Tropical–extratropical interactions related to upper-level troughs at low latitudes

Abstract

Momentum and kinetic energy fluxes associated with low-latitude transient disturbances at upper-levels play an important role in the general circulation of the atmosphere. They are related to eastward and equatorward propagating, positively tilted wave trains from the extratropics. Theoretical, modelling and observational studies show that this particular kind of tropical–extratropical interaction is most common in regions of mean upper-level westerlies at low latitudes, i.e. over the central and eastern Pacific and Atlantic Oceans during boreal winter and spring. The penetration of an upper-level trough into the Tropics is often associated with enhanced convection and the formation of an east- and poleward stretching elongated band of upper- and midlevel clouds, usually referred to as a ‘tropical plume’ (TP).

The present study provides an overview of various aspects related to the penetration of upper-level disturbances to low latitudes, including a description of the involved meteorological phenomena, climatological aspects, interannual variability, linear Rossby-wave and critical line theory, results from barotropic and higher-complexity modelling studies, the vertical structure of the disturbances as well as sources for the wave energy. In addition, the dynamical relation of the upper-troughs to convection, moisture transports and precipitation in the subtropics will be discussed. The paper concludes with a number of research perspectives for future investigations.

Introduction

The existence of important interactions between the Tropics and extratropics has been perceived for a long time (e.g. Bjerknes, 1969). Early studies have mainly investigated the influence of tropical heating anomalies on the extratropics on seasonal to interannual time-scales, for example in connection with the El Niño/Southern Oscillation (ENSO; e.g. Horel and Wallace, 1981). More recently, there is growing interest in the impact of large-scale intraseasonal tropical heating anomalies associated with the Madden–Julian Oscillation (MJO; e.g. Madden and Julian, 1994, Zhang, 2005) on the extratropical circulation (Weickmann et al., 1985, Ferranti et al., 1990, Bladé and Hartmann, 1995, Hendon et al., 2000, Jones et al., 2004). A prominent example of a tropical impact on midlatitudes on even smaller temporal and spatial scales is the transition of tropical cyclones into extratropical systems (e.g. Jones et al., 2003).

There are, however, also situations when disturbances from the extratropics affect low latitudes. This kind of interaction is most important on synoptic to submonthly time-scales and is confined to certain parts of the Tropics, in particular during the boreal cool season. Specific examples include lower-tropospheric cold-surge events over East Asia, which are connected to an intensification of convection over Indonesia (e.g. Chang et al., 1979, Boyle and Chen, 1987), and incursions of transient upper-tropospheric troughs into the Tropics. The latter phenomenon will be the focus of this paper.

It has been shown that low-latitude troughs are usually part of eastward and equatorward propagating coherent wave trains from the extratropics and that they can influence tropical convective activity (e.g. Liebmann and Hartmann, 1984, Kiladis and Weickmann, 1992b). Often elongated cloud bands form to the east of these troughs and then stretch from the Tropics into the subtropics or even midlatitudes. These bands are eye-catching signs of tropical–extratropical interactions in satellite imagery and have been termed ‘tropical plumes’ (TPs; McGuirk et al., 1988). In addition, studies of the general circulation of the atmosphere reveal that transient upper-level disturbances at low-latitudes accomplish a substantial part of the poleward transport of momentum and kinetic energy (e.g. Peixoto and Oort, 1992, their Figs. 11.7 and 13.8) making them an important element of the global and regional momentum balances (e.g. Kiladis and Feldstein, 1994).

The present paper aims at providing a broad overview of observational results and placing these in a theoretical background. Sections 2 Observations of tropical plumes, 3 Low-latitude upper-level troughs and their relation to convection and TPs contain a summary of the climatology and structural characteristics of TPs and low-latitude upper-troughs, respectively. Section 4 deals with aspects of Rossby waves and their propagation into low latitudes: (1) theory, (2) barotropic and more complex modelling, (3) vertical structure of observed waves and (4) source regions of wave energy. This section is meant to elucidate the strength and limitations of a purely linear view of the problem. In Section 5 the physical mechanisms relating low-latitude upper-disturbances with convection will be discussed, followed by an analysis of the associated moisture transports and precipitation patterns in Section 6. Given the relatively broad spectrum of views on these issues in the existing literature, a synoptic model is presented that tries to incorporate key points from various studies. The final Section 7 provides conclusions and lists open questions for future research.