Bower Amy S.

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Last Name
Bower
First Name
Amy S.
ORCID
0000-0003-0902-4984

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Author: Johns, William E. ×

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Equilibration and circulation of Red Sea Outflow water in the western Gulf of Aden

2005-11 , Bower, Amy S. , Johns, William E. , Fratantoni, David M. , Peters, Hartmut

Hydrographic, direct velocity, and subsurface float observations from the 2001 Red Sea Outflow Experiment (REDSOX) are analyzed to investigate the gravitational and dynamical adjustment of the Red Sea Outflow Water (RSOW) where it is injected into the open ocean in the western Gulf of Aden. During the winter REDSOX cruise, when outflow transport was large, several intermediate-depth salinity maxima (product waters) were formed from various bathymetrically confined branches of the outflow plume, ranging in depth from 400 to 800 m and in potential density from 27.0 to 27.5 σθ, a result of different mixing intensity along each branch. The outflow product waters were not dense enough to sink to the seafloor during either the summer or winter REDSOX cruises, but analysis of previous hydrographic and mooring data and results from a one-dimensional plume model suggest that they may be so during wintertime surges of strong outflow currents, or about 20% of the time during winter. Once vertically equilibrated in the Gulf of Aden, the shallowest RSOW was strongly influenced by mesoscale eddies that swept it farther into the gulf. The deeper RSOW was initially more confined by the walls of the Tadjura Rift, but eventually it escaped from the rift and was advected mainly southward along the continental slope. There was no evidence of a continuous boundary undercurrent of RSOW similar to the Mediterranean Undercurrent in the Gulf of Cadiz. This is explained by considering 1) the variability in outflow transport and 2) several different criteria for separation of a jet at a sharp corner, which indicate that the outflow currents should separate from the boundary where they are injected into the gulf.

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Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

2021-05-24 , Li, Feili , Lozier, M. Susan , Bacon, Sheldon , Bower, Amy S. , Cunningham, Stuart A. , de Jong, Marieke F. , deYoung, Brad , Fraser, Neil , Fried, Nora , Han, Guoqi , Holliday, Naomi Penny , Holte, James W. , Houpert, Loïc , Inall, Mark E. , Johns, William E. , Jones, Sam , Johnson, Clare , Karstensen, Johannes , Le Bras, Isabela A. , Lherminier, Pascale , Lin, Xiaopei , Mercier, Herlé , Oltmanns, Marilena , Pacini, Astrid , Petit, Tillys , Pickart, Robert S. , Rayner, Darren , Straneo, Fiamma , Thierry, Virginie , Visbeck, Martin , Yashayaev, Igor , Zhou, Chun

Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.

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Overturning in the Subpolar North Atlantic Program : a new international ocean observing system

2017-04-24 , Lozier, M. Susan , Bacon, Sheldon , Bower, Amy S. , Cunningham, Stuart A. , de Jong, Marieke Femke , de Steur, Laura , deYoung, Brad , Fischer, Jürgen , Gary, Stefan F. , Greenan, Blair J. W. , Heimbach, Patrick , Holliday, Naomi Penny , Houpert, Loïc , Inall, Mark E. , Johns, William E. , Johnson, Helen L. , Karstensen, Johannes , Li, Feili , Lin, Xiaopei , Mackay, Neill , Marshall, David P. , Mercier, Herlé , Myers, Paul G. , Pickart, Robert S. , Pillar, Helen R. , Straneo, Fiamma , Thierry, Virginie , Weller, Robert A. , Williams, Richard G. , Wilson, Christopher G. , Yang, Jiayan , Zhao, Jian , Zika, Jan D.

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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Somali Current rings in the eastern Gulf of Aden

2006-09-30 , Fratantoni, David M. , Bower, Amy S. , Johns, William E. , Peters, Hartmut

New satellite-based observations reveal that westward translating anticyclonic rings are generated as a portion of the Somali Current accelerates northward through the Socotra Passage near the mouth of the Gulf of Aden. Rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm/s, are comparable in overall diameter to the width of the Gulf of Aden (250 km), and translate westward into the gulf at 5–8 cm/s. Ring generation is most notable in satellite ocean color imagery in November immediately following the transition between southwest (boreal summer) and northeast (winter) monsoon regimes. The observed rings contain anomalous fluid within their core which reflects their origin in the equator-crossing Somali Current system. Estimates of Socotra Passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in May following the winter-to-summer monsoon transition. Cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together. Recent shipboard and Lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern Gulf of Aden.

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Mixing and entrainment in the Red Sea outflow plume. Part I : plume structure

2005-05 , Peters, Hartmut , Johns, William E. , Bower, Amy S. , Fratantoni, David M.

When the salty and heavy water of the Red Sea exits from the Strait of Bab el Mandeb, it continues downslope into the Gulf of Aden mainly along two channels. The 130-km-long “Northern Channel” (NC) is topographically confined and is typically only 5 km wide. In it, the Red Sea plume shows unanticipated patterns of vertical structure, turbulent mixing, and entrainment. Above the seafloor a 25–120-m-thick weakly stratified layer shows little dilution along the channel. Hence this bottom layer undergoes only weak entrainment. In contrast, a 35–285-m-thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing. It is thus the interface that exhibits the bulk of entrainment of the Red Sea plume in the NC. The interfacial layer also carries most of the overall plume transport, increasingly so with downstream distance. The “Southern Channel” (SC) is wider than the NC and is accessed from the latter by a sill about 33 m above the floor of the NC. Entrainment into the bottom layer of the SC is diagnosed to be strong near the entry into the SC such that the near-bottom density and salinity are smaller in the SC than in the NC at the same distance from Bab el Mandeb. In comparison with winter conditions, the authors encountered weaker outflow with shallower equilibration depths during the summer cruise. Bulk Froude numbers computed for the whole plume varied within the range 0.2–1. Local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment.

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Seasonality of the Meridional Overturning Circulation in the subpolar North Atlantic

2023-05-25 , Fu, Yao , Lozier, M Susan , Biló, Tiago Carrilho , Bower, Amy S. , Cunningham, Stuart A. , Cyr, Frédéric , de Jong, M. Femke , deYoung, Brad , Drysdale, Lewis , Fraser, Neil , Fried, Nora , Furey, Heather H. , Han, Guoqi , Handmann, Patricia , Holliday, N. Penny , Holte, James , Inall, Mark E. , Johns, William E. , Jones, Sam , Karstensen, Johannes , Li, Feili , Pacini, Astrid , Pickart, Robert S. , Rayner, Darren , Straneo, Fiammetta , Yashayaev, Igor

Understanding the variability of the Atlantic Meridional Overturning Circulation is essential for better predictions of our changing climate. Here we present an updated time series (August 2014 to June 2020) from the Overturning in the Subpolar North Atlantic Program. The 6-year time series allows us to observe the seasonality of the subpolar overturning and meridional heat and freshwater transports. The overturning peaks in late spring and reaches a minimum in early winter, with a peak-to-trough range of 9.0 Sv. The overturning seasonal timing can be explained by winter transformation and the export of dense water, modulated by a seasonally varying Ekman transport. Furthermore, over 55% of the total meridional freshwater transport variability can be explained by its seasonality, largely owing to overturning dynamics. Our results provide the first observational analysis of seasonality in the subpolar North Atlantic overturning and highlight its important contribution to the total overturning variability observed to date.

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