Current Meter Moorings Research Articles

Mixed Rossby Gravity Waves at Middepths of the Equatorial Indian Ocean

Abstract This study examines mixed Rossby gravity (MRG) waves at middepths (500–2000 m) of the Indian Ocean, using multiyear velocity time series obtained from current-meter moorings at 77°, 83°, and 93°E along the equator over the period 2000–19. These data are analyzed in combination with a high-resolution wind-forced ocean general circulation model. The spectrum of observed meridional velocity showed elevated energy over a wide range of periods from about 10 to 100 days with the spectral peak at a period of about 30 days. The model was able to simulate the characteristics of the observed spectrum. Further diagnostics determined that the detected variability is generally consistent with theoretical MRG waves in a resting ocean. Statistical analysis and a model sensitivity experiment identified distinct variations at three periods, where meridional velocity has sizable energy. The 14-day variability is wind driven and has a long zonal (∼3300 km) and vertical (∼4200 m) wavelength. The 28-day variability is excited by the dynamical instability of the background flow in the equatorial western Indian Ocean near the surface and propagates to the study area. It is characterized by a shorter zonal (∼1100 km) and vertical (∼2800 m) wavelength compared to 14-day variability. The 43-day variability has a zonal wavelength (∼820 km) comparable to the 28-day variability but does not show the tendency of propagation and is likely generated in situ through nonlinear interactions. These results show that various processes contribute to the excitation of MRG waves at middepths of the Indian Ocean. Significance Statement Mixed Rossby gravity (MRG) waves are a prominent mode of intraseasonal variability in tropical oceans. They are of climatic importance owing to their effects on turbulent mixing in the deep sea and how they affect the large-scale ocean circulation. This study examines MRG waves at the middepths of the equatorial Indian Ocean using a unique set of in situ current-meter measurements and a high-resolution ocean general circulation model. We describe characteristics such as horizontal and vertical wavelengths, frequency spectra, dispersion properties, and energy sources. Results show that wind forcing at the sea surface, dynamical instability of the background flow near the surface in the western Indian Ocean, and nonlinear interactions generate MRG waves at distinct periods and wavelengths.

Spatially Coherent Intraseasonal Velocity Fluctuations on the Western Antarctic Peninsula Shelf

AbstractWe analyze several year‐long moored current meter records collected on the midshelf and shelf break of the western Antarctic Peninsula in order to describe the major features of the intraseasonal circulation. An empirical orthogonal function (EOF) analysis of all current meter records in any given year reveals that over 49% of the circulation variance follows a spatially coherent, nearly depth‐invariant anticyclonic cell in the downstream vicinity of Marguerite Trough, qualitatively consistent with flow‐topography interaction with the canyon. An analysis of kinetic energy spectra supports the interpretation that the shelf break current is diverted shoreward at the location of the canyon and generates substantial counterclockwise energy at periods shorter than ~10 days as the flow is compressed by a downstream bank. The circulation is well correlated with the long‐shore wind stress both local and remote. Composite velocity profiles under upwelling and downwelling winds show that the change in the barotropic current under different wind states is comparable to or larger than the shear of the mean profile in all years. The barotropic velocity fluctuations are generally not coherent with subpycnocline heat content.

Latitudinal Structure of Solitons in the South China Sea

AbstractFour current-meter moorings and 12 pressure sensor–equipped inverted echo sounders (PIES) were deployed during summer 2011 in the South China Sea. The goal of the experiment was to obtain synoptic observations of the large-amplitude nonlinear internal waves from the near field to the far field as they propagated west-northwest across the sea. The program was unique because it was the first to observe the latitudinal variability of the wave crests in addition to the transformations along a single east–west transect. The waves were strongest down the center of the PIES array along roughly 20°45′N and were weaker off axis in both directions. Both a-waves and b-waves arrived earlier in the south than the north, but with different lag times indicating different propagation directions and therefore different sources. The waves were classified by their arrival patterns and source locations and not by their amplitude or packet structure. The Stanford Unstructured Nonhydrostatic Terrain-Following Adaptive Navier–Stokes Simulator (SUNTANS) model, calibrated against the array, showed that the a-waves developed out of the internal tide spawned in the southern portion of the Luzon Strait and the b-waves originated in the north. The northern tides were refracted and suffered large dissipative losses over the shallow portion of the western ridge, whereas the southern tides propagated west-northwest unimpeded, which resulted in a-waves that were larger and appeared sooner than the b-waves. The results were consistent with previous observations that can now be understood in light of the full three-dimensional structure of the internal waves and tides in the northeastern South China Sea.

Large dunes on the outer shelf off the Zambezi Delta, Mozambique: evidence for the existence of a Mozambique Current

The existence of a continuously flowing Mozambique Current, i.e. a western geostrophic boundary current flowing southwards along the shelf break of Mozambique, was until recently accepted by oceanographers studying ocean circulation in the south-western Indian Ocean. This concept was then cast into doubt based on long-term current measurements obtained from current-meter moorings deployed across the northern Mozambique Channel, which suggested that southward flow through the Mozambique Channel took place in the form of successive, southward migrating and counter-clockwise rotating eddies. Indeed, numerical modelling found that, if at all, strong currents on the outer shelf occurred for not more than 9 days per year. In the present study, the negation of the existence of a Mozambique Current is challenged by the discovery of a large (50 km long, 12 km wide) subaqueous dune field (with up to 10 m high dunes) on the outer shelf east of the modern Zambezi River delta at water depths between 50 and 100 m. Being interpreted as representing the current-modified, early Holocene Zambezi palaeo-delta, the dune field would have migrated southwards by at least 50 km from its former location since sea level recovered to its present-day position some 7 ka ago and after the former delta had been remoulded into a migrating dune field. Because a large dune field composed of actively migrating bedforms cannot be generated and maintained by currents restricted to a period of only 9 days per year, the validity of those earlier modelling results is questioned for the western margin of the flow field. Indeed, satellite images extracted from the Perpetual Ocean display of NASA, which show monthly time-integrated surface currents in the Mozambique Channel for the 5 month period from June–October 2006, support the proposition that strong flow on the outer Mozambican shelf occurs much more frequently than postulated by those modelling results. This is consistent with more recent modelling studies comparing the application of slippage and non-slippage approaches—they suggest that, when applying partial slippage, a western boundary current can exist simultaneously with the southward migrating eddies. Considering the evidence presented in this paper, it is concluded that a quasi-persistent, though seasonally variable Mozambique Current does exist.

The East Caledonian Current: A Case Example for the Intercomparison between AltiKa and In Situ Measurements in a Boundary Current

ABSTRACTThis paper presents an assessment of SARAL/AltiKa satellite altimeter for the monitoring of a tropical western boundary current in the south-western Pacific Ocean: the East Caledonian Current. We compare surface geostrophic current estimates obtained from two versions of AltiKa along-track sea level height (AVISO 1 Hz and PEACHI 40 Hz) with two kinds of dedicated in situ datasets harvested along the satellite ground tracks: one deep-ocean current-meter mooring deployed in the core of the boundary current and five glider transects. It is concluded that the AltiKa-derived current successfully captures the velocity of the boundary current, with a standard error of 11 cm/s with respect to the in situ data. It also appears important to reference AltiKa sea level anomaly to the latest mean dynamic topography available in our area. Doing so, Ka-band altimetry provides a satisfactory representation of the western boundary current. Thereby, it usefully contributes to observing its variability in such a remote and under-observed ocean region. However, the rather long repeat period of SARAL (35 days) in comparison to the high frequency variability seen in the flow velocity of the boundary current calls for a combined use of SARAL with the other satellite altimetry missions.

Spatiotemporal current variation of coastal-trapped waves west of the Noto Peninsula measured by using fishing boats

Spatiotemporal current variations of coastal-trapped waves (CTWs) were investigated by using a current dataset obtained from daily fishing operations west of the Noto Peninsula (NTP), Japan. Cross-shore lines located in southern, middle, and northern parts of the west coast of the NTP were designed to detect characteristics of CTWs with a time interval of a few days and about 5-km resolution in the cross-shore direction. Accuracy validation by using moored current meter data and sea level data demonstrated that the established dataset expresses accurate variations with periods of several days. The generation and propagation of a CTW event associated with a low-pressure zone passing north of the study area in late May 2010 were analyzed. Along-shore currents with the coast on the right strengthened in every line simultaneously with the domination of the southerly wind, and then weakened in order from south to north simultaneously with weakening of the southerly wind. Although the along-shore currents of the CTWs linearly decreased heading offshore along the south and middle lines, these currents broadened within about 50km from the coast along the north line, with small variations in the cross-shelf direction, with an increase in shelf width. These generation, propagation and current structure characteristics are clarified and interpreted by the characteristics of the estimated possible CTWs west of the NTP and numerical experiments, which reproduce wind-induced freely propagating CTWs. A change in the propagation characteristics and the structure of the CTWs associated with bottom topography indicates the possibility that adjustments can occur on the order of a few dozen kilometers.

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

Abstract The first multiyear continuous time series of Antarctic Circumpolar Current (ACC) baroclinic transport through Drake Passage measured by moored observations is presented. From 2007 to 2011, 19 current- and pressure-recording inverted echo sounders and 3 current-meter moorings were deployed in Drake Passage to monitor the transport during the cDrake experiment. Full-depth ACC baroclinic transport relative to the bottom has a mean strength of 127.7 ± 1.0 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard deviation of 8.1 Sv. Mean annual baroclinic transport is remarkably steady. About 65% of the baroclinic transport variance is associated with time periods shorter than 60 days with peaks at 20 and 55 days. Nearly 28% of apparent energy in the spectrum computed from transport subsampled at the 10-day repeat cycle of the Jason altimeter results from aliasing of high-frequency signals. Approximately 80% of the total baroclinic transport is carried by the Subantarctic Front and the Polar Front. Partitioning the baroclinic transport among neutral density γn layers gives 39.2 Sv for Subantarctic Surface Water and Antarctic Intermediate Water (γn < 27.5 kg m−3), 57.5 Sv for Upper Circumpolar Deep Water (27.5 < γn < 28.0 kg m−3), 27.7 Sv for Lower Circumpolar Deep Water (28.0 < γn < 28.2 kg m−3), and 3.3 Sv for Antarctic Bottom Water (γn > 28.2 kg m−3). The transport standard deviation in these layers decreases with depth (4.0, 3.1, 2.1, and 1.1 Sv, respectively). The transport associated with each of these water masses is statistically steady. The ACC baroclinic transport exhibits considerable variability and is a major contributor to total ACC transport variability.

Time-Mean Flow as the Prevailing Contribution to the Poleward Heat Flux across the Southern Flank of the Antarctic Circumpolar Current: A Case Study in the Fawn Trough, Kerguelen Plateau

Abstract The major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task.

Atmospherically Forced Exchange through the Bab el Mandeb Strait

Abstract The exchange between the Red Sea and the Indian Ocean on synoptic time scales (days to weeks) is investigated using moored current meter data collected in the strait of Bab el Mandeb from June 1995 to November 1996. Transport variations through the strait on these time scales can reach amplitudes of up to 0.6 Sv (1 Sv ≡ 106 m3 s−1), or nearly twice as large as the mean rate of exchange through the strait driven by annual evaporation over the Red Sea. The synoptic transport variability appears to be driven by two primary forcing mechanisms: 1) local wind stress variability over the strait and 2) variation in the large-scale barometric pressure over the Red Sea. Simple models of the forced response are developed and are shown to reproduce the essential features of the observations. The response to barometric pressure forcing over the Red Sea is fundamentally barotropic, whereas the response to along-strait winds is barotropic at high frequencies and tends toward a two-layer exchange at low frequencies. The responses to both types of forcing show enhanced amplitude at the Helmholtz resonance frequency for the Red Sea, which occurs at a period of about 5 days. A linear two-layer model, incorporating both types of forcing and a reasonable frictional parameterization, is shown to account for about 70% of the observed transport variance within the strait.

Observation of Semi-diurnal Internal Tides and Near-inertial Waves at the Shelf Break of the East China Sea

Semi-diurnal internal tides and near-inertial waves are investigated using moored current meter measurements at four sites along the shelf break of the East China Sea during August 1987 and May-June 1988. Each mooring is equipped with four current meters spanning from near surface to near bottom. Spectral analyses of all current data reveal dominant spectra at the semi-diurnal frequency band, where the upper and lower current measurements show out-of-phase relationship between them with significant coherences. These are consistent with typical characteristics of the first-mode semi-diurnal internal tide. Strong intensification of the near-bottom baroclinic currents is observed only at one site, where the ratio of the bottom slope to the slope of the internal-wave characteristics at the semi-diurnal frequency is close to unity. An energetic near-inertial wave event is observed during the first half of May-June 1988 observation at two mooring sites. Rotary spectra reveal that the most dominant signal is clockwise rotating motion at the near-inertial frequency band. Upward phase and downward energy propagations, shown in time-depth contour plots of near-inertial bandpass filtered currents, are confirmed by cross correlations between the upper- and lower-layer current measurements. The upward-propagating phase speed is estimated to be about 0.13 cm <TEX>$s^{-1}$</TEX> at both sites. Significant coherences and in-phase relationships of near-inertial currents at the same or similar depths between the two sites are observed in spite of their long distance of about 110 km.

Strong export of Antarctic Bottom Water east of the Kerguelen plateau

Deep western boundary currents east of the Antarctic Peninsula and the Kerguelen plateau are important pathways for transporting deep Antarctic water masses to the global ocean. An array of moored current meters, used to quantify the water transport in this system, reveals a flow that is stronger than any measured in a deep western boundary current at similar depths so far. The primary paths for the transport of Antarctic Bottom Water from the Southern Ocean into the global ocean are the deep western boundary currents east of the Antarctic Peninsula and the Kerguelen plateau1. Previous ship-based observations documented distinct water properties and velocities associated with a deep western boundary current in the Kerguelen region2,3,4,5,6,7, but the mean flow is as yet unconstrained. Here we report measurements from a coherent array of eight current-meter moorings that reveal a narrow and intense equatorward flow extending throughout the water column just east of the Kerguelen plateau. Velocities averaged over two years exceed 20 cm s−1 at depths of about 3,500 m, the strongest mean deep western boundary current flow yet observed at similar depths. We estimate the mean equatorward transport of water colder than 0 ∘C at 12.3±1.2×106 m3 s−1, partially compensated by poleward flow. We also estimate the net equatorward flow of water colder than 0.2 ∘C at about 8×106 m3 s−1, substantially higher than the 1.9×106 m3 s−1 reported from the boundary current that carries dense water from the Weddell Sea into the Atlantic Ocean north of the Falkland plateau8. We conclude that the Kerguelen deep western boundary current is a significant pathway of the global ocean’s deep overturning circulation.

The Deep Western Boundary Current at Cape Farewell: Results from a Moored Current Meter Array

Abstract An analysis is made of data from 30 Aanderaa recording current meters (RCMs) set on nine moorings located east of Cape Farewell, the southern tip of Greenland. The purpose of the measurements was to allow for the estimation of transport in the deep western boundary current (DWBC) below a depth of about 1500 m. The records commenced in September 2005 and lasted from 9.5 to 11.5 months. After calibration of the raw data, 12-h averages of temperature and current were derived and the latter employed to estimate the flow across and along the array direction. The 9.5-month average transport of water colder than 3°C was found to be 7.8 Sv (1 Sv ≡ 1 × 106 m3 s−1) with a standard error of 0.8 Sv. For water denser than σθ = 27.85 kg m−3, the transport is calculated as 4.5 Sv. Whether either of these values is significantly different from comparable measurements made 500 km upstream cannot be determined. In marked contrast, for σθ &amp;gt; 27.8 kg m−3, the transport is estimated as only 9.0 Sv, smaller than the widely accepted value of 13 Sv for nearby measurements made in 1978. A reevaluation of the calculations and assumptions made then allows one to determine the uncertainty of the earlier estimate and thereby conclude that the difference between the previous and present measurements is significant, that is, that the transport has decreased between 1978 and 2005–06. A weakening of the transport during the 9.5-month period is also observed, along with a warming and an increase in salinity in the core of the DWBC. These latter changes are shown to be consistent with interannual variability rather than a long-term trend.

Observed impact of upwelling events on water properties and biological activity off the southwest coast of New Caledonia

The upwelling events that follow strong trade wind episodes have been described in terms of their remarkable signature in the sea surface temperature southwest off New Caledonia. Upwelling brings deeper, and colder waters to the surface, causing 2–4°C drops in temperature in a few hours, followed by a slower relaxation over several days. Upwelling may sporadically bring nutrients to the surface under certain conditions, and increase the biological productivity. Two multidisciplinary hydrographic cruises allow the impact of upwelling on the chemical and biological properties of the water to be documented. Both cruises took place in austral summer (December 2004 and December 2005), but the first cruise occurred during a strong upwelling event, while the second cruise occurred in calm conditions. The water properties and planktonic composition show important contrasts, with a strong southeastward current (the “ALIS current of New Caledonia”) competing with the upwelling system. Our analysis suggests that, while observed productivities are far less than those of typical upwelling systems, some wind events in New Caledonia may contribute to biological activity. A currentmeter mooring, deployed during the second cruise, documents the ocean response to a changing wind field and the local impact of upwelling on currents and temperatures on the water column. The results are discussed, with the help of climatology, Argo float profiler data, satellite data and of a high-resolution numerical simulation.

Topographic Rossby waves in the Gulf of Mexico

Observations of topographic Rossby waves (TRW), using moored current meters, bottom pressure gauges, and Lagrangian RAFOS floats, are investigated for the deep basin of the Gulf of Mexico. Recent extensive measurement programs in many parts of the deep gulf, which were inspired by oil and gas industry explorations into ever deeper water, allow more comprehensive analyses of the propagation and dissipation of these deep planetary waves. The Gulf of Mexico circulation can be divided into two layers with the ∼800–1200 m upper layer being dominated by the Loop Current (LC) pulsations and shedding of large (diameters ∼300–400 km) anticyclonic eddies in the east, and the translation of these LC eddies across the basin to the west. These processes spawn smaller eddies of both signs through instabilities, and interactions with topography and other eddies to produce energetic surface layer flows that have a rich spectrum of orbit periods and diameters. In contrast, current variability below 1000 m often has the characteristics of TRWs, with periods ranging from ∼10–100 days and wavelengths of ∼50–200 km, showing almost depth-independent or slightly bottom intensified currents through the weakly stratified lower water column. These fluctuations are largely uncorrelated with simultaneous upper-layer eddy flows. TRWs must be generated through energy transfer from the upper-layer eddies to the lower layer by potential vorticity adjustments to changing depths of the bottom and the interface between the layers. Therefore, the LC and LC eddies are prime candidates as has been suggested by some model studies. Model simulations have also indicated that deep lower-layer eddies may be generated by the LC and LC eddy shedding processes. In the eastern gulf, the highest observed lower-layer kinetic energy was north of the Campeche Bank under the LC in a region that models have identified as having strong baroclinic instabilities. Part of the 60-day TRW signal propagates towards the Sigsbee Escarpment (a steep slope at the base of the northern continental slope), and the rest into the southern part of the eastern basin. Higher energy is observed along the escarpment between 89°W and 92°W than either under the northern part of the LC or further south in the deep basin, because of radiating TRWs from the western side of the LC. In the northern part of the LC, evidence was found in the observations that 20–30-day TRWs were connected with the upper layer through coherent signals of relative vorticity. The ∼90° phase lead of the lower over the upper-layer relative vorticity was consistent with baroclinic instability. Along the Sigsbee Escarpment, the TRWs are refracted and reflected so that little energy reaches the lower continental slope and a substantial mean flow is generated above the steepest part of the escarpment. RAFOS float tracks show that this mean flow continues along the escarpment to the west and into Mexican waters. This seems to be a principal pathway for deepwater parcels to be transported westward. Away from the slope RAFOS floats tend to oscillate in the same general area as if primarily responding to the deep wave field. Little evidence of westward translating lower-layer eddies was found in both the float tracks and the moored currents. In the western gulf, the highest deep energy levels are much less than in the central gulf, and are found seaward of the base of the slope. Otherwise, the situation is similar with TRWs propagating towards the slope, probably generated by the local upper-layer complex eddy field, being reflected and forcing a southward mean flow along the base of the Mexican slope. Amplitudes of the lower-layer fluctuations decay from the northwest corner towards the south.

Is the meander growth in the Brazil Current system off Southeast Brazil due to baroclinic instability?

Temporally-growing frontal meandering and occasional eddy-shedding is observed in the Brazil Current (BC) as it flows adjacent to the Brazilian Coast. No study of the dynamics of this phenomenon has been conducted to date in the region between 22 ° S and 25 ° S. Within this latitude range, the flow over the intermediate continental slope is marked by a current inversion at a depth that is associated with the Intermediate Western Boundary Current (IWBC). A time series analysis of 10-current-meter mooring data was used to describe a mean vertical profile for the BC-IWBC jet and a typical meander vertical structure. The latter was obtained by an empirical orthogonal function (EOF) analysis that showed a single mode explaining 82% of the total variance. This mode structure decayed sharply with depth, revealing that the meandering is much more vigorous within the BC domain than it is in the IWBC region. As the spectral analysis of the mode amplitude time series revealed no significant periods, we searched for dominant wavelengths. This search was done via a spatial EOF analysis on 51 thermal front patterns derived from digitized AVHRR images. Four modes were statistically significant at the 95% confidence level. Modes 3 and 4, which together explained 18% of the total variance, are associated with 266 and 338-km vorticity waves, respectively. With this new information derived from the data, the [Johns, W.E., 1988. One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras. Dyn. Atmos. Oceans 11, 323–350] one-dimensional quasi-geostrophic model was applied to the interpolated mean BC-IWBC jet. The results indicated that the BC system is indeed baroclinically unstable and that the wavelengths depicted in the thermal front analysis are associated with the most unstable waves produced by the model. Growth rates were about 0.06 (0.05) days −1for the 266-km (338-km) wave. Moreover, phase speeds for these waves were low compared to the surface BC velocity and may account for remarks in the literature about growing standing or stationary meanders off southeast Brazil. The theoretical vertical structure modes associated with these waves resembled very closely to the one obtained for the current-meter mooring EOF analysis. We interpret this agreement as a confirmation that baroclinic instability is an important mechanism in meander growth in the BC system.

Deep countercurrent beneath the Kuroshio in the Okinawa Trough

The Okinawa Trough is geographically separated into the northern and southern parts by the Kerama Gap, and the northern part is composed of the northern, central and southern subbasins formed by bights of the continental slope. Previous observations have indicated that a deep countercurrent is present beneath the Kuroshio on the continental slope in the northern Okinawa Trough. However, its persistence over time and its spatial structure over the entire basin have not been clarified to date. The present study examines moored current meter data on the continental slope in the southern and central subbasins of the northern Okinawa Trough for November 2004 to November 2006. Deep flows on the continental slope have a clear seasonality in the southern subbasin; eddy motions due to Kuroshio meanders are organized into a persistent countercurrent below the Kuroshio in the winter‐spring period. During this time, high‐frequency Kuroshio meanders with periods near 10 d are likely to diminish in the southern subbasin while low‐frequency Kuroshio meanders with periods of 1–3 months tend to dominate in the northern subbasin. In addition, a high‐resolution ocean general circulation model is used to explore the deep flow field over the entire Okinawa Trough. The model indicates that the deep flow field is stable in the southern Okinawa Trough over the year, whereas it is unstable in the northern Okinawa Trough particularly in the winter‐spring period. This results in a persistent countercurrent driven by cyclonic eddies on the continental slope in the northern Okinawa Trough.

Observability of the Irminger Sea circulation using variational data assimilation

This paper explores the prospects for constraining circulation and stratification estimates in the Irminger Sea and Denmark Strait using variational data assimilation. A regional circulation model with 9 km horizontal grid spacing, seven vertical levels and open boundaries is used. The circulation it simulates is realistic in several important respects and includes a chaotic mesoscale eddy field. Pulsation of the outflow of dense water through the Denmark Strait is also reasonably realistic, although entrainment and mixing of this water downstream is crudely represented. Identical twin assimilation experiments are performed to address the observability of the system with the existing in situ and satellite observing network in the absence of systematic errors. That is, the constraints exerted on the circulation and stratification by the data assimilation system are quantified. Synthetic data come from a reasonably realistic observing network that includes in situ hydrographic and current measurements, and remotely sensed surface temperature and sea level. Over a 15-day assimilation period in summer, the system fits all the observations to the relevant noise level, largely by correcting the initial conditions. Only modest changes in the time-dependent surface forcing and negligible changes to the open boundary conditions are needed. The system can accurately estimate sea surface height and sea surface temperature, correcting errors in the locations of surface fronts and eddies. The mid-depth salinity is almost completely unconstrained, however, whereas the velocity fields are somewhat constrained by the assimilation, especially by the remote-sensing data. Assimilation of satellite data enables us to accurately track the phase and amplitude of the overflow variability at the exit of Denmark Strait in the identical twin experiments. At other model outflow transects the assimilated solution is contaminated with spurious inertial oscillations. This is true even at a section offshore of Angmagssalik where a current-meter mooring array exists (in our experiments and in the real ocean). Nevertheless, improvement of the model dense outflow near the moorings is significant and propagates several hundred kilometres upstream and downstream. Copyright © 2006 Royal Meteorological Society

Impact of open ocean influence on a coastal model: The 1997–1998 El Niño off central California

Extensive moored current meters, atmospheric observations, and hydrographic surveys were obtained off central California during the 1997–1998 El Niño. In this study the temperature observations from current meter moorings and hydrographic surveys are assimilated into a coastal ocean model to study the seasonal circulation in the Santa Barbara Channel–Santa Maria Basin (SBC‐SMB). Mean coastal circulation was dominated by a poleward jet in fall 1997 at the peak of the El Niño but returned to the normal surface convergent pattern in fall 1998. The data‐assimilated model reproduces well the observed flow patterns in both years. The interannual flow variability is examined through sensitivity analysis of the effects of local forcing and open ocean influence. It is shown that the poleward jet is associated with enhanced California Countercurrent during El Niño, while the surface convergence is driven by local wind stress curl. Taking advantage of the extensive data set, the climatology is compared with the synoptic states. It is found that the climatology could cause significant negative impact.

Near-surface circulation in the southern Gulf of Cádiz

Abstract We have used several data sets (expandable bathythermograph sections, buoy trajectories, current-meter moorings, and surface wind stress) to investigate the temporal variation of the upper-thermocline (North Atlantic Central Waters) circulation patterns in the southern Gulf of Cadiz. The main data set consists of eight expandable bathythermograph sections (two per season) running between the Strait of Gibraltar and Cape Beddouza, just offshore the continental slope and approximately parallel to the Morocco coastline. A salinity–temperature polynomial, obtained from historical conductivity–temperature-depth data, is used to infer dynamic properties (salinity, density, dynamic height, geostrophic velocity and transport). Dynamic heights are calculated referred to the 1027.25 kg m - 3 neutral density surface, which we justify is a good reference level for this section. These observations are then combined with a process-oriented, one-layer, quasigeostrophic model for the northeastern corner of the North Atlantic subtropical gyre with modified eastern boundary conditions. The results indicate the existence of about 1 Sv onshore transport all year long into a 300 km long coastal transition zone off Morocco, which must follow south as a narrow current. During winter the flow is zonal towards the slope and recirculates south as a very narrow jet of less than 100 km width. During summer the flow probably becomes more intense but is deflected southeastward before reaching the slope, such that the onshore geostrophic transport into the coastal transition zone decreases and the recirculating band widens.

Tidal and near‐inertial peak variations around the diurnal critical latitude

Spectra from historic long‐term open‐ocean moored current meter data between latitudes 0° &lt; ∣ϕ∣ &lt; 45° reveal a significant drop in semidiurnal tidal band (D2) energy by ∼50% at ∣ϕ∣ ≈ 25–27°, whilst the peak near the local inertial frequency f is increased by a factor of ∼10 up to the level of D2‐energy at ∣ϕ∣ ≈ 28–30°, where f coincides with diurnal frequencies. The increase in f‐energy is accompanied by a red‐shift of the peak frequency to 0.97 ± 0.01f, or a poleward spreading of enhanced energy. This contrasts with more common blue‐shift. The enhancement may be the result of sub‐harmonic instability, as supported by sparse significant bicoherence at half‐D2, although i) systematic enhancement of diurnal tidal frequencies, notably M1, was not observed, ii) the latitudes of low D2‐energy and high f‐energy do not coincide. This may be due to a mix of coupled and independent waves, whilst the poleward trapping of sub‐f energy suggests non‐traditional effects.