Study of large scale unstable air sea interactions in the tropics using a coupled ocean atmosphere model

Abstract

Coupling between the ocean and the atmosphere plays an important role in determining the nature of short-term climate fluctuations in the tropics, such as the El Niño–Southern Oscillation (ENSO). Although the tropical ocean and the tropical atmosphere may be independently stable, coupling them can generate new unstable coupled modes. Some studies have shown that the coupled instability plays a crucial role in explaining the dominant periodicity of ENSO variability as well as the predictability of ENSO events.

The nature of the unstable coupled modes crucially depends on the physical processes considered in the model. The thermodynamical processes that determine the evolution of sea surface temperature (SST) anomalies in the ocean and the moist processes determining atmospheric heating are important. The inclusion of moist processes in the parameterization of atmospheric heating reduces the effective static stability and hence the phase speed of atmospheric waves. This facilitates the possibility of strong coupling between certain atmospheric and oceanic modes.

A detailed investigation into the importance of moist processes on coupled instability has not been conducted so far. Also, the dissipation rates used in simple atmospheric models vary by an order of magnitude. Hence, a detailed study of their importance is necessary. This study focuses attention on the dependence of the coupled unstable modes on various moist feedback mechanisms, namely, evaporation–wind feedback (EWF), convergence feedback (CF), and atmospheric dissipation.

The simple coupled model used is similar to the general thermodynamic model (Model III) used by Hirst (JAS, 43, 606–630, 1986). A linear first baroclinic mode atmosphere is coupled to a linear reduced-gravity ocean. The coupling between the ocean and the atmosphere occurs through: (i) atmospheric heating parameterized in terms of moist processes that depend on SST anomalies, and (ii) surface stresses parameterized in terms of atmospheric winds.

A wave-form solution is assumed in the zonal direction, and in the meridional direction the variables are expressed as a sum of parabolic cylinder functions. The resultant system of algebraic equations is solved as an eigenvalue problem.

Apart from the details of the model and method of solution, the dependence of the coupled instability of the tropical ocean–atmosphere system on variations of some crucial mean conditions is investigated in Chapter 2. It is shown that the conditions of the central Pacific are always favourable for instability growth, while those of the western Pacific cannot sustain instability growth. The eastern basin can support instability growth only if the thermocline is as deep as in the central Pacific (Selvarajan et al., 1991, Atmospheric Research, in press).

In the next two chapters, the influence of moist processes on the stability of the coupled system is investigated. It is shown that in the presence of the CF mechanism (or wave-CISK), the most dominant eastward-propagating interannual mode (UH mode) could become stationary or westward propagating. It also introduces several new unstable intraseasonal modes with a broad-band spectrum.

The other moist process of importance in the tropics is the EWF mechanism. Inclusion of this process increases the growth rate of the unstable modes. In the presence of these two mechanisms, a high-frequency mode is destabilized that appears to have resulted from coupling between an atmospheric Kelvin wave and an n = 1 oceanic inertia–gravity mode. These results may explain the behaviour of some complex coupled models and, to some extent, may also provide a mechanism for the observed aperiodicity of ENSO events (Goswami and Selvarajan, 1991, Geophysical Research Letters, 18, No. 6, 991–994).

In the next chapter, it is shown that for higher strengths of EWF, with reduced atmospheric dissipation, the frequencies of the interannual and intraseasonal modes become close to each other. Thus, it may be difficult to isolate a single interannual mode if there were not enough dissipation in the atmosphere.

The limiting cases of ocean thermodynamics, namely the upwelling limit and the advective limit, are also studied for a better understanding of the physics of coupled instability.