Abstract
A hot blob for near-surface water was identified eastward of New Zealand in the South Pacific in December 2019, which was the second strongest event on record in this region. Its sea surface temperature anomalies reached up to 5 °C, and the anomalous warming penetrated around 40 m deep vertically. From the atmospheric perspective, the anomalous high-pressure system from the surface up to 300 hPa lasted for about 50 days, accompanied by the blocking pattern at 500 hPa and a deep warming air column extending downward to the surface. A mixed-layer heat budget analysis revealed that the surface heat flux term was the primary factor contributing to the development of this hot blob, with more shortwave radiation due to the persistent high-pressure system and lack of clouds as well as higher temperature of the troposphere aloft denoted by sensible heat. The oceanic contribution including the horizontal advection and vertical entrainment was changeable and accounted for less than 50%. Moreover, we used the strongest hot blob event which peaked in December 2001 as another example to evaluate the robustness of results derived from the 2019 case. The results show similar circulation features and driving factors, which indicate the robustness of the above characteristics.
Highlights
Regional summer heatwaves have occurred more frequently over the globe since 2000 [1,2,3,4], which has attracted wide attention among researchers
This New Zealand “marine heatwave” brought tropical fish from 3000 km away [7]. This surge in ocean heat over a short period could have been difficult for local marine life if it had penetrated far beyond the surface. This record-breaking hot blob followed a marine heatwave two summers ago that propelled the hottest summer of New Zealand on record, more than 3 ◦ C above average sea surface temperature (SST), and led to tropical fish from Australia being found along the country’s coast [8]
The SST standard deviation (SD) in this region is large in December, indicating a relatively larger natural variability at this time
Summary
Regional summer heatwaves have occurred more frequently over the globe since 2000 [1,2,3,4], which has attracted wide attention among researchers. In December 2019, a hot blob ( termed “marine heatwave”) appeared and spanned at least a million square kilometers—an area nearly four times larger than New Zealand—in the South Pacific (Figure 1a) This remarkable spike in sea surface temperature (SST) reached up to 5 ◦ C above average across a massive patch and was located to the east of New Zealand, near the sparsely populated Chatham Islands archipelago. This surge in ocean heat over a short period could have been difficult for local marine life if it had penetrated far beyond the surface This record-breaking hot blob followed a marine heatwave two summers ago that propelled the hottest summer of New Zealand on record, more than 3 ◦ C above average SST, and led to tropical fish from Australia being found along the country’s coast [8]. We used another hot blob event in December 2001, the strongest event east of New Zealand on record, as another example to have a comparison and further prove the robustness of circulation anomalies and the leading driving factor derived from the recent December 2019 case
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