Research ArticleCLIMATOLOGY

Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: A test for future predictions

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Science Advances  04 Jun 2021:
Vol. 7, no. 23, eabg3848
DOI: 10.1126/sciadv.abg3848
  • Fig. 1 Site locations and climatology.

    (A) Location of proxy sites showing major rivers and geographic designations used in the text and figures. Site location colors denote records reflecting terrestrial precipitation isotopes (orange), wind strength (blue), and seawater isotopes (green). The same color scheme is used in Figs. 3, 4, and 6. (B) Seasonal precipitation climatology for purple box (80° to 99.5°E, 18° to 29°N). (C) BoB salinity (blue time series and box; 84.5° to 89.5°E, 16.5° to 20.5°N) compared to precipitation over land (purple time series and box) and precipitation over Mahanadi basin (D) (green time series; 82.5° to 85.5°E and 19.5° to 22.5°N). Precipitation climatology (96) and salinity climatology (97). Black outline is the Indian core monsoon zone.

  • Fig. 2 U1446 proxy records and spectra.

    Nine hundred thousand years marks shifts in the mean, variance, or spectral nature of various proxies, occurring near the midpoint of the mid-Pleistocene transition (MPT) between ~1150 and 700 ka ago. Spectra are for the 0- to 900-ka ago interval only. Red spectrum is the same in all cases, denoting ETP, an averaged record of normalized eccentricity, obliquity (tilt; T), and precession (June 21 perihelion) spanning the past 900 ka ago. Benthic and planktic δ18O [‰ Vienna Pee Dee belemnite (VPDB)], δ18Osw (‰ standard mean ocean water), and δ13Cwax (‰ VPDB). δ18Osw is corrected for both temperature and sea level change (see Materials and Methods).

  • Fig. 3 Phase wheel summaries of cross-spectral coherence and phase relationships at orbital frequency bands.

    Zero phase is set at June 21 perihelion for precession, denoting the timing of minimum precession (Pmin), maximum obliquity, and maximum eccentricity. (A) Positive phase (counterclockwise) represents leads, and negative phase (clockwise) represents lags. Red text and dots represent the timing of external forcing factors (hemispheric and seasonal insolation maxima and minima; at 0° and 180°) and internal forcing factors (global IV minima and GHG maxima; CO2). Vectors represent the timing of monsoon proxy responses within the (B) 100-ka eccentricity cycle, (C) the 23-ka precession cycle, and (D) the 41-ka obliquity cycle (averaged over the number of cycles present in a given record). All phase vectors plotted are coherent (>80 CI) with eccentricity, obliquity, and precession, with one exception; NW BoB δ18Osw coherence does not exceed 80 CI at eccentricity (0.78) but is strongly coherent with IV (0.90), so this phase is plotted relative to ice mininum. Phase errors are typically ±10° to 20° (table S3). Color-coded text at 0° and 180° (as in Figs. 1, 4, and 6) indicates the timing of model simulated responses to insolation-only forcing, green for maximum precipitation, blue for maximum winds, and orange for precipitation isotope minima. Proxy response vectors are similarly color coded (e.g., green for maximum precipitation/runoff). The gray vector and dashed line indicate the timing of SST maxima and G. ruber δ18O minima, respectively. SITIG, summer intertropical insolation gradient.

  • Fig. 4 As in Fig. 3 with the addition of proxy vectors from sites other than U1446 (Fig. 1).

    Proxy responses interpreted as responding to winds (blue), precipitation/runoff (green), and precipitation isotopic composition (orange) follow color coding of insolation-only simulation results (text at 0° and 180°) and that in Figs. 1, 3, and 6. Black vectors denote phase of the first derivative of the IV record, reflecting sea level (SL) change. The modifier ‘-TC’ in panel D indicates measurement at the thermocline, as opposed to measurement within the mixed-layer.

  • Fig. 5 Termination dynamics.

    (A) NW BoB precipitation/runoff (δ18Osw; red) relative to IV (benthic δ18O; blue). Terminations (T) 1 to 10 are denoted by vertical dashed blue lines. The midpoints of subsequent precipitation/runoff transitions to interglacial values are denoted by vertical dashed red lines. Red and blue dots indicate IV maxima and precipitation/runoff minima. (B) Same as in (A) but focusing on the ±15 ka surrounding IV terminations, for which the midpoints have been set to zero, with negative values indicating younger time. Thin dashed lines are individual records that have been averaged to yield thick lines. Arrow indicates the average lag between midpoints of IV terminations and midpoints of the subsequent proxy transitions. (C) Same as in (B) but showing EA YRV speleothem δ18O for T1 to T7 (50).

  • Fig. 6 Differences in average precipitation (June to August, mm per day) for precession (Pmin-Pmax) and obliquity (Tmax-Tmin) for each model.

    (A, B) European Community (EC). (C, D) Geophysical Fluid Dynamics Laboratory (GFDL). (E, F) Community Earth Systems Model (CESM). (G) Hadley Centre Coupled Model (HadCM3). Contours indicate values for Pmax and Tmin. Thick contour indicates 4000-m elevation for the Tibetan Plateau. Positive values (blue shading) indicate increased precipitation at Pmin and Tmax. Negative values (red shading) indicate increased precipitation at Pmax and Tmin. Figure after Bosmans et al. (60) Fig. 3 with proxy site locations superimposed and Indian core monsoon zone (63) outlined in (C) and (D). Site location colors denote records reflecting terrestrial precipitation isotopes (orange), wind strength (blue), and seawater isotopes (green), consistent with Figs. 1, 3, and 4.

Supplementary Materials

  • Supplementary Materials

    Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: A test for future predictions

    Steven C. Clemens, Masanobu Yamamoto, Kaustubh Thirumalai, Liviu Giosan, Julie N. Richey, Katrina Nilsson-Kerr, Yair Rosenthal, Pallavi Anand, Sarah M. McGrath

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    This PDF file includes:

    • Figs. S1 to S3
    • Tables S1 to S3

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