Tipping points in overturning circulation mediated by ocean mixing and the configuration and magnitude of the hydrological cycle: A simple model

Anand Gnanadesikan aMorton K. Blaustein Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA

Search for other papers by Anand Gnanadesikan in
Current site
Google Scholar
PubMed
Close
,
Gianluca Fabiani bModeling Engineering Risk and Complexity, Scuola Superiore Meridionale, Naples, Italy

Search for other papers by Gianluca Fabiani in
Current site
Google Scholar
PubMed
Close
,
Jingwen Liu aMorton K. Blaustein Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA

Search for other papers by Jingwen Liu in
Current site
Google Scholar
PubMed
Close
,
Renske Gelderloos aMorton K. Blaustein Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA
cDepartment of Hydraulic Engineering, Delft University of Technology, Delft, The Netherlands

Search for other papers by Renske Gelderloos in
Current site
Google Scholar
PubMed
Close
,
G. Jay Brett dJohns Hopkins Applied Physics Lab, Laurel, MD, USA

Search for other papers by G. Jay Brett in
Current site
Google Scholar
PubMed
Close
,
Yannis Kevrekidis eDepartment of Chemical Engineering, Johns Hopkins University, Baltimore, MD, USA

Search for other papers by Yannis Kevrekidis in
Current site
Google Scholar
PubMed
Close
,
Thomas Haine aMorton K. Blaustein Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA

Search for other papers by Thomas Haine in
Current site
Google Scholar
PubMed
Close
,
Marie-Aude Pradal aMorton K. Blaustein Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA

Search for other papers by Marie-Aude Pradal in
Current site
Google Scholar
PubMed
Close
,
Constaninos Siettos fDepartment of Applied Mathematics, University of Naples Federico II, Naples, Italy

Search for other papers by Constaninos Siettos in
Current site
Google Scholar
PubMed
Close
, and
Jennifer Sleeman dJohns Hopkins Applied Physics Lab, Laurel, MD, USA

Search for other papers by Jennifer Sleeman in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

In the modern ocean, transformation of light surface waters to dense deep waters primarily occurs in the Atlantic basin rather than in the North Pacific or Southern Oceans. The reasons for this remain unclear, as both models and paleoclimatic observations suggest that sinking can sometimes occur in the Pacific. We present a six-box model of the overturning that combines insights from a number of previous studies. A key determinant of the overturning configuration in our model is whether the Antarctic Intermediate Waters are denser than northern subpolar waters, something that depends on the magnitude and configuration of atmospheric freshwater transport. For the modern ocean, we find that although the interbasin atmospheric freshwater flux suppresses Pacific sinking, the poleward atmospheric freshwater flux out of the subtropics enhances it. When atmospheric temperatures are held fixed, North Pacific overturning can strengthen with either increases or decreases in the hydrological cycle, as well as under reversal of the interbasin freshwater flux. Tipping-point behavior, where small changes in the hydrological cycle may cause the dominant location of densification of light waters to switch between basins and the magnitude of overturning within a basin to exhibit large jumps, is seen in both transient and equilibrium states. This behavior is modulated by parameters such as the poorly constrained lateral diffusive mixing coefficient. If hydrological cycle amplitude is varied consistently with global temperature, northern polar amplification is necessary for the Atlantic overturning to collapse. Certain qualitative insights incorporated in the model can be validated using a fully-coupled climate model.

© 2024 American Meteorological Society. This is an Author Accepted Manuscript distributed under the terms of the default AMS reuse license. For information regarding reuse and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Anand Gnanadesikan, gnanades@jhu.edu

Abstract

In the modern ocean, transformation of light surface waters to dense deep waters primarily occurs in the Atlantic basin rather than in the North Pacific or Southern Oceans. The reasons for this remain unclear, as both models and paleoclimatic observations suggest that sinking can sometimes occur in the Pacific. We present a six-box model of the overturning that combines insights from a number of previous studies. A key determinant of the overturning configuration in our model is whether the Antarctic Intermediate Waters are denser than northern subpolar waters, something that depends on the magnitude and configuration of atmospheric freshwater transport. For the modern ocean, we find that although the interbasin atmospheric freshwater flux suppresses Pacific sinking, the poleward atmospheric freshwater flux out of the subtropics enhances it. When atmospheric temperatures are held fixed, North Pacific overturning can strengthen with either increases or decreases in the hydrological cycle, as well as under reversal of the interbasin freshwater flux. Tipping-point behavior, where small changes in the hydrological cycle may cause the dominant location of densification of light waters to switch between basins and the magnitude of overturning within a basin to exhibit large jumps, is seen in both transient and equilibrium states. This behavior is modulated by parameters such as the poorly constrained lateral diffusive mixing coefficient. If hydrological cycle amplitude is varied consistently with global temperature, northern polar amplification is necessary for the Atlantic overturning to collapse. Certain qualitative insights incorporated in the model can be validated using a fully-coupled climate model.

© 2024 American Meteorological Society. This is an Author Accepted Manuscript distributed under the terms of the default AMS reuse license. For information regarding reuse and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Anand Gnanadesikan, gnanades@jhu.edu
Save