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The Effect of Subtropical Cooling on the Amplitude of ENSO: A Numerical Study

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  • 1 NOAA–CIRES Climate Diagnostics Center, Boulder, Colorado
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Abstract

The effect of an enhanced subtropical surface cooling on El Niño–Southern Oscillation (ENSO) through the “ocean tunnel” is investigated using a coupled model. Here, the term “ocean tunnel” refers to the water pathway that connects the equatorial upwelling water to the subtropical/extratropical surface water. The subtropical cooling is introduced through a reduction of the radiative–convective equilibrium SST (SSTp) in that region. The SSTp for the equatorial region is kept fixed.

It is found that an enhanced cooling in the subtropics results in a regime with stronger ENSO. This is because an enhanced subtropical cooling reduces the temperature of the water feeding the equatorial undercurrent through the ocean tunnel. The resulting larger difference between the warm-pool SST and the temperature of the equatorial thermocline water—the source water for the equatorial upwelling—tends to increase the equatorial zonal SST contrast between the western and the eastern Pacific. In response to this destabilizing forcing to the coupled equatorial ocean–atmosphere, a stronger ENSO develops. ENSO is found to regulate the time-mean difference between the warm-pool SST and the temperature of the equatorial undercurrent. The findings provide further support for the “heat pump” hypothesis for ENSO, which states that ENSO is an instability driven by the meridional differential heating over the Pacific Ocean and that ENSO regulates the long-term stability of the coupled equatorial Pacific climate. The results also substantiate the notion that surface variability from higher latitudes may influence equatorial SST variability through the ocean tunnel.

Corresponding author address: Dr. De-Zheng Sun, NOAA–CIRES Climate Diagnostics Center, 325 Broadway, Boulder, CO 80304. Email: dezheng.sun@noaa.gov

Abstract

The effect of an enhanced subtropical surface cooling on El Niño–Southern Oscillation (ENSO) through the “ocean tunnel” is investigated using a coupled model. Here, the term “ocean tunnel” refers to the water pathway that connects the equatorial upwelling water to the subtropical/extratropical surface water. The subtropical cooling is introduced through a reduction of the radiative–convective equilibrium SST (SSTp) in that region. The SSTp for the equatorial region is kept fixed.

It is found that an enhanced cooling in the subtropics results in a regime with stronger ENSO. This is because an enhanced subtropical cooling reduces the temperature of the water feeding the equatorial undercurrent through the ocean tunnel. The resulting larger difference between the warm-pool SST and the temperature of the equatorial thermocline water—the source water for the equatorial upwelling—tends to increase the equatorial zonal SST contrast between the western and the eastern Pacific. In response to this destabilizing forcing to the coupled equatorial ocean–atmosphere, a stronger ENSO develops. ENSO is found to regulate the time-mean difference between the warm-pool SST and the temperature of the equatorial undercurrent. The findings provide further support for the “heat pump” hypothesis for ENSO, which states that ENSO is an instability driven by the meridional differential heating over the Pacific Ocean and that ENSO regulates the long-term stability of the coupled equatorial Pacific climate. The results also substantiate the notion that surface variability from higher latitudes may influence equatorial SST variability through the ocean tunnel.

Corresponding author address: Dr. De-Zheng Sun, NOAA–CIRES Climate Diagnostics Center, 325 Broadway, Boulder, CO 80304. Email: dezheng.sun@noaa.gov

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