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|
|September 14-30||Sally and Beta||EPO+, WPO+,
La Niña, AO+
The North American climate continued to be dominated by a ridge along the West Coast in September 2020. This ridge was associated with the positive phase of Pacific/North American pattern (PNA). It was somewhat unusual because La Niña continued to strengthen over the equatorial Pacific, and La Niña favors negative PNA patterns. The ridge persisted the anomalously hot and dry conditions that have remained over the Southwest since mid-April and fueled the fire season there. The ridge shifted eastward towards the end of the month allowing a pair of atmospheric rivers to bring rainfall to the Pacific Northwest. However, the broader ridge inhibited precipitation across the rest of the country, including the Southwest.
The key feature for the Central U.S. was a strong trough associated with a negative East Pacific Oscillation (EPO) around September 8-13. The trough extended from Hudson Bay to the Four Corners and brought very cold anomalies to the Great Plains and eastern Rockies during that period. This 6-day period drove the September mean temperature to be below-average for the Southern Plains. The trough also enhanced precipitation across the Central U.S. where it was generally dry for the remainder of the month.
The main climate driver for the eastern U.S. was a pair of tropical cyclones, Hurricane Sally and Tropical Storm Beta, which made landfall along the Gulf Coast in the latter half of the month. Sally in particular brought torrential rainfall from Alabama through Virginia with the heaviest amounts occurring in the Florida Panhandle. The rainfall from Beta was weaker and mainly fell over Texas and Louisiana. Beta was also notable as it was the ninth storm to make landfall along the contiguous U.S. in 2020, which ties 1916 for the most on record during a single season.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for September 2020
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for September 1-7 2020
A strongly positive PNA continued from the end of August into the beginning of September. The key feature of this PNA pattern was a ridge along the West Coast. That ridge continued the warm and dry conditions that have been in place over the Southwest since mid-April. The trough over the Upper Midwest brought cooler than normal conditions to that region, while the associated cold front created a swath of enhanced precipitation from Texas to the Mid-Atlantic.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for September 8-13 2020
The ridge moved northwestward towards Alaska during the second week of September as part of a strongly negative EPO. As a result, the warm anomalies along the West Coast weakened. The trough from the Midwest dug southwestward towards the Four Corners region bringing much colder than normal temperatures to the Great Plains and the eastern Rockies. Much of the region also saw above normal precipitation with that trough.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for September 14-30 2020
The trough over the Four Corners weakened during the middle of September, and the ridge moved back eastward over the Rockies. This ridge was weaker than the one earlier in the month, so the warm anomalies were not as strong. However, it was broader and the warm temperatures extended into the Northern Plains and the Upper Mississippi. This broad ridge suppressed precipitation across the contiguous U.S. The Pacific Northwest was wetter than normal due to two atmospheric rivers in the final days of the month, and the Southeast received heavy precipitation from Hurricane Sally and Tropical Storm Beta. The rest of the country was drier than normal under the ridge's influence.
- 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 strengthened during September 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These anomalies cooled to -0.8°C in September. The Southern Oscillation Index also remained strongly positive during September, which indicates that the atmosphere is coupled with the SST anomalies.
- Teleconnections (influence on weather): La Niña favors warmer and drier than normal conditions for the Central Plains and cool conditions along the West Coast. The Plains were particularly dry September 14-30, but the warmth was concentrated more towards the Northern Plains. During the remainder of the month, the warmth was primarily focused on the Western U.S. and to a lesser degree the Southeast.
- 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 generally weak during September and focused more towards phases 4 and 5 (Maritime Continent), primarily associated with the strengthening La Niña conditions.
- Teleconnections (influence on weather): The MJO's relatively weak strength during September limited its potential teleconnections. However, phases 4 and 5 (Maritime Continent) would typically favor warmer temperatures over the Central Plains, which mainly occurred September 8-13.
- 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 for most of September with the strongest positive values around September 1 and 26. It was briefly negative around September 8 and strongly negative on September 30. The positive monthly mean was primarily associated with the persistent ridge along the West Coast and to a lesser degree a trough near the Aleutians and an enhanced subtropical ridge over the Pacific. The positive PNA would typically also be associated with a trough over the Southeast, but that did not occur in September.
- Teleconnections (influence on weather): The PNA's teleconnections are relatively weak during September, but a positive PNA generally favors warmer temperatures along Pacific Northwest, which was apparent throughout the month.
- 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 positive during most of September with the strongest values around September 10 and 21. The monthly mean was positive due to the persistent trough near Greenland and the ridge over the North Atlantic. The stronger values September 8-13 were related to a ridge developing in the North Pacific as well.
- Teleconnections (influence on weather): A positive AO would typically be associated with the warmer than average temperatures over the Great Plains, but that was only observed September 14-30.
- 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 neutral at the beginning September, then strongly positive during September 7-21, but it became strongly negative by September 30. The monthly average was positive with the canonical trough over Greenland and ridge over the North Atlantic throughout the month.
- Teleconnections (influence on weather): The positive NAO during September typically favors warmer temperatures in the Northwest and cooler temperatures in the Southeast. That pattern was clear in the latter half of September even though the positive NAO weakened in final week.
- 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 oscillated around zero during September reaching its most negative value on September 10 and most positive value on September 20. The September average was near zero. The negative WPO around September 8-13 was associated with a trough near Kamchatka and a ridge near northern Japan. That pattern reversed in the latter half of the month when the WPO became positive.
- Teleconnections (influence on weather): The negative WPO is typically associated with cooler than normal temperatures over the Northern Rockies, which is consistent with the pattern observed September 8-13. Similarly, the positive WPO later in the month was consistent with the warm temperatures in that region.
- 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 strongly negative around September 8 and 30 and strongly positive around September 22. The average EPO was negative for September. The negative EPO was driven primarily by an extension of the ridge along the West Coast towards Alaska with a trough to its south, which was most apparent September 8-13. Later in the month, the ridge shifted eastward and the trough moved northward to Alaska, which led to the positive EPO the second half of the month.
- Teleconnections (influence on weather): The negative EPO favors cooler than normal temperatures across the Upper Midwest, which were observed September 8-13. Similarly, the positive EPO later in the month contributed to the warm temperatures there.