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

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.

Anthropogenic Aerosols, Greenhouse Gases, and the Uptake, Transport, and Storage of Excess Heat in the Climate System

“…In the NH, anomalous ocean heat transport moved heat equatorward from the subpolar zone of maximum OHU, resulting in a slightly greater increase in OHC across the middle‐to‐low latitudes. This is thought to be related to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), induced by the enhanced OHU and/or freshwater influx at high latitudes (Gregory et al, ; Yang et al, ). Heat was also transported equatorward from the (larger) OHU maximum centered around 55°S, but with a clear OHC maximum produced near 45°S.…”

“…The accumulated atmospheric and oceanic heat transport inferred from those values is also indicated, with features of the historical experiment labeled according to whether they are primarily driven by greenhouse gas (GHG) forcing (red text) or anthropogenic aerosol (AA) forcing (blue text). enhanced OHU and/or freshwater influx at high latitudes (Gregory et al, 2016;Yang et al, 2018). Heat was also transported equatorward from the (larger) OHU maximum centered around 55 • S, but with a clear OHC maximum produced near 45 • S. This meridional pattern of Southern Ocean warming is thought to be primarily driven by passive advection of the anomalous OHU by the climatological ocean current, with secondary influence from poleward-intensified zonal surface winds (Armour et al, 2016;Liu et al, 2018).…”

“…This result is consistent with a phenomenon known as Bjerknes compensation (Bjerknes, ), which states that changes in meridional atmosphere and ocean heat transport will be out‐of‐phase. While Bjerknes compensation was originally identified and studied in the context of stationary climates (e.g., Shaffrey & Sutton, ), it has been shown to operate in transient climate simulations forced by increased atmospheric GHGs (Huang & Zhang, ; Outten et al, ; Yang et al, ). The exact mechanism driving this near compensation in GHG‐forced climates is still not clear, although a hint can be gleaned from the meridional OHU pattern.…”

Roles of Surface Albedo, Surface Temperature and Carbon Dioxide in the Seasonal Variation of Arctic Amplification

“…Previous studies have suggested that the AA is a result of local forcing and feedbacks (Stuecker et al., 2018; hereafter ST18) and is mainly influenced by several factors, including increased carbon dioxide (CO 2 ; Zheng et al., 2019), surface albedo variation (A. Dai et al., 2019, hereafter DA19), surface warming (Pithan & Mautisten, 2014), vertical lapse rate variation (Goosse et al., 2018), water vapor variation, and cloud variation (Sejas et al., 2014). Increased CO 2 forcing impedes longwave radiation escaping the Earth system, enhances surface temperature and is considered the original reason for global warming (Yang et al., 2018). Surface albedo variation (mainly due to sea‐ice loss) allows more solar radiation to be absorbed at the surface of the polar ocean, which in turn enhances surface temperature and decreases sea ice (positive feedback; H. Park et al., 2018).…”

“…, vertical lapse rate variation (Goosse et al, 2018), water vapor variation, and cloud variation (Sejas et al, 2014). Increased CO 2 forcing impedes longwave radiation escaping the Earth system, enhances surface temperature and is considered the original reason for global warming (Yang et al, 2018). Surface albedo variation (mainly due to sea-ice loss) allows more solar radiation to be absorbed at the surface of the polar ocean, which in turn enhances surface temperature and decreases sea ice (positive feedback; H. Park et al, 2018).…”