A numerical study with a coupled model is performed to ascertain and understand the response of the amplitude of ENSO to cooling over the subtropical/extratropical ocean. The atmospheric component of this coupled model is an empirical atmospheric model. The surface heat flux into the ocean is assumed to be proportional to the difference between the radiative-convective equilibrium SST and the actual SST. The equatorial zonal wind stress is assumed to be proportional to the zonal SST gradients. The ocean component of the coupled model is a primitive equation model (the NCAR Pacific Basin Model) and therefore explicitly calculates the heat budget of the entire equatorial upper ocean.

Cooling in the substropical/extratropical ocean is introduced by reducing the radiative convective equilibrium SST in that region. The surface cooling is found to descend to the subsurface ocean and then propagates to the subsurface of the equatorial region. The colder temperature of the equatorial undercurrent then results in colder upwelling water. This further leads to a cooling in the central and eastern equatorial Pacific SST. When this cooling coincides with an ongoing La Nina, the equatorial zonal winds is enhanced which further depresses the thermocline in the western Pacific. The equatorial ocean then becomes more unstable and a stronger El Nino develops. The stronger El Nino cools the western Pacific and warms the eastern Pacific and thereby largely reverses the perturbation from the subsurface to the equatorial zonal SST contrast during the La Nina phase. The change in the time-mean SST contrast between the eastern and the western equatorial Pacific is found to be small. In the enhanced subtropical/extratropical cooling case, more heat is transported out of the equatorial Pacific to the higher latitudes and the transport becomes more episodic.

These findings further support the "heat-pump" hypothesis for ENSO. The hypothesis states that the magnitude of El Nino warming is proportional to the meridional differential heating over the Pacific: either an enhanced surface heating over the equatorial region or an enhanced cooling over the subtropical/extratropical ocean may result in stronger El Ninos. Moreover, El Nino may be a mechanism that prevents the time-mean state of the equatorial Pacific from becoming substantially unstable.