Impacts of extreme events on the ocean thermal structure over the North West Shelf of Australia: Part 1. The hydrodynamic drivers of the 2012/2013 marine heatwave; and Part 2. Effects of tropical cyclones on the ocean thermal balance.

Abstract

The North West Shelf (NWS) of Australia is an ecologically sensitive region susceptible to extreme ocean and atmospheric events driven by large-scale and climate variability. This thesis investigates the drivers of the 2012/2013 strong marine heatwave (MHW) and how tropical cyclones (TCs) affect the temperature and energy of the ocean in the NWS region. In part 1, a high-resolution regional numerical model based on the Regional Ocean Modelling System (ROMS) is implemented to simulate the variability of the upper ocean temperature and circulation in order to understand the mechanisms that drive the evolution of the 2012/2013 MHW. An upper ocean heat budget is used to quantify the roles of air-sea heat flux and ocean circulation in the development of extreme temperature anomalies over the continental shelf. Results indicate that during December 2012 an increase of net air-sea heat flux combined with positive horizontal advective heat flux anomaly led to development of the MHW. Despite a decrease in net air-sea flux warming, during the peak in January-February 2013 high-temperature anomalies were maintained by a combination of positive anomalies of heat advection and vertical mixing. The delayed onset of the 2012–2013 Australian monsoon resulted in alongshore wind anomalies on the NWS which in turn caused the advection and mixing anomalies. Part 2 focuses on characterizing the changes produced by TCs over the NWS of Australia on the ocean structure and ocean thermal energy budget using a 20-year composite of cyclone data (1996-2016) created from Bluelink ReANalysis data (BRAN). The passage of a TC can have strong effects over a large area as the strong winds cause vertical mixing and upwelling through the water column near the TC core as well as downwelling in the periphery of the TC circulation. Cold temperature anomalies develop at the ocean surface beneath the TC core in response to strong mixing and deep upwelling driven by surface divergence and Ekman pumping. In the TC outer circulation surface cold anomalies also develop. However, in the subsurface warm temperature anomalies appear below the mixed layer due to wind-driven mixing across the mixed layer. In the month following the passage of the TC, the surface cold temperature anomalies recover relatively quickly due to air-sea fluxes, while the subsurface warm anomalies take longer to recover, mostly due to a reduction in mixing once the TC has left the region. Different TC and ocean characteristics such as location with respect to the shelf edge, period of the year, intensity and translation speed, and the presence of a barrier layer are examined to assess their effects on thermal changes occurring in the ocean. Stronger cold anomalies develop at the surface over the shelf locations, over locations with a shallow or non-existent barrier layer, for strong TCs (category 3 or above on the Australian intensity category scale), and for slow-moving (< 4 m/s) TCs. The period of the year influences the recovery of the surface anomalies, which happens faster for TCs between December and February than for TCs that occur during or after March. The warm anomalies in the subsurface are stronger for more intense TCs and over locations with a shallow barrier layer as the vertical mixing processes are more efficient. Ocean heat content (OHC) integrated heat through the water column is examined as a proxy for the energy exchange between the atmosphere and ocean during the TC passage. The processes that produce changes in OHC during and after the passage of the TC are separated into regions near the TC core, in the TC outer structure, and at different depths in the ocean during the passage and the subsequent 30-day recovery period. The OHC in the top 500 m of the region within 1000 km from the TC centre recovers more than 30 days after the passage of the TC. During the passage of the TC the majority of the OHC losses are located beneath the TC core and in the surface layer, while the OHC increases in the subsurface layer in the region between 500 and 1000 km from the TC centre. When the temperature anomalies start to recover, the OHC increases at a stronger rate in the surface layer due to the air-sea heat fluxes and in the TC core region where the strong upwelling relaxes to downwelling. Strong TCs produce an overall increase in the OHC within a month after the TC passage, while weak TCs of categories 1 and 2 cause an overall decrease in the OHC.