XBT Measurements Research Articles

Imaging the Yellow Sea Bottom Cold Water from multichannel seismic data

The Yellow Sea Bottom Cold Water (YSBCW) is a large cold water mass lying in the deep part of the Yellow Sea during the warm season. We acquired multichannel seismic (MCS) data using an air gun source to image the structure of the YSBCW. The MCS data recorded reflections from sea water. The recognition of these reflections was confirmed by finite-difference seismic modeling in the frequency-domain. The seismic section from MCS data enabled discrimination of water masses distinctly separated by reflecting horizons. The structure of the water masses is fairly consistent with temperature-depth variations obtained using expandable bathythermograph (XBT) casts. The YSBCW is imaged as the lowermost water mass, maximally 40 m thick, that extends as a lens-like form along the sea bottom under the warm mixed layer. The correlation of XBT measurements and the seismic section indicates a rapid decrease in temperature from around 11 to 8 °C in the uppermost part of the YSBCW. A transition zone between the mixed layer and the underlying YSBCW is also defined. This transition zone has fairly uniform thickness of 14–18 m and marks an interval of rapid temperature drop, indicating vigorous thermal mixing. Our study demonstrates that MCS profiling is a useful and reliable tool for imaging fine structures in the shallow Yellow Sea.

Indication of recent warming process at the intermediate level in the Tyrrhenian Sea from SOOP XBT measurements

The Tyrrhenian Sea is a sub-basin of the western Mediterranean crossed by intermediate and deep waters from the eastern basin. Across this sub-basin, temperature profiles of the water column from expendable bathythermographs (XBT) have been acquired for sixteen years along transects realized thanks to the use of commercial vessels. Since 1999 an increase of temperature has been observed at intermediate depths even if interspersed with periods of decrease. This increase involves deeper and deeper depths along the years then involving the whole sub-basin in the range 200-800 m in September 2014 when largest anomalies over the whole period are found. The paper shows evidences of this rapid heating, giving insights into the origin and the diffusion of the warmer intermediate waters then showing its evolution in years and its relationship with the Eastern Mediterranean Transient.

The impact of historical biases on the XBT‐derived meridional overturning circulation estimates at 34°S

AbstractAn observational system based on high‐density expendable bathythermograph (XBT) data has provided the longest record of the South Atlantic meridional overturning and heat transport estimates across 34°S. Measurement biases are a point of concern for the capability of an XBT system to capture long‐term trends in volume and heat transports, and the impact of such biases on the meridional overturning estimates has never been quantified. In the present study, the sensitivity of the meridional overturning circulation and heat transport to uncertainties in XBT measurements is quantified under the framework of an eddy‐resolving model simulation. Results show that XBT measurement biases after 2010 can translate into small meridional overturning errors on the order of 3% or 0.38 Sv (1Sv = 106 m3 s−1), and 0.025 PW (1 PW = 1015 W) or 8% of the meridional heat transport in the model. Historical XBT‐derived trends in transport estimates across 34°S are stronger and statistically significant after the late 1990s, 0.3 Sv decade−1 and 0.02 PW decade−1. These trends are mostly due to the XBT linear depth bias, with smaller contributions associated with temperature and depth offsets from the historical record. Long‐term trends calculated from Simple Ocean Data Assimilation reanalysis, estimated as 0.1 Sv/decade and 0.006 PW/decade, are 3 times smaller than the XBT‐derived historical trends. Therefore, an adequate correction of historical XBT data is necessary for an early detection of trends in the meridional overturning circulation and heat transport.

Reducing Biases in XBT Measurements by Including Discrete Information from Pressure Switches

Abstract Biases in the depth estimation of expendable bathythermograph (XBT) measurements cause considerable errors in oceanic estimates of climate variables. Efforts are currently underway to improve XBT probes by including pressure switches. Information from these pressure measurements can be used to minimize errors in the XBT depth estimation. This paper presents a simple method to correct the XBT depth biases using a number of discrete pressure measurements. A blend of controlled simulations of XBT measurements and collocated XBT/CTD data is used along with statistical methods to estimate error parameters, and to optimize the use of pressure switches in terms of number of switches, optimal depth detection, and errors in the pressure switch measurements to most efficiently correct XBT profiles. The results show that given the typical XBT depth biases, using just two pressure switches is a reliable strategy for reducing depth errors, as it uses the least number of switches for an improved accuracy and reduces the variance of the resulting correction. Using only one pressure switch efficiently corrects XBT depth errors when the surface depth offset is small, its optimal location is at middepth (around or below 300 m), and the pressure switch measurement errors are insignificant. If two pressure switches are used, then results indicate that the measurements should be taken in the lower thermocline and deeper in the profile, at approximately 80 and 600 m, respectively, with an RMSE of approximately 1.6 m for pressure errors of 1 m.

Empirical Correction of XBT Data

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.

Sonic Layer Depth estimated from XBT temperatures and climatological salinities

AbstractSonic layer depth (SLD) plays an important role in antisubmarine warfare in terms of identifying the shadow zones for submarine safe parking. SLD is estimated from sound velocity profiles (SVP) which is in turn obtained from temperature and salinity (T/S) profiles. Given the limited availability of salinity data in comparison to temperature, SVPs need to be obtained from alternate methods. In the present work, to make use of voluminous temperature data sets from XBT, CTD and other source for estimating SLD, we propose a method of utilizing XBT measurements and World Ocean Atlas climatological salinities to compute SVP and then extract SLD. This approach is demonstrated by utilizing T/S data from Argo floats in the Arabian Sea (40° – 80° E and 0 – 30° N). SLD is estimated from SVP obtained from Argo T/S profiles first and again by replacing the Argo salinity with climatological salinity. It is found that in more than 90% of cases, SLD matched exactly, with the root mean square deviation ranging from 3 – 12 m with an average of 7 m.

Tracking bottom waters in the Southern Adriatic Sea applying seismic oceanography techniques

We present the first results from the seismic oceanography (SO) cruise ADRIASEISMIC where we successfully imaged thermohaline fine structures in the shallow water environment (50–150m) of the southern Adriatic Sea during March 2009 using a compact two GI-gun seismic source. The SO observations are complemented with traditional oceanographic and micro-structure measurements and show that SO can operate over almost the entire water column except (in our experimental layout) for the uppermost 50m. After processing to enhance the signal-to-noise ratio, the seismic reflection data have a vertical resolution of ~10m and a horizontal resolution of ~100m and provide a laterally continuous map of significant thermohaline boundaries that cannot be achieved with conventional physical oceanography measurements alone. ADRIASEISMIC specifically targeted structures in shallow waters, namely along the western margin of the southern Adriatic Sea, between the Gargano peninsula and the Bari canyon, and imaged the Northern Adriatic Dense Water (NAdDW), a bounded cold and relatively dense water mass flowing from the northern Adriatic Sea.The seismic data acquired in Bari canyon and offshore of the Gargano promontory show many regions of strongly reflecting shallow structures, and the incorporation of XBTs measurements with these data demonstrate that they can be interpreted in terms of temperature structures and gradients. In the Gargano region several warm water intrusive structures are mapped along with the offshore transitional edge of cold waters of strong NAdDW influence. In Bari Canyon, waters with NAdDW influence are further mapped extending over the shelf and off the slope into a 5km long tongue extending offshore between depths of 200 and 300m. More generally, even though neither cascading nor open-ocean deep convection process appeared to be evident during March 2009, the SO approach was able to map details of thermal features not resolved by even closely spaced XBT measurements.

Determination of dynamic heights in the Bay of Bengal from XBT profiles and climatological salinities

Dynamic height (DH) computations are necessary for the estimation of geostrophic currents, heat transport studies, identification of eddies and to validate the altimeter sea surface height (SSH) observations. To compute DHs we need the temperature and salinity profiles to a reasonable deeper level. But, temperature profiles alone, such as from XBT (expendable bathy thermograph), are more numerous than combined temperature and salinity measurements from CTD (conductivity, temperature and depth). To use these voluminous temperature measurements from XBT observations in the computation of DHs, temperature/salinity relations were developed in many oceanic regions. Since such an exercise cannot be carried out in the Bay of Bengal due to the absence of adequate CTD measurements, we have proposed a method of utilizing XBT measurements and climatological (Levitus and Boyer, 1994) salinities to compute the DHs. The approach is demonstrated by replacing the CTD salinities with the climatological salinities. The relative errors compared to the DH signals are 3.8%, 2.7% and 2.6% for 200, 700 and 1000 dbar levels, respectively. The DHs relative to 700 dbar computed using the XBT temperature and climatological salinity profiles are compared with the SSH observations from TOPEX altimeter. The standard deviation of the difference between the DHs computed after converting to geometrical units and SSH is 7.3 cm.

Seasonal and interannual evolution of the mixed layer in the Antarctic Zone south of Tasmania

Seasonal and interannual variations of the mixed layer properties in the Antarctic Zone (AZ) south of Tasmania are described using 7 WOCE/SR3 CTD sections and 8 years of summertime SURVOSTRAL XBT and thermosalinograph measurements between Tasmania and Antarctica. The AZ, which extends from the Polar Front (PF) to the Southern Antarctic Circumpolar Current Front (SACCF), is characterized by a 150 m deep layer of cold Winter Water (WW) overlayed in summer by warmer, fresher water mass known as Antarctic Surface Water (AASW). South of Tasmania, two branches of the PF divide the AZ into northern and southern zones with distinct water properties and variability. In the northern AZ (between the northern and southern branches of the PF), the mixed layer depth (MLD) is fairly constant in latitude, being 150 m deep in winter and around 40–60 m in summer. In the southern AZ, the winter MLD decreases from 150 m at the S-PF to 80 m at the SACCF and from 60 to 35 m in summer. Shallower mixed layers in the AZ-S are due to the decrease in the wind speed and stronger upwelling near the Antarctic Divergence. The WW MLD oscillates by ±15 m around its mean value and modest interannual changes are driven by winter wind stress anomalies. The mixed layer is on annual average 1.7 °C warmer, 0.06 fresher and 0.2 kg m −3 lighter in the northern AZ than in the southern AZ. The Levitus (1998) climatology is in agreement with the observed mean summer mixed layer temperature and salinity along the SURVOSTRAL line but underestimates the MLD by 10–20 m. The winter MLD in the climatology is also closed to that observed, but is 0.15 saltier than the observations along the AZ-N of the SR3 line. MLD, temperature and density show a strong seasonal cycle through the AZ while the mixed layer salinity is nearly constant throughout the year. During winter, the AZ MLD is associated with a halocline while during summer it coincides with a thermocline. Interannual variability of the AZ summer mixed layer is partly influenced by large scale processes such as the circumpolar wave which produces a warm anomaly during the summer 1996–1997, and partly by local mechanisms such as the retroflection of the S-PF which introduces cold water across the AZ-N.

Experimental testing of broadband decomposition of the time reversal operator (DORT) in shallow water

Results are shown from the application of DORT to at-sea data taken in an area south of Hudson Canyon off the New Jersey coast during Spring 2004. A vertical source/receiver array with 64 hydrophones spanning the water column of 100-m depth was operated in 500-Hz bands between 500 and 3500 Hz. Sets of four, five, or six beams were transmitted at small angles from horizontal and the response from an echo repeater was recorded on 64 channels. The resulting data were analyzed using broadband DORT. Specific transmissions were used to quantify the degradation of coherence due to temporal variability of the source/receiver, environment, and echo repeater. The isolation in the singular vectors of the echo repeater using sets of independent source beams is comparable to that from the coherence tests. The singular spectrum of the noise field is compared with the expected results from that of a random matrix. Backpropagation images based on at-sea data and propagation using in situ XBT measurements are shown. [Work supported by ONR.]

Assimilation of sea-surface temperature and altimetric observations during 1992–1993 into an eddy permitting primitive equation model of the North Atlantic Ocean

Sea-surface temperature (SST) and sea-surface height (SSH) observations collected from space between October 1992 and December 1993 have been assimilated into a realistic primitive equation model of the North Atlantic Ocean circulation at eddy permitting resolution. The assimilated SST data originate from AVHRR observations gathered and processed within the NASA Pathfinder project; the altimetric data consist of SSH maps computed as the sum of a time-invariant dynamic topography and gridded sea-level anomalies obtained by combining Topex/Poseidon and ERS altimeter data. The assimilation scheme is a reduced-rank Kalman filter derived from the Singular Evolutive Extended Kalman (SEEK) methodology [J. Mar. Syst. 16 (1998) 323], in which the error statistics is represented in a subspace of small dimension. The error subspace is initialized with a truncated series of Empirical Orthogonal Functions (EOFs) of the system variability. The analysis algorithm includes a mechanism to update the forecast error statistics adaptively using all pertinent informations from the innovation vector. Hindcast experiments have been conducted with a 1/3° model of the North Atlantic basin forced with ECMWF atmospheric reanalyses. The impact of the data assimilated during 1993 is assessed by examining how observed (SSH and SST) and nonobserved variables (such as velocity and thermohaline properties in the interior of the ocean) are modified by the assimilation scheme. Finally, the validation of the hindcast experiments with independent XBT measurements is performed in order to evaluate the objective skill of the procedure. The various diagnostics demonstrate the positive impact of the satellite data to hindcast the upper ocean circulation at eddy permitting resolution and the capacity of the scheme to estimate the geographic distribution of the forecast error.

Seasonal to interannual upper-ocean variability in the Drake Passage

Year-round monitoring of the upper-ocean temperature variability in Drake Passage has been undertaken since September 1996 through repeat expendable bathythermograph (XBT) surveys. The closely spaced measurements (6‐ 15 km apart) provide the e rst multi-year time series for examining seasonal to interannual variability in this Southern Ocean choke point. While the temperature sections reveal the seasonal variability in water mass formation of the upper layer, there was no seasonal signal evident below 200 m. Similarly, there was little seasonal cycle evident in the position of the Subantarctic Front, the Polar Front and the Southern Antarctic Circumpolar Current Front associated with the Antarctic Circumpolar Current (ACC) in Drake Passage. Mesoscale eddy features are readily identie able in the XBT sections and in some sparse salinity sections, as distinct alternating bands separated by near-vertical isotherms of cold and warm core temperatures. The eddies can also be tracked in concurrent maps of altimetric sea surface height, with time scales of ;35 days and diameters of 50‐ 100 km, following a north to north-east trajectory with the main path of ACC e ow through Drake Passage. Both the XBT and the altimetric data suggest the eddies are mainly cone ned to the Antarctic Polar Frontal Zone. To determine transport, an empirical relationship is derived between upper ocean XBT temperature and a baroclinic mass transport function from historical CTD casts collected in the Drake Passage. While in the temporal mean the strongest eastward transport is associated with the three major fronts in the ACC, the individual cruises strongly suggest a banded nature to the e ow through the passage. Some, although not all, of the eastward and westward bands of transport can be attributed to the presence of eddies. The high spatial resolution of the XBT measurements is more capable of distinguishing these countere ows than the typical 50 km resolution of historical hydrographic sections across Drake Passage. Commensurate with the position of the fronts, no real seasonal signal in Drake Passage transport is discernible, although there is substantial variability on interannual time scales. The Drake Passage XBT transport time series is strongly correlated to both zonal wind stress and wind stress curl in the southeast Pacie c Ocean.

Start-up transient of XBT measurement

The start-up transient of XBT measurements is statistically investigated for five types of recorder system: Z-60-16 II and III by Murayama Electric Co., Ltd., and MK-130 by Tsurumi Seiki Co., Ltd., which are commonly used in Japan, and Sippican MK-9 and MK-12, which are popular in the United States and other countries. The UNESCO recommendation to exclude XBT-measured temperatures in the top 3.7 m layer from the bathythermograph report is justified within a nominal precision of 0.2°C for Tsurumi Seiki T-7 probes. However, it is shown that the depth where the transient ceases differs for different types of recorder. If we require a more strict accuracy constraint of 0.02°C, the estimated depth of the transient is 2 m for MK-130, MK-9 and MK-12, 3 m for Z-60-16 III and 10 m for Z-60-16 II.

Estimates of Heat Content Variations from Sea Level Measurements in the Central and Western Tropical Pacific from 1979 to 1985

Abstract A method is developed to estimate oceanic heat content (0–300 m) variations from sea level measurements in the tropical Pacific. To this end, statistical relationships between heat content and steric level, used as a surrogate variable for the sea level, are derived from climatological data. These relationships are then applied on independent datasets and the predictive ability of the method is determined regionally by comparing heat content estimated from XBT and sea level measurements recorded in three tropical Pacific islands (Christmas Fanning and Truk) during the 1979–85 period. Good qualitative agreements are found between the two heat content estimates with correlations R = 0.78 to 0.94 and rms differences of average temperature of 0.25° to 0.50°C over an observed range of 6°C. Quantitative disagreements are observed in the central Pacific during the fall 1982 (El Ninc) period. These deficiencies in the method are found to be primarily due to intense and unusual salinity fluctuations at th...

Ocean acoustic tomography: Mesoscale velocity

The travel times of acoustic pulses transmitted in opposite directions over a 300‐km distance in mid‐ocean have been used to measure the fields of sound speed and absolute water velocity with mesoscale resolution. The experiment, which took place west of Bermuda during August and September 1983, consisted of transceivers on two moorings that sent and received signals every 2 hours for 21 days. Ray paths in opposite directions were found to be very nearly reciprocal; effects due to internal waves and mean currents were small. Sum travel times were inverted to obtain sound speed at 2‐day intervals, and differential travel times were inverted to obtain absolute current velocity u. The principal results are (1) range‐averaged sound speed perturbation δC(z) decreased over the 21 days by 5 ±0.2 m s−1 at 1000m depth, corresponding to a temperature decrease of approximately 1°C; similar (but lesser) changes are measured at other depths. The changes are associated with the first baroclinic mode. (2) The field δC(x,z) determined from expendable bathythermograph (XBT) measurements at the start of the experiment agrees with the field determined using the acoustic data within estimated errors of ± 1.6 m s−1. It is thus possible to obtain range‐dependent δC information from acoustic measurements made in a single vertical slice. (3) During the 21 days the range‐averaged barotropic velocity mode increased by a factor of 3, while the baroclinic mode decreased by a factor of 2. The estimated error in range‐averaged u(z) is ±2 cm s−1 above the main thermocline and ±1 cm s−1 below, while for range‐dependent u (x,z) the errors are 5 and 3 cm s−1, respectively. No significant range dependence in u (x,z) was found. The baroclinic velocity agrees with geostrophic velocities inferred from the XBT measurements at deployment and aircraft‐deployed XBT measurements 11 days after the end of the reciprocal transmissions.

Characteristics of thermohaline steps off the northeast coast of South America, July 1983

XBT and CTD measurements made in the Atlantic Ocean southeast of Barbados reveal a series of large thermohaline steps lying between the salinity maximum of the Subtropical Underwater at about 100 m and the salinity minimum of the Antarctic Intermediate Water at about 800 m. Historical data indicate that the steps are a permanent feature of the area extending over some 10 6 km 2. At two multicast stations in July 1983 the average step was 19.0 dbar in thickness, and the average interface between steps was 2.1 dbar thick. Mean potential temperature, salinity and potential density changes across the interfaces were, respectively, 0.526°C, 0.084 psu, and 0.030. Internal structure was noted in some of the steps, as well as features resembling intrusions. The low value of the density ratio, approximately 1.5, supports the hypothesis that the steps are caused by intense salt finger activity. Estimates of vertical fluxes of heat and salt through the staircases are large enough to suggest that the process plays a major role in vertical exchange across the thermocline in this region and, consequently, a major role in determining the T-S properties of the equatorial thermocline and of the Caribbean Current-Gulf Stream system.

XBT Measurements of Thermal Gradients in the MODE Eddy

Abstract Estimates of the repeatability and accuracy of XBT measurements were made using XBT data taken during MODE. The XBT observations of the depth of isotherms had systematic errors of up to 15 decibars (by comparison to simultaneous CTD drops) as well as random errors on the order of 8 declare. Maps of these observations show small-scale thermal structure which would imply sizeable increases in geostrophic shears.

Eddy Structure in the Central South Pacific Ocean

Two west-to-cast temperature sections (∼2500 km in length) have been constructed from closely spaced (∼37 km) XBT measurements in the central South Pacific collected during Legs J and K of the Pacific GEOSECS Expedition. For these portions of track passing through the islands of the Tuamotu Archipelago, the main thermocline exhibited 50 m station-to-station variability on a scale too short to be resolved. In the open ocean away from these islands, the variability was smaller, 10–20 m, with only one 200 km scale eddy spatially resolved.