What is the PDO and AMO?

If you read our blog, you know that I am really into long range forecasting.  I thought I would talk about a couple of different things that play a huge role on our weather, that you may have never heard about.  Forgive me if I get a little technical, but I will do my best to keep it in simple terms.

Below lies a good text and graphical depiction of what the PDO is all about.  The information is courtesy of the University of Washington:

The Pacific Decadal Oscillation (PDO)

Typical wintertime Sea Surface Temperature (colors),
 Sea Level Pressure (contours) and surface windstress (arrows) anomaly patterns during warm and cool phases of PDO
 

warm phase                                            cool phase


 The "Pacific Decadal Oscillation" (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time. Fisheries scientist Steven Hare coined the term "Pacific Decadal Oscillation" (PDO) in 1996 while researching connections between Alaska salmon production cycles and Pacific climate (his dissertation topic with advisor Robert Francis). Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics - the opposite is true for ENSO. Several independent studies find evidence for just two full PDO cycles in the past century: "cool" PDO regimes prevailed from 1890-1924 and again from 1947-1976, while "warm" PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990's. Shoshiro Minobe  has shown that 20th century PDO fluctuations were most energetic in two general periodicities, one from 15-to-25 years, and the other from 50-to-70 years.

Simply put, if the PDO is in a positive or warm phase, there are more frequent and stronger El Nino episodes.  Between 1980 and 2000, the PDO was very positive and thus we experienced more El Nino's and less drought.  Since roughly the year 2000, the PDO has been largely negative.  Thus, we have experienced more La Nina episodes and that has resulted in more drought years.  When I say "we" I am obviously talking about Colorado.  While this isn't the only player in our long range weather patterns, it certainly is influential.  Usually when we have an El Nino, we don't have to worry about drought.  When we have a La Nina episode, our drought concerns increase bigtime.  By following the PDO and it's phase changes, one can determine if we need to have drought thoughts on our mind or not.  

The warm phase of the PDO may also explain why the late 80's and 90's were some of the warmest times on record.  Thus, fueling global warming/climate change talk.  However, if you look back through history a negative PDO ( cold phase ) has the opposite impact.  The 1970's were an unusually cool time across the US and this is while the PDO was negative.  Nobody was worried about global warming or climate change at that point...lol.

What about the AMO?

The AMO is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years.

Below is a graph of the AMO since 1900:

Raw AMO Data since 1856

 

 

A warm or positive AMO usually contributes to more frequent and stronger hurricanes and generally warmer than normal temperatures for the Northern Hemisphere.  The opposite can be said for the negative or cold phase.

 

Below shows the different phases of both the PDO and AMO and how they can impact drought potential.  

 

Right now, the PDO is in a negative or cold phase and the AMO is positive.  If you buy the above graphic, we should be worried about the prospects of drought.  However, our developing El Nino is the "Wild Card".  El Nino usually give us wetter than normal conditions, and I don't see any reason to argue with that history.  Considering how nice and wet we've been this Summer, it may portend a rather active and snowy Winter.  That's what I am calling for anyway...

We will continue to examine these oscillations and different ones in the coming months.  Some of this material may also change your mind on rather you think climate change is human caused or if it is simply a natural cycle of the Earth.  I am siding with the latter...

Chief Meteorologist Brian Bledsoe

 

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