Luca Centurioni

Mean Dynamic Topography of the Ocean Derived from Satellite and Drifting Buoy Data Using Three Different Techniques

Abstract Presented here are three mean dynamic topography maps derived with different methodologies. The first method combines sea level observed by the high-accuracy satellite radar altimetry with the geoid model of the Gravity Recovery and Climate Experiment (GRACE), which has recently measured the earth’s gravity with unprecedented spatial resolution and accuracy. The second one synthesizes near-surface velocities from a network of ocean drifters, hydrographic profiles, and ocean winds sorted according to the horizontal scales. In the third method, these global datasets are used in the context of the ocean surface momentum balance. The second and third methods are used to improve accuracy of the dynamic topography on fine space scales poorly resolved in the first method. When they are used to compute a multiyear time-mean global ocean surface circulation on a 0.5° horizontal resolution, both contain very similar, new small-scale midocean current patterns. In particular, extensions of western boundary c...

Observations of Inflow of Philippine Sea Surface Water into the South China Sea through the Luzon Strait

Abstract Velocity observations near the surface made with Argos satellite-tracked drifters between 1989 and 2002 provide evidence of seasonal currents entering the South China Sea from the Philippine Sea through the Luzon Strait. The drifters cross the strait and reach the interior of the South China Sea only between October and January, with ensemble mean speeds of 0.7 ± 0.4 m s−1 and daily mean westward speeds that can exceed 1.65 m s−1. The majority of the drifters that continued to reside in the South China Sea made the entry within a westward current system located at ∼20°N that crossed the prevailing northward Kuroshio path. In other seasons, the drifters looped across the strait within the Kuroshio and exited along the south coast of Taiwan. During one intrusion event, satellite altimeters indicated that, directly west of the strait, anticyclonic and cyclonic eddies resided, respectively, north and south of the entering drifter track. The surface currents measured by the crossing drifters were much...

Permanent Meanders in the California Current System

Surface Velocity Program (SVP) drifter data from 1987 through 2005; Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) sea level anomalies; and NCEP reanalysis winds are used to assemble a time-averaged map of the 15-m-deep geostrophic velocity field in the California Current System seaward of about 50 km from the coast. The wind data are used to compute the Ekman currents, which are then subtracted from the drifter velocity measurements. The resulting proxy for geostrophic velocity anomalies computed from drifters and from satellite sea level measurements are combined to form an unbiased mean geostrophic circulation map. The result shows a California Current System that flows southward with four permanent meanders that can extend seaward for more than 800 km. Bands of alternating eastward and westward zonal currents are connected to the meanders and extend several thousand kilometers into the Pacific Ocean. This observed time-mean circulation and its associated eddy energy are compared to those produced by various high-resolution OGCM solutions: Regional Ocean Modeling System (ROMS; 5 km), Parallel Ocean Program model (POP; 1/10°), Hybrid Coordinate Ocean Model (HYCOM; 1/12°), and Naval Research Laboratory (NRL) Layered Ocean Model (NLOM; 1/32°). Simulations in closest agreement with observations come from ROMS, which also produces four meanders, geostrophic time-mean currents, and geostrophic eddy energy consistent with the observed values. The time-mean ageostrophic velocity in ROMS is strongest within the cyclonic part of the meanders and is similar to the ageostrophic velocity produced by nonlinear interaction of Ekman currents with the nearsurface vorticity field.

Removing Spurious Low-Frequency Variability in Drifter Velocities

Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or tether strain records. However, recent studies have revealed that a significant fraction of drifters believed to be drogued have actually lost their drogues, a problem that peaked in the mid-2000s before the majority of drifters in the global array switched from submergence to tether strain sensors. In this study, a methodology is applied to the data to automatically reanalyze drogue presence based on anomalous downwind ageostrophic motion. Results indicate that the downwind slip of undrogued drifters is approximately 50% higher than previously believed. The reanalyzed results no longer exhibit the dramatic and spurious interannual variations seen in the original data. These results, along with information from submergence/tether strain and transmission frequency variations, are now being used to conduct a systematic manual reevaluation of drogue presence for each drifter in the post-1992 dataset. Satellite-tracked drifting buoys (hereafter ‘‘drifters’’) of the Global Drifter Program (GDP) have been collecting near-surface ocean current observations in the tropical Pacific since 1979, with observations in the other basins also now spanning more than 15 years. The GDP is a branch of the National Oceanic and Atmospheric Administration’s (NOAA) Global Ocean Observing System and a scientific project of the Data Buoy Cooperation Panel, and is funded by NOAA’s Climate Program Office. Its objectives are to maintain a global array of ;1250 drifters and to provide a data processing system for scientific use of the resulting observations, which support short-term (seasonal to interannual) climate predictions, climate research, and climate monitoring. A subset of the driftersalso includes barometers for improved numerical weather forecasting efforts. The GDP works with a large number of national and international partners

Sea surface salinity under rain cells: SMOS satellite and in situ drifters observations

We study the signature of rainfall on S1cm, the sea surface salinity retrieved from the Soil Moisture and Ocean Salinity (SMOS) satellite mission first by comparing SMOS S1cm with ARGO sea surface salinity measured at about 5 m depth in the Intertropical Convergence Zone (ITCZ) and in the Southern Pacific Convergence Zone; second by investigating spatial variability of SMOS S1cm related to rainfall. The resulting estimated S1cm decrease associated with rainfall occurring within less than 1 h from the salinity measurement is close to −0.2 pss (mm h−1) −1. We estimate that rain induced roughness and atmospheric effects are responsible for no more than 20% of this value. We also study the signature of rainfall on sea surface salinity measured by surface drifters at 45 cm depth and find a decrease associated with rainfall of −0.21 (±0.14) pss (mm h−1) −1, consistent with SMOS observations. When averaged over one month, this rain associated salinity decrease is at most −0.2 in monthly 100 × 100 km2 pixels, and at most 40% of the difference between SMOS S1cm and interpolated in situ bulk salinity in pixels near the ITCZ. This suggests that more than half of this difference is related to the in situ products obtained from optimal interpolation and therefore influenced by smoothing and relaxation to climatology. Finally, further studies on the satellite-derived salinities should pay attention to that as well as to other sources of uncertainties in satellite measurements and not interpret fully the observed differences between in situ and satellite mapped products, as rain induced SSS variability.

Winter conditions in the Irminger Sea observed with profiling floats

Temperature/salinity profiles collected between 1994 and 2003 with profiling floats in the North Atlantic subpolar gyre are analyzed to investigate the hydrographic conditions in winter in the Irminger Sea. The salinity data can be calibrated against accurate profiles from ships obtained mostly during summer months and the resulting float profile salinity accuracy is of the order of 0.015. Between 1997 and 2003, when the North Atlantic Oscillation (NAO) index was generally low, the potential temperature and salinity of the Labrador Sea Water (LSW) core observed by the floats showed a positive trend, an indication of little or no deep convection. The float data show that in the Irminger Sea the thermal energy of the water column reaches the lowest values south and southwest of Cape Farewell, a place where deep convective events are likely to occur. The geostrophic velocity field at 15 m computed from drifting buoys and satellite measurements of sea level shows, for the same area, mean currents below 0.1 m s 1 and low levels of eddy kinetic energy. These factors, together with recent estimates of winter air-sea heat fluxes as high as 500 W m 2 for this region, are exploited to explore the evolution of the surface mixed layer using several one-dimensional models. The results suggest that the typical thickness of the surface mixed layer at the end of winter is of the order of 400 m. This is similar to observed values from floats.

The Evaluation of Salinity Measurements from PALACE Floats

Abstract Seven PALACE (Profiling Autonomous Lagrangian Circulation Explorer) floats were deployed in October 1996 in the Irminger Basin of the Atlantic Ocean as a U.K. contribution to the World Ocean Circulation Experiment. Of these floats, four were fitted with a conductivity–temperature–depth package. The floats were ballasted to drift at a depth of about 1500 m, above the Labrador Sea Water (LSW) cold and fresh extreme, and programmed to surface every 14 days. The floats made a profile during each ascent to the surface. The authors present a method to evaluate the performance of the conductivity sensors and to calibrate the float salinity data. Since the LSW appears to be relatively stable over a timescale of ∼1–2 months and a length scale of ∼50–100 km, the authors were able to make direct comparisons between the first year of float data and accurate ship-based measurements and, therefore, were able to correct for errors of the conductivity sensors. A correction was applied in all cases. The conductiv...

The nascent Kuroshio of Lamon Bay

A northward flowing current, emanating from the North Equatorial Current (NEC) bifurcation at the Philippine margin, enters Lamon Bay along Luzons eastern coast. There the NEC tropical water masses merge with subtropical water of the western North Pacific to form the Kuroshio. A northward flowing western boundary current is first observed near 16.5°N, marking the initiation of the Kuroshio. The current feeding into the nascent Kuroshio of Lamon Bay is bracketed by an anticyclonic dipole to its northeast and a cyclonic dipole to its southwest. Ship-based observational programs in the spring seasons of 2011 and 2012 detect a shift of the Lamon Bay thermohaline stratification with marked enrichment of NEC tropical thermocline water in 2012 relative to a dominant western North Pacific subtropical stratification of 2011. Temperature-salinity time series from moorings spanning the two ship-based observations identify the timing of the transition as December 2011. The NEC bifurcation was further south in May 2012 than in May 2011. We suggest that the more southern bifurcation in May 2012 induced increased NEC thermocline water injection into Lamon Bay and nascent Kuroshio, increasing the linkage of the western North Pacific subtropical and tropical thermoclines. This connection was reduced in May 2011 as the NEC bifurcation shifted into a more northerly position and western North Pacific subtropical thermocline dominated Lamon Bay stratification.

Observations of Large-Amplitude Nonlinear Internal Waves from a Drifting Array: Instruments and Methods

Abstract This paper presents a novel methodology applied to the observation of large-amplitude nonlinear internal waves in the upper ocean from an array of drifting instruments. The characteristics of the instrument used—an Autonomous Drifting Ocean Station with acoustic current profilers (ADOS-A), which is a drifting buoy with a 200-m-long thermistor chain, several profiling acoustic current meters, and a GPS—are discussed. The ADOS-A is lightweight and relatively inexpensive and can be deployed from aircrafts or from ships of opportunity. Three packets of large-amplitude, nonlinear internal waves were observed in the South China Sea. The speed and the direction of propagation of the waves are accurately determined and their characteristics and evolution over space and time scales comparable with those of the waves, as they propagate through the array, are discussed.

Advances in the Application of Surface Drifters

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.