• ChatNRG
  • Posts
  • December Weather Cheat Sheet: La Niña alongside Different Teleconnections

December Weather Cheat Sheet: La Niña alongside Different Teleconnections

Moderate-to-Weak La Niña and Teleconnections such as NAO, WPA, EPO, PNA, and AO

November 20, 2024

If you have further questions, just ask the bot
eg) What is the impact of a weak La Nina state with -EPO of the Eastern US in December?

Why is ENSO Important for December Weather?

  • El Niño: December during El Niño years often features milder temperatures in the northern U.S. and stormier weather in the southern U.S.

  • La Niña: December during La Niña years brings colder conditions to the northern U.S., drier weather in the South, and active storm tracks in the Pacific Northwest.

Key Characteristics of a Moderate to Weak La Niña

A La Niña occurs when sea surface temperatures (SSTs) in the central and eastern tropical Pacific are cooler than average by at least 0.5°C. For a moderate to weak La Niña, these anomalies are less extreme, typically ranging from -0.5°C to -1.0°C, which results in milder effects compared to strong La Niña years.

General Weather Impacts of a Moderate to Weak La Niña

  1. North America:

    • Pacific Northwest: Wetter and stormier with frequent low-pressure systems.

    • Midwest to Northeast: Colder-than-average conditions, with increased snowfall potential, especially near the Great Lakes.

    • Southern U.S. (Texas, Gulf Coast, Southeast): Warmer and drier than average, raising drought risks.

    • West Coast: Storm tracks may shift northward, leaving California drier than normal.

  2. Global Impacts:

    • Australia & Southeast Asia: Wetter conditions with increased risk of flooding.

    • South America: Drier-than-average weather in regions like northern Brazil, with wetter conditions in southern South America.

    • Africa: Wetter in East Africa but drier in southern regions.

How La Niña Interacts with Other Teleconnections

1. Interaction with the Pacific-North American Pattern (PNA):

  • -PNA (Typical Phase):

    • La Niña favors troughing in the West and ridging in the East, leading to stormy, wet conditions in the Pacific Northwest and milder, drier conditions in the Southeast.

    • During a weak/moderate La Niña, the negative PNA signal may be less dominant, allowing occasional cold air outbreaks into the eastern U.S. if other factors (e.g., AO or EPO) align.

  • +PNA (Opposing Phase):

    • If a positive PNA occurs, it can override the La Niña signal, leading to a ridge in the West and a trough in the East, amplifying cold outbreaks and snow in the central and eastern U.S.

2. Interaction with the Arctic Oscillation (AO):

  • +AO (Typical Phase)

    • La Niña often supports a stronger polar vortex in the Arctic, aligning with a positive AO that keeps Arctic air trapped at higher latitudes.

    • A moderate La Niña may occasionally weaken this relationship, allowing the AO to swing negative if other factors (e.g., Sudden Stratospheric Warming events) occur.

  • -AO:

    • A negative AO disrupts the polar vortex, allowing Arctic air to spill southward, which can intensify La Niña's cold impacts in the northern U.S.

3. Interaction with the North Atlantic Oscillation (NAO):

  • +NAO (Typical Phase)

    • La Niña often aligns with a +NAO, which keeps a strong jet stream over the North Atlantic, favoring milder, wetter winters in northern Europe and the eastern U.S.

  • -NAO:

    • A -NAO can override La Niña’s milder East Coast influence, creating blocking patterns that bring colder and snowier weather to the eastern U.S. and parts of Europe.

4. Interaction with the Eastern Pacific Oscillation (EPO):

  • -EPO (Typical Phase):

    • A -EPO is common during La Niña, with high pressure over Alaska forcing the jet stream southward. This amplifies cold air outbreaks in the northern and eastern U.S.

  • +EPO:

    • If a +EPO arises, it can counteract La Niña's cold signal, keeping Arctic air confined to the north and leading to milder conditions in the lower 48 states.

5. Interaction with Sudden Stratospheric Warming (SSW):

  • +WPO (Typical Phase):

    • During La Niña, the WPO often tends toward a positive phase, which aligns with the stronger trade winds and enhanced Walker Circulation typical of La Niña.

    • This supports a stable polar vortex and limits Arctic air intrusions into North America.

  • -WPO:

    • If the WPO turns negative during a La Niña winter, it disrupts the polar vortex and enhances the likelihood of Arctic outbreaks, particularly in the western and central U.S.

    • A -WPO can amplify La Niña’s cold air impacts in the northern U.S. and strengthen storm tracks in the Pacific Northwest.

6. Interaction with Sudden Stratospheric Warming (SSW):

  • SSW Events:

    • While not directly caused by La Niña, SSW events can disrupt the polar vortex, leading to prolonged cold spells in the mid-latitudes (e.g., eastern U.S., Europe).

    • During a La Niña winter, an SSW event can amplify the cold impacts already associated with a negative AO or negative EPO.

  • Without SSW:

    • In the absence of SSW, La Niña’s moderate-to-weak signal typically supports a more stable polar vortex, reducing the likelihood of severe Arctic outbreaks.

Key Takeaways for December 2024 with a Moderate-to-Weak La Niña:

  1. Dominant Weather Features:

    • Colder, wetter weather in the Pacific Northwest and northern U.S.

    • Warmer, drier conditions in the southern U.S.

    • Enhanced snow potential in the northern U.S., particularly the Midwest and Northeast.

  2. Teleconnection Interactions:

    • -EPO and/or -PNA reinforce La Niña’s cold impacts in the northern and eastern U.S.

    • -AO or -NAO can bring significant Arctic air outbreaks and snowstorms to the eastern U.S., overriding La Niña’s milder tendencies in the East.

    • +PNA or +EPO can weaken La Niña’s influence, keeping conditions milder in the eastern U.S.

  3. Uncertainty Factors:

    • The potential for SSW events could dramatically shift conditions in mid-to-late December, increasing Arctic air intrusions regardless of the ENSO signal.