Eurasian winter cooling in the warming

hiatus of 1998-2012

Chao Li, Bjorn Stevens, Jochem Marotzke

Max-Planck-Institut für Meteorologie

Hamburg

7 July 2015, OCFCC in Paris, France

(Email: chao.li@mpimet.mpg.de)

Introduction

Tropical Pacific cooling (e.g. Meehl et al., 2011; Kosaka and Xie, 2013)

Anomalously cold NH winter land surface (Cohen et al., 2012)

Incomplete observational coverage over Arctic (Cowtan and Way, 2013)

A possible artifact of data biases over the Ocean (Karl et al., 2015)

The present study:

• Investigate the relative magnitudes of the contributions of surface

temperature trends from different latitude bands to recent warming hiatus;

• Explore the cause of NH mid-latitude winter cooling with large ensembles

of sensitivity simulations with an atmospheric model.

Where does the warming hiatus of 1998-2012 come from?

Scaled zonal-mean surface temperature trends

• A pronounced NH winter cooling within 30°N~60°N;

• The warming hiatus is strongly influenced by a NH mid-latitude winter cooling.

What is the driver of the Eurasian winter cooling of 1998-2012?

Model and experimental design

Model:

ECHAM6: horizontal resolution T63 (1.9º), 47 vertical levels up to 0.01 hPa;

JSBACH: a subsystem for land and vegetation in ECHAM6

AMIP (20 realization)

90°N

90°S

ACLI (20 realization)

GCLI (20 realization)

Observed SST and sea ice in 1979-2012 Observed SST and

sea ice in 1979-2012

Climatology annual cycle of SST

Climatology annual cycle of SST

Standard CMIP5 AMIP

simulations

No SST variation in

Arctic

No SST variation

0°

Eurasia winter cooling versus DJF blocking frequency trend

AMIP: With SST/SIC variations

ACLI: no Arctic SST/SIC variations

GCLI: no SST/SIC variations

1998-2012

Ts trend blocking

AMIP/ACLI 1.52** 1.47**

AMIP/GCLI 1.34** 1.6**

ACLI/GCLI 0.88 1.08

Variances ratio

(** significant with bootstrapping test)

Conclusion

The GMST trend in 1998-2012 is strongly influenced by a pronounced

Eurasian winter cooling trend.

The observed Eurasian winter cooling trend over 1998-2012 arises

essentially from atmospheric internal variability and constitutes an extreme

climate event.

The Arctic sea-ice loss and SST changes do not drive systematic changes

in the NH large-scale circulation nor the Eurasian winter blocking frequency

in 1998-2012.

The dramatic change in Arctic sea ice and SSTs enhances the variability of

Eurasian winter climate and thus increases the probability of an extreme

Eurasian winter cooling trend.

Conclusion

Thanks for your attention!

The GMST trend in 1998-2012 is strongly influenced by a pronounced

Eurasian winter cooling trend.

The observed Eurasian winter cooling trend over 1998-2012 arises

essentially from atmospheric internal variability and constitutes an extreme

climate event.

The Arctic sea-ice loss and SST changes do not drive systematic changes

in the NH large-scale circulation nor the Eurasian winter blocking frequency

in 1998-2012.

The dramatic change in Arctic sea ice and SSTs enhances the variability of

Eurasian winter climate and thus increases the probability of an extreme

Eurasian winter cooling trend.

Monthly GMST trend

Warming trend in all

seasons.

Cooling trend in NH Winter

(DJF), which compensates

warming in other seasons.

Large-scale atmospheric circulation change

Arctic sea-ice change does not

drive systematical changes of NH

large-scale atmospheric circulation

in the past decades.

ECHAM6 can reasonably simulate

the variations of NH large-scale

atmospheric circulation changes.

NH mid-latitude DJF temperature trend

and NH blocking frequency