Atlantic Multidecadal Variability Modulates The Climate Response to ENSO

Abstract:

El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability in the tropics with global impacts on climate and populations. ENSO and its teleconnections exhibit decadal variability that may be influenced by low frequency modes in the Pacific and Atlantic oceans. These decadal variations can either mask or amplify the signal of externally forced trends in the climate system.

Here we investigate the impact of the Atlantic Multidecadal Variability (AMV) on ENSO and its climate impacts. We present a multi-model assessment using ensemble experiments from 9 coupled climate models in the Decadal Climate Prediction Project (DCPP) which restore North Atlantic sea surface temperatures (SST) towards observed AMV warm and cold phases. We identify ENSO events under the two AMV phases and compare their characteristics. Over the tropical Pacific, the AMV warm phase mutes the precipitation response to ENSO, and vice versa. However, the AMV modulation of ENSO-related SST is weak, suggesting the changes to ENSO precipitation are caused by an atmospheric pathway. We explore the hypothesis that the altered tropical Pacific precipitation signature of ENSO affects global teleconnections. During the warm AMV state, the extratropical climate response to ENSO is weaker due to a reduced ENSO variability, and vice versa during the cold AMV phase. The AMV modulates the temperature response to ENSO mainly over land during boreal winter, leading to relatively milder winters over North America and cooler winters over northern South America. The exception is Australia, where the warm AMV state amplifies the anomalously dry and warm response to El Niño during austral summer.