130 Midwinter minimum of eddy activity in an idealized GCM with a seasonal cycle

Thursday, 18 June 2015
Meridian Foyer/Summit (The Commons Hotel)
Hilla Afargan, Weizmann Institute of Science, Rehovot, Israel; and E. Galanti and Y. Kaspi

Over the Pacific, transient eddy activity exhibits a distinct minimum during midwinter, relative to fall and spring, despite the relatively strong jet and high baroclinicity during this season. Motivated by this phenomenon, we examine the relation between the strength and location of the subtropical jet and the intensity of the storm track using an aquaplanet idealized general circulation model. We analyze both perpetual and seasonally-varying conditions, implemented by seasonally varying the radiative parameters of the model. The model results are compared to NCEP reanalysis data between the years 1981-2010. We show that the midwinter minimum can be simulated in a zonally-symmetric configuration, without topography or land-sea contrasts. We find that this minimum is sensitive to the ratio of the planetary orbital period and the eddy time scales, and is more pronounced as we increase the seasonal time scale. We focus on the relationship between the strength of the subtropical jet (and the Hadley circulation), and eddy energetics in the midlatitudes. Our analysis suggests that during winters of stronger subtropical jet the decrease in eddy amplitude would be greater. Furthermore, while stronger subtropical jet could be associated with stronger meridional temperature gradients and more eddy activity in midlatitudes, perpetual simulations show that increasing the strength of the subtropical jet does not necessarily result in enhanced storm-track eddy activity in midlatitudes, and may depend on the latitude of the jet, as well as on its strength.
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