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Publication Date

2013-12-09

Availability

UM campus only

Embargo Period

2013-12-09

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2013-11-14

First Committee Member

Chidong Zhang

Second Committee Member

Benjamin Kirtman

Third Committee Member

Brian E. Mapes

Fourth Committee Member

Eli Tziperman

Abstract

A parameterization is designed to study the role of Madden-Julian Oscillation (MJO) and the modulation by sea surface temperature (SST) in El Niño—Southern Oscillation (ENSO). It is added to a coupled ocean-atmosphere model of intermediate complexity and to a coupled general circulation model (CGCM) that does not any generate internal MJO. The MJO parameters acquire values either additively, i.e. randomly from observed distributions, or multiplicatively, i.e. modulated by evolving model ENSO-SST, albeit with some stochasticity. Sensitivity tests using the intermediate model reveal the complexity of MJO-ENSO relationship: e.g. ENSO strengthens with increase in either the stochasticity of amplitude or the degree of its SST-modulation, but similar relationships are not evident for speed and zonal extent. Ensemble comparisons between simulations with and without SST-feedback suggest that SST feedback to the MJO acting in a stable regime can be responsible for the observed ENSO variance. The multiplicative case has a larger ensemble spread than the additive case, which manifests in a larger interdecadal variability of ENSO. Long-term simulations of the CGCM reveal that although the unrealistic 2-yr ENSO period is retained upon the inclusion of additive or multiplicative MJO, there is a disruption in the biennial regularity of ENSO. The timing and occurrence of El Niños become dependent on the timing of MJO activity, which manifests in unseasonal ENSO peaks and reduced interannual power. These constraints on El Niño are linked to the seasonal curtailment of eastward propagating upper ocean anomalies by the easterlies that intersperse MJO westerlies. The power is partially restored in the multiplicative case, where the enhancement of MJO westerlies during warming phase leads to stronger subsequent peaks. Re-simulations performed by switching the MJO presence or SST-dependence at a strategically chosen time demonstrate that the SST-feedback to the MJO has almost as much impact on the evolution of ENSO anomalies as the random component of the MJO.

Keywords

ENSO; MJO

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