On the joint role of subtropical atmospheric variability and equatorial subsurface heat content anomalies in initiating the onset of ENSO events

Previous research has shown that seasonal mean variations in both the subtropical/ extratropical sea level pressures over the central North Pacific and the subsurface heat content anomalies in the western equatorial Pacific are significantly related to the state of the El Nino - Southern Oscillation ( ENSO) 12 - 18 months later. Here we find that positive ( negative) subsurface temperature anomalies in the western equatorial Pacific during boreal summer/ fall, followed by negative ( positive) anomalies in the sea level pressure fields over the subtropical central North Pacific during boreal winter, tend to result in positive ( negative) mature ENSO events 12 - 15 months later ( i. e., during the following boreal winter). When the intervening sea level pressure anomalies are of the same sign as the preceding heat- content anomalies, the correlation between the heat- content anomalies and the following boreal- winter ENSO state disappears. There is still some relation between the boreal- winter sea level pressure anomalies and the ENSO state the following year when the two precursor patterns are of the same sign; however, the correlation is smaller and the ENSO events tend to be weaker. Additional analysis indicates that the two precursor fields are related to one another; however, the sea level pressure variations contain more unique information about, and provide better predictability of, the state of the following ENSO system than do the heat content anomalies.