Recent indications are that the frequency of extreme warming events in the ocean is increasing globally. In both 2015 and 2016, approximately one quarter of the ocean surface area experienced a marine heatwave that was either the longest or most intense ever recorded since global satellite records began in 1982. These events have devastated marine ecosystems globally but there is limited understanding of their physical drivers and the role of anthropogenic climate change. Individual marine heatwaves have been examined in terms of their definition, physical drivers and ecological impacts and inferences have been made to the role of climate change.
During the austral summer of 2015/16, sea surface temperatures (SSTs) off southeast Australia were up to 3–4 °C above climatological averages, the warmest on record for that region. At this time, temperature anomalies >1 °C were contiguous over an area nearly 21 times the size of Tasmania (1.4 × 106 km2), anomalies >2 °C over an area more than seven times the size of Tasmania (4.8 × 105 km2) and anomalies >3 °C over an area nearly half the size of Tasmania (3.2 × 104 km2). This event impacted regional biodiversity, such as the appearance of marine species normally found further north, and was a detrimental stressor on coastal fishery and aquaculture industries, including the abalone, Pacific oyster and Atlantic salmon industries. Even human interactions with the ocean were modified, where swimmers and surfers noted the unusual warmth of the waters around Tasmania; a region normally noted for its relatively cold waters for swimming.
The ocean off southeastern Australia is a global warming hotspot. The near-surface waters there are warming at nearly four times the global average rate and these increasing temperatures are seen as deep as 750 m. This warming has been linked to enhanced southward transport in the East Australian Current, driven by increased wind stress curl across the mid-latitude South Pacific. The extension of this current south of ca. 33°S consists of an unsteady train of mesoscale eddies, resulting in increased eddy mixing within the Tasman Sea. Future projections under anthropogenic climate change indicate continued strengthening of the southward transport in the East Australian Current Extension, linked with increased wind stress curl over the South Pacific, and a corresponding increase in the likelihood of extreme temperature events.
The study discusses the 2015/16 Tasman Sea marine heatwave from observations and ocean models, diagnoses its physical drivers and the role of anthropogenic climate change, and describes the ecological impacts that occurred.
The 2015/16 marine heatwave in the Tasman Sea was the longest (251 days) and most intense (+2.9 °C maximum anomaly) event on record in this region. This event was identifiable in daily remotely sensed SSTs, monthly gridded in situ-based SSTs and daily in situ near-shore sub-surface loggers.
Time series of temperature at a number of monitoring sites around Tasmania situated between 6 and 19 m depth indicated that the summer of 2015/2016 was the warmest on record at most sites since measurements began in 2004. The January averaged temperature (anomaly) at the IMOS National Reference Station at Maria Island (42.59°S,148.23°E) was 18.4 °C (+2.02 °C), which was more than 1 °C warmer than the previous record summer of 2011/12.
There is also keen interest in the why—that is, the cause of the event. This marine heatwave coincided with anomalous southward flows and enhanced eddy kinetic energy corresponding to a strengthened southward extension of the East Australian Current. A temperature budget, in which we consider horizontal advection and air–sea heat flux as possible physical drivers, indicated (anomalous) southward advection to be the primary driver of the anomalous temperatures, consistent with a stronger southward extension of the EAC.
Finally, there is interest in how climate change might influence or cause these events. An event attribution analysis using global climate models indicated that it was very likely to be that the occurrence of an event of this duration or intensity in the Tasman Sea region was ≥330 and ≥6.8 times as likely to be respectively due to the influences of anthropogenic climate change, compared with a naturally varying world. Impacts on marine ecosystems were varied and significant including an outbreak of Pacific Oyster Mortality Syndrome in Pacific oysters, mortality of blacklip abalone, poor performance of salmon aquaculture and intrusions by fish normally seen in warmer, more northerly waters. These multiple lines of evidence represent a comprehensive characterization of this extreme marine heatwave event.
The full paper can be read here: https://www.nature.com/articles/ncomms16101