How ocean ridges affect large-scale ocean circulation

Abstract A new basin-wide oscillation of the Mediterranean Sea is identified and analyzed using sea level observations from the Ocean Topography Experiment (TOPEX)/Poseidon satellite altimeter and a numerical ocean circulation model. More than 50% of the large-scale, nontidal, and non-pressure-driven variance of sea level can be attributed to this oscillation, which is nearly uniform in phase and amplitude across the entire basin. The oscillation has periods ranging from 10 days to several years and has a magnitude as large as 10 cm. The model suggests that the fluctuations are driven by winds at the Strait of Gibraltar and its neighboring region, including the Alboran Sea and a part of the Atlantic Ocean immediately to the west of the strait. Winds in this region force a net mass flux through the Strait of Gibraltar to which the Mediterranean Sea adjusts almost uniformly across its entire basin with depth-independent pressure perturbations. The wind-driven response can be explained in part by wind setup; a near-stationary balance is established between the along-strait wind in this forcing region and the sea level difference between the Mediterranean Sea and the Atlantic Ocean. The amplitude of this basin-wide wind-driven sea level fluctuation is inversely proportional to the setup region’s depth but is insensitive to its width including that of Gibraltar Strait. The wind-driven fluctuation is coherent with atmospheric pressure over the basin and contributes to the apparent deviation of the Mediterranean Sea from an inverse barometer response.

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A Diagnostic Model of the Nordic Seas and Arctic Ocean Circulation: Quantifying the Effects of a Variable Bottom Density along a Sloping Topography

Journal of Physical Oceanography ◽

10.1175/2008jpo3862.1 ◽

2008 ◽

Vol 38 (12) ◽

pp. 2685-2703 ◽

Cited By ~ 14

Author(s):

Signe Aaboe ◽

Ole Anders Nøst

Keyword(s):

Arctic Ocean ◽

Ocean Circulation ◽

Large Scale ◽

The Arctic ◽

Diagnostic Model ◽

Large Scale Circulation ◽

Nordic Seas ◽

Variable Bottom ◽

Canadian Basin ◽

Baroclinic Transport

Abstract A linear diagnostic model, solving for the time-mean large-scale circulation in the Nordic seas and Arctic Ocean, is presented. Solutions on depth contours that close within the Nordic seas and Arctic Ocean are found from vorticity balances integrated over the areas enclosed by the contours. Climatological data for wind stress and hydrography are used as input to the model, and the bottom geostrophic flow is assumed to follow depth contours. Comparison against velocity observations shows that the simplified dynamics in the model capture many aspects of the large-scale circulation. Special attention is given to the dynamical effects of an along-isobath varying bottom density, which leads to a transformation between barotropic and baroclinic transport. Along the continental slope, enclosing both the Nordic seas and Arctic Ocean, the along-slope barotropic transport has a maximum in the Nordic seas and a minimum in the Canadian Basin with a difference of 9 Sv (1 Sv ≡ 106 m3 s−1) between the two. This is caused by the relatively lower bottom densities in the Canadian Basin compared to the Nordic seas and suggests that most of the barotropic transport entering the Arctic Ocean through the Fram Strait is transformed to baroclinic transport. A conversion from barotropic to baroclinic flow may be highly important for the slope–basin exchange in the Nordic seas and Arctic Ocean. The model has obvious shortcomings due to its simplicity. However, the simplified physics and the agreement with observations make this model an excellent framework for understanding the large-scale circulation in the Nordic seas and Arctic Ocean.

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Structure of ocean circulation between the Galápagos Islands and Ecuador

Advances in Geosciences ◽

10.5194/adgeo-33-3-2013 ◽

2013 ◽

Vol 33 ◽

pp. 3-12 ◽

Cited By ~ 6

Author(s):

C. Collins ◽

A. Mascarenhas ◽

R. Martinez

Keyword(s):

Sea Level ◽

Ocean Circulation ◽

Large Scale ◽

Southern Oscillation ◽

Galapagos Islands ◽

Surface Flow ◽

Climate Indices ◽

Late Winter ◽

Coastal Current ◽

Galápagos Islands

Abstract. From 27 March to 5 April 2009, upper ocean velocities between the Galápagos Islands and Ecuador were measured using a vessel mounted ADCP. A region of possible strong cross-hemisphere exchange was observed immediately to the east of the Galápagos, where a shallow (200 m) 300 km wide northeastward surface flow transported 7–11 Sv. Underlying this strong northeastward surface current, a southward flowing undercurrent was observed which was at least 600 m thick, 100 km wide, and had an observed transport of 7–8 Sv. Next to the Ecuador coast, the shallow (< 200 m) Ecuador Coastal Current was observed to extend offshore 100 km with strongest flow, 0.33 m s−1, near the surface. Immediately to the west of the Ecuador Coastal Current, flow was directed eastward and southward into the beginnings of the Peru-Chile Countercurrent. The integral of the surface currents between the Galápagos and Ecuador agreed well with observed sea level differences. Although the correlation of the sea level differences with large scale climate indices (Niño3 and the Southern Oscillation Index) was significant, more than half of the sea level variability was not explained. Seasonal variability of the sea level difference indicated that sea level was 2 cm higher at the Galápagos during late winter and early spring, which could be associated with the pattern of northward surface flows observed by R/V Knorr.

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Upper-Bound General Circulation of the Ocean: a Theoretical Exposition

10.20944/preprints202109.0239.v1 ◽

2021 ◽

Author(s):

Hsien-Wang Ou

Keyword(s):

Ocean Circulation ◽

Upper Bound ◽

First Principles ◽

General Circulation ◽

Large Scale ◽

Companion Paper ◽

Jet Stream ◽

Equatorial Undercurrent ◽

Macroscopic Motion ◽

Dynamical Principle

This paper considers the general ocean circulation within the thermodynamical closure of our climate theory, which aims to deduce the generic climate state from first principles. The preceding papers of the theory have reduced planetary fluids to warm/cold masses and determined their bulk thermal properties, which provide prior constraints for the derivation of the upper-bound circulation when the potential vorticity is homogenized in moving masses. In a companion paper on the atmosphere, this upper bound is seen to reproduce the prevailing wind, forsaking therefore previous discordant explanations of the easterly trade and the polar jet stream. In this paper on the ocean, we again show that this upper bound may replicate broad features of the observed circulation, including a western-intensified subtropical gyre and a counter-rotating tropical gyre feeding the equatorial undercurrent. Together, we posit that PV homogenization may provide a unifying dynamical principle of the large-scale planetary circulation, which may be interpreted as the maximum macroscopic motion extractable by microscopic stirring --- within the confine of the thermal differentiation.

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Ocean circulation - Does large-scale ocean overturning circulation vary with climate change? [Past]

PAGES news ◽

10.22498/pages.20.1.15 ◽

2012 ◽

Vol 20 (1) ◽

pp. 15-15

Author(s):

Luke Skinner

Keyword(s):

Climate Change ◽

Ocean Circulation ◽

Large Scale ◽

Overturning Circulation

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Shifting ocean circulation warms the Subpolar North Atlantic since 2016

10.5194/egusphere-egu21-940 ◽

2021 ◽

Author(s):

Damien Desbruyères ◽

Léon Chafik ◽

Guillaume Maze

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Large Scale ◽

Deep Ocean ◽

Ocean Heat Uptake ◽

Driving Mechanisms ◽

Temperature Trends ◽

Meridional Overturning ◽

Ocean Observing

The Subpolar North Atlantic (SPNA) is known for rapid reversals of decadal temperature trends, with ramifications encompassing the large-scale meridional overturning and gyre circulations, Arctic heat and mass balances, or extreme continental weather. Here, we combine datasets derived from sustained ocean observing systems (satellite and in situ), and idealized observation-based modelling (advection-diffusion of a passive tracer) and machine learning technique (ocean profile clustering) to document and explain the most-recent and ongoing cooling-to-warming transition of the SPNA. Following a gradual cooling of the region that was persisting since 2006, a surface-intensified and large-scale warming sharply emerged in 2016 following an ocean circulation shift that enhanced the northeastward penetration of warm and saline waters from the western subtropics. Driving mechanisms and ramification for deep ocean heat uptake will be discussed.

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Radial Anisotropy and it's Relation to Fast and Slow Moving Tectonic Plates

10.5194/egusphere-egu21-13788 ◽

2021 ◽

Author(s):

Tak Ho ◽

Keith Priestley ◽

Eric Debayle

Keyword(s):

Upper Mantle ◽

Large Scale ◽

Oceanic Lithosphere ◽

Azimuthal Anisotropy ◽

Plate Motion ◽

Moho Depth ◽

Dispersion Characteristics ◽

Anisotropic Models ◽

Ocean Ridges ◽

Average Dispersion

We present a new radially anisotropic (&#958;)&#160;tomographic model for the upper mantle to transition zone depths derived from a large Rayleigh (~4.5 x 106&#160;paths) and Love (~0.7 x 106&#160;paths) wave path average dispersion curves with periods of 50-250 s and up to the fifth overtone. We first extract the path average dispersion characteristics from the waveforms. Dispersion characteristics for common paths (~0.3 x 106&#160;paths) are taken from the Love and Rayleigh datasets and jointly inverted for isotropic Vs&#160;and&#160;&#958;. CRUST1.0 is used for crustal corrections and a model similar to PREM is used as a starting model. Vs&#160;and&#160;&#958;&#160;are regionalised for a 3D model. The effects of azimuthal anisotropy are accounted for during the regionalisation. Our model confirms large-scale upper mantle features seen in previously published models, but a number of these features are better resolved because of the increased data density of the fundamental and higher modes coverage from which our&#160;&#958;(z) was derived. Synthetic tests show structures with radii of 400 km can be distinguished easily. Crustal perturbations of +/-10% to Vp, Vs&#160;and density, or perturbations to Moho depth of +/-10 km over regions of 400 km do not significantly change the model. The global average decreases from&#160;&#958;~1.06 below the Moho to&#160;&#958;~1 at ~275 km depth. At shallow depths beneath the oceans&#160;&#958;>1 as is seen in previously published global mantle radially anisotropic models. The thickness of this layer increases slightly with the increasing age of the oceanic lithosphere. At ~200 km and deeper depths below the fast-spreading East Pacific Rise and starting at somewhat greater depths beneath the slower spreading ridges,&#160;&#958;<1. At depths &#8805;200 km and deeper depths below most of the backarc basins of the western Pacific&#160;&#958;<1. The signature of mid-ocean ridges vanishes at about 150 km depth in Vs&#160;while it extends much deeper in the&#160;&#958;&#160;model suggesting that upwelling beneath mid-ocean ridges could be more deeply rooted than previously believed. The pattern of radially anisotropy we observe, when compared with the pattern of azimuthal anisotropy determined from Rayleigh waves, suggests that the shearing at the bottom of the plates is only sufficiently strong to cause large-scale preferential alignment of the crystals when the plate motion exceeds some critical value which Debayle and Ricard (2013) suggest is about 4 cm/yr.

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Detection of large-scale ocean circulation and tides

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1983.0047 ◽

1983 ◽

Vol 309 (1508) ◽

pp. 361-370 ◽

Cited By ~ 6

Keyword(s):

Heat Flux ◽

Surface Topography ◽

Ocean Circulation ◽

Satellite Altimetry ◽

Large Scale ◽

Precise Measurement ◽

Ocean Surface ◽

Ocean Topography ◽

Tidal Signal ◽

Nearly Continuous

As emphasized recently by Munk & Wunsch, the traditional methods of monitoring the ocean circulation give data too hopelessly aliased in space and time to permit a proper assessment of basin-wide dynamics and heat flux on climatic timescales. The prospect of nearly continuous recording of ocean-surface topography by satellite altimetry with suitable supporting measurements might make such assessments possible. The associated identification of the geocentric oceanic tidal signal in the data would be an additional bonus. The few weeks of altimetry recorded by Seasat gave a glimpse of the possibilities, but also clarified the areas where better precision and knowledge are needed. Further experience will be gained from currently projected multi-purpose satellites carrying altimeters, but serious knowledge of ocean circulation will result only from missions that are entirely dedicated to the precise measurement of ocean topography.

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