Synoptic Discussion - August 2020
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|
|Month||PNA+||La Niña, MJO
||NAO+, Hurricane Isaias
||La Niña, MJO, PNA+
||La Niña, MJO, Hurricane Laura
The primary climate driver during August was the positive phase of the Pacific/North American pattern (PNA) associated with a subtropical ridge in the Pacific, a trough in the Gulf of Alaska, the ridge in the western U.S., and the trough in the East. A positive PNA during the summer is typically associated with warmer conditions on both coasts, which happened in August. The positive PNA is also strongly tied to the La Niña conditions that developed during August, which often leads to more frequent positive PNA episodes. These effects of La Niña on the PNA were in turn modulated by an active Madden-Julian Oscillation (MJO). Both the MJO and the La Niña aligned in the second half of the month to provide favorable conditions for Atlantic tropical cyclone activity, including Hurricanes Marco and Laura.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for August 1-7 2020
August began with a typical positive PNA circulation that featured a subtropical ridge in the Pacific, a trough in the Gulf of Alaska, a ridge over the western U.S. and a trough in the East. Even though the western ridge was only moderately stronger than normal, it still contributed to above-normal temperatures throughout the Rockies and dry conditions through the western U.S. In the Southwest, this dry pattern has been in place since mid-April and would persist through August. Farther east, the trough was associated with below-normal temperatures in the Central Plains. Hurricane Isaias made landfall in the Carolinas on August 4 and brought heavy rain from the Mid-Atlantic to the Northeast.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for August 8-14 2020
The trough in the Gulf of Alaska shifted eastward to western Canada during August 8-14. It brought cooler temperatures to the Northwest, but it was too far north to bring any needed moisture to the western U.S. The ridge also shifted eastward, perhaps in response to a positive WPO. The warmest temperature anomalies were near western Texas and New England.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for August 15-21 2020
The positive PNA pattern was the primary driver again during August 15-21. The western ridge was strongest and farthest south during this period, which led to intense heat for the Southwest. Parts of Central California were more than 15°F above normal during this period, which exacerbated ongoing fires in the region. It also contributed to the high temperature of 130°F in Death Valley. If verified, this would be warmest August temperature on record and the third-warmest temperature on record for any month across the U.S. The trough in the East was associated with milder temperatures there, as well as anomalously wet conditions over the Southern Atlantic Coast.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for August 22-31 2020
The positive PNA strengthened again towards the end of August even if the pattern was somewhat weaker when averaged over August 22-31. It continued to fuel warmer than normal temperatures in the West, particularly for the Southern Rockies. Some much needed precipitation fell over portions of the Rockies, although California remained mostly dry. Above normal temperatures and precipitation were also observed around the Upper Mississippi Valley. Meanwhile, the Lower Mississippi received deluges from the back-to-back landfalls of Hurricanes Marco and Laura in Louisiana on August 24 and 27, respectively.
- 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 developed during August 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region increased to -0.6°C in August. The Southern Oscillation Index also increased rapidly in August, which indicates that the atmosphere is coupling with the SST anomalies. These developments combined with the expectation for La Niña conditions to continue through the winter prompted the Climate Prediction Center to issue a La Niña Advisory in early September.
- 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 dry conditions for the Central Plains were present during most of the month while the warmth was most pronounced during August 8-14 and 22-31. The West Coast was generally warm in August, but it was relatively cooler somewhat during August 8-14.
- 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 active during most of August beginning the month over the Maritime Continent (phase 4) and ending over the Indian Ocean (phase 2). It weakened somewhat as it crossed the western Pacific August 10-14 but strengthened again once it reached the Western Hemisphere in the middle of the month.
- Teleconnections (influence on weather): The MJO's in phases 4 and 5 (Maritime Continent) at the beginning of the month favored the warmer temperatures in the Central Plains and relatively cooler temperatures in the West Coast that occurred August 8-14. Similarly, the phase 8 (Western Hemisphere) in the middle of the month likely contributed to the warm anomalies in the Southwest during August 22-31. The MJO also strongly effects tropical cyclone activity, and the phase 8 was probably associated with the enhanced activity in the second half of the month over the eastern Pacific and the Atlantic.
- 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 the entire month of August. It was strongest August 1-12 and 27-31. The monthly mean was strongly positive. The circulation in August strongly resembles the PNA loading pattern with a ridge north of Hawaii, a trough in the Gulf of Alaska, a ridge over the West Coast, and a trough over the Southeast.
- Teleconnections (influence on weather): The PNA's teleconnections are weaker during the summer, but a positive PNA generally favors warmer temperatures along both the East and West Coasts. Both of these were 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 weak but generally negative for most of the month. The monthly mean was also negative but weak. The negative AO was driven primarily by a strong ridge near the North Pole and weaker troughs in the North Pacific and North Atlantic. Its amplitude was limited, however, by the ridges in the Central Pacific and Central Atlantic.
- Teleconnections (influence on weather): A negative AO would typically be associated with cooler than average temperatures in the Northern Plains and warmer than average temperatures near Texas. Texas was warm throughout the month, but the Northern Plains were cool only during August 1-7 when the AO was weakest.
- 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 moderately positive during the first half of August and moderately negative during the second half. The monthly mean was near zero. These were primarily associated with a trough over Greenland early in the month that later gave way to a ridge.
- Teleconnections (influence on weather): The positive NAO during August typically favors warmer temperatures in the Northern Rockies and cooler temperatures in the Southeast with the opposite happening during a negative NAO. The positive NAO pattern was generally observed August 1-7, but the negative pattern was less apparent.
- 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 moderately positive most of the month as was the monthly mean. The positive WPO was associated with a ridge near the Sea of Japan and a trough near Kamchatka. The trough was persistent throughout the month, but the ridge was strongest in the second half of August.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer than normal temperatures over the Northern Plains and cooler temperatures in the Southwest. That pattern was most apparent during August 1-7, and the WPO was weak but positive during August 1-6.
- 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 moderately positive until around August 10 and then was near zero for the rest of the month. The monthly average was moderately positive due primarily to the trough in the Gulf of Alaska, which was strongest during August 1-7. Its amplitude was limited by the lack of a strong subtropical ridge in the eastern Pacific and a ridge in northern Alaska during some parts of the month.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the Upper Midwest and Northeast. That pattern was most apparent August 8-14 although the EPO was already weakening at that time.