Changes in the seasonal cycle of the Atlantic meridional heat transport in a RCP 8.5 climate projection in MPI-ESM

Matthias Fischer,Daniela I V Domeisen,Johanna Baehr,Wolfgang A Müller

doi:10.5194/esd-8-129-2017

Abstract

Abstract. We investigate changes in the seasonal cycle of the Atlantic Ocean meridional heat transport (OHT) in a climate projection experiment with the Max Planck Institute Earth System Model (MPI-ESM) performed for the Coupled Model Intercomparison Project Phase 5 (CMIP5). Specifically, we compare a Representative Concentration Pathway (RCP) RCP 8.5 climate change scenario, covering the simulation period from 2005 to 2300, to a historical simulation, covering the simulation period from 1850 to 2005. In RCP 8.5, the OHT declines by 30–50 % in comparison to the historical simulation in the North Atlantic by the end of the 23rd century. The decline in the OHT is accompanied by a change in the seasonal cycle of the total OHT and its components. We decompose the OHT into overturning and gyre component. For the OHT seasonal cycle, we find a northward shift of 5° and latitude-dependent shifts between 1 and 6 months that are mainly associated with changes in the meridional velocity field. We find that the changes in the OHT seasonal cycle predominantly result from changes in the wind-driven surface circulation, which projects onto the overturning component of the OHT in the tropical and subtropical North Atlantic. This leads in turn to latitude-dependent shifts between 1 and 6 months in the overturning component. In comparison to the historical simulation, in the subpolar North Atlantic, in RCP 8.5 we find a reduction of the North Atlantic Deep Water formation and changes in the gyre heat transport result in a strongly weakened seasonal cycle with a weakened amplitude by the end of the 23rd century.

Highlights

Global surface temperatures are projected to warm – depending on the considered climate change scenario – intensively over the centuries (IPCC, 2013), accompanied by a projected shift in the amplitude and phase of the seasonal cycle of surface air temperatures (Dwyer et al, 2012)
Based on our analysis in the Max Planck Institute Earth System Model (MPI-ESM) Coupled Model Intercomparison Project Phase 5 (CMIP5) climate projection Representative Concentration Pathway (RCP) 8.5, we conclude for the Atlantic Ocean meridional heat transport that
2. these changes stem from a latitude-dependent altered seasonal cycle and a northward shift in the zonalmean zonal wind and the resulting changes in the surface wind field that lead to a shift by 1 to 5 months in the seasonal cycle of the Ekman heat transport and the overturning heat transport

Summary

Introduction

Global surface temperatures are projected to warm – depending on the considered climate change scenario – intensively over the centuries (IPCC, 2013), accompanied by a projected shift in the amplitude and phase of the seasonal cycle of surface air temperatures (Dwyer et al, 2012). The associated Atlantic Ocean meridional heat transport (OHT) is expected to weaken due to the direct linear relation of AMOC and OHT found in observations and model studies (Johns et al, 2011; Msadek et al, 2013). It is unclear how climate change along with a projected shift in the seasonal cycle of surface temperatures affects the seasonal cycle of the ocean circulation, and especially of the OHT.

Objectives

Findings

Discussion

Conclusion