The impact of surface heat fluxes associated with the North Pacific Oscillation on ENSO variability: testing the Seasonal Footprinting Mechanism

Recent studies suggest that extratropical-tropical interactions may be important for the excitation of ENSO events (Vimont et al. 2001, GRL; 2003a,b, J. Climate). Large fluctuations in internal atmospheric modes over the North Pacific in winter, especially the North Pacific Oscillation (NPO), impart an SST "footprint'' on the underlying ocean via changes in the surface heat fluxes. The SST footprint, which peaks in spring and persists through summer in the subtropics, impacts the atmosphere including zonal wind stress anomalies that extend onto the equator and thereby influences ENSO.

We test this “Seasonal Footprinting Mechanism” (SFM) hypothesis using an AGCM coupled to reduced gravity ocean (RGO) model in the tropical Pacific and a slab model over the remainder of the world's oceans. We have performed a set of 50 simulations initialized on November 1st using atmospheric and oceanic conditions obtained from different years in a 100-year control run of the coupled model. Surface heat fluxes associated with the NPO are then added to the SST tendency equation over the Pacific in each of these 50 experiments through the following March (first 5 months of the integration) and the model is integrated for 17 more months with out any additional forcing. The forcing was derived by regressing the surface fluxes on the principle component associated with the NPO (2nd EOF in the Pacific Sector) during NDJFM in an uncoupled AGCM simulation.

We assess the impact of the NPO-related heat flux forcing on ENSO variability by comparing the 50 experiments to the control run. Preliminary results indicate that the SFM contributes strongly to the development of El Niño events, as westerly winds develop in the central and western Pacific in summer and the NINO 3.4 SSTAs are ~0.5°C higher in the winter following when the SFM forcing was applied. In addition, of the 50 experiments 35 warmed in the ENSO region after the forcing was added and 19 became El Niño events (Nino3.4 SST > σ) while only 3 became La Niña events relative to the control. We will explore the processes through which the SFM favors the development of El Niño events.