Consecutive multiyear records of hourly ocean bottom temperature measurements are merged to produce new decade‐long time series at four depths ranging from 1,360 to 4,757 m within the northwest ...Argentine Basin at 34.5°S. Energetic temperature variations are found at a wide range of time scales. All sites exhibit fairly linear warming trends of approximately 0.02–0.04°C per decade over the period 2009–2019, although the trends are only statistically different from zero at the two deepest sites at depths of ~4,500–4,800 m. Near‐bottom temperatures from independent conductivity‐temperature‐depth profiles collected at these same locations every 6–24 months over the same decade show roughly consistent trends. Based on the distribution of spectral energies at the deepest sites and a Monte Carlo‐style analysis, sampling at least once per year is necessary to capture the significant warming trends over this decade to within 50% error bars at a 95% confidence limit.
Plain Language Summary
Quantifying global temperature changes requires observations of the full atmosphere‐ocean system; however, long‐term, continuous observations of temperature deep within the ocean are exceedingly rare. This study presents several decade‐long records of hourly temperature measurements from moored sensors 1 m above the seafloor in the northwestern Argentine Basin within the western South Atlantic Ocean. These sites, which range in depth from 1,360 to 4,757 m, show energetic temperature variations on daily to interannual time scales. The intensity of these variations is higher at the two shallower sites than is observed at the two deeper sites. In addition to the daily to interannual variations, long‐term warming trends are also detected over the period 2009–2019 at all four sites. The study also uses the hourly records at the two deeper sites to estimate how frequently the temperature at these locations must be observed in order to estimate the long‐term trends accurately.
Key Points
Decade‐long hourly records of ocean bottom temperature demonstrate a surprising amount of variability at a range of time scales
Two deep/abyssal Argentine Basin sites (>4,500 m) exhibit significant warming trends of 0.02°C ± 0.01°C per decade over the period 2009–2019
Sampling at least once a year is necessary to capture deep/abyssal temperature trends with at least ±50% accuracy (95% confidence limits)
More than forty years of Florida Current transport estimates are combined to study annual and longer-term variability in this important component of the MOC and subtropical gyre. A detailed analysis ...with error estimates illustrates the difficulties in extracting annual and longer time scale variability given the strong higher frequency energy present. The annual cycle represents less than 10% of the total Florida Current transport variance in a 16
yr segment of the record, while interannual (13–42 month) variability represents only 13% of the total and periods longer than 42 months represents less than 10% of the total. Given the observed high frequency variability of the Florida Current, in order to get a monthly mean that is accurate to within 0.5
Sv (one standard error level) more than 20 daily observations are needed. To obtain an estimate of the annual climatology that is “accurate” to within 20% of its own standard deviation, at least 24
yr of data is needed. More than 40 observations spread throughout a year are required to obtain an annual mean that is accurate to within 0.5
Sv. Despite these daunting data requirements, there is sufficient data now to evaluate both the annual cycle of the Florida Current transport with a high degree of accuracy and to begin to determine the longer period transport variability. Comparison of the Florida Current, NAO and wind stress curl records shows that a recently described Sverdrup-based mechanism explains a significant fraction of the long-period variability primarily during the 1986–1998 time window, with other mechanisms clearly dominating before and after.
The vigor of Atlantic meridional overturning circulation (MOC) is thought to be vulnerable to global warming, but its short-term temporal variability is unknown so changes inferred from sparse ...observations on the decadal time scale of recent climate change are uncertain. We combine continuous measurements of the MOC (beginning in 2004) using the purposefully designed transatlantic Rapid Climate Change array of moored instruments deployed along 26.5°N, with time series of Gulf Stream transport and surface-layer Ekman transport to quantify its intra-annual variability. The year-long average overturning is 18.7 ± 5.6 sverdrups (Sv) (range: 4.0 to 34.9 Sv, where 1 Sv = a flow of ocean water of 10⁶ cubic meters per second). Interannual changes in the overturning can be monitored with a resolution of 1.5 Sv.
The zonal structure and time variability of the abyssal flow (below 3,000 dbar) in the South Atlantic western boundary is investigated using a combination of moored observations and simultaneous ...hydrographic/velocity sections at 34.5°S during 2009–2018. Moored direct velocity measurements near the bottom show strong variability with a peak‐to‐peak range exceeding 80 cm/s and dominant signals at times scales of 1–2 months. Daily time series of the meridional absolute geostrophic volume transport computed from the moorings reveals a highly energetic record with a temporal standard deviation of 8.3 Sv and peak‐to‐peak variations of 49 Sv, suggesting a significant contribution of the abyssal layer flows to the Deep Western Boundary Current time variability. The absolute transport is mostly driven by barotropic changes that are dominated by variations in the bottom pressure ~650 km away from the continental slope.
Plain Language Summary
The coldest, densest waters of the world's oceans sink near Antarctica to depths greater than 3,000 m and flow into other basins along the complex seafloor of the abyss. These waters play a key role in redistributing heat and carbon throughout the globe on time scales of centuries to millennia and therefore have a profound impact on the Earth's climate. The gateway northward into the Atlantic Ocean is along the western edge of the South Atlantic. This study presents 9 years of unprecedented, continuous observations of the abyssal flow across 34.5°S off the South American coast together with deep current velocity measurements collected during eight oceanographic cruises in the region since 2009. By combining these observations, we provide a general picture of the abyssal circulation in the region and show that, rather than being slowly changing, it rapidly and strongly varies on time scales as short as 1–2 months.
Key Points
The abyssal flow is strongly variable, and its time variations greatly exceed the time mean
The meridional absolute volume transport in the abyssal layers is driven by bottom pressure changes taking place away from the western margin
The abyssal transport contributes significantly to the time variability of the Deep Western Boundary Current
The Florida Current (FC) contributes to Atlantic circulation by carrying the western boundary flow of the subtropical gyre and the upper branch of meridional overturning circulation. Repeated FC ...hydrographic (velocity, salinity, and temperature) sections during 1982–1987 and 2001–2015 characterize its water mass structure and associated transport variability. On average, FC volume transport comes from subtropical North Atlantic water (NAW, 44%), Antarctic Intermediate Water (AAIW, 14%), surface water (SW, 27%), and an indistinct source (Rem 15%), while salinity transport relative to the average salinity along 26°N comes from NAW (55%), AAIW (0.2%), SW (30%), and Rem (15%). From 1982–1987 to 2001–2015, NAW, AAIW, and Rem salinified by 0.03–0.16 g kg−1 and increased the salinity anomaly transport by 3%. These patterns imply that advective salt transport by the FC (1) is sensitive to subtropical North Atlantic variability and (2) is partially decoupled from the volumetric pathway of the upper overturning branch.
Key Points
Regular sampling of the Florida Current since 1982 resolves spatial patterns of salinity and velocity and decadal changes thereof
North Atlantic water contributes as much or more volume transport and salinity anomaly transport as South Atlantic water
All subsurface waters salinified between 1982–1987 and 2001–2015 and caused a 3% net increase in salinity anomaly transport
Plain Language Summary
Surface currents in the Atlantic Ocean flow northward as part of a global overturning circulation. Overturning transports heat and salt to northern latitudes as a major contribution to global climate. At 26°N in the Atlantic, almost all the northward flow is carried by the Florida Current (FC). This study uses temperature and salinity measurements of the FC in 1982–1987 and 2001–2015 to interpret what sets northward salt transport and how it changes over two decades. Below the surface, the FC has become much saltier by 0.05–0.01 g/kg. The net northward salt transport has increased by 3%, a small but significant amount. Though changes in volume transport have the potential for causing large changes in salt transport, the observations show instead that increased salt transport is driven primarily by saltier waters from the subtropical North Atlantic. FC salt transport appears to be partially decoupled from volume transport associated with overturning. These results provide evidence that changing stratification associated with saltier subtropical water does lead to changes to oceanic advection of salt on a basin scale.
The Atlantic component of the Meridional Overturning Circulation (AMOC) is a key contributor to the global meridional transport of volume, salt, and heat, and thus plays a central role in global ...climate. As part of ongoing efforts to monitor the intensity and variability of the AMOC in the South Atlantic, hydrographic sections have been regularly occupied since 2009 near the western boundary along a zonal line at 34.5°S. Here this high‐quality, high‐resolution data set is analyzed to establish the average hydrographic conditions of the northwestern Argentine Basin and the water mass spatial and temporal variability. The water mass analysis also reveals the pathways of the flow in this region, which are further corroborated by full‐depth direct velocity measurements. The repeated hydrographic sections capture an extremely rich vertical structure, characterized by seven distinct water mass layers of northern and southern origin, each with unique property signatures. Almost all of these layers exhibit a sharp zonally banded structure, which is indicative of recirculation cells offshore from the western boundary. The circulation at intermediate levels includes a previously undetected recirculation cell confined very close to the western boundary and superimposed on the classical intermediate water pathway beneath the South Atlantic subtropical gyre. The deep level flow is characterized by the Deep Western Boundary Current (DWBC) and a northward recirculation ~500 km east from the slope.
Key Points
An unprecedented set of in situ observations is used to determine the pathways of the water masses close to the western boundary near 34°S
The water masses present a well‐defined zonal structure characterized by primary types close to the slope and subvarieties farther offshore
Previously undetected recirculation cells and strong meandering are observed at the intermediate and deep levels
Data from three independent and extensive field programs in the Straits of Florida, the Mid-Atlantic Bight, and near the Southeast Newfoundland Ridge are reanalyzed and compared with results from ...other historical studies to highlight the downstream evolution of several characteristics of the Gulf Stream's mean flow and variability. The three locations represent distinct dynamical regimes: a tightly confined jet in a channel; a freely meandering jet; and a topographically controlled jet on a boundary. Despite these differing dynamical regimes, the Gulf Stream in these areas exhibits many similarities. There are also anticipated and important differences, such as the loss of the warm core of the current by 42°N and the decrease in the cross-frontal gradient of potential vorticity as the current flows northward. As the Gulf Stream evolves it undergoes major changes in transport, both in magnitude and structure. The rate of inflow up to 60°W and outflow thereafter are generally uniform, but do exhibit some remarkable short-scale variations. As the Gulf Stream flows northward the vertical coherence of the flow changes, with the Florida Current and North Atlantic Current segments of the Gulf Stream exhibiting distinct upper and deep flows that are incoherent, while in the Mid-Atlantic Bight the Gulf Stream exhibits flows in three layers each of which tends to be incoherent with the other layers at most periods. These coherence characteristics are exhibited in both Eulerian and stream coordinates. The observed lack of vertical coherence indicates that great caution must be exercised in interpreting proxies for Gulf Stream structure and flow from vertically-limited or remote observations.
•Analysis of new and historical data sets along the length of the Gulf Stream pathway allows for the development of a consistent understanding of the flow and property changes as the current evolves downstream.•Surface and deep flows within the Gulf Stream are generally incoherent for most time scales between one day and one year, regardless of the location along the Gulf Stream pathway.•The Gulf Stream flow at 27°N and 42°N appears to operate in two independent layers, whereas at 38°N the Gulf Stream flow exists as three independent layers.•Proper understanding of Gulf Stream variability requires simultaneous observations throughout the water column.•Transport exchange between the Gulf Stream and the bounding recirculations to the north and south appears to have a maximum for inflow near 68°W based on analysis of data sets in the region.
The vigor of Atlantic meridional overturning circulation (MOC) is thought to be vulnerable to global warming, but its short-term temporal variability is unknown so changes inferred from sparse ...observations on the decadal time scale of recent climate change are uncertain. We combine continuous measurements of the MOC (beginning in 2004) using the purposefully designed transatlantic Rapid Climate Change array of moored instruments deployed along 26.5 degrees N, with time series of Gulf Stream transport and surface-layer Ekman transport to quantify its intra-annual variability. The year-long average overturning is 18.7 +/- 5.6 sverdrups (Sv) (range: 4.0 to 34.9 Sv, where 1 Sv = a flow of ocean water of 10(6) cubic meters per second). Interannual changes in the overturning can be monitored with a resolution of 1.5 Sv.
The rapid climate change programme (RAPID) has established a prototype system to continuously observe the strength and structure of the Atlantic meridional overturning circulation (MOC) at 26.5°N. ...Here we provide a detailed description of the RAPID-MOC monitoring array and how it has evolved during the first four deployment years, as well as an overview of the main findings so far. The RAPID-MOC monitoring array measures: (1) Gulf Stream transport through Florida Strait by cable and repeat direct velocity measurements; (2) Ekman transports by satellite scatterometer measurements; (3) Deep Western Boundary Currents by direct velocity measurements; (4) the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the Mid-Atlantic Ridge; and (5) barotropic fluctuations using bottom pressure recorders. The array became operational in late March 2004 and is expected to continue until at least 2014. The first 4 years of observations (April 2004–April 2008) have provided an unprecedented insight into the MOC structure and variability. We show that the zonally integrated meridional flow tends to conserve mass, with the fluctuations of the different transport components largely compensating at periods longer than 10 days. We take this as experimental confirmation of the monitoring strategy, which was initially tested in numerical models. The MOC at 26.5°N is characterised by a large variability—even on timescales as short as weeks to months. The mean maximum MOC transport for the first 4 years of observations is 18.7
Sv with a standard deviation of 4.8
Sv. The mechanisms causing the MOC variability are not yet fully understood. Part of the observed MOC variability consists of a seasonal cycle, which can be linked to the seasonal variability of the wind stress curl close to the African coast. Close to the western boundary, fluctuations in the Gulf Stream and in the North Atlantic Deep Water (NADW) coincide with bottom pressure variations at the western margin, thus suggesting a barotropic compensation. Ongoing and future research will put these local transport variations into a wider spatial and climatic context.
The Atlantic Meridional Overturning Circulation (AMOC) extends from the Southern Ocean to the northern North Atlantic, transporting heat northwards throughout the South and North Atlantic, and ...sinking carbon and nutrients into the deep ocean. Climate models indicate that changes to the AMOC both herald and drive climate shifts. Intensive trans-basin AMOC observational systems have been put in place to continuously monitor meridional volume transport variability, and in some cases, heat, freshwater and carbon transport. These observational programs have been used to diagnose the magnitude and origins of transport variability, and to investigate impacts of variability on essential climate variables such as sea surface temperature, ocean heat content and coastal sea level. AMOC observing approaches vary between the different systems, ranging from trans-basin arrays (OSNAP, RAPID 26 N, 11 S, SAMBA 34.5 N) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16 N). In this paper, we outline the different approaches (aims, strengths and limitations) and summarize the key results to date. We also discuss alternate approaches for capturing AMOC variability including direct estimates (e.g., using sea level, bottom pressure, and hydrography from Lagrangian floats), indirect estimates applying budgetary approaches, state estimates or ocean reanalyses, and proxies. Based on the existing observations and their results, and the potential of new observational and formal synthesis approaches, we make suggestions as to how to evaluate a comprehensive, future-proof observational network of the AMOC to deepen our understanding of the AMOC and its role in global climate.
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