Understanding Bjerknes Compensation in Meridional Heat Transports and the Role of Freshwater in a Warming Climate

Qianzi Yang,Haijun Yang,Qin Wen,Jie Yao,Yingying Zhao

doi:10.1175/jcli-d-17-0587.1

Abstract

The Bjerknes compensation (BJC) under global warming is studied using a simple box model and a coupled Earth system model. The BJC states the out-of-phase changes in the meridional atmosphere and ocean heat transports. Results suggest that the BJC can occur during the transient period of global warming. During the transient period, the sea ice melting in the high latitudes can cause a significant weakening of the Atlantic meridional overturning circulation (AMOC), resulting in a cooling in the North Atlantic. The meridional contrast of sea surface temperature would be enhanced, and this can eventually enhance the Hadley cell and storm-track activities in the Northern Hemisphere. Accompanied by changes in both ocean and atmosphere circulations, the northward ocean heat transport in the Atlantic is decreased while the northward atmosphere heat transport is increased, and the BJC occurs in the Northern Hemisphere. Once the freshwater influx into the North Atlantic Ocean stops, or the ocean even loses freshwater because of strong heating in the high latitudes, the AMOC would recover. Both the atmosphere and ocean heat transports would be enhanced, and they can eventually recover to the state of the control run, leading to the BJC to become invalid. The above processes are clearly demonstrated in the coupled model CO2 experiment. Since it is difficult to separate the freshwater effect from the heating effect in the coupled model, a simple box model is used to understand the BJC mechanism and freshwater’s role under global warming. In a warming climate, the freshwater flux into the ocean can cool the global surface temperature, mitigating the temperature rise. Box model experiments indicate clearly that it is the freshwater flux into the North Atlantic that causes out-of-phase changes in the atmosphere and ocean heat transports, which eventually plays a stabilizing role in global climate change.

Highlights

The fundamental mechanisms maintaining energy balance of Earth’s climate system are climate feedback and meridional heat transport (MHT)
We show that the Bjerknes compensation (BJC) is valid during the transient period of global warming, because the change in ocean thermohaline is mainly attributed to freshwater perturbation instead of heating perturbation
We studied the BJC in a warming climate using a coupled Earth system model and a coupled box model

Summary

Introduction

The fundamental mechanisms maintaining energy balance of Earth’s climate system are climate feedback and meridional heat transport (MHT) The former, defined as the relationship between the net radiation flux at the top of the atmosphere (TOA) and surface temperature, determines the local net energy budget in the vertical direction. The Earth system gains additional energy because of the enhanced greenhouse effect, increasing the ocean heat content These violate the prerequisite to the BJC occurrence (Bjerknes 1964). Our results from a coupled climate model experiment forced by doubled CO2 show that the BJC can still be valid to some extent, even during the transient period of global warming (Fig. 1). We show that the BJC is valid during the transient period of global warming, because the change in ocean thermohaline is mainly attributed to freshwater perturbation instead of heating perturbation.

Coupled models and experiments

Freshwater’s role revealed by the box model

Conclusions and discussion

A Coupled Box Model

Findings

A2 B1 B2 cr0 D1 S0 w g x