The initial value problem for tropical perturbations to a baroclinic atmosphere

Abstract

AbstractLarge‐scale wave propagation ideas have proved extremely valuable in interpreting real and model data. These ideas have been developed in the context of barotropic models, and here the extension to baroclinic atmospheres is investigated. The dispersion of waves from an initial, large‐scale, tropical perturbation to a baroclinic basic flow is studied using a direct approach. A 15‐level, spectral, primitive equation model is integrated forward in time. Even in baroclinically unstable flows it is found that for a period, normally of order 12 days, the model yields what can be considered to be the direct response to the tropical perturbations. After this time baroclinic instability dominates. The basic flows are a resting atmosphere, a climatological westerly, an equatorial longitudinal modification to this, and a 3‐D climatological December‐February state. For the perturbations used here, equatorial Kelvin waves play only a very small role. The large‐scale equatorial Rossby wave shows only small dispersion and tends to move with a Doppler‐shifted phase speed which allows it to ‘accumulate’ to the east of an equatorial maximum of 10 ms−1 or more. Rossby wavetrains propagate into the middle and high latitude westerly regions. With the climatological flow, perturbations in the Indian Ocean‐west Pacific sector trigger a modal structure in the North Pacific and wavetrain across North America very much as barotropic theory would suggest and similar to observed Pacific‐North America (PNA) patterns. In such a flow the middle latitude waves tend to extend to the equator in the westerly regions of the east Pacific and Atlantic, in agreement with observed behaviour.