Comparative Evaluation of Niño1+2 and Niño3.4 Indices in Terms of ENSO Effects Over the Euro-Mediterranean Region

Abstract

Global or regional impacts of El Niño Southern Oscillation (ENSO) have predominantly been investigated through the Niño3.4 index, representing the sea surface temperature (SST) variations in the central Tropical Pacific. In this study, we comparatively evaluated the usefulness of Niño1+2, a relatively less utilised index that represents SST variability in the Eastern Tropical Pacific. In our analyses, we focused on ENSO impacts on Euro-Mediterranean (EM) climate variability during boreal winter, using data from the NCEP/NCAR Reanalysis. The correlation analysis involving Niño1+2 depicts more distinct temperature and precipitation patterns over the EM region. Amongst the SST-based Niño indices, it has the highest correlation with the East Atlantic index (0.47, statistically significant at > 99% confidence level), a prominent regional teleconnection associated primarily with the strength of the East Atlantic ridge, which produces dipole-type climate patterns between East Atlantic/Western Europe and Central/Eastern Mediterranean. Moreover, its lagged correlations with the following spring (0.39), summer (0.31), and autumn (0.36) are all statistically significant at ≥ 99% confidence levels. The composite analysis shows that different Niño regions have distinct effects on atmospheric circulation and climate in the EM region. The Niño1+2 index is particularly helpful in identifying the years when warm SST anomalies of El Niño extend to the Eastern Equatorial Pacific, which results in a reversal of temperatures across the EM region. Thus, this study suggests that Niño1+2 is a useful index for studying climate variability and predictability in the EM region, especially when used in conjunction with other Niño indices, as it captures some ENSO features that they may not encompass.