Meridional movement of Pacific winds and their role in ENSO event onset and termination

Abstract

During the mature phase of El Nino-Southern Oscillation (ENSO) events, near the end of the calendar year, there is a southward shift of the zonal wind anomalies, which are centred around the equator prior to the event peak. This thesis investigates the role of this meridional wind movement in the termination of ENSO events by using simple and complex climate models. Previous studies have shown that ENSO's anomalous wind stress, including this southward shift (SWS), can be reconstructed with the two leading Empirical Orthogonal Functions (EOFs) of wind stresses over the tropical Pacific. Here a hybrid coupled model is developed, featuring a statistical atmosphere based on these first two EOFs coupled to a shallow water model ocean. The addition of the SWS enhances the termination of El Nino events, making the events shorter, while it does not appear to play an important role on the duration of La Nina events. Thus, the SWS is partly responsible for seasonal synchronization of ENSO events. This thesis also examines the representation of the SWS in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Although the models that capture the SWS also simulate many more strong El Nino and La Nina events peaking at the correct time of the year, the overall seasonal synchronization is still underestimated. This is attributed to underestimated changes in warm water volume during moderate El Nino events, so that these events display relatively poor seasonal synchronization. Several significant differences between the models with and without the SWS are identified including biases in the magnitude and spatial distribution of precipitation and sea surface temperature anomalies during ENSO. Aiming to understand the physical mechanisms leading up to the extreme 2015-16 El Nino in relation to the two previous extreme events (1997-98 and 1982-83), we found a persistent location of the westerly wind stress anomalies north of the equator during the two years prior to the event peak. As a result of this meridional asymmetry, the anomalous southward ocean flow during this period, in cooperation with warmer subsurface water over the central equatorial Pacific, led to the large event magnitude.