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|
|January 8-17||PNA-|| NAO+, AO+, MJO
January 2020 was notable for being the fifth warmest on record over the Contiguous U.S. (CONUS). That warmth was largely consistent with a strongly positive Arctic Oscillation (AO). The positive AO is associated with an enhanced cyclonic circulation around the North Pole, which tends to concentrated the coldest air in the Arctic. That pattern tends to result in warmer temperatures for most of the midlatitudes including the CONUS. As is typical during a positive AO, the warmth was particularly concentrated in the near the Great Lakes in association with an upper-level ridge there. Cooler temperatures filtered into the Northern Plains around the middle of the month, but the warmth returned towards the end of January.
The precipitation patterns were driven primarily by a positive East Pacific Oscillation (EPO) during January 2020. The key features of this pattern are a trough in the Gulf of Alaska and a ridge to its south. The trough helped funnel moisture onshore into the Pacific Northwest, which propelled Washington to its fourth wettest January. Meanwhile, the ridge brought anomalously dry conditions to the southwest. The trough also moved eastward in the middle of the month in association with a negative Pacific/North American (PNA) pattern. That configuration brought exceptional moisture to the central CONUS, which led to the sixth wettest January in Oklahoma.
The upper-level circulation was particularly zonal during the first week of January. That is common with the positive EPO that was associated with a trough in the Gulf of Alaska and a ridge to its south. That combination strengthens and lengthens the Pacific jet. The associated onshore flow brought heavy precipitation to the Pacific Northwest. The rest of the contiguous U.S. was generally dry except for the Southeast. Temperatures were also much above normal over most of the country, but particularly the Northern Plains.
The circulation during January 8-17 was a typical negative PNA pattern featuring a ridge in the central North Pacific, a trough in western Canada, and a ridge over the east-southeastern U.S. This pattern shifted the warm anomalies eastward and concentrated from along the southeast and East Coast from Texas to New England. The trough brought significant precipitation to the central part of the country. It also maintained the onshore flow and enhanced precipitation in the Pacific Northwest.
A positive WPO became the dominant circulation signal January 28-23. This pattern was primarily associated with a trough over eastern Russia and a ridge over the western Pacific. The trough extended across the Arctic to the Gulf of Alaska, and it helped bring cold air into the Northern Plains. It also continued the onshore flow into the Pacific Northwest, while a weaker trough brought enhanced precipitation to the upper Mississippi Valley.
The positive EPO returned at the end of January. This pattern again brought warmer-than-average temperatures across the Contiguous U.S., particularly in the North–Central U.S. The trough in the Gulf of Alaska brought more extensive precipitation to the Pacific Northwest, while a ridge to its south brought dry conditions to the Southwest.
- 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: The ocean and atmosphere system continued to be in ENSO-neutral during January 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These have been moderately warm since October after being near-zero in August and September. They warmed further in January, but a much longer period of warming along with more substantial atmospheric coupling would be required for a return to El Niño conditions.
- Teleconnections (influence on weather): The teleconnections with ENSO are muted during near-neutral conditions. An active El Niño would have favored cooler than normal temperatures along the East Coast and the Southern U.S., but most of the month had almost the opposite pattern. It would have also favored wetter conditions along the East Coast and the Pacific Northwest, the latter of which were a key feature in January 2020.
- 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 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 active during January. It began the month over the Maritime Continent (Phase 4) and amplified as it slowly moved eastward across the Pacific Ocean (Phases 5-7). It weakened towards the end of the month over the Western Hemisphere (Phase 8).
- Teleconnections (influence on weather): The MJO over the Maritime Continent (Phase 4) early in the month favored warmer temperatures in the Eastern U.S. that transpired during January 8-17.
- 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 strongly negative during January 8-17 with near-zero values before and after. The upper-level circulation exhibited a canonical negative PNA wavetrain with a ridge over the central North Pacific, a trough in western Canada, and a ridge in the eastern U.S. On average, the PNA was negative during January 2020.
- Teleconnections (influence on weather): The negative PNA during January 8-17 favored above-normal temperatures along the West Coast and cooler temperatures over the Southeast, which was very consistent with what happened.
- 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 positive for the first half of January and moderately positive the second half. Averaged over the month, it was strongly positive, primarily associated with an anomalously strong Arctic cyclone over the North Pole and enhanced ridging throughout the midlatitudes, particularly the central North Pacific. The midlatitude ridges faded in the second half of the month, which tempered the positive AO.
- Teleconnections (influence on weather): The positive AO is associated with warmer than average temperatures over the Mississippi Valley, which were present for the first half of the month (January 1-7 and January 8-17).
- 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 positive for most of January, particularly around January 4-13. It turned briefly negative at the end of the month, but the monthly average was strongly positive. The positive NAO was largely due to the trough near Greenland along with ridges over Europe and eastern Canada. Both the Greenland trough and the European ridge weakened near the end of the month, resulting in the negative NAO then.
- Teleconnections (influence on weather): The positive NAO would typically be associated with warmer temperatures over the Southeast, which were especially apparent in the middle of the month.
- Description: The WPO teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the East Asian jet stream in the western Pacific.
- Status: The daily WPO index was generally weak but positive. It was moderately positive around January 20, and the monthly average was positive. The positive WPO was primarily associated with ridges over Eastern Asia and the Central Pacific. The intensification around January 20 was associated with a canonical trough extending from The WPO intensified when that ridge was coupled with a trough over eastern Russia during January 18-23.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer than normal temperatures over the Northern Plains, which occurred January 18-23.
- Description: The EPO pattern identifies variations in the strength and location of the eastern Pacific jet stream. During the positive phase, the jet is stronger and shifted southward. The negative phase is associated with an Alaskan ridge that weakens the jet. The EPO is closely related to the East Pacific–North Pacific (EP–NP) teleconnection pattern, although the two are defined with opposite signs.
- Status: The daily EPO index was generally positive early and late in the month but neutral in the middle. Averaged over January, the EPO was strongly positive in association with the trough in the Gulf of Alaska and the ridge to its south. Both features were strongest at the beginning and end of the month.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the Upper Midwest and Northeast. A similar pattern emerged when the EPO was positive early and late in January.