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, FsSSTp-SST, ?x
  SSTE -SSTw
• Ocean component The NCAR Pacific basin model
• Sun, D.-Z., 2003, J. Climate, 16, 185-205
• Sun, 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
Warm-pool
Warm-pool
Tw
Tc
Undercurrent
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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
Existing Paradigm
CO2
2xCO2
Mean Climate
ENSO
A Revised Paradigm
2xCO2
ENSO
Mean Climate
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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