An analysis of warm pool and cold tongue El Nios: air-sea coupling processes, global influences, and recent trends

The differences in tropical air-sea interactions and global climate connection as well as the hindcast skills for the warm pool (WP) and cold tongue (CT) El Nios are investigated based on observed, (re)analyzed, and model hindcast data. The robustness of observed global climate connection is established from the model simulations. Lastly, variations of atmosphere and ocean conditions in the recent decades, and their possible connection with the frequency increase of the WP El Nio are discussed. Consistent with previous results, our individual case study and composite results suggest that stronger (weaker) and more eastward extended (westward confined) westerly wind along the equatorial Pacific in early months of a year is associated with active (suppressed) air-sea interaction over the cold tongue/the Intertropical Convergence Zone complex, as well as more (less) intensive oceanic thermocline feedback, favoring the CT (WP) El Nio development. The preceding westerly wind signal and air-sea interaction differences may be responsible for the predication skill difference with higher (lower) overall hindcast skill for the CT (WP) El Nio in the Climate Forecast System of National Centers for Environmental Prediction. Our model experiments show that, in addition to the tropics, the eastern Pacific, North America and North Atlantic are the major regions having robust climate differences between the CT and WP El Nios. Nevertheless, the climate contrasts seem not robust over the Eurasian continent. Also, the frequency increase of the WP El Nio in the recent decades may not be directly connected with the linear trend of the tropical climate.