The Role of ENSO in Regulating its Background State
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Transcript of The Role of ENSO in Regulating its Background State
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