Upwelling front and two-cell circulation in a two-dimensional model

The intensification of upwelling front and two-cell circulation is studied numerically in a two dimensional level model. Upwelling front is set initially with longshore geostrophic flow. The uniform wind stress forces the ocean which has an infinite north-south coast line. Two-cell circulation, downwelling just inshore-side of the front and upwelling offshore-side, is induced, and the front is intensified. It is found that the intensification is occurred in the inshore-side of the front, and the intensification is basically due to the deviation from the thermal-wind balance, as is shown bySuginohara (1977). It is found that the inshore-side cell intrudes under the pycnocline. It seems to reproduce the observed two-cell circulation.     

Response of Equatorial Pacific Mean Temperature Field to Intraseasonal Wind Forcing

The effects of intra-seasonal wind forcing on the mean field of the tropical Pacific Ocean has been studied using an ocean general circulation model (GCM). Idealized intra-seasonal zonal wind forcing with zero mean, which propagates eastward, induces net eastward jets at the equator that shift the warm water pool to the east. The mean temperature of the upper 200 m of the ocean increases off the equator and decreases at the equator. The change is independent of the propagation speed of the intra-seasonal wind forcing. The magnitude of the change depends on the amplitude and the period of the forcing, and the ocean structure, while the spatial pattern is independent of these parameters. A simple shallow water model is used to explain these changes. It is found that the term responsible for the enhanced eastward Equatorial jet is the Reynolds stress term, which arises from a phase shift of the zonal current due to friction. The resultant convergence of eastward momentum on the equator and geostrophic adjustment of the interface to the change of zonal current brings about the thermal redistribution of the upper ocean seen in the GCM.     

Equatorward Spreading of a Passive Tracer with Application to North Pacific Interdecadal Temperature Variations

A simulation is conducted with a realistic ocean general circulation model to investigate the three dimensional spreading of a passive tracer prescribed at the sea surface with the same distribution as the interdecadal sea surface temperature (SST) anomalies observed in the North Pacific. The tracers reaching the equator have the same sign as the major oval-shaped SST anomaly pattern in the central North Pacific but with a magnitude reduced less than 10% of the mid-latitude SST anomaly. The mixing both with the water containing SST anomalies of an opposite sign off the west coast of North America, and with the Southern Hemisphere thermocline water both contribute to the reduced equatorial amplitude. On the way to the equator in the southwestern part of the subtropical gyre, the subducted water is replenished by tracers leaking from the recirculation region to the north. The simulated passive tracer field in the subsurface layers agrees with the observed interdecadal temperature anomalies, suggesting the relevance of the processes studied here to the thermocline variability in the real North Pacific.