Waveguides and the remote response to tropical SST anomalies

Waveguiding properties of atmospheric dynamical processes have profound influences on the structure of disturbances excited by interannual tropical heating anomalies. Two instances of this are well-known, namely the bending of planetary wavetrains away from the poles caused by beta and the ducting of energy out of the tropics through the climatological westerlies of the eastern tropical Pacific. In this investigation we study a third waveguide, the subtropical jet, which tends to trap medium scale planetary waves in such a way as to inhibit meridional propagation and enhance zonal propagation. The waveguide makes it possible for interannual disturbances to propagate completely around the globe so that some SST anomalies produce responses at locations much farther from the source than conventional propagation mechanisms predict.

Analysis of boreal winter observations and atmospheric general model simulations is undertaken to test whether the remote connections predicted by linear theory as a consequence of subtropical jet waveguiding actually occur. Results conclusively demonstrate that there is covariability at locations in the subtropical jet complex on opposite sides of the globe. The structure of this covariability matches the zonal wave five signature expected from waveguiding considerations. Of equal interest is a pronounced quasi-annular component to the jet-trapped covariability that is not anticipated by linear theory.

GCM experiments specially designed to determine which distributions of tropical heating excite global responses via the waveguide are then studied. The results indicate that heating in a band of longitudes between the Indian Ocean and western tropical Pacific are especially adept at producing a response that spans the Northern Hemisphere. Of special interest is the great sensitivity found to the exact longitude of the heating near the dateline with sources west of 165W producing a circumglobal response and sources to the east producing a more conventional arching pattern confined to the Pacific and North American sector. This suggests that some ENSO events should be more global in character than others.

Observational analysis of boreal winter seasonal anomalies supports many of the findings from the modeling component of the study. In nature activity in the waveguide is associated with SST anomalies in the Indian and Pacific Oceans. Western tropical Pacific SST anomalies are associated with midlatitude anomalies that are m