SAPRISE annual meeting, University of Exeter, 24-26 June 2013

1. 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

2. 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.

3. 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).

4. 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).

5. 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.

6. 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.

7. SST, v925, sh925, LH flux & SAT relationships

8. 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.

9. 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.

10. 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).