Supporting Information forContribution of anthropogenic aerosols to persistent La Niña-likeconditions in the early 21st centuryYen-Ting Hwang1, Shang-Ping Xie2, Po-Ju Chen1,3, Hung-Yi Tseng1, and Clara Deser4
1Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

2

Scripps Institute of Oceanography, University of California San Diego, La Jolla, California, USA

3

Department of Atmospheric, Oceanic, & Earth Sciences, George Mason University, Fairfox, Virginia, USA

4

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

Corresponding authors: Yen-Ting Hwang and Clara Deser
Emails: ythwang@ntu.edu.tw, cdeser@ucar.edu

This PDF file includes:
Supporting text
Figures S1 to S8
SI References

1b.

c.

d.

e.

f.

Depth (m)Depth (m)a.

Figure S1. The geographic distribution of sulfate aerosol emission during (a) 1980s and(b) 2020s. (c)(e) The fast and slow responses to sudden removal of the aerosol forcing in
the experiment using the sulfate aerosol emission during 1980s. (d)(f) The fast and slow
responses in the experiment using the sulfate aerosol emission during 2020s.

2Depth (m)Figure S2. Similar panels to the Figure 3 but for the fast response (year 1-3 minus control)in the idealized aerosol experiments using tropical sulfate aerosol emission. The tropicalonly aerosol experiment, containing 15 ensemble members, involved a time invariant
sulfate aerosol emission between 10oS~10oN at 1980s condition, which is suddenly
introduced in the beginning of the experiment.

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Figure S3. Temporally integrated, anomalous heat budget (in K) in the top 50 meter of
equatorial Pacific region (5oS–5oN, 150oE-60oW) during years 1-3. Bars from the left
include: the tendency term (total change in oceanic temperature), surface flux, heat
convergence from the horizontal, meridional, and vertical directions, and the residual term.
Detailed derivation of the heat budget can be found in Eq. 2.2-2.6 in Tseng et al. 20231. In
brief, the anomalous temperature (relative to control) at year 3 (the first bar on the left) canbe attributed to the contributions from the five bars on the right.

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Depth (m)Depth (m)Figure S4. The meridional depth-profile of (top) fast and (bottom) slow potential
temperature responses over Pacific in the idealized aerosol experiment. From the left to theright panels, we show the potential temperature anomalies (shaded) at 160 oE, 160oW, and120oW. The light grey lines denote the isopycnals of the control simulation, and the blacklines represent the σ0 = 24~27 kg/m3 isopycnals.

5Figure S5. As in Figure S3, but for 0-200m (darker grey bars) and 0-400m (lighter greybars) heat budgets in degree K in the equatorial Pacific region (5oS–5oN, 150oE-60oW)
during years 1–10.

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Figure S6. The heat convergence from the meridional convergence (the first two bars on
the left, the forth term in Figure S5) is further decomposed into contributions ofanomalous temperature, velocity, and a nonlinear term, with the contribution from thenorthern and the southern boundaries plotted as light purple and light orange bars.

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a.

b.

c.

d.

Figure S7. Evolution of (a) the globally averaged sulfate aerosol emission in CESM LE(plotted in black, molecule/cm2/s) and its time differential (plotted in red,molecule/cm2/s2), and the pattern correlation between the 20-year running trend oftropical Pacific SST in (b) ALL minus XAERindus (c) ALL (d) HadISST (observation)and the fast response (plotted in red) and the slow response (plotted in black) in theidealized experiment (the patterns plotted in Figure 3). The light black and red lines in(b)-(d) denote the time that the 20-year SST running trends are not statistically significant
over more than 70% of the tropical Pacific region.

8CESM-LE ALL minus XAERindusb.

depth (m)a.

d.

depth (m)c.

Figure S8. Zonal and meridional mean anomalous potential temperature in the equatorial
Pacific (5oS~5oN; Pacific basin) in (a) CESM LE ALL minus XAERindus, (b) CESM LE
ALL, and (c) ORAS5 (Ocean Reanalysis System 5 by the European Center for MediumRange Weather Forecasts2). (d) The averaged anomalous potential temperature in thesubsurface equatorial Pacific (5oS~5oN; Pacific basin; 200~400m). The anomalies inCESM LE are computed using the multi-century control simulation with preindustrialcondition. The anomalies in ORAS5 are calculated with the base period 1958~1978, a spinup period treated as a backward extension in ORAS5 system. All of the anomalies aresmoothed by a 5-year running mean in CESM AER (All minus XAERindus) and CESM
LE, and by 3-year running mean in ORAS5.

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SI References

1. H.-Y. Tseng et al., Fast and Slow Responses of the Tropical Pacific to Radiative
Forcing in Northern High Latitudes. Journal of Climate, 1-31 (2023).
2. Zuo, Hao, et al. "The ECMWF operational ensemble reanalysis–analysis system forocean and sea ice: a description of the system and assessment." Ocean
science 15.3, 779-808, (2019).

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