Early emergence and determinants of human-induced Walker circulation weakening

Figures

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  Fig. 1. Long-term changes in the Walker circulation for 1980–2099 under different emission scenarios. Spatial distribution of linear trends in (a) sea level pressure, (c) vertical velocity at 500 hPa (positive values refer to downward velocity), (e) velocity potential at 200 hPa, and (g) area-averaged vertical velocity over 5°S–5°N under the SSP2-4.5 scenario. Black contours denote the climatological mean from 1980–2014. White slant hatchings denote areas where more than 80% of the models agree in sign of the trends. b, d, f and h are the same as (a), (c), (e), and (g) but under the SSP5-8.5 scenario. Boxes in (a–f) indicate regions used to define the Walker circulation intensity indices and the zonal SST gradient (see Methods).

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  Fig. 2. Comparison of detectability of the Walker circulation intensity change at different tropospheric levels. a Signal-to-noise ratio of the Walker circulation intensity change relative to the 1980–2014 baseline calculated based on the sea-level pressure gradient (dSLP). Blue (red) thick line denotes the ensemble mean of models under the SSP2-4.5 (SSP5-8.5) scenario, while blue (red) shading indicates the model spread. The gray dashed lines denote the signal-to-noise ratio of 1 or -1. b Time of emergence (ToE) for annual-mean Walker circulation intensity change based on the dSLP index, under the SSP2-4.5 (blue bars) and SSP5-8.5 (red bars) scenarios. Lighter blue and red bars indicate models in which no emergence of the forced Walker circulation intensity change is detected by the end of the 21st century for the respective scenarios. c–f are the same as (a–b) but based on the 500 hPa vertical velocity gradient (dW500) and the maximum absolute value in the 200 hPa velocity potential over the western tropical Pacific (VPmax), respectively.

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  Fig. 3. Time evolution of the signal-to-noise ratio and the intensity of Walker circulation in the abrupt4×CO2 experiment. a, c and e Time series of the signal-to-noise ratio of the Walker circulation change in the abrupt4×CO2 experiment based on the sea-level pressure gradient (dSLP), the 500 hPa vertical velocity gradient (dW500), and the maximum absolute value in the 200 hPa velocity potential (VPmax), respectively. b, d and f Time series of the Walker circulation intensity in the pre-industrial control experiment (black line) and the abrupt4×CO2 experiment (red line) based on dSLP, dW500, and VPmax indices, respectively. Red shading indicates the model spread in the abrupt4×CO2 experiment.

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  Fig. 4. Signal-to-noise ratio of the Walker circulation change in response to CO2 forcing at different tropospheric levels in the abrupt4×CO2 experiment. a–b Spatial distribution of the signal-to-noise ratio of sea level pressure (SLP) for the first 10 years and the last 50 years. Shown are the multimodel ensemble mean. White slant hatchings denote areas where more than 80% of the models agree on the sign of changes. c–f are the same as (a–b) but for vertical velocity at 500 hPa (W500) and velocity potential at 200 hPa (VP200), respectively. Boxes indicate regions used to define the Walker circulation intensity indices.

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  Fig. 5. Sources of uncertainty in ToE for the Walker circulation change. a Likely range of time of emergence (ToE) determined by forced signal (x-axis) and internal variability (y-axis) based on sea-level pressure gradient (dSLP) in CMIP6 models under the SSP5-8.5 scenario. b Intermodel relationship between ToE and slow response of the Walker circulation (See Methods) in the abrupt4×CO2 experiment based on the dSLP index. The intermodel correlation coefficient and p-value are shown in the upper-left corner of the panels, with p < 0.01 denoting a significant correlation. c–f are the same as (a–b) but for the 500 hPa vertical velocity gradient (dW500) and the maximum absolute value in the 200-hPa velocity potential over the western tropical Pacific (VPmax), respectively. Gray shading in (b), (d) and (f) indicates models without ToE.

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  Fig. 6. Verifying the role of the ocean thermostat mechanism. a Initial and (b) long-term sea surface temperature (SST) anomaly patterns in the abrupt4×CO2 experiments for Group-L models. c–d The same as (a–b) but for Group-E models. The SST anomalies are derived by comparing them to the piControl experiments and then subtracting the average ocean warming within the 40°S–40°N, 60°E–60°W region. Shown are the ensemble mean. White slant hatchings denote areas where all the models agree in sign.