Cold Cyclonic Eddy Research Articles

Observation of Statistical Characteristics and Vertical Structures of Surface Warm Cyclonic Eddies and Cold Anticyclonic Eddies in the North Pacific Subtropical Countercurrent Region

Conventional wisdom about mesoscale eddies is that cyclonic (anticyclonic) eddies are commonly associated with cold(warm) surface cores. Nevertheless, plenties of surface warm cyclonic eddies (WCEs) and cold anticyclonic eddies (CAEs) in the North Pacific Subtropical Countercurrent (STCC) region are observed by a synergistic investigation based on data from satellite altimetry, microwave radiometer, and Argo float profiles in this study. The results indicate that these two types of abnormal eddies (WCEs and CAEs) are prevalent in the STCC region, comprising approximately 30% of all eddies detected via satellite observations. We then analyze their spatial-temporal distribution characteristics and composite vertical structures. A statistical comparison with surface cold cyclonic eddies (CCEs) and warm anticyclonic eddies (WAEs) reveals notable differences between the anomalous and typical eddies. Additionally, we present the composite vertical structures of temperature and salinity anomalies for the anomalous eddies across five delineated subregions within an eddy-coordinate system. Furthermore, the close relationship between these abnormal eddies and subsurface-intensified mesoscale eddies are discussed.

Characteristics of surface physical and biogeochemical parameters within mesoscale eddies in the Southern Ocean

Abstract. Using satellite sea surface temperature (SST) and chlorophyll a (Chl a) as well as observation-based reconstruction of dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2) from 1996 to 2015, we investigate the modulation mechanisms of eddies on surface physical and biogeochemical parameters in the Southern Ocean (SO). About one-quarter of eddies are observed to be “abnormal” (cold anticyclonic and warm cyclonic eddies) in the SO, which show opposite SST signatures to “normal” eddies (warm anticyclonic and cold cyclonic eddies). The study finds that the modification of abnormal eddies on physical and biogeochemical parameters is significant and differs from normal eddies due to the combined effects of eddy pumping and eddy-induced Ekman pumping. Normal and abnormal eddies have opposite DIC anomalies, contrary to the SST anomalies. Moreover, the contributions of abnormal eddies to pCO2 are about 2.7 times higher than normal eddies in regions where abnormal eddies dominate. Although Chl a anomalies in normal and abnormal eddies show similar patterns and signals, eddy-induced Ekman pumping attenuates the magnitudes of Chl a anomalies within abnormal eddies. In addition to the variation of the same parameter within different eddies, the dominant eddy-driven mechanisms for different parameters within the same kind of eddies also vary. The strength of the eddy stirring effect on different parameters is the primary factor causing these differences, attributed to variations in the magnitudes of horizontal parameter gradients. Understanding the role of abnormal eddies and the complexity of eddy-driven processes is crucial for accurately estimating the influence of mesoscale eddies on physical and biogeochemical processes in the SO, which is essential for simulating and predicting biogeochemical dynamics and carbon cycling in the region.

Interactions between cold cyclonic eddies and a western boundary current modulate marine heatwaves

Marine heatwaves are known to cause severe ecosystem damage and therefore have received attention in recent years. However, the focus has tended to be on global (surface) studies, but not coastal waters. Cyclonic eddies are important and underappreciated components in the eddy-dominated western boundary current system, but their impacts on the path of the western boundary currents have largely been unexplored. Here we show that cold cyclonic eddies can modulate the most intense coastal marine heatwaves on record inshore of the East Australian Current. We show that the marine heatwave was driven and modulated by the lateral movement of the western boundary current jet and cyclonic eddies. This study reveals that the interplay of cyclonic eddies and a western boundary current can drive coastal ocean warming, paving the way for future investigations into eddy interactions and the evolution of coastal marine heatwaves in other western boundary current regions.

The generation mechanism of cold eddies and the related heat flux exchanges in the upper ocean during two sequential tropical cyclones

The impacts of two sequential tropical cyclones (TCs), Kyarr and Maha, [from October 24 to November 06, 2019, over the Arabian Sea (AS)] on upper ocean environments were investigated using multiple satellite observations, Argo float profiles and numerical model outputs. To obtain a realistic TC strength, the Weather Research and Forecasting (WRF) model was used to reproduce Kyarr and Maha. During Kyarr and Maha, three distinct cold patches were observed at the sea surface with a maximum sea surface cooling of approximately 5°C. The comparison between WRF model simulation results and ERA5 wind field showed that the WRF model simulation indicated high simulation accuracy with respect to the SST decrease in the AS under the influence of Kyarr and Maha’s wind stress curls. Meanwhile, concentration of chlorophyll a (chl-a) and positive relative vorticity of sea surface also appeared in the three cold patch areas. Through the use of eddy detection algorithms, three mesoscale cold cyclonic eddies were identified along the track of TC Kyarr, and the locations of these cold eddies were highly correlated with three obvious negative sea surface height anomalies (SSHAs). The radii of the three cold eddies were 69 km, 50 km, and 41 km. With a focus on the thermodynamic responses of the three cold eddy fields to Kyarr and Maha, the central regions of the three cold eddies were explored. The central regions of the three cold eddies exhibited relatively shallow mixed-layer depths (MLDs) and low mixed-layer temperatures (MLTs). The depth integrated heat (DIH) content was also calculated to explore the heat flux exchanges occurring in different layers in the upper 200 m of the centre of each eddy. The results showed that DIH in each eddy centre varied by one order of magnitude, accounting for between 127.3 MJ m-2 and 1220.0 MJ m-2 of heat loss. This study suggests that the effect of long forcing time on intense positive wind stress curls can produce upwelling caused by Ekman response, which is the main influencing factor of the three cold eddies generation mechanism. At the same time, the positive relative vorticity injected into the sea surface also has some contribution. TC-induced vertical mixing and upwelling (strengthened by unstable structures inside the cold eddies) cause substantial redistribution of the DIH, and related heat flux exchanges at different layers occur in the eddy fields.

Enhanced Oceanic Environmental Responses and Feedbacks to Super Typhoon Nida (2009) during the Sudden-Turning Stage

The ocean surface and subsurface biophysical responses and their feedbacks to super typhoon Nida were comprehensively investigated based on a substantial dataset of multiple-satellite observations, Argo profiles, and reanalysis data. Nida experienced two Category 5 stages: a rapid intensification stage that was fast moving along a straight-line track, and a rapid weakening stage that was slowly moving along a sharp-left sudden-turning track. During the straight-line stage, Nida caused an average sea surface temperature (SST) cooling of 1.44 °C and a chlorophyll-a (chl-a) concentration increase of 0.03 mg m−3. During the sudden-turning stage, cyclonic sudden-turning induced a strong cold cyclonic eddy (SSHA < −60 cm) by strong upwelling, which caused the maximum SST cooling of 6.68 °C and a long-lasting chl-a bloom of 0.6 mg m−3 on the left-hand side of the track, resulting in substantial impacts on the ocean ecological environment. Furthermore, the enhanced ocean cold wake and the longer air–sea interaction in turn decreased the average inner-core SST of 4 °C and the corresponding enthalpy flux of 780 W m−2, which induced a notable negative feedback to the typhoon intensity by weakening it from Category 5 to Category 2. The left bias response and notable negative feedback are special due to sharp-left sudden-turning of typhoon. Comparing with the previously found slow translation speed (~4 m s−1) of significant ocean response, the negative feedback requires even more restriction of translation speed (<2 m s−1) and sharp sudden-turning could effectively relax restrictions by making equivalent translation speed lower and air-sea interaction time longer. Our findings point out that there are some unique features in ocean–typhoon interactions under sudden-turning and/or lingering tracks comparing with ordinary tracks.

Derivation of the thermal characteristics of mesoscale eddies

This study aims at explaining the relationship between thermodynamic characteristics and direction of rotation of mesoscale eddies (MEs). The geometric characteristics of the MEs are under the following assumptions: the structure of the MEs is symmetrical, and changes of oceanic physical variables are close to linear features in the radial direction in the ME regions. Based on these assumptions, by using primitive equations without friction under a cylindrical coordinate system, the thermodynamic characteristics of the MEs are derived, showing that the conventional relationship of warm anticyclonic eddies with high sea surface height (SSH) and cold cyclonic eddies with low SSH is not consistent with the SSH and sea surface temperature (SST) observations of eddies. The results show that the symmetrical form is an ideal approximation for the geometric characteristics of MEs. In consideration of the above assumptions, there are advantages for derivation of the characteristics of the MEs under a cylindrical coordinate.

Application of PMF receptor model merging with PAHs signatures for source apportionment of black carbon in the continental shelf surface sediments of the Bohai and Yellow Seas, China

AbstractBlack carbon (BC) and polycyclic aromatic hydrocarbons (PAHs) are byproducts generated from the incomplete combustion of organic materials, including fossil fuels and biomass. The similar production processes shared by BC and PAHs provide the possibility to infer the BC sources using the PAHs signatures. This study successfully utilized data sets of BC and PAHs analyzed from the continental shelf surface sediments of the Bohai and Yellow Seas to a standard receptor model of positive matrix factorization (PMF) to apportion the sources of BC in the sediment matrix. Results showed that combustion of fossil fuels (i.e., coal and oil/petroleum) accounted for an average level of 83 ± 5% of the total BC preserved, which was significantly higher than that from the biomass burning (17 ± 5%). The spatial distributions of the fossil BC concentrations and percentages differed significantly from those of the biomass BC, implying their different geochemical behaviors in the continental shelf regimes and further emphasizing the importance to effectively differentiate between fossil BC and biomass BC. In addition to the relative proportions of the BC subtypes (char‐BC/soot‐BC), the regional‐specific hydrodynamic conditions, including the cold cyclonic eddy, resuspension and coastal current, also exerted a significant influence on these spatial variations.

Heterotrophic prokaryote distribution along a 2300 km transect in the North Pacific subtropical gyre during a strong La Niña conditions: relationship between distribution and hydrological conditions

Abstract. The spatial distribution of heterotrophic prokaryotes was investigated during the Tokyo–Palau cruise in the western part of the North Pacific subtropical gyre (NPSG) along a north–south transect between 33.60 and 13.25° N. The cruise was conducted in three different hydrological areas identified as the Kuroshio region, the subtropical gyre area and the transition zone. Two eddies were crossed along the transect: one cold-core cyclonic eddy and one warm-core anticyclonic eddy and distributions of the heterotrophic prokaryotes were recorded. By using analytical flow cytometry and a nucleic acid staining protocol, heterotrophic prokaryotes were discriminated into three subgroups depending on their nucleic acid content (low, high and very high nucleic acid contents labelled LNA, HNA and VHNA, respectively). Statistical analyses performed on the data set showed that LNA, mainly associated with low temperature and low salinity, were dominant in all the hydrological regions. In contrast, HNA distribution seemed to be associated with temperature, salinity, Chl a and silicic acid. A latitudinal increase in the HNA / LNA ratio was observed along the north–south transect and was related to higher phosphate and nitrate concentrations. However, the opposite relationship observed for the VHNA / HNA ratio suggested that the link between nucleic acid content and oligotrophic conditions is not linear, underlying the complexity of the biodiversity in the VHNA, HNA and LNA subgroups. In the Kuroshio Current, it is suggested that the high concentration of heterotrophic prokaryotes observed at station 4 was linked to the path of the cold cyclonic eddy core. In contrast, it is thought that low concentrations of heterotrophic prokaryotes in the warm core of the anticyclonic gyre (Sta. 9) are related to the low nutrient concentrations measured in the seawater column. Our results showed that the high variability between the various heterotrophic prokaryote cluster abundances depend both on the mesoscale structures and the oligotrophic gradient.

Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

Abstract. The distribution of ultraphytoplankton was investigated in the western North Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El Niño Southern Oscillation (ENSO). Observations were conducted in a north-south transect (33.6–13.25° N) along the 141.5° E meridian in order to study the ultraplankton assemblages in various oligotrophic conditions. Analyses were performed at the single cell level by analytical flow cytometry. Five ultraphytoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes, nanoeukaryotes and nanocyanobacteria-like) defined by their optical properties were enumerated in three different areas visited during the cruise: the Kuroshio region, the subtropical Pacific gyre and a transition zone between the subtropical Pacific gyre and the Warm pool. Prochlorococcus outnumbered the other photoautotrophs in all the investigated areas. However, in terms of carbon biomass, an increase in the relative contribution of Synechococcus, picoeukaryotes and nanoeukaryotes was observed from the centre of the subtropical gyre to the Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes observed at the surface suggested an increase in nutrients likely due to the vicinity of a cold cyclonic eddy. In contrast, in the salinity front along the isohaline 35 and anticyclonic eddy located around 22.83° N, the mainly constant distribution of Prochlorococcus from the surface down to 150 m characterised the dominance by these microorganisms in high salinity and temperature zone. Results suggested that the distribution of nanocyanobacteria-like is also closely linked to the salinity front rather than low phosphate concentration. The maximum abundance of ultraphytoplankton was located above the SubTropical Counter Current (STCC) at depths > 100 m where higher nutrient concentrations were measured. Finally, comparison of the ultraphytoplankton concentrations during El Niño (from the literature) and La Niña (this study) conditions seems to demonstrate that La Niña conditions lead to higher concentrations of Synechococcus in the Subtropical gyre and a lower abundance of Synechococcus in the Kuroshio region. Our results suggest that the west part of NPSG is a complex area, where different water masses, salinity fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton assemblages in the upper layer of the water column.

Dynamics of Kuroshio path variations in a high‐resolution general circulation model

A long‐lived (2‐year) typical large meander (LM) path of the Kuroshio is reproduced in a high‐resolution general circulation model (GCM) simulation with realistic topography, and the life cycle (formation, maintenance, and disappearance) of the LM is studied. The formation of the LM is triggered by a pair of upper layer anticyclonic and cyclonic eddies on the Kuroshio, with the cyclonic one on the downstream side. Upon reaching the western side of the Shikoku Basin south of Japan these anomalies induce a lower layer anticyclonic eddy. They enhance each other while they are carried eastward by the Kuroshio. Specifically, the southward flow on the eastern side of the lower anticyclonic eddy carries the upper cold cyclonic eddy offshore, resulting in the southward deflection of the Kuroshio path (large meander). These processes can be understood as a growth of baroclinic instability. A stationary LM is established when the westward tendency, caused by the beta effect owing to the large meridional displacement, counterbalances the eastward tendency, caused by the combination of advection and vortex induction owing to the curvature of the Kuroshio path. Large‐scale disturbances from the upstream tend to strengthen the LM through baroclinic energy conversion. In contrast, small‐scale disturbances from the upstream grow locally along the meandering Kuroshio front through baroclinic energy conversion. They extract cold waters from the cold cyclonic eddy of the LM as they flow away to the east. The horizontal extent of the cold cyclonic eddy is reduced, and the meander is carried eastward since the advection effect dominates over the beta effect that weakens as a result of the smaller meridional extent.

A 3-D mesoscale map of primary production at the Antarctic Polar Front: results of a diagnostic model

A diagnostic model is established to estimate synoptically the mesoscale distribution of primary production at the Antarctic Polar Front (APF). The model domain is a 3-D box, centred at roughly 50 S and 10 E, of about 1 degree latitude and 2 degree longitude horizontal extent, and of 300 m depth. The box was surveyed in high resolution during austral summer 1995/1996 with a towed undulating vehicle and by complementary ship-based measurements. Measurements of global solar radiation, of the underwater light field, and of the chlorophyll concentration from the survey are used as input variables for the model. The model is based on photosynthesis-light relationships, with parameters taken from in vitro incubations performed during the survey. The model results show mesoscale patches of elevated primary production along a meander of the APF, and lowest production in a cold cyclonic eddy south of the front. Production is confined to a shallower depth range in the front than outside, due to self-shading effects from generally higher mixed-layer chlorophyll concentrations. Self-shading effects account for variations of the percent light depths, and of the saturation light depth, by a factor of 2 within the survey area. Primary production at the surface varies horizontally between 7 and 56 mg C m^-3 d^-1, with a mean of 26 mg C m^-3 d^-1, and vertically integrated production ranges from 295 to 975 mg C m^-2 d^-1, with an areal mean of 585 mg C m^-2 d^-1. Changes by a factor of 2 in integrated production occur on horizontal scales as small as 10 km. Production rates also differ significantly between days as a result of changes in global solar radiation.

Mesoscale frontal dynamics: shaping the environment of primary production in the Antarctic Circumpolar Current

Abstract The frontal regions of the Antarctic Circumpolar Current (ACC) differ from other parts of the ACC due to higher phytoplankton concentrations and primary production rates. We hypothesise that the enhancement of primary production results from the mesoscale frontal dynamics, in particular the cross-front circulation related to baroclinic instability. The hypothesis is corroborated by data collected in austral summer 1995/1996 at the Antarctic Polar Front between 1°W and 12°E during quasi-synoptic surveys with a measuring system combining towed and vessel-mounted instruments. Further confirmation is obtained from moored current meters and an earlier section worked across the front in austral winter 1992. The quasi-synoptic surveys cover a meander structure of the front and a cold cyclonic eddy located to its south. The highest chlorophyll concentrations, correlating with enhanced primary production rates, are found in a band of mesoscale patches aligned with the front and in a tongue extending southward from the front along the leading edge of a meander ridge. The increased chlorophyll concentrations at the meander edge are explained by confluence of surface water, which carries with it the phytoplankton grown in favourable light conditions. While mesoscale surface confluence structures the synoptic chlorophyll distribution pattern, the mean enhancement of primary production at the front can be attributed to the influence of cross-front circulation on stratification. Ageostrophic cross-front circulation related to mesoscale upwelling and downwelling was identified at a site of eddy/front interaction. Consistent with the principle of potential vorticity conservation, upwelling was found to occur on the anticyclonic, equatorward side of the jet and downwelling on the cyclonic, poleward side in the frontogenetic situation. The associated cross-front circulation is characterised by poleward motion of light water at the surface and a reversed flow of dense water at greater depth; thus it contributes to stratification and thereby to a more favourable photic environment for the phytoplankton growing in the shallower mixed layer. While the cross-front circulation varies on horizontal scales of 100 km and time scales of days to weeks, it is constrained to sites of available potential energy, i.e. fronts marked by sloping isopycnals. With mixed-layer shallowing by cross-front circulation, we suggest a mechanism to explain the enhancement of primary production by mesoscale frontal dynamics in regions and during periods of light limitation. This complements the understanding of the importance of frontal dynamics for primary production, hitherto focussed on mesoscale upwelling of nutrients in the oligotrophic regime.

Time and spatial variability of sea surface temperature in the Gulf of California

Fourteen years of satellite images (1983–1996) are used to examine the variability of sea surface temperature (SST) in the Gulf of California. The study focussed on the semiannual, annual and interannual scales and on the average. On average, SST decreases from the mouth to the head and its variability increases. The annual scale is responsible for most of the temporal variability, which oscillates in phase with minor north-south variations. The northern gulf shows the formation of warm anticyclonic eddies during winter and cold cyclonic eddies during summer. The spring transition shows a cyclonic eddy closer to the mainland side of the gulf; the autumn transition shows a not well-defined anticyclonic eddy. The SST around the island region is always colder than the rest of the gulf. The lateral variability in the central and southern regions is associated with upwelling phenomena. The semiannual and annual amplitudes increase to the north by a factor of two with respect to the southern region. On the interannual scale, the 1988–1989 and 1992–1993 events reach all the gulf. Both events appear first in the south and island regions, and the signals are more intense at the islands than the rest of the gulf. This behavior has not been reported before. The 1985, 1987 and 1990 events show a "normal" evolution, i.e., the warm waters appear in the south and gradually progress into the gulf.

Dynamics of the Brazil‐Malvinas Confluence based on inverted echo sounders and altimetry

We use data from Geosat altimeter and from 10 inverted echo sounder (IES) moorings deployed in the SW Atlantic Ocean off the Argentine continental shelf to investigate several aspects of the dynamics of the upper layer in the Brazil‐Malvinas Confluence region. We use the altimeter data to estimate the sea height anomalies at each IES location and use the IES data to compute the upper layer thickness, taken in this work to go to the depth of the 8°C isotherm. We first discuss the sea height and upper layer thickness variations caused by the passage of the Brazil Current, Malvinas Current, and warm anticyclonic and cold cyclonic eddies. We introduce a two‐layer model in which we decompose the sea height into its baroclinic and barotropic contributions. We then propose a method to monitor the thickness of the upper layer and the barotropic and baroclinic transports as a function of the sea height anomalies and the statistics of the upper layer thickness and reduced gravity for the region. We compute the reduced gravity values from the slope of a linear fit between the sea height anomalies and the upper layer thicknesses. We estimate the reduced gravity values for this region to range from 0.005 to 0.011 m s−2. We also estimate the mean barotropic sea height difference using two methods: conservation of mass and conservation of potential vorticity. Finally, we compute the time series for the baroclinic and barotropic transports during the Geosat Exact Repeal Mission time period. Our results suggest that the mean baroclinic transport in the upper layer decreases from 12 Sv at around 35°S to 7 Sv at 37°S. Our results also indicate that there is a significant barotropic contribution to the upper layer transport in the confluence region.

Eddies southwest of the Denmark Strait

Numerous cold cyclonic eddies (20–40 km diameter) have been observed in satellite IR imagery (NOAA-11, Ch 3 and 4) along the edge of the East Greenland Shelf to the southwest of the Denmark Strait. The eddies move southwestward along the southeast edge of the East Greenland Current. The path of the eddies is coincident with the location of numerous deep current meter observations from previous surveys during which the strong pulses of overflow water (recorded speeds of over 150 cm s−1) had spectra peaked at periods of 1.5−2.5 days. It is suggested that the pulses are the signal observed from energetic, propagating cyclonic vortices extending up to the surface layer and formed by vortex stretching associated with the sinking of dense overflow water downstream from the sill of the Denmark Strait and along the shelf slope. The signature of the vortices at the sea surface appears in the IR imagery as a cold cyclonic eddy often exhibiting a pronounced spiral configuration. Because the entire water column in the slope region would be subjected to strong mixing by the vortices, considerable modification of not only the cold overflow water but also the intermediate and surface layers would occur.

Lower continental slope cyclonic eddies in the central Gulf of Mexico

Current meters, an inverted echo sounder, hydrography, drifters, and satellite imagery are used to characterize relatively small (100‐ to 150‐km diameter) cold cyclonic eddies in the central basin and on the lower Louisiana slope of the Gulf of Mexico. These cyclones are shown to be long‐lived (6 months or more), have limited movements when compared with Loop Current anticyclones, and be fairly vigorous, with upper layer currents of 30–50 cm s−1. They usually have only small temperature differences with surrounding water masses in the upper 50 to 100 m of the water column and are therefore not readily apparent in satellite thermal imagery. The largest isotherm displacements occur over depths of 200–800 m. In one case, a cyclone was traced from the deep basin near 92°W to the northern slope as a major anticyclone propagated southwestward through the gulf. The presence of cyclones and anticyclones on the Louisiana slope is consistent with observed current meter measurements in the upper half of the water column that have long (several months) periods and approximately equal variances in the cross‐slope and along‐slope directions.

Response of the Eastern Tropical Pacific to Meridional Migration of the ITCZ: The Generation of the Costa Rica Dome

Abstract A regional ocean circulation model with fine horizontal resolution has been developed in order to obtain a coherent seasonal picture of the eastern tropical Pacific off Central America. The Costa Rice Dome with a cyclonic circulation grows rapidly in late spring off the Gulf of Papagayo and matures in summer and early fall in accord with the northward migration of the ITCZ. In winter strong northern converging in the southernmost ITCZ from three passes in Central America excite warm anticyclones nonlinear eddies confined in the upper layer. Those anticyclones are identified as the intermediate geostrophic eddies by Matsuura and Yamagata. The Costa Rica Dome is eroded in winter and early spring by the westward propagating warm anticyclones but, at the same time, a new embryo of the dome begins to evolve from the westward propagating cold cyclonic eddy excited off the Gulf of Papagayo by the northers. The Costa Rica Dome can be maintained by the winds with the cyclonic wind stress curl in summer on...

Transports, frontal motions and eddies at the Brazil-Malvinas currents confluence

Three inverted echo sounders were deployed to study the dynamics of the western South Atlantic Brazil-Malvinas Confluence region. From the original travel time data obtained with the sounders, the dynamic height of the surface relative to 800 m at the three locations was obtained and geostrophic velocities between sites calculated. The time-averaged geostrophic velocity of the Brazil Current (relative to 800 m) during the observed period is 35 cm s −1 (±2.2 cm s −1) and the mean transport relative to 800 m is 11 Sv. For the Malvinas Current return flow, the mean geostrophic velocity is 9 cm s −1 (±2.0 cm s −1) and the mean associated transport 3.5 Sv. Cold intrusions are observed in offshore records during the 17-month recorded period, some of them associated with cold cyclonic eddies with available potential energy of 6.3 × 10 15 J. Using a simple model the location of the thermal front is obtained from the time series of the depth of the thermocline. Results show that the main motion of the front is an east-west displacement for distances of about 100 km; the observed period occurs with a periodicity of 12 months and is related to a variability in the latitude of maximum northward penetration of the Malvinas Current. Motions of the front, with a period of 1–2 months, are probably related to a north-south variation of the latitude of return of the Brazil Current.

A continental shelf upwelling event off Vancouver Island as revealed by satellite infrared imagery

A series of nine relatively cloud-free infrared satellite images, of the coastal ocean off Vancouver Island, reveals the evolution of sea-surface temperature patterns during a 16-day period of upwelling favorable winds in the summer of 1980. Early in the upwelling event, the cold water in the north was restricted to a narrow band, while in the south cold surface water extended out to the continental shelf break. This southern feature is believed to be an expression of a semipermanent, cold cyclonic eddy (Freeland and Denman, 1982). As upwelling continued, the cold water boundary propagated offshore at about 10 km/day eventually passing beyond the shelf break. Short-lived (2-3 days) meanders were observed in the northern front with length scales consistent with variations in local bottom topography and coastline irregularities. After wind reduction, the coldest band parted the coast and propagated offshore.