Detection and attribution of upper-tropospheric warming over the tropical western Pacific

Abstract

Both model and satellite observations suggest that the warming in the tropical upper troposphere is more pronounced than in the lower troposphere. This study describes the tropical upper-tropospheric-warming (UTW) in the ECMWF Reanalysis Interim dataset, and reveals radiative and non-radiative processes contributing to the long-term change in UTW. The maximum intensity of the UTW is found between 250 and 400 hPa with a warming rate about 3K/decade over the tropical Atlantic and western Pacific oceans. The warming is more prominent from boreal winter to early spring than the rest of the year. In the tropical western Pacific, where the warmest ocean water resides, the long-term change in the annual mean UTW is mainly contributed by the variations of atmospheric deep convection and the oceanic dynamical process and heat storage (Ocn) with the latter playing a more important role. The UTW caused by the deep convection is mainly through the latent heat release associated with enhanced upward motion and condensation. The UTW related to Ocn is mainly a result of enhanced upward long-wave radiation from the ocean surface due to the large heat storage in the thermocline. The variations in Ocn mainly contribute to the UTW from winter to early spring, while the changes in deep convection play a more essential role from summer to fall. Also shown by the differences in deep convection, a strengthening of the Walker circulation since the early 2000s is also found in the reanalysis, which is most apparent in boreal summer and fall over the tropical Pacific, facilitating the deep convection to dominate.