Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Fedorov, Alexey

Abstract

This thesis examines several key elements of the tropical ocean-atmosphere system on various timescales and their interactions, including tropical cyclones (TCs), Madden-Julian Oscillation (MJO), westerly wind bursts (WWBs) and El Niño. The MJO is the dominant intraseasonal mode (periods of 20-100 days) of the tropical atmosphere, which serves as a major source of predictability for subseasonal to seasonal weather forecasts in the tropics and beyond. It also modulates TC genesis, and on certain conditions generates WWBs which contribute to El Niño development. The generation of WWBs, on the other hand, is modulated by El Niño evolution. As the most important interannual mode (periods of 2-7 years) of the climate system, the El Niño-Southern Oscillation (ENSO) has huge global impacts. It is therefore crucial to understand better the interactions of these key elements of the tropical ocean-atmosphere system. In this thesis, we first study the adjustment of the tropical atmosphere to localized sea surface warming using a Lagrangian atmospheric model (LAM) that simulates a realistic MJO. Transient idealized warm sea surface temperature (SST) anomalies of different aspect ratios and magnitudes are imposed in the equatorial Indian Ocean during MJO-neutral conditions. We observe a robust generation of an MJO event, which becomes a key element of atmospheric adjustment along with the expected Kelvin and Rossby waves. The upper-level quadrupole circulation characteristic of the MJO also becomes evident around day 14. Two anticyclonic gyres are found to be generated by the Gill-type response to convective heating, and two cyclonic gyres are forced by the excited Kelvin waves and extratropical Rossby wave trains. We further confirm the results using a higher model hierarchy, the Super-Parameterized Community Atmosphere Model (SPCAM). Additionally, using moisture and moist static energy (MSE) budget analyses, we find different mechanisms for the excitation and eastward propagation of the MJO. These findings highlight the role of the MJO in the moist atmospheric adjustment to localized tropical heating, with implications for the MJO mechanisms and prediction. We next investigate a link between the MJO and WWBs that involves TCs generated within the MJO sufficiently close to the equator. Using reanalysis and observational datasets, we find that during the El Niño onset stage, warm background SST anomalies in the western-central equatorial Pacific draw the MJO convective envelope in the southern hemisphere closer to the equator. As a result, the MJO westerly wind anomalies together with TCs embedded in the MJO induce strong WWBs (during ENSO neutral conditions the MJO usually takes a more southerly route and cannot induce strong bursts). Subsequently, during the development stage of El Niño, anomalous warming in the central-eastern equatorial Pacific, steers the MJO path, now in the northern hemisphere, toward the equator, strengthening the MJO signal over the central equatorial Pacific. Consequently, TCs modulated by the MJO move closer to the equator and farther east, facilitating WWB generation. The role of the MJO in generating WWBs is further validated by conducting LAM atmospheric experiments; we also find that under favorable conditions (e.g., warm SST anomalies), stronger MJO events tend to generate WWBs. Finally, we explore potential changes of WWBs under anthropogenic global warming, considering their critical role in the ENSO dynamics. In particular, we investigate how the enhanced eastern equatorial Pacific warming pattern, emerging in future coupled climate model projections, can affect WWB characteristics in an atmospheric general circulation model, Community Atmosphere Model (CAM6). We find that during El Niño onset (December to April), the simulated integrated WWB intensity under global warming decreases slightly by 8% (not statistically significant). During El Niño development (May to November) however, the integrated WWB intensity increases by 41%, mainly caused by a higher frequency of TC occurrence in the central tropical Pacific (within 15°N/S). No significant eastward shift of WWBs is observed during El Niño onset and development, which is caused by the weakening of trade winds in the central-eastern equatorial Pacific. We anticipate that the increase of WWB intensity would lead to a stronger ENSO and even stronger increase in WWBs in a coupled model.

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