EF

oceanlecture11enso2025

Upwelling and El Niño

El Niño Events

Notable El Niño events reported:

  • November 1977

  • February 1999

La Niña indicates cooler ocean conditions and can appear after an El Niño. Laguna Niguel, CA, 1998: Key geographical location referenced, potentially in relation to upwelling or climate events occurring at that time.

Ekman Transport

Review of key concepts:

  • Balance between wind stress and Coriolis effect.

  • Net water flow is 90° to the right of the wind in the Northern Hemisphere.

  • Motion typically extends to ~100 m depth due to friction.

I. Upwelling

Upwelling refers to the upward vertical flow of water, replacing displaced surface waters with deeper waters, generally from 100-200 m depths. Upwelling waters are cold and nutrient-rich (phosphorus and nitrogen). Nutrient abundance enhances biological productivity, especially algal growth.

Notable Upwelling Zones

Areas significant for upwelling include:

  • Subpolar gyres (discussed previously in lectures).

  • Antarctic Circumpolar Current (ACC): deep upwelling noted in earlier discussions.

  • Equator regions.

  • Coasts, especially the eastern boundaries of subtropical gyres.

Coastal Upwelling

Equatorward winds generally parallel the eastern boundaries of subtropical gyres. This leads to Ekman transport away from the shore, causing surface waters to be replaced by colder, subsurface waters.

Coastal Upwelling: California

Data showing chlorophyll and sea surface temperature in Central California on June 12, 1993, highlighting temperature variations and possible upwelling influence.

Cold Tongue Sea Surface Temperatures

Equatorial upwelling highlighted by diverging surface waters near the equator. Due to Coriolis having opposite effects in each hemisphere.

Earth’s Climate Patterns

Idealized Flow:

  • Symmetrical atmospheric cells.

Climatology:

  • Average climate effects related to land and seasonal changes.

Interannual Oscillations:

  • Interaction between winds and oceans can lead to persistent deviations from typical climatology.

II. El Niño-Southern Oscillation (ENSO)

ENSO represents significant natural interannual weather and climate variations globally. Driven by changes in coupled atmosphere-ocean circulation, especially over the equatorial Pacific.

A. Equatorial Pacific Mean State (Climatology)

Characterization of ocean temperatures:

  • Cold tongue forms where cold waters upwell in the eastern Pacific.

  • On the equator, water flows in the direction of the wind, with negligible Coriolis effect.

  • Warm waters accumulate in the western equatorial Pacific, creating a warm pool.

Monthly Mean Ocean Temperatures (°C)

East-west cross-section detailing Pacific water temperature at varying depths, illustrating significant temperature gradients from surface to deep water.

Atmospheric Dynamics

Walker Circulation:

  • Zonal airflow over the equator with significant strength over the Pacific.

  • Winds flow from high to low pressure areas, with negligible Coriolis effect.

  • Positive feedback observed with sea surface temperatures affecting wind patterns.

C. Southern Oscillation

Varied terrestrial and oceanic conditions indicated by oscillations in atmospheric pressure differences (SO Index) between Tahiti and Darwin, Australia. Periodicity of oscillation: approximately every 2-7 years.

D. ENSO Events Definition

Events categorized based on the persistence of warm or cold anomalies influenced by oceanic conditions. Previous significant El Niño events: 1982-83, 1997-98, 2015-16.

E. Global Air Temperature Changes

El Niño effects on global air temperatures: Typically raises Earth's average air temperature. La Niña and neutral conditions facilitate greater heat uptake by the cooling ocean.

F. Teleconnections

Climate effects experienced in distant regions, transmitted through atmospheric changes. Example: In the Southwest US, El Niño years are associated with wetter winter/spring conditions, while La Niña years may result in drought.

ENSO Prediction (Forecasting)

Prediction models based on current conditions demonstrate probabilities of ENSO states (La Niña, Neutral, and El Niño) throughout seasons. NOAA Climate Prediction: Atmospheric circulation patterns consistent with ongoing El Niño events. Expectation of transitioning to ENSO-neutral conditions in upcoming seasons.

Depth Profiles and Thermohaline Data

Information on deep ocean circulation characteristics described in terms of temperature and salinity across various depths. Insights into the distribution of ocean properties and how they may influence larger climatic events.