Modulation of intraseasonal (25-70 day) processes by the superimposed ENSO cycle across the Pacific basin

Jon Matthew Schrage, Purdue University

Abstract

In recent years, many observational studies of the intraseasonal Madden-Julian Oscillation (MJO) have been conducted. Similarly, a vast number of papers describe the interannual changes to the atmosphere and oceans as part of the El Nino-Southern Oscillation (ENSO) phenomenon. However, very little work has been done to document the effects ENSO has on the MJO and its concomitant circulation anomalies. With the advent of the fifteen-year ECMWF Reanalysis (ERA) data archive, it is now possible to stratify southern summer seasons with respect to the phase of ENSO, and still have large-enough data samples to meaningfully examine the intraseasonal processes. In this thesis, cursory reviews of ENSO and the MJO are given, followed by a description of the effects of interannual variability on intraseasonal convection along the equator. During the El Nino years, the MJO is found to propagate farther eastward across the tropical Pacific, whereas it tends to become quasi-stationary over the western Pacific Warm Pool region in other years. Changes in the MJO's upper-level outflow and the associated low-level westerly wind bursts during El Nino are examined. Changes in downstream extratropical teleconnections are also documented. Tropical MJO events generally established a region of enhanced upper-level westerlies over the North Pacific, but the effect occurs later and farther to the south in El Nino years. The South Pacific Convergence Zone (SPCZ) is found to propagate eastward with the tropical MJO convection. This effect is not seen in non-El Nino situations, despite the fact that many studies have shown that the passage of MJO convection almost always excites convection in the SPCZ. One possible interpretation of this result is that the MJO/SPCZ linkage may be stronger in El Nino years.

Degree

Ph.D.

Advisors

Vincent, Purdue University.

Subject Area

Atmosphere

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