Abstract
Tropical instability waves (TIWs) arise from shear instabilities of equatorial Pacific Ocean currents and are important for the tropical climate and the El Niño–Southern Oscillation. Yet the long-term evolution of TIW activity under climate change remains unclear due to the difficulty in estimating equatorial current velocity. Here we use in situ, satellite altimeter and sea surface temperature observations along with a realistic eddy-resolving ocean simulation to show that TIW activity has intensified in the central equatorial Pacific at ∼12 ± 6% per decade over the recent three decades. The extended satellite data and the ocean model simulation show that the increased TIW activity is probably caused by an enhanced cross-equatorial asymmetric warming in the eastern tropical Pacific. The intensified TIWs lead to increased eddy dynamic heating effects of ∼70% since the 1990s near the equator, with implications for predicting and projecting tropical Pacific climate changes.
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Data availability
The NOAA OISST v.2.1 is available at https://www.ncei.noaa.gov/data/sea-surface-temperature-optimum-interpolation/v2.1/. The AVISO SSH is available at https://resources.marine.copernicus.eu/product-detail/SEALEVEL_GLO_PHY_L4_NRT_OBSERVATIONS_008_046/DATA-ACCESS. The OSCAR v.2.0 is available at https://doi.org/10.5067/OSCAR-25F20. The NOAA NBS v.2.0 wind is available at https://www.ncei.noaa.gov/products/blended-sea-winds. The TAO observations are available at https://www.pmel.noaa.gov/tao/drupal/disdel/. The drifter-derived climatological mean zonal currents are available at https://www.aoml.noaa.gov/phod/gdp/mean_velocity.php. The reconstruction dataset for temperature and salinity is available at http://www.ocean.iap.ac.cn/?navAnchor=home. The OFES2 simulation and 10-member ensemble output are available at https://www.jamstec.go.jp/ofes/ofes2.html. The GLORYS12V1 is available at https://data.marine.copernicus.eu/product/GLOBAL_MULTIYEAR_PHY_001_030/description. The ERSST v.5 is available at http://apdrc.soest.hawaii.edu/dods/public_data/NOAA_SST/ERSST/monthly_v5. The ECMWF ORAS5 reanalysis is available at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-oras5?tab=form. The JMA JRA55-do reanalysis is available at https://climate.mri-jma.go.jp/pub/ocean/JRA55-do/. The CMIP6 HighResMIP1 ensemble is available at https://esgf-node.llnl.gov/projects/cmip6/. The processed data68 used in this study are publicly available in MATLAB format at https://doi.org/10.6084/m9.figshare.24633639.
Code availability
The diagnostic model67 for the TIW surface currents is available at https://doi.org/10.6084/m9.figshare.24633606.v1 via MATLAB codes. All analyses and figures68 in this manuscript are reproducible via MATLAB codes found in the repository (https://doi.org/10.6084/m9.figshare.24633639).
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Acknowledgements
M.W. and Y.D. at SCSIO/CAS were supported by the National Natural Science Foundation of China (42090042, 42106024) and the Chinese Academy of Sciences (133244KYSB20190031, 183311KYSB20200015, LTOZZ2203, SCSIO202201). S.-P.X. was supported by the US NSF (AGS 2105654). H.S. was supported by JSPS KAKENHI Grant Number JP19H05701. The numerical computation was supported by the Earth Simulator in the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and the High-Performance Computing Division in the South China Sea Institute of Oceanology, Guangzhou, China.
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M.W., S.-P.X. and Y.D. conceived the study. M.W. conducted the analyses, wrote the initial manuscript draft and produced all figures. H.S. conducted the global ocean model simulation (OFES2), and M.N. conducted the OFES2 ensemble simulations. All authors contributed to the interpretation of the results and the writing and revision of the paper.
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Extended data
Extended Data Fig. 1 Observed trends of equatorial wavenumber-frequency spectra.
1993–2021 trends (color shading) are superimposed with the mean states (contour, in log scale): a, SSTy variance spectrum, contour initiating from – 0.8 with interval of 0.2, (10−6 °C m−1)2; b, SST variance spectrum, contour initiating from – 2.6 with interval of 0.2, °C2; c, SSH-derived surface kinetic energy (KE) spectrum, contour initiating from – 0.2 with interval of 0.2, J m−3; d, SSH variance spectrum, contour initiating from – 1.4 with interval of 0.2, cm2. The period is shown on the y-axis in red text. Red dashed box represents the TIW spectral domain with westward wavelength ≤ 3000 km and period ≤ 40 days. Black dashed curves denote dispersion relations of free equatorial waves of the first baroclinic mode, with the phase velocity of long gravity wave at 2.8 m s−1: ‘R0’, ‘R1–3’ and ‘K’ denote the mixed Rossby-gravity, first–third meridional-mode Rossby, and Kelvin waves, respectively. The trends are based on the 9-year running average of the spectra that are calculated over 160°W–100°W at each year, then averaged between (1°S–1°N) / (2°S–2°N) for (a,c) / (b,d). Dotted regions pass the 95% confidence level based on the modified Mann–Kendall test.
Extended Data Fig. 2 Trends based on annual mean time series.
a,b, Satellite-observed \(\overline{{{{\rm{SST}}^{\prime} }_{y}}^{2}}\), c,d, altimeter-derived EKE, and e,f, OFES2-simulated surface EKE. Left panels represent time series averaged in the dashed box of right panels (160°W–100°W and 1°S–1°N): annual (thin blue curves) and 9-yr running mean (bold blue curves); Dashed lines denote their fitted linear trends (thin magenta/red for annual mean over 1993–2015/2021, and bold red for 9-yr running mean over 1993–2021). T, p, and n represent the trends (per decade) ± uncertainties (estimated as the s.e.m.), p values, and n numbers for the statistics. Right panels represent 1993–2021 trends based the annual mean time series.
Extended Data Fig. 3 Trends in \({{{{\boldsymbol{(}}{\overline{{\mathbf{SST}}}}_{\boldsymbol{yy}}{\boldsymbol{)}}}^{\mathbf{2}}}}\), \({{\overline{{\boldsymbol{v}^{\prime}}}^{\mathbf{2}}}}\), \({{\overline{{\boldsymbol{v}^{\prime}}}^{\mathbf{2}} {({\overline{{\mathbf{SST}}}}_{\boldsymbol{yy}})}^{\mathbf{2}}}}\), and \({\overline{{{{\mathbf{SST}}}^{\prime}}}_{\boldsymbol{y}}^{\mathbf{2}}}\) of Eq. 3.
a,e,i, \(({{\overline{{\rm{SST}}}}_{{yy}}})^{2}\). b,f,j, \({{\overline{{{v}^{\prime}}}^{{2}}}}\). c,g,k, \({{\overline{{{v}^{\prime}}}^{{2}} {({\overline{\rm{{SST}}}}_{{yy}})}^{{2}}}}\). d,h,l, \({\overline{{{\rm{{SST}}^{\prime}}_{{y}}}^{{2}}}}\). a–d, e–h and i–l are on the basis of satellite observations, GLORYS12V1 reanalysis (1993–2020), and OFES2 simulation over 1993–2021, respectively. They were averaged between 160°W–100°W, standardized by maximum values of the climatological mean in meridional direction. Data are represented as the trends ± uncertainties (grey shading, estimated as the s.e.m.) at the equator, with n numbers shown. The results demonstrate the proportional relationship: \(\overline{{{{\rm{SST}}^{\prime}}} }_{y}^{2}\propto \overline{{{v^{\prime}}}}^{2}{{ \overline{{\rm{SST}}}}_{{yy}}}^{2}\), at the equator (Eq. 3, Methods).
Extended Data Fig. 4 Validation of EKE with in situ observations.
Upper and lower panels denote two sites at 140°W and 110°W equator of the TAO array, respectively. a,d, Depth-temporal variation of subsurface EKE from TAO; Black curves denote thermocline defined as the depth of 20 °C isotherm (D20). b,e, Multidecadal mean of EKE profile from altimeter (dots on the surface), OFES2 (in CMYK color), and TAO (in RGB color) between 1993–2006 (blue curves) and 2007–2020 (red curves); Color shading denotes the standard deviation of the time series. c,f, Interannual variability of EKE from altimeter (red), OFES2 at 30 m depth (blue), and TAO at 30 m depth (black), 1993–2020 mean removed. The r, p, and n denote the correlation coefficients, p values, and n numbers of the altimeter and OFES2 EKE time series with TAO based on the Student’s t test.
Extended Data Fig. 5 Meridional structures of \({\overline{{{{\mathbf{SSH}}^{\prime}}}}_{\boldsymbol{y}}^{\mathbf{2}}}\), EKE, surface zonal current, and SST trends.
Left/right panels are trends/mean over 1993–2021, averaged between 160°W–100°W. a,b, \({\overline{{{\rm{{SSH}}^{\prime}}}}_{{y}}^{{2}}}\). c,d, EKE. e,f, surface zonal current. g,h, SST. They are on the basis of satellite observation or derivation (red), GLORYS12V1 reanalysis (magenta), and OFES2 simulation (blue). Black curve in f denotes surface zonal currents from historical drifter trajectories66. Trends marked with circles pass the 95% confidence level based on the modified Mann–Kendall test.
Extended Data Fig. 6 Mean states of EKE budget based on OFES2.
Similar to Fig. 3, but for the climatological mean. a, Mean EKE and the budget terms (shaded bars); Error bars denote uncertainties as the standard deviations between 1993–2021 mostly due to interannual variability; Data are represented as the mean ± uncertainties, with n numbers of 24, 28, 26, 24, and 25 from left to right. b–d, Meridional transects of mean EKE (colour shading) with zonal current (contours, cm s−1) (b), BTR (colour shading) with (1 − Uyy/β) (contours, dimensionless) (c) and BCR (colour shading) with temperature (contour, °C) (d), 160° W–100° W averaged. e, 9-yr running mean time series of EKE (red), BTR (solid magenta), BCR (dashed magenta) averaged between 160°W–100°W, 2°S–2°N, and 0–50 m depth, and SEC speed (black) averaged between 0°–4°N.
Extended Data Fig. 7 Trends of upper ocean stratifications in the equatorial Pacific.
1993–2021 trends (color shading) are superimposed with the mean states (contour, same unit as the trends without ‘/ dec’): a,b, Temperature; c,d, Vertical stratification (buoyancy frequency square, \({N}^{2}=-{b}_{z}\), where the buoyancy \(b=-\frac{\rho }{{\rho }_{0}}g\)); e,f, Meridional buoyancy gradient square (\({{b}_{y}}^{2}\)). \({{b}_{y}}^{2}\) is calculated using monthly data before the average. Left/right panels are from IAP reconstruction/OFES2, averaged between 160°W–100°W. Dashed and solid black curves are climatological mean mixed layer depth (MLD) and D20, respectively. Note that the left and right have different ranges of colorbar and contour intervals in order to magnify the structure of the trends.
Extended Data Fig. 8 Trends of the tropical Pacific climate and ocean surface currents in OFES2 simulation.
Same as Fig. 5a,c but for the OFES2 simulation. a, 1993–2021 trends of simulated SST (colour shading) and 10-m wind from JRA55-do reanalysis (vector, accelerating in bold red; decelerating in thin blue). b, 1993–2021 trends of simulated SSH (colour shading) and surface currents (vector, accelerating in bold red; decelerating in thin blue). The JRA55-do reanalysis is the atmospheric forcing of the OFES2 simulation.
Extended Data Fig. 9 CMIP6 HighResMIP1-simulated and projected \({\overline{{{{\mathbf{SST}}^{\prime}}}}_{\boldsymbol{y}}^{\mathbf{2}}}\) trends.
a, Climatological mean as a function of latitude; b, Long-term evolution (9-yr running mean, standardized by the 1993–2014 mean). Scatter diagrams of the standardized \(\overline{{{{\rm{SST}}^{\prime}}}}_{y}^{2}\) (y axis) vs. \({{\overline{{\rm{SST}}}}_{{yy}}}^{2}\) (x axis): c, the 1993–2014 mean (standardized by the observation), d, the 1993–2014 and e, 2015–2050 trends (markers with black edge pass the 95% confidence level), averaged between 160°W–100°W and 1°S–1°N. Black curves in a,b and grey pentagrams in c,d denote the observation. Bold colored curves (blue/red denotes historical run/future projection) in a,b, and grey squares in c,d,e denote multi-model ensemble mean (MME) with spreads (1 s.d. of ensemble with n number of 6). The used CMIP6 HighResMIP1 ensemble consists of six coupled atmosphere-ocean models (thin curves or colored markers) with historical radiative forcing (Hist-1950 experiment: 1950–2014) and future SSP585 forcing (Highres-future experiment: 2015–2050).
Extended Data Fig. 10 Trends of TIW-induced EDH.
1993–2021 trends (color shading) are superimposed with the mean states (contour, same unit as the trends without ‘/ dec’): a, Satellite-derived surface EDH; b, 160°W–100°W average of a along with the uncertainties in percentage form, standardized by maximum values (∼2 °C month−1) of the climatological mean in the meridional direction; Data are represented as the trends ± uncertainties (estimated as the s.e.m.) at the equator, with n numbers of 25. c, Meridional transect of OFES2-simulated subsurface EDH averaged between 160°W–100°W. The OFES2 simulation underestimates the EDH trends because the \(\overline{{{{\rm{SST}}^{\prime}}}}_{y}^{2}\) trend is biased to be negative (Extended Data Fig. 3l). The OFES2 results suggest that the heating effects are amplified near the surface.
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Wang, M., Xie, SP., Sasaki, H. et al. Intensification of Pacific tropical instability waves over the recent three decades. Nat. Clim. Chang. 14, 163–170 (2024). https://doi.org/10.1038/s41558-023-01915-x
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DOI: https://doi.org/10.1038/s41558-023-01915-x
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