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