FY 2023

Coupled atmosphere–ocean variations on timescales of days observed in the western tropical Pacific warm pool during mid-March 2020

Nagano, A., B. Geng, K.J. Richards, M.F. Cronin, K. Taniguchi, M. Katsumata, and I. Ueki

J. Geophys. Res., 127, e2022JC019032, doi: 10.1029/2022JC019032, View open access article online at AGU (external link) (2022)

From 11 to 12 March 2020, we observed atmosphere and ocean variations during a temporal warming of the western tropical Pacific warm pool and the associated active convection on the equator by the R/V Mirai and unmanned surface vehicles, known as Wave Gliders. The warming, which extended the warm pool eastward at a speed of approximately 2.9 m s−1, was found to be caused mainly by high sensitivity of the temperature of the thin mixed layer to shortwave and longwave radiation flux. During the warming, westward-propagating inertia-gravity wave disturbances in addition to intensified northerly wind intermittently enhanced turbulent latent heat release into the atmosphere (>150 W m−2), leading to heavy rainfall from midnight to morning, so-called diurnal dancing, in the eastern region of the warm pool. The strong air–sea coupling was related to the zonal wind variation trapped near the sea level, which caused their convergences, also intensified atmospheric convection. Subsequently (13–14 March), due to the atmospheric density stratification and wind speed changes, atmospheric disturbances radiated upward and ceased to be coupled with the ocean.

Plain Language Summary. The western tropical Pacific is called the tropical Pacific warm pool. We observed atmosphere and ocean variations during a temporal warming of the warm pool in mid-March 2020 by the R/V Mirai and unmanned surface vehicles, known as Wave Gliders. The warming of the warm pool is found to be caused by solar-radiation heating of the thin surface mixed layer from 11 to 12 March. During the warming period, westward-propagating zonal wind disturbances trapped near the sea level in addition to strong northerly wind brought about substantial latent heat release from the ocean into the atmosphere, which caused active convection associated with heavy rainfall from midnight to morning. Subsequently, because the atmospheric density stratification and wind speed changed, generated atmospheric disturbances escaped upward from the sea level and stopped being coupled with the ocean. The strength of the diurnal atmosphere and ocean coupling appears to be related to the changes of the atmospheric background conditions.