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

Abstract

Abstract The extratropical response to El Niño in late fall departs considerably from the canonical El Niño signal. Observational analysis suggests that this response is modulated by anomalous forcing in the tropical west Pacific (TWP), so that a strong fall El Niño 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 Niño 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 Niño 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 Niño teleconnection. This hypothesis is partly supported by regionally coupled ensemble GCM simulations for the 1950–99 period, in which prescribed observed El Niño 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 Niño atmospheric bridge. To separate the deterministic signal driven by TWP coupling from that associated with inter–El Niño 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 Niño event, with coupling limited to the TWP and tropical Indian Ocean, is carried out. Together, the model findings suggest that the extratropical El Niño 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 Niño 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.