Atmosphere Ocean Science Colloquium

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Sea Surface Temperatures, Planetary Rotation Rate, and Precipitation in the African Sahel and the Intertropical Convergence Zone

Speaker: Spencer Hill, Caltech

Location: Warren Weaver Hall 1302

Date: Wednesday, November 9, 2016, 3:30 p.m.

Synopsis:

I discuss precipitation associated with tropical meridional overturning circulations in two contexts, in the Sahel region of Africa in comprehensive atmospheric general circulation models (AGCMs) and in the zonal mean Intertropical Convergence Zone (ITCZ) in a moist idealized AGCM. The well-documented severe Sahelian drying with uniform SST warming in the NOAA Geophysical Fluid Dynamics Laboratory AM2.1 AGCM is eliminated when the default convective parameterization is replaced with an alternate. The drying is commensurate with MSE convergence due to suppressed ascent balanced by MSE divergence due to increased dry advection from the Sahara. These qualitative energetic responses to uniform warming are shared by five other GFDL models and ten CMIP5 models Climatological values and interannual variability in observations and reanalyses suggest that drying in AM2.1 is exacerbated by an overly top-heavy ascent profile and positive feedbacks through cloud radiative properties. In an idealized aquaplanet moist GCM with perpetual solstice solar forcing, the ITCZ is located in the summer hemisphere subtropics, despite insolation and boundary layer MSE maximizing at the summer pole. Conversely, forced with Earth's normal annual cycle of insolation, the ITCZ migrates seasonally farther poleward into the summer hemisphere as the planetary rotation rate is decreased; the global precipitation maximum is at the summer pole for rotation rates 1/8 that of Earth's and slower. I discuss the extent to which prevailing theoretical paradigms -- both energetic and momentum-based -- capture the ITCZ dynamics under these exotic conditions and our attempts at improving them.