On large‐scale circulations in convecting atmospheres

TLDR

Conventional thinking treats convection as a heat source for large‑scale circulations while the latter supply water vapor, but the true problem is the spatial and temporal distribution of subcloud‑layer entropy. The study challenges this view and proposes an alternative paradigm in which convection is nearly in statistical equilibrium with its environment. Under this paradigm, convection controls the vertical temperature profile, tying temperature directly to subcloud‑layer entropy rather than to heating. We find that subcloud‑layer entropy is governed by sea surface temperature, surface wind speed, and large‑scale vertical velocity, and that recognizing this control yields a simple, physically consistent view of large‑scale flows, while convection reduces effective static stability by about an order of magnitude and dampens all circulations.

Abstract

Abstract The dominant thinking about the interaction between large‐scale atmospheric circulations and moist convection holds that convection acts as a heat source for the large‐scale circulations, while the latter supply water vapour to the convection. We show that this idea has led to fundamental misconceptions about this interaction, and offer an alternative paradigm, based on the idea that convection is nearly in statistical equilibrium with its environment. According to the alternative paradigm, the vertical temperature profile itself, rather than the heating, is controlled by the convection, which ties the temperature directly to the subcloud‐layer entropy. The understanding of large‐scale circulations in convecting atmospheres can, therefore, be regarded as a problem of understanding the distribution in space and time of the subcloud‐layer entropy. We show that the subcloud‐layer entropy is controlled by the sea surface temperature, the surface wind speed, and the large‐scale vertical velocity in the convecting layer, and demonstrate how the recognition of this control leads to a simple, physically consistent view of large‐scale flows, ranging from the Hadley and Walker circulations to the 30–50‐day oscillation. In particular, we argue that the direct effect of convection on large‐scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations, and to damp all of them.