Winter is just around the corner. What will it be like? If you want to cut to the chase, scroll down to the bottom. If you want some interesting details, read on.
There are two major long term ocean temperature effects that play key roles in global weather situations. If you read my blog regularly, I refer to the Pacific Decadal Oscillation(PDO) as a key player in our weather. Another, that I don’t mention as often is the Atlantic Multidecadal Oscillation(AMO).
The AMO and PDO are long term cycles. Both take several decades to oscillate from warm to cold and back again to warm. These compare to the relatively short term El Nino Southern Oscillation (ENSO). ENSO includes La Nina, El Nino, and La Nada. These cycles tend to shorter cycles of several months.
In the graphs below, the positive (red) portions of the graph are the warm phases and indicate positive temperature anomalies in the the northern Pacific and Atlantic Oceans.
Pacific Decadal Oscillation Index - University of Washington
Atlantic Multidecadal Oscillation - Wikipedia from NOAA data
Notice the time scales are different in the two graphs. Here are current shorter term graphs of both indices.
Atlantic Multidecadal Oscillation since 1981
Pacific Decadal Oscillation since 1981
I find that most experts believe that the PDO shifted to a cool or negative phase in 1998 and remains there today. Also, as I have read more most believe the AMO shifted to a warm phase in 1995. So, who cares? You should. The NorthAtlantic and North Pacific have very strong influences on our weather patterns.
Here is a 2003 article from McGabe, et al, Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. It’s an interesting article that helps explain the drought we have seen for the last decade or so. This chart provides a summary of the outcome of their research.
Drought frequency (in percent of years) for positive and negative regimes of the PDO and AMO. (A) Positive PDO, negative AMO. (B) Negative PDO, negative AMO. (C) Positive PDO, positive AMO. (D) Negative PDO, positive AMO.
The 4 maps represent that possible combinations of positive and negative AMO’s and PDO’s. The red areas show a greater than 30 percent likelihood of a drought. The blue areas show a lesss than 20 percent chance of drought. For the current combination of positive AMO and negative PDO, we would look to image D in the lower right hand corner of this chart. This is the region with the highest likelihood of drought.
So, back to my analog year for this year. The AMO was very positive at this time of year. The PDO shifted from very positive to negative. While the chart above doesn’t show the most recent months, this is very similar to this year. The AMO is currently very positive and the PDO has aggressively shifted to negative. Some say that this PDO shift is the most aggressive ever recorded.
The outlook is for the AMO to stay positive, or warm, the PDO to stay negative and a strengthening La Nina (cool phase ENSO). Here is the sea surface temperature anomaly outook for December 2010 to February 2011 from the International Research Institute for Climate and Society.
Global sea surface temperature anomaly outlook for December 2010 to February 2011
The current La Nina is forecasted to continue to deepen through the winter. This is a major La Nina event that should cause global temperatures to be significantly cooler this winter. This is apparently the most aggressive shift and deepest La Nina since 1955.
El Nino Southern Oscillation Index (ENSO) forecast (IRI)
So, I continue to see this winter being quite dry. I am still up in the air about temperature. December 1998 to February 1999 precipitation in Flagstaff was 1.18 inches, 5.74 inches below normal. This winter could be on par with those amounts.
Keep in mind that last year at this time, the forecast was for a strengthening La Nina and a very dry winter. That obviously didn’t happen. There is still lots of time for something to change.