scholarly journals Coupled Ocean–Atmosphere Responses to Recent Freshwater Flux Changes over the Kuroshio–Oyashio Extension Region

2011 ◽  
Vol 24 (5) ◽  
pp. 1507-1524 ◽  
Author(s):  
Liping Zhang ◽  
Lixin Wu ◽  
Jiaxu Zhang
Keyword(s):  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Freshwater Flux ◽  
Boundary Current ◽  
The West ◽  
The North ◽  
Ocean Atmosphere ◽  
Set Up ◽  
The Kuroshio

Abstract Observations have indicated a trend of freshwater loss in the global western boundary current extension regions over several recent decades. In this paper, the coupled ocean–atmosphere response to the observed freshwater flux trend [defined as evaporation minus precipitation (EmP)] over the Kuroshio–Oyashio Extension (KOE) region is studied in a series of coupled model experiments. The model explicitly demonstrates that the positive EmP forcing in the KOE region can set up a cyclonic gyre straddling the subtropical and the subpolar gyre, which induces anomalous southward cold advection in the west and northward warm advection in the interior. This leads to the formation of a temperature dipole in the midlatitudes with a cooling in the west and a warming in the east. With the positive EmP forcing in the KOE, the response of the extratropical atmospheric circulation in the North Pacific sector is characterized by an equivalent barotropic low originating primarily from the western tropical Pacific changes and countered by the extratropical SST forcing. The positive EmP forcing also strengthens the tropical zonal SST gradient and thus ENSO through several competing processes including the surface-coupled wind–evaporative–SST (WES) mechanism, subduction of extratropical warm anomalies, and spinup of the density-driven meridional overturning circulation. Applications to recent Pacific climate changes are discussed.

scholarly journals Circulation, Heat, and Freshwater Transport at 36°N in the Atlantic

2010 ◽  
Vol 40 (12) ◽  
pp. 2661-2678 ◽  
Author(s):  
Elaine L. McDonagh ◽  
Paula McLeod ◽  
Brian A. King ◽  
Harry L. Bryden ◽  
Sinhué Torres Valdés
Keyword(s):  
North Atlantic ◽  
Western Boundary ◽  
Freshwater Flux ◽  
Bering Strait ◽  
Boundary Current ◽  
Sea Transport ◽  
The North ◽  
Freshwater Transport ◽  
The Pacific ◽  
Mid Atlantic Ridge

Abstract In May and June 2005, a transatlantic hydrographic section along 36°N was occupied. A velocity field is calculated using inverse methods. The derived 36°N circulation has an overturning transport (maximum in the overturning streamfunction) of 16.6 Sv (1 Sv ≡ 106 m3 s−1) at 1070 m. The heat transport across the section, 1.14 ± 0.12 PW, is partitioned into overturning and horizontal heat transports of 0.75 and 0.39 PW, respectively. The horizontal heat flux is set by variability at the gyre rather than by mesoscale. The freshwater flux across the section is 1.55 ± 0.18 Sv southward based on a 0.8-Sv flow from the Pacific through the Bering Strait at a salinity of 32.5 psu. The oceanic divergence of freshwater implies a net input of freshwater to the ocean of 0.75 Sv over the North Atlantic and Arctic between 36°N and the Bering Strait. Most (85%) of the recently ventilated upper North Atlantic Deep Water (water originating in the Labrador Sea) transport across the section occurs in the deep western boundary current rather than being associated with an interior pathway to the west of the mid-Atlantic ridge.

scholarly journals Western Boundary of the North Atlantic Subtropical Gyre: Decadal Change

2021 ◽  
Author(s):  
Daniel Santana-Toscano ◽  
M. Dolores Pérez-Hernández ◽  
Verónica Caínzos ◽  
Melania Cubas Armas ◽  
Cristina Arumí-Planas ◽  
...  
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Subtropical Gyre ◽  
North Equatorial Current ◽  
Western Boundary ◽  
Boundary Current ◽  
The North ◽  
North Brazil ◽  
Different Seasons ◽  
Using Data

<p>The A20 is a meridional hydrographic section located at 52ºW on the western North Atlantic Subtropical Gyre that encloses the path of the water masses of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with LADCP-SADCP data and the velocities from an inverse box model, the circulation of the western North Atlantic Subtropical Gyre is estimated. The main poleward current of the AMOC is the Gulf Stream (GS) which carries 129.0±10.5 Sv in 2003 and 110.4±12.2 Sv in 2012. Due to the seasonality, the GS position is shifted southward in 2012 - relative to that of 2003 - as both cruises took place in different seasons. In opposite direction, the Deep Western Boundary Current (DWBC) crosses the section twice, first at 39.3-43.2ºN (-34.9±7.5 Sv in 2003 and -25.3±9.4 Sv in 2012) and then at 7.0-11.7ºN (42.0±8.0 Sv in 2003 and 48.0±8.1 Sv in 2012). Additionally, two zonal currents contribute with westward transport below 20ºN: the North Equatorial Current and the North Brazil Current; with a net value of -28.0±4.1 Sv in 2003 and -36.7±3.6 Sv in 2012.</p>

scholarly journals How Much Has the North Atlantic Ocean Overturning Circulation Changed in the Last 50 Years?

2014 ◽  
Vol 27 (16) ◽  
pp. 6325-6342 ◽  
Author(s):  
Simon F. B. Tett ◽  
Toby J. Sherwin ◽  
Amrita Shravat ◽  
Oliver Browne
Keyword(s):  
North Atlantic ◽  
Mixed Layer Depth ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
The North ◽  
Ocean Reanalyses ◽  
The North Atlantic

Abstract Volume transports from six ocean reanalyses are compared with four sets of in situ observations: across the Greenland–Scotland ridge (GSR), in the Labrador Sea boundary current, in the deep western boundary current at 43°N, and in the Atlantic meridional overturning circulation (AMOC) at 26°N in the North Atlantic. The higher-resolution reanalyses (on the order of ¼° × ¼°) are better at reproducing the circulation pattern in the subpolar gyre than those with lower resolution (on the order of 1°). Simple Ocean Data Assimilation (SODA) and Estimating the Circulation and Climate of the Ocean (ECCO)–Jet Propulsion Laboratory (JPL) produce transports at 26°N that are close to those observed [17 Sv (1 Sv ≡ 106 m3 s−1)]. ECCO, version 2, and SODA produce northward transports across the GSR (observed transport of 8.2 Sv) that are 22% and 29% too big, respectively. By contrast, the low-resolution reanalyses have transports that are either too small [by 31% for ECCO-JPL and 49% for Ocean Reanalysis, system 3 (ORA-S3)] or much too large [Decadal Prediction System (DePreSys)]. SODA had the best simulations of mixed layer depth and with two coarse grid long-term reanalyses (DePreSys and ORA-S3) is used to examine changes in North Atlantic circulation from 1960 to 2008. Its results suggest that the AMOC increased by about 20% at 26°N while transport across the GSR hardly altered. The other (less reliable) long-term reanalyses also had small changes across the GSR but changes of +10% and −20%, respectively, at 26°N. Thus, it appears that changes in the overturning circulation at 26°N are decoupled from the flow across the GSR. It is recommended that transport observations should not be assimilated in ocean reanalyses but used for validation instead.

scholarly journals Influence of Salinity and Temperature Gradients on the Variability of the North Brazil Undercurrent

2021 ◽  
Vol 8 ◽  
Author(s):  
Hao Liu ◽  
Zexun Wei ◽  
Ingo Richter ◽  
Xunwei Nie ◽  
Chuanshun Li
Keyword(s):  
Interannual Variability ◽  
Time Scales ◽  
Linear Trend ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Temperature Gradients ◽  
Western Boundary ◽  
Boundary Current ◽  
The North ◽  
North Brazil

The North Brazil Undercurrent (NBUC) is a narrow (<1°) northward western boundary current in the tropical South Atlantic Ocean. It carries a large volume of water (>16 Sv) and plays an important role in the Atlantic Meridional Overturning Circulation and the South Atlantic Subtropical Cell. Strong salinity and temperature fronts occur over the NBUC region. The role of temperature and salinity gradients on the genesis of NBUC variability has never been explored. This study uses three high-resolution (≤0.1°) and one low-resolution (=0.25°) model outputs to explore the linear trend of NBUC transport and its variability on annual and interannual time scales. We find that the linear trend and interannual variability of the geostrophic NBUC transport show large discrepancies among the datasets. Thus, the linear trend and variability of the geostrophic NBUC are associated with model configuration. We also find that the relative contributions of salinity and temperature gradients to the geostrophic shear of the NBUC are not model dependent. Salinity-based and temperature-based geostrophic NBUC transports tend to be opposite-signed on all time scales. Despite the limited salinity and temperature profiles, the model results are consistent with the in-situ observations on the annual cycle and interannual time scales. This study shows the relationship of salinity-based and temperature-based geostrophic NBUC variations in the annual and interannual variability and trend among different models and highlights the equal important roles of temperature and salinity in driving the variability of NBUC transport.

scholarly journals Size Matters: Another Reason Why the Atlantic Is Saltier than the Pacific

2017 ◽  
Vol 47 (11) ◽  
pp. 2843-2859 ◽  
Author(s):  
C. S. Jones ◽  
Paola Cessi
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Subpolar Gyre ◽  
Boundary Current ◽  
Surface Salinity ◽  
The North ◽  
The North Atlantic

AbstractThe surface salinity in the North Atlantic controls the position of the sinking branch of the meridional overturning circulation (MOC); the North Atlantic has higher salinity, so deep-water formation occurs there rather than in the North Pacific. Here, it is shown that in a 3D primitive equation model of two basins of different widths connected by a reentrant channel, there is a preference for sinking in the narrow basin even under zonally uniform surface forcing. This preference is linked to the details of the velocity and salinity fields in the “sinking” basin. The southward western boundary current associated with the wind-driven subpolar gyre has higher velocity in the wide basin than in the narrow basin. It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of freshwater is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward western boundary current associated with the wind-driven subpolar gyre, bringing salty water from lower latitudes northward and enabling deep-water mass formation.

scholarly journals Gap-Leaping Western Boundary Current in a Circular Tank Model

2007 ◽  
Vol 37 (6) ◽  
pp. 1488-1495 ◽  
Author(s):  
Vitalii A. Sheremet ◽  
Joseph Kuehl
Keyword(s):  
Numerical Models ◽  
Flow Patterns ◽  
Curvilinear Coordinates ◽  
Laboratory Model ◽  
Western Boundary ◽  
Test Case ◽  
Boundary Current ◽  
Tank Model ◽  
Set Up ◽  
The Kuroshio

Abstract An oceanographically generic problem of the interaction of a boundary current with bathymetric features such as a gap in the ridge or a strait between two islands is considered. Multiple flow patterns (penetrating or leaping the gap) and hysteresis (dependence on prior evolution) may exist in such systems. Examples include the Gulf Stream leaping from the Yucatan to Florida and the Kuroshio leaping from Luzon to Taiwan. Using numerical analysis, Sheremet earlier found that multiple steady states can be explained by variation in the balance between the inertia (which promotes leaping state) and the β effect (which promotes penetrating state). In the present work a verification of the multiple states and hysteresis in a laboratory model are offered. To set up a gap-leaping current, a circular tank with a sloping bottom (simulating the β effect) is used, and the flow is driven using a new method of pumping fluid through sponges (thus generating a Sverdrup flow in the interior). A semicircular ridge with a gap is inserted into the western part of the tank. Using a dye release flow visualization method, the existence of multiple flow patterns over varying boundary current transport values differing by a factor of more than 2 are dramatically shown. An associated numerical model in bipolar curvilinear coordinates, which allows for the matching of all the boundaries, reproduces the laboratory results very well. This idealized problem offers a very useful geophysical test case for numerical models involving flow separation and reattachment.

scholarly journals Ship Routing Utilizing Strong Ocean Currents

2013 ◽  
Vol 66 (6) ◽  
pp. 825-835 ◽  
Author(s):  
Yu-Chia Chang ◽  
Ruo-Shan Tseng ◽  
Guan-Yu Chen ◽  
Peter C Chu ◽  
Yung-Ting Shen
Keyword(s):  
Flow Speed ◽  
Current Data ◽  
Ocean Currents ◽  
Surface Velocity ◽  
Western Boundary ◽  
Boundary Current ◽  
Ship Routing ◽  
Slow Steaming ◽  
The North ◽  
The Kuroshio

From the Surface Velocity Program (SVP) drifter current data, a detailed and complete track of strong ocean currents in the north-western Pacific is provided using the bin average method. The focus of this study is on the Kuroshio, the strong western boundary current of the North Pacific flowing northward along the east coast of Taiwan and then turning eastward off southern Japan. With its average flow speed of about 2 knots, the Kuroshio can significantly increase the ship's speed for a “super-slow-steaming” container ship travelling at speeds of 12 knots between the ports of Southeast Asia and Japan. By properly utilizing knowledge of strong ocean currents to follow the Kuroshio on the northbound runs and avoid it on the return trip, considerable fuel can be saved and the transit time can be reduced. In the future, the detailed Kuroshio saving-energy route could be built into electronic chart systems for all navigators and shipping routers.

The penetration of Labrador Slope Water to Cape Hatteras and its role in Gulf Stream Dynamics

2020 ◽  
Author(s):  
Adrian New ◽  
David Smeed ◽  
Adam Blaker ◽  
Jenny Mecking
Keyword(s):  
Gulf Stream ◽  
Water Mass ◽  
Gulf Of Maine ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
The North ◽  
The Us ◽  
Cape Hatteras ◽  
Northern Side

<p>Labrador Slope Water is known to exist in the Slope Sea off the US eastern shelf as a relatively fresh and cool water mass deriving from the Labrador Current further north, and is present between the upper layer US shelf-derived water masses and the deeper Deep Western Boundary current waters, typically near 400-600m. This LSLW  is investigated in the EN4 database and shown to penetrate as far south as Cape Hatteras (74-75°W), having previously only been described as far west as the Gulf of Maine (66°W). We then examine, using both EN4 and Line W observations, the changes of this water mass between 2005-2008, when the strength of Atlantic Meridional Overturning Circulation (AMOC) measured by the RAPID array at 26°N, was high, and 2009-2015, when the AMOC was low. We show that in the AMOC high period, there was a larger volume of the LSSW present on the northern side of the Gulf Stream system which resulted in an increased meridional slope of the isopycnals near these depths, commensurate with increased geostrophic transport, and also in a more southerly position, of the Gulf Stream after separation at Cape Hatteras. The LSLW could therefore play an important role in decadal timescale variations in the North Atlantic climate system through its impact on the Gulf Stream and AMOC.</p>

scholarly journals Structure, variability, and dynamics of the West Greenland Boundary Current System

2022 ◽  
Author(s):  
◽  
Astrid Pacini
Keyword(s):  
Current System ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Small Scale ◽  
Western Boundary ◽  
Coastal Current ◽  
Boundary Current ◽  
The West ◽  
West Greenland ◽  
Atlantic Origin

The ventilation of intermediate waters in the Labrador Sea has important implications for the strength of the Atlantic Meridional Overturning Circulation. Boundary current-interior interactions regulate the exchange of properties between the slope and the basin, which in turn regulates the magnitude of interior convection and the export of ventilated waters from the subpolar gyre. This thesis characterizes theWest Greenland Boundary Current System near Cape Farewell across a range of spatio-temporal scales. The boundary current system is composed of three velocity cores: (1) the West Greenland Coastal Current (WGCC), transporting Greenland and Arctic meltwaters on the shelf; (2) the West Greenland Current (WGC), which advects warm, saline Atlantic-origin water at depth, meltwaters at the surface, and newly-ventilated Labrador Sea Water (LSW); and (3) the Deep Western Boundary Current, which carries dense overflow waters ventilated in the Nordic Seas. The seasonal presence of the LSW and Atlantic-origin water are dictated by air-sea buoyancy forcing, while the seasonality of the WGCC is governed by remote wind forcing and the propagation of coastally trapped waves from East Greenland. Using mooring data and hydrographic surveys, we demonstrate mid-depth intensified cyclones generated at Denmark Strait are found offshore of the WGC and enhance the overflow water transport at synoptic timescales. Using mooring, hydrographic, and satellite data, we demonstrate that the WGC undergoes extensive meandering due to baroclinic instability that is enhanced in winter due to LSW formation adjacent to the current. This leads to the production of small-scale, anticyclonic eddies that can account for the entirety of wintertime heat loss within the Labrador Sea. The meanders are shown to trigger the formation of Irminger Rings downstream. Using mooring, hydrographic, atmospheric, and Lagrangian data, and a mixing model, we find that strong atmospheric storms known as forward tip jets cause upwelling at the shelfbreak that triggers offshore export of freshwater. This freshwater flux can explain the observed lack of ventilation in the eastern Labrador Sea. Together, this thesis documents previously unobserved interannual, seasonal, and synoptic-scale variability and dynamics within the West Greenland boundary current system that must be accounted for in future modeling.

Excavations at Fishbourne, 1963. Third Interim Report

1964 ◽  
Vol 44 (1) ◽  
pp. 1-8
Keyword(s):  
Interim Report ◽  
The West ◽  
Civic Society ◽  
North West ◽  
The Third ◽  
North East ◽  
The North ◽  
The Future ◽  
Set Up

Early in 1963 much of the land occupied by the Roman building at Fishbourne was purchased by Mr. I. D. Margary, M.A., F.S.A., and was given to the Sussex Archaeological Trust. The Fishbourne Committee of the trust was set up to administer the future of the site. The third season's excavation, carried out at the desire of this committee, was again organized by the Chichester Civic Society.1 About fifty volunteers a day were employed from 24th July to 3rd September. Excavation concentrated upon three main areas; the orchard south of the east wing excavated in 1962, the west end of the north wing, and the west wing. In addition, trial trenches were dug at the north-east and north-west extremities of the building and in the area to the north of the north wing. The work of supervision was carried out by Miss F. Pierce, M.A., Mr. B. Morley, Mr. A. B. Norton, B.A., and Mr. J. P. Wild, B.A. Photography was organized by Mr. D. B. Baker and Mrs. F. A. Cunliffe took charge of the pottery and finds.

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