1.5
The Midlatitude Atmospheric Bridge to the North Pacific
Grant Branstator, NCAR, Boulder, CO
It has been well established that a significant fraction of wintertime interannual variability of midlatitude North Pacific SST can be traced to the remote influence of the tropical Pacific Ocean by way of diabatically stimulated meridionally propagating Rossby waves. But we now know that there is a class of Rossby waves that propagate primarily in the zonal direction because they are trapped in the climatological subtropical midtropospheric jet. These features are most conspicuous over southern Asia and the North Pacific and are most prominently manifested in the so-called Circumglobal Waveguide Pattern that connects interannual variability at widely separated locations throughout the Northern Hemisphere. Because they extend to the surface over the oceans, it would seem that these zonally oriented disturbances have the potential to provide an alternative means by which the North Pacific Ocean can be remotely influenced, namely by events in midlatitudes, especially in southern Asia. In this study we investigate this possibility.
Using output from a control integration of NCAR's climate model, CCSM3, we have confirmed that the jet-trapped patterns of atmospheric variability earlier seen in atmospheric models and in nature are prominent in this coupled simulation and that they produce marked interannual atmospheric variability near the surface of the North Pacific. Furthermore, by way of correlation and lagged correlation analysis we have found that they are associated with underlying SST anomalies that are of similar magnitude to those produced from the tropics in reaction to ENSO. Moreover, additional analysis of surface fluxes and lag/lead statistics indicates that the atmosphere is primarily driving the midlatitude ocean anomalies. Because by construction these patterns are not induced by ENSO but rather appear to be initiated far upstream over Asia and even Europe, the results appear to confirm that midlatitude events, by way of this midlatitude bridge, can be as important as tropical Pacific events in producing interannual North Pacific Ocean variability. Additional analysis suggests that though the atmosphere is the primary driver the interaction between the media is truly two-way in that the resulting oceanic anomalies appear to support long-lived overlying near surface atmosphere anomalies.
To the extent that the observational record allows, a similar analysis of data from nature has been carried out. There too it is apparent that zonally propagating atmospheric disturbances have a significant influence on the interannual variability of North Pacific SST during winter. In particular the Circumglobal Waveguide Pattern is responsible for a recurring east-west oriented dipole near the dateline and 30N.
Session 1, Coupled Ocean–Atmosphere Basin–Scale And Decadal Variability
Monday, 30 January 2006, 9:00 AM-11:45 AM, A309
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