December 2023 Monthly Synoptic Discussion

December 2023 Synoptic Discussion | National Centers for Environmental Information (NCEI)

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)
- December 1-7
Madden-Julian Oscillation (MJO) over the Maritime Continent
- Negative AO
- Positive WPO
- December 8-24
- Positive AO
- Positive WPO
- Positive North Atlantic Oscillation (NAO)
- December 25-31
- Positive Pacific/North American (PNA) pattern
- Negative WPO
- Negative AO
- Positive

Summary

The North American circulation was heavily influenced by a large and persistent ridge located over much of the U.S. and Canada. The ridge was centered over central Canada, but shifted east towards Ontario and the upper U.S. Midwest and northeast towards the end of the month. The negative AO at the beginning and end of the month likely influenced the more dynamic weather patterns across the U.S. during those periods. Broad ridging occurred over the U.S. during the middle portion of the month, with warmer than normal temperatures across most of the U.S. as a result. This corresponded with a positive AO during the same period.

Submonthly Evolution

December 1-7

During the early part of December, there was a dynamic synoptic pattern, with a ridge moving across much of the United States. This led to warmer than normal conditions on average across the U.S. during the first week of the month. A negative AO was experienced during the early part of the month, which is typically associated with more ridges and troughs, which was observed. Precipitation was limited across most of the central U.S., but there was enhanced precipitation over the Pacific Northwest during the first week of December.

December 8-24

During the middle part of December, a large and persistent ridge developed over the central U.S. and Canada. This ridge led to much warmer than normal conditions across most of the United States throughout the middle of the month. This is associated with the positive AO that was observed during the same period. Precipitation was enhanced over the East Coast of the U.S. and also across the southern and central plains.

December 25-31

The pattern at the end of December was dominated by the large and persistent ridge shifting east over Ontario and the upper Midwest/New England. This led to much warmer than normal conditions occurring over the upper Midwest and New England during the end of the month. As was the case at the beginning of the month the negative phase of the AO returned. The end of December also was characterized by a positive WPO and PNA. Much of the western half of the United States experienced very little precipitation during the final week of December, whereas storm systems led to precipitation occurring over the northern Great Plains and the east coast.

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 December. The most common metric for ENSO is the SST anomalies in the equatorial Pacific. These strengthened to around +2.0°C during December. The Southern Oscillation Index, which measures the atmospheric response, remained negative. 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. The cool anomalies were mostly offset by the strong ridging, except over the southeast U.S. It is also associated with wetter conditions for the southeastern U.S. and drier conditions for the northern U.S. This pattern was observed during most of December.

MJO index for December 2023. Each dot represents the value for a given day with select dates labeled in red.

MJO index for December 2023. Each dot represents the value for a given day with select dates labeled in red.

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 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 December in phase 3-4 (Maritime Continent) and nearly circumnavigated the globe moving eastward to phase 1-2 (Africa) by the end of the month.
- Teleconnections (influence on weather): During December, phases 3, 4, and 5 of the MJO favor warm anomalies near the Missouri Valley and eastern U.S. Phase 1 is associated with wet conditions near the Missouri Valley. Neither of these patterns were apparent during December.

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 weak during most of December with positive values in the late part of the month. The monthly mean was positive.
- Teleconnections (influence on weather): The positive PNA generally favors warmer than normal conditions for the Pacific Northwest and cooler for the Southeast. Cooler-than-normal temperatures in the Southeast occurred during the positive PNA pattern, which was observed near Texas December 25-31.

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 was negative at the beginning and end of December with positive values during the middle of the month. The monthly mean was near zero.
- Teleconnections (influence on weather): The positive AO is typically associated with warm anomalies near the northern Plains, which were observed December 8-24. The cool anomalies over the northern Plains typically seen with negative AO were not observed at the beginning or end of the month. 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 middle part 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 negative at the beginning of the month and was neutral/positive thereafter. The monthly mean was positive.
- 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 last three weeks of the month.

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 positive for the majority of December, with a short exception in early December. The monthly mean was strongly positive.
- 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 much of the second half of December. The positive WPO is also associated with wetter conditions across the Missouri Valley. The positive WPO may have contributed to the rainfall over the central U.S. during the latter part of December.

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 was strongly positive early in the month and oscillated between positive and negative during the rest of the month. The monthly mean was positive.
- 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 towards the middle and late part of the month.

Citing This Report NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for December 2023, published online January 2024, retrieved on January 13, 2024 from

https://www.ncei.noaa.gov/ access/ monitoring/ monthly-report/ synoptic/ 202312.