Northeast Pacific Warm Pool Explained

NEPWP

I first noticed an area of warm water in the Northeast Pacific in the cold Winter of 2013-2014. I call it the Northeast Pacific Warm Pool (NEPWP) and created an anomaly data from University of Hawaii-APDRC LAS8 for public.

The NEPWP Region is between 35°N and 50°N and 165°W and 135°W. It goes back to 1870 and anomaly is based on 1981-2010 average. Keep in mind, any data before 1950 should be taken with a grain of salt as they are not directly measured. Here is the NEPWP Table.

NEPWP Anomaly Value
Strong Cool <= 1.50
Moderate Cool -1.00 to -1.499
Weak Cool -0.5 to -0.999
Neutral -0.499 to 0.499
Weak Warm 0.5 to 0.999
Moderate Warm 1.00 to 1.499
Strong Warm >= 1.50

NEPWP Anomaly Value (Multiplied By 10)
Strong Cool <= -15.00
Moderate Cool -10.00 to 14.999
Weak Cool -5.00 to -9.999
Neutral -4.999 to 4.999
Weak Warm 5.00 to 9.999
Moderate Warm 10.00 to 14.999
Strong Warm >= 15.00

Here is what the 500 millibar (18,000 feet) Geopotential Height anomaly looked like for 2013-2014, which had the strongest warm NEPWP.

20132014Winter_500mbGeopotentialHeight_Anomaly_NH

Notice a great deal of ridging over Alaska, Gulf of Alaska, and Siberia. The ridging over Alaska is the negative phase of East Pacific Oscillation (EPO). It is the Pacific equivalent to the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). So, how does NEPWP and EPO correlate. Here is data for meteorological winter (December to February), spring (March to May), summer (June to August), and fall (September to November). The data is from 1948 to 2014 up to Summer. The EPO link is daily, but I converted it to monthly.

DJF NEPWP EPO
r = -0.45
p < 0.05

MAM NEPWP EPO
r = -0.47
p < 0.05

JJA NEPWP EPO
r = -0.37
p < 0.05

SON NEPWP EPO
r = -0.40
p < 0.05

NEPWP and EPO are strongly correlated. Cooler NEPWP is more favorable for positive EPO, while warmer NEPWP is more favorable for negative EPO. Warmer water allows ridging to develop over Alaska and Northwestern Canada. NEPWP could give us an idea about EPO even before 1948. A warm NEPWP have negative EPO, while cool NEPWP have positive EPO. Of course, correlation does not mean causation, but it is a factor. You can have negative EPO during cool NEPWP like in February 1989.

The ridging allows cold air to come down to the Lower 48, which lead to an Arctic blast and cold winters. As the Lower 48 froze, Alaska, Northern Canada, and Western US are warm due to the ridging. The previous two winters were quite warm compared to this past winter. Here is a surface temperature anomaly map of Winter 2013-2014.

20132014Winter_SurfaceTemperature_Anomaly_NH

Notice that the Eastern two-thirds of North America is quite cold. The polar regions are unusually warm as it is with the Western US. The ridging made those areas warm and caused the jet stream to pull in cold air that is normally in the polar regions. The ridging also plays a role in precipitation as a map shows which areas are drier and wetter than normal.

20132014Winter_PrepRate_Anomaly_NH

The Western US is very dry. No surprise they are in a drought, especially in California, which is really dire. The ridging shunts the low pressure systems away from California and Western US. Interesting to note that Alaska is not dry despite the ridging, but the Bering Sea and western point of Aleutian Islands are dry. Interesting fact, the Aleutian Islands are the westernmost and easternmost point in America as it crosses the 180 degree longitude. Some of the Aleutian Islands are west of 180 degrees longitude in the Eastern Hemisphere.

Wonder what the geopotential height anomaly is like at 18,000 feet (500 millibar level) during a moderate and strong warm NEPWP. I define moderate warm NEPWP is 1.0 to 1.499 or 10.0 to 14.999 above 1981 to 2010 mean, while strong warm NEPWP as 1.50 or 15.0 and higher above 1981 to 2010 mean. Here is a map of what it looks like. All anomaly maps from here are for winter from December to February since 1870. It is top 10 warmest NEPWP between December to February. Data is from 20th Century Reanalysis Monthly Composites.

DJF_Top10WarmNEPac_500mbGeopotentialHeight_Anomaly_NH

Notice ridging over Alaska, the warm water allows ridging to form, which is a negative EPO. There is troughing over Northern Canada and Greenland, which is a positive NAO. Interesting to note there is ridging over the Southeast US. How is winter temperature like during a moderate and strong warm phase of NEPWP?

DJF_Top10WarmNEPac_SurfaceTemperature_Anomaly_NH

Interesting to note that Canada and Northern US is very cold when there is a moderate and strong NEPWP. The Southern US is warm, which is rather interesting. Overall, Southern US is warmer and Northern US and Canada is cooler during the winter with a moderate and strong NEPWP in place. The troughing over Canada and Greenland keeps Canada and Northern US colder, which bottles up cold air. A positive NAO gives the Southern US a warmer than normal winter. Opposite when NAO is negative, which is colder than normal for Southern US.

Wonder what a warm NEPWP does to rainfall? Here is a precipitation rate map.

DJF_Top10WarmNEPac_PrepRate_Anomaly_NH

The Western US and Canada is very dry. That is no surprise as California is in a severe drought at the time of this article. Parts of the Southeast, Midwest, to East Coast is wetter than normal. Overall, a warm NEPWP does not have much impact on precipitation in the winter.

Regardless, there have been big freezes and major winter events when there is a moderate and strong warm NEPWP. Late January to early February 1949 had a huge Arctic blast. January 1962 had a huge Arctic blast which is considered one of the big freezes in history as it set temperature and surface pressure records. It occurred when America saw back to back cold winters in the late 1970s. A year after the December 1989 freeze, another huge freeze hit in December 1990, which hit the Western US. An epic White Christmas came in 2004 for Texas.

It is opposite when the NEPWP is in a cool phase. I define moderate cool NEPWP is -0.5 to -0.999 or -5.0 to -9.99 below 1981 to 2010 mean, while strong cool NEPWP as -1.5 or -15 and below 1981 to 2010 mean.

DJF_Top10CoolNEPac_500mbGeopotentialHeight_Anomaly_NH

There is troughing over Alaska, Siberia, and Southeastern US. There is ridging over Northern Canada and Greenland. Troughing over Alaska is a positive EPO, while ridging over Northern Canada and Greenland is a negative NAO. So, how is temperature affected by cool phase of NEPWP? Here is a temperature anomaly map.

DJF_Top10CoolNEPac_SurfaceTemperature_Anomaly_NH

It is not as warm during cool NEPWP. As mentioned, since there is ridging over Northeastern Canada and Greenland, it shunts the cooler air down south. The ridging over Northeastern Canada and Greenland is a negative NAO. As a result of ridging, Northeast Canada and Greenland is warmer than normal. It is cold over Alaska, Northern Asia, and Siberia due the trough locking up cold air. Does cool NEPWP have the opposite affect on precipitation from warm NEPWP?

DJF_Top10CoolNEPac_PrepRate_Anomaly_NH

Western US and Canada are wet, which is opposite from warm NEPWP. Southeast Texas, Southeast, and Northeast have wetter than normal winter. The Northeast is not affected by cool or warm NEPWP. It has more of an affect west of the Mississippi River. Cool and warm NEPWP have impact on rainfall on different areas. It could dry in one area and wet in the other area.

Even when the NEPWP is in a moderate to strong cool phase, freezes and major winter events have happened. The Winter of 1902-1903, 1903-1904, and 1904-1905 were cold for America. They happened back to back, like the 1910s and 1970s.

It shows that NEPWP is not the only factor when it comes to winter weather. There are El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO), NAO, AO, and EPO. NEPWP is certainly a factor and it influences EPO.