Enhanced warming of the subtropical mode water in the North Pacific and North Atlantic

Transcription

1 In the format provided by the authors and unedited. SUPPLEMENTARY INFORMATION DOI: /NCLIMATE3371 Enhanced warming of the subtropical mode water in the North Pacific and North Atlantic Shusaku Sugimoto 1 *, Kimio Hanawa 2, Tomowo Watanabe 3, Toshio Suga 2 and Shang-Ping Xie 4 Over the past six decades, the subtropical surface ocean widespread. In situ subsurface data were sparse prior to the 1 Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai , Japan. 2 Department of Geophysics, Graduate School of Science, Tohoku University, Sendai , Japan. 3 National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Yokohama , Japan. 4 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093, USA. * sugimoto@pol.gp.tohoku.ac.jp This PDF file includes: Supplementary Discussion 1. Relationship between the subtropical mode water temperature and climate modes Supplementary Table S1 Supplementary Figures S1 to S10 NATURE CLIMATE CHANGE Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

2 1. Relationship between the subtropical mode water temperature and climate modes It has been reported that the North Atlantic Subtropical Mode Water (NASTMW) temperature is modulated strongly with winter heat loss associated with the North Atlantic Oscillation (NAO) (ref. 6, 37). Actually, the winter sea surface temperature (SST) in the NASTMW formation region is significantly correlated with NAO index with no lags (Fig. S10). Past works have pointed out the delayed (about 5 7 years) response of North Pacific Subtropical Mode Water (NPSTMW) temperature to the Pacific Decadal Oscillation (PDO)-related wind forcing in the central North Pacific (ref. 38, 39); the subtropical gyre spin-up associated with the intensification of westerlies at a positive phase of PDO induces an increase in warm water advection by the Kuroshio with a lag of 3 to 5 years (ref ), resulting in positive temperature anomalies in the NPSTMW formation region about 2 years later (ref. 41, 43). We examined the relationship between the PDO and winter SST in the NPSTMW formation region, by performing a lag correlation analysis. Results represented that the SST is correlated significantly with the PDO index with a lag of 6 years (Fig. S10), as pointed out by past works (ref. 38). As displayed in Fig. S10, the correlation coefficient between the SST in the NPSTMW formation region and PDO index is not so high. This means that other processes influence the winter SST variability in the NPSTMW formation region; ocean heat loss due to the winter monsoon and monsoon-induced southward Ekman transport (ref ) and vertical entrainment process attributable to variations in strength of subsurface stratification (ref 27, 48, 49). To gain better understanding of NPSTMW temperature variability, more research is needed. But it is beyond the scope of this study focusing change of NPSTMW temperature. Additional References for Supplementary Discussion 31. Locarnini, R. A. et al. Temperature. Vol. 1, World Ocean Atlas 2013, NOAA Atlas NESDIS 73, 40 pp. (2013). 32. Zweng, M. M. et al. Salinity. Vol. 2, World Ocean Atlas NOAA Atlas NESDIS 74, 39 pp. (2013). 2

3 33. Monterey, G. & Levitus, S. Seasonal Variability of Mixed Layer Depth for the World Ocean. NOAA Atlas NESDIS 14, 100 pp. (1997). 34. Compo, G. P. et al. The Twentieth Century Reanalysis Project. Q. J. R. Meteorol. Soc. 137, 1 28 (2011). 35. Hersbach, H. et al. ERA-20CM: A twentieth-century atmospheric model ensemble. Q. J. R. Meteorol. Soc. 141, (2015). 36. Garcia, H. E. & Gordon, L. I. Oxygen solubility in seawater: better fitting equations. Limnol. Oceanogr. 37, (1992). 37. Joyce, T. M., Deser, C. & Spall, M. A. The relation between decadal variability of subtropical mode water and the North Atlantic Oscillation. J. Clim. 13, (2000). 38. Hanawa, K. & Kamada, J. Variability of core layer temperature (CLT) of the North Pacific subtropical mode water. Geophys. Res. Lett. 28, (2001). 39. Sugimoto, S. & Hanawa, K. Impact of remote reemergence of the subtropical mode water on winter SST variation in the central North Pacific. J. Clim. 20, (2007). 40. Deser, C., Alexander, M. A. & Timlin, M. S. Evidence for a wind-driven intensification of the Kuroshio Current Extension from the 1970s to the 1980s. J. Clim. 12, (1999). 41. Yasuda, T. & Kitamura, Y. Long-term variability of North Pacific subtropical mode water in response to spin-up of the subtropical gyre. J. Oceanogr. 59, (2003). 42. Sugimoto, S. et al. Temporal variations of the net Kuroshio transport and its relation to atmospheric variations. J. Oceanogr. 66, (2010). 43. Vivier, F., Kelly, K. A., & Thompson, L. Heat budget in the Kuroshio Extension region: J. Phys. Oceanogr. 32, (2002). 44. Suga, T., & Hanawa, K. The subtropical mode water circulation in the North Pacific. J. Phys. Oceanogr. 25, (1995). 45. Yasuda, T. & Hanawa, K. Decadal changes in the mode waters in the midlatitude North Pacific. J. Phys. Oceanogr. 27, (1997). 46. Taneda, T., Suga, T. & Hanawa, K. Subtropical mode water variation in the northwestern part of the North Pacific sub- tropical gyre. J. Geophys. Res. 105, 3

4 (2000). 47. Hanawa, K. & Yoritaka, H. North Pacific subtropical mode waters observed in long XBT cross sections along 32.5 N line. J. Oceanogr. 57, (2001). 48. Qiu, B., Chen, S., & Hacker, P. Effect of mesoscale eddies on subtropical mode water variability from the Kuroshio Extension System Study (KESS). J. Phys. Oceanogr. 37, (2007). 49. Iwamaru, H., Kobashi, F. & Iwasaka, N. Temporal variations of the winter mixed layer south of the Kuroshio Extension. J. Oceanogr. 66, (2010). 4

5 Supplementary Table S1. List of 22 CMIP5 climate models used in this study Model Name Institute Country Ocean Grid Period used (Lon Lat) ACCESS1.3 Commonwealth Scientific and Australia Industrial Research Organization/Bureau of Meteorology BCC-CSM1.1 Beijing Climate Center China CanESM2 Canadian Centre for Climate Canada Modelling and Analysis CCSM4 National Center for Atmospheric USA Research CNRM-CM5 Centre National de Recherches France Météorologiques/Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique CSIRO-Mk3.6.0 Commonwealth Scientific and Australia Industrial Research Organization/Queensland Climate Change Centre of Excellence FGOALS-g2 Institute of Atmospheric Physics, China Chinese Academy of Sciences GFDL-CM3 Geophysical Fluid Dynamics USA GFDL-ESM2G Laboratory GFDL-ESM2M GISS-E2-H NASA/GISS (Goddard Institute for USA GISS-E2-R Space Studies) HadGEM2-CC Met Office Hadley Centre UK HadGEM2-ES IPSL-CM5A-MR Institute Pierre Simon Laplace France IPSL-CM5B-LR MIROC-ESM Atmosphere and Ocean Research Japan Institute (The University of Tokyo)/National Institute for Environmental Studies/Japan Agency for Marine-Earth Science and Technology MPI-ESM-LR Max Planck Institute for Meteorology Germany MPI-ESM-MR MRI-CGCM3 Meteorological Research Institute Japan MRI-ESM NorESM1-M Bjerknes Centre for Climate Research, Norwegian Meteorological Institute Norway

6 NPSTMW-NASTMW distribution La t it u de 50N 40N 30 N 30N 30 N section 33 N section E 180 Longitude 120W 60W Longitude 0 Depth [m] Depth [m] 20N 33 N 700 Supplementary Figure S1. Geographical distribution of NPSTMW and NASTMW. Upper panel displays thickness of the NPSTMW and NASTMW in June, from the World Ocean Atlas 2013 version 2 (WOA13v2, ref. 31, 32). Lower two panels represent cross sections of potential temperature in June along 30 N in the North Pacific and 33 N in the North Atlantic, from the WOA13v2. Thick solid contours indicate θ (potential temperature) of 15 C and 20 C, which are the defined upper and lower temperature boundaries of NPSTMW and NASTMW. Blue shading denotes the core layer of low vertical temperature gradient < 1.5 C (100 m) 1. 6

7 Mixed layer depth in winter Supplementary Figure S2. Mixed layer depth in late winter (February March). The mixed layer depth is defined as the shallowest depth at which potential density increases by kg m-3 from 10 m depth and that potential temperature changes by 0.5 C from 10 m depth, following Monterey and Levitus (ref. 33), from the WOA13v2. The contours represent annual-mean dynamic height at 200 m (with a contour interval of 0.1 m2 s-2) relative to the 1000-dbar level. Black rectangles represent the NPSTMW formation region (135 E 155 E, 28 N 35 N) and NASTMW formation region (40 W 70 W, 34 N 40 N), which encompass the region where the mode water properties are set and the region where the mode water is subducted. 7

8 a Number of profiles in NPSTMW region 1, Argo WOD13 FRA JODC b Number of profiles in NASTMW region 1, Argo WOD Supplementary Figure S3. Number of profiles used for calculation of NPSTMW and NASTMW temperature. Number of profiles, color-coded by data source, in which the core temperature was detected in the NPSTMW distribution region (133 E 160 E, 26 N 35 N) and NASTMW distribution region (40 W 75 W, 23 N 37 N) from May to December of each year. 8

9 a HadISST1 b ERSST4 c Kaplan2 d HadSST2 e Minobe-SST Supplementary Figure S4. Trends in SST. Annual-mean SST trends for five datasets: a) HadISST1, b) ERSST4, c) Kaplan2, d) HadSST2, and e) Minobe-SST. Gray stippling indicates trends that are not statistically significant at 90% confidence level. White grids represent insufficient data. Black rectangles represent the North Pacific subtropics (NPST; 130 E 180, 20 N 33 N) and North Atlantic subtropics (NAST; 30 W 75 W, 20 N 37 N). 9

10 Winter a HadISST1 b ERSST4 c Kaplan2 d HadSST2 e Minobe-SST Supplementary Figure S5. Trends in winter SST. Same as Supplementary Fig. S4, except for winter (January March) SST: black rectangles represent the NPSTMW and NASTMW formation regions. 10

11 Winter upward heat release trends [W m 2 100yr 1 ] NPSTMW formation region NASTMW formation region NOAA20CRv2 ERA20c Supplementary Figure S6. Trends in winter upward heat release in the NPSTMW and NASTMW formation regions. Winter (January March) trends of net surface heat flux, which is a sum of the shortwave radiation flux, longwave radiation flux, latent heat flux, and sensible heat flux, for two atmospheric reanalysis datasets: 1) the National Oceanic and Atmospheric Administration (NOAA)/Cooperative Institute for Research in Environmental Sciences 20th Century Reanalysis version 2 (NOAA20CRv2, ref. 34) on a 2 (longitude) 2 (latitude) grid (green bars) and 2) the European Centre for Medium-Range Weather Forecasts (ECMWF) 20th Century Reanalysis (ERA20c, ref. 35) on a 1 1 grid (orange bars). A positive value indicates an increased heat release from the ocean. Error bars indicate ranges of 90% confidence. Solid error bars represent statistically significant trends exceeding 90% confidence level. 11

12 a NPSTMW vertical temperature gradient [ C 100m 1 ] b NASTMW vertical temperature gradient [ C 100m 1 ] c Trends [ C 100m 1 100yr 1 ] NPSTMW NASTMW Supplementary Figure S7. Trends in NPSTMW and NASTMW stratification. a, ly time series (see Methods) of the core vertical temperature gradient of NPSTMW. Shading represents ranges of 90% confidence, estimated from anomaly values with at least five profiles. White circles denote values when there were fewer than five profiles. b, Same as a, except for NASTMW. c, Same as Supplementary Fig. S6, except displaying trends in the core vertical temperature gradient of NPSTMW and NASTMW. All trends are significant at 90% confidence level. 12

13 a Number of profiles in NPSTMW region 1, b Number of profiles in NASTMW region 1, Supplementary Figure S8. Number of profiles used for calculation of dissolved oxygen and O 2 saturation in NPSTMW and NASTMW. Same as Supplementary Fig. S3, except displaying the number of profiles used to calculate dissolved oxygen and O 2 saturation from the World Ocean Database 2013 (WOD13, ref. 19). 13

14 a NPSTMW dissolved oxygen b NASTMW dissolved oxygen c Trends DO O 2 * NPSTMW DO O 2 * NASTMW Supplementary Figure S9. Trends in NPSTMW and NASTMW dissolved oxygen content and saturation value of O 2. a, b, Same as Supplementary Fig. S7, except displaying the core dissolved oxygen content. Gray line represents O 2 saturation (O 2 *) of NPSTMW and NASTMW. O 2 * is estimated based on Garcia and Gordon (ref. 36). c, Same as Supplementary Fig. S6, except displaying trends in dissolved oxygen (DO) content (dark gray bars) and O 2 saturation (O 2 *) (light gray bars) of NPSTMW and NASTMW. All trends are significant at 90% confidence level. 14

15 Correlation coefficient Lag [years] Supplementary Figure S10. Lag relationship between climate modes and SST in mode water formation regions. Lag-correlation coefficients of (red line) winter SST in the NPSTMW formation region versus PDO index with different lags and (blue line) winter SST in the NASTMW formation region versus NAO index with different lags. Lag means the climate mode indices leading the SST. White circles represent significant at 90% confidence level. 15