The correspondence of sea-surface temperature (SST) anomalies to changes in antecedent large-scale sea level pressure anomalies is investigated using reanalysis data. By statistically examining linearly coupled precursor sea level pressure fields and subsequent SST fields for different lag periods, it is possible to isolate a precursor mode of sea level pressure (SLP) variability in the central subtropical North Pacific that precedes variations in the January--March El Ni¿o/Southern Oscillation (ENSO) by approximately 12--15 months. A sea level pressure index, which captures the important characteristics of this precursor mode of variability, is developed and evaluated. It is shown that both analyzed and observed versions of the index are significantly correlated with the January--March ENSO one year later. The SLP index is then used to examine the evolution of the surface circulation and temperature structures leading up to mature ENSO events. Initially, the January--March subtropical North Pacific SLP anomalies are associated with changes in the intensity of the subtropical trade wind regime over the North Pacific, as well as with SST anomalies over the eastern equatorial Pacific and subtropical central Pacific. In agreement with the correlation statistics associated with the SLP and lagged NINO3.4 indices, both the sea level pressure field and the SST field subsequently develop ENSO-like structures over the course of the following year. Significant discussion of these results and pertinent areas of future research are provided within the broader context of the ENSO system.