After over a year of waiting and watching for the infamous El Nino to form, it now appears that the atmosphere is finally responding to the presence of abnormally warm waters off the coast of Ecuador. These warm waters, the textbook signal of the El Nino phenomenon, have been in place for several months now, and look to persist in coming months.
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| Climate Prediction Center |
The above image shows observed sea surface temperatures across the Equatorial Pacific on the top panel, with observed sea surface temperature anomalies on the bottom panel. On that bottom panel, we see a large swath of oranges (warmer than normal water temperatures) extending from the coast of Ecuador, all the way into Micronesia, relatively close to northeast Australia. It is that swath of warm waters that tells us the El Nino is present in the oceans.
However, things get messy quickly. In order for an El Nino to 'function' properly, we need it to appear in the oceans and in the atmosphere. For the last year-plus, we haven't gotten the atmosphere to cooperate; if you recall, typical El Nino conditions include wetter than normal conditions in the South, warmer weather in the North, and some drier weather in the Ohio Valley, as the graphic below indicates.
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| CPC |
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| CPC |
Suddenly, in these last couple weeks, we've gotten some pretty noticeable indications that the El Nino is now present in both the oceans and the atmosphere. The most significant indicator, with respect to the United States (since the El Nino affects the world as a whole), is most certainly the heavy rains in the South.
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| CPC |
This chart shows accumulated precipitation in millimeters from April 26th to May 25th, 2015. Notice the significant precipitation stretching from the Central Plains down to the Gulf Coast, most prevalent in Oklahoma and Texas, both of which are dealing with deadly floods as I type this. Looking at the average precipitation anomaly chart for April-May-June in the composite graphics earlier in this post, we can see that the Southern Plains are typically very wet during El Nino summers. In addition, we've also seen some relative drying-out in the Southeast and Ohio Valley regions, both of which tend to see below-normal precipitation during the April-May-June period of a 'typical' El Nino.
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| CPC |
Another signal of the El Nino is the presence of enhanced convection over the Equatorial Pacific, the same region we analyzed in the observed sea surface temperature charts. Notice the swath of blue on the OLR (Outgoing Longwave Radiation) anomaly panel (bottom panel) over the Equatorial Pacific. That blue swath indicates above-average thunderstorm activity, in the same sense that oranges and reds signify below-average thunderstorm activity. The warm waters of the El Nino in the Equatorial Pacific help create enhanced thunderstorm activity in the area, which is showing up on thunderstorm monitoring charts above.
What does this all mean going forward?
We're solidly entrenched in an El Nino pattern at this juncture. I'm planning additional research later this summer, but it's rather likely that this very wet pattern will continue in the Southern Plains throughout the summer, as El Nino's tend to allow such wet patterns to continue across the South and the West. It's also expected that the East and Central US will see a higher threat of a cooler summer, enhanced by the sea surface temperature configuration off the west coast of North America.
As far as precipitation, outside of the South Plains, some continued drying may be anticipated in the Southeast and Ohio Valley. Don't be surprised to see portions of the East US slip into a drought as we move ahead in the year, while drought conditions may very well ease in the West.
Andrew
