The Response of the Tropical Atmosphere to an Idealized Equatorial Continent. Results from TRACMIP

Are there aspects of a monsoon that distinguish it from the ITCZ in fundamental ways? Is the defining characteristic of a monsoon its association with a summertime continental heat low and reversing regional circulation; the substantial contrast between a rainy and a dry season; or the rapidity of the transition between the dry and rainy seasons?

The Tropical Rain belts with and Annual cycle and Continent Model Inter-comparison Project (TRACMIP) approached this debate by assuming that the presence of a tropical continent would generate a monsoon and that the comparison between the continental and oceanic climates would answer the question of what are the foundational characteristics of a monsoon. This approach extends to a full-physics, multi-model framework the seminal study of Chou, Neelin, and Su (QJRS, 2001): the control set up is an aquaplanet with a slab ocean of uniform mixed layer depth and a simple representation of a constant ocean heat transport, while the monsoon set up includes a continent that is geometrically simple and with only a few, minimal parameterization of land process: reduced evaporation, increased albedo, reduced heat capacity, and a shutoff of ocean heat transport.

The annual mean response of the tropical atmosphere to the introduction of the continent is most pronounced downstream from the continent itself, where a broad equatorial region of cold SST and negative precipitation anomalies extends for one and half time the width of the continent. The oceanic precipitation anomalies drive most of the regional Gill-like response in the tropics and the Rossby wave train into the extra tropics. We attribute the SST cooling to a feedback between specific humidity, longwave radiation, and temperature initiated by advection of dry continental air over the ocean; therefore, we conclude that, for the annual mean response, the most important property of the idealized continent is the reduction in evaporation, which is linked to the representation of soil and vegetation properties. Over the continent, the mean temperature response is controlled by the imposed zeroing out of the (implied) oceanic heat transport, and rainfall anomalies follow, with wetter condition over warm surface and vice-versa.

This study reveals what aspects of the land idealization are key to the mean climate and thus suggests which physical properties of land ought to be considered in an ``elegant hierarchy'' of land models.