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March 31, 2020
Anatomy of a heatwave: how Antarctica recorded a 20.75°C day last month
While the world rightfully focuses on the COVID-19 pandemic, the planet is still warming.
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While the world rightfully focuses on the COVID-19 pandemic, the planet is still warming. This summer鈥檚 Antarctic weather, as elsewhere in the world, was unprecedented in the observed record.
Our research, published today in , describes the recent heatwave in Antarctica. Beginning in late spring east of the Antarctic Peninsula, it circumnavigated the continent over the next four months. Some of our team spent the summer in Antarctica observing these temperatures and the effect on natural systems, witnessing the heatwave first-hand.
Antarctica may be isolated from the rest of the continents by the Southern Ocean, but has worldwide impacts. It drives the , a constant system of deep-ocean circulation which transfers oceanic heat around the planet, and its melting ice sheet adds to global sea level rise.
Antarctica represents the simple, extreme end of conditions for life. It can be seen as a 鈥榗anary in the mine鈥�, demonstrating patterns of change we can expect to see elsewhere.
A heatwave in the coldest place on Earth
Most of Antarctica is ice-covered, but there are small ice-free oases, predominantly on the coast. Collectively 0.44% of the continent, these unique areas are for penguins and other seabirds, mosses, lichens, lakes, ponds and associated invertebrates.
This summer, Casey Research Station, in the Windmill Islands oasis, experienced its first recorded heat wave. For three days, minimum temperatures exceeded zero and daily maximums were all above 7.5掳C. On January 24, its highest was recorded, almost 7掳C above Casey鈥檚 30-year mean for the month.
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The arrival of warm, moist air during this weather event brought rain to Davis Research Station in the normally frigid, ice-free desert of the Vestfold Hills. The warm conditions triggered extensive meltwater pools and surface streams on local glaciers. These, together with melting snowbanks, contributed to high-flowing rivers and flooding lakes.
By February, most heat was concentrated in the Antarctic Peninsula at the northernmost part of the continent. A new Antarctic was recorded on February 6 at Argentina鈥檚 Esperanza research station on the Peninsula - almost 1掳C above the previous record. Three days later this was eclipsed when at Brazil鈥檚 Marambio station, on Seymour Island east of the Peninsula.
What caused the heatwave?
The pace of warming from global climate change has been generally slower in East Antarctica compared with West Antarctica and the Antarctic Peninsula. This is in part due to the , which has occurred in spring over Antarctica since the late 1970s.
The hole has tended to strengthen jet stream winds over the promoting a generally of the Southern Annular Mode in summer. This means the Southern Ocean鈥檚 westerly wind belt has tended to stay close to Antarctica at that time of year creating a seasonal 鈥榮hield鈥�, reducing the transfer of warm air from the Earth鈥檚 temperate regions to Antarctica.
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But during the spring of 2019 a over Antarctica significantly reduced the size of the ozone hole. This helped to support a more 鈥榥egative鈥� state of the Southern Annular Mode and weakened the shield.
Other factors in late 2019 may have also helped to warm Antarctica. The Indian Ocean Dipole was in a strong 鈥榩ositive鈥� state due to a . This meant that water in the western Indian Ocean was warmer than normal. Air rising from this and other warm ocean patches in the Pacific Ocean provided energy sources that altered the path of weather systems and helped to disturb and warm the stratosphere.
Is a warming Antarctica good or bad?
Localised flooding appeared to benefit some Vestfold Hills鈥� moss banks which were previously very . Prior to the flood event, most mosses were grey and moribund, but one month later many moss shoots were green.
Given the generally cold conditions of Antarctica, the warmth may have benefited the flora (mosses, lichens and two vascular plants), and microbes and invertebrates, but only where liquid water formed. Areas in the Vestfold Hills away from the flooding became more drought-stressed over the summer.
High temperatures may have caused heat stress in some organisms. Antarctic mosses and lichens are often dark in colour, allowing sunlight to be absorbed to create warm microclimates. This is a great strategy when temperatures are just above freezing, but heat stress can occur once 10掳C is exceeded.
On King George Island, near the Antarctic Peninsula, our measurements showed that in January 2019 moss surface temperatures only exceeded 14掳C for 3% of the time, but in 2020 this increased fourfold (to 12% of the time).
Based on our experience from previous anomalous hot Antarctic summers, we can expect many biological impacts, positive and negative, in coming years. The most recent event highlights the connectedness of our climate systems: from the surface to the stratosphere, and from the monsoon tropics to the southernmost continent.
Under climate change, extreme events are predicted to increase in frequency and severity, and Antarctica is not immune.
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