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Frajka-Williams, E, Ansorge IJ, Baehr J, Bryden HL, Chidichimo MP, Cunningham SA, Danabasoglu G, Dong SF, Donohue KA, Elipot S, Heimbach P, Holliday NP, Hummels R, Jackson LC, Karstensen J, Lankhorst M, Le Bras IA, Lozier MS, McDonagh EL, Meinen CS, Mercier H, Moat BI, Perez RC, Piecuch CG, Rhein M, Srokosz MA, Trenberth KE, Bacon S, Forget G, Goni G, Kieke D, Koelling J, Lamont T, McCarthy GD, Mertens C, Send U, Smeed DA, Speich S, van den Berg M, Volkov D, Wilson C.  2019.  Atlantic Meridional Overturning Circulation: Observed transport and variability. Frontiers in Marine Science. 6   10.3389/fmars.2019.00260   AbstractWebsite

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 degrees N, 11 degrees S, SAMBA 34.5 degrees S) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16 degrees 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 autonomous profiling 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.

Sutton, AJ, Feely RA, Maenner-Jones S, Musielwicz S, Osborne J, Dietrich C, Monacci N, Cross J, Bott R, Kozyr A, Andersson AJ, Bates NR, Cai WJ, Cronin MF, DeCarlo EH, Hales B, Howden SD, Lee CM, Manzello DP, McPhaden MJ, Melendez M, Mickett JB, Newton JA, Noakes SE, Noh JH, Olafsdottir SR, Salisbury JE, Send U, Trull TW, Vandemark DC, Weller RA.  2019.  Autonomous seawater pCO(2) and pH time series from 40 surface buoys and the emergence of anthropogenic trends. Earth System Science Data. 11:421-439.   10.5194/essd-11-421-2019   AbstractWebsite

Ship-based time series, some now approaching over 3 decades long, are critical climate records that have dramatically improved our ability to characterize natural and anthropogenic drivers of ocean carbon dioxide (CO2) uptake and biogeochemical processes. Advancements in autonomous marine carbon sensors and technologies over the last 2 decades have led to the expansion of observations at fixed time series sites, thereby improving the capability of characterizing sub-seasonal variability in the ocean. Here, we present a data product of 40 individual autonomous moored surface ocean pCO(2) (partial pressure of CO2) time series established between 2004 and 2013, 17 also include autonomous pH measurements. These time series characterize a wide range of surface ocean carbonate conditions in different oceanic (17 sites), coastal (13 sites), and coral reef (10 sites) regimes. A time of trend emergence (ToE) methodology applied to the time series that exhibit well-constrained daily to interannual variability and an estimate of decadal variability indicates that the length of sustained observations necessary to detect statistically significant anthropogenic trends varies by marine environment. The ToE estimates for seawater pCO(2) and pH range from 8 to 15 years at the open ocean sites, 16 to 41 years at the coastal sites, and 9 to 22 years at the coral reef sites. Only two open ocean pCO(2) time series, Woods Hole Oceanographic Institution Hawaii Ocean Time-series Station (WHOTS) in the subtropical North Pacific and Stratus in the South Pacific gyre, have been deployed longer than the estimated trend detection time and, for these, deseasoned monthly means show estimated anthropogenic trends of 1.9 +/- 0.3 and 1.6 +/- 0.3 mu atm yr(-1), respectively. In the future, it is possible that updates to this product will allow for the estimation of anthropogenic trends at more sites; however, the product currently provides a valuable tool in an accessible format for evaluating climatology and natural variability of surface ocean carbonate chemistry in a variety of regions. Data are available at https.//doi. org/10.7289/V5DB8043 and https.// (Sutton et al., 2018).

Frajka-Williams, E, Lankhorst M, Koelling J, Send U.  2018.  Coherent circulation changes in the deep North Atlantic from 16 degrees N and 26 degrees N transport arrays. Journal of Geophysical Research-Oceans. 123:3427-3443.   10.1029/2018jc013949   AbstractWebsite

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.

Koelling, J, Wallace DWR, Send U, Karstensen J.  2017.  Intense oceanic uptake of oxygen during 2014-2015 winter convection in the Labrador Sea. Geophysical Research Letters. 44:7855-7864.   10.1002/2017gl073933   AbstractWebsite

Measurements of near-surface oxygen (O-2) concentrations and mixed layer depth from the K1 mooring in the central Labrador Sea are used to calculate the change in column-integrated (0-1700 m) O-2 content over the deep convection winter 2014/2015. During the mixed layer deepening period, November 2014 to April 2015, the oxygen content increased by 24.3 +/- 3.4 mol m(-2), 40% higher than previous results from winters with weaker convection. By estimating the contribution of respiration and lateral transport on the oxygen budget, the cumulative air-sea gas exchange is derived. The O2 uptake of 29.1 +/- 3.8 mol m(-2), driven by persistent undersaturation (>= 5%) and strong atmospheric forcing, is substantially higher than predicted by standard (nonbubble) gas exchange parameterizations, whereas most bubble-resolving parameterizations predict higher uptake, comparable to our results. Generally large but varying mixed layer depths and strong heat and momentum fluxes make the Labrador Sea an ideal test bed for process studies aimed at improving gas exchange parameterizations.

Send, U, Testor P.  2017.  Direct observations reveal the deep circulation of the western Mediterranean Sea. Journal of Geophysical Research-Oceans. 122:10091-10098.   10.1002/2016jc012679   AbstractWebsite

Direct observations of the deep water circulation in the western Mediterranean Sea are presented, based on the analysis of autonomous profiling floats drifting at 1,200 and 1,900 m depth during the 1997-2002 period. The amount of water circulating in the basin is quantified, revealing several distinct gyres and boundary currents. It was also possible to follow the spreading of the newly formed Western Mediterranean Deep Water (nWMDW) and Tyrrhenian Deep Water (TDW), two main components of the deep water in the western Mediterranean, from their origin, based on their temperature and salinity signature. Both boundary currents and isolated eddies carrying the water into the interior are important for this.

Anutaliya, A, Send U, McClean JL, Sprintall J, Rainville L, Lee CM, Jinadasa SUP, Wallcraft AJ, Metzger EJ.  2017.  An undercurrent off the east coast of Sri Lanka. Ocean Science. 13:1035-1044.   10.5194/os-13-1035-2017   AbstractWebsite

The existence of a seasonally varying undercurrent along 8 degrees N off the east coast of Sri Lanka is inferred from shipboard hydrography, Argo floats, glider measurements, and two ocean general circulation model simulations. Together, they reveal an undercurrent below 100-200 m flowing in the opposite direction to the surface current, which is most pronounced during boreal spring and summer and switches direction between these two seasons. The volume transport of the undercurrent (200-1000 m layer) can be more than 10 Sv in either direction, exceeding the transport of 1-6 Sv carried by the surface current (0-200 m layer). The undercurrent transports relatively fresher water southward during spring, while it advects more saline water northward along the east coast of Sri Lanka during summer. Although the undercurrent is potentially a pathway of salt exchange between the Arabian Sea and the Bay of Bengal, the observations and the ocean general circulation models suggest that the salinity contrast between seasons and between the boundary current and interior is less than 0.09 in the subsurface layer, suggesting a small salt transport by the undercurrent of less than 4 % of the salinity deficit in the Bay of Bengal.

Ganachaud, A, Cravatte S, Sprintall J, Germineaud C, Alberty M, Jeandel C, Eldin G, Metzl N, Bonnet S, Benavides M, Heimburger LE, Lefevre J, Michael S, Resing J, Queroue F, Sarthou G, Rodier M, Berthelot H, Baurand F, Grelet J, Hasegawa T, Kessler W, Kilepak M, Lacan F, Privat E, Send U, Van Beek P, Souhaut M, Sonke JE.  2017.  The Solomon Sea: its circulation, chemistry, geochemistry and biology explored during two oceanographic cruises. Elementa-Science of the Anthropocene. 5   10.1525/elementa.221   AbstractWebsite

The semi-enclosed Solomon Sea in the southwestern tropical Pacific is on the pathway of a major oceanic circuit connecting the subtropics to the equator via energetic western boundary currents. Waters transiting through this area replenish the Pacific Warm Pool and ultimately feed the equatorial current system, in particular the equatorial undercurrent. In addition to dynamical transformations, water masses undergo nutrient and micronutrient enrichment when coming in contact with the coasts, impacting the productivity of the downstream equatorial region. Broadscale observing systems are not well suited for describing the fine-scale currents and water masses properties in the Solomon Sea, leaving it relatively unexplored. Two multidisciplinary oceanographic cruises were conducted in the Solomon Sea region, the first in July-August 2012 and the second in March 2014, by investigators from France and the United States. The experimental approach combined physical, chemical, geochemical and biogeochemical analyses, providing access to a wide range of space and time scales of the circulation. This collection of data allows describing the fine-scale structure of the currents and the water properties, transformations and mixing from the surface to the sill depth in the Solomon Sea and in the straits connecting it to the equator. Ocean-margin exchanges were documented through a comprehensive sampling of trace elements and isotopes as efficient tracers of natural fertilization processes. As air chemistry is largely impacted by the regional volcanic plumes, rainwater pH was also sampled. Dinitrogen fixation rates were measured and found to be among the highest in the global ocean, highlighting this region as a hot spot of nitrogen fixation. This study provides an overview of the climatic context during both cruises and the physical circulation and water masses properties. It provides a comprehensive description of all measurements made onboard, and presents preliminary results, aiming to serve as a reference for further physical, geochemical and biogeochemical studies.

Wijesekera, HW, Shroyer E, Tandon A, Ravichandran M, Sengupta D, Jinadasa SUP, Fernando HJS, Agrawal N, Arulananthan K, Bhat GS, Baumgartner M, Buckley J, Centurioni L, Conry P, Farrar TJ, Gordon AL, Hormann V, Jarosz E, Jensen TG, Johnston S, Lankhorst M, Lee CM, Leo LS, Lozovatsky I, Lucas AJ, MacKinnon J, Mahadevan A, Nash J, Omand MM, Pham H, Pinkel R, Rainville L, Ramachandran S, Rudnick DL, Sarkar S, Send U, Sharma R, Simmons H, Stafford KM, Laurent LS, Venayagamoorthy K, Venkatesan R, Teague WJ, Wang DW, Waterhouse AF, Weller R, Whalen CB.  2016.  ASIRI: An Ocean–Atmosphere Initiative for Bay of Bengal. Bulletin of the American Meteorological Society. 97:1859-1884.   10.1175/bams-d-14-00197.1   Abstract

Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

Sutton, AJ, Sabine CL, Feely RA, Cai WJ, Cronin MF, McPhaden MJ, Morell JM, Newton JA, Noh JH, Olafsdottir SR, Salisbury JE, Send U, Vandemark DC, Weller RA.  2016.  Using present-day observations to detect when anthropogenic change forces surface ocean carbonate chemistry outside preindustrial bounds. Biogeosciences. 13:5065-5083.   10.5194/bg-13-5065-2016   AbstractWebsite

One of the major challenges to assessing the impact of ocean acidification on marine life is detecting and interpreting long-term change in the context of natural variability. This study addresses this need through a global synthesis of monthly pH and aragonite saturation state (Omega(arag)) climatologies for 12 open ocean, coastal, and coral reef locations using 3-hourly moored observations of surface seawater partial pressure of CO2 and pH collected together since as early as 2010. Mooring observations suggest open ocean subtropical and subarctic sites experience present-day surface pH and Omega(arag) conditions outside the bounds of preindustrial variability throughout most, if not all, of the year. In general, coastal mooring sites experience more natural variability and thus, more overlap with preindustrial conditions; however, present-day Omega(arag) conditions surpass biologically relevant thresholds associated with ocean acidification impacts on Mytilus californianus (Omega(arag) < 1.8) and Crassostrea gigas (Omega(arag) < 2.0) larvae in the California Current Ecosystem (CCE) and Mya arenaria larvae in the Gulf of Maine (Omega(arag) < 1.6). At the most variable mooring locations in coastal systems of the CCE, subseasonal conditions approached Omega(arag) = 1. Global and regional models and data syntheses of ship-based observations tended to underestimate seasonal variability compared to mooring observations. Efforts such as this to characterize all patterns of pH and Omega(arag) variability and change at key locations are fundamental to assessing present-day biological impacts of ocean acidification, further improving experimental design to interrogate organism response under real-world conditions, and improving predictive models and vulnerability assessments seeking to quantify the broader impacts of ocean acidification.

Lee, CM, Jinadasa SUP, Anutaliya A, Centurioni LR, Fernando HJS, Hormann V, Lankhorst M, Rainville L, Send U, Wijesekera HW.  2016.  Collaborative observations of boundary currents, water mass variability, and monsoon response in the southern Bay of Bengal. Oceanography. 29:102-111.   10.5670/oceanog.2016.43   AbstractWebsite

The region surrounding Sri Lanka modulates monsoon-driven exchange between the Bay of Bengal and the Arabian Sea. Here, local circulation impacts the pathways followed by the boundary currents that drive exchange, thereby modulating mixing and water mass transformation. From 2013 to 2016, an international partnership conducted sustained measurements around the periphery of Sri Lanka, with the goal of understanding how circulation and mixing in this critical region modulate exchange between the Bay of Bengal and the Arabian Sea. Observations from satellite remote sensing, surface drifters, gliders, current meter moorings, and Pressure Inverted Echo Sounders capture seasonally reversing monsoon currents off the southern tip of Sri Lanka, trace the wintertime freshwater export pathway of the East India Coastal Current, and document the deflection of currents running along the east coast of Sri Lanka by cyclonic and anticyclonic eddies. Measurements also reveal energetic interleaving, indicative of mixing and stirring associated with these flows. Circulation inferred from satellite remote sensing and drifter tracks sometimes differs from that indicated by in situ sections, pointing to the need for observing systems that employ complementary approaches toward understanding this region.

Legler, DM, Freeland HJ, Lumpkin R, Ball G, McPhaden MJ, North S, Crowley R, Goni GJ, Send U, Merrifield MA.  2015.  The current status of the real-time in situ Global Ocean Observing System for operational oceanography. Journal of Operational Oceanography. 8:S189-S200.   10.1080/1755876x.2015.1049883   AbstractWebsite

The GODAE-OceanView collection of papers primarily concerns the development of ocean data-assimilation models for operational oceanography. However, these models cannot function without a secure supply of in situ ocean data in near real-time. Several projects and programmes supply such data. The purpose of this paper is to review these data sources and describe the history, present status, future and robustness of these programmes. The conclusion is that though challenges continue with some components of the Global Ocean Observing System, overall the system continues to evolve and improve. The data are available in real-time to drive assimilation models, and expectations are increasing for more observational data. The prospects for the next 10 years seem to be good. All of the systems are evolving and there is little doubt that the Global Ocean Observing System will look different 10 years from now as new technologies emerge and capabilities improve.

Ganachaud, A, Cravatte S, Melet A, Schiller A, Holbrook NJ, Sloyan BM, Widlansky MJ, Bowen M, Verron J, Wiles P, Ridgway K, Sutton P, Sprintall J, Steinberg C, Brassington G, Cai W, Davis R, Gasparin F, Gourdeau L, Hasegawa T, Kessler W, Maes C, Takahashi K, Richards KJ, Send U.  2014.  The Southwest Pacific Ocean circulation and climate experiment (SPICE). Journal of Geophysical Research-Oceans. 119:7660-7686.   10.1002/2013jc009678   AbstractWebsite

The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions. Key Points Southwest Pacific WBCs transport large volumes toward the equator and the pole Pathways are complex; water properties tend to erode during the transit Variations due to seasons, ENSO and the SPCZ modulate the relative WBC strengths

Martz, T, Send U, Ohman MD, Takeshita Y, Bresnahan P, Kim HJ, Nam S.  2014.  Dynamic variability of biogeochemical ratios in the Southern California Current System. Geophysical Research Letters. 41:2496-2501.   10.1002/2014gl059332   AbstractWebsite

We use autonomous nitrate (NO3-), oxygen (O-2), and dissolved inorganic carbon (DIC) observations to examine the relationship between ratios of C:N:O at an upwelling site in the Southern California Current System. Mean ratios and 95% confidence intervals observed by sensors over 8 months were NO3-:O-2=-0.110.002, NO3-:DIC=0.140.001, and DIC:O-2=-0.830.01, in good agreement with Redfield ratios. Variability in the ratios on the weekly time scale is attributable to shifts in biological demand and nutrient availability and shown to exhibit a spectrum of values ranging from near 100% New Production to 100% Regenerated Production.

Fan, X, Send U, Testor P, Karstensen J, Lherminier P.  2013.  Observations of Irminger Sea Anticyclonic Eddies. Journal of Physical Oceanography. 43:805-823.   10.1175/jpo-d-11-0155.1   AbstractWebsite

Mesoscale anticyclonic eddies in the Irminger Sea are observed using a mooring and a glider. Between 2002 and 2009, the mooring observed 53 anticyclones. Using a kinematic model, objective estimates of eddy length scales and velocity structure are made for 16 eddies. Anticyclones had a mean core diameter of 12 km, and their mean peak observed azimuthal speed was 0.1 m s(-1). They had core salinities and potential temperatures of 34.91-34.98 and 4.488-5.34 degrees C, respectively, making them warm and salty features. These properties represent a typical salinity anomaly of 0.03 and a temperature anomaly of 0.28 degrees C from noneddy values. All eddies had small (<< 1) Rossby numbers. In 2006, the glider observed two anticyclones having diameters of about 20 km and peak azimuthal speeds of about 0.3 m s(-1). Similar salinity anomalies were detected throughout the Irminger Sea by floats profiling in anticyclones. Two formation regions for the eddies are identified: one to the west of the Reykjanes Ridge and the other off the East Greenland Irminger Current near Cape Farewell close to the mooring. Observations indicate that eddies formed in the former region are larger than eddies observed at the mooring. A clear increase in eddy salinity is observed between 2002 and 2009. The observed breakup of these eddies in winter implies that they are a source of salt for the central gyre. The anticyclones are similar to those found in both the Labrador Sea and Norwegian Sea, making them a ubiquitous feature of the subpolar North Atlantic basins.

Ohman, MD, Rudnick DL, Chekalyuk A, Davis RE, Feely RA, Kahru M, Kim HJ, Landry MR, Martz TR, Sabine CL, Send U.  2013.  Autonomous ocean measurements in the California Current ecosystem. Oceanography. 26:18-25. AbstractWebsite

Event-scale phenomena, of limited temporal duration or restricted spatial extent, often play a disproportionately large role in ecological processes occurring in the ocean water column. Nutrient and gas fluxes, upwelling and downwelling, transport of biogeochemically important elements, predator-prey interactions, and other processes may be markedly influenced by such events, which are inadequately resolved from infrequent ship surveys. The advent of autonomous instrumentation, including underwater gliders, profiling floats, surface drifters, enhanced moorings, coastal high-frequency radars, and satellite remote sensing, now provides the capability to resolve such phenomena and assess their role in structuring pelagic ecosystems. These methods are especially valuable when integrated together, and with shipboard calibration measurements and experimental programs.

Send, U, Fowler G, Siddall G, Beanlands B, Pittman M, Waldmann C, Karstensen J, Lampitt R.  2013.  SeaCycler: A moored open-ocean profiling system for the upper ocean in extended self-contained deployments. Journal of Atmospheric and Oceanic Technology. 30:1555-1565.   10.1175/jtech-d-11-00168.1   AbstractWebsite

The upper ocean, including the biologically productive euphotic zone and the mixed layer, has great relevance for studies of physical, biogeochemical, and ecosystem processes and their interaction. Observing this layer with a continuous presence, sampling many of the relevant variables, and with sufficient vertical resolution, has remained a challenge. Here a system is presented that can be deployed on the top of deep-ocean moorings, with a drive mechanism at depths of 150-200 m, which mechanically winches a large sensor float and smaller communications float tethered above it to the surface and back down again, typically twice per day for periods up to 1 year. The sensor float can carry several sizeable sensors, and it has enough buoyancy to reach the near surface and for the communications float to pierce the surface even in the presence of strong currents. The system can survive mooring blowover to 1000-m depth. The battery-powered design is made possible by using a balanced energy-conserving principle. Reliability is enhanced with a drive assembly that employs a single rotating part that has no slip rings or rotating seals. The profiling bodies can break the surface to sample the near-surface layer and to establish satellite communication for data relay or reception of new commands. An inductive pass-through mode allows communication with other mooring components throughout the water column beneath the system. A number of successful demonstration deployments have been completed.

Send, U, Regier L, Jones B.  2013.  Use of underwater gliders for acoustic data retrieval from subsurface oceanographic instrumentation and bidirectional communication in the deep ocean. Journal of Atmospheric and Oceanic Technology. 30:984-998.   10.1175/jtech-d-11-00169.1   AbstractWebsite

Many fixed oceanographic instruments and observing systems are deployed in the water column or on the seafloor for extended periods of time without any expression at the sea surface. To routinely communicate with such subsurface instruments in the deep ocean, here a system is presented that uses underwater gliders and commercially available acoustic modems for this task and its use is demonstrated with subsurface moorings and inverted echo sounders plus bottom pressure sensor (PIES). One recent glider mission spent 31 days in data retrieval dives, capturing 2 MB of error-free subsurface data. To acquire this volume, a total of 2.65 MB(including all retransmissions) were sent, with a success rate of 75%. A model for the energy usage of each phase of modem function was derived from laboratory measurements. While the model predicts that the glider would expend 0.21 J to acquire each data byte, the actual consumption of the glider in the field is 0.49 J byte(-1). The inefficiency is due to overhead associated with establishment of the acoustic link and with the resending of data that is received with errors. Including all the time for negotiating the acoustic link and for the retransmission of erroneous data, the net data throughput are around 3 bytes s(-1) in spite of the modem operating at 140 to 600 baud. Even with these limitations, the technique has shown to be useful and is being utilized routinely in a research project in the California Current to obtain data from horizontal distances up to 7 km from an instrument at depths up to 4000 m, transferring on average 6 kB of data in a day of acoustic communications.

Howard, J, Babij E, Griffis R, Helmuth B, Himes-Cornell A, Niemier P, Orbach M, Petes L, Allen S, Auad G, Auer C, Beard R, Boatman M, Bond N, Boyer T, Brown D, Clay P, Crane K, Cross S, Dalton M, Diamond J, Diaz R, Dortch Q, Duffy E, Fauquier D, Fisher W, Graham M, Halpern B, Hansen L, Hayum B, Herrick S, Hollowed A, Hutchins D, Jewett E, Jin D, Knowlton N, Kotowicz D, Kristiansen T, Little P, Lopez C, Loring P, Lumpkin R, Mace A, Mengerink K, Morrison JR, Murray J, Norman K, O'Donnell J, Overland J, Parsons R, Pettigrew N, Pfeiffer L, Pidgeon E, Plummer M, Polovina J, Quintrell J, Rowles T, Runge J, Rust M, Sanford E, Send U, Singer M, Speir C, Stanitski D, Thornber C, Wilson C, Xue Y.  2013.  Oceans and marine resources in a changing climate. Oceanography and Marine Biology: An Annual Review, Vol 51. 51( Hughes RN, Hughes DJ, Eds.).:71-192., Boca Raton: Crc Press-Taylor & Francis Group Abstract

The United States is an ocean nation-our past, present, and future are inextricably connected to and dependent on oceans and marine resources. Marine ecosystems provide many important services, including jobs, food, transportation routes, recreational opportunities, health benefits, climate regulation, and cultural heritage that affect people, communities, and economies across the United States and internationally every day. There is a wealth of information documenting the strong linkages between the planet's climate and ocean systems, as well as how changes in the climate system can produce changes in the physical, chemical, and biological characteristics of ocean ecosystems on a variety of spatial and temporal scales. There is relatively little information on how these climate-driven changes in ocean ecosystems may have an impact on ocean services and uses, although it is predicted that ocean-dependent users, communities, and economies will likely become increasingly vulnerable in a changing climate. Based on our current understanding and future projections of the planet's ocean systems, it is likely that Marine ecosystems will continue to be affected by anthropogenic-driven climate change into the future. This review describes how these impacts are set in motion through a suite of changes in ocean physical, chemical, and biological components and processes in US waters and the significant implications of these changes for ocean users and the communities and economies that depend on healthy oceans. US international partnerships, management challenges, opportunities, and knowledge gaps are also discussed. Effectively preparing for and responding to climate-driven changes in the ocean will require both limiting future change through reductions of greenhouse gases and adapting to the changes that we can no longer avoid.

Send, U, Nam S.  2012.  Relaxation from upwelling: The effect on dissolved oxygen on the continental shelf. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007517   AbstractWebsite

Continental shelves in upwelling regimes are subject to sequences of upwelling and relaxation events, each on timescales of order 1 week typically. These episodes have pronounced impacts on the temperature and density structure on the shelves and also on the along-shore and cross-shore flow regimes. It had previously been demonstrated that relaxation phases advect warm water along shore (poleward) from regions of less intense upwelling, thus adding to the heat balance in upwelling locations and providing a rectification of oscillating heat fluxes. In the current paper it is shown that relaxations also modify the dissolved oxygen (DO) budget of the lower layers. On a narrow shelf, this provides enhanced DO values due to near-surface exposure, while on a wide shelf decreased DO concentrations are created due to oxygen consumption on the inner shelf. The resulting variations along the coast can lead to along-shore advection of high or low DO during a relaxation event. Observations are presented from moorings off San Diego and Del Mar, which show large departures from density-correlated DO values during relaxations and which support the proposed mechanisms.

Baringer, MO, Cunningham SA, Meinen CS, Garzoli S, Willis J, Lankhorst M, Macdonald A, Send U, Hobbs WR, Frajka-Williams E, Kanzow TO, Rayner D, Johns WE, Marotzke J.  2012.  Meridional overturning circulation observations in the subtropical North Atlantic. State of the Climate in 2011. 93( Blunden J, Arndt DS, Eds.).:S78-S81.: American Meteorological Society   10.1175/2012BAMSStateoftheClimate.1   Abstract
Nam, S, Kim HJ, Send U.  2011.  Amplification of hypoxic and acidic events by La Nina conditions on the continental shelf off California. Geophysical Research Letters. 38   10.1029/2011gl049549   AbstractWebsite

Low-oxygen and low-pH events are an increasing concern and threat in the Eastern Pacific coastal waters, and can be lethal for benthic and demersal organisms on the continental shelf. The normal seasonal cycle includes uplifting of isopycnals during upwelling in spring, which brings low-oxygen and low-pH water onto the shelf. Five years of continuous observations of subsurface dissolved oxygen off Southern California, reveal large additional oxygen deficiencies relative to the seasonal cycle during the latest La Nina event. While some changes in oxygen related to the isopycnal depression/uplifting during El Nino/La Nina are not unexpected, the observed oxygen changes are 2-3 times larger than what can be explained by cross-shore exchanges. In late summer 2010, oxygen levels at mid-depth of the water column reached values of 2.5 ml/L, which is much lower than normal oxygen levels at this time of the seasons, 4-5 ml/L. The extra uplifting of isopycnals related to the La Nina event can explain oxygen reductions only to roughly 3.5 ml/L. We find that the additional oxygen decrease beyond that is strongly correlated with decreased subsurface primary production and strengthened poleward flows by the California Undercurrent. The combined actions of these three processes created a La Nina-caused oxygen decrease as large and as long as the normal seasonal minimum during upwelling period in spring, but later in the year. With a different timing of a La Nina, the seasonal oxygen minimum and the La Nina anomaly could overlap to potentially create hypoxic events of previously not observed magnitudes. Citation: Nam, S., H.-J. Kim, and U. Send (2011), Amplification of hypoxic and acidic events by La Nina conditions on the continental shelf off California, Geophys. Res. Lett., 38, L22602, doi:10.1029/2011GL049549.

Nam, S, Send U.  2011.  Direct evidence of deep water intrusions onto the continental shelf via surging internal tides. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006692   AbstractWebsite

Near-bottom diurnal thermocline shoaling and diurnal baroclinic currents were observed from time series data of water temperature, salinity, pressure, and velocity collected from August 2006 to January 2007 in the nearshore zone off the Huntington Beach where the local inertial frequency is higher than the diurnal frequency (poleward of the diurnal critical latitude). During the stratified season, the deep offshore cold (dense) water, when shoaled into shallow water along the bottom, was often trapped for a few hours and dissipated in the nearshore region (water depth of 10-20 m) during the ebb phase of the diurnal surface tide. It appears that perturbations propagating onshore along the shoaled isopycnals can form an upslope surging front and an internal bolus. Our data allow the application of theoretical/lab criteria for internal hydraulic jumps, internal bolus formation, and internal wave breaking and all criteria are fulfilled during a part of the tidal shoaling cycle. Nonlinear advection associated with the internal boluses causes higher (close to omega(-3)) spectral falloff rate of near-bottom temperature with frequency omega than the canonical Garret-Munk spectra (omega(-2)) in the range of 0.1-1.0 cph, implying strong scattering of tidal energy toward smaller scales. We are able to directly calculate the offshore eddy heat flux by cold water intrusion onto the shelf resulting from this process. Similar impacts and implications are expected for the biogeochemical quantities, as well as for the role of subinertial internal waves on turbulent mixing in the immediate proximity of the generating region, i.e., sloping bottom.

Hofmann, GE, Smith JE, Johnson KS, Send U, Levin LA, Micheli F, Paytan A, Price NN, Peterson B, Takeshita Y, Matson PG, Crook ED, Kroeker KJ, Gambi MC, Rivest EB, Frieder CA, Yu PC, Martz TR.  2011.  High-frequency dynamics of ocean pH: a multi-ecosystem comparison. Plos One. 6   10.1371/journal.pone.0028983   AbstractWebsite

The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO(2), reveal sensitivities and responses of individual species, one missing link in the OA story results from a chronic lack of pH data specific to a given species' natural habitat. Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO2, often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100. Our data provide a first step toward crystallizing the biophysical link between environmental history of pH exposure and physiological resilience of marine organisms to fluctuations in seawater CO2. Knowledge of this spatial and temporal variation in seawater chemistry allows us to improve the design of OA experiments: we can test organisms with a priori expectations of their tolerance guardrails, based on their natural range of exposure. Such hypothesis-testing will provide a deeper understanding of the effects of OA. Both intuitively simple to understand and powerfully informative, these and similar comparative time series can help guide management efforts to identify areas of marine habitat that can serve as refugia to acidification as well as areas that are particularly vulnerable to future ocean change.

Send, U, Lankhorst M, Kanzow T.  2011.  Observation of decadal change in the Atlantic meridional overturning circulation using 10 years of continuous transport data. Geophysical Research Letters. 38   10.1029/2011gl049801   AbstractWebsite

The meridional overturning circulation (MOC) represents the main mechanism for the oceanic northward heat transport in the Atlantic, and fluctuations of this circulation are believed to have major impacts on northern hemisphere climate. While numerical ocean and climate models and paleo-records show large variability in this circulation, the use of direct observations of the MOC for detecting climate-timescale changes has proven difficult so far. This report presents the first observational record of MOC measurements that is continuous and sufficiently long to exhibit decadal-scale changes, here a decrease by 20% over the observational period (Jan. 2000-June 2009) and large interannual changes in the flow and its vertical structure. Data are from a mooring array at 16 degrees N (Meridional Overturning Variability Experiment, MOVE). The observed change agrees with the amplitude of multi-decadal natural fluctuations seen in numerical ocean and climate models. Knowledge of the existence and phasing of such internal cycles provides multi-decadal climate predictability. Recently, some numerical model simulations have produced results that show a weakening of the MOC since the 1990's and observational confirmation of this now is a high priority. Citation: Send, U., M. Lankhorst, and T. Kanzow (2011), Observation of decadal change in the Atlantic meridional overturning circulation using 10 years of continuous transport data, Geophys. Res. Lett., 38, L24606, doi:10.1029/2011GL049801.

Dobricic, S, Pinardi N, Testor P, Send U.  2010.  Impact of data assimilation of glider observations in the Ionian Sea (Eastern Mediterranean). Dynamics of Atmospheres and Oceans. 50:78-92.   10.1016/j.dynatmoce.2010.01.001   AbstractWebsite

Glider observations of temperature, salinity and vertically averaged velocity in the Ionian Sea (Eastern Mediterranean Sea), made in the period October 2004-December 2004, were assimilated into an operational forecasting model together with other in situ and satellite observations. The study area has a high spatial and temporal variability of near surface dynamics, characterized by the entrance of the Atlantic Ionian Stream (MS) into the Northern Ionian Sea. The impact of glider observations on the estimation of the circulation is studied, and it is found that their assimilation locally improves the prediction of temperature, salinity, velocity and surface elevation fields. However, only the assimilation of temperature and salinity together with the vertically averaged velocity improves the forecast of all observed parameters. It is also found that glider observations rapidly impact the analyses even remotely, and the remote impacts on the analyses remain several months after the presence of the glider. The study emphasizes the importance of assimilating as much as possible all available information from gliders, especially in dynamically complex areas. (C) 2010 Elsevier B.V. All rights reserved.