Atmospheric Dynamics and Seasonal Impacts of El Nino and La Nina

Typical Winter Configurations: Under normal or "ENSO-neutral" conditions, North America is primarily influenced by three major jet streams:
- The Subtropical Jet Stream.
- The Pacific Jet Stream.
- The Polar Jet Stream.
Geographic Influence of the Pacific Jet Stream: On average, the Pacific Jet Stream is directed toward the Pacific Northwest.
- This positioning explains why the Pacific Northwest typically receives significantly higher annual rainfall than California.
- While the jet stream occasionally migrates south to bring storms into Northern and Southern California, the general climatic state of California is drier than the Northwest.
- In recent years (relative to the lecture recording), California has been in a sustained drought, making the state drier than historical averages.
Other Jet Streams: While the Pacific Jet Stream is the primary driver for West Coast weather, the Subtropical and Polar jet streams predominantly affect the rest of the United States.

Southward Shift: During a typical wintertime El Niño pattern, the Pacific Jet Stream moves south of its neutral position.
- Instead of tracking storms into the Pacific Northwest, the jet stream directs them toward Southern California, the Southern United States, Mexico, and the Gulf of Mexico.
- This shift results in this entire southern region becoming exceptionally wet.
Regional Variability in El Niño:
- In cases of a "Big Niño" (very strong event), the jet stream may sit slightly further north than in a moderate event, potentially affecting more of Northern California and the Northern United States.
- However, the Pacific Northwest typically remains outside this track during El Niño, causing that region to become warmer and drier than normal due to the lack of storm tracking.
Jet Stream Intensity: The Pacific Jet Stream during El Niño is not just repositioned; it is also physically stronger and characterized by faster wind speeds.

The Atlantic Basin: An El Niño year is generally considered unfavorable for Atlantic hurricane development.
- Wind Shear: The strong Pacific Jet Stream cutting across the United States and into the Atlantic creates significant "wind shear."
- Thunderstorm Disruption: Hurricanes are composed of a series of thunderstorms rotating around central low pressure. Normally, thunderstorms develop vertically with gentle surrounding winds.
- In El Niño years, the high-velocity winds of the jet stream destroy the vertical development of thunderstorms, preventing them from taking hold and forming hurricanes.
The Pacific Basin: conversely, the Pacific Ocean often sees increased hurricane activity during El Niño.
- Tropics in the Pacific experience warmer waters, which fuel storm development.
- The disruptive wind shear seen in the Atlantic is not present south of the United States in the Pacific, allowing storms to intensify.

The Misconception of Continuous Rain: Public perception often equates El Niño with constant rain. However, dry spells are a normal and documented occurrence even during very strong events.
Historical Data on California Dry Spells:
- 1982–1983 El Niño: Recorded a $23\text{-day}$ dry spell in the winter. Despite this, the season was famous for massive rainy periods, flooding, and substantial damage.
- 1997–1998 El Niño: Recorded an $18\text{-day}$ dry spell. This year is remembered as a "very good year" for California rainfall, demonstrating that dry spells do not signify the failure of the El Niño pattern.
- 2015–2016 El Niño: (At the time of recording) The instructor advises against interpreting current dry spells as a problem, as historical data suggests more rain will follow before the event concludes.

Formation Sequence: La Niña events do not occur in isolation. They typically follow an El Niño event, though it is not a guaranteed progression.
- Possible sequences include: Normal → El Niño → La Niña OR Normal → El Niño → Normal.
- Science does not yet fully understand why La Niña only follows El Niño and never a normal year directly.
Northward Jet Stream Shift: During La Niña, the Pacific Jet Stream moves further north than its normal position.
- Pacific Northwest Impact: The region becomes much wetter than usual. The jet stream becomes more vigorous, leading to flooding because the local geology cannot manage the excess water volume beyond normal high-rainfall conditions.
- California Impact: Northern California may experience some of this increased precipitation depending on the tracking variability. However, Southern California usually remains dry during La Niña (which is consistent with its natural state as a dry region).

Temperature Gradients: In La Niña conditions, the Equatorial Pacific has cooler water and air.
- Arctic regions (near Alaska) remain cold.
- The temperature difference (gradient) between the tropics and the Arctic is less extreme than usual because the tropics are cooler than normal.
Pressure and Wind Speed:
- A smaller temperature gradient results in a less steep pressure gradient.
- Result: This creates a weaker, slower-moving jet stream.
- Because the storms within this jet stream move more slowly, they have the capacity to "dump" more water in specific locations, contributing to the increased flooding observed in the Pacific Northwest during La Niña.