Eddy formation and propagation in the eastern tropical Pacific

Abstract

Observations of eddies in the eastern tropical Pacific from TOPEX altimetry data show that there are seasonal and interannual variations in eddy activity. Comparisons between time of eddy formation and corresponding wind data show that not all eddies are caused by winds blowing offshore from the coast of Central America. Plots of eddy tracks from TOPEX data show that some of these eddies last for over 6 months and travel more than 250 of longitude toward the west. Others go more towards the equator and dissipate quickly. A General Circulation Model is used to study the formation and propagation aspects of these eddies. Results from experiments exploring the formation mechanism show that high frequency wind bursts are sufficient but not necessary for eddy formation in the eastern tropical Pacific. Eddy activity remains almost the same if only the annual harmonic of the wind field is used to force the model. Forcing the model with only the high frequency wind component produces almost no eddies. The formation of eddies during periods of weak offshore winds suggests other possible mechanisms, such as unstable mean flows, for the formation of the eddies. Experiments done to study the propagation of the eddies show that the eddies are greatly affected by the structure of the background flow. Eddies formed in September or October encounter a strong westward flowing current and do not dissipate rapidly. These eddies do not travel south beyond the region of shear between the currents. They last for more than 6 months and travel westward for more than 250 of longitude. Eddies formed in March and April encounter a strong eastward flow dissipate quickly and propagate towards the equator where they disappear. These eddies last for less than four months and cover less than 150 of longitude. Eddies generated in January show properties between these two extreme cases.