Following the warmest January in recorded history, February's expected above-normal global land/ocean surface temperatures could result in the warmest meteorological winter in 141 years of record-keeping.
Meteorological winter in the Northern Hemisphere, which began on Dec. 1, 2019, and will end Feb. 29, 2020, consists of what are usually the three coldest months of the year. But the December 2019 through February 2020 stretch has been historically warm, according to the National Oceanic and Atmospheric Administration (NOAA). The last month of 2019 was the second-warmest December in history and February has been warm across the globe, as well.
"Based on the global temperatures I see so far through the 23rd, February 2020 probably will end up once again among the top 10 warmest Februarys on record," said AccuWeather senior meteorologist Brett Anderson.
February 2019 was the fifth-warmest February on record and the warmest was February 2016 when the average global land/ocean surface temperature rose to 56.08 degrees Fahrenheit, 2.18 degrees above the 20th century average (53.9 F).
As for the United States, a combination of factors led to the contiguous U.S. experiencing higher-than-normal temperatures from December 2019 through February 2020. A polar vortex that remained above the polar region without weakening and a strong positive phase of the Arctic Oscillation (AO) were among the contributing elements.
The AO is an atmospheric phenomenon that can govern weather and climate patterns across mid- and high-latitude areas not only in North America but also across Europe and Asia in the Northern Hemisphere, and it is especially pronounced during winter.
The AO is a measurement of the air pressure closer to the surface and can be in sync or couple with the polar vortex, which is measured higher up in the atmosphere. When the air pressure is low over the pole and higher over the middle latitudes, a strong counter-clockwise wind flow occurs around the pole. This causes a milder westerly flow of air across the U.S. and southern Canada, which will cut off and isolate the Arctic air to the north.
"When you have a strong polar vortex that remains in the polar region, it tends to keep frigid air pent up so that it is difficult for long-lasting outbreaks of frigid conditions to reach the middle latitudes, including portions of the Midwest and Northeast," said AccuWeather lead long-range meteorologist Paul Pastelok.
"There are several reasons for the warmth - including the positive AO that remained positioned above the Arctic Circle - and at least two to three had to work side by side to bring the extreme high temperature averages that we saw in January and early February," Pastelok added.
Also, warm sea surface temperature anomalies in the western Atlantic Ocean and Gulf of Mexico, as well as warmer lake temperatures, less ice and a modification of air masses around the Great Lakes and Ohio Valley contributed to the mild pattern in the East. The strong southwest Atlantic upper high became the dominant feature, along with the positive AO and a transient pattern that led to eastern Pacific air overtaking much of the nation during the heart of the winter.
The contiguous U.S. had its fifth warmest January on record (35.51 F) and no state ranked average or below average for the month. January is the coldest month of the year on average in the contiguous U.S.; the average January temperature over the 20th century was 30.12 F.
Not every state, however, has had warm air. This meteorological winter, some of the major cities in Alaska, which had its warmest year on record in 2019, "can be ranked among the top 10 historically for coldest winters," said Pastelok. For the same reasons that the contiguous U.S. is running warmer than normal, Alaska - not a part of the contiguous U.S. - has been experiencing a colder-than-normal pattern since late December.
The average temperature in Fairbanks was 5.9 degrees Fahrenheit below average (which is -10.8 F) from Dec. 1 through Feb. 24, and it was 4 degrees below average (3.3 F) in Nome. In 2019 for the same time period, the average temperature was 3.9 degrees Fahrenheit above normal in Fairbanks and 2.7 F above average in Nome.
An extended break to the warm pattern does not appear likely soon for the contiguous U.S. as the start to meteorological spring on March 1 nears, according to AccuWeather meteorologists.
"There is good consensus that the unusually strong positive phase of the AO will persist well into March," Anderson said. "This favors the polar vortex remaining over the polar region with just brief spells of colder air into the central and eastern parts of North America over the coming weeks as milder conditions dominate. The cold shot coming into the eastern half of the country late this week may be the coldest that we will have for a while."