"We have records going back to 1766 and we have nothing like this," said Dame Juliet Slingo the U.K Met offices chief meteorologist of the record rainfall and storminess this January. "all the evidence suggests there is a link to climate change" she said, without ruling out the possibility of a role for natural variability. Last winter, a sudden stratospheric warming in early January triggered a cold and stormy late winter and spring. Early this January a sudden stratospheric warming appeared to be beginning but the stratospheric polar vortex squeezed into a very cold tight windy cigar shape, bringing Siberian air towards North America, intense storms towards Europe and extreme drought to California.
Departure from normal for the position and intensity of the stratospheric polar vortex. The polar vortex was more intense than normal and displaced towards North America.
The stratospheric polar vortex at 70 mb was pushed towards north America by subsiding air from intense tropical convection above the west Pacific ocean. This squeeze created much stronger than normal winds over the north Atlantic ocean, generating intense storms that hit Europe.
Meteorologists map the height at which a weather balloon reaches a specific air pressure to analyze the forces driving the jet stream and the dynamics of the atmosphere. Because warm air is lighter than cold air, it rises up in the tropics and the atmosphere is much thicker in the tropics. Cold air forms dense high pressure areas near the surface in the polar regions in winter. The thickness of the atmosphere at the winter pole is much less than the thickness in the tropics. Strong winds are found where the thickness drops rapidly at the boundary with polar air. This is where the polar jet stream is located. The strong gradient in the maps in the north Atlantic is where the jet stream was much stronger than normal this winter. Note that the jet stream in the Atlantic tracks above the Gulf Stream which has been much warmer than normal this winter. The heat of the Gulf Stream helps intensify the height gradient in the atmosphere and the strong winds aloft, producing intense storms.
The polar night jet in the upper stratosphere was twice its normal speed in December and January. The polar vortex had an unusual elongated shape from Nov. 1 2013 to Feb. 1, 2014, extending anomalously towards North America.
Intense tropical convection in the tropical Pacific squeezed the stratospheric polar vortex bringing Arctic air over north America and severe storms to western Europe from November 1 2013 to February 1, 2014.
The U.K. Met Office highlighted the exceptionally intense stratospheric polar vortex and its effects in their report on this winter's exceptional weather.
Above the troposphere, westerly winter winds in the polar night jet stream were very strong during December and January. The polar night jet exceeded twice its normal strength at times during the winter, reaching speeds in excess of 100m/s in the upper stratosphere. A strengthening of the polar night jet often precedes periods of a strong Atlantic jet stream below and a positive North Atlantic Oscillation pattern, as was seen during the whole December to January period and consistent with the increased winter storminess this year.
The deep polar vortex in the stratosphere directly affected the lower atmosphere at 500mb, half way up in the atmosphere, where weather happens. An amplified storm track formed along an exceptionally strong jet stream over the north Atlantic from North America to Europe. A "ridiculously resistant" ridge, bringing record heat to the west coast and Alaska and blocking storms to California, was strengthened by subsidence from the stratosphere. The 500mb anomalies were extreme.
So what might have caused this weird weather and strange stratospheric squeeze? One factor is a natural cycle of winds in the stratosphere called the quasi-biennial oscillation. The winds were in the westerly QBO phase this year. This would tend to cause the intensification in the polar night jet in the stratospheric polar vortex because it is a westerly wind, too. Moreover, increasing greenhouse gas levels have been predicted to cool the stratosphere by reducing the temperature of heat radiating from the top of the lower atmosphere. A cooling stratosphere and warming troposphere is one of the predicted signatures of climate change caused by increasing levels of anthropogenic greenhouse gases.
The squeeze, however, likely has a source deep in the tropics. Warm water has piled up in the west Pacific ocean as trade winds intensified over the past 20 years. This very deep pool of warm water is causing tropical convection to intensify in the tropical Pacific and Indonesian region, increasing the flow of air from the tropics into the stratosphere. This air rises high in the stratosphere in the tropics then sinks near the poles. It can't reach the winter pole because it is blocked by the cold night air which creates an intense vortex However, this warm sinking air is likely the cause of the warm anomaly over the north Pacific ocean which has squeezed the polar vortex and strengthened the ridge causing the California drought.
This pool of very warm water is easily identified by a large bulge in sea level in the tropical western Pacific ocean and eastern Indian ocean. Note that there is also a bulge in the Atlantic side of the Arctic ocean caused by abnormally warm water north of Norway. Much warmer than normal water temperatures in the Gulf stream and the Norwegian Atlantic current and the Atlantic side of the Arctic ocean are also likely intensifying Atlantic storms.
Sea surface heights have risen rapidly over the past 20 years in the tropical western Pacific ocean as trade winds have strengthened and the warm pool has warmed and deepened.
The Climate Prediction Center maps areas of rising air in the tropics. This winter, the convection in the Australian-Indonesian monsoon has been stronger than normal creating stronger than normal upward motion. This abnormally intense convection is increasing the rate of flow of air into the stratosphere over the tropical warm pool that shows up on sea level maps. Moreover, it is intensifying the tropospheric flow across the tropics increasing subsidence in the tropical Atlantic ocean contributing to the extreme drought in Brazil that is parching Brazil's largest city, Sao Paolo.
This map of velocity potential at 200mb shows where air has been rising up faster than normal for the past 90 days around Australia and Indonesia (green) and sinking faster than normal (brown) over the tropical Atlantic ocean.
The intensification of tropical convection that is driving the build up of heat in the western Pacific and Indian oceans may be driven by climate change.
This process which appears to be slowing the rate of surface warming may be more destructive than benign, if the Pacific thermostat hypothesis, discussed by Michael Mann, is correct.
We showed in recent work, for example, that the same mechanisms described above may help to explain many of the now-better-established features of the medieval climate anomaly. For example, the La Niña-like temperature pattern in the tropical Pacific we have discerned for the MCA [Medieval Climate Anomaly] when solar output was high and volcanic eruptions were few, seems to be consistent with the tropical Pacific thermostat mechanism.
Finally, this matter illustrates how scientific uncertainty is not necessarily our friend when it comes to projected climate change impacts, and it provides a good example of true healthy scientific skepticism (as opposed to the sort of denialism/contrarianism that is too often passed off as 'skepticism'):
The implications of this seemingly innocuous finding are not trivial. It suggests the possibility that heating by increased greenhouse gas concentrations could lead to a more La Niña-like state of the climate, associated, for example, with intensified drought in the desert Southwest and increased Atlantic hurricane activity. If the minority of climate models that produce that response are correct, we might see a greater exacerbation of these effects than the IPCC currently projects.
The intensified ocean currents caused by the intensified tropical circulation and increasing La Nina like conditions will bring more oceanic heat towards the Arctic, leading to increased rates of sea ice loss. I strongly suspect that the trend of rapid loss of Arctic sea ice is related to the increase of oceanic heat. If this is true, there will be a rapid positive thermal feedback in the Arctic as late summer sea ice collapses over the next decade.
Europe has seen huge surf and massive coastal damage from these storms. This video shows incredible footage of surfers taking on some of the world's biggest waves in Portugal.
Nazaré Blow Up from SURFPortugal Mag on Vimeo.
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