El Nino Southern Oscillation (ENSO)

Flashcards about ENSO

ENSO

• Largest pattern of climate variability on the planet;

• prime example of ocean and atmosphere interaction

• leading to near global alterations to rainfall, winds, pressure, and temperature in tropics and beyond.

• Vallis, 2011

Climate variability

• A feature of complex systems that can be unforced, arising through interactions within the system without external forcing.

• does not require external forcing

• Vallis, 2011

Tropical Pacific Mean State

• Characterised by higher SSTs in the western Pacific near Australia and lower SSTs in the eastern Pacific near the Americas, with a gradient between them.

• Vallis, 2011

West Pacific Warm Pool

• The region of warm temperatures in the western Pacific.

• Vallis, 2011

Subtropical Anticyclones

• Air spinning clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere near surface trade winds that converge in the Intertropical Convergence Zone.

• Baede et al., 2018

Walker Circulation

• Linked to the gradient in SST across the Pacific, contributing to high rainfall, especially near the Maritime continent.

• Baede et al., 2018

Thermocline

• The transition zone below the surface separating mixed warm ocean water with cold deep ocean

• deeper in the west and shallower in the east

• associated with upwelling of cold ocean water.

• Baede et al., 2018

La Nina

An exacerbation of normal conditions over the Pacific, with the western Pacific getting warmer and the eastern Pacific getting colder, linked to a stronger Walker circulation.

leads to stronger upwelling in the eastern Pacific and deeper thermocline in the western pacific

Vallis, 2011

El Nino

• A phase where warm water spreads out over the central Pacific, leading to cooler temperatures in the west and a more equalized thermocline across the Pacific.

• warmth not concentrated in the west +thermocline no longer steep

• Vallis, 2011

Sea Surface Temperature Anomalies

• linked to big reduction in the upwelling in the Pacific

• Vallis, 2011

Thermocline Depth Anomalies

• Variations in the depth of the thermocline across the Pacific, with a shallower gradient during El Nino and a stronger gradient during La Nina.

• Vallis, 2011

Pressure Anomalies

• Weakened low-pressure region over the west Pacific (positive anomaly) during El Nino and negative anomalies in the eastern Pacific.

• pressure gradient is what’s driving these winds

• Vallis, 2011

Southern Oscillation Index (SOI)

• A measure of the difference in surface pressure between Darwin and Tahiti used to track ENSO events

• Positive SOI values tend towards La Nina, while negative values tend towards El Nino.

• Wang et al., 2012

TAO/TRITON Array

• A set of 70 moorings in the tropical Pacific that measure wind speed and direction, relative humidity, air temperature, and ocean temperatures at various depths to track the evolution of the tropical Pacific Ocean state.

• Wang et al., 2012

Multivariate ENSO Index (MEI)

A more comprehensive measure of the atmospheric-oceanic alterations in the ENSO cycle

Determined based on six variables: SLP (sea level pressure, surface zonal winds, surface meridional winds, SSTs, surface air temperature, and cloudiness

Wang et al., 2012

Teleconnections

• Remote impacts of ENSO, connecting climate anomalies in one place to another.

• Vallis, 2011

Global impacts of ENSO

• Tend to be higher in El Nino years/months and lower in La Nina years/months.

• El Niño years temperatures tend to grow throughout the event and La Niña

• less rain over Australia, More rainfall over South America

• Vallis, 2011

Case study: ENSO

• 2003-4 severe monsoon for India predicted

• but instead there was a drought

• Kumar et al., 2006

ENSO and Walker Circulation

• ENSO alters the east-west Walker circulation across the globe, controlling the strength and location of subsiding and rising air near the equator.

• Vallis, 2011

ENSO and Hadley Circulation

• Excess latent heating near the equator during El Nino can intensify uplift, resulting in a more vigorous Hadley circulation.

• Vallis, 2011