Abstract
AbstractDaily initialized coupled and uncoupled numerical weather prediction (NWP) forecasts from the global Met Office Unified Model (MetUM) are compared for the 2016 Indian summer monsoon. Three MetUM configurations are used: atmosphere only (ATM), coupled to a mixed‐layer ocean model (KPP), and coupled to a dynamical ocean model (NEMO). The analysis focuses on the impact of air–sea coupling, particularly in the Bay of Bengal (BoB), on NWP for monsoon rainfall. Seasonal‐mean biases in all three configurations are highly consistent and driven by errors in atmospheric processes. Rainfall is initially overestimated over India, but underestimated over the BoB, the latter associated with too much short‐wave radiation and too little cloud cover in MetUM. The excess short‐wave radiation (>40 W·m−2 over the northwest BoB) is partially compensated by additional latent cooling, primarily due to overestimated surface wind speeds. In NEMO and KPP, coupling improves the timing of intraseasonal active and break phases over India, primarily the end of these phases, which are systematically too late in ATM. NEMO and KPP show a more realistic intraseasonal local phase relationship between sea surface temperature (SST) and rainfall throughout the BoB, but no configuration reproduces the observed significant lagged relationship between BoB SST and Indian rainfall. The lack of this relationship may be partly attributed to weak heat flux feedbacks to northern BoB SST, with the forecast short‐wave feedback having systematically the wrong sign (positive) compared to satellite radiation, and thus contributing to SST warming at all lead times. Based on these MetUM forecasts, there is a limited impact of coupling on NWP for monsoon rainfall, both for the mean rainfall and intraseasonal variability. Further research to improve NWP for monsoon rainfall should focus on reducing MetUM atmospheric systematical biases.
Highlights
Indian summer monsoon rainfall provides a critical freshwater resource for more than 1 billion people on the Indian subcontinent
The analysis focuses on the impact of air–sea coupling, in the Bay of Bengal (BoB), on numerical weather prediction (NWP) for monsoon rainfall
Over the BoB there is a wet bias at early lead times, especially in the west, with a dry bias emerging in the east and spreading westwards until it covers most of the Bay
Summary
Indian summer monsoon rainfall provides a critical freshwater resource for more than 1 billion people on the Indian subcontinent. Synoptic variability is controlled by monsoon lows and depressions, which typically form in the Bay of Bengal (BoB) and track along the monsoon trough that extends northwest–southeast across India (e.g., Sikka, 1977; Boos et al, 2015). These systems cause considerable rainfall, with recent estimates suggesting that depressions are responsible for 10–20% of rainfall across central India (Hunt and Fletcher, 2019). Active BSISO phases over India are associated with stronger westerly winds, higher rainfall over central India, reduced rainfall over southern and northeastern India, and an increase in the frequency of monsoon lows and depressions (e.g., Goswami et al, 2003; Krishnamurthy and Shukla, 2007)
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