NOTES AND CORRESPONDENCE A Quantitative Evaluation of ENSO Indices

El Nino-Southern Oscillation (ENSO) is a natural, coupled atmospheric-oceanic cycle that occurs in the tropical Pacific Ocean on an approximate timescale of 2-7 yr. ENSO events have been shown in previous studies to be related to regional extremes in weather (e.g., hurricane occurrences, frequency and severity of tornadoes, droughts, and floods). The teleconnection of ENSO events to extreme weather events means that the ability to classify an event as El Nino or La Nina is of interest in scientific and other applications. ENSO is most often classified using indices that indicate the warmth and coolness of equatorial tropical Pacific Ocean sea surface temperatures (SSTs). Another commonly used index is based on sea level pressure differences measured across the tropical Pacific Ocean. More recently, other indices have been proposed and have been shown to be effective in describing ENSO events. There is currently no consensus within the scientific community as to which of many indices best captures ENSO phases. The goal of this study is to compare several commonly used ENSO indices and to determine whether or not one index is superior in defining ENSO events; or alter- natively, to determine which indices are best for various applications. The response and sensitivity of the SST-based indices and pressure-based indices are compared. The Nino-4 index has a relatively weak response to El Nino; the Nino-1 12 index has a relatively strong response to La Nina. Analysis of the sensitivity of the indices relative to one another suggests that the choice of index to use in ENSO studies is dependent upon the phase of ENSO that is to be studied. The Japan Meteorological Agency (JMA) index is found to be more sensitive to La Nina events than all other indices. The Southern Oscillation, Nino-3.4, and Nino-4 indices are almost equally sensitive to El Nino events and are more sensitive than the JMA, Nino-1 12, and Nino-3 indices.