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Author Topic: Broadening of Agulhas and other western boundary currents  (Read 2340 times)

sidd

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Broadening of Agulhas and other western boundary currents
« on: November 09, 2016, 09:26:36 PM »
I think this is important, but i dont know why i think so. I do know that Agulhas affects AMOC, and perhaps vice versa. Kuroshio and East Australia current are broadening also.

doi:10.1038/nature19853

"These results indicate that intensifying winds may be increasing the eddy kinetic energy of boundary currents, rather than their mean flow. This could act to decrease poleward heat transport and increase cross-frontal exchange of nutrients and pollutants between the coastal ocean and the deep ocean."

Below, EKE is eddy kinetic energy.

  "Our results, together with recent analyses in other western ­boundary currents, suggest that intensifying winds may act to increase the EKE of boundary currents, rather than their mean flow. This ­hypothesis draws parallels with the eddy compensation hypothesis for the Antarctic Circumpolar Current in the Southern Ocean, where eddies appear to dampen the effect of increased wind energy input on the mean flow [Ref. 30.] In essence, while winds tend to accelerate the flow and steepen ­isopycnals, eddies mix laterally across the current to slump the ­isopycnals. Coupling between eddies and the atmosphere has also been shown to influence this frontal balance [Ref. 31.] The implication of ­broadening boundary currents is a more porous divide between the continental shelves and the open ocean, leading to greater mixing and cross-frontal exchange. In the Agulhas Current, these changes could also enhance upwelling over the shelf, since the strongest upwelling events are driven by meanders. These implications are in contrast to those of an intensifying flow, which would tend to dampen cross-­frontal mixing and increase meridional heat transport.

  If western boundary currents are not strengthening, observed patterns of surface warming [Ref. 5,9,10] must be explained by a poleward shift of the ocean gyres."

sidd

Pmt111500

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Re: Broadening of Agulhas and other western boundary currents
« Reply #1 on: November 11, 2016, 11:31:01 AM »
I don't see how this couldn't be correct as the ghgs mix up rather fast at least in nh, so the logical result would be higher heat in the equatorial currents which feed the western boundary currents. The other possibility could be western boundary currents would heat up also, but if this is not observed then this must be true, no?

AbruptSLR

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Re: Broadening of Agulhas and other western boundary currents
« Reply #2 on: November 11, 2016, 05:27:04 PM »
The linked reference indicates that the "destabilization point" for meanders of the Gulf Stream have shifted westward:

M. Andres (28 September 2016), "On the recent destabilization of the Gulf Stream path downstream of Cape Hatteras", Geophysical Research Letters, DOI: 10.1002/2016GL069966


http://onlinelibrary.wiley.com/doi/10.1002/2016GL069966/full

Abstract: "Mapped satellite altimetry reveals interannual variability in the position of initiation of Gulf Stream meanders downstream of Cape Hatteras. The longitude where the Gulf Stream begins meandering varies by 1500 km. There has been a general trend for the destabilization point to shift west, and 5 of the last 6 years had a Gulf Stream destabilization point upstream of the New England Seamounts. Independent in situ data suggest that this shift has increased both upper-ocean/deep-ocean interaction events at Line W and open-ocean/shelf interactions across the Middle Atlantic Bight (MAB) shelf break. Mooring data and along-track altimetry indicate a recent increase in the number of deep cyclones that stir Deep Western Boundary Current waters from the MAB slope into the deep interior. Temperature profiles from the Oleander Program suggest that recent enhanced warming of the MAB shelf may be related to shifts in the Gulf Stream's destabilization point."


See also:
https://eos.org/research-spotlights/gulf-stream-destabilization-point-is-on-the-move?utm_source=eos&utm_medium=email&utm_campaign=EosBuzz111116

Extract: "In a new study, Andres reports that the location where the Gulf Stream meanders begin to amplify has migrated westward over the past 2 decades. Satellite, mooring, and shipboard observations of the current’s flow from 1993 to 2014 show that this “destabilization point” has shifted west at a rate of about 25 kilometers per year.

The author hypothesizes that the cause of the migration could be related to interactions between the Gulf Stream and the Deep Western Boundary Current, which flows toward the equator and crosses beneath the Gulf Stream near Cape Hatteras. Future research could determine why the destabilization point is on the move and what its full implications, such as potential effects on marine ecosystems, will be."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

Pmt111500

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Re: Broadening of Agulhas and other western boundary currents
« Reply #3 on: November 20, 2016, 08:56:50 AM »
Actually this could be good news for the heat budget of the planet. Rather have the heat in surface than in the deep storage from where it may rise to do nasty surprises?
« Last Edit: November 20, 2016, 09:40:07 AM by Pmt111500 »

sidd

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Re: Broadening of Agulhas and other western boundary currents
« Reply #4 on: January 17, 2017, 02:16:48 AM »
There is a discussion paper at ocean-sci-discuss about freshening below the thermocline and salinification above, and some discussion of Agulhas and weakening of South Indian ocean westerlies at

http://www.ocean-sci-discuss.net/os-2016-54/

They attribute to intensified hydrological cycle.

sidd