Formation and Limiting Mechanisms for Very High Sea Surface Temperature: Linking the Dynamics and the Thermodynamics

Abstract

The present study composites atmosphere and ocean conditions associated with ocean hot spots. Ocean hot spots are defined as regions where SST exceeds 29.75°C and that have an area greater than 1 ? 106 km2. The composite atmosphere includes surface flux parameters, deep convection and cloud amounts, cloud radiative forcing, and analysis fields from the National Meteorological Center (NMC) and European Centre for Medium-Range Weather Forecasts weather forecasting systems. The composite ocean includes sea level height and the temperature and velocity structures down to 720 m from the NMC ocean forecasting system. These fields are composited for the months before, during, and after the appearance of hot spots in order to develop a four-dimensional picture of the atmosphere and ocean conditions that are associated with the formation and the decay of these very high ocean surface temperatures. The analysis indicates that the formation of these hot spots is largely confined to the region within the 28°C isotherm of the long-term mean SST, with greatest concentrations occurring in the western Pacific warm pool. Extended analysis of the warm-pool hot spots (0°?10°S, 156°?176°W) indicates strong influences from interannual, annual, and 30?60 day timescales, with La Niña conditions appearing to inhibit formation, southern summer favoring formation, and the descending (ascending) phase of the Madden?Julian oscillation (MJO) favoring formation (decay). This interaction with the MJO indicates how internal, or remotely forced, atmospheric variability, in addition to local feedbacks, may he playing a role to help limit SST. Furthermore, the out-of-phase relationship between SST and deep convection associated with this variability suggests the possibility of a positive feedback mechanism for the MJO. With respect to the surface heat budget, the data indicate that during the hot spot evolution, the convective perturbations to the surface shortwave exceed those for evaporation by at least a factor of 2. The composite ocean conditions indicate that the rapidly varying atmospheric conditions associated with the hot spot evolution induce significant changes below the surface layer of the ocean as well. These changes appear to be primarily linked to the onset of westerly wind bursts associated with the enhanced deep convection. Removing the El Niño time periods from the composites indicates that the composite mean is more dependent on the interannual state than the composite atmosphere. These results indicate that the ocean should not be rendered too simple with respect to understanding the limiting mechanisms of high SST.