SAPRISE annual meeting, University of Exeter, 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 low- level 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 , 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).