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.