Typhoon-induced sea surface cooling during the 2011 and 2012 typhoon seasons: observational evidence and numerical investigations of the sea surface cooling effect using typhoon simulations

Understanding oceanic responses to typhoons and the impacts those responses have on the typhoons themselves is important so that typhoon predictions performed using numerical models and typhoon forecasts can be improved. However, in situ oceanic observations underneath typhoons are still limited. To gain a deep understanding of the oceanic response and estimate the magnitude of its impact, three profiling floats were deployed in the western North Pacific during the 2011 and 2012 typhoon seasons. The daily observations showed that the sea surface cooled by more than 2°C in typhoons Ma-on and Muifa in 2011, and typhoons Bolaven and Parapiroon in 2012. The response was different at different float locations relative to the typhoon center, that is, the response within 100 km of the typhoon center was different to the response more than 100 km from the center on the right- or left-hand sides of the typhoon track, even though the response was affected by pre-existing oceanic conditions, precipitation, and the typhoon intensity. The salinity and temperature profiles were also considerably different before, during, and after the passage of a typhoon. To determine the impacts of typhoon-induced sea surface cooling on typhoon predictions, the impacts of the four typhoons were numerically evaluated using an atmosphere-wave-ocean coupled model. The coupled model simulated sea surface cooling and the resultant increases in the central pressures caused by the passages of the typhoons reasonably well. When axisymmetrically simulated, the mean sea surface cooling beneath a typhoon decreased the latent heat fluxes by 24% to 47%. A larger cooling effect gave a larger decrease in the latent heat flux only during the intensification phase. The decrease in the latent heat flux affected the inner core structure, particularly in the inflow boundary layer and around the eyewall. The cooling effect significantly affected the track simulation only for Typhoon Muifa, which had the weakest zonal steering flow of the four typhoons. These results suggest that making more frequent typhoon observations using profiling floats, further developing oceanic analysis techniques, and improving our understanding of typhoon-ocean interactions are required to produce more accurate typhoon predictions.