Multi-decadal trend patterns of the Indian

Ocean sea level since the 1950s: contribution

from external forcing

Weiqing Han

The University of Colorado at Boulder

CESM workshop, June 17-19, 2019, Boulder, Co

From: Han W., Detlef Stammer, G. A. Meehl, Aixue Hu, Frank Sienz and Lei

Zhang 2018: Climate, 6(2), 51, https://doi.org/10.3390/cli6020051

Background & goal

Previous studies: distinct pattern

of sea level trend since the 1950s;

Indian Ocean warming asso. with

anthropogenic forcing may have

contributed; this effect however,

has never been quantified

GoalQuantify the effects of internal climate variability vs external

forcing on the observed sea level trend pattern since the 1950s,

using observational and reanalysis data to detect sea level trend

& large ensemble climate model experiments to assess external

forcing: Max-Planck Institute of Meteorology (MPI) 100

members & NCAR CESM1 40 members

India

Seychellese-Chagos

Thermocline Ridge

The observed multi-decadal trend

Linear trend:1958-2005 (global SLR removed)

Seychelles-Chagos sea level fall:

ORAS4: -1.74 mm/yr

SODA: -0.73 mm/yr

(~42% of ORAS4)

lack obs to constrain models

Halosteric sea level & deeper

ocean below 700m are also

important

Results

Linear trend:1993–2010

(global SLR removed)

Pattern correlation with

satellite AVISO r:

ORAS4: 0.84

SODA: 0.48

Caveat: ORAS4 assimilated

satellite SSHA but SODA

did not.

External forcing: large-ensemble climate model results

Linear trend:1958–2005

(global SLR removed)

Externally forced:

West IO: contributes to

observed sea level fall

EIO: fall – does not

contribute to observed SLR

Results

NSSIP AGCM experiments: surface

wind and Ekman pumping velocity

forced by linear trend of tropical

Indian Ocean warming: positive

Ekman pumping velocity

2S-20S region – favors sea level fall

CMIP5 models bias: west-east gradient

Annamalai et al. (2017):

10-4 cm/s/century

It is likely that MPI is more

reasonable.

Results: 1958-2005 trend

MPI: −0.29 ± 0.04 mm year −1: ~17±2.3% ORAS4

100 ~40±5.5% SODA

~24±3.3% (ORAS4+SODA)/2

MPI+CESM1: −0.23 ± 0.03 mm year −1 : ~19±2.4%

140 (ORAS4+SODA)/2

West: Tropical South IO (Seychelles) (50° E–85° E, 17° S–

5° S)

Obs-ORAS4 −1.74 ± 0.12

Obs-SODA −0.73 ± 0.12

Obs-mean −1.23 ± 0.12

MPI 100 −0.29 ± 0.04

CESM1 40 −0.08 ± 0.07

Model-mean −0.23 ± 0.03

4. Summary

• For the observed Indian Ocean multi-decadal sea level trends

from 1958-2005 (global SLR removed), effects of internal

variability dominate external forcing;

• Using ORAS4+SODA average to represent observations, and the

ensemble mean of 140 members (100 from MPI and 40 from

CESM1) to represent external forcing, natural variability

accounts for ~80% & external forcing ~20% of observed falling

trend over the Seychelles Island region

Thank you!