Low-frequency intraseasonal variability in a zonally symmetric aquaplanet model

Das, Surajit and Sengupta, Debasis and Chakraborty, A and Sukhatme, Jai and Murtugudde, Raghu (2016) Low-frequency intraseasonal variability in a zonally symmetric aquaplanet model. In: METEOROLOGY AND ATMOSPHERIC PHYSICS, 128 (6). pp. 697-713.

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Abstract

We use the aquaplanet version of the community atmospheric model, with perpetual spring equinox forcing and zonally symmetric sea surface temperature (SST), to study tropical intraseasonal oscillations (ISOs). In the first two experiments, we specify zonally symmetric SST profiles that mimic observed climatological July and January SSTs as surface boundary conditions. In the January SST simulation, we find a zonal wavenumber 1 mode with dominant period of 60 days, moving east at about 6 m s(-1). This mode, which resembles the Madden-Julian oscillation (MJO), is absent in the July SST case, although convectively coupled Kelvin waves are prominent in both experiments. To further investigate the influence of tropical SST on ISO and convectively coupled equatorial waves, we conduct experiments with idealised symmetric SST profiles having different widths of warm ocean centered at the equator. In the narrowest SST experiment, the variance of moist activity is predominantly in weather-scale Kelvin waves. When the latitudinal extent of warm SST is comparable to or larger than the equatorial Rossby radius, we find a dominant low frequency (50-80 days) eastward mode that resembles the MJO, as in the January SST experiment. We also find westward propagating waves with intraseasonal (30-120 days) periods and zonal wavenumber 1-3; the structure of these signals projects onto equatorially trapped Rossby waves with meridional mode numbers 1, 3 and 5, associated with convection that is symmetric about the equator. In addition, the model generates 30-80 days westward moving signals with zonal wavenumber 4-7, particularly in the narrow SST experiment. Although these waves are seen in the wavenumber-frequency spectra in the equatorial region, they have largest amplitude in the middle and high latitudes. Thus, our study shows that wider, meridionally symmetric SST profiles support a strong MJO-like eastward propagation, and even in an aquaplanet setting, westward propagating Rossby waves comprise a large portion of tropical intra-seasonal variability.

Item Type:	Journal Article
Publication:	METEOROLOGY AND ATMOSPHERIC PHYSICS
Additional Information:	Copy right for this article belongs to the SPRINGER WIEN, SACHSENPLATZ 4-6, PO BOX 89, A-1201 WIEN, AUSTRIA
Department/Centre:	Division of Mechanical Sciences > Divecha Centre for Climate Change Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Date Deposited:	07 Dec 2016 07:10
Last Modified:	07 Dec 2016 07:10
URI:	http://eprints.iisc.ac.in/id/eprint/55527

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