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Roles of air–sea coupling and horizontal resolution in the climate model simulation of Indian monsoon low pressure systems


The roles of air–sea coupling and horizontal resolution in the representation of Indian monsoon low pressure systems (LPS) in Met Office Unified Model (MetUM) global climate simulations are investigated. To avoid the generally large sea surface temperature (SST) biases in standard coupled atmosphere–ocean global climate models (GCMs), the analysis is performed on experiments from an atmosphere model coupled to a mixed-layer ocean model (MetUM-GOML2), which allows coupling to be applied regionally as well as globally, while constraining the ocean mean state in coupled regions. Compared to the standard AMIP-style MetUM atmosphere-only simulations, the MetUM-GOML2 simulations produce more monsoon LPS, which is attributed to effects of relatively small remaining (Indian Ocean) SST biases that somewhat strengthen the atmospheric monsoon base state. However, the MetUM-GOML2 simulations, all starting from the same atmospheric and oceanic base state, allow for an idealised approach to evaluate the relative effects of coupling and resolution. When the effects of SST biases are excluded, global coupling has a neutral impact on the number of LPS formed, while the associated rainfall is somewhat reduced due to a local negative air–sea feedback reducing the strength of atmospheric convection and weakening individual LPS. The MetUM-GOML2 simulations show particular sensitivity to localised coupling in the Indian and Pacific Oceans, which appears to enhance the effect of monsoon LPS. Although, in contrast to the global coupling comparison, the comparison of regionally coupled simulations is affected by both differences in interannual SST variability and SST biases, and it is likely that this causes at least part of the positive effects from Indian and Pacific Ocean coupling. More importantly, however, is that the effects of air–sea coupling are substantially smaller than the positive effects of the increase in horizontal resolution from N96 (approx. 200 km) to N216 (approx. 90 km). The resolution effect is also larger than that seen in older MetUM configurations.

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This work and its contributors (Richard Levine and Gill Martin) was supported through the Weather and Climate Science for Service Partnership (WCSSP) India, a collaborative initiative between the Met Office, supported by the UK Government’s Newton Fund, and the Indian Ministry of Earth Sciences (MoES). Nicholas Klingaman was supported by an Independent Research Fellowship from the Natural Environment Research Council (NE/L010976/1) and by the NERC/Global Challenges Research Fund programme Atmospheric hazards in developing countries: risk assessment and early warnings (ACREW). Simon Peatman was supported by the NERC Bay of Bengal Boundary Layer Experiment project of the (NE/L013800/1). ERA5 figures in this paper have been generated using Copernicus Climate Change Service Information 2020. The authors would like to thank the two anonymous reviewers for suggestions that helped improve the manuscript.

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Correspondence to Richard C. Levine.

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Levine, R.C., Klingaman, N.P., Peatman, S.C. et al. Roles of air–sea coupling and horizontal resolution in the climate model simulation of Indian monsoon low pressure systems. Clim Dyn 56, 1203–1226 (2021).

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  • Indian Monsoon
  • Global Climate Model
  • Low Pressure Systems
  • Air–sea coupling
  • Horizontal resolution