The Impact of ENSO on the Extratropics

Abstract

The impact of tropical remote forcing on the extratropics is examined with ideal El Niño-Southern Oscillation (ENSO) forcing in the tropical Pacific. A set of numerical experiments are described in which perfectly periodic ENSO is prescribed in the tropical Pacific as a lower boundary condition, and a slab mixed layer ocean model is coupled to an Atmospheric General Circulation Model (AGCM) in all other ocean basins. First, the role of subtropical air-sea coupling is investigated by changing the tropical Pacific forcing region, i.e., narrow (10°S~10°N) vs. broad (20°S~20°N) forcing region. When the tropical Pacific is prescribed to be only the climatological annual cycle, different SST in the subtropics results. This, in turn, leads to different atmospheric motions, and consequently affects the adjacent extratropical atmosphere. The effect is limited to the Pacific basin only. When the tropical Pacific includes ENSO, meridionally broad structure of SST forcing intensifies the meridional atmospheric circulation in the North Pacific basin, and, hence, the extratropical response to ENSO increases. Secondly, the relationship between remote ENSO forcing and seasonality is examined. Here we compare the response to perfectly periodic ENSO forcing that peaks in boreal summer versus boreal winter. The results indicate that the maximum extratropical response to the ENSO is determined by the local seasonality rather than the temporal phase of ENSO. When the peak of ENSO is in boreal summer, the surface heat flux in the North Pacific is maximized in the boreal winter, six months earlier than the peak of ENSO. At the same time, the evolution of SST in the South Pacific is very similar to that of the observed North Pacific. The tropical atmosphere linearly responds to the prescribed SST forcing, but the atmospheric bridge connecting the deep tropics to the extratropics occurs in specific seasons. Lastly, we examined how the extratropical response to ENSO varies depending on the period of ENSO. Ideally periodic two, four, and six year ENSO period experiments were performed. When the ENSO responses mature in the North Pacific, the composite patterns are similar among experiments, but the variance is sensitive to the ENSO periodicity. The extratropical response to ENSO is damped by local air-sea interaction. This local damping has a time-scale that is considerable longer than one year. Hence, the high frequency ENSO, in which the time-scale between successive ENSO events is shorter than the local damping process, results in increased variances of the ENSO forced pattern. In addition, the La Niña forced pattern persists longer than the El Niño forced pattern.