SST Diurnal Variability in the Climate Forecast System and Its Influence on Low Frequency Variability

Abstract

The diurnal cycle of sea surface temperature (SST) is one of the fundamental periodic variations in the earth climate system. It is generated by the upper ocean's response to large diurnal change of the incoming solar radiation and other local metrological conditions such as wind stress, precipitation, etc. In situ observations and satellite remote sensing have indicated that the SST day-night difference can be highly varied from daily to intraseasonal time scales and the amplitude of the variation can be up to 2-3 degrees C under certain conditions. The potential impacts of diurnal SST variation on the weather and climate deserve further investigation. In particularly, it has been hypothesized that the episodic enhancement of the SST diurnal amplitude actively affects organized convections over the western Pacific warm pool and modulates the tropical atmospheric intraseasonal variability. The hypothesis, however, cannot be verified with current coupled ocean-atmosphere general circulation models (CGCMs) because the SST diurnal cycle has yet to be efficiently resolved in these global climate models. We believe that realistic representation of SST diurnal cycle in CGCMs is important because such enhancement may improve the capability of these models to simulate weather and climate. The major difficulty in simulating the SST diurnal cycle is due to the inability of current CGCM oceanic components to resolve the transient diurnal mixed layer evolving within the upper few meters of sea water. Here we adopt two sub-layer parameterization schemes, which are designed by other scientists based on the diurnal mixed layer theories but have not been implemented in CGCMs for climate research, to the Climate Forecast System (CFS) developed by National Center for Environmental Prediction (NCEP). Both parameterization schemes have been fully tested; multi-year simulations and several sensitivity experiments have been conducted. The SST diurnal cycle has been objectively extracted from the simulations and compared with The Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA/COARE) observations and an empirically reconstructed dataset of daily SST diurnal variation based on the International Satellite Cloud Climatology Project (ISCCP) and Special Sensor Microwave/Imager (SSM/I). The results show that the CFS with sub-layer parameterizations can reproduce the characteristics of the spatial pattern and temporal variation of SST diurnal cycle to a large extent. Furthermore, we study the diurnal SST effects on the mean state and low frequency variability, with an emphasis on the atmospheric intraseasonal oscillation, through a comparison of the error patterns of the model simulation including the diurnal mixed layer process with the original CFS simulation without diurnal variation. A possible mechanism for the feedback of SST diurnal variability on tropical atmospheric intraseasonal variability has been discussed.