# Presentation: 796586

## ENSO Coupled Ocean-Atmosphere Dynamicsand its Representation in Coupled Climate Models

- _Antonietta Capotondi_
- NOAA/ Physical Science Division
- University of Colorado/Cires/CDC

## Outline

- Review coupled ocean-atmosphere interactions associated with ENSO events
- Introduce simple dynamical models used to explain the oscillatory nature of ENSO (_delayed_ and _recharge_ oscillators)
- Examine how ENSO is represented in state-of-the-art climate models, with focus on the temporal structure of ENSO.

## El Niño-Southern Oscillation (ENSO)

- Originally detected as an unusual warming (El Niño) or cooling (La Niña) of the ocean along the coast of Peru, occurring every 3-6 years during boreal winter.
- Natural phenomenon in which the system oscillates between warmer-than-average to colder-than-average conditions.

## El Niño-Southern Oscillation (ENSO)

- Warming of the surface water is associated with variations in precipitation, surface winds, and sea level pressure. The _Southern Oscillation_ is the pressure difference (see-saw) between Tahiti and Darwin (central-western tropical Pacific_x0004_) which varies during El Niño/La Niña events.
- ENSO is an ocean-atmosphere coupled phenomenon: changes in the ocean induce changes in the atmosphere, and the latter feed back to the ocean.

## Tropical Pacific mean conditions

- Warm pool

- Cold tongue

## Vertical temperature section along the EquatorTropical Atmosphere Ocean (TAO) data (1980-2000)

- Depth

## Sea Surface Temperatures

**Equatorial cold tongue is weaker than average or absent during El Niño, resulting in positive SST anomalies**

**Equatorial cold tongue is stronger than average during La Niña, resulting in negative SST anomalies**

## Standard deviation of interannual SST anomalies

- Standard deviation of x(t)=

## El Niño cycle 

- The changes in SST affect the distribution of tropical rainfall and atmospheric circulation features

## Southern Oscillation – Low Index Phase (El Niño)

**High**

- Equatorial Easterlies – weaker than average

**Low**

**Reduced Precipitation**

**SLP – below average**

**T**

**D**

**SLP – above average**

**Enhanced Precipitation**

## Ocean adjustment to wind changes

- The ocean adjusts to wind changes through wave propagation: equatorial and extraequatorial (Rossby) waves.
- Equatorial waves include:
- _Kelvin waves_. They propagate eastward along the equator
- _Rossby waves_. They propagate westward

## Ocean adjustment

- Wind anomaly

- Kelvin waves propagate eastward from the area of wind anomalies and make the thermocline become deeper in the east. This is a positive feedback known as Bijerknes feedback.
- Rossby waves propagate westward, and reflect at the coast as eastward propagating Kelvin waves, providing a negative feedback.

## Ocean adjustment through wave propagationThe “delayed oscillator” paradigm

- Evolution of thermocline depth along the equator

## The “delayed oscillator” paradigmThe role of off-equatorial waves

## The “reduced-gravity” model

- h

*(u,v*)

- Momentum equations:

- p = pressure, F=forcing,D=dissipation

- Continuity equation:

- =mean density, f=Coriolis paarameter

## The “recharge oscillator”:A simple paradigm for the El Niño cycle

## Does the real ocean behave as a “recharge oscillator”?Empirical orthogonal function analysis of subsurface temperature data (Meinen and McPhaden 2000)

## Models

- CCSM3  (USA)
- GFDL-CM2.0 (USA)
- GISS-EH (USA)
- PCM (USA)
- IPSL-CM4 (France)
- CNRM-CM3 (France)
- UKMO-HadCM3 (UK)
- MRI-CGCM2.3.2 (Japan)
- CSIRO-CM3 (Australia)
- _Ocean analyses:_
- GFDL/ARCs (1980-2000)
- INGV (1958-2000)

- Models have different grids and resolutions for both oceanic and atmospheric components.

## ENSO in coupled climate modelsSST standard deviations

- All the models have SST variations which are displaced too further west. Some models have very little variations along the eastern coast.

## Niño Regions

## Thermocline VariabilityDepth of 15°C isotherm (Z15)

## EOFs of Z15

## EOFs of Z15

- PCM

- MRI

- INGV

- GISS-EH

## EOFs of Z15

- UKMO-HadCM3

- CSIRO

- CNRM

- INGV

## Phase relationship between the Z15 modes

- INGV

- CCSM3

- UKMO-HadCM3

- PC2-PC1 lag-correlation

## ENSO in coupled climate models 2Evolution of the Niño3.4 index

## Spectral characteristicsNino3.4 index (5°S-5°N, 170°W-120°W)

## Wind forcing

- _Meridional width of the wind stress anomalies_: if the wind stress extends over a broader range of latitudes the adjustment timescale of the equatorial ocean increases.
- _Longitudinal position of the wind stress anomalies_: it influences the zonal advective feedback. Eastward displacement of the wind stress anomalies favors ENSO growth and longer duration.

- Studies performed with intermediate coupled models (Kirtman et al. 1997, An and Wang 2000) have shown that the spatial structure of the wind stress anomalies can influence the ENSO timescale (see also Wittenberg et al. 2005, Deser et al. 2005).

## Regression of τx upon the Niño3.4 indexNCEP/NCAR Reanalyses

## Regression of τx upon the Niño3.4 index

- CCSM3

- GFDL-CM2.0

- IPSL-CM4

- NCEP

## Regression of τx upon the Niño3.4 index

- GISS-EH

- PCM

- MRI

## Regression of τx upon the Niño3.4 index

- UKMO-HadCM3

- CSIRO

- CNRM

## Dependency of period upon structure of anomalous wind stress

## Influence of meridional width of xRegression of the curl() upon the Nino3.4 index

## Influence of meridional width of xEOF2 of thermocline depth

- INGV

- CCSM3

- HadCM3

- 10°N

- 10°S

## Conclusions

- The representation of ENSO in the coupled climate models has considerably improved in the last generation of models.
- Inaccuracies in the representation of ENSO include _spatial_ _structure_ of the variability, as well as its _temporal_ _evolution_. In the models, ENSO events tend to occur too frequently and too regularly than in nature.
- Wind stress forcing in the CGCMs has a narrower meridional scale and is displaced westward compared to observations. Both factors can lead to a shorter timescale for ENSO.