No end in sight for our very active weather pattern

Are you surprised by the active weather pattern that has developed throughout New Jersey this year?  From powerful blizzards in the winter to tornado producing thunderstorms in the summer, central New Jersey especially has been under the constant barrage of rather dangerous weather conditions over the past year.  So what is behind all of this active weather?

Well, let me first state that no weather pattern is driven by one feature, but a combination of features in the atmosphere, the oceans, and even in outer space that all have an influence on our climate and current weather patterns.  The easy way out as many like to do is point to the use of your car or your "carbon foot print" and blame any major storm on "climate change".  The atmosphere is far too complex to be driven by CO2 alone if at all.

I have broken down three factors, that I'm betting you don't hear much about, that has had an impact on the changes we've seen in New Jersey and throughout North America over the past year.  

The first factor is a change we have seen in the sea surface temperatures in the Pacific.  The Pacific Decadal Oscillation or PDO changes switches from a warm phase to a cold phase every 20 to 30 years.  The past 30 years, the PDO has been in a warm phase which means the Pacific Ocean sea surface temperatures are averaging above normal.  A warm or positive PDO state leads to strong east based El Nino (towards South America) or weak west based La Nina (towards the central Pacific) phases in the Tropical Pacific and a more progressive Pacific jet stream in the higher latitudes.  The combination of these features tends to produce more warm winters for North America on average.  

However, now the PDO has transitioned to a cold phase, which has a significant impact on the climate in North America.    First, a negative PDO helps establishes cold sea surface temperatures in the Gulf of Alaska, which helps to create a more amplified jet stream leading to a powerful upper low off the West coast called a negative Eastern Pacific Oscillation (EPO).  Basically when the EPO goes negative, a very strong ridge is able to build over western North America, which can lead to a trough in eastern North America.  Another impact from a negative PDO state is that La Nina patterns are more likely to develop in the Tropical Pacific and the orientation of El Nino and La Nina change as well.  La Nina can become very strong in a negative PDO phase with an east based orientation while El Nino typically is weak and west based in orientation.  Note that in the winter of 2009/10, the Tropical Pacific had a west based El Nino and last winter La Nina was east based!  The changes in orientation impacts where thunderstorms develop in the tropical Pacific, which influences the position and strength of the Jet Stream or storm track over the United States!

The next factor that is influencing our current pattern is called the Atlantic Multidecadal Oscillation or AMO.  The AMO has two phases, cold and warm, and is an oscillation between above normal and below normal sea surface temperatures in the Atlantic.  When the AMO is negative, the jet stream is typically more progressive or fast moving over eastern North America and the north Atlantic leading to a fast moving pattern and also warm winters.  When the AMO is positive, the Polar jet stream slows down and buckles, creating blocking upper level features like powerful upper lows over eastern Canada and the north Atlantic.  With warmer sea surface temperatures, tropical development is far more likely as well as the Polar jet stream is locked over Canada and favorable atmospheric and sea surface conditions develop.  Also, due to an upper level ridge more likely to be in place over the Plains to the western Atlantic, there is a greater potential for droughts and even dust storms in the Plains.  The a known positive AMO phase in the 1930's to 1950's lead to several droughts in this area which enhanced the "Dust Bowl" disaster in the Plains.  

Finally, the state of the stratosphere the past year is key to connecting all of these other factors together in my opinion.  The stratosphere has been very cold of late over the Mid Latitudes and relatively warm over the northern Latitudes.  How does this impact the troposphere where we live?  Well, think of the stratosphere like a compressor in an air conditioner.  

When the stratosphere is cold, that part of the atmosphere contracts, which means the troposphere can expand.  When the troposphere expands, hot air masses can become enhanced and expand.  Also, when the stratosphere is cold, the atmosphere becomes far more unstable which leads to powerful thunderstorms and stronger upper level wind shear in the summer and more dynamic, convective winter storms in the winter.  Sounds familiar, doesn't it!

When the stratosphere is warm, the opposite develops.  The stratosphere expands while the troposphere contracts leading to cold air masses building and a high potential for high latitude blocking pattens.  Over the past two winters, the stratosphere in the higher latitudes has been warm leading to a prolonged negative Arctic Oscillation and North Atlantic Oscillation, which forces cold air from the Arctic into the eastern United States and makes winter storms more likely to develop.  

So let's put all of this together!  The PDO is in a negative phase which tends to produce a ridge over western North America and a trough over eastern North America.  Meanwhile, the AMO is negative, which enhances high latitude blocking in eastern Canada and the northern Atlantic, enhances tropical development potential, and leads to a development of a strong ridge over the Plains, Southeast, and western Atlantic.  Finally, the cold state of the stratosphere in the mid latitude is enhancing the strength of the ridge meanwhile the warm state of the stratosphere is enhancing the troughs over the higher latitudes!  Wouldn't you know it, that's EXACTLY the type of pattern we've been seeing, if one takes into consideration seasonal influences, for the past year and this current atmospheric set up is not expected to change for some time.