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.
Overview
Key Drivers
- Month
- El Niño
- Oscillating Arctic Oscillation (AO)
- Positive West Pacific Oscillation (WPO)
-
February 1-13
- Positive WPO
- Positive Pacific/North American (PNA) pattern
- Negative AO
-
February 14-19
- Positive PNA
- Oscillating AO
- Negative East Pacific Oscillation (EPO)
-
February 20-29
- Positive AO
- Positive North Atlantic Oscillation (NAO)
- Positive WPO
- Positive EPO
Summary
The North American circulation at the beginning of the month was characterized by strong ridging that brought very warm conditions across large swaths of the U.S. Some ridging also returned at the end of the month, which continued to lead to very warm conditions, especially over the eastern half of the U.S. During the middle of the month, a transient trough brought cooler and more dynamic weather conditions across the country. The warmer temperatures that prevailed across much of the U.S. were likely associated with a positive AO during the beginning and end of the month. The positive WPO that persisted throughout the month likely also had some influence on the warm weather conditions.
Submonthly Evolution
February 1-13
During the first two weeks of February, there was a strong ridge located over much of the northern and eastern U.S. that was centered over Canada. As a result, there were unusually warm temperatures over much of the northern and eastern U.S. Ridging is typically experienced in these regions when the WPO is positive and the AO is positive, and both were observed during the early part of the month. Enhanced precipitation occurred across the southwestern U.S., with drier conditions over the northern U.S. during the early part of February.
February 14-19
During the middle part of February, atmospheric drivers were much more dynamic, leading to more transient weather patterns being experienced across much of the country. The AO was oscillating during this part of the month, which could explain some of the more dynamic conditions experienced. As a result, the overall temperature signals during the middle of the month were much more muted than during the early and late parts of the month. Dry conditions were experienced over the southern U.S. with some precipitation occurring over the northern and western U.S.
February 20-29
The overall temperature pattern at the end of February mirrored the pattern seen during the early part of the month with warmer than normal temperatures returning to much of the U.S., especially from Texas to the Midwest. A ridge over the southwestern U.S. which extended into Mexico and another ridge extending from the Canadian Maritimes helped to facilitate the warmer weather patterns experienced across the country. There was once again a positive AO during the latter portion of the month, which is associated with warm conditions over the Midwest and eastern U.S. The end of February also was characterized by a positive WPO. Much of the U.S. was drier than normal during the end of February, though some precipitation did occur over the western U.S. and the Northeast.
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: El Niño conditions continued during February. The most common metric for ENSO is the SST anomalies in the equatorial Pacific. These values were around +1.8°C during February. The Southern Oscillation Index, which measures the atmospheric response, was around -1.4. Together, the coupled ocean-atmosphere system was consistent with El Niño.
- Teleconnections (influence on weather): El Niño favors cooler temperatures for most of the contiguous U.S. Ridging offset these conditions during the majority of the month as February was much warmer than average across the U.S. El Niño is also associated with wetter conditions for the southeastern U.S. and the West Coast along with drier conditions for the northern U.S. This pattern was observed during the beginning of February.
MJO: Madden-Julian Oscillation
- Description: The MJO is the biggest source of subseasonal (20-60 day) tropical variability. It typically develops as a large envelope of tropical thunderstorms 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. The MJO index identifies both the MJO's amplitude and the longitude of its strongest rainfall. MJO amplitude is represented by the distance from the center of the figure with values outside the circle are considered active. The eight phases around the circle indicate the MJO's location.
- Status: The MJO index began February in phase 7 and remained there until the middle part of the month, after which the MJO was weak.
- Teleconnections (influence on weather): During February, phase 7 of the MJO does not have a significant influence on temperature patterns over the U.S. Phase 7 is associated with wet conditions near Missouri and Wisconsin; which was not strongly observed during the early part 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 weakly positive during the first two thirds of February with negative values in the late part of the month. The monthly mean was near zero.
- Teleconnections (influence on weather): The positive PNA generally favors warmer than normal conditions for the Pacific Northwest and cooler for the Southeast. These patterns were generally not observed during February. The positive PNA also favors drier than normal conditions for the Pacific Northwest and east of the Mississippi River. These conditions were partially observed during the first two thirds of the month.
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 index began the month strongly positive, but quickly shifted to being strongly negative until the final half of the month. During the final half of the month, the AO returned to being strongly positive. The monthly mean was 0.7.
- Teleconnections (influence on weather): The positive AO is typically associated with warm anomalies over the central and eastern U.S., which were observed during early and late February. During the middle part of the month, more normal temperature conditions were experienced over large portions of the U.S., which was likely related to the shift to a negative AO during the same period. There were also drier than normal conditions across much of the southern U.S. corresponding to the negative AO, which favors drier conditions over the desert Southwest and the Lower Mississippi River Valley. The positive AO is also associated with dry conditions for the southern and eastern U.S. It also favors wet conditions for the Pacific Northwest. These patterns were observed during the end of the month.
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 index was weakly positive for most of the first half of the month and was more strongly positive during the second half of February. The monthly mean was 1.4.
- Teleconnections (influence on weather): The NAO is positively correlated with temperatures for the eastern U.S. The positive NAO was consistent with the warm conditions seen over much of the U.S. during the month of February.
WPO: West Pacific Oscillation
- 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 strongly positive during the majority of February. The monthly mean was 1.3.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer temperatures near the Ohio Valley and the Northeast. This pattern was observed during the beginning and end of February. The positive WPO is also associated with wetter conditions across the Mid-Mississippi to Ohio Valleys. The positive WPO may have contributed to the rainfall over the central and southeastern U.S. during the first two weeks of the month.
EPO: East Pacific Oscillation
- 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 oscillated between positive and negative during the first third of the month, was strongly negative during the middle third of the month, and strongly positive at the end of the month. The monthly mean was weakly negative.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the northern and eastern U.S. The observed patterns matched the positive EPO patterns across the Upper Midwest to the Northeast during the end of the month.
