SEARCHING FOR A NEW ENSO SST INDEX: THE TWO-BOX METHOD

Abstract

Operational Niño indices in the Eastern Pacific region use a one-box method to calculate sea surface temperature anomalies for identifying El Niño and La Niña events. A new sea surface temperature index method is presented here, which we call the Niño Difference Index. The definition calls for an additional sea surface temperature box to be placed in the Maritime Continent to use in conjunction with an Eastern Pacific sea surface temperature box. Our two-box method measures the sea surface temperature gradient in the tropical Pacific region, a hallmark feature of ENSO events since the sea surface temperature gradient weakens (strengthens) during an El Niño (La Niña) event. The definition of the Niño Difference Index has a more fundamental connection to ENSO since Niño indices only measure the sea surface temperature anomalies in a localized area of the tropical Pacific which have a strong sea surface temperature response during El Niño/La Niña events. The Niño Difference Index also relates to the shift in the locations of strong atmospheric convection in the tropical Pacific because the it is a measure of where convection migrates to/away from during an ENSO event. Niño Difference Index definitions are searched for using the local and remote response of precipitation to ENSO to use this measure of the atmospheric response and see if different regions of the globe find different index definitions that are optimal. Southern Oscillation Index data is also incorporated in our search process to search for Niño Difference Index options. Once Niño Difference Index options are narrowed down to a small subset, a series of final correlation tests is done using different ENSO metrics. This will determine the strength of the final Niño Difference Index options relationship with ENSO compared to the operational Niño indices.