SAPRISE annual meeting, University of Exeter, June 2013 The role of the Indian peninsula in the Asian monsoon (HadGEM3-A GA3) Andy Tuner, Gill Martin, Richard Levine

Motivation Much work on role of Tibetan Plateau (e.g. Li & Yanai, 1996), Himalayas (e.g. Boos & Kuang, 2010) in development and maintenance of Asian summer monsoon. East African highlands also play a role (Slingo et al., 2006). Strong convection over Bay of Bengal maintained by surface barrier layer (Shenoi et al., 2005). Idealised studies into role relative role of MSE ventilation, Rossby wave dynamics, land surface etc (Chou et al., 2001; Chou & Neelin, 2003). However little on role of Indian peninsula in GCMs.

AMIP runs of of perturbed land surface. First remove whole peninsular India (underlying SSTs are defined in PCMDI/AMIP forcing data, approximating zonal mean between Arabian Sea and Bay of Bengal). Experiments also test role of orography (western Ghats) and surface (lake versus bare soil) separately. Further experiments looking at the Indo-Gangetic Plains given position of the monsoon trough and land-use change/irrigation there (Koster hotspot). Experimental set up

Summary of experiments

Standard land surface classification Grid box can be subdivided into 9 tiles, either as a combination of any of the 8 shown or as land ice. C3 grasses dominate.

Monsoon simulation in HadGEM3-A Dry biases over much of central India, particularly in the northeast peninsula. Weak Somali jet and flow around the trough consistent with the dry biases. India dry bias and WEIO wet bias consistent with CMIP3/5 models (but stronger).

CMIP3/CMIP5 precipitation biases CMIP3 and CMIP5 models show large dry biases over India but wet biases over the WEIO and Maritime Continent in boreal summer. Beware counterintuitive colour scale. Sperber, Annamalai, Kang, Kitoh, Moise, Turner, Wang and Zhou (2012) Climate Dynamics.

Peninsula (pen) experiments: removal, bare/lake & orography

Sensitivity to IGP June MSLP and surface temperature

Timestep analysis of rainfall Wet surface experiments show enhanced deep convection at high rain rates, and reduction in mid-level convection.

Role of monsoon depressions Monsoon depressions examined in lake_SA and no_SA compared to control. Depressions play a much more significant role in wet surface experiments.

Behaviour of monsoon ISV Northward propagating monsoon ISV tested after Lin et al. (2008). Lanczos-filtered day daily precipitation data averaged 50-70°E lag-correlated with point in Bay of Bengal. Wet surface experiments show evidence of more coherent northward propagating modes.

Seasonal cycle of turbulent fluxes and net energy convergence into the column Wet surface leads to dominance of LH rather than SH. no_pen (sea) LH is a result of convolution of seasonal cycles of surface winds and SST. lake_pen shows enhanced convergence of F net into atmospheric column.

Typical boundary layer types in summer 7 possible boundary layer types in Lock scheme. Summer control simulation lacks cumulus boundary layer, consistent with lack of deep convection.

Diurnal cycle of surface temperature DC further enhanced in bare_pen experiment. DC suppressed in wet surface experiments.

Diurnal cycle of boundary layer, convection and turbulent heat fluxes Diurnal cycle of BL (control) Diurnal cycle of BL (no_pen) Diurnal cycle of LH, SH

Comparison of BL diurnal cycle Wetting the surface dominates while bare soil or orography make little difference.

Summary Perturbations to peninsular South Asia have dramatic impact on monsoon in HadGEM3-A (e.g. larger than those in convective entrainment change experiments). Replacing land surface with sea does not collapse the monsoon since “land-sea” contrast is important further north and at higher levels also. Additional supply of latent heating at surface strengthens monsoon rainfall. Important role of IGP in positioning of monsoon trough and determining the monsoon circulation. Further work plans to test role of peninsula in coupled framework (FAMOUS).