Abstract
Marine heatwaves (MHWs) are extreme ocean warming events that can have devastating impacts, from biological mortalities to irreversible redistributions within the ocean ecosystem. MHWs are an added concern because they are expected to increase in frequency and duration. To date, our understanding of these extreme ocean temperature events is mainly limited to the surface layers, despite some of the consequences they are known to have on the deep marine environment. In this paper, using data from sea surface temperature (SST) and in situ observations from Argo floats, we investigate the anomalous water characteristics during MHWs down to 2000 m in the western Tasman Sea which is located off the east coast of Australia. Focusing on their vertical extensions, characteristics and potential drivers, we break MHWs down into three categories (1) shallow [0 – 150 m], (2) intermediate [150 – 800 m], and (3) deep events [> 800 m]. Only shallow events show a relationship between surface temperature anomalies and depth extent, in agreement with a likely surface origin in response to anomalous air-sea fluxes. By contrast, deep events have greater and deeper maximum temperature anomalies than their surface signal (mean of almost 3.4oC at 165 m depth) and are more frequent than expected (>45%), dominating MHWs in winter. They predominantly occur within warm core eddies, which are deep mesoscale anticyclonic structures carrying warm water-mass from the East Australian Current. This study highlights the importance of MHWs down to 2000 m and the influence of oceanographic circulation on their characteristics. Consequently, we recommend a complementary analysis of sea level anomalies and SST be conducted to improve the prediction of MHW characteristics and impacts, both physical and biological.
Highlights
Marine heatwaves (MHWs) are extreme climate events of anomalously warm ocean temperature that can have profound impacts on marine species, ecosystem distribution and as a result, socioeconomics
Note that the peak for MHW depth that occurs in the higher range depths is likely due to the influence of the profiles which are warmer than climatology all the way down to the deepest (2000 m) Argo measurement
23% of the total MHWs are restricted to the surface layers, while almost half (∼45%) show a significant temperature anomaly deeper than 800 m
Summary
Marine heatwaves (MHWs) are extreme climate events of anomalously warm ocean temperature that can have profound impacts on marine species, ecosystem distribution and as a result, socioeconomics. Analysis of sea surface temperatures (SSTs) showed that some of these biological changes are due to anomalous, discrete and persistent warming which leads to the introduction of warm water species and causes biological changes among residing species in the affected depths (Wernberg et al, 2013). Oliver et al (2018a) found that from 1925 to 2016, surface MHWs have increased globally in frequency and duration by 34 and 17%, respectively. Frölicher et al (2018) found that MHW duration doubled from 1982 to 2016 and attributed 87% of MHWs to human-induced warming. The observed and anticipated increases in MHWs underscore the need for a rapid improvement in understanding MHWs and how to manage them
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