Summer sea-ice extent still retreating – minimum in sight
The second half of February was marked by extreme weather conditions, with particularly warm temperatures over the Arctic and exceptionally low temperatures over Eurasia and North America. This situation lasted into April, with temperatures in the East Siberian Sea up to 10 °C higher than the long-term average for that month. The air temperatures in the east Greenland Sea were up to 5 °C and in Baffin Bay 3 °C higher than the long-term average. In the summer, the Arctic, particularly in the East Siberian Sea continued to be warmer than the long-term average, although in other parts of the Arctic, the temperatures were within the normal range. January to May 2018 are so far the warmest months in the Chukchi Sea and Bering Street (see figure 6). Figures 7 and 8 show a summary of the atmospheric condition in the Arctic for the period January to August 2018. Atmospheric circulation in the Arctic in summer 2018 The conditions in the Arctic in summer 2018 are closely linked to the extreme heat and drought in Germany this summer. This was caused by what is known as atmospheric blocking, a stable high over Scandinavia that forces Atlantic low-pressure systems towards the north or south, repeatedly driving warm and dry air towards the western part of Europe. Climate models of future scenarios show that climate change can increase the likelihood of such weather conditions. Warming at the northern latitudes leads to dwindling snow and ice in the Arctic, which because of its white colour reflects a large part of the sun’s radiation. If there is less ice and snow, the sun shines directly onto the Arctic Ocean and the land, which are darker and so absorb the radiation, causing them to become warmer. Warming in the Arctic increases and progresses much more rapidly than at mid-latitudes (this is known as “Arctic amplification”). This results in the temperature difference between the Arctic and the middle latitudes becoming smaller. However, the contrast in temperature between the Arctic and our mid-latitudes is the driver for the weather in Germany and Europe, as a whole. Low-pressure systems develop to balance out the temperature differences. The front of a low-pressure area pushes warm air northwards, while the back pushes cold air to the south. If the temperature contrast is lower, the low-pressure system is weaker so that it can be more easily blocked by a strong high-pressure system. What we have observed this summer is a change in the jet stream, a strong band of winds that controls low-pressure areas. The jet stream moves in waves around the globe’s middle latitudes. If a region is in the jet stream’s trough then frequent low-pressure areas occur there; if the area is in a peak, high-pressure influences dominate. “According to forecasts from various climate models, the jet stream will become weaker due to a reduced temperature gradient between the high and mid-latitudes, and the peaks and troughs will move more slowly. Correspondingly, they will alternate less frequently over Europe, so that in the future high pressure areas will remain over the same area for increasing lengths of time periods.”, Dr. Monica Ionita-Scholz climatologist from AWI states out. If, for example, the peak remains over Scandinavia, it keeps the low-pressure area away from us for a longer period – like we experienced this summer.
Figure 9: Geopotential height (500hPa) for July 2018 (left) relative to long-term mean (1981-2000) for the Arctic; integrated water vapor transport for July 2018 (middle) and for January to August (right) (source: www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html)