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Impact of coupling between IFS Atmospheric and Ocean models

HRES (ECMWF forecasts cycle 43r3 and earlier) was not coupled with NEMO but retained the initial sea-surface temperature anomalies throughout the forecast period.  HRES tended to overdeepen deepen relatively slow-moving tropical cyclones (tropical cyclones too much.  This was due to the lack of ocean/atmosphere coupling in HRES which often kept the ocean too warm when it should be cooling).  In reality the strong winds and slow movement would induce turbulent mixing of the very warm surface waters with cooler waters from deeper in the ocean, reducing sea-surface temperatures, and hence inducing less deepening of the storm.  ENS was and continues to be coupled with the ocean and didn't suffer from this problem.

Fig8.1.10.11: Comparison of a coupled HRES (used in ECMWF forecasts from cycle 45r1 released June 2018) and an uncoupled HRES (used in ECMWF forecasts cycle 43r3 and earlier) forecast following tropical cyclone NEOGURI. Left: Sea-surface temperatures in °C; Right: Central pressures for NEOGOURI in hPa; Uncoupled HRES - Blue, Coupled HRES - Red, Observed values - Black. HRES data time 00UTC 6 July 2014.

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