Asymmetric modulation of ENSO teleconnections by the interdecadal Pacific oscillation

Remote influences of ENSO are known to vary with different phases of the interdecadal Pacific oscillation (IPO). Here, observational and reanalysis data from 1920 to 2014 are analyzed to present a global synthesis of the IPO's modulation on ENSOteleconnections, followed by a modeling investigation. Regressions of surface air temperature T, precipitation P, and atmospheric circulations upon IPO and ENSO indices reveal substantial differences between ENSO and IPO teleconnections to regional T and P in terms of spatial pattern, magnitude, and seasonality. The IPO's modulation on ENSOteleconnections asymmetrically varies with both IPO and ENSO phases. For a given ENSO phase, IPO's modulations are not symmetric between its two phases; for a given IPO SST anomaly, its influence depends on whether it is superimposed on El Niño, La Niña, or neutral ENSO. The IPO modulations are linked to the atmospheric response to tropical SST anomalies, manifested in the local Hadley circulation and the local Walker circulation at low latitudes and the Rossby wave train in the extratropics, including the Pacific-North American (PNA) pattern in the Northern Hemisphere.Aset of numerical experiments usingCAM5 forced with different combinations of the IPO- and ENSO-related SSTs further shows that the asymmetric modulation arises from the nonlinear Clausius- Clapeyron relation, so that the atmospheric circulation response to the same IPO-induced SST departure is larger during a warm rather than a cold ENSO phase, and the response to a warm IPO state is larger than that to a cold IPO state. The asymmetry depends primarily on the tropical Pacific mean state and tropical SST anomalies and secondarily on extratropical SST anomalies.