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Abstract
The impact of El Niño and Southern Oscillation (ENSO) on different types of atmospheric transient activity is studied, based on data from an ensemble general circulation model (GCM) experiment and the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) Reanalysis Project. In particular, the Madden-Julian Oscillation (MJO), North Pacific low-frequency (subseasonal/intraseasonal) circulation anomalies and summertime tropical synoptic-scale disturbances are considered. Extensive comparisons between reanalyses results and those based on ensemble GCM integrations are carried out.
The MJO activity from the GCM and reanalyses agree in several aspects, such as their spectral characteristics, regions with large amplitudes and their phase speed. MJO convection within the central Pacific is enhanced during warm ENSO events, suppressed during cold events. Positive values of the instability index, based on the vertical gradient of the moist static energy, are found to precede MJO convection at the equator. During warm events, there are stronger signals of this index over the central Pacific, which could be important for the further eastward penetration of convective anomalies in the region.
The life cycle of low-frequency circulation anomalies over the North Pacific is studied. During cold ENSO events, eddies in general are stronger, with more prominent dispersion downstream over the west coast of U.S. GCM results show that stronger incoming wave activity from the subtropics is accompanied by stronger intraseasonal convection in the tropical western Pacific, suggesting that the MJO plays a role on how ENSO affects extratropical low-frequency activity.
The GCM is able to reproduce qualitatively the propagation and growth/decay behavior of tropical summertime synoptic-scale disturbances. During warm ENSO events, storm tracks over the western Pacific are displaced eastward. Stronger barotropic conversion from the mean to eddy kinetic energy is related to increased low-level cyclonic wind shear in the region. Finally, equatorward energy dispersion from eddies is also located more to the east in the warm phase of ENSO. The relationship between the structure of the eddies and their related dispersion pattern is examined.
