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 NOAA ERSST v3. 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. Here is the Northeast Pacific Warm Pool data I created from 1870 to 2014. Keep in mind, any data before 1950 should be taken with a grain of salt as they are not directly measured.
Here is what the 500 millibar (18,000 feet) Geopotential Height anomaly looked like this past winter winter.
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 caustation, but it is a factor.
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.
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.
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 0.5 to 0.899 or 5.0 to 8.99 above 1981 to 2010 mean, while strong warm NEPWP as 0.9 or 9.0 and 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 and since 1950.
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?
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 Greeland 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.
Wonder what a warm NEPWP does to rainfall? Here is a precipitation rate map.
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. Winter of 1978-1979 is one of the coldest winters on record for America since 1895. It occurred when America saw back to back cold winters in the late 1970s. December 1989 had a huge freeze that is on the level of 1886, 1895, 1899, and 1983. 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 -0.899 or -5.0 to -8.99 below 1981 to 2010 mean, while strong cool NEPWP as -0.9 or -9.0 and below 1981 to 2010 mean. 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.
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?
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. Late January to early February 1951 had a huge freeze that covered North America and is one of the greatest freezes between 1949 to 1962. Houston had the longest below 32°F for 5 days in the 1951 freeze!
It shows that NEPWP is not the only factor when it comes to winter weather. There is 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.