Extratropical Forcing of Tropical Pacific Convection during Northern Winter

Abstract

Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific. Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the development of a subtropical jet downstream of the disturbance. The SPCZ disturbance is not strongly tilted, but is still accompanied by a strong poleward flux of heat and momentum. Evidence for the occurrence of cross-equatorial wave dispersion in the eastern Pacific during northern winter is also presented. These observations are consistent with theory and modeling of Rossby waves in a westerly basic state extending from the midlatitudes into the tropics.