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2016
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.

2015
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.

2014
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.

2013
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.

2012
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
n/a
2011
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.

2010
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.

Testor, P, Meyers G, Pattiaratchi C, Bachmayer R, Hayes D, Pouliquen S, Petit de la Villeon L, Carval T, Ganachaud A, Gourdeau L, Mortier L, Claustre H, Taillandier V, Lherminier P, Terre T, Visbeck M, Karstensen J, Krahmann G, Alvarez A, Rixen M, Poulain P-M, Osterhus S, Tintore J, Ruiz S, Garau B, Smeed D, Griffiths G, Merckelbach L, Sherwin T, Schmid C, Barth JA, Schofield O, Glenn S, Kohut J, Perry MJ, Eriksen C, Send U, Davis R, Rudnick D, Sherman J, Jones C, Webb D, Lee C, Owens B.  2010.  Gliders as a component of future observing systems. Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 2), . ( Hall J, Harrison DE, Stammer D, Eds.)., Venice, Italy: ESA Publication WPP-306   10.5270/OceanObs09.cwp.89   Abstract

The aim of this community white paper is to make recommendations for a glider component of a global ocean observing system. We first recommend the adoption of an ARGO-like (Global array of free-drifting profiling floats) data system for gliders. Then, we argue that combining glider deployments with the other components (ships, moorings, floats and satellites) will considerably enhance our capacity for observing the ocean by filling gaps left by the other observing systems. Gliders could be deployed to sample most of the western and eastern boundary circulations and the regional seas (around 20 basins in the world) which are not well covered by the present global ocean observing system and near fixed-point time series stations. These plans already involve people scattered around the world in Australia, Canada, Cyprus, France, Germany, Italy, Norway, Spain, UK, and the USA, and will certainly expand to many other countries. A rough estimate of resources required is about 13M$/Euro for ~20+ gliders permanently at sea during five years in the world ocean, based on present scientific infrastructures.

Send, U, Davis R, Fischer J, Imawaki S, Kessler W, Meinen C, Owens B, Roemmich D, Rossby T, Rudnick D, Toole J, Wijffels S, Beal L.  2010.  A global boundary current circulation observing network. Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 2), . ( Hall J, Harrison DE, Stammer D, Eds.)., Venice, Italy: ESA Publication WPP-306   10.5270/OceanObs09.cwp.78   Abstract

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.

2009
Lankhorst, M, Fratantoni D, Ollitrault M, Richardson P, Send U, Zenk W.  2009.  The mid-depth circulation of the northwestern tropical Atlantic observed by floats. Deep-Sea Research Part I-Oceanographic Research Papers. 56:1615-1632.   10.1016/j.dsr.2009.06.002   AbstractWebsite

A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600-1050dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200-2050dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e.g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7Sv across 16 degrees N between the Antilles islands and the Mid-Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADW east of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow. (C) 2009 Elsevier Ltd. All rights reserved.

2008
Kortzinger, A, Send U, Wallace DWR, Karstensen J, DeGrandpre M.  2008.  Seasonal cycle of O2 and pCO2 in the central Labrador Sea: Atmospheric, biological, and physical implications. Global Biogeochemical Cycles. 22   10.1029/2007gb003029   AbstractWebsite

We present full 2004-2005 seasonal cycles of CO(2) partial pressure (pCO(2)) and dissolved oxygen (O(2)) in surface waters at a time series site in the central Labrador Sea (56.5 degrees N, 52.6 degrees W) and use these data to calculate annual net air-sea fluxes of CO(2) and O(2) as well as atmospheric potential oxygen (APO). The region is characterized by a net CO(2) sink (2.7 +/- 0.8 mol CO(2) m(-2) yr(-1)) that is mediated to a major extent by biological carbon drawdown during spring/summer. During wintertime, surface waters approach equilibrium with atmospheric CO(2). Oxygen changes from marked undersaturation of about 6% during wintertime to strong supersaturation by up to 10% during the spring/summer bloom. Overall, the central Labrador Sea acts as an O(2) sink of 10.0 +/- 3.1 mol m(-2) yr(-1). The combined CO(2) and O(2) sink functions give rise to a sizable APO flux of 13.0 +/- 4.0 mol m(-2) yr(-1) into surface waters of the central Labrador Sea. A mixed layer carbon budget yields a net community production of 4.0 +/- 0.8 mol C m(-2) during the 2005 productive season about one third of which appears to undergo subsurface respiration in a depth range that is reventilated during the following winter. The timing of the spring bloom is discussed and eddies from the West Greenland Current are thought to be associated with the triggering of the bloom. Finally, we use CO(2) and O(2) mixed layer dynamics during the 2005 spring bloom to evaluate a suite of prominent wind speed-dependent parameterizations for the gas transfer coefficient. We find very good agreement with those parameterizations which yield higher transfer coefficients at wind speeds above 10 m s(-1).

Kanzow, T, Send U, McCartney M.  2008.  On the variability of the deep meridional transports in the tropical North Atlantic. Deep-Sea Research Part I-Oceanographic Research Papers. 55:1601-1623.   10.1016/j.dsr.2008.07.011   AbstractWebsite

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.

Kortzinger, A, Send U, Lampitt RS, Hartman S, Wallace DWR, Karstensen J, Villagarcia MG, Llinas O, DeGrandpre MD.  2008.  The seasonal pCO2 cycle at 49°N/16.5°W in the northeastern Atlantic Ocean and what it tells us about biological productivity. Journal of Geophysical Research-Oceans. 113   10.1029/2007jc004347   AbstractWebsite

A 2-year record of mixed layer measurements of CO(2) partial pressure (pCO(2)), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49 degrees N/16.5 degrees W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO(2) levels by about 40 +/- 15 mu atm, which gives rise to a perennial CO(2) sink of 3.2 +/- 1.3 mol m(-2) a(-1). The seasonal pCO(2) 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 CO(2) 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.

2007
Schmidt, S, Send U.  2007.  Origin and composition of seasonal Labrador sea freshwater. Journal of Physical Oceanography. 37:1445-1454.   10.1175/jpo3065.1   AbstractWebsite

The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring-summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic ( via the East Greenland Current), which implies some predictability on longer time scales.