Abstract:
There exists a pronounced asymmetry between the amplitudes of central Pacific (CP) and eastern Pacific (EP) El Niño events. The present study examines such an asymmetry and its relationship with the North Pacific sea surface temperature anomalies (SSTA). Results indicate that the weaker CP El Niño amplitude can be attributed to the weaker anomalous zonal wind response to the east-west equatorial SSTA gradient during its growing phase compared with EP El Niño. Furthermore, the occurrence of CP El Niño is closely associated with southwesterly surface wind anomalies in the subtropical North Pacific, as well as ocean warming reminiscent of the North Pacific Gyre Oscillation (NPGO) pattern in its vicinity. Both the observations as well as the pacemaker experiments with a coupled global climate model suggest that the anomalous low-level southwesterlies, induced by the North Pacific Oscillation (NPO)-like atmospheric variability, can enhance positive SSTA signals and extend them southwestward to the central equatorial Pacific via the wind–evaporation–SST (WES) feedback. This will further attenuate the atmospheric response to zonal SSTA gradient, and hence weaken the amplitude of CP El Niño. In addition, observations and numerical experiments indicate that both of the Aleutian Low (AL) mode and NPO mode are dominant over the North Pacific and alternated on the interdecadal time-scale. When the NPO mode is dominant, it become more effective at playing a role in triggering an El Niño in the central Pacific via the seasonal footprinting mechanism over the subtropical northeastern Pacific. Therefore, anomalous low-level southwesterlies over the subtropical North Pacific can effectively act as a conduit for tropical–subtropical air–sea interaction in that region, and can play an important role in limiting the intensity of CP El Niño.
