Abstract
Abstract A mechanism is presented, based on multiscale interactions via nonlinear wind-induced surface heat exchange (WISHE), that produces eastward-propagating, intraseasonal convective anomalies in the tropical atmosphere. Simulations of convectively coupled disturbances are presented in two intermediate-complexity atmospheric models. One is a shallow water model with a simple WISHE-motivated heating term. The other model is also based on a first baroclinic mode but has an additional prognostic equation for humidity and a simple representation of moist convection based on a quasi-equilibrium approximation. In spite of many differences between the models, they robustly produce a coherent signal in westerly winds and convection that travels eastward at 4–10 m s−1. It is shown here that this slow signal is a forced response to an eastward-propagating Yanai (mixed Rossby–gravity) wave group. The response takes the form of a forced Kelvin wave that is driven nonlinearly, via WISHE, by meridional wind anomalies of the Yanai wave group and that travels considerably more slowly than the free convectively coupled Kelvin waves in these models. The Yanai waves are destabilized in the models used here by WISHE in the presence of mean easterlies, but more generally they could also be excited by stratiform instability in the absence of mean easterlies so that the mechanism described here could also operate without mean easterlies. Similarities to and differences from the Madden–Julian oscillation (MJO) and convectively coupled tropical waves are discussed.
Highlights
It has been argued that wind-induced surface heat exchange (WISHE) (Emanuel 1987; Neelin et al 1987), by which ocean evaporation enhanced by surface winds forces atmospheric convection, plays a central role in organizing tropical intraseasonal variability (e.g., Sobel et al 2008)
This paper identifies a novel mechanism in which the equatorial winds due to Yanai [mixed Rossby–gravity (MRG)] wave groups, through nonlinear WISHE, induce enhanced surface heat exchange
The signal is coherent with a group of long Yanai waves that has a group velocity similar to that of the slow forced Kelvin wave
Summary
It has been argued that wind-induced surface heat exchange (WISHE) (Emanuel 1987; Neelin et al 1987), by which ocean evaporation enhanced by surface winds forces atmospheric convection, plays a central role in organizing tropical intraseasonal variability (e.g., Sobel et al 2008). Emanuel (1987) and Neelin et al (1987) found unstable Kelvin wave–like solutions due to the coupling of convection, large-scale winds, and surface heat fluxes in linear models, and they suggested that these solutions might explain the dominant pattern of tropical intraseasonal variability: the Madden–Julian oscillation (MJO; Madden and Julian 1971, 1994). Emanuel (1987) and Neelin et al (1987) found unstable Kelvin wave–like solutions due to the coupling of convection, large-scale winds, and surface heat fluxes in linear models, and they suggested that these solutions might explain the dominant pattern of tropical intraseasonal variability: the Madden–Julian oscillation (MJO; Madden and Julian 1971, 1994).
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