The impact of ENSO on behavior of types of transient
activities
Francis Tam
GFDL
Abstract |
The impact of ENSO on intraseasonal and tropical synoptic-scale activities is studied, using both NCEP-NCAR reanalyses and data based on a GCM experiment. The latter consists of an ensemble of integrations, with the SST over deep tropical eastern Pacific varying interannually, but assigned to its climatological values in regions located otherwise. During El Nino years, enhancement of intraseasonal activities over the Pacific, east of their climatologically active regions, is found for both GCM and reanalyses datasets. Depending on the phase of ENSO, circulation anomalies associated with the MJO exhibit different degree of eastward propagation. Intraseasonal convective signals are able to penetrate more eastward during warm events; during cold events, convection halts at about 150E and fails to continue its propagation. Simple diagnostics show that such a change could be related to the low-level mean moisture distribution during different ENSO phases. How ENSO affects extratropical intraseasonal variability is also studied. A recurrent pattern of low-frequency circulation is to found to be related with both tropical intraseasonal activities as well as westward propagating perturbations reported by Branstator (1987) and Kushnir (1987). The MJO-related part takes the form of an extratropical wavetrain, and this pattern is sensitive to the ENSO cycle, which can be accounted by different MJO convection during ENSO. While the extratropical basic state is an important factor controlling variability of low-frequency activities, results here show that the latter could also be influenced by changes of tropical forcing. Finally, the impact of ENSO on tropical synoptic-scale activities during boreal summer is considered. Simple methods are used to obtain information such as wavelengths, phase and group velocities, concerning these wave-like disturbances. In the western Pacific, their storm tracks are displaced eastward during the El Nino years, accompanying changes of the location of the monsoon trough. Interestingly, TD-type disturbances located in off-equator regions can excite waves that look like equatorial modes, by means of southeastward dispersion. Its significance on tropical-wave dynamics will be discussed. |
1:30 PM/ DSRC 1D 403
(Note: special day and time)