Estimating the Deep Overturning Transport Variability at 26°N Using Bottom Pressure Recorders

Worthington, Emma; Frajka‐Williams, Eleanor; McCarthy, Gerard

doi:10.1029/2018jc014221

Cited by 9 publications

(11 citation statements)

References 35 publications

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“…The deep changes inferred from GRACE remain southward if the analysis is extended across the entire width of the Atlantic to mimic the coverage of the RAPID array. The disagreement between GRACE and RAPID trends has been noted previously and attributed to possible errors in the decadal variability in GRACE data (Worthington et al, 2019). To account for the difference between trends in RAPID and the GRACE JPL RL05.1 M product, the error in the decadal trends of the zonal ocean mass gradient would have to be 23.3 mm per decade.…”

Section: Net Circulation Change West Of the Marmentioning

confidence: 85%

Decadal Strengthening of Interior Flow of North Atlantic Deep Water Observed by GRACE Satellites

2020

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The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides information on changes to the Earth's gravity field, including ocean mass. Long-term trends in GRACE data are often considered unreliable due to uncertainties in the corrections made to calculate ocean mass from the raw measurements. Here, we use an independent estimate of ocean mass from satellite altimetry and in situ density data from five mooring sites and repeat hydrography to validate trends in GRACE over the North Atlantic, finding substantial agreement between the methods. The root-mean-square difference between ocean mass changes calculated with this method from the mooring data and those measured by GRACE is 3.5 mm per decade, much lower than the mean signal of 15.6 ± 1.8 mm per decade for GRACE and 17.8 ± 5.2 mm per decade for the altimetry-mooring estimate. The GRACE ocean mass data are then used to study the change in the deep circulation of the North Atlantic between the 1 April 2002 to 31 March 2009 and 1 April 2010 to 31 March 2017 periods, revealing a large-scale anticyclonic circulation anomaly off the North American coast. The change is associated with an increase of 13.9 ± 3.3 Sv (1 Sv = 10 6 m 3 s −1) of southward North Atlantic Deep Water flow in the interior between 30 • N and 40 • N, largely balanced by a northward anomaly of 10.7 ± 3.3 Sv for the boundary circulation. This implies an increased importance of interior pathways compared to the Deep Western Boundary Current for the spreading of North Atlantic Deep Water, which constitutes the lower limb of the Atlantic Meridional Overturning Circulation. Plain Language Summary The Gravity Recovery and Climate Experiment (GRACE) satellites measure the Earth's gravity field, which can be used to study ocean currents. In this study, we validate long-term trends in the data using independent ocean measurements to confirm that they reflect real changes in the ocean. In the Atlantic Ocean, North Atlantic Deep Water flows southward from its formation region near Greenland, spreading throughout the ocean basin. The flow of North Atlantic Deep Water is a crucial part of Earth's climate system, making it an important process to understand. Most of the water spreads southward in the so-called Deep Western Boundary Current along the continental shelves of North America, but there is also a significant flow further offshore in the interior of the basin. We use the GRACE data to measure changes to these two pathways between the 2002-2009 and 2010-2017 periods. The results show that southward flow in the interior has strengthened during this time, with opposite changes near the continent. This marks a significant change to how this water spreads from the formation region.

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Section: Net Circulation Change West Of the Marmentioning

confidence: 85%

Decadal Strengthening of Interior Flow of North Atlantic Deep Water Observed by GRACE Satellites

2020

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“…While progress is made in solving the problems of bottom pressure sensors on longer time scales (e.g. Kajikawa & Kobata, 2014;Worthington et al, 2019), the advantage of our method is that the BPRs are less expensive and easier to deploy than full-height mooring arrays. Additionally, we can fall back on more than 20 years of shipboard hydrographic measurements in the tropical South Atlantic -at the western (e.g.…”

Section: Summary and Discussionmentioning

confidence: 99%

Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

Herrford¹,

Brandt²,

Kanzow³

et al. 2020

Preprint

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Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11° S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The observed seasonal cycles of the AMOC, its components as well as the Western Boundary Current as observed with current meter moorings are in general good agreement with results of an ocean general circulation model. The seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, slightly stronger at the western boundary. In the model, seasonal AMOC variability at 11° S is governed by the variability in the eastern basin. Here, long Rossby waves originating from equatorial forcing are known to be radiated from the Angolan continental slope and propagate westward into the basin interior. The contribution of the western basin to AMOC seasonal variability is instead comparably weak as transport variability due to locally forced Rossby waves is mainly compensated by the Western Boundary Current. Our analyses indicate, that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes being not properly represented in the model, uncertainties in the wind forcing are particularly relevant for AMOC estimates at 11° S.

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“…Annual timeseries (1950-2018) of volume-averaged salinity for the total water column (a), upper 500 m layer (b), and 500 m-bottom layer (c) over the South Atlantic subtropical (34° S-7° S, yellow), tropical (7° S-10° N, green), North Atlantic subtropical (10° N-40° N, red), and sub-polar (40° N-65° N, blue) regions budget in the area bounded to the South by 26.5° N where changes in AMOC and FWT can be more reliably estimated, and the northern boundary of the Atlantic at 65° N. Taking into account the 26.5° N-65° N area averaged E − P change (from OAFlux/GPCP), the equivalent 3-D freshwater content change (inferred from the total depth volume-averaged salinity change in En4) and the estimated FWT change at 26.5° N from McDonagh et al(2015), we may estimate FWT change at the northern Atlantic boundary at 65° N needed to close the anomalous freshwater budget for this area over 1979-2018. In our calculations we ignore any change of freshwater transports at the Gibraltar Strait, as the mean net freshwater transport there is an order of magnitude less than the estimated change in FWT at 26.5° N.Recent estimates based on RAPID array measurements demonstrate a decreasing AMOC strength at 26.5° N over 20045° N over -20165° N over (Smeed et al 2018Worthington et al 2019). Over 2004-2013, estimates of FWT at 26.5° N also show that southward FWT has decreased with the AMOC(McDonagh et al 2015), although the time series reveals inter-annual variability of ~ 0.2 Sv.…”

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confidence: 99%

Changing water cycle and freshwater transports in the Atlantic Ocean in observations and CMIP5 models

et al. 2020

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Observations over the last 40 years show that the Atlantic Ocean salinity pattern has amplified, likely in response to changes in the atmospheric branch of the global water cycle. Observational estimates of oceanic meridional freshwater transport (FWT) at 26.5° N indicate a large increase over the last few decades, during an apparent decrease in the Atlantic Meridional Overturning Circulation (AMOC). However, there is limited observation based information at other latitudes. The relative importance of changing FWT divergence in these trends remains uncertain. Ten models from the Coupled Model Intercomparison Project Phase 5 are analysed for AMOC, FWT, water cycle, and salinity changes over 1950-2100. Over this timescale, strong trends in the water cycle and oceanic freshwater transports emerge, a part of anthropogenic climate change. Results show that as the water cycle amplifies with warming, FWT strengthens (more southward freshwater transport) throughout the Atlantic sector over the 21st century. FWT strengthens in the North Atlantic subtropical region in spite of declining AMOC, as the long-term trend is dominated by salinity change. The AMOC decline also induces a southward shift of the Inter-Tropical Convergence Zone and a dipole pattern of precipitation change over the tropical region. The consequent decrease in freshwater input north of the equator together with increasing net evaporation lead to strong salinification of the North Atlantic subtropical region, enhancing net northward salt transport. This opposes the influence of further AMOC weakening and results in intensifying southward freshwater transports across the entire Atlantic.

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Estimating the Deep Overturning Transport Variability at 26°N Using Bottom Pressure Recorders

Cited by 9 publications

References 35 publications

Decadal Strengthening of Interior Flow of North Atlantic Deep Water Observed by GRACE Satellites

Decadal Strengthening of Interior Flow of North Atlantic Deep Water Observed by GRACE Satellites

Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

Changing water cycle and freshwater transports in the Atlantic Ocean in observations and CMIP5 models

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