Documentation of a highly ENSO-related SST region in the equatorial Pacific

A new ENSO SST index is documented that is strongly correlated to the core ENSO phenomenon. The SST anomaly in much of the east-central and eastern tropical Pacific is closely related to ENSO. However the anomaly from approximately the centre of the eastern half of the equatorial Pacific westward to near the date line is suggested to be most strongly ENSO-related when darn spanning the most recent several decades are used. This is the case both with respect to (1) strength of association with other oceanic/atmospheric ENSO-related anomalies (both simultaneously and as a time-delayed predictand), and (2) impact on remote worldwide climate anomalies. This observational insight was lacking in the early 1980s when the four ''Nino'' regions were developed. While a firmer dynamical foundation for this regional preference still needs to be established the region straddling Nino 3 and Nino 4 may be regarded as an appropriate general SST index of the ENSO state by researchers, diagnosticians and forecasters. A dataset of this index, called ''Nino 3.4'' (5 degrees N-5 degrees S, 120-170 degrees W), is maintained an the Internet, shown in the Climate Diagnostics Bulletin, and provided in the Appendix of this note.