ENSO Asymmetry ry and Subsurface Nonlinear Dynamical Heating in in Reanalysis and CMIP5 Cli limate Models Michiya Hayashi * and Fei-Fei Jin University of Hawaii at Manoa, Honolulu, HI
Asymmetry between El Niño & La Niña Amplitude: El Niño > La Niña Structure: More eastern-Pacific El Niño Positive SST skewness in Niño-3 region … Climate models generally have less skill…Why? Subsurface ocean is positively skewed more! (Cai et al. 2015) Niño-3 SST 1 𝑂 ∑ 𝑈− 𝑈 3 > 0 Skewness = 𝜏 3 (Hayashi & Jin 2017) (Burgers & Stephenson 1999)
ENSO cycle with intense subsurface variability Subsurface warming El Niño SST growth Subsurface cooling El Niño decay La Niña growth Subsurface asymmetry SST asymmetry … What causes subsurface asymmetry? (Timmermann et al. 2018, Nature: ENSO complexity)
Nonlinear dynamical heating (NDH) SST & ocean current’s normal condition totally collapsed during strong ENSO events Linear dynamical heating (LDH) Large dynamical nonlinear advection Enhanced ENSO asymmetry … Subsurface dynamics is also nonlinear! NDH Residuals (Jin et al. 2003) (Jin et al. 2003)
Questions: How does the subsurface NDH affect ENSO asymmetry? Is the subsurface NDH critical for ENSO asymmetry in climate models? Key points : Subsurface NDH reduces La Niña cooling by 30% (in ocean reanalysis) Subsurface NDH is highly correlated with ENSO asymmetry in CMIP5 climate models NDH mean (ORAS3, SODA331, GODAS) Data: ORAS3, SODA3.3.1, GODAS, CMIP5 (historical) Method: Heat budget analysis in the EP subsurface NDH standard deviation LDH NDH
Subsurface NDH & Equatorial Undercurrent (EUC) Subsurface Niño-3 SST anomaly (shade) NDH Subsurface zonal current ~EUC (Hayashi & Jin 2017) The subsurface NDH is positively skewed very large during strong El Niño events accompanied by halted EUC
LDH vs NDH: NDH reduces the linear cooling by 30% Zonal NDH component only (ORAS3 data, Hayashi & Jin 2017) Regression coeff. of NDH to LDH is −0.33 for LDH<0 (cooling) NDH reduces the subsurface cooling by 30% from linear tendency mostly explained by zonal NDH
Roles of subsurface NDH Strong El Niño composite (1982/83, 1997/98) on El Niño/La Niña events Tsub ’ Usub ’ SST’ Pos. Tsub ’ reaches the east edge Year 2 Neg. Tsub ’ is weaker & stalled at 150˚ W Westward Usub ’ in decay period Year 1 Subsurface NDH LDH ＋ NDH LDH NDH reduces the cooling in decay period is persistently positive in EP (150- 90˚W) Year 2 contributes to both the enhanced warming & stalled cooling in the easternmost Pacific Positive skewness of Tsub ’ in EP Year 1 Amplitude/structure ENSO asymmetry Subsurface temp. tendency [ ℃ /month] (ORAS3 data, Hayashi & Jin 2017)
CMIP5 models simulate ENSO asymmetry poorly (Niño-3 SST skewness) p-value <0.01 ENSO asymmetry Skewness 1 𝑈 3 𝑂 ∑ 𝑈− 𝜏 3 ENSO variability (Niño- 3 SST standard deviation, ˚C) ENSO amplitude is close to the reanalysis at the multi-model mean ENSO skewness cannot be distinguished from zero statistically ENSO skewness increases with amplitude but apart from observations
Subsurface NDH controls ENSO asymmetry in CMIP5 (Niño-3 SST skewness) p-value <0.00001 ENSO asymmetry Skewness 1 𝑈 3 𝑂 ∑ 𝑈− 𝜏 3 Subsurface NDH variability (Standard deviation , ˚ C/month) The subsurface NDH variability is highly correlated with ENSO skewness (R=0.80, p<0.00001) close to the reanalysis in only a few CMIP5 models
Why is the subsurface NDH too weak in CMIP5? Reanalysis mean CMIP5 mean Reanalysis mean CMIP5 mean Reanalysis mean CMIP5 mean CMIP5 Tsub ’ st.d. is not far from the reanalysis CMIP5 Usub ’ st.d. is about half of the reanalysis Too weak EUC variability may lead to weak NDH & ENSO asymmetry
Summary The NDH reduces the subsurface cooling by 30%, causing less surface EP cooling for La Niña The subsurface NDH leads asymmetry in ENSO magnitude/structure CMIP5 ENSO asymmetry highly depends on the subsurface NDH variability Too weak EUC variability may be responsible for weak ENSO asymmetry via the NDH ENSO asymmetry Main References Jin, F.-F., et al. (2003, GRL). Strong El Niño events and nonlinear dynamical heating. Hayashi, M., & Jin, F.-F. (2017, GRL). Subsurface nonlinear dynamical heating and ENSO asymmetry. Timmermann, A., et al. (2018, Nature). El Niño-southern oscillation complexity. Hayashi, M., & Jin, F.-F. (submitted). Acknowledgements M.H. was supported by JSPS Overseas Research Fellowships #201860671. Subsurface NDH variability
12C.4 ENSO Asymmetry ry and Subsurface Nonli linear Dynamic ical l Heatin ing in in Reanalysis and CMIP IP5 Clim limate Models • Thursday, 10 January 2019: 9:15 AM • North 129B (Phoenix Convention Center - West and North Buildings) • Michiya Hayashi , Univ. of Hawaii, Honolulu, HI; and F. F. Jin • A relationship between the El Niño-Southern Oscillation (ENSO) asymmetry and the nonlinear dynamical heating (NDH) in the subsurface ocean along the thermocline is delineated using ocean reanalysis datasets and outputs of climate models. The ENSO asymmetry is measured by the skewness of sea surface temperature (SST) anomalies in the Niño-3 region (90- 150˚W, 5˚S - 5˚N). The subsurface NDH, which is defined as the nonlinear advective terms, was calculated based on a heat budget analysis in the equatorial Pacific Ocean using ocean reanalysis products (ORAS3, SODA3.3.1, and GODAS) and preindustrial control simulations of CMIP5 climate models. The standard deviations of the Niño-3 SST simulated in CMIP5 models are comparable with the reanalysis. Nevertheless, the simulated levels of the Niño-3 SST skewness, in general, are much less than that in the reanalysis. The levels of skewness in both the Niño-3 SST and eastern-Pacific subsurface temperature are almost linearly related to the standard deviation of the eastern-Pacific subsurface NDH. This relation suggests that the ENSO asymmetry may be controlled by the subsurface NDH variability. As the latter is mainly contributed by its zonal component, weak variability in the equatorial undercurrent may contribute to weak ENSO asymmetries in CMIP5.
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