XBT Sections Research Articles

Cold core eddies and fronts of the Antarctic Circumpolar Current south of New Zealand from in situ and satellite data

The meridional heat flux required to balance the heat lost by ocean to atmosphere at high latitudes must be accomplished by some mechanism other than mean advection and the heat flux by eddies crossing the Antarctic Circumpolar Current (ACC) may be a candidate. In this study, the positions of the main ACC fronts are determined based on 23 expendable bathythermographs (XBT) transects collected from 1994 to 2010 and are compared with those detected through satellite altimetry. Then, cold core anomalies in XBT sections are identified and altimetry is used to follow the spatial‐temporal evolution of these cold, low sea level anomalies. Mean values of main parameters, such as speed (0.35 km/h), lifetime (79 weeks), and diameter (105 km), are estimated. Moreover, estimations of rotational speed (0.9–76.8 cm/s), ocean surface layer heat content along temperature sections and eddy available heat anomaly (mean value −9.74 × 109 Jm−2) give a wider description of the detected eddies. In our study area, the spawning of eddies is found to occur downstream of the Southeast Indian Ridge and in correspondence of the polar front (PF) with regard to the ACC frontal structure. The contribution of eddies to the global heat budget is not only linked to their ability to cross the ACC fronts but also to the capacity of keeping partially unaltered the properties of water inside them. Analysis of the relation between the translation and rotational speeds shows that a typical eddy may effectively be a significant part (0.8%) of the net meridional heat transport across the PF with a mean heat content/anomaly of −7.65 × 1019 J.

Atlantic meridional heat transports in two ocean reanalyses evaluated against the RAPID array

The Atlantic meridional overturning circulation in two versions of the NEMO ° global ocean model has been compared with the RAPID transport array at 26°N. Both model versions reproduce the mean MOC strength well although the Florida Straits flows differ because the pathway of the Gulf Stream is not strongly constrained at this resolution. Both models however have a mean meridional heat transport of 1.07PW, much lower than the 1.35PW from RAPID observations in April 2004–October 2007. Much of the heat transport discrepancy is due to lower transports in summer across the MidOcean (Bahamas‐Africa) section, due to stronger southward geostrophic flows in the top 100 m where the water is warmest. Seasonal thermocline changes increase temperature differences across the basin driving stronger geostrophic shear, but this effect is much weaker in the top 100 m of the RAPID velocity data. The effect accounts for a reduction of 1.1Sv in MOC and 0.1PW in heat transports. The rest of the discrepancy comes from lower Ekman transports from using ERAInterim winds instead of QuikSCAT, a smaller zonally varying “Eddy” heat transport component, estimated from repeat XBT sections in the observations, and the southward throughflow in the model. Other differences in depth structure of the model MOC and RAPID observations are described but have much less impact on heat transports.

Using satellite altimetry to correct mean temperature and salinity fields derived from Argo floats in the ocean regions around Australia

We present results from a suite of methods using in situ temperature and salinity data, and satellite altimetric observations to obtain an enhanced set of mean fields of temperature, salinity (down to 2000-m depth) and steric height (0/2000 m) for a time-specific period (1992–2007). Firstly, the improved global sampling resulting from the introduction of the Argo program, enables a representative determination of the large-scale mean oceanic structure. However, shortcomings in the coverage remain. High variability western boundary current eddy fields, continental slope and shelf boundaries may all be below their optimal sampling requirements. We describe a simple method to supplement and improve standard spatial interpolation schemes and apply them to the available data within the waters surrounding Australia (100°E–180°W; 50°S–10°N). This region includes a major current system, the East Australian Current (EAC), complex topography, unique boundary currents such as the Leeuwin Current, and large ENSO related interannual variability in the southwest Pacific. We use satellite altimetry sea level anomalies (SLA) to directly correct sampling errors in in situ derived mean surface steric height and subsurface temperature and salinity fields. The surface correction is projected through the water column (using an empirical model) to modify the mean subsurface temperature and salinity fields. The errors inherent in all these calculations are examined. The spatial distribution of the barotropic–baroclinic balance is obtained for the region and a ‘baroclinic factor’ to convert the altimetry SLA into an equivalent in situ height is determined. The mean fields in the EAC region are compared with independent estimates on repeated XBT sections, a mooring array and full-depth CTD transects.

Three-dimensional finestructure and vertical microstructure in thermohaline intrusions.

Thermohaline intrusions have intrigued physical oceanographers since the first XBT section was published by Spilhaus in 1939. Characterized by vertical gradients of temperature and salinity having compensating density effects, intrusions are meters to tens of meters thick and extend laterally for hundreds of meters to many kilometers. Boundaries often have TS staircases produced by salt fingering or diffusive layering. Large diapycnal fluxes have been inferred from sections showing intrusions sloping across isopycnals, but these inferences are questionable, as isopycnal movements can give the illusion of diapycnal motion. To address these issues, in 2007 we used a depth-cycling towed body, a neutrally buoyant Lagrangian float, and microstructure profilers to map and follow intrusions in the subtropical front north of Hawaii. All intrusions were connected to the front, i.e., none was isolated lenses, some as long, thin folds, and others resembled tongues. About 10 m thick, the sheets protruded 10 km into the warm, saline side of the front, were coherent laterally for 10–30 km, and lasted at least 1 week, evolving at both inertial (23 h) and sub-inertial (∼10 day) time scales. They appeared to be formed by advective deformation of the front driven by sub-mesoscale two-dimensional motions.

Transport and variability of the Antarctic Circumpolar Current south of Africa

Data from five CTD and 18 XBT sections are used to estimate the baroclinic transport (referenced to 2500 dbar) of the ACC south of Africa. Surface dynamic height is derived from XBT data by establishing an empirical relationship between vertically integrated temperature and surface dynamic height calculated from CTD data. This temperature‐derived dynamic height data compare closely with dynamic heights calculated from CTD data (average RMS difference = 0.05 dyn m). A second empirical relationship between surface dynamic height and cumulative baroclinic transport is defined, allowing us to study a more extensive time series of baroclinic transport derived from upper ocean temperature sections. From 18 XBT transects of the ACC, the average baroclinic transport, relative to 2500 dbar, is estimated at 90 ± 2.4 Sv. This estimate is comparable to baroclinic transport values calculated from CTD data. We then extend the baroclinic transport time‐series by applying an empirical relationship between dynamic height and cumulative baroclinic transport to weekly maps of absolute dynamic topography derived from satellite altimetry, between 14 October 1992 and 23 May 2007. The estimated mean baroclinic transport of the ACC, obtained this way, is 84.7 ± 3.0 Sv. These transports agree well with simultaneous in‐situ estimates (RMS difference in net transport = 5.2 Sv). This suggests that sea level anomalies largely reflect baroclinic transport changes above 2500 dbar.

Fronts, water masses and heat content variability in the Western Indian sector of the Southern Ocean during austral summer 2004

High density CTD and XBT sections were covered from 35° to 56°S along 45°E and 57°30′E to investigate the morphology of the main fronts in the southwest Indian Ocean, as a part of the Indian pilot expedition to the Southern Ocean on board ORV Sagar Kanya. Northern branch of the Subtropical Front (NSTF) was observed at ∼ 35°30′S along 45°E. Along 57°30′E, the signature of the Agulhas Return Front (ARF) + Subtropical Front (STF) was identified with a rapid decrease in surface temperature between 43°30′ and 45°S and it is located with a southward shift compared to that at 45°E. The Subantarctic Front (SAF) was distinguished as two fronts as northern SAF (SAF1) and southern SAF (SAF2) along both the meridional sections. Polar Front1 (PF1) was identified between 49° and 50°S whereas Polar Front2 (PF2) was identified between 52° and 54°S along 45°E. This study reveals a southward shift of the oceanic fronts (ARF + STF) from west to east, with a maximum southward displacement of > 2° latitude at 57°30′E. A novel finding of this study is that along 45°E, SAF1 merged with ARF and SSTF and SAF2 ∼ 4° latitude southwards from the merged fronts whereas along 57°30′E, SAF1 was not identified as a merged front with ARF and STF as opposed to earlier studies [Belkin, I.M., Gordon, A.L., 1996. Southern Ocean fronts from the Greenwich Meridian to Tasmania. Journal of Geophysical Research 101, 3675–3696]. The thermocline region was absent south of PF. An enhancement in the mixed layer thickness from 42° to 52°S occurred in association with the strengthening of the wind forcing. Major water masses like Subtropical Surface Water, Subantarctic Surface Water, Mode Water, Antarctic Intermediate Water, Circumpolar Deep Water and Antarctic Bottom Water were identified along 45°E. Upper-ocean heat-content computation revealed a remarkable drop of 989 × 10 7 J m −2 at ∼ 42°S and 1405 × 10 7 J m −2 at ∼ 44°S along 45° and 57°30′E, respectively. We believe that this sudden drop in heat content affects the meridional heat transfer which is crucial to the regional climatic variability.

Effect of Salinity on Estimating Geostrophic Transport of the Indonesian Throughflow along the IX1 XBT Section

Geostrophic transport of the Indonesian Throughflow (ITF) is estimated from optimally-interpolated temperature data along a frequently repeated expendable bathythermograph (XBT) section between Fremantle, Australia and Sunda Strait, Indonesia and from two historical temperature-salinity (T/S) relationship products, CSIRO Atlas for Regional Seas (CARS) and Levitus (1982). The annual mean ITF geostrophic transport relative to 400 in during 1984-2001 is estimated to be 4.6 Sv using the CARS T/S relationship, which is about 20% higher than that found using the Levitus T/S relationship. This transport increment is due to the fact that the CARS T/S relationship, which incorporates more recent hydrographic data, better resolves the low-salinity signature of the ITF water. Isothermal averaging in the CARS T/S relationship may also improve representations of the water mass signatures in deep layers.

On the sampling timescale required to reliably monitor interannual variability in the Antarctic circumpolar transport

Numerous field programs have been conducted with the intention of monitoring the transport variability of the Antarctic Circumpolar Current (ACC). Some programs are ongoing, with further efforts planned. Here we address, for the first time, the sampling frequency required for reliable monitoring of the ACC at interannual periods. We find that, for all practical purposes, sampling with a frequency significantly higher than weekly is required to obtain reliable annual means; coarser sampling leads to serious aliasing and degradation of the true temporal progression of transport. Monitoring changes in the seasonality of the ACC requires even more rapid sampling. In practice, hydrographic sections, repeat XBT sections and even altimetry along a single repeat groundtrack will fail to capture the true interannual variability; instead, in situ instrumentation (fixed gauges, moorings or bottom lander‐based equipment) is required. Combinations of techniques may be useful, if they include data from fixed in situ sources.

South Pacific Eastern Subtropical Mode Water*

Abstract The structure, formation, and destruction of South Pacific Eastern Subtropical Mode Water (SPESTMW) are analyzed. Geographic extent and water properties are discussed by using high-quality CTD sections collected between 1991 and 1996. Defined as having a planetary potential vorticity magnitude of less than 3 × 10−10 m−1 s−1, SPESMTW has a volume of about 1.1 × 1015 m3, estimated from CTD data. The ventilation of this mode water is described by using data from a high-resolution XBT section in concert with 30-month time series from profiling CTD floats, some of the first Argo deployments. Published subduction rates allow a mode-water formation rate estimate of 8.7 × 106 m3 s−1. Combining this estimate with the volume yields a residence time of about 4 years. The density-compensating covarying patterns of late winter surface temperature and salinity in the ventilation region of SPESTMW are shown to contribute to the strength of the mode water. However, while the destabilizing salinity gradient in SP...

Variability of the depth of the 20°C isotherm along 6°N in the Bay of Bengal: its response to remote and local forcing and its relation to satellite SSH variability

Abstract The depth of the 20°C isotherm (hereafter referred to as D20) derived from expendable bathythermograph (XBT) measurements along 6°N in the southern Bay of Bengal (hereafter referred to as the Bay) shows pronounced seasonal-to-interannual variability and profound westward propagation. But not all these features are well depicted by satellite observations of sea-surface height (SSH). The cause of the thermocline variability for the period of 1988–1999 and the relationship to SSH variability were analyzed in this study with the addition of wind and salinity data sets. Local Ekman pumping and remote Kelvin/Rossby wave propagation are the two leading forcing mechanisms for the thermocline variability at the chosen XBT section. On seasonal timescales, the phase of the D20 semiannual variations is determined by a remote forcing signal, but the amplitude is determined by interaction with the annual background that is preconditioned by the local Ekman pumping. The local background has the phase oscillating in a way that it enhances the westward propagation of the remote semiannual signal during the first half of the year but suppresses the semiannual signal during the second half of the year. On interannual timescales, the D20 at 6°N is most sensitive to the remote equatorial wind variability, with local forcing variability being a secondary contributor. This study shows that the calculation of the Ekman pumping velocity should include the contribution from not only wind-stress curl but also zonal wind stress due to the β-effect at 6°N. Omitting the latter would augment the effect of local forcing. The relationship between satellite SSH and D20 is not linear in the Bay, as large salinity variation contributes considerably to the SSH variability. Compared to the structure of seasonal D20 anomalies, the SSH anomalies are more zonally aligned, less westward propagating, and have a weak semiannual signal. A relationship between the sea level and thermocline variation is derived by using a two-layered system that includes seasonal variations of temperature and salinity in upper-ocean density. This allows the structure of the satellite SSH to be examined from the XBT temperature and the salinity climatology and enables salinity effect to be quantified. At 6°N the amplitude of the SSH variations induced by the salinity effect is about 3 cm, equivalent to the amplitude induced by the temperature effect.

Comparison between XBT data and TOPEX/Poseidon satellite altimetry in the Ligurian-Tyrrhenian area

Abstract. From September 1999 to December 2000, eXpendable Bathy-Thermograph (XBT) profiles were collected along the Genova-Palermo shipping route in the framework of the Mediterranean Forecasting System Pilot Project (MFSPP). The route is virtually coincident with track 0044 of the TOPEX/Poseidon satellite altimeter, crossing the Ligurian and Tyrrhenian basins in an approximate N–S direction. This allows a direct comparison between XBT and altimetry, whose findings are presented in this paper. XBT sections reveal the presence of the major features of the regional circulation, namely the eastern boundary of the Ligurian gyre, the Bonifacio gyre and the Modified Atlantic Water inflow along the Sicily coast. Twenty-two comparisons of steric heights derived from the XBT data set with concurrent realizations of single-pass altimetric heights are made. The overall correlation is around 0.55 with an RMS difference of less than 3 cm. In the Tyrrhenian Sea the spectra are remarkably similar in shape, but in general the altimetric heights contain more energy. This difference is explained in terms of oceanographic signals, which are captured with a different intensity by the satellite altimeter and XBTs, as well as computational errors. On scales larger than 100 km, the data sets are also significantly coherent, with increasing coherence values at longer wavelengths. The XBTs were dropped every 18–20 km along the track: as a consequence, the spacing scale was unable to resolve adequately the internal radius of deformation (< 20 km). Furthermore, few XBT drops were carried out in the Ligurian Sea, due to the limited north-south extent of this basin, so the comparison is problematic there. On the contrary, the major features observed in the XBT data in the Tyrrhenian Sea are also detected by TOPEX/Poseidon. The manuscript is completed by a discussion on how to integrate the two data sets, in order to extract additional information. In particular, the results emphasize their complementariety in providing a dynamically complete description of the observed structures. Key words. Oceanography: general (descriptive and regional oceanography) Oceanography: physical (sea level variations; instruments and techniques)

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.

Interannual upper ocean variability in the tropical Indian Ocean

Empirical orthogonal function (EOF) analysis is used to characterize interannual upper ocean temperature variability since mid‐1980s along two repeated XBT sections across the Indian Ocean. The mode 1 EOF loadings have deepened thermocline in the western tropical Indian Ocean and a shoal thermocline in the east/southeast Indian Ocean during the 1994 and 1997/98 Indian Ocean dipole (IOD) events. Reversed anomalies occur during 1988/89, 1996, and 1998/99. The mode 2 EOF loadings only have significant magnitudes related to the IOD, with upwelling (downwelling) along the Sumatra‐Java coast and downwelling (upwelling) in the southern subtropics leading (lagging) the IOD peaks. Lagged correlations between sea level anomaly and westward wind stress anomaly along the equatorial Indian Ocean (an IOD index) support the EOF analysis: the two EOF modes are linked by propagation and reflection of subtropical Rossby waves and equatorial Kelvin waves, which also contribute to reverse the IOD through a delayed action.

Transition regions and their role in the relationship between sea surface height and subsurface temperature structure in the Atlantic Ocean

Expendable bathythermograph (XBT) profiles and TOPEX/Poseidon altimeter data (T/P) are compared for the years 1993 through 1997 to determine how much can be understood about water column variability from XBT's given only sea height anomalies (SHA) from T/P. Our focus is on the annual cycle along two well sampled XBT sections in the Atlantic Ocean from 10°S to 40°N. Regions of transition are identified that separate the mid‐latitudes where surface buoyancy fluxes dominate the forcing of sea level, from those in the equatorial region where thermocline effects dominate. Zones of transition occur in the vicinity of troughs where small fluctuations in SHA belie the true nature of water column variability. Here, surface and thermocline variability tend to cancel each other. Thus, the character of SHA in transition regions emphasizes how important direct observations can be in interpreting satellite altimetric observations correctly when both surface and thermocline variability are important but are compensating in nature.

On the Variability of the Tropical Pacific Thermal Structure during the 1979–96 Period, as Deduced from XBT Sections

The thermal structure variability of the tropical Pacific is investigated using an objective analysis of about 250 000 temperature profiles (mainly XBT) collected during the 1979–96 time period. Mean conditions and seasonal variability are briefly described to set the context, and temperature anomalies are constructed relative to a mean seasonal cycle to focus on the ENSO (El Niño–Southern Oscillation) timescale. Heat content anomalies (0–450 m) built up in the western equatorial basin prior to the 1982–83, 1986–87, and 1997–98 El Niño events but not clearly prior to the 1991–92, 1993, and 1994–95 events, which are thus found “atypical.” Low-frequency migration of temperature anomalies located at the mean thermocline depth is evidenced eastward in the equatorial band, as well as westward along a narrow zonal band located slightly north of the mean position of the intertropical convergence zone (say, 10°–20°N). This indicates that ENSO-related temperature anomalies in the subsurface ocean are present generally in the western equatorial Pacific about 1–2 years before the appearance of temperature anomalies in the eastern equatorial Pacific. Similarly, this indicates that subsurface temperature anomalies tend to be present in the eastern Pacific basin around 14°N about 1–2 years before the appearance of temperature anomalies in the western Pacific basin at the same latitude. The likely mechanisms responsible for these migrations and the possible link between the eastward and westward migration are discussed.

Mesoscale variability in the South China Sea from the TOPEX/Poseidon altimetry data

Using wavelet transform we studied the mesoscale variability in the South China Sea (SCS) by analyzing 5-yr (October 1992 to August 1997) TOPEX/Poseidon (T/P) altimetry data. Our analysis suggests that mesoscale variability inside the SCS is weaker than that outside the SCS in the Kuroshio. It is found that despite the large temporal variation in the mesoscale variability, there are two narrow bands of significant mesoscale variability north of 10°N throughout much of the 5-yr period. The stronger one lies along the western boundary, while the weaker one is oriented in a southwest-to-northeast direction across the central SCS. In the rest of the SCS, the mesoscale variability is much weaker. In light of the numerical simulation by Metzger and Hurlburt (1996, Journal of Geophysical Research, 101, 12,331–12,352) and an XBT section along 15°N, the broad characteristic structure of the mesoscale variability indicates that the large-scale mean circulation in the SCS is primarily a cyclonic gyre north of about 10°N. In addition to the mesoscale variability, analysis of both the T/P and the XBT data indicates that there also exists significant intra-annual variability within similar geographic locations. The intra-annual variability is found to be primarily a subsurface feature with a very weak surface signature.

Some aspects of time variability of the Mediterranean Water off south Portugal

Mediterranean Water contributes significantly to the hydrological properties and circulation of the intermediate layers of the North Atlantic, yet the temporal variability of the Mediterranean Undercurrent is still not well understood, even at intra-annual time scales. The present study addresses the evolution of the temperature field in the layers of Mediterranean Water off south Portugal during one year (July 1993–July 1994), obtained with an almost weekly repetition of a high-resolution expendable bathythermograph (XBT) section in the vicinity of Portimão Canyon, during the AMUSE experiment. The information obtained with this set of data was complemented by velocity observations of the Mediterranean Undercurrent with RAFOS floats launched simultaneously with the XBT lines. The typical structure of the Mediterranean Undercurrent along the slope of the Gulf of Cadiz, consisting of a westward jet associated with the cores of the thermohaline structure, was observed in almost 60% of the 24 repetitions of the XBT section. The results show that this typical structure in the velocity was not always accompanied by a typical thermal structure. Anomalous behaviour of the Undercurrent – southward or southwestward flow or local recirculation – occurred only in winter and corresponded to some anomalies in the thermal field associated with the Mediterranean Water. The occurrence of warm features offshore of and separated from the main body of warm water found against the slope off south Portugal seemed to be connected with episodes of changes in direction and speed of the Undercurrent. Several scales of time variability within the Mediterranean Water layers were identified ranging from a week to a month. A possible seasonal trend in the temperature of these layers was apparent in the XBT data and seemed also to be found in time series obtained with moored current meters.

Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy‐resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (λ<500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long‐wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (λ>500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin‐wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal‐to‐noise ratio is highest in the western quarter of the transect. Basin‐wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9×104 m2 s−1 RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8°C at 200 m to 0.3°C at 400 m. Geostrophic volume transport, 0–800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (106 m3 s−1) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.

Lagrangian Observations of Meddy Formation during A Mediterranean Undercurrent Seeding Experiment*

Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15–20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3–7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

Seasonal evolution of upper ocean thermal structure between Tasmania and Antarctica

We describe the upper ocean thermal structure between Tasmania and Antarctica based on thirteen repeat temeprature sections occupied between 1991 and 1994. The sections cross three main fronts. The subtropical front is found between Tasmania and the South Tasman Rise in each of the sections. The subantarctic front (SAF) is composed of two parts, which have distinct thermohaline signatures and behave somewhat independently: the northern part, associated with the 6–8°C isotherms, is characterised by large meridional gradients of both temperature and salinity; the southern part is associated with a weaker meridional temperature gradient and negligible salinity gradient between the 3° and 5°C isotherms. The northern part of the SAF is located between 50°S and 51°S in each of the sections, but the position of the southern part of the SAF is more variable with time. A cold core eddy or meander is found north of the SAF throughout the 1993–1994 austral summer. The polar front (PF) is found near 53°S in all sections. Dynamic height is estimated for each of the XBT sections by exploiting the tight correlation in this region between vertically-integrated temperature and dynamic height. Dynamic height decreases relatively smoothly with latitude between 50°S and 53°S, so that the SAF, PF and the water between the two fronts forms a broad belt of eastward flow relative to a deeper level. The difference in dynamic height at the sea surface relative to 2000 m is 1.03 dyn m between 47°S and 60°S and is constant through the 1993–1994 austral summer to within the accuracy of the method (rms error ≈ 0.07 dyn m). The dynamic height expression of the cold core eddy reaches a maximum of 0.23 dyn m in February 1994. The upper 100 m of the water column warms by about 1.6°C between December and March south of 54°S, corresponding to an average warming rate of 95 W m −2. Changes in heat content at other latitudes are dominated by meridional shifts of the fronts, and no clear seasonal trend can be identified.