BECOME-GYRE: BjerknEs COMpEnsation and the sub-polar GYRE

Elucidate the multi-decadal variability of the sub-polar gyre and its relationship to merridional heat transport variations in the North Atlantic. 

Objectives

  1. Identification and analysis of the sub-polar gyre and its links to atmospheric flow in multiple CMIP5 models
  2. Publication on the links between Bjerknes Compensation and the sub-polar gyre in CMIP5 models
  3. (Stretch Goal) Develop a clear mechanism underlying the multi-decadal variability of Bjerknes Compensation

Project Summary

Bjerknes Compensation – the idea that anomalies of heat transport in the atmosphere and ocean must be approximately equal and opposite – has now been well established in the scientific literature [Shaffery and Sutton, 2006; Jungclaus and Koenigk, 2010; Outten and Esau, (submitted)]. However, studies of Bjerknes Compensation in climate models have all identified a multi-decadal variability in the heat transport anomalies, with a period of approximately 60-80 years. The mechanism underlying this multi-decadal variability has not been clearly identified. Consensus has been forming in recent years that to give rise to the observed variability, the atmosphere and ocean must feed back onto one another. It has been well established in the literature how the ocean affects the atmosphere, through reduced sea ice cover in the Greenland and Barents Seas leading to strong fluxes into the atmosphere, sharply rising air temperatures and the formation of a thermal low. But how the atmosphere in turn feeds back into the ocean on these multi-decadal timescales is still a hotly debated topic.

It has previously been shown that blocking at high latitudes in the North Atlantic sector results in shifted storm tracks and changes in winds that lead to weakened ocean gyres [Häkkinen et al., 2011]. Outten and Esau [submitted] found changes in the large-scale flow over the North Atlantic associated with Bjerknes Compensation, and a strong correlation (R=-0.52, p≤0.05) between the variability in atmospheric heat transport anomalies and the strength of the sub-polar gyre. The sub-polar gyre is an important component of the Atlantic meridional overturning circulation (AMOC) and thus the ocean heat transport in the North Atlantic, so this link could be part of the missing mechanism for how the atmosphere feeds back onto the ocean on multi-decadal timescales. A recent study in Science [Clement et al. 2015] also determined that the Atlantic Multi-decadal Oscillation (AMO) is not linked to AMOC as had previously been thought, but to stochastic changes in the atmosphere, thus adding weight to the idea of the atmosphere influencing the ocean on multi-decadal timescales. 

The project will take advantage of the analysis in the on going MEDEVAC project to perform a side study focussed on the links between Bjerknes Compensation and the sub-polar gyre. At least 10 CMIP5 models will be analysed for the strength and variability of their sub-polar gyres, with more included in the study as the ESGF comes back online. This variability will then be linked to changes in large-scale atmospheric flow and through this to the changes in the Bjerknes Compensation. Investigations into the other multi-decadal variability signals in the North Atlantic will also be conducted. The overarching aim will be to elucidate on the mechanism by which the atmosphere drives the ocean heat transport on multi-decadal timescales and to further develop our understanding of large-scale atmosphere-ocean interactions between the North Atlantic and Arctic. This will target the RG2 and RG4 strategic objectives of SKD. It is also expected that the results of this study will compliment the results of the MEDEVAC project and feed back into the discussions of that consortium.

 
Project Details
Acronym: 
BECOME-GYRE
NERSC Principal Investigator: 
Stephen Outten
Coordinating Institute: 
Nansen Environmental and Remote Sensing Center
Project Status: 
Completed