Anatomy and Decadal Evolution of the Pacific Subtropical–Tropical Cells (STCs)*

2005 ◽  
Vol 18 (18) ◽  
pp. 3739-3758 ◽  
Author(s):  
Antonietta Capotondi ◽  
Michael A. Alexander ◽  
Clara Deser ◽  
Michael J. McPhaden
Keyword(s):  
Time Scales ◽  
General Circulation ◽  
Phase Relationship ◽  
Circulation Model ◽  
Adjustment Process ◽  
Western Boundary ◽  
Boundary Current ◽  
Meridional Transport ◽  
The Pacific ◽  
Equatorial Upwelling

Abstract The output from an ocean general circulation model driven by observed surface forcing (1958–97) is used to examine the evolution and relative timing of the different branches of the Pacific Subtropical–Tropical Cells (STCs) at both interannual and decadal time scales, with emphasis on the 1976–77 climate shift. The STCs consist of equatorward pycnocline transports in the ocean interior and in the western boundary current, equatorial upwelling, and poleward flow in the surface Ekman layer. The interior pycnocline transports exhibit a decreasing trend after the mid-1970s, in agreement with observational transport estimates, and are largely anticorrelated with both the Ekman transports and the boundary current transports at the same latitudes. The boundary current changes tend to compensate for the interior changes at both interannual and decadal time scales. The meridional transport convergence across 9°S and 9°N as well as the equatorial upwelling are strongly correlated with the changes in sea surface temperature (SST) in the central and eastern equatorial Pacific. However, meridional transport variations do not occur simultaneously at each longitude, so that to understand the phase relationship between transport and SST variations it is important to consider the baroclinic ocean adjustment through westward-propagating Rossby waves. The anticorrelation between boundary current changes and interior transport changes can also be understood in terms of the baroclinic adjustment process. In this simulation, the pycnocline transport variations appear to be primarily confined within the Tropics, with maxima around 10°S and 13°N, and related to the local wind forcing; a somewhat different perspective from previous studies that have emphasized the role of wind variations in the subtropics.

scholarly journals Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales

2011 ◽  
Vol 24 (21) ◽  
pp. 5652-5670 ◽  
Author(s):  
Thierry Penduff ◽  
Mélanie Juza ◽  
Bernard Barnier ◽  
Jan Zika ◽  
William K. Dewar ◽  
...  
Keyword(s):  
Sea Level ◽  
Time Scales ◽  
General Circulation ◽  
Large Scale ◽  
Full Range ◽  
Circulation Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Intrinsic Variability ◽  
Boundary Current

Abstract This paper evaluates in a realistic context the local contributions of direct atmospheric forcing and intrinsic oceanic processes on interannual sea level anomalies (SLAs). A ¼° global ocean–sea ice general circulation model, driven over 47 yr by the full range of atmospheric time scales, is quantitatively assessed against altimetry and shown to reproduce most observed features of the interannual SLA variability from 1993 to 2004. Comparing this simulation with a second driven only by the climatological annual cycle reveals that the intrinsic part of the total interannual SLA variance exceeds 40% over half of the open-ocean area and exceeds 80% over one-fifth of it. This intrinsic contribution is particularly strong in eddy-active regions (more than 70%–80% in the Southern Ocean and western boundary current extensions) as predicted by idealized studies, as well as within the 20°–35° latitude bands. The atmosphere directly forces most of the interannual SLA variance at low latitudes and in most midlatitude eastern basins, in particular north of about 40°N in the Pacific. The interannual SLA variance is almost entirely due to intrinsic processes south of the Antarctic Circumpolar Current in the Indian Ocean sector, while half of this variance is forced by the atmosphere north of it. The same simulations were performed and analyzed at 2° resolution as well: switching to this laminar regime yields a comparable forced variability (large-scale distribution and magnitude) but almost suppresses the intrinsic variability. This likely explains why laminar ocean models largely underestimate the interannual SLA variance.

scholarly journals Coherence of Western Boundary Pressure at the RAPID WAVE Array: Boundary Wave Adjustments or Deep Western Boundary Current Advection?

2013 ◽  
Vol 43 (4) ◽  
pp. 744-765 ◽  
Author(s):  
Shane Elipot ◽  
Chris Hughes ◽  
Sofia Olhede ◽  
John Toole
Keyword(s):  
Continental Slope ◽  
Time Scales ◽  
Propagation Speed ◽  
Large Error ◽  
Western Boundary ◽  
Ocean Bottom ◽  
Boundary Current ◽  
Coherent Signals ◽  
Woods Hole Oceanographic Institution ◽  
Meridional Transport

Abstract This study investigates the coherence between ocean bottom pressure signals at the Rapid Climate Change programme (RAPID) West Atlantic Variability Experiment (WAVE) array on the western North Atlantic continental slope, including the Woods Hole Oceanographic Institution Line W. Highly coherent pressure signals propagate southwestward along the slope, at speeds in excess of 128 m s−1, consistent with expectations of barotropic Kelvin-like waves. Coherent signals are also evidenced in the smaller pressure differences relative to 1000-m depth, which are expected to be associated with depth-dependent basinwide meridional transport variations or an overturning circulation. These signals are coherent and almost in phase for all time scales from 3.6 years down to 3 months. Coherence is still seen at shorter time scales for which group delay estimates are consistent with a propagation speed of about 1 m s−1 over 990 km of continental slope but with large error bounds on the speed. This is roughly consistent with expectations for propagation of coastally trapped waves, though somewhat slower than expected. A comparison with both Eulerian currents and Lagrangian float measurements shows that the coherence is inconsistent with a propagation of signals by advection, except possibly on time scales longer than 6 months.

Recent Changes in the Pacific Subtropical Cells Inferred from an Eddy-Resolving Ocean Circulation Model*

2007 ◽  
Vol 37 (5) ◽  
pp. 1340-1356 ◽  
Author(s):  
Wei Cheng ◽  
Michael J. McPhaden ◽  
Dongxiao Zhang ◽  
E. Joseph Metzger
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Control Volume ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Volume Transport ◽  
Western Boundary ◽  
Equatorial Undercurrent ◽  
Model Driven ◽  
The Pacific

Abstract In this study the subtropical cells (STC) in the Pacific Ocean are analyzed using an eddy-resolving ocean general circulation model driven by atmospheric forcing for the years 1992–2003. In particular, the authors seek to identify decadal changes in the STCs in the model and to compare them with observations in order to understand the consequences of such changes for the equatorial ocean heat and mass budgets. The simulation shows a trend toward increasing pycnocline volume transport at 9°N and 9°S across the basin from 1992 to 2003. This increase [4.9 ± 1.0 Sv (Sv ≡ 106 m3 s−1)] is in qualitative agreement with observations and is attributed primarily to changes in the interior ocean transport, which are partially compensated by opposing western boundary transports. The subtropical meridional volume transport convergence anomalies in the model pycnocline are found to be consistent with anomalous volume transports in both the observed and modeled Equatorial Undercurrent, as well as with the magnitude of simulated anomalous upwelling transport at the base of the mixed layer in the eastern Pacific. As a result of the increased circulation intensity, heat transport divergence through the lateral boundaries of the tropical control volume (defined as the region between 9°N and 9°S, and from the surface to σθ = 25.3 isopycnal) increases, leading to a cooling of the tropical upper ocean despite the fact that net surface heat flux into the control volume has increased in the same time. As such, these results suggest that wind-driven changes in ocean transports associated with the subtropical cells play a central role in regulating tropical Pacific climate variability on decadal time scales.

Evaluation of Interior Circulation in a High-Resolution Global Ocean Model. Part I: Deep and Bottom Waters

2004 ◽  
Vol 34 (12) ◽  
pp. 2592-2614 ◽  
Author(s):  
Alexander Sen Gupta ◽  
Matthew H. England
Keyword(s):  
Time Scales ◽  
Mixed Layer ◽  
Deep Water ◽  
General Circulation ◽  
Bottom Water ◽  
Circulation Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Model Skill ◽  
The Antarctic

Abstract Global watermass ventilation pathways and time scales are investigated using an “eddy permitting” (¼°) offline tracer model. Unlike previous Lagrangian trajectory studies, here an offline model based on a complete tracer equation that includes three-dimensional advection and mixing is employed. In doing so, the authors are able to meaningfully simulate chlorofluorocarbon (CFC) uptake and assess model skill against observation. This is the first time an eddy-permitting model has been subjected to such an assessment of interior ocean ventilation. The offline model is forced by seasonally varying prescribed velocity, temperature, and salinity fields of a state-of-the-art ocean general circulation model. A seasonally varying mixed layer parameterization is incorporated to account for the degradation of surface convection processes resulting from the temporal averaging. A series of CFC simulations are assessed against observations to investigate interdecadal-time-scale ventilation using a variety of mixed layer criteria. Simulated tracer inventories and penetration depths are in good agreement with observations, especially for thermocline, mode, and surface waters. Deep water from the Labrador Sea is well represented, forming a distinct deep western boundary current that branches at the equator, although concentrations are lower than observed. The formation of bottom water, which occurs around the Antarctic margin, is also generally too weak, although there is excellent qualitative agreement with observations in the region of the Ross and Weddell Seas. Multicentury ventilation of the outflow of North Atlantic Deep Water and bottom water from the Antarctic Margin are investigated using 1000-yr passive tracer experiments with specified interior source regions. The model captures many of the detailed pathways evident from observations, with much of the discrepancy accounted for by differences between actual and modeled topography. A comparison between model-derived “tracer age” and Δ14C “advection age” provides a semiquantitative assessment of model skill at these longer time scales.

scholarly journals Variability and Coherence of the Agulhas Undercurrent in a High-Resolution Ocean General Circulation Model

2009 ◽  
Vol 39 (10) ◽  
pp. 2417-2435 ◽  
Author(s):  
A. Biastoch ◽  
L. M. Beal ◽  
J. R. E. Lutjeharms ◽  
T. G. D. Casal
Keyword(s):  
High Resolution ◽  
General Circulation ◽  
Current System ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Ocean Model ◽  
Western Boundary ◽  
Boundary Current ◽  
Sensitivity Experiment ◽  
Agulhas Current

Abstract The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.

scholarly journals Local Sensitivities of the Gulf Stream Separation

2017 ◽  
Vol 47 (2) ◽  
pp. 353-373 ◽  
Author(s):  
Joseph Schoonover ◽  
William K. Dewar ◽  
Nicolas Wienders ◽  
Bruno Deremble
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Climate Modeling ◽  
Circulation Model ◽  
Ocean Modeling ◽  
Western Boundary ◽  
Massachusetts Institute Of Technology ◽  
Boundary Current ◽  
Institute Of Technology ◽  
The Impact

AbstractRobust and accurate Gulf Stream separation remains an unsolved problem in general circulation modeling whose resolution will positively impact the ocean and climate modeling communities. Oceanographic literature does not face a shortage of plausible hypotheses that attempt to explain the dynamics of the Gulf Stream separation, yet a single theory that the community agrees on is missing. In this paper, the authors investigate the impact of the deep western boundary current (DWBC), coastline curvature, and continental shelf steepening on the Gulf Stream separation within regional configurations of the Massachusetts Institute of Technology General Circulation Model. Artificial modifications to the regional bathymetry are introduced to investigate the sensitivity of the separation to each of these factors. Metrics for subsurface separation detection confirm the direct link between flow separation and the surface expression of the Gulf Stream in the Mid-Atlantic Bight. It is shown that the Gulf Stream separation and mean surface position are most sensitive to the continental slope steepening, consistent with a theory proposed by Melvin Stern in 1998. In contrast, the Gulf Stream separation exhibits minimal sensitivity to the presence of the DWBC and coastline curvature. The implications of these results to the development of a “separation recipe” for ocean modeling are discussed. This study concludes adequate topographic resolution is a necessary, but not sufficient, condition for proper Gulf Stream separation.

scholarly journals Seasonality in Transition Scale from Balanced to Unbalanced Motions in the World Ocean

2018 ◽  
Vol 48 (3) ◽  
pp. 591-605 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen ◽  
Patrice Klein ◽  
Jinbo Wang ◽  
Hector Torres ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Seasonal Changes ◽  
General Circulation ◽  
World Ocean ◽  
Circulation Model ◽  
Internal Tides ◽  
Western Boundary ◽  
Massachusetts Institute Of Technology ◽  
Boundary Current ◽  
Tropical Oceans

AbstractThe transition scale Lt from balanced geostrophic motions to unbalanced wave motions, including near-inertial flows, internal tides, and inertia–gravity wave continuum, is explored using the output from a global 1/48° horizontal resolution Massachusetts Institute of Technology general circulation model (MITgcm) simulation. Defined as the wavelength with equal balanced and unbalanced motion kinetic energy (KE) spectral density, Lt is detected to be geographically highly inhomogeneous: it falls below 40 km in the western boundary current and Antarctic Circumpolar Current regions, increases to 40–100 km in the interior subtropical and subpolar gyres, and exceeds, in general, 200 km in the tropical oceans. With the exception of the Pacific and Indian sectors of the Southern Ocean, the seasonal KE fluctuations of the surface balanced and unbalanced motions are out of phase because of the occurrence of mixed layer instability in winter and trapping of unbalanced motion KE in shallow mixed layer in summer. The combined effect of these seasonal changes renders Lt to be 20 km during winter in 80% of the Northern Hemisphere oceans between 25° and 45°N and all of the Southern Hemisphere oceans south of 25°S. The transition scale’s geographical and seasonal changes are highly relevant to the forthcoming Surface Water and Ocean Topography (SWOT) mission. To improve the detection of balanced submesoscale signals from SWOT, especially in the tropical oceans, efforts to remove stationary internal tidal signals are called for.

scholarly journals Labrador Slope Water Connects the Subarctic with the Gulf Stream

2021 ◽  
Author(s):  
Adrian New ◽  
David Smeed ◽  
Arnaud Czaja ◽  
Adam Blaker ◽  
Jenny Mecking ◽  
...  
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Circulation Model ◽  
Western Boundary Current ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Western Slope ◽  
Western Boundary ◽  
Boundary Current ◽  
Shelf Slope

<p>Labrador Slope Water (LSLW) is found in the Slope Sea on the US-Canadian eastern shelf-slope as a relatively fresh and cool water mass, lying between the upper layer water masses and those carried by the Deep Western Boundary Current. It originates from the Labrador Current and has previously only been reported in the Eastern Slope Sea (east of 66°W). We here use the EN4 gridded database and the Line W hydrographic observations to show for the first time that the LSLW also penetrates into the Western Slope Sea, bringing it into close contact with the Gulf Stream. We also show that the LSLW spreads across the entire Slope Sea north of the Gulf Stream, and is both fresher and thicker when the Atlantic Meridional Overturning Circulation (AMOC) is high at the RAPID array at 26°N. The fresher, thicker LSLW is likely to contribute an additional 1.5 Sv of Gulf Stream transport. The spreading of the LSLW is also investigated in a high-resolution ocean general circulation model (NEMO), and is found to occur both as a western boundary current and through the extrusion of filaments following interaction with Gulf Stream meanders and eddies. The mechanism results in downward vertical motion as the filaments are entrained into the Gulf Stream. We conclude that the LSLW (rather than the deeper Labrador Sea Water) provides the intermediate depth water masses which maintain the density contrast here which partly drives the Gulf Stream, and that the transport of the LSLW from the Labrador shelf-slope offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting high latitude changes in the Subarctic with subsequent variability in the Gulf Stream and AMOC.</p>

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