The Late Fall Extratropical Response to ENSO: Sensitivity to Coupling and Convection in the Tropical West Pacific

The extratropical response to El Nino in late fall departs considerably from the canonical El Nino signal. Observational analysis suggests that this response is modulated by anomalous forcing in the tropical west Pacific (TWP), so that a strong fall El Nino teleconnection is more likely when warm SST conditions and/or enhanced convection prevail in the TWP. While these TWP SST anomalies may arise from noise and/or long-term variability, they may also be generated by differences between El Nino events, through variations in the tropical atmospheric bridge. This bridge typically drives subsidence west of the date line and enhanced trade winds over the far TWP, which cool the ocean. In late fall, however, some relatively weaker and/ or more eastward-shifted El Nino events produce a correspondingly weakened and displaced tropical bridge, which results in no surface cooling and enhanced convection in the TWP. Because the North Pacific circulation is very sensitive to forcing from the TWP at this time of year, the final outcome is a strong extratropical El Nino teleconnection. This hypothesis is partly supported by regionally coupled ensemble GCM simulations for the 1950-99 period, in which prescribed observed El Nino SST anomalies in the eastern/central equatorial Pacific and an oceanic mixed layer model elsewhere coexist, so that the TWP is allowed to interact with the El Nino atmospheric bridge. To separate the deterministic signal driven by TWP coupling from that associated with inter-El Nino differences and from the noise due to intrinsic TWP convection variability (not induced by local SST anomalies), a second large-ensemble (100) simulation of the 1997/98 El Nino event, with coupling limited to the TWP and tropical Indian Ocean, is carried out. Together, the model findings suggest that the extratropical El Nino teleconnection during late fall is very sensitive to convective forcing in the TWP and that coupling-induced warming in the TWP may enhance this El Nino teleconnection by promoting convection in this critical TWP region. A more general implication is that diagnostic studies using December February (DJF) seasonal averages may obscure some important aspects of climate anomalies associated with forcing in the tropical Pacific.