds
and strong
convergence into
WEIO.
Subsurface behaviour
Mean subsurface temperature structure along 64-68E
(shaded) and composite difference of cold minus warm
models (contours).
Mixed layer
too deep in
northern
Arabian Sea,
consistent
with
excessive
surface
winds.
Summary
Strong relation between excess winter monsoon winds,
cold dry air advection & cold SSTs in northern AS.
Blame laid on cold SAT over land north of Arabian Sea
– cause still needs to be determined.
Published as Marathayil et al. (2013) Environ. Res.
Letts. 8 014028, open access.
SST biases can lead to subsequent poor South Asian
monsoon (Levine & Turner, 2012; Levine et al., 2013).
Future plans:
Explore in CMIP5 & examine Arabian Sea SST–WEIO
precipitation–IOD interactions to follow up on
Marathayil (2013, PhD thesis).