Sunday, 16 July 2017

The hot Tasman Sea during the el-NIno

For many months I was reporting this phenomen simply by looking at Climate Reanalyzer. Meanwhile there was absolute silence about this from NIW, NZ government and media.


The few mentions in the media linked this to the el-Nino but never mentioned any link to climate change.

Turns out I was 100 % right on this all along.

Wasn’t that hard really.

The unprecedented 2015/16 Tasman Sea marine heatwave



29 May, 2017

Abstract

The Tasman Sea off southeast Australia exhibited its longest and most intense marine heatwave ever recorded in 2015/16. Here we report on several inter-related aspects of this event: observed characteristics, physical drivers, ecological impacts and the role of climate change. This marine heatwave lasted for 251 days reaching a maximum intensity of 2.9 °C above climatology. The anomalous warming is dominated by anomalous convergence of heat linked to the southward flowing East Australian Current. Ecosystem impacts range from new disease outbreaks in farmed shellfish, mortality of wild molluscs and out-of-range species observations. Global climate models indicate it is very likely to be that the occurrence of an extreme warming event of this duration or intensity in this region is respectively ≥330 times and ≥6.8 times as likely to be due to the influence of anthropogenic climate change. Climate projections indicate that event likelihoods will increase in the future, due to increasing anthropogenic influences.


Introduction

Recent indications are that the frequency of extreme warming events in the ocean is increasing globally1. In both 2015 and 2016, approximately one quarter of the ocean surface area experienced a marine heatwave (MHW; based on the definition by Hobday et al.2) that was either the longest or most intense ever recorded since global satellite records began in 1982 (Supplementary Fig. 1). These events have devastated marine ecosystems globally but there is limited understanding of their physical drivers and the role of anthropogenic climate change. Individual MHWs have been examined in terms of their definition2, physical drivers3,4,5,6,7 and ecological impacts7,8,9,10,11 and inferences have been made to the role of climate change9,10,11,12.
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.
Marine ecosystems are strongly influenced by extreme climatic events including heatwaves8,9, cold snaps13, storms14 and floods15,16. MHWs, which can be caused by a combination of atmospheric and oceanographic processes, have led to a range of ecological impacts, including mass mortality of abalone (off Western Australia17), benthic habitat loss (Mediterranean Sea8) and altered human use of the ocean (that is, fisheries; northwest Atlantic and off Western Australia10,11). In the coastal waters off eastern Tasmania, a complete die-off of giant kelp (Macrocystis pyrifera) was reported during a warm weather event in 1988 (ref. 18) and may have been associated with a MHW.
The ocean off southeastern Australia is a global warming hotspot19. The near-surface waters there are warming at nearly four times the global average rate20,21 and these increasing temperatures are seen as deep as 750 m22. 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 Pacific21,23. 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 Sea24. 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 25,26,27,28.
This study discusses the 2015/16 Tasman Sea MHW from observations and ocean models, diagnoses its physical drivers and the role of anthropogenic climate change, and describes the ecological impacts that occurred. We investigate the hypotheses that the MHW in the Tasman Sea during austral summer 2015/16 was driven by anomalous southward transport in the East Australian Current and that anthropogenic climate change increased the likelihood of such an event. Our approach involves a synthesis of observations, theory and numerical models. First, we describe the event from remotely-sensed SST measurements as well as near-shore in situ sub-surface temperature and velocity measurements. Second, we determine the primary physical drivers of the MHW using ocean model estimates to determine the relative contributions of horizontal temperature advection and air–sea heat fluxes. Third, we use global climate models to estimate the increased risk of MHWs in the Tasman Sea, with the duration and intensity of this particular event observed in the summer of 2015/16, due to anthropogenic climate change. Finally, we document how the 2015/16 MHW affected regional coastal ecosystems, including the billion dollar aquaculture and fisheries industries.


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