Tropical climate regimes and global change in a simple setting

Abstract

In 1941 Isaac Asimov published the short story "Nightfall" about Lagash, a planet surrounded by six suns. The suns (almost) never set on Lagash, bathing it in more or less constant sunshine. Fifty-nine years later, Ben Kirtman and Ed Schneider published a General Circulation Model (GCM) simulation of such a planet ("A spontaneously generated tropical stmospheric general circulation", J. Atmos. Sci., 2000). They assumed a 50 meter slab ocean everywhere and globally uniform insolation. They did this to discover some truths about the role of planetary rotation, and rotation alone, in organizing the general circulation. Not surprisingly they found that deep convection happens preferentially in the Tropics.

We repeat this work with a different GCM, the NCAR Community Climate Model (CCM3.10), and extend it significantly by exploring the model's behavior under a range of the insolation parameter. The main finding is that several Tropical climate regimes exist in this model, and that this influences the global climate sensitivity of this planet. Each regime involves a different spatial organization of precipitation in the Tropics: a double ITCZ, a single ITCZ (more or less) on the Equator, a single, variable off-Equatorial ITCZ, and a stable, strongly asymmetric ITCZ. We emphasize that all these regimes were obtained by varying only the insolation in a single GCM with a single, standard set of parameterizations.

I'll talk about some mechanisms for the maintenance of these ITCZ structures, the role of coupling to the ocean mixed-layer, and the relevance of these results to theories of ITCZ location. I'll also present an analysis of the global climate sensitivity of this hypothetical planet as insolation is increased and the planet warms and discuss the implications for more realistic climate change simulations.