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Lee, DK, Centurioni L.  2018.  Water following characteristics of Global Drifter Program drifters with and without subsurface float. Deep-Sea Research Part I-Oceanographic Research Papers. 137:20-29.   10.1016/j.dsr.2018.05.003   AbstractWebsite

In the Northeast Pacific (40-50 degrees N, 135-180 degrees W), the angle between the wind friction velocity (u) and the current observed by drifters with subsurface float (Global Drifter Program Version 1; GDP-V1) was found to be consistently 10-15 degrees larger at all frequencies than the angle observed by drifters without subsurface float (Global Drifter Program Version 2; GDP-V2). To investigate the cause, cross-spectral analysis and vector regression between the wind and current were performed after carefully screening drifter tracks to study the wind-driven currents observed by drifters with different configurations. Vector regression analysis between the wind and current revealed that the angle of wind-driven current observed by GDP-V1 drifters was 10-13 degrees larger for u < 1.5cms(-1) compared to that observed by GDP-V2 drifters. One possible explanation for a smaller angle between wind and current from drifters without subsurface float is the shallowing of observed depth due to the shrinking of the holey sock drogue induced by surface wave action. The depth of the current observed by GDP-V2 drifters during the winter was estimated using the observed angle and the e-folding depth calculated from the angle at 15 m by GDP-V1 drifters. In the winter, the mean depth of the wind-driven current observed by GDP-V2 drifters, which have been deployed since the early 2000s, was approximately in the range of 8-10 m depending on the estimation of the e-folding depth either from the angle change or from the amplitude decay in the Ekman layer. Except for the friction velocity exceeding 1.5cms(-1), a nearly constant amplitude between surface current and friction velocity at all friction velocity ranges is another finding in our study.

Chao, Y, Farrara JD, Zhang HC, Armenta KJ, Centurioni L, Chavez F, Girton JB, Rudnick D, Walter RK.  2018.  Development, implementation, and validation of a California coastal ocean modeling, data assimilation, and forecasting system. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 151:49-63.   10.1016/j.dsr2.2017.04.013   AbstractWebsite

A three-dimensional, near real-time data-assimilative modeling system for the California coastal ocean is presented. The system consists of a Regional Ocean Modeling System (ROMS) forced by the North American Mesoscale Forecast System (NAM). The ocean model has a horizontal resolution of approximately three kilometers and utilizes a multi-scale three-dimensional variational (3DVAR) data assimilation methodology. The system is run in near real-time to produce a nowcast every six hours and a 72-hour forecast every day. The performance of this nowcast system is presented using results from a six-year period of 2009-2015. The ROMS results are first compared with the assimilated data as a consistency check. RMS differences in observed satellite infrared sea surface temperatures (SST) and vertical profiles of temperature between observations and ROMS nowcasts were found to be mostly less than 0.5 degrees C, while the RMS differences in vertical profiles of salinity between observations and ROMS nowcasts were found to be 0.09 or less. The RMS differences in SST show a distinct seasonal cycle that mirrors the number of observations available: the nowcast is less skillful with larger RMS differences during the summer months when there are less infrared SST observations due to the presence of low-level clouds. The larger differences during summer were found primarily along the northern and central coasts in upwelling regions where strong gradients exist between colder upwelled waters nearshore and warmer offshore waters. RMS differences between HF radar surface current observations and ROMS nowcasts were approximately 7-8 cm s-(1), which is about 30% of the time mean current speeds in this region. The RMS differences in sea surface height (SSH) between the AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic) altimetric satellite observations and ROMS nowcasts were about 2 cm. In addition, the system realistically reproduces the interannual variability in temperatures at the M1 mooring (122.03 degrees W, 36.75 degrees N) in Monterey Bay, including the strong warming of the California coastal ocean during 2014. The ROMS nowcasts were then validated against independent observations. A comparison of the ROMS nowcast with independent profile observations of temperature and salinity shows RMS differences of 0.7 to 0.92 degrees C and 0.13 to 0.17, which are larger (by up to a factor of 2) than the differences found in the comparisons with assimilated data. Validation of the depth-averaged currents derived from Spray gliders shows that the flow patterns associated with California Current and California Undercurrent/Davidson current systems and their seasonal variations are qualitatively reproduced by the ROMS modeling system. Lastly, the impact of two recent upgrades to the system is quantified. Switching the lateral boundary conditions from a U.S. west coast regional model to the global HYCOM (HYbrid Coordinate Ocean Model) model results in an improvement in the simulation of the seasonal and interannual variations in the SSH, especially south of Pt. Conception (120.47 degrees W, 34.45 degrees N). The assimilation of altimetric satellite SSH data also results in an improvement in the model surface currents when compared to independent surface drifter observations.

Centurioni, LR.  2018.  Drifter Technology and Impacts for Sea Surface Temperature, Sea-Level Pressure, and Ocean Circulation Studies. Observing the Oceans in Real Time. ( Venkatesan R, Tandon A, D'Asaro E, Atmanand MA, Eds.).:37-57., Cham: Springer International Publishing   10.1007/978-3-319-66493-4_3   Abstract

The purpose of this chapter is twofold. First, we illustrate the technology used by the Lagrangian drifters deployed by Global Drifter Program (GDP), which is the principal component of the Global Drifter Array; second, we review and summarize the most recent studies on the impact of drifter data for calibration and validation of sea surface temperature (SST) satellite products, Numerical Weather Prediction (NWP) and climate studies, tropical cyclones (TCs)-ocean interaction and ocean circulation studies. Several types of drifters are described, starting from the simplest configuration that measures SST and sea-level atmospheric pressure (SLP), continuing with special drifters designed to measure sea surface salinity (SSS) and sea-level wind (SLW), and ending with air-deployable drifting thermistor chains that measure the temperature of the upper 150 m of the ocean, which are used to study the interaction of the ocean’s mixed layer with TCs. We also discuss the implications of the satellite telecommunication technology on the accuracy of drifter’s geolocation and on the timeliness of the near real-time data stream.

Goni, GJ, Todd RE, Jayne SR, Halliwell G, Glenn S, Dong J, Curry R, Domingues R, Bringas F, Centurioni L, Di Marco SF, Miles T, Morell J, Pomales L, Kim HS, Robbins PE, Gawarkiewicz GG, Wilkin J, Heiderich J, Baltes B, Cione JJ, Seroka G, Knee K, Sanabia ER.  2017.  Autonomous and Lagrangian Ocean Observations for Atlantic Tropical Cyclone Studies and Forecasts. Oceanography. 30:92-103.   10.5670/oceanog.2017.227   AbstractWebsite

The tropical Atlantic basin is one of seven global regions where tropical cyclones (TCs) commonly originate, intensify, and affect highly populated coastal areas. Under appropriate atmospheric conditions, TC intensification can be linked to upper-ocean properties. Errors in Atlantic TC intensification forecasts have not been significantly reduced during the last 25 years. The combined use of in situ and satellite observations, particularly of temperature and salinity ahead of TCs, has the potential to improve the representation of the ocean, more accurately initialize hurricane intensity forecast models, and identify areas where TCs may intensify. However, a sustained in situ ocean observing system in the tropical North Atlantic Ocean and Caribbean Sea dedicated to measuring subsurface temperature, salinity, and density fields in support of TC intensity studies and forecasts has yet to be designed and implemented. Autonomous and Lagrangian platforms and sensors offer cost-effective opportunities to accomplish this objective. Here, we highlight recent efforts to use autonomous platforms and sensors, including surface drifters, profiling floats, underwater gliders, and dropsondes, to better understand air-sea processes during high-wind events, particularly those geared toward improving hurricane intensity forecasts. Real-time data availability is key for assimilation into numerical weather forecast models.

Lindstrom, EJ, Shcherbina AY, Rainville L, Farrar JT, Centurioni LR, Dong SF, D'Asaro EA, Eriksen C, Fratantoni DM, Hodges BA, Hormann V, Kessler WS, Lee CM, Riser SC, St Laurent L, Volkov DL.  2017.  Autonomous Multi-Platform Observations During the Salinity Processes in the Upper-ocean Regional Study. Oceanography. 30:38-48.   10.5670/oceanog.2017.218   AbstractWebsite

The Salinity Processes in the Upper-ocean Regional Study ( SPURS) aims to understand the patterns and variability of sea surface salinity. In order to capture the wide range of spatial and temporal scales associated with processes controlling salinity in the upper ocean, research vessels delivered autonomous instruments to remote sites, one in the North Atlantic and one in the Eastern Pacific. Instruments sampled for one complete annual cycle at each of these two sites, which are subject to contrasting atmospheric forcing. The SPURS field programs coordinated sampling from many different platforms, using a mix of Lagrangian and Eulerian approaches. This article discusses the motivations, implementation, and first results of the SPURS-1 and SPURS-2 programs.

Centurioni, LR, Hormann V, Talley LD, Arzeno I, Beal L, Caruso M, Conry P, Echols R, Fernando HJS, Giddings SN, Gordon A, Graber H, Harcourt RR, Jayne SR, Jensen TG, Lee CM, Lermusiaux PFJ, L'Hegaret P, Lucas AJ, Mahadevan A, McClean JL, Pawlak G, Rainville L, Riser SC, Seo H, Shcherbina AY, Skyllingstad E, Sprintall J, Subrahmanyam B, Terrill E, Todd RE, Trott C, Ulloa HN, Wang H.  2017.  Northern Arabian Sea Circulation Autonomous Research (NASCar): A research initiative based on autonomous sensors. Oceanography. 30:74-87.   10.5670/oceanog.2017.224   AbstractWebsite

The Arabian Sea circulation is forced by strong monsoonal winds and is characterized by vigorous seasonally reversing currents, extreme differences in sea surface salinity, localized substantial upwelling, and widespread submesoscale thermohaline structures. Its complicated sea surface temperature patterns are important for the onset and evolution of the Asian monsoon. This article describes a program that aims to elucidate the role of upper-ocean processes and atmospheric feedbacks in setting the sea surface temperature properties of the region. The wide range of spatial and temporal scales and the difficulty of accessing much of the region with ships due to piracy motivated a novel approach based on state-of-the-art autonomous ocean sensors and platforms. The extensive data set that is being collected, combined with numerical models and remote sensing data, confirms the role of planetary waves in the reversal of the Somali Current system. These data also document the fast response of the upper equatorial ocean to monsoon winds through changes in temperature and salinity and the connectivity of the surface currents across the northern Indian Ocean. New observations of thermohaline interleaving structures and mixing in setting the surface temperature properties of the northern Arabian Sea are also discussed.

Centurioni, L, Hornayi A, Cardinali C, Charpentier E, Lumpkin R.  2017.  A global ocean observing system for measuring sea level atmospheric pressure: Effects and impacts on numerical weather prediction. Bulletin of the American Meteorological Society. 98:231-238.   10.1175/bams-d-15-00080.1   AbstractWebsite
Horanyi, A, Cardinali C, Centurioni L.  2017.  The global numerical weather prediction impact of mean-sea-level pressure observations from drifting buoys. Quarterly Journal of the Royal Meteorological Society. 143:974-985.   10.1002/qj.2981   AbstractWebsite

Observing System Experiments have been used to evaluate the forecast impact of sea-level pressure observations from drifting buoys. Two seasons have been selected with different synoptic weather characteristics, but similar amount of buoy observations. Control and denial experiments were performed with and without assimilating drifting buoys' sea-level pressure observations. The denial experiments withdraw around 95% of the total surface pressure measurements from buoys; the remaining 5% are provided by moored buoys. Changes in the forecast performance are evaluated in terms of root mean-squared error and anomaly correlation scores. Adjoint diagnostic tools are also used to estimate the observations' contribution to the analysis and forecast. The lack of drifter surface pressure observations has a large and significant detrimental impact on the mean-sea-level pressure, temperature and wind fields. The signal is detectable not only near to the surface but throughout the troposphere up to 250 hPa. Drifter surface pressure observations contribute to decrease the total global forecast error by approximately 3%. In particular, case-studies reveal that drifting buoy observations can be especially important to reduce the forecast error on complex or rapidly evolving cyclogenesis. All the diagnostics performed indicate that drifting buoys are essential ingredients of WMO's Global Observing System.

Lumpkin, R, Ozgokmen T, Centurioni L, Annual R.  2017.  Advances in the application of surface drifters. Annual Review of Marine Sciences, Vol 9. 9:59-81., Palo Alto: Annual Reviews   10.1146/annurev-marine-010816-060641   Abstract

Surface drifting buoys, or drifters, are used in oceanographic and climate research, oil spill tracking, weather forecasting, search and rescue operations, calibration and validation of velocities from high-frequency radar and from altimeters, iceberg tracking, and support of offshore drilling operations. In this review, we present a brief history of drifters, from the message in a bottle to the latest satellite-tracked, multisensor drifters. We discuss the different types of drifters currently used for research and operations as well as drifter designs in development. We conclude with a discussion of the various properties that can be observed with drifters, with heavy emphasis on a critical process that cannot adequately be observed by any other instrument: dispersion in the upper ocean, driven by turbulence at scales from waves through the submesoscale to the large-scale geostrophic eddies.

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.

Lozovatsky, I, Wijesekera H, Jarosz E, Lilover MJ, Pirro A, Silver Z, Centurioni L, Fernando HJS.  2016.  A snapshot of internal waves and hydrodynamic instabilities in the southern Bay of Bengal. Journal of Geophysical Research-Oceans. 121:5898-5915.   10.1002/2016jc011697   AbstractWebsite

Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI-EBoB Program portray the characteristics of high-frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20 h time series of CTD, ADCP, and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z=54 m were taken to the east of Sri Lanka. Internal waves of periods similar to 10-40 min were recorded at all depths below a shallow (similar to 20-30 m) surface mixed layer in the background of an 8 m amplitude internal tide. The absolute values of vertical displacements associated with high-frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high-amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding similar to 5 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internalwave- induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri < 0.25 and about 10% for Ri < 0.36 (K-H billows). The probability of the generation of asymmetric breaking internal waves and Holmboe instabilities is above similar to 25%.

Chang, YC, Tseng RS, Chu PC, Chen JM, Centurioni LR.  2016.  Observed strong currents under global tropical cyclones. Journal of Marine Systems. 159:33-40.   10.1016/j.jmarsys.2016.03.001   AbstractWebsite

Global data from drifters of the Surface Velocity Program (Niiler, 2001) and tropical cyclones (TCs) from the Joint Typhoon Warning Center and National Hurricane Center were analyzed to demonstrate strong ocean currents and their characteristics under various storm intensities in the Northern Hemisphere (NH) and in the Southern Hemisphere (SH). Mean TC's translation speed (U-h) is faster in the NH (similar to 4.7 m s(-1)) than in the SH (similar to 4.0 m s(-1)), owing to the fact that TCs are more intense in the NH than in the SH. The rightward (leftward) bias of ocean mixed-layer (OML) velocity occurs in the NH (SH). As a result of this slower Uh and thus a smaller Froude number in the SH, the flow patterns in the SH under the same intensity levels of TCs are more symmetric relative to the TC center and the OML velocities are stronger. This study provides the first characterization of the near-surface OML velocity response to all recorded TCs in the SH from direct velocity measurements. (C) 2016 Elsevier B.V. All rights reserved.

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.

Wijesekera, HW, Teague WJ, Jarosz E, Wang DW, Jensen TG, Jinadasa SUP, Fernando HJS, Centurioni LR, Hallock ZR, Shroyer EL, Moum JN.  2016.  Observations of currents over the deep southern Bay of Bengal-with a little luck. Oceanography. 29:112-123.   10.5670/oceanog.2016.44   AbstractWebsite

Long-term time series of velocity, hydrographic, and turbulence fields were collected from a six-element subsurface mooring array in the southern Bay of Bengal. The moorings, deployed in December 2013 and recovered in August 2015, were entangled with commercial fishing nets and lines, while top subsurface buoys ended up being serendipitously closer to the surface than planned. In spite of these unexpected events, almost all the sensors and data were recovered. The moorings provided currents between 6 m and 500 m depths from acoustic Doppler current profilers, supplemented by hydrographic data and turbulent dissipation rates at selected depths. The observations captured the summer and winter monsoon currents, eddies, and intraseasonal oscillations. Near-surface currents as large as 1.75 m s(-1) were observed in July 2014. Currents stronger than 0.5 m s(-1) were confined to the upper 200 m. Observations of currents, temperature, and sea surface height (SSH) fields revealed eddylike features with positive and negative SSH anomalies (similar to 20 cm) moving westward at speeds of about 0.1 m s(-1). Intraseasonal oscillations with periods of 30 to 90 days were strongest near the surface. For the duration of the deployment, root-mean-square velocity fluctuations were about 0.1 m s(-1) near the surface but decayed with depth and became nearly uniform (similar to 0.03-0.06 m s(-1)) below 100 m.

Hormann, V, Centurioni LR, Mahadevan A, Essink S, D'Asaro EA, Kumar BP.  2016.  Variability of near-surface circulation and sea surface salinity observed from lagrangian drifters in the northern Bay of Bengal during the waning 2015 southwest monsoon. Oceanography. 29:124-133.   10.5670/oceanog.2016.45   AbstractWebsite

A dedicated drifter experiment was conducted in the northern Bay of Bengal during the 2015 waning southwest monsoon. To sample a variety of spatiotemporal scales, a total of 36 salinity drifters and 10 standard drifters were deployed in a tight array across a freshwater front. The salinity drifters carried for the first time a revised sensor algorithm, and its performance during the 2015 field experiment is very encouraging for future efforts. Most of the drifters were quickly entrained in a mesoscale feature centered at about 16.5 degrees N, 89 degrees E and stayed close together during the first month of observations. While the eddy was associated with rather homogeneous temperature and salinity characteristics, much larger variability was found outside of it toward the coastline, and some of the observed salinity patches had amplitudes in excess of 1.5 psu. To particularly quantify the smaller spatiotemporal scales, an autocorrelation analysis of the drifter salinities for the first two deployment days was performed, indicating not only spatial scales of less than 5 km but also temporal variations of the order of a few hours. The hydrographic measurements were complemented by first estimates of kinematic properties from the drifter clusters, however, more work is needed to link the different observed characteristics.

Lumpkin, R, Centurioni L, Perez RC.  2016.  Fulfilling observing system implementation requirements with the global drifter array. Journal of Atmospheric and Oceanic Technology. 33:685-695.   10.1175/jtech-d-15-0255.1   AbstractWebsite

The Global Ocean Observing System (GOOS) requirements for in situ surface temperature and velocity measurements call for observations at 5 degrees x 5 degrees resolution. A key component of the GOOS that measures these essential climate variables is the global array of surface drifters. In this study, statistical observing system sampling experiments are performed to evaluate how many drifters are required to achieve the GOOS requirements, both with and without the presence of a completed global tropical moored buoy array at 5 degrees S-5 degrees N. The statistics for these simulations are derived from the evolution of the actual global drifter array. It is concluded that drifters should be deployed within the near-equatorial band even though that band is also in principle covered by the tropical moored array, as the benefits of not doing so are marginal. It is also concluded that an optimal design half-life for the drifters is similar to 450 days, neglecting external sources of death, such as running aground or being picked up. Finally, it is concluded that comparing the drifter array size to the number of static 5 degrees x 5 degrees open-ocean bins is not an ideal performance indicator for system evaluation; a better performance indicator is the fraction of 5 degrees x 5 degrees open-ocean bins sampled, neglecting bins with high drifter death rates.

Menna, M, Faye S, Poulain PM, Centurioni L, Lazar A, Gaye A, Sow B, Dagorne D.  2016.  Upwelling features off the coast of north-western Africa in 2009-2013. Bollettino Di Geofisica Teorica Ed Applicata. 57:71-86.   10.4430/bgta0164   AbstractWebsite

Satellite data (images of sea surface temperature and chlorophyll-a), ocean surface wind products, Lagrangian observations (surface drifters) and other ancillary data (upwelling index) are used to describe the upwelling seasons off NW Africa during 2009-2013, with particular focus on the coasts of Senegal and Mauritania. The impact of the upwelling is characterised by a comparative analysis, carried out in terms of wind-induced upwelling and water/ecosystem response to this forcing, of five geographical sectors detected in the study area. The wind forcing analysis shows the most favourable upwelling conditions in the period December-June in the southern sectors (south of 16 degrees N), and from February to October in the northern sectors (north of 18 degrees N). Southern sectors are strongly influenced by wind forcing, whereas to the north the upwelling also occurs during the months with low Ekman transport values. The analysis of the sea surface temperature and chlorophyll-a concentration confirms the existence of an upwelling season during winter-spring in the south, and emphasizes the different behaviours between the northern and southern sectors. Drifter tracks allow the addition of details about the flow of cold water offshore and alongshore. In particular, they describe the westward transport of cold water, by means of energetic filaments rooted at specific locations along the coast, north of Cape Vert and the south-SW ward transport of the coastal water south of Cape Vert.

Postacchini, M, Centurioni LR, Braasch L, Brocchini M, Vicinanza D.  2016.  Lagrangian observations of waves and currents from the River Drifter. Ieee Journal of Oceanic Engineering. 41:94-104.   10.1109/joe.2015.2418171   AbstractWebsite

The working principle and the capabilities of a new platform called the River Drifter are here presented. This technology has applications in the study of the hydrodynamics of coastal areas, rivers, and lakes. The River Drifter was designed for shallow water applications (1 m and deeper) to collect concurrent measurements of surface currents, three-dimensional velocity profiles underneath the device, water depth, and salinity. Here, we discuss how water level displacements can be inferred and used to measure the swell characteristics and to also correct the measured velocity. We also show how the local vorticity field can be computed. As an example application, we describe a study whose goal was to investigate the fate of a polluted river plume and how two River Drifters initially following the same path are characterized by very different final trajectories. The different behaviors of the two drifters are explained in terms of the local flow dynamics, which are strongly influenced by the seabed morphology, forcing the River Drifters to move in different directions.

Lien, RC, Ma B, Lee CM, Sanford TB, Mensah V, Centurioni LR, Cornuelle BD, Gopalakrishnan G, Gordon AL, Chang MH, Jayne SR, Yang YJ.  2015.  The Kuroshio and Luzon undercurrent east of Luzon Island. Oceanography. 28:54-63.   10.5670/oceanog.2015.81   AbstractWebsite

Current structure, transport, and water mass properties of the northward-flowing Kuroshio and the southward-flowing Luzon Undercurrent (LU) were observed for nearly one year, June 8, 2012-June 4, 2013, across the Kuroshio path at 18.75 degrees N. Observations were made from four platforms: an array of six subsurface ADCP moorings, two Seagliders, fivepressure inverted echo sounders (PIES), and five horizontal electric field (HEF) sensors, providing the most detailed time series of the Kuroshio and Luzon Undercurrent water properties to date. Ocean state estimates of the western boundary current system were performed using the MIT general circulation model-four-dimensional variational assimilation (MITgcm-4D-Var) system. Prominent Kuroshio features from observations are simulated well by the numerical model. Annual mean Kuroshio transport, averaged over all platforms, is similar to 16 Sv with a standard deviation similar to 4 Sv. Kuroshio and LU transports and water mass pathways east of Luzon are revealed by Seaglider measurements. In a layer above the salinity maximum associated with North Pacific Tropical Water (NPTW), Kuroshio transport is similar to 7 Sv and contains North Equatorial Current (NEC) and Western Philippine Sea (WPS) waters, with an insignificant amount of South China Sea water on the shallow western flank. In an intermediate layer containing the core of the NPTW, Kuroshio transport is similar to 10 Sv, consisting mostly of NEC water. In the lower layer of the Kuroshio, transport is similar to 1.5 Sv of mostly North Pacific Intermediate Water (NPIW) as a part of WPS waters. Annual mean Luzon Undercurrent southward transport integrated to 1,000 m depth is similar to 2.7 Sv with a standard deviation similar to 2 Sv, carrying solely WPS waters below the salinity minimum of the NPIW. The transport of the western boundary current integrated over the full ocean depth east of Luzon Island is similar to 14 +/- 4.5 Sv. Sources of the water masses in the Kuroshio and Luzon Undercurrent are confirmed qualitatively by the numerical model.

Yang, YJ, Jan S, Chang MH, Wang J, Mensah V, Kuo TH, Tsai CJ, Lee CY, Andres M, Centurioni LR, Tseng YH, Liang WD, Lai JW.  2015.  Mean structure and fluctuations of the Kuroshio East of Taiwan from in situ and remote observations. Oceanography. 28:74-83.   10.5670/oceanog.2015.83   AbstractWebsite

The Kuroshio is important to climate, weather prediction, and fishery management along the northeast coast of Asia because it transports tremendous heat, salt, and energy from east of the Philippines to waters southeast of Japan. In the middle of its journey northward, the Kuroshio's velocity mean and its variability east of Taiwan crucially affect its downstream variability. To improve understanding of the Kuroshio there, multiple platforms were used to collect intensive observations off Taiwan during the three-year Observations of the Kuroshio Transports and their Variability (OKTV) program (2012-2015). Mean Kuroshio velocity transects show two velocity maxima southeast of Taiwan, with the primary velocity core on the onshore side of the Kuroshio exhibiting a mean maximum velocity of similar to 1.2 m s(-1). The two cores then merge and move at a single velocity maximum of similar to 1 m s(-1) east of Taiwan. Standard deviations of both the directly measured poleward (v) and zonal (u) velocities are similar to 0.4 m s(-1) in the Kuroshio main stream. Water mass exchange in the Kuroshio east of Taiwan was found to be complicated, as it includes water of Kuroshio origin, South China Sea Water, and West Philippine Sea Water, and it vitally affects heat, salt, and nutrient inputs to the East China Sea. Impinging eddies and typhoons are two of the principal causes of variability in the Kuroshio. This study's models are more consistent with the observed Kuroshio than with high-frequency radar measurements.

Andres, M, Jan S, Sanford TB, Mensah V, Centurioni LR, Book JW.  2015.  Mean structure and variability of the Kuroshio from Northeastern Taiwan to Southwestern Japan. Oceanography. 28:84-95.   10.5670/oceanog.2015.84   AbstractWebsite

In the subtropical western North Pacific Ocean, the Kuroshio delivers heat, salt, and momentum poleward, much like its North Atlantic analog, the Gulf Stream. Though the Kuroshio generally flows along the western boundary from Taiwan to southeastern Japan as an "attached" current, the Kuroshio's strength, vertical structure, and horizontal position undergo significant temporal and spatial variability along this entire route. Ubiquitous mesoscale eddies and complicated topography associated with a string of marginal seas combine to make the western North Pacific a region with complex circulation. Here, we synthesize results from the recent US Origins of the Kuroshio and Mindanao Currents and Taiwan Observations of Kuroshio Transport Variability observational programs with previous findings to build a comprehensive picture of the Kuroshio on its route from northeastern Taiwan to southeastern Japan, where the current finally transitions from a western boundary current into the Kuroshio Extension, a vigorously meandering free jet.

Poulain, PM, Centurioni L.  2015.  Direct measurements of World Ocean tidal currents with surface drifters. Journal of Geophysical Research-Oceans. 120:6986-7003.   10.1002/2015jc010818   AbstractWebsite

Velocities of surface drifters are analyzed to study tidal currents throughout the World Ocean. The global drifter data set spanning the period 1979-2013 is used to describe the geographical structure of the surface tidal currents at global scale with a resolution of 2 degrees. Harmonic analysis is performed with two semidiurnal, two diurnal, and four inferred tidal constituents. Tidal current characteristics (amplitude of semimajor axis, rotary coefficient, tidal ellipse inclination, and Greenwich phase) are mapped over the World Ocean from direct observations. The M2 currents dominate on all the shallow continental shelves with magnitude exceeding 60 cm/s. They are also substantial (4-5 cm/s) over the main deep topographic features such as the Mid-Atlantic Ridge, the Southwest Indian Ridge, and the Mariana Ridge. The S2 currents have amplitudes typically half the size of the M2 currents, with a maximum of about 30 cm/s. The K1 and O1 currents are important in many shallow seas. They are large in the vicinity of the turning latitudes near 30 degrees N/S where they merge with inertial motions of the same frequency. They are also substantial in the South China Sea and Philippine Sea. Maps of rotary coefficients indicate that all tidal motions are essentially clockwise (anticlockwise) in the Northern (Southern) Hemisphere. The rotary coefficient of the tidal currents is compared with the theory of freely and meridionally propagating baroclinic inertia-gravity waves. The Greenwich phase of the M2 constituent has large-scale coherent propagation patterns which could be interpreted as the propagation of the barotropic tide.

Wijesekera, HW, Jensen TG, Jarosz E, Teague WJ, Metzger EJ, Wang DW, Jinadasa SUP, Arulananthan K, Centurioni LR, Fernando HJS.  2015.  Southern Bay of Bengal currents and salinity intrusions during the northeast monsoon. Journal of Geophysical Research-Oceans. 120:6897-6913.   10.1002/2015jc010744   AbstractWebsite

Shipboard velocity and hydrographic profiles collected in December 2013 along with drifter observations, satellite altimetry, global ocean nowcast/forecast products, and coupled model simulations were used to examine the circulation in the southern Bay of Bengal as part of ongoing international research efforts in the region. The observations captured the southward flowing East India Coastal Current (EICC) off southeast India and east of Sri Lanka. The EICC was approximately 100 km wide, with speeds exceeding 1 m s(-1) in the upper 75 m. East of the EICC, a subsurface-intensified 300 km-wide, northward current was observed, with maximum speeds as high as 1 m s(-1) between 50 m and 75 m. The EICC moved low-salinity water out of the bay and the subsurface northward flow carried high-salinity water into the bay during typical northeast monsoon conditions during a time period when the central equatorial Indian Ocean was experiencing a westerly wind burst related to the Madden-Julian Oscillation (MJO) event. While the northward subsurface high-salinity flow has previously been observed during the southwest monsoon, it was observed during the northeast monsoon. The observations are consistent with northward high-salinity subsurface flow in numerical model solutions. The analysis suggests that direct forcing along the equator may play a significant role for high-salinity intrusions east of Sri Lanka.

Alford, MH, Peacock T, MacKinnon JA, Nash JD, Buijsman MC, Centuroni LR, Chao SY, Chang MH, Farmer DM, Fringer OB, Fu KH, Gallacher PC, Graber HC, Helfrich KR, Jachec SM, Jackson CR, Klymak JM, Ko DS, Jan S, Johnston TMS, Legg S, Lee IH, Lien RC, Mercier MJ, Moum JN, Musgrave R, Park JH, Pickering AI, Pinkel R, Rainville L, Ramp SR, Rudnick DL, Sarkar S, Scotti A, Simmons HL, St Laurent LC, Venayagamoorthy SK, Hwang Y, Wang J, Yang YJ, Paluszkiewicz T, Tang TY.  2015.  The formation and fate of internal waves in the South China Sea. Nature. 521:65-U381.   10.1038/nature14399   AbstractWebsite

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis(1), sediment and pollutant transport(2) and acoustic transmission(3); they also pose hazards for man-made structures in the ocean(4). Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking(5), making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects(6,7). For over a decade, studies(8-11) have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

Schmitt, RW, Asher W, Bingham F, Carton J, Centurioni L, Farrar T, Gordon A, Hodges B, Jessup A, Kessler WS, Rainville L, Shcherbina A.  2015.  From salty to fresh: Salinity Processes in the Upper-ocean Regional Study-2 (SPURS-2): Diagnosing the physics of a rainfall-dominated salinity minimum. Oceanography. 28:150-159.   10.5670/oceanog.2015.15   AbstractWebsite

One of the notable features of the global ocean is that the salinity of the North Atlantic is about 1 psu higher than that of the North Pacific. This contrast is thought to be due to one of the large asymmetries in the global water cycle: the transport of water vapor by the trade winds across Central America and the lack of any comparable transport into the Atlantic from the Sahara Desert. Net evaporation serves to maintain high Atlantic salinities, and net precipitation lowers those in the Pacific. Because the effects on upper-ocean physics are markedly different in the evaporating and precipitating regimes, the next phase of research in the Salinity Processes in the Upper-ocean Regional Study (SPURS) must address a high rainfall region. It seemed especially appropriate to focus on the eastern tropical Pacific that is freshened by the water vapor carried from the Atlantic. In a sense, the SPURS-2 Pacific region will be looking at the downstream fate of the freshwater carried out of the SPURS-1 North Atlantic region. Rainfall tends to lower surface density and thus inhibit vertical mixing, leading to quite different physical structure and dynamics in the upper ocean. Here, we discuss the motivations for the location of SPURS-2 and the scientific questions we hope to address.