Delineating the Seasonally Modulated Nonlinear Feedback Onto ENSO From Tropical Instability Waves. Issue 7 (2nd April 2020)

• Record Type: Journal Article
• Authors: Xue, Aoyun; Jin, Fei‐Fei; Zhang, Wenjun; Boucharel, Julien; Zhao, Sen; Yuan, Xinyi
• Is Part Of: Geophysical research letters. Volume 47:Issue 7(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-04-02
• Subjects: ENSO -- tropical instability waves -- NDH -- combination tone -- rectification effects -- nonlinear feedback; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL085863
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Tropical instability waves (TIWs), the dominant form of eddy variability in the tropics, have a peak period at about 5 weeks and are strongly modulated by both the seasonal cycle and El Niño–Southern Oscillation (ENSO). In this study, we first demonstrated that TIW‐induced nonlinear dynamical heating (NDH) is basically proportional to the TIW amplitude depicted by a complex index for TIW. We further delineated that this NDH, capturing the seasonally modulated nonlinear feedback of TIW activity onto ENSO, is well approximated by a theoretical formulation derived analytically from a simple linear stochastic model for the TIW index. The results of this study may be useful for the climate community to evaluate and understand the TIW‐ENSO multiscale interaction. Plain Language Summary: Tropical instability waves (TIWs) are westward propagating high‐frequency waves having a main period about 5 weeks. Their activity is strongly modulated by the cold tongue annual cycle and El Niño–Southern Oscillation (ENSO). At the same time, TIW activity as a whole systematically transports heat meridionally from warm to cold regions, and thus when they are modulated by ENSO, they can have a nonlinear rectification effect on ENSO in return. We find that the TIW‐induced rectification effect on ENSO can be related to the amplitude of a simple index that captures the main propagative wavy feature of TIW. This feedback effect prevents the growth of La Niña (El Niño) events by promoting a warming (cooling) through meridional convergence of TIW heat transport. Finally, we introduce a theoretical formulation for TIW‐induced effect by adopting a simple linear stochastic model for TIW focused on the complex index for TIW. This validated formulation shall be useful, for instance, for evaluating and understanding the climate model's ability in simulating the TIW‐ENSO multiscale interaction. Key Points: Nonlinear dynamical heating (NDH) due to tropical instability waves (TIWs) is largely proportional to the amplitude of a simple TIW index TIW feedback onto El Niño–Southern Oscillation through TIW‐induced NDH is nonlinear and strongly seasonal dependent A theoretical derived simple expression for this feedback is in agreement with the deduced result from the reanalysis data