Satellite integrated water vapor (IWV) images over the central and eastern Pacific frequently show bands of enhanced water vapor on order of 200 km width and several 1000's km length. These bands are associated with the low-level jet of wintertime extratropical cyclones, which produce most of the U.S. West Coast's annual precipitation. In many cases, these bands extend far enough south that it appears possible that a tropical moisture source may be contributing to West Coast precipitation. However, from the satellite images alone it is not possible to determine whether these bands originate from tropical moisture advection or solely from local low-level moisture convergence. To address the question of the source of moisture in these storms, six case studies of IWV bands have been selected between 1997-2001, all of which produced significant precipitation in California. These six cases include El Nino, La Nina, and neutral ENSO conditions, and occurred during the Caljet and Pacjet field programs.

Numerical simulations using trajectory analysis are carried out using the NCAR/Penn State mesoscale model to physically interpret the satellite IWV images. The preliminary results from these simulations indicates that the ability of these extra-tropical cyclones to tap tropical moisture depends on the strength of the Hadley circulation. In the case study from a neutral ENSO year, the flow associated with mid-latitude storms penetrates the sinking branch of the Hadley circulation, creating a break in the sinking branch somewhere between 120W and 160W. As a result of this, low-level moisture over the tropical ocean feeds into the extra-tropical storm. On the other hand, during an El Nino year, the sinking branch of the Hadley circulation is enhanced in such a way that it acts as a wall to block air parcels over the eastern tropical Pacific from moving northward to interact with the mid-latitude storms. These simulations provide a detailed picture of how low-level moisture over the eastern tropical Pacific can feed the mid-latitude storms that produce significant precipitation over the US west coast. They also provide a better understanding of how the transport of low-level moisture from the low-latitudes to mid-latitudes changes with the ENSO cycle.