The Role of ENSO in Regulating its Background State

De-Zheng SunTao Zhang

CU/CIRES/Climate Diagnostics Center &NOAA/Earth System Research Laboratory

Boulder, Coloradohttp://www.cdc.noaa.gov/people/dezheng.sun

Background

• Both the oscillator theory and the stochastic theory of ENSO have the background state prescribed, leaving the question whether ENSO in turn plays a role in determining the background state unaddressed.

• Answering this question, however, is critical for a number of climatic issues including understanding the response of ENSO to global warming and diagnosing the causes of tropical biases in coupled GCM simulations.

Our Hypothesis

• The recurrent occurrence of El Nino and La Nina events--ENSO--may be a mechanism that prevents the time-mean state of the coupled tropical Pacific ocean-atmosphere from becoming substantially unstable.

The Methodology

• Conducting perturbation experiments in pairs with a coupled model; Turning off ENSO in one of the two experiments; Contrasting the differences in the response to the perturbations between the case with ENSO and the case without ENSO.

• The perturbations are enhanced tropical heating or enhanced extratropical cooling.

The Model• Atmospheric component: empirical, Fs~SSTp-SST, x ~SSTE -SSTw

• Ocean component: The NCAR Pacific basin model

Sun, D.-Z., 2003, J. Climate, 16, 185-205Sun, D.-Z., T. Zhang, S.-I. Shin, 2004, J. Climate, 17, 3786-3798

A key parameter that measures the stability of the coupled tropical ocean-atmosphere system

Tc

Warm-pool

Undercurrent

Tw

Warm-pool

Response of Tw-Tc with and without ENSO

Tropical Heating Experiments

Response of the equatorial ocean temperature to tropical heating

Without ENSO With ENSO

Response of ENSO Amplitude to Tropical Heating

Equatorial Ocean Temperature Response During La Nina and El Nino

Destabilizing the tropics from the extratropics: the “ocean-tunnel”

Water constituting the equatorial undercurrent and therefore the upwelling water in the equatorial Pacific comes from the subtropical/extra-tropical region (McCreary and Lu 1994, Pedlosky 1987)

Response in the upper ocean temperature to extratropical cooling

Without ENSO With ENSO

Response of ENSO amplitude to extratropical cooling

Response in the upper ocean temperatureto extratropical cooling

Without ENSO With ENSO

Conclusion

ENSO acts as a basin-scale heat “mixer” that prevents any significant increase from occurring in the time-mean difference between the warm-pool SST (Tw) and the temperature of the thermocline water (Tc).

A New Paradigm for Understanding How ENSO Responds to Global Warming

CO2

2xCO2 ENSOMean Climate

2xCO2 Mean Climate ENSO

Existing Paradigm:

A Revised Paradigm:

Implications

Climate models that do not have good simulations of ENSO may not give reliable predictions of the response of the mean climate to global warming.

The excessive cold-tongue in the coupled GCM simulations of the time-mean tropical Pacific SST may be a consequence of the underestimate of the ENSO activity in these models.

Our existing paradigm to understand the response of ENSO to global warming needs to be modified.

Response of Tw-Tc without and with ENSO

Extratropical Cooling Experiments

Ocean Temperature Difference During La Nina and El Nino

120oE-160oE

160oE-210oE

120oE-160oE

Meridional Structure of Ocean Temperature Response DuringLa Nina and El Nino

120oE-160oE

160oE-210oE

210oE-290oE

Response in the upper ocean temperature to tropical heating

Without ENSO With ENSO

Mean temperature response to tropical heating: a meridional view

Temperature differences during La Nina: a meridional view

120oE-160oE

120oE-160oE

160oE-210oE

210oE-290oE

Temperature differences during El Nino: a meridional view

120oE-160oE

160oE-210oE

210oE-290oE

Ocean Temperature Response During La Nina and El Nino

Mean temperature response to subtropical cooling

Ocean Temperature Response During La Nina and El Nino