Variability and Long‐Term Trends in the Shelf Circulation Off Eastern Tasmania

Abstract

This study investigates trends and interannual variability of the marine climate across the continental shelf off eastern Tasmania for 1993-2016. This region is a hotspot for global warming and biodiversity. Eastern Tasmania lies at the boundary between two ocean currents (the East Australian Current Extension, or EAC Extension, and the Zeehan Current, ZC) leading to the local marine climate exhibiting trends and variability from both boundary currents. A numerical ocean model is used to provide high-resolution (∼2 km) estimates of the temperature, salinity, and circulation for the region. Results indicate significant positive trends in temperature, salinity, and southward flow over the shelf, consistent with an increasing EAC Extension. These trends are particularly strong in autumn, indicating a lengthening of the warm season. The interannual variability in the EAC Extension and ZC was quantified by a simple index, based on a modal analysis of surface circulation, indicating the relative dominance of each current. Strong EAC years were related to significantly more summertime marine heat wave days. Large-scale remote drivers of variability were considered, and we found weak but significant links with El Niño-Southern Oscillation and Tasman Sea Blocking. El Niño-Southern Oscillation was found to modulate the EAC Extension in summer with a El Niño leading to enhanced southward flow and warming over the shelf. Tasman Sea Blocking was found to drive enhanced southward surface flow, particularly in winter. Nonetheless, large-scale forcing modes explain less than 25% of the total variability in the EAC-ZC system indicating that most of the variability is internally generated. Plain Language SummaryThe near-surface waters off eastern Tasmania represent both a global warming and a biodiversity hotspot, and marine ecosystems there are under significant stress. The sensitivity of local ecosystems to both warming and transient marine heat wave events has been observed but spatiotemporal patterns of historical temperature and salinity variability are less well known. A numerical ocean model is used here to provide high-resolution estimates of the seawater temperatures, salinity, and circulation over the eastern Tasmanian continental shelf. We perform a systematic analysis of multidecadal-scale trends and interannual variability off eastern Tasmania over the 24-year period from 1993 to 2016. We find a significant increase in East Australian Current (EAC) Extension penetration into the region with an associated increase in temperature and salinity throughout the water column, particularly in summer and autumn. The variability of the circulation on the shelf is found to be dominated by the interplay of the EAC Extension and Zeehan Current, and an annual index is developed indicating the relative strength of these two currents over time. In both summer and winter we find that an anomalously strong EAC Extension (Zeehan Current) leads to an increased (decreased) probability of marine heat wave days.

A global assessment of marine heatwaves and their drivers

“…Nonetheless, most of these relationships are weak suggesting that a substantial fraction of MHWs in WBC systems may be internally generated. For example, off southeast Australia most of the interannual variance of the western boundary current system appears to be unrelated to large-scale climate modes 57 and instead may be due to the internal variability arising from shorter time scale instabilities generated from local forcing, interacting, and cascading across scales 58 .…”

Multi-decadal decline in cover of giant kelp Macrocystis pyrifera at the southern limit of its Australian range

“…The most dominant oceanic currents to impact Tasmania are Australia's east and west continental boundary currents, the East Australian Current (EAC) and the Zeehan Current (ZC; the southern-most extension of the Leeuwin Current). Both currents are characterised by warmer, high-salinity but nutrient-poor water that flows southwards along the east (EAC) and west (ZC) coasts (Ridgway & Condie 2004, Oliver & Holbrook 2018. In Tasmania, the EAC is an erratic eddy-driven flow that is strongest in summer, while the ZC demonstrates a more consistent flow that peaks in winter (Oliver & Holbrook 2018).…”

“…Both currents are characterised by warmer, high-salinity but nutrient-poor water that flows southwards along the east (EAC) and west (ZC) coasts (Ridgway & Condie 2004, Oliver & Holbrook 2018. In Tasmania, the EAC is an erratic eddy-driven flow that is strongest in summer, while the ZC demonstrates a more consistent flow that peaks in winter (Oliver & Holbrook 2018). In transitional stages where neither current dominates, the influence of incursions of the Antarctic Circumpolar Current (ACC) are recognised by its cold and relatively nutrient-rich signature (Bucha nan et al 2014).…”

“…Seasonal cycles in temperature and nutrients are superimposed onto inter-annual cycles corresponding to the ENSO. ENSO describes changes to the circulation of the South Pacific Gyre, which cycles through El Niño and La Niña events every 8−12 yr. La Niña events result in a strengthening of the EAC and ZC and subsequent warming of coastal waters off Tasmania, while El Niño events are characterised by weakened currents and cooler waters (Feng et al 2003, Oliver & Holbrook 2018.…”

Observational Insight Into the Subsurface Anomalies of Marine Heatwaves

“…Being a global warming hotspot, the western Tasman Sea (Figure 1) is a region where these effects are acutely experienced. This is due to climate change-induced shifts in the westerly winds which affect the East Australian Current (EAC) flow and strengthen the EAC extension through the Tasman Sea, generating one of the largest warming rates in the southern hemisphere (Cai et al, 2005;Wu et al, 2012;Sloyan and O'Kane, 2015;Oliver and Holbrook, 2018). The EAC is the western boundary current within the South Pacific subtropical gyre.…”