Comparison Of Response To Thermal Forcing And Wind Stress Forcing For Equatorial Pacific

Numerical experiments are conducted using a global tripolar grid ocean general circulation model derived from the Geophysical Fluid Wind stresss Laboratory (GFDL) Modular Ocean Model (M0M4) with high resolution and 50 vertical levels to investigate the response of sea temperature and zonal current interannual variations in the upper equatorial Pacific Ocean to wind stress forcing, thermal forcing and the two integrated forcing, respectively , and to test the ability of the model simulating ENSO circle though describing the evolution characteristics of the associated warm and cold events for the period 1980-2000.The forcing fields are from the wind stress fields and thermal fields calculated by the standard formulae within the model by the use of the monthly mean atmospheric data such as 2-m air temperature, 2-m specific humidity, sea lecel pressure, surface wind speed, and the monthly mean surface flux data including clear sky downward longwave flux and net shortwave radiation taken from the NCEP/NCAR reanalysis dataset and the relatived monthly mean climatology from the German OMIP dataset.The conclusions at the sea surface are as follows: (l)The distribution and propagating eastward and westward of sea surface temperature anomalies(SSTA) , and the annual variations that characterize ENSO circle of the zonal current velocity, are all simulated successfully in the wind stress forcing experiment, but how intensive these anomalies are and where their maximum variability arises is quite different from the observed values. In this experiment , the simulated strong El Nino/La Nina episodes are better than the weak ones and the simulated El Nino episodes have a bit advantages over the La Nina ones. (2)Compared to the wind tress experiment, although the simulated SSTA is weaker in the thermal forcing, the four modest or strong El Nino and two strong La Nina episodes are still well simulated, and phase differences between thesimulated warm(cold) episodes and obseved ones are less, whereas the simulated amplitude of the strong El Nino is much weaker and that of other ones is comparable in the two experiments. It is shown that SSTA critical effect of the themal forcing appears in the east over the equatorial Pacific.(3) The simulated strength and distribution is substantially improved and approximates the observed values in the integrated wind stress and thermal forcing experiment.It is suggested that the influence on SST of the thermal forcing can not be neglected in the equatorial Pacific, especially in the eastern Pacific, and the simulation of the phase and amplitude of SSTA in Nino3 is so well simulated that it is very close to the observed values as result of the added thermal forcing.Furthermore it is also found that the model can reproduce the entire process of onset of El Nino/La Nina events in the sub-surface, whether in wind stress forcing or in thermal forcing , i.e. the buildup of sub-surface sea warm (cold) water anomalies in the equaorial western Pacific—propagating eastward and upward along the ocean climatic thermocline—occurrence of SSTA in the equaorial eastern Pacific, whereas the integrated forcing is added to the model, the wind stress forcing plays a dominant role and the thermal forcing takes second place in the process. In addition, for the zonal current velocity anomalies, the effect of the thermal forcing is negligible with respect to that of the wind stress forcing whether at the surface or in the sub-surface.