NOAA's National Centers for Environmental Information
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.
Summary
| Time Period | Key Driver | Other Drivers |
|---|---|---|
| Month | PNA | La Niña, AO- |
| January 1-13 | PNA+ | La Niña |
| January 14-22 | La Niña | |
| January 23-31 | AO- | NAO-, PNA- |
The January circulation pattern was strongly modulated by the phase of the Pacific/North American pattern (PNA), which was positive during the first half of the month and strongly negative during the final week of January. As part of the positive PNA wavetrain, a ridge extended from the tropical Pacific to across Canada. This ridge was associated with warm and dry conditions across much of the contiguous U.S. but particularly the Northern Plains.
The pattern in January was particularly exceptional because of the ongoing La Niña event and a strongly negative phase of the Arctic Oscillation (AO). Both of these typically favor cool anomalies for the Northern Plains. However, the ridge was too strong and persistent to allow for the typical cold air outbreaks. This particular La Niña event has also been shifted farther west than most years, which may also have favored these warmer temperatures.
The strongly negative AO was associated in part with a stratospheric warming event that happened during late December through January. During that time, the stratospheric polar vortex warmed dramatically and its jet stream weakened. The vortex usually blows from west to east around the north pole. Three times during January, it slowed to the point where it reversed direction.
Like the negative AO itself, stratospheric warming events tend be associated with more cold air outbreaks in North America and Europe. Europe did experience cold anomalies associated with a deep and persistent trough there, but the aforementioned ridge precluded cold anomalies over North America. Stratospheric warming events are also associated with more intense winter storms. This event may have played a role in the atmospheric river that hit California around January 27, and the subsequent snowstorm in the Midwest and ultimately a major nor'easter around February 1.
Monthly Mean
Submonthly Evolution
January 1-13
The first half of January was driven by a positive PNA pattern that featured an enhanced subtropical Pacific ridge, a trough near Alaska, and a ridge from the Southwest U.S. throughout Canada. The ridge over Canada was farther east than typical for a positive PNA and was associated with the strong warm anomalies over the Northern Plains. The southwestward extension of the ridge continued the drought conditions over Southern California.
January 14-22
During the middle of January, the positive PNA weakened. The trough that had been near Alaska in previous weeks receded across the Bering Strait, and the ridge over western Canada strengthened southwestward. As a result, the warm anomalies continued over the northern U.S., but warm anomalies also developed over the Southwest. Conditions remained dry across most of the country.
January 23-31
The weather patterns became more unstable during the final week of January. The PNA shifted to its negative phase with a trough along the West Coast. That trough brought an atmospheric river to the Southwest and produced heavy rain, flooding, and mudslides over the drought-stricken region. The upper-level trough associated with that storm continued eastward as a significant snowstorm across the Great Plains. It later spawned a major nor'easter around February 1.
Atmospheric Drivers
ENSO: El Niño Southern Oscillation
- 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 January 2021. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These continued their gradual warming in January but remained near -1°C, which was still indicative of an ongoing La Niña. The Southern Oscillation Index, which measures the atmospheric response, remained strongly positive in January, 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 northern regions. These patterns were generally reversed during January 2021. However, the cool SST anomalies are focused more on the Central Pacific than is typical for La Niña, which is usually centered more in the eastern Pacific. That shift would be more consistent with the warm signal observed over the Northern Plains based on MJO composites.
MJO: Madden-Julian Oscillation
- Description: The MJO is the biggest source of subseasonal (31-130 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 index was generally weak during January. The index began the month over the Indian Ocean and then transitioned to the western Pacific at the end of the month. However, these shifts were driven by low-frequency drivers. The convection over the Indian Ocean was associated with the ongoing La Niña and the signal over the western Pacific came from a persistent trough near Hawaii.
- Teleconnections (influence on weather): The lack of a strong propagating MJO limited its potential teleconnections in January. However, the projection onto phase 3 of the La Niña signal would have favored the warm anomalies over the Northern Plains that were observed most of the month.
PNA: Pacific/North American pattern
- 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 positive during the first half of January and then transitioned to strongly negative by the end of the month. The monthly mean was near zero. The positive PNA was associated with a ridge over the subtropical Pacific, a trough near Alaska, and a ridge over the northern U.S. The ridge was shifted farther east than normal for a positive PNA. The negative PNA late in the month was associated with a ridge near the Aleutians and a trough along the West Coast.
- Teleconnections (influence on weather): The positive PNA generally favors warmer conditions in the Pacific Northwest and cooler in the Southeast. The observed temperatures in the first half of January generally followed that pattern. However, the warmth was shifted farther east and the cold was farther west. Similarly, the negative PNA late in the month was likely associated with the cool anomalies for the northern U.S. and the warmth for the Southeast observed January 23-31.
AO: Arctic Oscillation
- 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 most of January. The monthly mean was also strongly negative and tied 1960 for the 7th lowest January value since 1950. The negative AO was associated with ridges near Greenland and the Barents Sea along with troughs near the Aleutians, the North Atlantic, and western Europe. It also likely related to a strong and prolonged warming of the stratospheric polar vortex.
- Teleconnections (influence on weather): Temperatures over the eastern half of the U.S. are typically cooler than normal during a negative AO. That pattern was only observed January 14-22, and even then it was weaker than expected for a strongly negative AO. The persistent ridge over eastern Canada associated with the positive PNA likely prevented the cold air outbreaks that would typically accompany the negative AO.
NAO: North Atlantic Oscillation
- 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 was weak but negative during all of January, and the monthly mean NAO was negative. The negative NAO was primarily associated with the ridge near Greenland and the troughs over the North Atlantic and western Europe.
- Teleconnections (influence on weather): The negative NAO typically favors cooler than normal conditions and increased storminess over the eastern U.S. These were only observed during late January.