Can ENSO-Like Convection Force an ENSO-Like Extratropical Response on Subseasonal Time Scales?

A daily El Niño–Southern Oscillation (ENSO) index is developed based on precipitation rate and is used to investigate subseasonal time-scale extratropical circulation anomalies associated with ENSO-like convective heating. The index, referred to as the El Niño precipitation index (ENPI), is anomalously positive when there is El Niño–like convection. Conversely, the ENPI is anomalously negative when there is La Niña–like convection. It is found that when precipitation becomes El Niño–like (La Niña–like) on subseasonal time scales, the 300-hPa geopotential height field over the North Pacific and western North America becomes El Niño–like (La Niña–like) within 5–10 days. The composites show a small association with the MJO. These results are supported by previous modeling studies, which show that the response over the North Pacific and western North America to an equatorial Pacific heating anomaly occurs within about one week. This suggests that the mean seasonal extratropical response to El Niño (La Niña) may in effect simply be the average of the subseasonal response to subseasonally varying El Niño–like (La Niña–like) convective heating. Implications for subseasonal to seasonal forecasting are discussed.