On the discrepancy between observed and CMIP5 multi-model simulated Barents Sea winter sea ice decline

Dawei Li,Rong Zhang,Thomas R Knutson

doi:10.1038/ncomms14991

Abstract

This study aims to understand the relative roles of external forcing versus internal climate variability in causing the observed Barents Sea winter sea ice extent (SIE) decline since 1979. We identify major discrepancies in the spatial patterns of winter Northern Hemisphere sea ice concentration trends over the satellite period between observations and CMIP5 multi-model mean externally forced response. The CMIP5 externally forced decline in Barents Sea winter SIE is much weaker than that observed. Across CMIP5 ensemble members, March Barents Sea SIE trends have little correlation with global mean surface air temperature trends, but are strongly anti-correlated with trends in Atlantic heat transport across the Barents Sea Opening (BSO). Further comparison with control simulations from coupled climate models suggests that enhanced Atlantic heat transport across the BSO associated with regional internal variability may have played a leading role in the observed decline in winter Barents Sea SIE since 1979.

Highlights

This study aims to understand the relative roles of external forcing versus internal climate variability in causing the observed Barents Sea winter sea ice extent (SIE) decline since 1979
We would like to address the following questions: What are the relative roles of changes in external radiative forcing versus internal variability in causing the observed winter Barents Sea SIE decline over the satellite period? Are there any substantial discrepancies between the observed and coupled model intercomparison project phase 5 (CMIP5)-simulated externally forced response in winter Northern Hemisphere (NH) sea ice decline, given that the internal variability of Atlantic heat transport into the Arctic is largely removed in the CMIP5 multimodel ensemble mean forced response? The work presented here aims to provide insights on these questions by comparing observations with the CMIP5 multi-model ensemble simulations forced by changes in external radiative forcing as well as with long pre-industrial control simulations under constant radiative forcing
We show that a comparison of the observed and CMIP5 multi-model-simulated time series of March NH SIE anomalies alone is insufficient for understanding the cause of the observed winter NH sea ice declining trend over the satellite period

Summary

Introduction

This study aims to understand the relative roles of external forcing versus internal climate variability in causing the observed Barents Sea winter sea ice extent (SIE) decline since 1979. Further comparison with control simulations from coupled climate models suggests that enhanced Atlantic heat transport across the BSO associated with regional internal variability may have played a leading role in the observed decline in winter Barents Sea SIE since 1979. Recent work suggests that winter Barents Sea SIE is well correlated with summer Arctic SIE in both coupled climate model simulations and observations, suggesting an important role for enhanced Atlantic heat transport into the Arctic in both winter and summer Arctic sea ice declines[13]. We would like to address the following questions: What are the relative roles of changes in external radiative forcing versus internal variability in causing the observed winter Barents Sea SIE decline over the satellite period? Our analysis suggests that enhanced Atlantic heat transport into the Barents Sea associated with regional internal variability may have played a leading role in the observed winter Barents Sea SIE decline since 1979

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