To understand why El Niño is frequently followed by La Niña while the opposite occurs rarely, we analyze the inherent asymmetry in a delayed negative feedback loop in the framework of delayed oscillator theory using observational data. The asymmetrical response of the ocean wave to wind is much larger than that of the wind intensity to sea surface temperature (SST) and that of the subsurface temperature to thermocline. Strong oceanic response during El Niño compared to La Niña is presumably due to the relatively shallow mean thermocline over the western Pacific, which efficiently traps the atmospheric momentum in the shallow upper ocean, and the asymmetrical wind pattern response to SST. A modified delayed oscillator model experiment verifies that the asymmetrical ocean wave response to wind is more important in the asymmetrical transition of El Niño and La Niña than asymmetrical wind intensity response to SST and asymmetrical subsurface temperature response to thermocline.
Bibliographical noteFunding Information:
We thank Boris Dewitte and Roman Olson for helpful comments about wave dynamics and significance test. This research was supported by Basic Science Research Program through National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and future Planning (2014R1A2A1A11049497). Websites for the data sets used are (1) ERSSTv4 (http://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v4.html), (2) SODAv2.2.4 (http://sodaserver.tamu.edu/assim/SODA_2.2.4/), (3) NCEP-1 (http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html), and (4) ECDAv3.1 (http://apdrc.soest.hawaii.edu/dods/public_data/GFDL/ecda_v3.1).
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)