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G. Reverdin

Abstract

Surface temperature, salinity, and density are examined in the northeastern part of the North Atlantic subpolar gyre over the last 115 years of measurements. This region presents coherent variability in space but also between different seasons, with relatively small trends and large multidecadal variability. The most significant trend is a lowering in surface density. Multidecadal variability in T and S is large and is usually similar, with the largest difference between the two in the 1920s and a tendency of T to lead S. Multidecadal T and S are correlated with the winter North Atlantic Oscillation (NAO) index at 0 or 1-yr lag for T and 0 to 3-yr lag for S. This suggests a strong contribution of advection. The lag between T and S is also suggestive of a contribution of air–sea fluxes of heat or freshwater, but probably more so at high frequencies than at the multidecadal time scales. Salinity higher frequency is correlated with NAO at a 2–3-yr lag, whereas T higher frequency variability presents no correlation with NAO at any lag. This suggests different relations between seasonal NAO indices and air–sea heat fluxes patterns in this region before and after 1960; also the advective signal is more clearly identified in salinity in this region.

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G. Reverdin and G. Sommeria

Abstract

During Summer 1979, 88 super-pressure constant-level balloons were launched at the nominal flight level of 900 mb from the Seychelles Islands and from the northern tip fo the Malagasy Republic as part of the French participation in the MONEX experiment. Balloons tracked by the Argos system on board TIROSN and NOAA-6 satellites provide good estimates of the wind velocity and of the Lagrangian acceleration of air masses within the planetary boundary layer (PBL). With additional wind and pressure data, a study of the balance of forces and of the vertical wind structure in the planetary boundary layer has been performed, including the computation of the vertical transport of horizontal momentum by turbulent mixing. The accuracy of this estimate is marginal in equatorial regions, but sufficient in the Northern Hemisphere to derive a friction coefficient. The meridional variation of the wind veering within the planetary boundary layer is interpreted using a simple one-dimensional model. Veering is weak in the vicinity of the equator and appears to be of the Ekman type more than 10° away. If one follows a cross-equatorial trajectory, the transition in the direction of the veering occurs ∼5° north of the equator. This provides a measure of the relative importance of advective terms compared to local forcing terms in the dynamics of the tropical boundary layer.

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F. Vauclair, Y. du Penhoat, and G. Reverdin

Abstract

The mass and heat budgets of the warm upper-ocean layer are investigated in the equatorial Atlantic using in situ observations during the period 1979–99, which encompassed a series of warm events in the equatorial Atlantic. The warm water layer is defined as the layer having an in situ temperature higher than 20°C, which is within the core of the equatorial thermocline. The geostrophic transport is calculated by combining gridded temperatures with historical salinity data. The Ekman transport is estimated from observed wind data or model- based wind products. The change in warm water volume is then compared with the horizontal mass convergence, and the residuals are determined. The heat budget of the upper layer is investigated in the same way. Three regions are considered: the equatorial band between 8°N and 8°S to study the meridional redistribution of the warm water and two boxes (western and eastern boxes) to investigate the zonal redistribution of the warm water. Mass and heat budget variability in the equatorial band is discussed in relation to the zonal wind variability. The authors discovered that during the development of an equatorial warm event the meridional net divergence first decreases, reaching its minimum as the warm event matures. Meridional divergence increases again as conditions become normal in the equatorial band. The vertical velocity through the 20°C isotherm also reveals variations consistent with this scenario. Cross-isotherm mass transport decreases during warm events. The heat budget residual is more difficult to interpret. The average value is consistent with heat loss through turbulent mixing at the base (20° isotherm), but the fluctuations are most likely noise, resulting mainly from the limited accuracy of the model surface heat fluxes used.

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G. Reverdin, F. Marin, B. Bourlès, and P. Lherminier

Abstract

Data from French cruises in 1999–2007, a period during which Deep Blue (DB) or T7 expendable bathythermographs (XBTs) were deployed, and for which ancillary temperature data are available in the northeast Atlantic and equatorial Atlantic regions, are examined. There was a total of 16 cruises with XBTs launched between conductivity–temperature–depth (CTD) stations; during most of these, as well as during three additional cruises that were also considered, intake temperature was measured. XBT data from two voluntary observing ships in the North Atlantic subpolar gyre for which intake temperature was measured were also investigated. There is an XBT cold bias due to stirring of a stratified upper layer by the ship, resulting in differences between XBT temperatures at 3–5 m and intake measurements. This is most pronounced for midlatitude spring or summer cruises, when it averages about 0.10°C. When these situations are removed, the comparisons clearly indicate positive biases in XBT temperature measurements in 1999–2006, with individual cruise averages generally between 0° and 0.1°C, and a tendency to have larger biases when surface temperature is high. In addition, a positive depth-estimate bias of the XBTs in the upper thermocline (on the order of 4 m) is identified, as well as a depth overestimation through the profile, averaging 1.7% (1.2%) for the equatorial (midlatitude) cruises (with respect to a previously published depth estimate).

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G. Reverdin, P. Rual, Y. Du Penhoat, and Y. Gouriou

Abstract

A set of temperature profiles from expendable bathythermographs collected from 1980 to April 1988 along two ship routes transecting the equatorial Atlantic from 11°N to 11°S is analyzed to inter the vertical structure of the annual variability of the temperature and the currents in the upper ocean.

During the average seasonal cycle, the vertical isotherm displacements occur earlier below 300 meters than near the surface at most locations within 4 degrees of the equator. At the equator the amplitude of the displacements does not decrease with depth in the upper 500 meters. This still holds down to 700 meters, but there are less data at these depths. The lead of the deeper isotherm displacements with respect to those in the upper thermocline implies that there is a contribution to the pressure forces from these layers that is not in phase with the contribution of the upper thermocline. This also suggests that the energy source of the seasonal variability is close to the surface. Dynamic height and geostrophic currents relative to 400 db are also estimated. A seasonal cycle is found on the subsurface currents, which vary by up to a factor two during the cycle.

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M. Hamon, G. Reverdin, and P.-Y. Le Traon

Abstract

The authors use a collocation method between XBT and CTD/Ocean Station Data (OSD; including bottle cast and low-resolution CTD) from World Ocean Database 2005 (WOD2005) to statistically correct the XBT fall rate. An analysis of the annual median bias on depth shows that it is necessary to apply a thermal correction, a second-order correction on the depth, as well as a depth offset representing measurement errors during XBT deployment. Data were separated into several categories: shallow and deep XBTs and below or above 10°C of vertically averaged ocean temperatures (in the top 400 m). Also, XBT measurements in the western Pacific between 1968 and 1985 were processed separately because of large regional biases. The estimated corrections deviate from other published estimates with some large variations in time of both linear and curvature terms in the depth corrections, and less time variation of the temperature correction for the deep XBTs. This analysis of heat content derived from corrected XBTs provides at first order a similar variability to other estimates from corrected XBTs and mechanical bathythermographs (MBTs). It shows a fairly prominent trend in 0–700-m ocean heat content of 0.39 × 1022 J yr−1 between 1970 and 2008.

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J. P. Duvel, C. Basdevant, H. Bellenger, G. Reverdin, A. Vargas, and J. Vialard

During the Validation of the Aeroclipper System under Convective Occurrences (VASCO) test experiment in January and February 2007, eight Aeroclipper prototypes were launched from Mahe Island in the tropical Indian Ocean. The Aeroclipper is a streamlined balloon maintained in the atmospheric surface layer by a guide rope dragging on the ocean surface. While requiring some design improvements, these prototypes showed good potential for the exploration of the tropical air-sea interface, even under rough cyclonic conditions.

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G. Reverdin, J. Boutin, A. Lourenco, P. Blouch, J. Rolland, P. P. Niiler, W. Scuba, and A. F. Rios

Abstract

Sea surface salinity (SSS) data were collected in the Bay of Biscay between April and November 2005. The major source of data is 15 surface drifters deployed during the COSMOS experiment in early April and early May 2005 [12 from the Scripps Instution of Oceanography (SIO) and 3 from METOCEAN]. This is complemented by thermosalinograph (TSG) data from four French research vessels and four merchant vessels, from salinity profiles collected by Argo profiling floats and CTD casts, and from surface samples during two cruises. Time during the two cruises was dedicated to direct inspection of the drifters, recovering some, and providing validation data. This dataset provides a unique opportunity to estimate the accuracy of the SSS data and to evaluate the long-term performance of the drifter salinities. Some of the TSG SSS data were noisy, presumably from bubbles. The TSG data from the research vessels needed to be corrected from biases, which are very commonly larger than 0.1 pss-78 (practical salinity scale), and which in some instances evolved quickly from day to day. These corrections are only available when samples were collected or ancillary data are available (e.g., from CTD profiles). The resulting accuracy of the corrected TSG dataset, which varies strongly in time, is discussed. The surface drifter SSS data presented anomalous daytime values during days with strong surface warming. These data had to be excluded from the dataset. The drifter SSS presented initial biases in the range 0.009 to −0.026 pss-78. The (usually) negative bias increased by an average of −0.007 pss-78 during the average 65-day period before the COSMOS-2 cruise on 22–27 June. High chlorophyll derived from satellite ocean color, and therefore high density of phytoplanktonic cells, is observed in Medium Resolution Imaging Spectrometer (MERIS)/Moderate Resolution Imaging Spectroradiometer (MODIS) composites during part of the period, in particular in late April or early May. No correlation was found between the change in bias and the estimated surface chlorophyll. Evolution during the following summer months is harder to ascertain. For three buoys, there is little change in bias, but for two others, there could have been an increase in bias by up to 0.03 or 0.04 pss-78 during July–August. Seven drifters were recovered in the autumn, which provide recovery or postrecovery estimates of the biases, suggesting in three cases (out of seven) a large (0.02–0.03 pss-78) increase in bias during the autumn months, but no significant increase for the other four drifters.

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D. Bourras, H. Branger, G. Reverdin, L. Marié, R. Cambra, L. Baggio, C. Caudoux, G. Caudal, S. Morisset, N. Geyskens, A. Weill, and D. Hauser

Abstract

The present paper describes a new type of floating platform that was specifically designed for estimating air–sea fluxes, investigating turbulence characteristics in the atmospheric surface boundary layer, and studying wind–wave interactions. With its design, it can be deployed in the open ocean or in shallow-water areas. The system is designed to be used from a research vessel. It can operate for ~10 h as a drifting wave rider and 3 h under power. Turbulence and meteorological instrument packages are placed at a low altitude (1–1.5 m). It was deployed for validation purposes during the Front de Marée, Variabilité (FROMVAR), 2011 experiment off the west coast of Brittany, France. Wind friction velocity and surface turbulent buoyancy flux were estimated using eddy covariance, spectral, bulk, and profile methods. The comparisons of the four methods show a reasonable agreement except for the spectral buoyancy flux. This suggests that the platform design is correct. Also, the wind measured at a fixed height above the sea shows spectral coherence with wave heights, such that wind and swell are in phase, with the largest wind values on top of swell crests. This result in qualitative agreement with current model predictions supports the capability of the Ocean Coupled to Atmosphere, Research at the Interface with a Novel Autonomous platform (OCARINA) to investigate wind–swell interactions.

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G. Reverdin, J. Boutin, N. Martin, A. Lourenco, P. Bouruet-Aubertot, A. Lavin, J. Mader, P. Blouch, J. Rolland, F. Gaillard, and P. Lazure

Abstract

The accuracy of temperature measurements from drifters is first examined for 16 drifters (manufactured either by Metocean Data Systems or by Pacific Gyre) deployed with two temperature sensors in the tropical or North Atlantic Ocean. One of these sensors is the SST thermistor commonly used on Surface Velocity Program (SVP) drifters since the late 1980s; whereas the other sensor is a platinum temperature probe associated with a Seabird conductivity cell. The authors find (for 19 separate deployments) an average positive offset of the SST thermistor measurements in 17 out of 19 cases, exceeding 0.1°C in five instances. Among the five drifters that were at sea for a year or more, two present a large trend in this offset (0.10° and −0.10°C yr−1); and in two other cases, there is a clear annual cycle of the offset, suggesting a dependency on temperature. Offsets in 9 out of 12 drifters with sea time longer than 4 months present a negative trend, but the average trend is not significantly different from zero. The study also examined 29 drifters from four manufacturers equipped only with the usual SST thermistor, but for which either a precise initial temperature measurement was available or a float was attached to provide accurate temperature measurements (for a duration on the order of a month). These comparisons often identify SST biases at or soon after deployment. This initial bias is null (or slightly negative) for the set of Clearwater Instrumentation’s drifters, it is very small for two out of three sets of Technocean drifters, and positive for the third one, as well as for the set of Pacific Gyre drifters (on the order of 0.05°C).

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