Supplementary Information for:

12345

Critical role of biomass burning aerosols in enhanced historical

6Indian Ocean warming

7Yiqun Tian1, Shineng Hu1*, Clara Deser28910

11112

Division of Earth and Climate Sciences, Nicholas School of the Environment, Duke University,Durham, NC, USA

2

Climate and Global Dynamics, National Center for Atmospheric Research, Boulder, CO, USA

1314
15
16
17181920
2122
23
24252627*

28

Division of Earth and Climate Sciences, Nicholas School of the Environment, Duke University

Corresponding author: Shineng Hu (shineng.hu@duke.edu)

2930Supplementary Figure 1 | Observed tropical SST trends since 1925. Observed tropical SST

31

trends (K/century) during 1925-2021, averaged across four SST datasets (ERSSTv5, HadISST,

32

COBE-SST, Kaplan SST). The black contour line represents the isopleth of the tropical-mean

33

SST trend (value shown on the top-right corner). This figure is the same as Fig. 1a but with only34

the regions where all four datasets agree on the sign of the SST trend being shown. White35

hatches represent the regions that are 99% significant based on a t-test.

3637

Supplementary Figure 2 | Observed tropical SST trends in different datasets. a-c, SST

38

trends in HadISST during the periods of a 1925-2021, b 1925-2005, and c 1950-2021. d-f, SST

39

trends during 1925-2021 for d ERSST v5, e COBE-SST, and f Kaplan SST v2. The black40

contour line represents the isopleth of the tropical-mean SST trend (value shown on the top-right41corner). White hatches in a-f represent the regions that are 99% significant based on a t-test.

4243

Supplementary Figure 3 | Observed and model simulated variations in annual-mean TIO

44

SST since 1925. a, TIO absolute SST (K) and b, TIO relative SST (K) with respect to the45reference period 1925-1944. In both panels, the grey lines are for each CESM1-LE all-forcing

46

ensemble member and the black line is for ensemble-mean in the all-forcing experiments, and47

the colored lines are for different observational SST datasets. Relative SST is defined as absolute48

SST minus tropical-mean SST.

49
50

Supplementary Figure 4| Model simulated tropical SST trend pattern against observations.

51In all panels, the horizontal axis is for TIO relative SST trends during 1925-2005, and the52

vertical axis is for the pattern correlation of tropical SST trends within 20°S-20°N between an

53

individual ensemble member (grey) or ensemble-mean (black) of CESM1 LE all-forcing

54

experiments and observations. The only difference among the four panels is the observational

55

dataset. a, HadISST. b, ERSSTv5. c, Kaplan SST v2. d, COBESST V2. Relative SST is defined56

as absolute SST minus tropical-mean SST.

5758Supplementary Figure 5 | BMB aerosol changes in the historical period. a, Aerosol optical

59

depth (AOD) trends during 1925-2005 associated with BMB aerosol changes. b, AOD variations

60in specific regions associated with BMB aerosol changes. Boxes used for averaging AOD are

61

shown in a. White hatches in a represent the regions that are 99% significant based on a t-test.

6263

Supplementary Figure 6 | BMB-induced tropical SST trends. BMB-induced tropical SST

64##trends (K/century) during 1925-2005 associated with a total ( 𝑇!"), b clear-sky (𝑇!",%&'()), and c65#cloud (𝑇!",%&*+,) shortwave radiative flux changes.

66
67

Supplementary Figure 7 | Climate changes simulated by CESM1 LE all-forcing

68

experiments. a, Ensemble-mean rainfall trends (colors; mm/d/century) overlaid by69

climatological rainfall (contours; mm/d) and b, Ensemble-mean ocean salinity trends

70(g/kg/century) during 1925-2005. c, Ensemble-mean 850 mb zonal wind trends (colors;71

m/s/century) overlaid by climatological 850 mb zonal wind (contours; m/s) and d, Ensemble-72

mean rainfall trends (mm/d/century) during 1925-2005. White hatches in a-d represent the73

regions that are 99% significant based on a t-test.

74
75

Supplementary Figure 8 | NAO changes in the single-forcing CESM1 LE. a, Sea level

76

pressure anomalies associated with the positive phase of NAO. b, Probability distribution

77

functions of NAO trends across all the members in each set with vertical lines highlighting the78

ensemble averages. c-f, Ensemble-mean NAO series in ALL, BMB, GHG, and AAER,

79

respectively.

80
81

Supplementary Figure 9 | Observed and model simulated TIO relative SST series. a, 21-82

year running mean for TIO relative SST variations (°C) since 1920 for observations (grey lines)

83

and ensemble means from the single-forcing CESM1 LE. The grey and green shadings are one

84

standard deviation among ensemble members for ALL and BMB, respectively.