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Abstract
When two or more extreme events co-occur, we call them compound events. There is growing interest in such events because many types of extremes are becoming more frequent - giving rise to more compound events - and because compound events may have more impact than the sum of parts. The overall aim of this thesis is to provide a statistical characterisation and physical understanding of the mechanisms driving compound terrestrial and marine heat extremes.
First, the relationship between adjacent coastal marine and THWs around Australia is quantified using observation and reanalysis data. A significant increase in the number of THW days is found in the presence of adjacent co-occurring marine heatwaves along the coastal belt of Australia. Moreover, synoptic conditions driving THWs, at three locations around Australia, are conducive to warming the ocean, which would increase the likelihood of a marine heatwave. However, the prior ocean state must also be conducive to reach marine heatwave conditions. These findings suggest that co-occurring terrestrial and marine heatwaves co-occur more frequently than chance would dictate, and that large scale synoptics may be favourable for both coastal terrestrial and marine heatwaves.
Following this, the focus was shifted to a specific region to isolate the influence of a single marine heatwave event on terrestrial warming. The ocean off south-eastern Australia has recently experienced a string of major marine heatwaves including the 2015/16 Tasman Sea marine heatwave. This was an unprecedented event that lasted for over eight months. It is possible that marine heatwaves can enhance temperatures of adjacent coastal areas through mechanisms such as advection and local radiative changes. This hypothesis was tested by carrying out a large ensemble of simulations forced by the 2015/16 Tasman Sea sea surface temperature anomalies using a regional atmosphere model, and examining how this influenced land temperatures and local circulation. It was found that sea surface temperature anomalies drive seasonally dependent significant warming (up to 1°C), primarily in coastal southeast Australia, Tasmania, and New Zealand, from September to February. Most of the coastal warming is consistent with the advection of anomalous heat from the marine heatwave regions as opposed to local radiative effects or adiabatic heating.
Terrestrial heat extremes are most dangerous to human health when combined with high relative humidity levels. Extreme heat stress from high heat and humidity, is increasingly becoming an issue in South Asia, the Middle East, and coastal southwest North America. These three regions are near high sea surface temperatures. While the initial components of the thesis focused on extreme temperature events, this final study investigated the combined effect of extreme heat during marine heatwaves. By using observation and reanalyses data for regions surrounding Mumbai, Karachi, Kuwait, Doha, Miami and New Orleans, a higher number of extreme heat stress days was found during marine heatwaves, compared to normal ocean conditions. Typically, onshore winds advect hot and humid air from marine heatwave regions to the adjacent coastal cities which can exacerbate the heat stress at these locations. It was also found that temperature contributes most to the heat stress metric over land while relative humidity tends to play a larger role over the ocean. These results show that regions which are already susceptible to high heat stress, are likely to experience more extreme heat stress events during a marine heatwave.
This thesis has helped improve our understanding of compound extreme heat events including co-occurring marine/ THWs and marine heatwaves with extreme heat stress conditions. Compound extreme events are much less common than extremes in single variables and so have been harder to study. With anthropogenic global warming, these events will become more frequent making the study of compound events more relevant.
