ENSO evolution asymmetry: EP versus CP El Niño

Abstract

Through an oceanic mixed-layer heat budget analysis, the dominant processes contributing to the largest decay rate (− 0.37 °C/mon) in EP El Nino, the moderate delay rate (− 0.22 °C/mon) in CP El Nino and the smallest decay rate (0.13 °C/mon) in La Nina, are identified. The result shows that both dynamic (wind induced equatorial ocean waves and thermocline changes) and thermodynamic (net surface solar radiation and latent heat flux changes) processes contribute to a fast decay and thus phase transition in EP El Nino composite, whereas the thermodynamic process has less effect on the decay rate for both CP El Nino and La Nina due to the westward shift of sea surface temperature anomaly (SSTA) centers. Thus, the difference in surface wind stress forcing is critical in contributing to evolution asymmetry between CP El Nino and La Nina, while the difference in both the wind stress and heat flux anomalies contribute to evolution asymmetry between EP El Nino and La Nina. It is interesting to note that El Nino induced anomalous anticyclone over the western North Pacific is stronger and shifts more toward the east during EP El Nino than during CP El Nino, while compared to CP El Nino, the center of an anomalous cyclone during La Nina shifts further to the west. As a consequence, both EP and CP El Nino decay fast and transform into a La Nina episode in the subsequent year, whereas La Nina has a much slower decay rate and re-develops in the second year.