Interview with Judah Cohen: “I try to use variability in the polar vortex for mid and longer term forecasts”

Rubén Vázquez

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 How would you summarize this Winter 2016/2017?

     Overall it was another warm winter.  One exception was Europe where it was cold (outside of Northern Europe).  But despite the warm winter a lot of snow did fall.  For the United States, it was a relatively wet winter, which helped with the drought especially in California.

Rubén Vázquez

 Has it behaved as you expected?

      I was expecting a colder winter across the Northern Hemisphere than was observed, especially in the Eastern US, where it was another mild winter.  I was pleased with our European forecast but not at all with our US forecast.

Juan Taboada

   Every week you give an analysis of the situation and a forecast of the expected impacts both for short and mid-term horizon. Are there areas of north hemisphere in which mid-term forecast (10-15 days) is better? perhaps other areas in which the skill of this mid-term forecast is not useful?

      I try to use variability in the polar vortex for mid and longer term forecasts. The polar vortex is a deep low-pressure center in the upper atmosphere that usually sits over the Pole during the winter months and is encircled or surrounded by a fast-flowing ribbon or river of air. Air flows counterclockwise around low pressure in the Northern Hemisphere or in a west to east direction.   When the polar vortex is strong the weather tends to be mild across the Northern Hemisphere including the Eastern US and Europe and when it is weak it tends to be colder across the Northern Hemisphere. This relationship can be used for long range forecast.  However I believe it is more useful for the Eastern US than the Western US.  I also think that it is useful for Europe.

Juan Taboada

   With your experience of some decades looking at Artic conditions, do you think that there are significant tendencies on the dynamics of the area, such as more (or less) polar stratospheric breaks, sudden stratospheric wamings, etc?

     The polar stratosphere has gotten warmer in recent decades in mid to late winter and this is related to a weakening of the polar vortex also known as a sudden stratospheric warming.  A weaker polar vortex often results in more severe winter weather across the Northern Hemisphere including the Eastern US and Europe.

Juan Taboada

   Do you have any idea or statistic about in which areas of the north hemisphere your weekly mid-term forecasts are better fulfilled?

     My weekly discussions are very broad and discuss the weather on a broad scale and I don’t have a good sense where the discussions have been more accurate.  However, if I were to guess I would say those regions most sensitive to variability in the polar vortex including the Eastern US, Europe and East Asia are the regions that my discussions are most applicable.

Rubén Vázquez

   At meteovigo we believe that climate change is accelerating in recent years. With changes in circulation such as the jet stream which tends to be more southerly. We believe this was what happened in the small ice age which led to abrupt changes of time. What’s your opinion about it?

     I have published papers related to this idea.  My arguments have been that rapid change in the Arctic including warmer temperatures, less sea ice and more snowfall in autumn has contributed to a weaker polar vortex in mid to late winter.  When the polar vortex is weak the Jet Stream tends to be displaced more southward favoring more severe winter weather including cold air outbreaks and heavy snowfalls.

Rubén Vázquez

 Do you think that next winters this circulation can bring us stronger snowstorms?

    Heavy snowfalls have increased across the Eastern US and it is plausible that the same is happening in parts of Europe.  Though I don’t fully understand the reason for the heavier snowfalls I suspect that they are possibly related to Arctic change and I see no reason for this trend to change.

Juan Taboada

   Seasonal forecast is an important topic for mid latitudes in north hemisphere. Many efforts and studies have been devoted in the last decades to improve the skill of these forecast. Your field of expertise is in the Artic, but have you ever thought about interactions of the variability in the Arctic with other world areas, such as Equatorial Pacific? Taking into account that stratosphere plays a significant role in the response to El Niño in Europe perhaps changes in stratosphere in the Arctic could affect this response.

   I have focused on the Arctic but I explore the entire globe for any source of predictability of long term weather.  I have shown in my research that extensive snow cover and reduced sea ice favor a weaker polar vortex.  But I am interested in the impacts of the tropics on the polar vortex as well.  Others have shown that El Niño also favors a weak polar vortex, so I consider this as well in my long-range forecasts.

Rubén Vázquez

  A good part of the answers to the seasonal forecast study is at the pole. How far do you think we can make improvements in the future by finding out more about the Arctic? 

   I do believe that the biggest advances in seasonal forecasts will come from a better understanding how variability in the Arctic influences weather in the mid-latitudes.  I do hope that there is a focus on the relationship between Arctic change and mid-latitude weather in the coming years.

Rubén Vázquez

  What do you like the most about your work as a climatologist?

 I am fascinated by the weather and learning how the weather works, gaining an understanding for weather systems brings me never ending satisfaction.  Also, I enjoy the challenge of making long range weather forecasts and I try my best to be right.

Rubén Vázquez

  Have you ever heard of Galicia (Spain) and its wonderful beaches and seafood? 

 I have heard of Galicia and I even visited Ourense once for a workshop on weather.  But it was in the late fall and it rained a lot, certainly not beach weather.  But I remember it was a beautiful location.

Rubén Vázquez

 We’re going to put you in a bind. If we offer you a week with the family with all cost paid in the snow or on the beaches of Galicia (Spain) with seafood. Which option would you choose?

 I love to ski but as I get older I do enjoy the beaches and the warm weather more.  I think if it involved my whole family then I would definitely choose Galicia.  I know that my whole family would prefer the beaches of Galicia than the snow!

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