The Annual Cycle of Circulation of the Southwest Subtropical Pacific, Analyzed in an Ocean GCM*

Abstract An ocean GCM, interpreted in light of linear models and sparse observations, is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest Pacific Ocean. The simple structure of annual wind stress curl over the South Pacific produces a large region of uniformly phased, stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year, directing flow anomalies alternately toward and away from the boundary at its northern end, near 10°S. The response of the western boundary currents is to redistribute these anomalies northward toward the equator and southward to the subtropical gyre, a redistribution that is determined principally by linear Rossby processes, not boundary dynamics. When the subtropical gyre and South Equatorial Current (SEC) are strong (in the second half of the year), the result is both increased equatorward transport of the New Guinea Coastal Current and poleward transport anomalies along the entire Australian coast. Because of this opposite phasing of boundary current anomalies across 10°S, annual migration of the bifurcation point of the total SEC, near 18°S in the mean, has no significance regarding variability of transport from subtropics to equator.

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Mean Conditions and Seasonality of the West Greenland Boundary Current System near Cape Farewell

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0086.1 ◽

2020 ◽

Vol 50 (10) ◽

pp. 2849-2871

Author(s):

Astrid Pacini ◽

Robert S. Pickart ◽

Frank Bahr ◽

Daniel J. Torres ◽

Andrée L. Ramsey ◽

...

Keyword(s):

Current System ◽

Labrador Sea ◽

Western Boundary ◽

Coastal Current ◽

Boundary Current ◽

The West ◽

West Greenland ◽

Irminger Sea ◽

West Greenland Current ◽

Recirculation Gyre

AbstractThe structure, transport, and seasonal variability of the West Greenland boundary current system near Cape Farewell are investigated using a high-resolution mooring array deployed from 2014 to 2018. The boundary current system is comprised of three components: the West Greenland Coastal Current, which advects cold and fresh Upper Polar Water (UPW); the West Greenland Current, which transports warm and salty Irminger Water (IW) along the upper slope and UPW at the surface; and the Deep Western Boundary Current, which advects dense overflow waters. Labrador Sea Water (LSW) is prevalent at the seaward side of the array within an offshore recirculation gyre and at the base of the West Greenland Current. The 4-yr mean transport of the full boundary current system is 31.1 ± 7.4 Sv (1 Sv ≡ 106 m3 s−1), with no clear seasonal signal. However, the individual water mass components exhibit seasonal cycles in hydrographic properties and transport. LSW penetrates the boundary current locally, through entrainment/mixing from the adjacent recirculation gyre, and also enters the current upstream in the Irminger Sea. IW is modified through air–sea interaction during winter along the length of its trajectory around the Irminger Sea, which converts some of the water to LSW. This, together with the seasonal increase in LSW entering the current, results in an anticorrelation in transport between these two water masses. The seasonality in UPW transport can be explained by remote wind forcing and subsequent adjustment via coastal trapped waves. Our results provide the first quantitatively robust observational description of the boundary current in the eastern Labrador Sea.

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

10.1575/1912/27886 ◽

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.

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IMPACTS OF SOUTHERN HEMISPHERE WESTERLIES ON THE BRAZIL CURRENT AT 30ºS

Brazilian Journal of Geophysics ◽

10.22564/rbgf.v36i3.1959 ◽

2018 ◽

Vol 36 (3) ◽

pp. 1

Author(s):

Jessica S. Carvalho ◽

Fabricio S. C. Oliveira ◽

Edmo J. D. Campos

Keyword(s):

Wind Stress ◽

South Atlantic ◽

Western Boundary Current ◽

Subtropical Gyre ◽

Brazilian Coast ◽

Western Boundary ◽

Boundary Current ◽

The South ◽

Wind Stress Curl ◽

Brazil Current

ABSTRACT. Previous studies have pointed out an intensification of the global western boundary currents induced by changes in the wind-stress curl patterns over the oceans. The Brazil Current (BC) is the western boundary current into the South Atlantic Subtropical Gyre, which flows southwards along the Brazilian coast. A numerical model is used to investigate the response of BC to this change in wind forcing between 1960-2010, across 30ºS. The results found here support the increasing trend noticed in the wind-stress curl and a poleward migration of the South Hemisphere westerlies in the past decades. The residual transport of BC at 30_S is composed by its southward main flow and the northward branch of a recirculation cell (Rec) east of the BC. Both the BC and Rec transports showed a decrease trend of 0.10 Sv dec1 and 0.28 Sv dec1, respectively. It suggests a southward migration of Rec in response to changes in the westerlies. The results also indicate a relative intensification in the western boundary transport and a strengthening in the South Atlantic Subtropical Gyre. Keywords: Western Boundary Current, Meridional Transport, HYCOM.RESUMO. Estudos anteriores apontam para uma intensificação das correntes de contorno oeste globais induzidas por mudanças no rotacional do estresse do vento sobre os oceanos. A Corrente do Brasil (CB) é a corrente de contorno oeste do giro Subtropical do Atlântico Sul, que flui para sul ao longo da costa brasileira. Um modelo numérico é usado para investigar a resposta da CB às mudanças na forçante do vento entre 1960-2010, ao longo de 30_S. Os resultados encontrados aqui suportam a tendência de aumento observada no rotacional do estresse do vento e a migração para o polo dos ventos de oeste do hemisfério sul nas ultimas décadas. O transporte residual da CB em 30ºS é composto pelo seu fluxo principal para sul e o braço para norte de uma célula de recirculação (Rec) a leste da CB. Ambos os transportes da CB e Rec mostraram uma tendência de redução de 0,10 Sv dec1 e 0,28 Sv dec1, respectivamente. Isto sugere uma migração para sul da Rec em resposta às mudanças dos ventos de oeste. Os resultados também indicam uma relativa intensificação do transporte na borda oeste e um fortalecimento do giro Subtropical do Atlântico Sul.Palavras-chave: Corrente de Contorno Oeste, Transporte Meridional, HYCOM.

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Resolution convergence and sensitivity studies with North Atlantic circulation models. Part I: The western boundary current system

Ocean Modelling ◽

10.1016/j.ocemod.2006.08.005 ◽

2007 ◽

Vol 16 (3-4) ◽

pp. 141-159 ◽

Cited By ~ 86

Author(s):

Frank O. Bryan ◽

Matthew W. Hecht ◽

Richard D. Smith

Keyword(s):

North Atlantic ◽

Current System ◽

Western Boundary Current ◽

Western Boundary ◽

Boundary Current ◽

Sensitivity Studies

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Combining Observations from Multiple Platforms across the Kuroshio Northeast of Luzon: A Highlight on PIES Data

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-16-0095.1 ◽

2016 ◽

Vol 33 (10) ◽

pp. 2185-2203 ◽

Cited By ~ 7

Author(s):

Vigan Mensah ◽

Magdalena Andres ◽

Ren-Chieh Lien ◽

Barry Ma ◽

Craig M. Lee ◽

...

Keyword(s):

Time Series ◽

Current Velocity ◽

Mean Field ◽

Volume Transport ◽

Velocity Shear ◽

Western Boundary ◽

Thermocline Depth ◽

Boundary Current ◽

Processing Scheme ◽

The Kuroshio

AbstractThis study presents amended procedures to process and map data collected by pressure-sensor-equipped inverted echo sounders (PIESs) in western boundary current regions. The modifications to the existing methodology, applied to observations of the Kuroshio from a PIES array deployed northeast of Luzon, Philippines, consist of substituting a hydrography-based mean travel time field for the PIES-based mean field and using two distinct gravest empirical mode (GEM) lookup tables across the front that separate water masses of South China Sea and North Pacific origin. In addition, this study presents a method to use time-mean velocities from acoustic Doppler current profilers (ADCPs) to reference (or “level”) the PIES-recorded pressures in order to obtain time series of absolute geostrophic velocity. Results derived from the PIES observations processed with the hydrography-based mean field and two GEMs are compared with hydrographic profiles sampled by Seagliders during the PIES observation period and with current velocity measured concurrently by a collocated ADCP array. The updated processing scheme leads to a 41% error decrease in the determination of the thermocline depth across the current, a 22% error decrease in baroclinic current velocity shear, and a 61% error decrease in baroclinic volume transports. The absolute volume transport time series derived from the leveled PIES array compares well with that obtained directly from the ADCPs with a root-mean-square difference of 3.0 Sv (1 Sv ≡ 106 m3 s–1), which is mainly attributed to the influence of ageostrophic processes on the ADCP-measured velocities that cannot be calculated from the PIES observations.

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Positive Regional Sea Level Anomalies in Southern Brazil Due to Changes in Austral Atmospheric Circulation

10.21203/rs.3.rs-706527/v1 ◽

2021 ◽

Author(s):

Venisse Schossler ◽

Francisco Aquino ◽

Jefferson Simões ◽

Pedro Reis ◽

Denilson Viana

Keyword(s):

Sea Level ◽

Wind Stress ◽

Southern Annular Mode ◽

Altimeter Data ◽

Western Boundary ◽

Positive Trend ◽

Boundary Current ◽

Wind Stress Curl ◽

Sea Level Anomalies ◽

Regional Sea Level

Abstract Pressure gradients and winds play an important role in Southern Hemisphere (SH) sea levels, which are currently associated with the positive trend of the Southern Annular Mode (SAM). This study investigated regional sea level anomalies (SLAs) in the southern coast Brazil using altimeter data (1993–2019), post-processed by the X-TRACK (CTOH/LEGOS). We observed a negative SLA from 1993 to 2009 and a positive SLA from 2010 to 2019, with upward trends throughout the evaluation period. We analyzed wind stress curl, pressure, and wind fields at sea level (FNMOC and ERA 5, respectively) in addition to sea surface temperature and height anomalies (SSTA/SSHA-OISST) in the South Atlantic Ocean (SAO) for 1993–2009 and 2010–2019. In relation to the first period, the second shows the enhancement in Hadley and Walker cells and trade winds, in addition to greater SSTA and SSHA in SAO. The SAO subtropical gyre and zonal winds at 45°S contribute to the intensification of the western boundary current. A greater pressure gradient between the SAO surface and the southeast of South America is noteworthy. Regionally, the positive SAM brings an increase in sea level to the study area, caused by greater wind stress and variability in heat flows.

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Characterizing ERA-Interim and ERA5 surface wind biases using ASCAT

Ocean Science ◽

10.5194/os-15-831-2019 ◽

2019 ◽

Vol 15 (3) ◽

pp. 831-852 ◽

Cited By ~ 32

Author(s):

Maria Belmonte Rivas ◽

Ad Stoffelen

Keyword(s):

Large Scale ◽

Surface Wind ◽

Atmospheric Stability ◽

Ocean Current ◽

Western Boundary ◽

Boundary Current ◽

Wind Stress Curl ◽

Meridional Winds ◽

Mesoscale Convective ◽

Transient Wind

Abstract. This paper analyzes the differences between ERA-Interim and ERA5 surface winds fields relative to Advanced Scatterometer (ASCAT) ocean vector wind observations, after adjustment for the effects of atmospheric stability and density, using stress-equivalent winds (U10S) and air–sea relative motion using ocean current velocities. In terms of instantaneous root mean square (rms) wind speed agreement, ERA5 winds show a 20 % improvement relative to ERA-Interim and a performance similar to that of currently operational ECMWF forecasts. ERA5 also performs better than ERA-Interim in terms of mean and transient wind errors, wind divergence and wind stress curl biases. Yet, both ERA products show systematic errors in the partition of the wind kinetic energy into zonal and meridional, mean and transient components. ERA winds are characterized by excessive mean zonal winds (westerlies) with too-weak mean poleward flows in the midlatitudes and too-weak mean meridional winds (trades) in the tropics. ERA stress curl is too cyclonic in midlatitudes and high latitudes, with implications for Ekman upwelling estimates, and lacks detail in the representation of sea surface temperature (SST) gradient effects (along the equatorial cold tongues and Western Boundary Current (WBC) jets) and mesoscale convective airflows (along the Intertropical Convergence Zone and the warm flanks for the WBC jets). It is conjectured that large-scale mean wind biases in ERA are related to their lack of high-frequency (transient wind) variability, which should be promoting residual meridional circulations in the Ferrel and Hadley cells.

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The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations

Geoscientific Model Development ◽

10.5194/gmd-12-3329-2019 ◽

2019 ◽

Vol 12 (7) ◽

pp. 3329-3355 ◽

Cited By ~ 6

Author(s):

Franziska U. Schwarzkopf ◽

Arne Biastoch ◽

Claus W. Böning ◽

Jérôme Chanut ◽

Jonathan V. Durgadoo ◽

...

Keyword(s):

High Resolution ◽

Boundary Conditions ◽

Current System ◽

Western Boundary Current ◽

Global Ocean ◽

Western Boundary ◽

Boundary Current ◽

The South ◽

Agulhas Current ◽

Mesoscale Dynamics

Abstract. The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.

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A three-dimensional model of the wind-driven ocean circulation

Journal of Fluid Mechanics ◽

10.1017/s0022112068002193 ◽

1968 ◽

Vol 34 (4) ◽

pp. 721-734 ◽

Cited By ~ 5

Author(s):

J. A. Johnson

Keyword(s):

Ocean Circulation ◽

Three Dimensional ◽

Ekman Layer ◽

Return Flow ◽

Western Boundary ◽

Dimensional Model ◽

Boundary Current ◽

Three Dimensional Model ◽

Wind Stress Curl ◽

Interior Flow

A linear three-dimensional model of the wind-driven ocean circulation is treated by boundary-layer methods. The interior flow, below the Ekman layer, differs from the classical gyres of Munk (1950). There is a north-eastwards transport of fluid from the western boundary current of the southern gyre across the latitude of zero wind stress curl into the northern gyre. A return flow in the Ekman layer preserves continuity.

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