The Role of ENSO in Regulating its Background State
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The Role of ENSO in Regulating its
Background State
De-Zheng Sun
Tao Zhang
CU/CIRES/Climate Diagnostics Center
&NOAA/Earth System Research Laboratory
Boulder, Colorado
http://www.cdc.noaa.gov/people/dezheng.sun
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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.
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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.
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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.
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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
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A key parameter that measures the stability of the coupled tropical ocean-atmosphere system
Tc
Warm-pool
Undercurrent
Tw
Warm-pool
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Response of Tw-Tc with and without ENSO
Tropical Heating Experiments
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Response of the equatorial ocean temperature to tropical heating
Without ENSO With ENSO
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Response of ENSO Amplitude to Tropical Heating
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Equatorial Ocean Temperature Response During La Nina and El Nino
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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)
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Response in the upper ocean temperature to extratropical cooling
Without ENSO With ENSO
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Response of ENSO amplitude to extratropical cooling
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Response in the upper ocean temperatureto extratropical cooling
Without ENSO With ENSO
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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).
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A New Paradigm for Understanding How ENSO Responds to Global Warming
CO2
2xCO2 ENSOMean Climate
2xCO2 Mean Climate ENSO
Existing Paradigm:
A Revised Paradigm:
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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.
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Response of Tw-Tc without and with ENSO
Extratropical Cooling Experiments
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Ocean Temperature Difference During La Nina and El Nino
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120oE-160oE
160oE-210oE
120oE-160oE
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Meridional Structure of Ocean Temperature Response DuringLa Nina and El Nino
120oE-160oE
160oE-210oE
210oE-290oE
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Response in the upper ocean temperature to tropical heating
Without ENSO With ENSO
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Mean temperature response to tropical heating: a meridional view
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Temperature differences during La Nina: a meridional view
120oE-160oE
120oE-160oE
160oE-210oE
210oE-290oE
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Temperature differences during El Nino: a meridional view
120oE-160oE
160oE-210oE
210oE-290oE
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Ocean Temperature Response During La Nina and El Nino
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Mean temperature response to subtropical cooling
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Ocean Temperature Response During La Nina and El Nino