Modulation of the seasonal footprinting mechanism by the boreal spring Arctic Oscillation

Previous studies suggest that wintertime North Pacific Oscillation (NPO) is able to force an El Nino event during the following winter via the seasonal footprinting mechanism (SFM). In this study, we present evidence that springtime Arctic Oscillation (AO) has a significant modulation effect on the connection between the NPO and El Nino. When the spring AO is positive, a positive wintertime NPO can result in significant El Nino-like warming anomalies via the SFM. However, when the spring AO is negative, the connection of the NPO and the El Nino is not robust at all. Thus, the phase of spring AO should be taken into account when using NPO as a predictor for El Nino. Further analysis reveals that the mechanism of the AO modulation may be through changing the underlying SST footprinting over the subtropical northeastern Pacific.