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- | MJO, Hurricanes |
| October 1-7 | NAO- | |
| October 8-14 | La Niña | PNA+, Delta |
| October 15-31 | PNA- | MJO, Zeta |
The dominant pattern during October 2020 was a negative phase of the Pacific/North American pattern (PNA), although the observed wavetrain was shifted farther east than normal. It featured a trough near Hawaii, a ridge from Alaska to the West Coast, a trough over the Midwest, and a ridge along the East Coast. The ridge along the West Coast has been a fixture since mid-April and continued the drought and fire conditions over the Southwest.
The trough was particularly strong in the second half of October, due in part to the Madden-Julian Oscillation (MJO) moving across the Pacific then. It was associated with colder than normal temperatures across the Central U.S., including near-record cold temperatures from the Rockies to the western Great Lakes. The trough was also associated with a persistent frontal zone that brought significant precipitation from Oklahoma to Ohio, including an unusual October ice storm for the central and southern Plains.
The other major climate drivers during October were a pair of hurricanes, Delta and Zeta, that made landfall over Louisiana. Both storms brought heavy rainfall across the Southeast. They also contributed to the list of records set by the 2020 Atlantic Hurricane season. Through November 1, eleven named storms made landfall over the contiguous U.S., which broke the previous record of nine in 1916. Five of those storms, including Delta and Zeta, made landfall in Louisiana, which is the most ever for any state in one year. Through October, the 2020 hurricane season had produced a total of 27 named storms, which was three ahead of the pace set by 2005. The 2020 season went on to break 2005's record seasonal number of 28 with Tropical Storm Theta on November 10.
Monthly Mean
Submonthly Evolution
October 1-7
The primary driver during the first week of October was a ridge over the western U.S. and a trough to its east. The ridge continued the extended warm and dry conditions for the Southwest in particular. The trough was associated with a negative NAO and brought cooler conditions to the eastern U.S. Despite the upper-level trough, a persistent high-pressure system suppressed precipitation outside of the Great Lakes region.
October 8-14
The circulation became more zonal during October 8-14 with a broad ridge over the U.S. and a trough to its north over Canada. The ridge was associated with warmer than normal temperatures for most of the contiguous U.S., particularly the Central Plains. The trough also brought a series of atmospheric rivers to the Pacific Northwest with significant rain and snow. In the Southeast, Hurricane Delta made landfall about 12 miles from where Hurricane Laura had just six weeks earlier. Delta brought significant rainfall across the Southeast.
October 15-31
The negative PNA wavetrain became more dominant during the second half of the month with a ridge off the West Coast, a trough through the Midwest, and another ridge along the East Coast. The West Coast ridge allowed the warm and dry conditions to persist. The trough was associated with a prolonged cold event to the Central U.S. On the east side of that trough, a stationary frontal zone brought persistent precipitation along a swath from Oklahoma to Ohio. Hurricane Zeta became the fifth named storm to make landfall in Louisiana in 2020, the most ever for any state in a single season. It brought more rainfall to the same region as Hurricane Delta a couple weeks earlier.
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 October 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These continued cooling to -1.2°C in October, which is indicative of a moderately strong La Niña. The Southern Oscillation Index, which measures the atmospheric response, actually weakened during October due to competing influences from subseasonal variability like the Madden–Julian Oscillation (MJO).
- Teleconnections (influence on weather): La Niña favors warmer than normal temperatures for the Midwest and drier than normal conditions for the Lower Mississippi. This pattern was most apparent during October 8-14, although Hurricane Delta brought significant rainfall to the Lower Mississippi.
MJO: Madden-Julian Oscillation
- Description: The MJO is the biggest source of subseasonal (31-70 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 moderately active during October. It began the month over the Maritime Continent (phase 5) and then moved across the Pacific (phases 6/7) near the end of the month.
- Teleconnections (influence on weather): The MJO in Phases 5/6 initially enhances the La Niña teleconnections. However, as the MJO moves eastward in subsequent weeks, it can lead to cold anomalies over the Great Plains. The timing is faster than normal, but this propagation may have contributed to the pattern in the second half of October.
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 negative for most of October except for a strongly positive episode at the beginning of the month and a weaker positive event around October 9. The monthly average was negative. The primary wavetrain for the month featured a trough near Hawaii, a ridge near Alaska, a trough near Hudson Bay, and a ridge near the East Coast. This pattern is shifted eastward relative to a canonical negative PNA and was strongest during the second half of the month.
- Teleconnections (influence on weather): The negative PNA generally favors cooler temperatures along Pacific Northwest and warmer in the Southeast. The warmth over the Southeast was most apparent in the second half of the month. Consistent with the shifted wavetrain, the cold anomalies moved eastward to the Great Plains.
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 generally negative until around October 21 and positive thereafter. The amplitude was generally weak throughout October, so the monthly mean was near zero. The ridge near the North Pole would have favored a negative AO, but the ridges in the North Pacific and North Atlantic favored positive values. These competing signals essentially cancelled each other.
- Teleconnections (influence on weather): Temperatures over the Northern Plains are typically warmer than normal during a positive AO and cooler than normal during a negative AO. Neither of these were observed in phase with the AO during October.
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: Similar to the AO, the daily NAO was negative until around October 20 and generally positive thereafter. The negative values early in the month were stronger for the NAO than the AO, so the October mean NAO was negative. It was driven primarily by the ridge near Greenland and troughs over Europe and near the Hudson Bay. However, a persistent ridge over the North Atlantic weakened the NAO signal.
- Teleconnections (influence on weather): The NAO's teleconnections with the U.S. are generally weak during the Fall. During Winter, the negative NAO would have favored cooler than normal conditions over the Eastern U.S., as was observed during October 1-7.