What Affects the Location of Tropical Precipitation? An Idealized Modeling Perspective

Abstract:

Greater precipitation falls in the northern hemisphere (NH) tropics than in the southern hemisphere (SH) tropics over the course of the year. Here, we examine how Andes topography, ocean heat fluxes, and the presence of land affect the location of tropical precipitation in an idealized aquaplanet version of GFDL's AM2.1 GCM. We find that all three of these features influence which hemisphere has greater precipitation, though to different extents. The addition of Andes topography leads to a small regional northward shift of rainfall, but realistic ocean heat transport leads to a much larger and more global northward shift of precipitation. In simulations with an idealized NH subtropical continent, the location of tropical precipitation depends on the land albedo and continent width. To demonstrate this, we place a single Sahara-like rectangular continent with a width of 60-degrees and an albedo of 0.3 into the NH. This simulation has greater SH tropical precipitation, despite the NH continent's hot surface: the high land albedo increases the NH deficit of energy, creating an anomalous Hadley circulation that imports energy to the NH and moisture to the SH. As a result, the hemisphere with greater temperature is not the hemisphere with greater precipitation. However, simulations with wider continents and the same fixed albedo shift precipitation into the NH as the continent widens. While northward ocean heat transport is likely the reason for the observed annual NH peak in tropical precipitation, these experiments show that continents and topography also play an important role in modifying its location.