Synoptic Discussion - February 2021
Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.
|Time Period||Key Driver||Other Drivers|
|February 1-6||AO-||La Niña|
|February 7-21||AO-||La Niña|
|February 22-28||AO+||La Niña|
The North American climate in February was strongly modulated by extreme variability in the Arctic Oscillation (AO). A strongly negative AO at the beginning of the month was associated in part with a stratospheric warming event that happened during late December through January. The AO reached -5.3 on February 10-11, which tied February 5, 1978 and February 13, 1969 for the lowest daily February value since 1950. In addition to this extreme negative value, the AO also experienced large volatility in February. During the middle of the month, it rapidly switched sign and ended the month strongly positive.
The negative AO is typically associated with cooler than normal temperatures over the Northern Plains. These anomalies were particularly extreme and long-lived during the middle of February. A large swath of the Plains was more than 30°F below normal for the two-week average February 7-21. This event set numerous records and was the coldest event in the Contiguous U.S. in 30 years. It also caused significant strain on infrastructure and led to wide-spread power outages, particularly in Texas. The long duration of the event was driven in part by a ridge over northern Canada. Coupled with the trough over the Plains, that ridge produced a blocking pattern that weakened the jet stream and precluded the cold air from moving out of the region.
This cold event was particularly unusual in the context of climate change. The most recent National Climate Assessment reported very high confidence that climate change is making cold waves like the one in February weaker and less common. Those findings are consistent between both the historical record and also future model projections. On the other hand, it is less well understood how drivers like the AO or the blocking pattern might be influenced by climate change.
The negative AO and the cold wave were both also consistent with the ongoing La Niña event. However, this event is not particularly strong and could not account for the strength of the AO and temperature anomalies. The other key climate driver during February was a strong ridge over the Gulf of Alaska. The southeastern extension of this ridge limited the transport of moisture into the Southwest and prolonged the drought conditions there. It also likely contributed to the strength of the trough downstream over the Plains.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for February 1-6 2021
February began with cold anomalies across the eastern U.S. in the wake of a major winter storm from the end of January. That storm had become a strong nor'easter by the beginning of February and brought heavy snow to the Northeast. A ridge extending from the Southwest to the Gulf of Alaska brought warm anomalies to the Rockies and prohibited precipitation over the Southwest. A cold front swept through the Plains February 4-5 and brought the leading edge of the Arctic air that would dominate the region for the next two weeks.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for February 7-21 2021
During the middle of February, the contiguous U.S. experienced the strongest cold wave in 30 years. Note the larger contour interval needed to illustrate the temperature anomalies during this major event. A large swath of the Plains experienced temperatures that averaged more than 30°F below normal over this 14-day period. These extreme temperatures broke numerous records across the region.
The frigid temperatures were associated with a broad and stationary trough over the Northern Plains. A ridge to its north over northern Canada created a blocking pattern that weakened the jet stream and contributed to the long duration of the event. That pattern was also associated with a strongly negative AO, which peaked at -5.3 on February 10-11. These values tied February 5, 1978 and February 13, 1969 for the strongest negative AO in February since 1950.
The ridge over northern Canada weakened around the middle of the month as the negative AO rapidly weakened. This allowed for more propagating weather patterns. However, instead of moving the cold air out of the U.S., it initially brought more shortwave troughs from the Arctic that reinforced the cold. It also led to several winter storms in the Southeast and resulted in the largest Contiguous U.S. snow cover in the recording period starting October 2003. The widespread snow cover included most of Texas. One of these storms even spawned an EF-3 tornado in eastern North Carolina.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for February 22-28 2021
A growing positive AO during the final week of February helped to bring an end to the cold wave. It strengthened the jet stream and finally moved the broad trough and its associated cold temperatures northeastward. A ridge anchored over the Southeast brought early spring-like temperatures. However, the more active weather pattern also brought Denver, CO its strongest snowstorm since February 2016.
- Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed “ENSO-neutral” conditions. These variations in SST change the locations of the Pacific's largest thunderstorms, which can in turn change circulation patterns around the globe.
- Status: La Niña conditions persisted during February 2021. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These remained near -1°C in February, which was indicative of the ongoing La Niña. The Southern Oscillation Index, which measures the atmospheric response, remained positive in February, which indicates a robust atmospheric response to the La Niña ocean conditions.
- Teleconnections (influence on weather): La Niña favors warmer than normal temperatures for the Southeast and cooler than normal for the Northern Plains. It also favors drier than normal conditions across the southern U.S., and wetter than normal for the Ohio Valley and the Pacific Northwest. The temperature pattern was present throughout February, although the observed cold wave was stronger than could be explained by the La Niña alone.
- Description: The MJO is the biggest source of subseasonal (31-60 day) tropical variability. It typically develops as a large envelope of tropical thunderstorms that develops over the Indian Ocean that then moves eastward. Like ENSO, the MJO's effects on tropical rainfall is so strong that it can alter the atmospheric circulation around the globe. The thunderstorms decay when they cross the Pacific, but the associated winds can often continue across the Western Hemisphere to initiate the next MJO in the Indian Ocean. The MJO is episodic, meaning that it is not always active. Most indices for tracking the MJO identify both the MJO's amplitude and the longitude of its strongest rainfall, usually described as one of eight phases.
- Status: The MJO was weak during February. The MJO index was confined to phases 6/7 (Western Pacific) during the entire month, largely driven by the La Niña signal. However, a weaker propagating signal began the month in the Western Hemisphere and moved eastward to the western Pacific during February.
- Teleconnections (influence on weather): The lack of a strong propagating MJO limited its potential teleconnections in February. However, the projection onto phase 7 of the La Niña signal would have favored the warm anomalies over the Northern Plains that give way to cold anomalies in the following weeks. Marginal warm anomalies were observed in the first days of the month, and the cold was dominant thereafter.
- Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
- Status: The daily PNA index was near zero for most of February with some weak positive values in mid-February and weak negative values near the end of the month. The monthly mean was near zero. The ridge over the Gulf of Alaska and the trough over the Northern Plains was reminiscent of a negative PNA, but they were displaced from their canonical locations and lacked the rest of the PNA wavetrain.
- Teleconnections (influence on weather): A negative PNA would have favored cooler conditions in the Pacific Northwest and warmer in the Southeast. However, the lack of a strong PNA signal in February precluded any significant impacts.
- Description: The AO teleconnection pattern generally measures the pressure difference between the low pressure over the North Pole and the higher pressures in the subtropical ridges. This pressure difference is larger during a positive AO, resulting in a stronger midlatitude jet. When the AO is negative, the jet is weaker and will have larger troughs and ridges.
- Status: The daily AO was strongly negative for the first half of February before rapidly transitioning to strongly positive at the end of the month. The negative AO peaked on February 10-11 at -5.3, which ties February 5, 1978 and February 13, 1969 for the lowest February value on record. It was also lower than 99.9% of all days since 1950. The peak of the positive AO was +2.7 on February 26, which was greater than 97% of all days since 1950. The AO monthly mean was negative. The negative AO in February was associated with persistent ridging near the north pole throughout the month. This anomalously weak tropospheric polar vortex may have been related to the sudden stratospheric warming events in January. The positive values near the end of the month were associated with troughs over northern Canada and central Russia along with the strong ridge in the Gulf of Alaska.
- Teleconnections (influence on weather): Temperatures over the eastern half of the U.S. are typically cooler than normal during a negative AO, particularly over the Northern Plains. That pattern was prominent throughout the month but particularly during the severe cold wave during the middle of the month.
- Description: The NAO teleconnection pattern relates the pressure over the sub-polar low near Greenland and Iceland with the subtropical high over the Central Atlantic. It significantly affects the weather on both sides of the Atlantic.
- Status: The daily NAO had weak negative values during the first half of the month and moderate positive values toward the end of February. The monthly mean was weakly positive. The low amplitude NAO was unusual given the high amplitude AO. The mean circulation for February resembled a negative NAO with a ridge over Greenland and a trough over the North Atlantic. However, both features were father north than is typical for a negative NAO. The ridges over the East Coast and western Europe were also more consistent with a positive NAO.
- Teleconnections (influence on weather): The negative NAO typically favors cooler than normal conditions and increased storminess over the eastern U.S. The cold anomalies were concentrated farther west during February in part due to the ridge near the East Coast.