Indian Ocean warming reduces Pacific warming response to
anthropogenic greenhouse gases
Lei Zhang1, Weiqing Han1, Kristopher B. Karnauskas1,2, Gerald A. Meehl3, Aixue Hu3, Nan
Rosenbloom3, Toshiaki Shinoda4
1 Dept. of Atmospheric and Oceanic Sciences, Univ. of Colorado, Boulder, CO, USA
2 Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, CO, USA
3 National Center for Atmospheric Research (NCAR), Boulder, CO, USA
4 Dept. of Physical and Environmental Sciences, Texas A&M Univ., Corpus Christi, Texas, USA
NOAA
SST trend during 1920-2013
La Niña-like (?) warming pattern in observations
Introduction: SST trend pattern
SST trend during 1920-2013
Uncertainties across different observational SST data sets
SST trend during 1920-2013
SST trend during 1920-2013
IO > PO
IO > PO
IO > PO
IO <~ PO
In observations, the Indian Ocean warming trend is greater than the tropical Pacific, but
this interbasin warming contrast is not present in climate models;
What is the role of the Indian Ocean warming in affecting the tropical Pacific response to
the anthropogenic greenhouse gas effect?
Science questions
Indian Ocean-Global Atmosphere (IOGA) pacemaker experiments
Experiment design
• Model: NCAR CESM1
• Simulation period: 1920-2013
• External forcing: the same as CMIP5 historical run (1920-2005) and RCP8.5 (2006-2013)
• Ten member ensemble
• Indian Ocean SST is restored toward ERSSTv3b
CESM Large-Ensemble (CESM-LE) experiments
Differences between IOGA and CESM-LE are caused by Indian
Ocean SST warming
In IOGA (IO SST + Ext):
- Pacific easterly trades are enhanced
- Warming in the tropical Pacific is small
In CESM-LE (Ext):
- Pacific Walker circulation is weakened
- Similar magnitude of SST warming in the
tropical Indian and Pacific Oceans
In IOGA – CESM-LE (IO SST):
- Indian Ocean warming strengthens the Pacific
easterly trades and causes Pacific SST cooling
How does warming in the Indian Ocean affect Pacific easterly trades?
IOGA: IO SST + Ext
CESM-LE: Ext
IOGA – CESM-LE:IO SST
Why there is no interbasin warming contrast in climate models?
Natural climate variability
Natural climate variability +Model spread
Conclusions
• Observations show a greater warming trend in the tropical Indian Ocean than in
the tropical Pacific, but the interbasin warming contrast is not present in climate
models.
• By analyzing IOGA and CESM-LE, we investigate the role of Indian Ocean warming
in affecting changes in the tropical Pacific under global warming. In IOGA, warming
in the Indian Ocean induces negative SLP anomalies in the basin, enhancing the
zonal SLP gradient in the tropical Pacific. As a result, the Pacific easterly trades are
strengthened, which further causes SST cooling in the Pacific.
• Underestimation of the interbasin warming contrast is a common model bias and is
unlikely due to influence of natural climate variability.
Thanks!
NOAA