Tropical-extratropical Interaction and the Wintertime Climatological ITCZ Precipitation Maximum over the east Pacific

During northern winter, the eastern North Pacific ITCZ is characterized as the most narrow convergence zone on Earth, with its heaviest precipitation confined to a zonally oriented region of less than 200 km width in the meridional direction, even in the mean. It is well established that the southern boundary of the ITCZ is controlled to a large extent by the strong meridional gradient in sea surface temperature (SST), but the cause for the more diffuse northern boundary has not been completely explained. A striking feature of this portion of the ITCZ is a local precipitation maximum that is centered at around 8N and between 140W-120W during November through April. This precipitation maximum is actually collocated with an SST minimum along that latitude, strongly suggesting that its origin is dynamical rather than directly driven by boundary forcing. We show that at least part of this climatological precipitation pattern originates as a rectified signal of forcing by wave energy originating in the extratropical storm track of the North Pacific. Such a connection has been well-established in the past, and we show that these transients can lead to a less well defined northern boundary. In this study we document the impact of this tropical-extratropical interaction on the momentum and moisture budgets through an analysis of the local E Vector and its impact on the general circulation through zonally averaged E-P flux diagnostics. The origin of the wave activity is also explored through an analysis of the preferred direction of “wave breaking” within the Pacific storm track that leads to the equatorward propagation of wave energy.