Assessing Extratropical Impact on Observed Tropical Climate Variability

The extratropical influence on the observed events of El Niño-Southern Oscillation (ENSO) variability of the last few decades is assessed by constraining the extratropical atmospheric variability in two coupled general circulation models (FOAM, GFDLCM2.1) using the regional coupled data assimilation in the extratropics. The ensemble mean ENSO response to extratropical atmospheric forcing, which is systematically and quantitatively studied through a series of experiments, indicates robust extratropical influence on some observed ENSO events. Furthermore, an event-by-event quantitative analysis shows significant differences of the extratropical influence among the observed ENSO events, both in its own strength and in its relation to tropical precursors such as the equatorial Pacific heat content anomaly. Preliminary experiments also show that the extratropical influences the tropics via mainly the atmospheric teleconnection and secondarily the coupled teleconnection. These studies provides a dynamic quantitative assessment of the extratropical influence on observed ENSO variability on an event-by-event basis.