Abstract
Marine heatwaves (MHWs) are extreme ocean temperature events that have wide ranging and pervasive impacts on marine ecosystems, with flow-on effects for aquaculture and fishery industries worldwide. Understanding the local and large-scale processes that contribute to their generation is important to aid in the development of forecasts on sub-seasonal to decadal time scales. However, to date, many MHW studies have focused on open-ocean events as opposed to those in coastal oceans (<20 km from the coast or <250 m depth), partially due to data scarcity in these areas. Using a synthesis of remotely sensed, in situ (surface and subsurface) and model-derived data, this work investigates the trends, variability and drivers of MHWs across the coastal and shelf seas of Aotearoa / New Zealand (AoNZ).
An investigation of long-term trends and drivers of MHWs in shallow water marine ecosystems (SWMEs) was performed for two AoNZ sites, an open coast and semi-enclosed harbour near the northern and southern limits of AoNZ respectively, utilising the two longest (>50 year) daily in situ ocean temperature records in the Southern Hemisphere. Positive trends in MHW days and annual MHW duration were revealed for the southern site, while the northern site has experienced a decrease in maximum MHW intensity. When investigating the co-occurrence of events between the nearshore coastal and offshore waters, it was found that the open coast site was more likely to have co-occurring MHWs than the semi-enclosed harbour. The association between MHW occurrence and anomalous atmospheric properties was evaluated using data from an atmospheric reanalysis. This suggested that anomalous low wind speed and reduced latent heat loss associated with blocking high-pressure systems are important factors contributing to MHW development in SWMEs.
Large interannual variability in MHW occurrence around AoNZ was noted at the two coastal sites. To better understand the drivers of this variability, 40 years of remotely sensed sea surface temperature data was used to detect summer MHWs occurring across the New Zealand Territorial Sea (NZTS, <12 nautical miles from land) with a focus on key aquaculture areas. An EOF analysis revealed that inter-annual variability in summer MHWs was dominated by a single mode of variability, accounting for 50% of variability in MHW metrics nationally and 40-80% at a regional scale. This mode was characterised by a widespread increase in MHW properties across the NZTS. A regression analysis further showed that this mode is associated with anomalous high pressure to the east of AoNZ, a shoaling of the mixed layer, and reduced wind stress. When investigating links to large-scale climate modes known to impact ocean temperature around AoNZ, the Southern Annular Mode is found to be significantly correlated with MHW development in the NZTS with a lead time of 2-3 months prior to summer, highlighting a potentially useful source of predictability for summer MHWs around AoNZ.
A case study investigation of a long-lasting MHW event that occurred in a large (>170 km) coastal embayment during 2021/22 was undertaken to understand key drivers at a coastal scale. A satellite-analysed SST product shows the event persisted at the surface across the embayment for more than six months. An analysis of the depth-structure of this event was completed using data from a novel, fishing-vessel based observation network that provides high spatio-temporal coverage of the shelf sea surrounding AoNZ over 2021-present. These observations captured the development of a surface-intensified MHW during early summer that was restricted to the mixed layer, which subsequently deepened and extended throughout the water column to reach 150 m in early autumn and persisted for most the year. A volume-averaged heat budget was constructed for the event using output from a regional ocean model to study the physical mechanisms controlling its evolution. The evolution of the event is shown to be related to compounding impacts of (i) anomalous air-sea heat fluxes over the summer period and (ii) downwelling-favourable winds during late-summer and early autumn 2022.
This thesis goes some way to increasing understanding of MHWs around AoNZ, with many of the results relevant to understanding of MHWs occurring in shelf seas globally. Trends in MHW properties are broadly consistent with SST warming trends around AoNZ, with the interannual variability in MHW properties associated with persistent anomalous high pressure systems. Occurrence of MHWs was linked to the Southern Annular Mode, with this mode of climate variability proposed as a likely predictor of MHW risk. While some consideration has been made of a notable subsurface MHW in the AoNZ shelf sea, further work is required to better understand the nature of subsurface MHWs both around AoNZ and in coastal and shelf seas globally. Additionally, the findings from this thesis should be considered when creating MHW forecasting tools and completing feasibility and longevity assessment of aquaculture and fishing endeavours to help ensure future-proofing of these industries. It is hoped that the work presented here is built upon in future studies of MHWs globally.