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Agriculture Weather Forecast - The Effects of Pacific Decadal Oscillation & the Atlantic Mutidecadal Oscillation

published: June 28th 2010
by: Weather Wise By Brian Bledsoe

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Hey folks!  Glad to talk to you again.  Hope all are enjoying their Summer so far.  We just got into a stretch of nice and warm weather up here in Colorado.  Heck, I still had to talk about snow in my nightly weather reports as late as the second week in May.  Glad that is over... 

 In my last article, we talked about how El Nino is done and how we are transitioning toward La Nina.  We also talked about how La Nina usually isn't kind to us in the western and southern Plains, when it comes to moisture totals.  Unfortunately, I am about to talk about more bad news regarding the potential onset of drought.  It has to do with the phase of both the Pacific and Atlantic Oceans.
 

 First, the Pacific:  The Pacific Decadal Oscillation or PDO for short, deals with the long term flucuation of the sea surface temperatures in the Pacific Ocean.  It changes phases every 20 to 30 years, and has a warm and a cold phase.  The warm phase favors the development of more El Nino's.  Good for moisture.  The cold phase favors the development of more La Nina's.  Not so good for moisture.

 warm phase                                                                 cool phase

Below is a graph of the PDO phase for the past 100 or so years:

 As you can see, we are in the cold phase. You may ask yourself, "Then why did we just have an El Nino"?  El Nino's can occur when the Pacific is in the cold phase, they are just shorter and weaker.  Remember, La Nina is soon going to take hold... 

 Secondly, the Atlantic:  The Atlantic Multidecadal Oscillation or AMO for short, deals with the sea surface temperature trend in the Atlantic Ocean.  It is much like the PDO in it changes phase about every 30 years.  Not much reference to El Nino or La Nina here, but it still has a huge impact on our weather patterns.  I.E.  the warm phase of the AMO favors more active hurricane seasons, since hurricanes thrive in warmer than normal water in the Atlantic. 

 Below is the last 100+ years of the AMO:

Notice how we are currently in the warm phase of the AMO.  Now you may be asking yourself where I am going with this.  Here's where I am going with this...  The more we look at the oceans, understand the oceans, and recognize their trends and changes, the more reliable our long range forecasts will be.  The graphic below illustrates my point:

The blue shaded areas in the above maps indicate a considerable reduction in drought frequency.  The red shaded areas indicate a considerable increase in drought frequency.  Thus, when the PDO is in the negative or cold phase and the AMO is in the warm or positive phase, the liklihood of drought increases greatly over all of the Rocky Mountain states and the Desert Southwest, all of Texas, most of Louisiana, and the majority of the Corn Belt.  -PDO and +AMO not good...  You can look at the other maps and see what the different phases add up to, but I am focused on the -PDO and + AMO, because that is what we are currently dealing with. 

 With the onset of La Nina all but a certainty, the combination of the above phases will likely end our nice moist streak of the Spring.  The above information continues to reinforce my forecast for Summer drought and heat to return for most of the western and southern Plains and western Gulf Coast States.  Granted, tropical storm activity can change that trend, and we will likely have an above normal tropical season.  However, the overall trend after hurricane season, suggests that we shouldn't plan on another wet Fall and Winter.

 If you have a weather question, feel free to email me at BBledsoe@southernlivestock.com.