**3. ENSO (Teleconnection) Indices**

A teleconnection index is used to describe the temporal behavior of a particular mode of climate variability such as ENSO. Essentially statistical constructs, teleconnection indices are presented in the form of a single number for the temporal scale of interest (e.g., monthly, annual) with the assumption that a specific index captures the range of often complex, ocean and/or atmospheric process interactions that give rise to what is a multifaceted mode of climatic variability. While each teleconnection index has a commonly accepted acronym (Table 1), there may be various versions of a particular index because different methods, data sets, atmosphere and ocean variables, criteria, and sampling periods might have been used in their derivation [6].

A plethora of ENSO indices have been developed to measure, monitor, and summarise ENSO status. These can be broadly divided into atmospheric, oceanic and blended indices. Some of the more common ENSO atmospheric and oceanic indices, reported monthly by NOAA in its Climate Diagnostics Bulletin, are presented in Table 2.

**Table 2.** El Niño Southern Oscillation (ENSO) indices commonly reported by NOAA on a monthly basis. Modified from the Climate Diagnostics Bulletin http://www.cpc.ncep.noaa.gov/products/CDB.


<sup>1</sup> Positive (negative) values of 850-hPa zonal wind indices imply easterly (westerly) anomalies. <sup>2</sup> Currently NOAA uses 1981–2010 as the base period. <sup>3</sup> Positive (negative) values of 200-hPa zonal wind index imply westerly (easterly) anomalies. <sup>4</sup> Positive (negative) values indicate large amounts of outgoing longwave radiation.
