Structure, variability, and forcing of the East Asian Subtropical Jet

The availability of the long-term reanalyzed National Centers for Environmental Prediction (NCEP)/ National Center for Atmospheric Research (NCAR) gridded dataset creates the opportunity to reexamine the structure and time evolution of the East Asian subtropical jet (EA STJ) with updated data. The EA STJ is a dominant feature of the Northern Hemisphere wintertime circulation and is observed to play an important role in tropical-extratropical interactions. Its entrance region becomes anchored to the Himalayan plateau during the cool season (Oct-Apr) while its eastward extent shows considerable interannual and intraseasonal variability during this time of year.

Using the NCEP/NCAR reanalysis data we have documented the structure and the aforementioned time variability on several differing scales. Included in this aspect of our study is an examination of the structure and variability of the EA STJ as a function of the phase of the Southern Oscillation. We have observed, for example, that during El Nino years the jet propagates eastward across the Pacific while during La Nina years the jet's eastward extent is limited. Furthermore, the eastward propagation during El Nino years is not steady in that the exit region makes repeated excursions into the Eastern Pacific/Western North America.

We have also begun preliminary investigation of the physical mechanisms that act to modulate the EA STJ. These mechanisms include the role of confluence to the lee of the Himalayan plateau and the part played by anomalous tropical convection in forcing both space and time variability in the EA STJ. Our results show the confluence of differing air masses east of the Himalaya is associated with an acceleration of the jet over East Asia and the Western Pacific while the eastward extension of the jet coincides with anomalous convection in the tropical Pacific.