When it Comes to Drought Relief, El Niño isn't the Only Player

We have addressed El Niño in previous posts, and what it could potentially mean or not mean, in the coming months. While El Niño gets all the publicity, there are other forces at work that act to drive our weather and climate.

Two of those such forces are the Pacific Decadal Oscillation and the Atlantic Multi-decadal Oscillation. These are long term oscillations (they change phase about every 25-35 years) that will many times govern whether we see more El Niño or La Niña episodes over a period of years. Recently, there have been some important shifts in both of these oscillations that I believe will play a huge role in the type of weather we see for the next several months.

Pacific Decadal Oscillation

The above graphic shows the different phases of the PDO since 1950. The orange spikes are the PDO in the warm phase, and the blue spikes are the PDO in the cold phase. This is a short sample of the PDO (we will look at a longer one later), but shows that from 1950-1978 the PDO was in a predominantly cold phase. In 1978, the PDO shifted to a predominantly warm phase, which generally lasted until about 2005. During this time, there were multiple El Niño episodes, including two super El Niños. . . 1982-83 and 1997. In 2005, the PDO shifted to a predominantly cold phase, which is currently where we reside. See the graphic below which breaks down the PDO phase since 2000:

Notice that since 2000, and especially since 2005 we have had very few warm PDO periods. We have had very few El Niño episodes, and quite a few La Niña episodes. Why is that? Well, when the PDO is in a warm phase, more frequent and longer lasting El Niños occur. When the PDO is in a cold phase, more frequent and longer lasting La Niñas are favored. Recently, the PDO has shifted to a weak and likely short lived warm/positive mode. I broke down the PDO by the numbers since 2005 and this is what I found:

Since January, the PDO has been getting steadily more positive. Why is this significant? See below as the graphic shows what is favored with each PDO phase and the relative ocean temperatures associated with each.

Being that the PDO has shifted to a slightly warm/positive phase, we are more than likely on target to experience an El Niño episode. During an El Niño, history shows that the southern third of the country is usually wetter and cooler than normal.

The 1990s was a very wet decade. The PDO was warm at that time and several El Niño episodes occurred, which likely contributed to the 90s being such a wet decade. Since the PDO shifted to a negative phase around 2005, more La Niña episodes have occurred. Thus, more droughty years than not. . . directly related to the PDO. When will the PDO change to a positive/warm phase and stay that way? Likely another 20 years before that happens. . .

Atlantic Multi-Decadal Oscillation
The graph below shows the predominant AMO phases since 1880. The red spikes indicate the AMO in a warm/positive phase, while the blue spikes indicate the AMO in a cool/negative phase.

Overall, the AMO has been positive since about 1995. Recently, the AMO has taken a slight and likely short lived turn to a negative phase. I say short lived, because we likely still have about 4-10 more years of the AMO being predominantly positive/warm, before a full phase shift to a coo/negative phase takes place. I am using history as my guide for this.

Off and on since 2005 we have struggled with drought. During that time, the PDO has been negative and the AMO has been positive. Recently, the PDO has turned slightly positive and the AMO has turned slightly negative. I broke down the AMO numerically since 2005 and the recent change is certainly notable.

The back half of 2010 through most of 2011, areas from Southeast Colorado southward into Texas suffered through one of the worst droughts on record. In late 2011, the AMO shifted to a slight negative/cold phase. I believe this rather stark change in the AMO coupled with a slight warming of the PDO, created an avenue for drought relief. In fact, compared to what the drought looked like in August of 2011, the drought was largely erased by the drought was mostly erased by February of 2012. The images below tell this story.

While there were some areas that were still dealing with drought, it was nothing like it was just a few months earlier. Coincidence that the drought broke when the PDO and AMO blinked? I think not.

McCabe (2004) did an amazing job highlighting drought frequency when the oceans are in their particular phase. See the map below:

As I have shown, since 2005 we've been in the -PDO +AMO phase (lower right). Notice all of the red shading, which indicates higher than normal drought frequency. Currently, we are in a short lived switch favoring +PDO and -AMO (upper left). Notice all of the blue shading. . . that blue shading represents greatly reduced drought frequency. I believe that it is no coincidence that since we have been experiencing this recent change in the PDO and AMO phase, our weather pattern has been very active since April.

How long will our likely short lived detour last? Tough to say, but I would give it at least 6-9 months before we revert back to the predominant phase of each ocean (-PDO and +AMO).

I spoke in San Diego and Waco this past week on this very subject. I stressed that wetter than normal conditions are likely going to remain less frequent and will be shorter lasting than they used to be for Colorado (Southwest 1/3 of the U.S. too). The drier times are going to be more frequent and last longer. When the Atlantic completes its shift to a negative/cold phase in another 4-10 years, we will be in the phase highlighted by McCabe on the lower left part of the map (-PDO and -AMO).

History suggests that while some parts of the southwest third of the country get drought relief, we still have a higher than normal drought frequency for most of Colorado and the Western High Plains. Under that shift, drought frequency greatly lessens for most of Texas. I stress to anyone that relies on water for their business, that they need a drought plan and one that they are not afraid to execute. Because while some areas that have been riddled with drought are seeing some of the best moisture they have seen in years this early in the season, we will go back to drought at some point. The oceans and history strongly suggest that it is inevitable.

Brian Bledsoe

Brian Bledsoe is Weather5280’s climate and long-range forecast specialist. Brian is chief meteorologist at KKTV in Colorado Springs. Follow him on Twitter @BrianBledsoe

Colorado Springs, CO
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