A Coupled Ocean–Atmosphere Instability of Relevance to the Seasonal Cycle

Abstract

Recent observational studies have suggested that interactions between the atmosphere and the ocean play an important role in the pronounced annual cycle of the eastern equatorial Pacific and Atlantic Oceans. The key to this atmosphere–ocean interaction is a positive feedback between the surface winds and the local SST gradients in the cold tongue/ITCZ complex regions, which leads to an instability in the coupled system. By means of linear instability analyses and numerical model experiments, such an instability mechanism is explored in a simple coupled ocean-atmosphere system. The instability analysis yields a family of antisymmetric and symmetric unstable SST modes. The antisymmetric mode has the most rapid growth rate. The most unstable antisymmetric mode occurs at zero wavenumber and has zero frequency. The symmetric SST mode, although its growth rate is smaller, has a structure at annual period that appears to resemble the observed westward propagating feature in the annual cycle of near-equatorial zonal wind and SST. Unlike the ENSO type of coupled unstable modes, the modes of relevance to the seasonal cycle do not involve changes in the thermocline depth. The growth rates of these modes are linearly proportional to the mean vertical temperature gradient and inversely proportional to the depth of mean thermocline in the ocean. Because of the shallow thermocline and strong subsurface thermal gradients in the eastern Pacific and Atlantic Oceans, these coupled unstable modes strongly influence the seasonal cycles of those regions. On the basis of theoretical analyses and the observational evidence, it is suggested that the antisymmetric SST mode may be instrumental in rapidly reestablishing the cold tongues in the eastern Pacific and Atlantic Oceans during the Northern Hemisphere summer, whereas the symmetric SST mode contributes to the westward propagating feature in the annual cycle of near-equatorial zonal winds and SST.