Strange summer extremes around the globe

7/25/2015 AMOC , Arctic , extreme weather , Jet Stream 0 Comments

In June this year we saw record heat in South America and Western United States but the weather here in Scandinavia has been very rainy and a bit cold this summer. How come? 

The heat wave that struck Pakistan in June resulted in many deaths and temperatures reached 40-44 degrees Celsius in Portugal and Spain at the end of the month. As we can see on the map below, some regions of the world have experienced high temperature anomalies for June. Venezuela, Argentina and Colombia noted new heat records, and massive forest fires have been burning in Canada and Alaska, while we here in Scandinavia have experienced a very rainy summer. It's like weather patterns are getting stuck, perhaps they are?

Source: SMHI (2015)

That is at least one credible theory discussed by Francis & Vavrus (2015). According to the authors new metrics and evidence support a linkage between rapid Arctic warming, relative to Northern hemisphere mid-latitudes, and more frequent wavy jet-stream configurations that favour persistent weather patterns. Results suggest that as the Arctic continues to warm faster than elsewhere on the planet in response to rising greenhouse-gas concentrations the frequency of extreme weather events caused by persistent jet-stream patterns will increase. In that context, heat waves in Pakistan, forest fires in Alaska and Canada and heavy precipitation events in Scandinavia makes sense. Because the temperature gradient between the north pole and the equator has decreased, as more ice melts due to warming, wind patterns start forming more strange patterns that tend to “get stuck”. 

The Arctic has warmed approximately twice as fast as the Northern mid-latitudes since the 1990s, a phenomenon known as Arctic amplification. This disproportionate temperature rise has influenced large wind circulation patterns, i.e. the polar jet stream, slowing them down. In areas and seasons which temperature gradient have weakened due to Arctic amplification we can now observe wavier and meandering jet-stream patterns.

In another study Moore et al. (2015) points out that the loss of sea ice in Greenland and Iceland Seas is affecting the production of dense water that forms the deepest part of the Atlantic ocean current (AMOC) that carries warm water from the tropics northward to Europe and the North Atlantic and cold water southwards. The authors state that this could potentially lead to colder climate in northwest Europe.

Because the Greenland Sea provides much of the mid-depth water that fills the Nordic Seas the observed change in ocean mixing, from once deep mixing to now only shallow mixing, could potentially change the temperature and salinity characteristics of these seas. A slowing down of the Atlantic ocean mixing currents could have dramatic impacts on the climate of the North Atlantic and western Europe. In particular, it would reduce the volume of warm water transported at the surface towards western Europe. More research is needed before we can really understand how the slowing down of North Atlantic ocean currents could impact European climate. But one thing is sure, we are experiencing some weird climate these days and we have only warmed the planet to about 0.85 degrees Celsius, imagine what 2 degrees would be like!