North Pacific subtropical mode water is controlled by the Atlantic Multidecadal Variability

Abstract

North Pacific subtropical mode water, a vertically homogeneous thermocline water mass, occupies the entire subtropical Western Pacific Ocean. It transports mass, heat and nutrients from the surface to the subsurface ocean, providing memory of climate variability1,2,3,4,5,6,7,8,9,10,11. Decadal variability of the mode water temperature has been attributed to the Pacific Decadal Oscillation (PDO)2,3,12,13, but this is based on short data records. Here, using long records of observations, we show that decadal-to-multidecadal variability of the mode water mean temperature is instead controlled by the Atlantic Multidecadal Variability (AMV). During an AMV-positive phase, warm sea surface temperatures (SSTs) in the North Atlantic Ocean weaken the subtropical North Pacific westerlies; the associated anomalous easterlies in the subtropical west Pacific4,5 drive a northward Ekman transport of warm water to the mode water formation area. Subduction of the warm water increases mode water temperature, influencing Northwestern Pacific upper ocean heat content and fish catches. A long pre-industrial model simulation with multiple AMV cycles and a pacemaker experiment support this mechanism—the AMV forcing alone can drive decadal variability of the mode water. Thus, the AMV provides important memory for prediction of decadal climate and ecosystem variability in the Pacific Ocean.

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Fig. 1: North Pacific subtropical mode water and its relationship with the AMV.
Fig. 2: Connection between the North Pacific subtropical mode water and the AMV.
Fig. 3: Temperature anomaly induced by AMV propagates from surface to subsurface.
Fig. 4: Mechanism and influence of the North Pacific subtropical mode water.

Data availability

Data related to this paper can be downloaded from the following: Ishii data, https://rda.ucar.edu/datasets/ds285.3/; EN4 data, https://www.metoffice.gov.uk/hadobs/en4/; IAP data, http://159.226.119.60/cheng/; SODA data, https://climatedataguide.ucar.edu/climate-data/soda-simple-ocean-data-assimilation; NCEP data, https://www.esrl.noaa.gov/psd/; the PDO index, https://www.ncdc.noaa.gov/teleconnections/pdo/data.csv; the AMV index, https://climatedataguide.ucar.edu/climate-data/atlantic-multi-decadal-oscillation-AMO-index-station-based; the data of the PI-control experiment and pacemaker experiment are available at Ocean and Atmosphere Data Center of Ocean University of China, http://coadc.ouc.edu.cn/download/b.e10.B1850CN.f09_g16.001/ and http://coadc.ouc.edu.cn/download/Pacemaker_EXP/, respectively.

Code availability

The CESM, developed by the NCAR, can be downloaded from http://www2.cesm.ucar.edu/.

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Acknowledgements

X.L. is supported by China’s National Key Research and Development Projects (2016YFA0601803) and the National Natural Science Foundation of China (41925025 and U1606402). B.W. is supported by the China Scholarship Council (201806330010). L.Y. thanks the NOAA for support for her study on climate change and variability.

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Contributions

X.L., B.W. and L.Y. conceived the study and B.W. performed data analysis. B.W. and X.L. wrote the initial manuscript. All authors contributed to interpreting results, discussion of the associated dynamics and improvement of this paper.

Corresponding author

Correspondence to Xiaopei Lin.

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The authors declare no competing interests.

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Peer review information Nature Climate Change thanks Xichen Li and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 North Pacific subtropical mode water and its relationship with the PDO.

a, Time series of detrend mode water temperature anomaly (black from Ishii data, blue from EN4 data, yellow from IAP data, purple from SODA data) and the normalized PDO index (red) smoothed by a 7-year low-pass filter. The shadings for each line show the one standard deviations of their time series. The grey shadings indicate a cold period (1970–1980) and a warm period (2000–2010) of the mode water, respectively. b, Lead-lag correlation between the low-pass filtered detrend mode water temperature anomaly (black from Ishii data, blue from EN4 data, yellow from IAP data) and the PDO index during the period of 1945–2012. c, Same as (b), but only for the period of 1978–2012.

Extended Data Fig. 2 Variation of the mode water mean depth and thickness.

a, Mean temperature gradient (shading, unit in oC/10 m) during the cold period derived from the Ishii data. Black solid contours show the mean potential temperature for the cold period (1970–1980) and the dashed contours are their climatology. The marked 16 °C and 18 °C contours are the defined upper and lower temperature boundaries of the mode water, and the marked 12 °C contours present the main thermocline depth. The white contour is the 1.5 °C/100 m line, which is the vertical temperature gradient criterion of the mode water (see Methods). b, Same as (a), but for the warm period 2000-2010. c, Time series of detrend mode water thickness (unit in m, black) from the Ishii data and the normalized AMV index (red) for the period 1945–2012 (thin lines) and 7-year running averages (thick lines). Correlation coefficient between the mean depth and AMV at zero lag is -0.85. d, Same as (c), but for the mode water mean depth (unit in m, black). Correlation coefficient between the mode water thickness and AMV at zero lag is 0.83.

Extended Data Fig. 3 Variation of the mixed layer depth and main thermocline depth in the mode water area.

a, Time series of detrend mixed layer depth (MLD, unit in m, black) from the Ishii data and the normalized AMV index (red) for the period 1945-2012 (thin lines) and 7-year running averages (thick lines). Correlation coefficient between the MLD and AMV at zero lag is -0.80. b, MLD anomaly (unit in m) during the cold period. The contour intervals are 2 m and the thick contours indicate the value of 4 m. c, MLD anomaly (unit in m) during the warm period. The contour intervals are 2 m and the thick contours indicate the value of -4 m. d, Same as (a), but for the main thermocline depth (MTD, see Methods, unit in m). Correlation coefficient between the MTD and AMV at zero lag is -0.82. e, MTD anomaly (unit in m) during the cold period. The contour intervals are 5 m and the thick contours indicate the value of 5 m. f, MTD anomaly (unit in m) during the warm period. The contour intervals are 5 m and the thick contours indicate the value of -5 m.

Extended Data Fig. 4 Model supports for the mechanism of the mode water controlled by the AMV.

a, Regressions of the SST (shading, unit in oC) and surface wind filed (vector, unit in m/s) with respect to the AMV index derived from the pre-industrial model simulation (PI-Control EXP.). Green line indicates the transect A (150°E, 20°-50°N). b, Temperature regressed upon the AMV index, derived from the PI-Control EXP., along transect A. c, Time series of mode water mean temperature and the normalized model AMV index (red) from the PI-Control EXP. smoothed by a 7-year low-pass filter. Correlation coefficient between the mode water mean temperature and the model AMV at zero lag is 0.76. The shading shows the one standard deviations of its time series.

Extended Data Fig. 5 Mechanism of the North Pacific subtropical mode water for the cold period.

a, Net heat flux (Qnet) anomaly (units in W/m2) in the cold period of 1970-1980 derived from the NCEP data set. The black box indicates the formation area (130°E-180°, 28°-35°N) and the subduction area (130°E-180°, 20°-28°N) of the mode water. b, SST anomalies induced by Ekman heat transport (shading, units in oC) and anomalous wind fields (vector, unit in m/s) in the mode water formation and subduction area during the cold period 1970-1980 derived from the Ishii and NCEP data sets.

Extended Data Fig. 6 Pacemaker model experiment forced by the positive AMV SSTs.

In the Atlantic Ocean, observed SST trend (unit in oC/36 yr) from 1979-2014 was added in the modelled mixed layer temperature in the restoring forcing. In the other areas, the mixed-layer temperature was restored to the model climatology. Then the climate response to the restored ocean temperature was calculated by the difference between the Pacemaker experiment and a control experiment, in which the mixed-layer temperature was restored to the model climatology.

Extended Data Fig. 7 Transient simulation for the mechanism of the mode water controlled by the AMV.

a, Anomalous SST (shading, unit in oC) from the Pacemaker model experiment (Pacemaker EXP.) for the AMV positive phase (mode water warm phase) in the first year. Green line indicates the transect A (150°E, 20°-50°N). b, Temperature anomaly derived from Pacemaker EXP. during the AMV positive phase, along transect A, in the first year. c, d and e, f, Same as (a), (b), but for the second year and last 10 years. Contours show the mean temperature for each year and the marked 16 oC and 20 oC contours are the defined upper and lower temperature boundaries of the mode water in the Pacemaker EXP., and the marked 12 oC contours present the main thermocline depth (see Methods).

Extended Data Fig. 8 Vertical structure of the ocean heat content.

a, Ocean heat content density anomalies (OHCa, shading, unit in × 106 J/m3) along transect A during cold period derived from the Ishii data. Black solid contours show the mean potential temperature for the cold period (1970-1980) and the dashed contours are their climatology. The marked 16 oC and 18 oC contours are the defined upper and lower temperature boundaries of the mode water, and the marked 12 oC contours present the main thermocline depth. The white contour is the 1.5 oC/100 m line, which is the vertical temperature gradient criterion of the mode water (see Methods).

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Wu, B., Lin, X. & Yu, L. North Pacific subtropical mode water is controlled by the Atlantic Multidecadal Variability. Nat. Clim. Chang. 10, 238–243 (2020). https://doi.org/10.1038/s41558-020-0692-5

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