Antarctica is the highest, driest, coldest, windiest and whitest continent on earth. But how warm does it get?
That was the question posed last year to the World Meteorological Organization (WMO), the United Nations funded body that oversees meteorology and weather observations worldwide. I am part of a WMO panel on weather and climate extremes that was asked to provide an answer, with our findings announced earlier this month (March 2).
The word “warm” is not one usually associated with Antarctica. It is far better known for being cold. The lowest temperature ever observed on Earth, -89.2 °C, was recorded at a Russian research station in central Antarctica, on 21 July 1983 in the middle of the dark Antarctic winter. But even Antarctica has a summer season, with 24-hour sunlight, when temperatures can rise above freezing, especially around the coast.
Why should we care about extreme high temperatures in the Antarctic? To understand our changing climate, it’s important to know just how much the weather and climate can vary across the globe, including in Antarctica. While most of the globe has been warming in the past century, a lot of the Antarctic continent has not. Plus the area of sea ice that freezes on the coastal ocean surface has been growing slowly over the past 30 years. Understanding temperature and sea ice trends around Antarctica is a major research effort right now. All research on how the climate is changing is built upon high-quality records of weather and climate observations. The newly defined records give the international community a benchmark for comparison with future observations in a changing climate.
The Antarctic continent is huge, around 14 million square kilometres and roughly twice the size of Australia. Most of it is covered in ice, several kilometres thick in many regions. Since temperatures decrease with altitude, the high ice surface is bound to be much colder than coastal regions near sea level. There are also several islands dotted around the southern oceans that are included in the official UN definition of the “Antarctic region”, which is all land and permanent ice south of 60°S latitude. When working out temperature records, the exact region under consideration must be worked out first. After much discussion, we decided to come up with three high-temperature extremes, one for the Antarctic region south of 60°S, one for all of continental Antarctica, and one for “the ice”. The third region was taken to be all areas above 2500 metre altitude, which includes most of the high East Antarctic plateau.
Apart from the decrease of temperature with height in the atmosphere, temperatures are mostly controlled by latitude. The Equatorial region is the warmest and high latitudes are coldest, because sunlight is most intense in the tropics and least intense near the poles. Outside the tropics, the temperature on a given day is controlled by sunshine and wind direction. In the Southern Hemisphere, winds from the north bring warmer air while winds from the south bring cooler air. So we might expect to see Antarctic temperature records set at the lowest-latitude locations, on days when the wind is from the north, and that’s essentially what has been found.
Looking first at the full region south of 60°S, the record high temperature is 19.8°C, set on 30 January 1982 on Signy Island. Signy is a small island lying between the Antarctic Peninsula and the southern tip of South America. It is just south of 60 degrees’ latitude and on the day of the record the wind was blowing from the northwest. One other factor pushed the local temperature higher again: the “foehn effect”. This is where air rises over a mountain, loses its moisture as rain falls in the rising air (releasing heat in the process), then compresses and warms as it descends downwind of the mountain. Signy is a mountainous island and in a northwesterly the recording station is downwind of the mountains. This is the same mechanism that results in very warm nor’west days in Canterbury, downwind of the Southern Alps.
For the continent of Antarctica, the record highest temperature recorded is 17.5°C, on 24 March 2015 at Esperanza research station near the northern tip of the Antarctic Peninsula. At 63°24’S, it’s almost the northernmost point on the Antarctic continent. It is not far from Signy Island and subject to many of the same kinds of weather conditions. Once again, a strong northwest wind flow was blowing over the mountains that run the length of the Antarctic Peninsula, and the eastern coastal site of Esperanza experienced the strong warming that can occur in “foehn” conditions.
The third and final record high temperature, for sites above 2500 metre altitude, was measured half a hemisphere away at Automatic Weather Station (AWS) site D-80 located inland of the Adélie Coast in east Antarctica. D-80 is at an altitude of 2500 metres, so just inside the boundary for regions of at least 2500 metre altitude. The air temperature rose to a balmy -7°C on 28 December 1980. Two things contributed to the high temperature on the day: northerly winds bringing warm air from over the Indian Ocean and clear sunny skies contributing to surface warming.
Temperature records can be assessed only where there’s a weather station and a record of observations, and in Antarctica this limits things quite a bit. Because of the inhospitable nature of the climate and the difficulty of maintaining weather observations in most parts of the continent, there’s only a small number of sites to choose from and most of those are around the coast. Higher temperatures may have been experienced in other locations, but if it wasn’t measured, we’ll never know.
For the future, climate models project significant warming over the continent of Antarctica, with loss of seasonal sea ice. Continued monitoring of temperatures over Antarctica and surrounding oceans is vital for our understanding of climate change over the southern pole and worldwide. Sooner or later, one of those new high temperature records is bound to be broken.
The Commission for Climatology committee consisted of polar science and climate experts from Argentina, Spain, Morocco, New Zealand, the United Kingdom and the United States.
Full details of the assessment can be found in the online issue of Eos: Earth and Space Science News of the American Geophysical Union.