Diagnosing the Influence of Mesoscale Eddy Fluxes on the Deep Western Boundary Current in the 1/10° STORM/NCEP Simulation

Jackson

et al. 2020

J Adv Model Earth Syst

A multimodel, multiresolution ensemble using Coupled Model Intercomparison Project Phase 6 (CMIP6) High Resolution Model Intercomparison Project (HighResMIP) coupled experiments is used to assess the performance of key aspects of the North Atlantic circulation. The Atlantic Meridional Overturning Circulation (AMOC), and related heat transport, tends to become stronger as ocean model resolution is enhanced, better agreeing with observations at 26.5°N. However, for most models the circulation remains too shallow compared to observations and has a smaller temperature contrast between the northward and southward limbs of the AMOC. These biases cause the northward heat transport to be systematically too low for a given overturning strength. The higher‐resolution models also tend to have too much deep mixing in the subpolar gyre. In the period 2015–2050 the overturning circulation tends to decline more rapidly in the higher‐resolution models, which is related to both the mean state and to the subpolar gyre contribution to deep water formation. The main part of the decline comes from the Florida Current component of the circulation. Such large declines in AMOC are not seen in the models with resolutions more typically used for climate studies, suggesting an enhanced risk for Northern Hemisphere climate change. However, only a small number of different ocean models are included in the study.

Section: Resultsmentioning

confidence: 99%

Sensitivity of the Atlantic Meridional Overturning Circulation to Model Resolution in CMIP6 HighResMIP Simulations and Implications for Future Changes

Jackson

et al. 2020

J Adv Model Earth Syst

“…However, tracer‐based velocity estimates are about one order of magnitude lower than current‐meter measurements of the DWBC speed (Lee et al., 1996; Rhein et al., 2015). Observations (Kanzow et al., 2008; Lee et al., 1996) and models (Lüschow et al., 2019) suggest that the deep flow is subject to strong temporal variability associated with meanders of the DWBC and eddies. Leaman and Vertes (1996) find eddies triggered by the marked topography of San Salvador (∼24°N) to reduce the mean southward velocity of RAFOS floats compared with their speed within the DWBC itself.…”

Section: Introductionmentioning

confidence: 99%

On the Variability of the DWBC Transport Between 26.5°N and 16°N in an Eddy‐Rich Ocean Model

2021

The Atlantic Meridional Overturning Circulation (AMOC) transports warm surface water northwards. It is responsible for the northward ocean heat transport with a pronounced impact on the large scale climate, especially in Europe (Srokosz & Bryden, 2015). At depth the AMOC exports cold water of North Atlantic origin (North Atlantic Deep Water; NADW) to the south, which carries anthropogenic carbon dioxide (CO 2 ) and other trace gases (Pickart et al., 1989;Sabine et al., 2004) away from the uptake regions in the north. A main component of the AMOC's deep limb is the Deep Western Boundary Current (DWBC; Rhein et al., 2015). While north of ∼26°N interior pathways away from the western boundary were shown to exist, the flow of NADW seems to be concentrated in a coherent DWBC further south (Bower et al., 2009;Gary et al., 2011). This potentially leads to a faster spreading of tracers and trace gases in the subtropical North Atlantic (STNA). However, tracer-based velocity estimates are about one order of magnitude lower than current-meter measurements of the DWBC speed (

Journal of Physical Oceanography

“…Eliminating w between Eq. (3) and the time averaged density budget, which has been found to be reasonably approximated by u Á =r ' 2= Á u 0 r 0 near the DWBC (Lüschow et al 2019), yields the along-stream averaged potential vorticity equation…”

Section: B Eddy Thickness Fluxesmentioning

confidence: 99%

“…The STORM configuration with its tripolar grid has two coordinate poles in the Northern Hemisphere over Asia and North America, ensuring an essentially uniform resolution. STORM's ability to represent large parts of the mesoscale eddy field was demonstrated for mesoscale eddies in general (e.g., von Storch et al 2012;von Storch et al 2016) and also for deep eddies in particular (Lüschow et al 2019). The GR15 configuration uses a bipolar grid with a resolution of about 180 km in our region of interest at low latitudes, rendering it mostly free of eddies.…”

Section: Experiments Designmentioning

confidence: 99%

“…When going down in the water column, R increases for strong and deep eddies because the eddy swirl speed V 0 stays more or less constant with depth (Lüschow et al 2019) while variations in density r 0 decrease with depth like the surrounding stratification does. Thus, the relative importance of the EVF can be expected to increase with depth for vertically coherent eddies such as those in the DWBC (Lüschow et al 2019).…”

Section: B Eddy Thickness Fluxesmentioning

confidence: 99%

See 1 more Smart Citation

Overturning Response to a Surface Wind Stress Doubling in an Eddying and a Non-Eddying Ocean

Lüschow

Marotzke

Storch

2021

Self Cite

In this paper, the overturning responses to wind stress changes of an eddying and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%, 1 Sv = 106 m3s−1). In contrast, the DWBC responds with a speedup in the non-eddying and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies which are sourced from the upper ocean provide an excess input of relative vorticity which then actively forces the DWBC mean flow to slow down.