The effect of an enhanced subtropical surface cooling on El Nin˜o–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.