Dynamics of ocean circulation and air-sea interaction in the Southeast Indian Ocean and their impact on Ningaloo Niño
Extreme ocean warmings associated with the Ningaloo Niño have had significant impacts on regional climate and the health of the marine ecosystem in the Southeast Indian Ocean. The generation and development of the Ningaloo Niño are caused by a combination of atmospheric forcing and oceanic processes, including air-sea heat fluxes and the heat transport associated with the Leeuwin Current (LC). In addition, the large-scale climate variability in the tropics can also affect the Ningaloo Niño via atmosphere and ocean teleconnections. In this dissertation, the variability of the Southeast Indian Ocean, including the air-sea flux and LC variability, is investigated systematically using observations, reanalysis, and numerical model experiments to advance our understanding of the driving mechanism of the Ningaloo Niño. Firstly, the air-sea heat flux variability during the Ningaloo Niño is analyzed using six major air-sea heat flux datasets. One of the major sources of uncertainties in the latent heat flux climatology is the bulk flux algorithm. Over the life cycle of Ningaloo Niño, the anomalous latent heat flux is dominant in the net surface heat flux variations, and the uncertainties in latent heat flux anomaly largely depend on the phase of the Ningaloo Niño. During the developing and peak phase, the contribution of air-sea heat flux to the surface warming has large uncertainties, which are primarily caused by the differences in the sea surface temperature. However, during the decay phase, large negative latent heat flux anomalies (cooling the ocean) are found in all datasets, indicating the important role of latent heat flux in damping anomalous warming during the recovery phase. Secondly, the sensitivity of model resolution on the climatology and variability of the LC is evaluated in an eddy-permitting and eddy-resolving Ocean General Circulation Model (OGCM). The magnitude and structure of the mean LC are more realistic in the high-resolution (eddy-resolving, 1/12°) OGCM experiment. During the 2010-2011 Ningaloo Niño, the high-resolution experiment simulates a stronger LC, which leads to a warmer ocean temperature off the west coast of Australia. Lastly, the effect of the continental shelf and slope on the LC and Ningaloo Niño are investigated using a series of high-resolution Indo-Pacific OGCM experiments. The “control” experiment uses a realistic bottom topography along the west coast of Australia, whereas the sensitivity (“no-shelf”) experiment uses a modified topography with no continental shelf and slope near the coast. The LC in the no-shelf experiment is located closer to the coast, and the strength is decreased by about 28% compared to the control experiment. During the 2010-2011 Ningaloo Niño, stronger enhancements of the LC are detected in the control experiment, which lead to a 26% increase in the upper 50 m ocean temperature. The analysis of ocean dynamical processes indicates that the shelf-slope topography can effectively trap the positive sea level anomaly at the coast and suppress the Rossby wave radiation from the coast, thereby maintaining a stronger LC.