Boundary currents along the Sri Lankan eastern and southern coasts serve as a pathway for salt exchange between the Bay of Bengal and the Arabian Sea basins in the northern Indian Ocean, which are characterized by their contrasting salinities. Measurements from two pairs of pressure-sensing inverted echo sounders (PIES) deployed along the Sri Lankan eastern and southern coasts as well as satellite measurements are used to understand the variability of these boundary currents and the associated salt transport. The volume transport in the surface (0-200-m depth) layer exhibits a seasonal cycle associated with the monsoonal wind reversal and interannual variability associated with the Indian Ocean dipole (IOD). In this layer, the boundary currents transport low-salinity water out of the Bay of Bengal during the northeast monsoon and transport high-salinity water into the Bay of Bengal during the fall monsoon transition of some years (e.g., 2015 and 2018). The Bay of Bengal salt input increases during the 2016 negative IOD as the eastward flow of high-salinity water during the fall monsoon transition intensifies, whereas the effect of the 2015/16 El Nino on the Bay of Bengal salt input is still unclear. The time-mean eddy salt flux over the upper 200 m estimated for the April 2015-March 2019 period along the eastern coast accounts for 9% of the salt budget required to balance an estimated 0.13 Sv (1 Sv equivalent to 10(6) m(3) s(-1)) of annual freshwater input into the Bay of Bengal. The time-mean eddy salt flux over the upper 200 m estimated for the December 2015-November 2019 period along the southern coast accounts for 27% of that same salt budget. SIGNIFICANCE STATEMENT: In the northern Indian Ocean, the highly saline Arabian Sea undergoes extreme evaporation while the Bay of Bengal (BoB) receives excess freshwater input. The focus of this study is the role of the observed time-variable circulation around Sri Lanka that permits the exchange between these basins to maintain their salinity distributions. The circulation fluctuates seasonally following the monsoon wind reversal and interannually in response to large-scale climate modes. The BoB freshwater export around Sri Lanka occurs during the northeast monsoon, whereas saline water import occurs during the fall monsoon transition of some years. However, rapid changes in both water volume transport and salt exchange can occur. The circulation over 0-200-m depth transports similar to 9%-27% of the BoB salt budget.

The concentration of oxygen exerts major controls on life in the ocean, and its distribution in the ocean and atmosphere carries information about biological productivity, transports of mass and heat, ocean deoxygenation and global carbon sinks. Our understanding of processes underlying oxygen distributions, their key features and variability is often lacking. Here we investigate the magnitude, variability and uncertainty of the air–sea flux of oxygen, carbon dioxide and atmospheric potential oxygen over an annual cycle in the Labrador Sea. We demonstrate that two-thirds of the annual oxygen uptake occurs over only 40 days in winter and is associated with a bubble-mediated component of air–sea gas transfer linked to episodic high winds, strong cooling and deep convective mixing. By neglecting the bubble-mediated flux component, global models may underestimate oxygen and atmospheric potential oxygen uptake in regions of convective deep-water formation by up to an order of magnitude. Uncertainties in wind speed products lead to additional major (up to 80%) uncertainty in air–sea fluxes in these critical regions. Our findings may help explain observation–model discrepancies in distributions of atmospheric potential oxygen and imply that oxygen levels in the deep ocean are more sensitive to climate change than currently thought.

The cruise was organized into 2 legs, the first without the equipment in the container and the second, with the equipment after the container was picked up in Almería.

The meridional overturning circulation (MOC) has been measured by boundary arrays in the Atlantic since 2000. Over the past decade of measurements, however, the reported tendencies in overturning circulation strength have differed between 168N and 268N. Here we investigate these differences by diagnosing their origin in the observed hydrography, finding that both arrays show deep waters (below 1,100 dbar) at the western boundary becoming fresher and less dense. The associated change in geopotential thickness is about 0.15 m s between 2004–2009 and 2010–2014, with the shift occurring between 2009 and 2010 and earlier at 268N than 168N. In the absence of a similar density change on the east of the Atlantic, this middepth reduction in water density at the west would drive an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at 268N and 3.9 Sv at 168N. These transport anomalies result in an intensifying tendency in the MOC estimate at 168N, but at 268N, the method of correcting the geostrophic reference level results in an opposing (reducing) tendency of the MOC. The results indicate that both arrays are observing coherent, low-frequency changes, but that there remain discrepancies in the methods of addressing the geostrophic reference level for boundary arrays measuring ocean circulation. Plain Language Summary The Atlantic Meridional Overturning Circulation (MOC), sometimes known as the great ocean conveyor, moves heat northwards in the Atlantic in the top 1km of the oceans, with deep water moving southward at depth. Observational programs have been continuously monitoring how quickly this ocean circulation is moving at various latitudes in the Atlantic since the early 2000s. In this paper, we compare measurements of the MOC at 26N and 16N to try to understand how the circulation has been changing during more than a decade of observations. We find that estimates of the overall MOC strength indicate a decreasing circulation strength at 26N, but increasing at 16N. This difference can be traced to choices made during the calculation of the MOC, but that the raw observations at the two latitudes show coherent changes on interannual and longer timescales.

The meridional overturning circulation (MOC) has been measured by boundary arrays in the Atlantic since 2000. Over the past decade of measurements, however, the reported tendencies in overturning circulation strength have differed between 16 degrees N and 26 degrees N. Here we investigate these differences by diagnosing their origin in the observed hydrography, finding that both arrays show deep waters (below 1,100 dbar) at the western boundary becoming fresher and less dense. The associated change in geopotential thickness is about 0.15 m(2) s(-2) between 2004-2009 and 2010-2014, with the shift occurring between 2009 and 2010 and earlier at 26 degrees N than 16 degrees N. In the absence of a similar density change on the east of the Atlantic, this middepth reduction in water density at the west would drive an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at 26 degrees N and 3.9 Sv at 16 degrees N. These transport anomalies result in an intensifying tendency in the MOC estimate at 16 degrees N, but at 26 degrees N, the method of correcting the geostrophic reference level results in an opposing (reducing) tendency of the MOC. The results indicate that both arrays are observing coherent, low-frequency changes, but that there remain discrepancies in the methods of addressing the geostrophic reference level for boundary arrays measuring ocean circulation. Plain Language Summary The Atlantic Meridional Overturning Circulation (MOC), sometimes known as the great ocean conveyor, moves heat northwards in the Atlantic in the top 1km of the oceans, with deep water moving southward at depth. Observational programs have been continuously monitoring how quickly this ocean circulation is moving at various latitudes in the Atlantic since the early 2000s. In this paper, we compare measurements of the MOC at 26N and 16N to try to understand how the circulation has been changing during more than a decade of observations. We find that estimates of the overall MOC strength indicate a decreasing circulation strength at 26N, but increasing at 16N. This difference can be traced to choices made during the calculation of the MOC, but that the raw observations at the two latitudes show coherent changes on interannual and longer timescales.

In situ boundary arrays have been installed in the North Atlantic to measure the large-scale ocean circulation. Here, we use measurements at the western edge of the North Atlantic at 16^∘N and 26^∘N to investigate low-frequency variations in deep densities and their associated influence on ocean transports. At both latitudes, deep waters (below 1100 dbar) at the western boundary are becoming fresher and less dense. The associated change in geopotential thickness is about 0.15 ^2^-2 between 2004-2009 and 2010-2014, with the shift occurring between 2009-2010 and earlier at 26^∘N than 16^∘N. Without a similar density change on the east of the Atlantic, a mid-depth reduction in water density at the west drives an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at 26^∘N and 3.9 Sv at 16^∘N. While these transport anomalies result in an intensifying tendency in the meridional overturning circulation (MOC) estimate at 16^∘N, the method of applying a zero net mass transport constraint at 26^∘N results in an opposing (reducing) tendency of the MOC.

: The long-term goal is to investigate the boundary-current and inter-basin ocean circulation which governs the conditions and variability in Bay of Bengal. For this, the flow around Sri Lanka is critical since it exchanges salt and freshwater between the Bay of Bengal and the Arabian Sea.

Abstract. The concentration of dissolved oxygen in seawater is routinely measured using a standardized titration method that involves analysis shortly after the water sample has been collected. However, none of the existing procedural documents are specific about how soon after collection the titration has to be done. Here, we report on a small number of samples where duplicates were collected and one batch was titrated within days after collection, while the other batch was stored for several weeks before titration. In addition, for a subset of the samples a third batch was taken that was stored like the others but with a particular chemical already added before storage. Comparison between the batches confirms that there is no significant difference between the ones that were stored and the ones that were analyzed sooner, indicating that a month-long storage period is acceptable. The implication of this is that such oxygen samples do not necessarily have to be analyzed while still on the ship; instead, it is possible to transport them ashore for analysis there.

Western and eastern boundary currents are key regions for understanding and monitoring the ocean's influence on and response to climate change processes.Yet the present global ocean observing is poorly suited for capturing the small scales, intense currents, often large vertical extent, the eddy-rich conditions, and (for eastern boundary currents) the biogeochemical and ecosystem variables needed.This paper reviews available technologies and methods, none of which can satisfy all requirements for the needed observing system.Therefore, merged hybrid approaches are proposed, which need to be evaluated in each case since conditions vary strongly.A global network is presented as a vision, of which 50% has at least partial implementations already.

The global overturning circulation (OC) and its deep branch strongly influence phenomena of direct interest to society, including climate change and variability, sea level, temperature and rainfall patterns over land, global biogeochemical cycles and marine productivity.Observations of the deep ocean remain scarce, limiting our ability to understand and predict the overturning and deep circulations, their response to changes in forcing, and the impact of changes in the deep ocean on marine ecosystems, biogeochemical cycles and global climate.However, substantial progress has been made in recent years, including quantitative estimates of the strength of the global overturning circulation; the first time series measurements of the Atlantic meridional overturning circulation; evidence for changes in temperature, salinity and carbon in the deep ocean; and a deeper understanding of the role of the deep ocean and OC in low-frequency climate variability.These advances provide a guide to the design and implementation of a sustained observing system for the deep ocean and OC.We outline a strategy for sustained observations of the deep ocean that begins with tools available now, primarily repeat hydrography and moored arrays spanning deep boundary currents, key passages, and ocean basins where feasible.New technologies -including profiling floats and gliders capable of sampling the full ocean depth, long-duration moorings with data telemetry, and new sensors are needed to complete a comprehensive observing system for the deep ocean.

A 2‐year record of mixed layer measurements of CO2 partial pressure (pCO2), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49°N/16.5°W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO2 levels by about 40 ± 15 μatm, which gives rise to a perennial CO2 sink of 3.2 ± 1.3 mol m−2 a−1. The seasonal pCO2 cycle is characterized by a summer minimum (winter maximum), which is due to the dominance of biological forcing over physical forcing. Our data document a rapid transition from deep mixing to shallow summer stratification. At the onset of shallow stratification, up to one third of the mixed layer net community production during the productive season had already been accomplished. The combination of high prestratification productivity and rapid onset of stratification appears to have caused the observed particle flux peak early in the season. Mixed layer deepening during fall and winter reventilated CO2 from subsurface respiration of newly exported organic matter, thereby negating more than one third of the carbon drawdown by net community production in the mixed layer. Chemical signatures of both net community production and respiration are indicative of carbon overconsumption, the effects of which may be restricted, though, to the upper ocean. A comparison of the estimated net community production with satellite‐based estimates of net primary production shows fundamental discrepancies in the timing of ocean productivity.

A 5-year-long time series of meridional transport below 1180d bar-zonally integrated across a section spanning, the western basin of the tropical North Atlantic-is analyzed. It has been inferred from (i) zonally integrated meridional geostrophic transports derived from density and bottom pressure measurements at the end points of a 1000 km wide section bounded by the base of the western continental rise and the Mid-Atlantic Ridge and (ii) mooring-based direct current meter measurements over the steep Lesser Antilles continental rise. The southward time mean transport of North Atlantic Deep Water (NADW) transport is 15.9Sv. The vertical shear of the geostrophic transport profiles in the western and eastern part of the section each show two layers of maximum southward transport within the NADW. The transport time series reveals changes of 7.7 Sv rms at periods of 1 month and longer, at times showing changes of up to 405v within a month's time. The baroclinic (internal) contribution of the geostrophic flow (relative to 4950 dbar), yields fluctuations of 6.6 Sv rms. Adding transports over the steep continental rise reduces the overall transport variability to 5.2 Sv rms. As a result of this reduction in shorter-period variability, the lower-frequency variability becomes more pronounced, part of which is expected to be linked to the meridional overturning circulation (MOC). The transport variability is consistent with baroclinic Rossby waves (at periods between 3 and 9 months), dominating in the eastern and central part of the section, and with changes in deep western boundary current (DWBC) strength, DWBC re-circulation patterns and eddies that become important in the western part of the section. The reference-level (external) geostrophic transport variability displays long-wavelength (> 2000 km) fluctuations of 7.5 Sv rms on periods less than 2 weeks that are consistent with barotropic Rossby waves.Numerical model simulations imply that the observed zonally integrated deep transport changes in the western basin have moderate skill in sensing changes in the MOC and in meridional heat transport, and that a now implemented extension of the array's integration scale into the eastern basin of the Atlantic would substantially improve the performance of the array as an MOC observing system. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.

The Labrador Sea is an important area of deep water formation and is hypothesized to be a significant sink for atmospheric CO 2 to the deep ocean. Here we examine the dynamics of the CO 2 system in the Labrador Sea using time‐series data obtained from instrumentation deployed on a mooring near the former Ocean Weather Station Bravo. A 1‐D model is used to determine the air‐sea CO 2 uptake and penetration of the CO 2 into intermediate waters. The results support that mixed‐layer p CO 2 remained undersaturated throughout most of the year, ranging from 220 μ atm in mid‐summer to 375 μ atm in the late spring. Net community production in the summer offset the increase in p CO 2 expected from heating and air‐sea uptake. In the fall and winter, cooling counterbalanced a predicted increase in p CO 2 from vertical convection and air‐sea uptake. The predicted annual mean air to sea flux was 4.6 mol m −2 yr −1 resulting in an annual uptake of 0.011 ± 0.005 Pg C from the atmosphere within the convection region. In 2001, approximately half of the atmospheric CO 2 penetrated below 500 m due to deep convection.