bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical modeling of the mean exchange through the Strait of Gibraltar
Authors: Sannino G. , Bargagli A. , Artale V.
Published: October 2002

The three‐dimensional Princeton Ocean Model is used to investigate the mean flow and the hydraulics regime in the Strait of Gibraltar. The model makes use of a coast‐following curvilinear orthogonal grid that includes the Gulf of Cadiz and the Alboran Sea, with very high resolution in the strait (∼500 m). A lock‐exchange initial condition is used: The western part of the model domain is filled with Atlantic water, whereas the eastern part is filled with Mediterranean water. A conservative, nondiffusive, and nondispersive numerical scheme for the horizontal tracers advection has been implemented to simulate the free‐flow adjustment for the lock‐exchange initial condition. Predicted velocities, interface depth, water, and salinity transports are comparable with observed data. Model results describe circulation of the Mediterranean outflow in detail and demonstrate the presence of a mixing layer between the Atlantic and Mediterranean water. The hydraulic regime is analyzed, calculating the composite Froude number for three layers within the strait. Results indicate the presence of a hydraulic control at Camarinal Sill and in the northern part of Tarifa Narrows. In order to understand if the Strait of Gibraltar is in the submaximal or maximal regime we have formulated a cross‐strait mean composite Froude number. This formulation allowed us to limit the values of the composite Froude number in the range of 1.2–1.5 at Camarinal Sill and between 0.4 and 0.8 at Tarifa Narrows. We have concluded that the mean exchange simulated by the model is in submaximal regime.
Influence of wind stress and ambient flow on a high discharge river plume
Authors: García Berdeal I. , Hickey B. M. , Kawase M.
Published: October 2002

The response of a high discharge river plume to an alongshore ambient flow and wind forcing is studied with a three‐dimensional numerical model. The study extends prior model studies of plumes by including (1) a very large volume discharge (14,000 m3 s−1, about twice the maximum used in other models), (2) ambient flow in a direction opposite to that of the propagation of coastally trapped waves, and (3) a sequence of wind direction reversals. The magnitude of the ambient flow, wind stress, estuary width, and river outflow are based on typical values for the Columbia River on the Washington coast. The model results challenge two longstanding notions about the Columbia plume; first, that plume orientation is in a relatively stable southwest position in summer. For example, with average discharge conditions (7000 m3 s−1) a summertime downwelling event can erode the southwestward plume and advect it to the north of the river mouth over several days. Second, the plume is not always unidirectional; branches can occur both upstream and downstream of the river mouth simultaneously. The model also provides an explanation for the observation that the plume rarely tends southward during the winter season; in contrast to summer conditions the rotational tendency of the plume and the ambient flow are in the same direction, so that wind stress must be be significant (>1.4 dynes cm−2 for at least 2 days) to reverse the plume direction. Distinct anticyclonic freshwater pools form in modeled plumes both north and south of the river mouth under steady forcing conditions when ambient flow is present. The scale of modeled pools is consistent with features observed in the Columbia plume.
Injection of carbon from the shelf to offshore beneath the euphotic zone in the California Current
Authors: Barth John A. , Cowles Timothy J. , Kosro P. Michael , Shearman R. Kipp , Huyer Adriana , Smith Robert L.
Published: October 2002

High concentrations of chlorophyll are found in the California Current System over 300 km offshore, far from the productive coastal upwelling region, and between 150 and 250 m, well below the depth to which photosynthetically active solar radiation penetrates. This exceptionally deep chlorophyll feature is formed near the coast and transported offshore in the meandering California Current jet. Chlorophyll is forced downward along sloping density surfaces through conservation of potential vorticity along the meandering jet path. Thus mesoscale physical dynamics serve to inject large amounts of carbon, e.g., 2400 t as reported here, per event from regions of active coastal upwelling into the adjacent deep ocean, a process that must be considered when computing oceanic carbon budgets.
Nonlinear multichannel singular spectrum analysis of the tropical Pacific climate variability using a neural network approach
Authors: Hsieh William W. , Wu Aiming
Published: October 2002

Singular spectrum analysis (SSA), a linear (univariate and multivariate) time series technique, performs principal component analysis (PCA) on an augmented data set containing the original data and time‐lagged copies of the data. Neural network theory has meanwhile allowed PCA to be generalized to nonlinear PCA (NLPCA). In this paper, NLPCA is further extended to perform nonlinear SSA (NLSSA): First, SSA is applied to reduce the dimension of the data set; the leading principal components (PCs) of the SSA then become inputs to an NLPCA network (with a circular node at the bottleneck). This network performs the NLSSA by nonlinearly combining all the input SSA PCs. The NLSSA is applied to the tropical Pacific sea surface temperature anomaly (SSTA) field and to the sea level pressure anomaly (SLPA) field for the 1950–2000 period. Unlike SSA modes, which display warm and cool periods of similar duration and intensity, NLSSA mode 1 shows the warm periods to be shorter and more intense than the cool periods, as observed for the El Niño‐Southern Oscillation. Also, in SSTA NLSSA mode 1 the peak warm event is centered in the eastern equatorial Pacific, while the peak cool event is located around the central equatorial Pacific, an asymmetry not found in the individual SSA modes. A quasi‐triennial wave of about a 39 month period is found in NLSSA mode 2 of the SSTA and of the SLPA.
Wind‐forced dynamics at the estuary‐shelf interface of a large coastal plain estuary
Authors: Janzen C. D. , Wong K.‐C.
Published: October 2002

Wind‐current relationships at the mouth of a large coastal plain estuary are examined using seasonal current, wind, and sea level records. Observations indicate a small portion of subtidal current is forced by along‐shelf (AL) wind‐induced coastal sea level fluctuations (remote forcing). However, subtidal fluctuations on the order of 10 cm/s appear to be largely forced by along‐estuary wind stress (local forcing). Local forcing dominates the wind‐driven subtidal current during principal AL and along‐estuary wind conditions, but along‐estuary wind stress accentuates the local effect while diminishing the remote effect. A simple two‐dimensional analytical model helps clarify the competition between remote and local wind forcing of current near the estuary mouth, supporting the existence of a strong bi‐directional current that is locally wind‐driven, regardless of principal wind forcing. Both observed and model results indicate that the relative orientation of the wind stress with respect to the estuary and the adjacent shelf are important in determining the roles of remote and local forcing. Findings from this study challenge the application of traditional coastal pumping models for estuary‐shelf exchange and have implications for larval recruitment and exchange of material between estuaries and inner shelves.
Deep lenses of circumpolar water in the Argentine Basin
Authors: Arhan M. , Carton X. , Piola A. , Zenk W.
Published: October 2002

Three deep anticyclonic eddies of a species only reported once before [Gordon and Greengrove, 1986] were intersected by hydrographic lines of the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) programs in the Argentine Basin. The vortices are centered near 3500 m depth at the interface between North Atlantic Deep Water and Bottom Water. They have ∼1500‐m‐thick cores containing Lower Circumpolar Deep Water and a dynamic influence that may span up to two thirds of the water column. As one eddy was observed just downstream of the western termination of the Falkland Escarpment, a destabilization of the deep boundary current by the sudden slope relaxation is suggested as a potential cause of eddy formation. Besides isopycnal interleaving at the eddy perimeters, strongly eroded core properties in the upper parts of the lenses, associated with low density ratios, hint at double diffusion at the top of the structures as another major decay mechanism. The presence of an eddy in the northern Argentine Basin shows the possibility for a northward drift of the vortices, in this basin at least. Deep events in recent current measurements from the Vema Channel are presented that raise the question of further equatorward motion to the Brazil Basin.
Observations of nearshore infragravity waves: Seaward and shoreward propagating components
Authors: Sheremet A. , Guza R. T. , Elgar S. , Herbers T. H. C.
Published: October 2002

The variation of seaward and shoreward infragravity energy fluxes across the shoaling and surf zones of a gently sloping sandy beach is estimated from field observations and related to forcing by groups of sea and swell, dissipation, and shoreline reflection. Data from collocated pressure and velocity sensors deployed between 1 and 6 m water depth are combined, using the assumption of cross‐shore propagation, to decompose the infragravity wave field into shoreward and seaward propagating components. Seaward of the surf zone, shoreward propagating infragravity waves are amplified by nonlinear interactions with groups of sea and swell, and the shoreward infragravity energy flux increases in the onshore direction. In the surf zone, nonlinear phase coupling between infragravity waves and groups of sea and swell decreases, as does the shoreward infragravity energy flux, consistent with the cessation of nonlinear forcing and the increased importance of infragravity wave dissipation. Seaward propagating infragravity waves are not phase coupled to incident wave groups, and their energy levels suggest strong infragravity wave reflection near the shoreline. The cross‐shore variation of the seaward energy flux is weaker than that of the shoreward flux, resulting in cross‐shore variation of the squared infragravity reflection coefficient (ratio of seaward to shoreward energy flux) between about 0.4 and 1.5.
On the total input of Antarctic waters to the deep ocean: A preliminary estimate from chlorofluorocarbon measurements
Authors: Orsi Alejandro H. , Smethie William M. , Bullister John L.
Published: October 2002

Deep ocean inventories of dissolved chlorofluorocarbon‐11 (CFC‐11) along representative sections off Antarctica provide the first estimate of the overall strength of all dense water sources in the Southern Ocean. Their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the Thermohaline Circulation (THC). The bottom layer is supplied via sinking of Antarctic Bottom Water (AABW) produced at a few continental shelves. The middle layer receives the offshore injection of ventilated Modified Circumpolar Deep Water (MCDW) produced along much of the lengthy Antarctic Slope Front. The top layer is ventilated by northward export of Antarctic Surface Water into the Upper Circumpolar Deep Water of the Antarctic Circumpolar Current. Average Southern Ocean inputs to the upper two layers of the deep ocean for the 1970–1990 period are derived on the basis of the CFC‐11 distributions along meridional sections, the mean CFC‐11 saturations of all water mass ingredients, and the inferred mixing leading to production of dense source waters. About 5.4 ± 1.7 Sv (1 Sv = 106 m3s−1) of near‐freezing Shelf Water ventilate the bottom layer, and 4.7 ± 1.7 Sv and 3.6 ± 1.3 Sv of cold Antarctic Surface Water ventilate the middle and top layers. Therefore the total contribution of ventilated Southern Ocean waters to the lower limb of the global THC is about 14 Sv. This is close to the about 17 Sv estimated for North Atlantic near‐surface sources from CFC‐11 inventories. Their entrainment of 7.4 ± 2.4 Sv of CFC‐poor subsurface Lower Circumpolar Deep Water during the formation and sinking of AABW and MCDW raises the total Southern Ocean input to the deep ocean to about 21 Sv.
Anatomy of turbulence in a narrow and strongly stratified estuary
Authors: Etemad‐Shahidi Amir , Imberger Jorg
Published: October 2002

More than 400 profiles of (salinity) density and velocity microstructure with 923 turbulent patches were collected over 4 days from an anchored boat with the Portable Flux Profiler (PFP) in the Swan River estuary during the wet season. By using the PFP the buoyancy flux was measured directly and estimated indirectly for this flow. Overturns were suppressed by the strong stratification, and large‐scale anisotropy was correlated with turbulent Froude number. The direct measurements of the buoyancy flux exhibited both positive (downgradient) and negative (upgradient) values within the turbulent patches. The net average mixing efficiency was 0.04, less than the commonly used value of 0.17. Therefore the rate of vertical transport was small even though the dissipation levels within the turbulent patches were relatively high. The magnitude of the above mentioned competing fluxes increased with decreasing turbulent Froude numbers even though the net flux when bin‐averaged with respect to turbulent Froude number remained close to zero.
Possible errors in CO2 air‐sea transfer velocity from deliberate tracer releases and eddy covariance measurements due to near‐surface concentration gradients
Authors: Jacobs Cor , Friis Kjeld Jørgen , Nightingale Phil , Upstill‐Goddard Rob , Larsen Søren , Oost Wiebe
Published: October 2002

The air‐sea transfer velocity of CO2, kCO2, from a deliberate tracer experiment (DTE) and from eddy covariance measurements (ECM) during ASGAMAGE differed on average by a factor of ∼2.5, in spite of the fact that both methods were applied with maximum care. In an attempt to explain this difference, implicit assumptions were investigated critically using complementary one‐dimensional (1‐D) and 2‐D models for the upper oceanic boundary layer and atmospheric surface layer. Simulations of a DTE and ECM facilitate examination of the possible impact of near‐surface vertical gradients, horizontal inhomogeneity, and instationarity of the tracer concentration on the results from field experiments. The simulations suggest that vertical tracer concentration gradients due to physical processes might occur outside the diffusive boundary and that these might have escaped prior observation due to their confinement to the upper meter of the water column. Such gradients could then cause DTE to typically underestimate kCO2 by 10–25%, depending on ambient conditions, such as light penetration. Buffering of the seawater CO2 system renders ECM much less sensitive to near‐surface gradients. However, inhomogeneity of the CO2 concentration in water might cause scatter in ECM due to inhomogeneity‐related flux divergence in air. Additionally, in the case of instationarity a complex and strongly varying fine structure of vertical CO2 concentration profiles in water could cause inconsistencies with CO2 flux observations in air. Both mechanisms might explain several tens of percents of the scatter in the ECM, while systematic effects are ∼10%.
Distribution and interannual variability of dense water production from coastal polynyas on the Chukchi Shelf
Authors: Winsor Peter , Chapman David C.
Published: October 2002

Dense water formation from coastal polynyas on the Chukchi Shelf is examined using a primitive‐equation ocean model forced by surface buoyancy fluxes from a time‐dependent polynya model for the winters of the 1978–1998 period. The model is forced by either meteorological observations or National Center for Environmental Prediction (NCEP) reanalysis data. During the 21‐year period the surface forcing and the dense water production vary by a factor of 2. Using meteorological observations, most winters produce substantial amounts of water with a density anomaly of at least 1.2 kg m−3 (salinity increase of 1.5 psu), while using NCEP forcing, density anomalies typically reach 0.8 kg m−3 (1.0 psu) but are rarely as large as 1.2 kg m−3. With meteorological forcing, the production of dense water is fairly uniform throughout the entire period, whereas with NCEP forcing, nearly all of the water with density anomaly greater than 1.2 kg m−3 is formed during the early part of the modeled period (1978–1984), with the density anomaly rarely reaching 1 kg m−3 in the later part (1987 and onwards). Interannual variability is high with large differences between successive winters in both ice production and dense water production. Most of the observed variability can be explained by varying wind fields, with offshore winds creating polynyas between 30 and 75% of the time during November to April. Using a climatologically based mean initial salinity of 31.6 psu, we show that maximum salinities produced rarely exceed 33.5 psu. Furthermore, on the basis of moored observations in Bering Strait, we conclude that the interannual variability of the initial salinity is of the same magnitude as the interannual variability in dense water formation, and thus both are equally important in determining whether or not winter water is dense enough to contribute to the cold halocline layer of the Arctic Ocean. Winters with high fractions of offshore winds can produce anomalies up to 1.8 kg m−3 (2.2 psu). This, together with the varying initial salinity (density) fields, can achieve waters with maximum salinities up to 35.4 psu. Finally, we find that the derived ice volumes and dense water productions are highly sensitive to the forcing (meteorological vs. NCEP), and comparison with in situ observations is highly recommended.
Hydrographic conditions off northern Chile during the 1996–1998 La Niña and El Niño events
Authors: Blanco Jose Luis , Carr Mary‐Elena , Thomas Andrew C. , Strub P. Ted
Published: October 2002

The evolution of oceanographic conditions in the upwelling region off northern Chile (18°–24°S) between 1996 and 1998 (including the 1997–1998 El Niño) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5°S), Iquique (20.5°S), and Antofagasta (23.5°S) and a time series of vertical temperature profiles off Iquique. Spatial patterns of sea surface temperature and salinity from May 1996 to March 1997 followed a normal seasonal progression, though conditions were anomalously cool and fresh. Starting in March 1997, positive anomalies in sea level and sea surface temperature propagated along the South American coast to 37°S. Maximum sea level anomalies occurred in two peaks in May–July 1997 and October 1997 to February 1998, separated by a relaxation period. Maximum anomalies (2°C and 0.1 practical salinity units (psu)) extended to 400 m in December 1997 within 50 km of the coast. March 1998 presented the largest surface anomalies (>4°C and 0.6 psu). Strong poleward flow (20–35 cm s−1) occurred to 400 m or deeper during both sea level maxima and weaker (10 cm s−1) equatorward flow followed each peak. By May 1998, SST had returned to the climatological mean, and flow was equatorward next to the coast. However, offshore salinity remained anomalously high owing to a tongue of subtropical water extending southeast along the Peruvian coast. Conditions off northern Chile returned to normal between August and December 1998. The timing of the anomalies suggests a connection to equatorial waves. The progression of the 1997–1998 El Niño was very similar to that of 1982–1983, though with different timing with respect to seasons.
The seasonal cycle of meridional heat transport at 24°N in the North Pacific and in the global ocean
Authors: Zhang D. , Johns W. E. , Lee T. N.
Published: October 2002

An improved estimate of the Kuroshio transport and its seasonal variation derived from the World Ocean Circulation Experiment (WOCE) moored current meter array east of Taiwan (referred to as PCM‐1) is combined with various wind products and available hydrographic data in the ocean interior to determine the meridional heat transport at 24°N in the North Pacific. The resulting seasonal cycle of meridional heat transport has a minimum heat flux of ∼0 PW in January and February and a broad maximum in the second half of the year, with a peak of 1.1 PW in July and a secondary maximum of 1.0 PW in November. The annual mean heat transport is 0.62 PW, with an uncertainty of 0.2 PW. Of the total heat transport, 0.37 PW is associated with the “overturning” circulation, and 0.25 PW is contributed by the horizontal “gyre” circulation. The Parallel Ocean Program (POP) model simulation of the meridional heat flux at 24°N compares favorably with the observations with regard to both the seasonal variation and the annual mean value. The vertical and horizontal cells are found to contribute about 3/5 and 2/5 of the total heat transport, respectively, and they are confined in the upper ocean on the annual mean and longer timescales. However, for the seasonal variation the vertical cell dominates the variation and involves circulation changes through the entire water column, while the horizontal cell heat flux remains nearly constant year‐round. The new estimate of meridional ocean heat flux across 24°N in the Pacific is combined with an updated estimate in the Atlantic at this latitude to yield a total oceanic heat flux across the latitude circle of 24°N of 2.1 ± 0.4 PW, with an annual cycle that ranges from 1.1 PW in February to 2.8 PW in August. This is the first such estimate of the seasonal cycle of the world ocean heat flux across 24°N from direct oceanographic observations.
Navidad development in the southern Bay of Biscay: Climate change and swoddy structure from remote sensing and in situ measurements
Authors: Garcia‐Soto Carlos , Pingree Robin D. , Valdés Luis
Published: October 2002

The warm water extension of the Iberian Poleward Current off northern Spain (“Navidad”) was examined in the advanced very high resolution radiometer (AVHRR) satellite archive (1979–2000) and in a time series of January sea surface temperatures. Winter warming in the southern Bay of Biscay during Navidad years was correlated with low values of the North Atlantic Oscillation (NAO) Index for the preceding months (November–December). Exceptional Navidad development and winter warming were observed during January 1990, January 1996, and January 1998, and extensive measurements were gathered for these 3 years. The Eastern Boundary poleward warming was found to extend from Portugal to Norway in exceptional Navidad years. The long‐term changes of SST in the Navidad region (1967–2000) were analyzed in the frame of the decadal variation for the Celtic shelf (1890–2000), which showed an increase of 1°C over the century. During January 1990 and January 1996, cloud‐free images of Navidad showed a pronounced production of slope water oceanic eddies (swoddies) in the SE corner of the Bay of Biscay. The small‐scale properties of one of these swoddies (F90) were analyzed during summer from measurements at sea (RRS Discovery 193 cruise; July 1990). SeaSoar sections showed the physical structure of the swoddy core. The distribution and abundance of chlorophyll a associated with the swoddy was determined. It is shown that a swoddy has a higher chlorophyll a maximum in the seasonal thermocline than associated cyclones. Simultaneous satellite observations of the sensors AVHRR, Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS), and altimeter (combined TOPEX/Poseidon/ERS‐2 data) were used to analyze the remote sensing properties of three summer swoddy‐like eddies during August 1998. The summer SeaWiFS chlorophyll a concentration at the center of the eddy near 45.5°N, 6°W (AE6) was appraised against a time series (September 1997 to April 2001) of SeaWiFS chlorophyll a concentration representative of a central region of the Bay of Biscay.
Nearshore physical processes and bio‐optical properties in the New York Bight
Authors: Chang G. C. , Dickey T. D. , Schofield O. M. , Weidemann A. D. , Boss E. , Pegau W. S. , Moline M. A. , Glenn S. M.
Published: October 2002

Temporal and spatial variability of physical, biological, and optical properties on scales of minutes to months and meters to ∼50 km are examined using an extensive data set collected on the New York Bight continental shelf during the Hyperspectral Coastal Ocean Dynamics Experiment. Measurements from a midshelf mooring and bottom tripod (∼25 km offshore, 24 m water depth) and two nearshore profiling nodes (∼5 km offshore, 15 m water depth) are utilized to quantify and correlate midshelf and nearshore variability. Towed shipboard undulating profilers and a high‐frequency radar (CODAR) array provide complementary spatial data. We show that phytoplankton and dissolved matter each accounted for roughly 50% of total absorption (440 nm) at midshelf. In contrast, particulate compared to gelbstoff absorption dominated total absorption at the nearshore location. A relatively high‐salinity, low‐temperature, low particulate coastal jet decreased turbidity nearshore and advected lower‐salinity, higher‐chlorophyll waters to the midshelf region, resulting in increased biomass at midshelf. Small‐scale (order of a few kilometers) convergence and divergence zones formed from the interaction of semidiurnal tides with mean currents and a water mass/turbidity front. The front resulted in increased decorrelation scales from nearshore (∼1 day) toward midshelf (2–3 days) for optical and biological parameters. We conclude that optical and biological variability and distributions at midshelf and nearshore locations were influenced mainly by semidiurnal tides and the coastal jet. We present insights into nearshore coastal processes and their effects on biology and optics as well as for the design of future nearshore interdisciplinary coastal programs.
Assessment of mesoscale eddy parameterizations for a single‐basin coarse‐resolution ocean model
Authors: Solovev Michael , Stone Peter H. , Malanotte‐Rizzoli Paola
Published: October 2002

In this study we explore the ability of three parameterization schemes to represent mesoscale eddies' effects in a single, idealized ocean basin. We first carry out an eddy‐resolving simulation with an ocean general circulation model (GCM). The three schemes tested are the traditional Fickian diffusion and the Green‐Stone (GS) and Gent‐McWilliams (GM) parameterizations. Evaluation of the divergence of the eddy heat flux from the reference simulation in the time‐averaged thermal balance shows that eddies contribute significantly to the balance only in a geographically limited area adjacent to the western boundary around the separation point of the midlatitude jet. For this region, diagnostic tests are carried out comparing the local properties of the eddy heat flux evaluated from the fine‐resolution eddy‐resolving reference simulation with the parameterized fluxes diagnosed according to the three schemes. Evaluation of the Fickian diffusion shows that the components of the eddy heat flux are not down the temperature gradient in about half of the domain. The GS parameterization is assessed by examining the relative orientation of the isopycnal surface and the eddy heat flux vector. The relative orientation falls outside the range implicitly assumed by the GS parameterization in about half of the domain. Similarly, the GM scheme simulates the wrong sign for the divergence of the eddy heat flux in about half of the domain. Comparison of the divergences predicted with the schemes in the most energetic subdomain of the western region shows that all of them have some skill but does not identify any one scheme as being superior to the others, and two of them, GS and GM, fail in the western boundary current by predicting the wrong sign of the divergence. The incorrect orientation of the eddy heat flux vector in the test of the GS scheme implies that mean flow kinetic energy is an important source of eddy energy, and this is a likely reason why the GS and GM schemes do not do better. We carry out a further assessment of the parameterization schemes by implementing them in an ocean GCM in coarse‐resolution experiments, thus mimicking the actual procedure used in climate simulations. All the parameterization schemes can be tuned to reproduce with some accuracy some of the climatological diagnostic quantities, hence their relative success in climatological coarse‐resolution simulations. However, once again, no one scheme is noticeably better than the others. We note that our results only apply to parameterizations of eddies generated in western boundary currents and their extensions.
Monochromatic and random wave breaking at blocking points
Authors: Chawla Arun , Kirby James T.
Published: October 2002

In this paper we study the energy dissipation due to current‐limited wave breaking in monochromatic and random waves with the help of experimental tests. The opposing currents are strong enough for wave blocking to occur. A modified bore model is used to simulate the dissipation rate in the monochromatic waves, and an empirical bulk dissipation formula for wave breaking in random waves is proposed. The effects of wave blocking on the dynamics of the wave field are also discussed.
Sea surface temperature and sea surface height variability in the North Pacific Ocean from 1993 to 1999
Authors: Casey Kenneth S. , Adamec David
Published: October 2002

Correlations between advanced very high resolution radiometer Pathfinder sea surface temperature (SST) and TOPEX sea surface height (SSH) anomalies are calculated for the 7 year period 1993–1999 in the equatorial and North Pacific. These correlations provide measures of SSH and SST covariability and are obtained using an empirical orthogonal function analysis separately on the two fields and a singular value decomposition (SVD) method on both fields simultaneously. These correlations reveal distinct SST‐SSH states: a warm, elevated period from January 1993 to December 1994; a cooler, lower phase spanning January 1995 through December 1996; and the strongly variable El Niño‐Southern Oscillation (ENSO) period spanning 1997–1999. The relationships between these SST‐SSH states and the dominant interannual and interdecadal modes of Pacific variability are investigated through correlations with indices for the Southern Oscillation, Pacific‐North American (PNA) pattern, and Pacific Decadal Oscillation (PDO). When including the tropics, significant correlations between the first SVD mode of the SST‐SSH covariability and the interannual ENSO pattern and interdecadal PDO pattern are evident. Restricting the focus to the extratropical North Pacific reveals significant correlations with only the interdecadal PDO pattern. While the first mode reveals connections to the ENSO and PDO patterns, the second SVD mode indicates significant correlations to the PNA pattern for both the tropical and extratropical Pacific.
Secondary circulation in a region of flow curvature: Relationship with tidal forcing and river discharge
Authors: Chant Robert J.
Published: October 2002

On the basis of a 301 day acoustic Doppler current profiler mooring in an estuarine tidal strait the strength and structure of secondary circulation in a region of flow curvature is related to variations in tidal forcing and river discharge. During low‐flow conditions the structure of secondary flow is consistent with a centrifugally forced helical flow, with bottom flow toward the inside of the bend and surface flow toward the outside of the bend. The strength of secondary flow increases linearly with tidal range and is consistent with a vertical eddy viscosity that is linearly dependent on tidal current speed. During times of high river discharge the strength of secondary flow is significantly reduced, and its vertical structure undergoes a fundamental change over the spring/neap cycle. During spring tides the classic helical flow pattern is evident, albeit weaker than during low‐flow conditions. However, during neap tides a more complex two‐cell structure is evident. The change between these two states occurs with a spring/neap transition in the subtidal flow, indicating that it is also accompanied by changing stratification. Simple scaling analysis suggests that during weakly stratified conditions, secondary circulation will influence stream‐wise dynamics and dispersion for channels with widths on order or less than 0.1 H/Cd, where H is the water column depth and Cd is a quadratic bottom drag coefficient. In contrast, during highly stratified conditions, lateral excursions due to secondary flows are limited to approximately one tenth of the channel's width and are an ineffective lateral mixing agent.
A general circulation model study of the dynamics of the upper ocean circulation of the South China Sea
Authors: Yang Haijun , Liu Qinyu , Liu Zhengyu , Wang Dongxiao , Liu Xiongbin
Published: October 2002

The temporal‐spatial structure and the formation mechanisms of the South China Sea (SCS) circulation are studied using the Princeton Ocean Model (POM). The model well reproduces the observed sea surface height (SSH) annual cycle and some current system such as the SCS Western Boundary Current (SCSWBC) system and the Kuroshio Loop Current (KLC). Four sensitivity experiments are carried out to reveal the dynamic mechanisms of the SCS circulation. The results show that most of the seasonal variability of the SCS is controlled predominantly by wind forcing. The Kuroshio affects the mean SSH significantly but contribute little to the variability. The SCSWBC system consists of the SCS Warm Current (SCSWC) and the Vietnam Coastal Current (VCC). In winter the SCSWBC splits into two branches separated at ∼14°–18°N. The northern branch is the SCSWC flowing northward and the southern branch is the VCC flowing southward. In summer, however, the SCSWBC flows unidirectionally northward from the Karimata Strait to the Taiwan Strait. The temporal variation of the SCSWBC, especially the VCC, is determined by the wind forcing over the interior SCS. Buoyancy forcing can strengthen (weaken) the summer (winter) SCSWBC. The KLC is crucial to the SCS circulation north of 18°N. It ultimately determines the appearance of the SCSWC in winter.
Low‐frequency variations in global mean sea level: 1950–2000
Authors: Chambers D. P. , Mehlhaff C. A. , Urban T. J. , Fujii D. , Nerem R. S.
Published: October 2002

Low‐frequency variability in global mean sea level (GMSL) is studied for the period 1950–2000 by interpolating sparse tide gauge data to a global grid using empirical orthogonal functions (EOFs) of sea level variability determined from TOPEX/Poseidon (T/P) altimeter data. Results are based on data with long‐term trends removed. The fact that the results do not have secular trends is an artifact of the analysis and should not be interpreted as an indication that sea level is not rising. The EOF reconstruction technique is discussed, and the resulting GMSL time series is compared to GMSL time series from Geosat and T/P altimetry and proxy GMSL time series estimated from global sea surface temperature data. The error assessment suggests the accuracy of the GMSL time series reconstructed from the tide gauge data is 2–4 mm RMS for a 1 year running mean smoothing and about 1 mm for a 5 year running mean smoothing. Several El Niño/La Niña events are evident in the GMSL, as well as significant low‐frequency variability at a 10–12 year period. GMSL appears to have been generally lower than normal in the late 1960s, throughout the 1970s, and in the 1980s. GMSL appears to have been generally higher than normal in the late 1950s and early 1960s and in the early 1980s and has been rising throughout the 1990s, when T/P is observing. The implication of the low‐frequency signals on the determination the secular rate of GMSL from satellite altimetry is discussed.
Influence of the tropical Atlantic versus the tropical Pacific on Caribbean rainfall
Authors: Taylor Michael A. , Enfield David B. , Chen A. Anthony
Published: October 2002

The Caribbean rainfall season runs from May through November and is distinctly bimodal in nature. The bimodality allows for a convenient division into an early season (May–June–July) and a late season (August–September–October). Evidence suggests that interannual variability in the early season is influenced strongly by anomalies in the sea surface temperatures of the tropical North Atlantic, with positive anomalies over a narrow latitudinal band (0°–20°N) being associated with enhanced Caribbean rainfall. The coincidence of this band with the main development region for tropical waves suggests a modification of the development of the waves by the warmer tropical Atlantic. The strong influence of the tropical North Atlantic wanes in the late season, with the equatorial Pacific and equatorial Atlantic becoming more significant modulators of interannual variability. The spatial pattern of significant correlation suggests strongly the influence of the El Niño/La Niña phenomenon, with a warm Pacific associated with a depressed late season and vice versa. There additionally seems to be a robust relationship between late season Caribbean rainfall and an east‐west gradient of sea surface temperature (SST) between the two equatorial oceanic basins. Oppositely signed SST anomalies in the NINO3 region and the central equatorial Atlantic (0°–15°W, 5°S–5°N) are well correlated with Caribbean rainfall for this period.
Calculation of the Bowen ratio in the tropical Pacific using sea surface temperature data
Authors: Jo Young‐Heon , Yan Xiao‐Hai , Pan Jiayi , He Ming‐Xia , Liu W. Timothy
Published: October 2002

The equilibrium Bowen ratio (Bo*) can be estimated empirically from the sea surface temperature (SST) only when the air is saturated with water vapor at the sea surface. However, since most of the sea surface is not saturated with water vapor, several studies have been conducted for the empirical Bowen ratio (BoE = Bo*(SST)), which was formulated with many sources of data from many areas. Hicks and Hess' [1977] linear regression formula for the BoE as a function of SST alone was used to estimate the empirical Bowen ratio in the tropical Pacific. Climatological and long‐term time series Optimum Interpolation Sea Surface Temperature (OISST) data was used for the climatological study of the BoE. The climatological mean Bowen ratio showed the regions, where the latent heat flux was most dominant. The Bowen ratio in the western Pacific warm pool (WPWP) area (O (0.04)) was 10% smaller than those in the upwelling coastal areas (O (0.14)). Along 5°N and 5°S, a 0.7 correlation coefficient between the BoE and the bulk formulated Bowen ratio (BoB), using climatological data, was found in the weak wind zone of the doldrums. BoB is the ratio of the bulk formulated sensible heat flux to the bulk formulated latent heat flux. EOF analysis was used to find out dominant temporal and spatial signals of the BoE. The first temporal EOF of the BoE, using 156 months (1986–1999) of OISST anomaly was related to El Niño events with an approximate 12‐month lag when compared to the El Niño‐Southern Oscillation (ENSO) index (SOI). The second and third temporal EOF showed annual and interannual signals, respectively. In order to estimate the discrepancy between the BoE and the BoB and to correct for the wind effect and the humidity effect, long‐term time series of Tropical Atmosphere Ocean (TAO) data was used. Our results showed that the eastern Pacific demonstrated a stronger correlation between the BoB and the BoE than the western Pacific. However, the Bowen ratio differences (BoD = |BoE − BoB|) were small in the western Pacific, and a larger margin of error was found at the eastern Pacific, where ocean dynamics perturb the upper ocean heat balance. The sensitivity of the humidity and the winds on the BoE was tested using linear correlation of the BoD while varying the winds and humidity. The BoD was small, especially in the southwest Pacific, where there are weak winds and high levels of the humidity. Thus the BoE in our study will be more useful to study involving heat fluxes in the WPWP area rather than those in the eastern Pacific. The correction of the humidity and the wind was applied using a linear regression of the humidity and the winds. After the corrections, the BoD was reduced by an average of 70.2%. Using TAO data, we obtained BoE = 0.033Bo* in the western Pacific with Bo*'s standard deviation of 0.008 at sea surface temperature 29.27°C ± 0.54°C.
Heat flux carried by the Antarctic Circumpolar Current mean flow
Authors: Sun Che , Watts D. Randolph
Published: October 2002

A stream function projection of historical hydrographic data is applied to study the heat flux problem in the Antarctic Circumpolar Current (ACC). The ACC is defined as a circumpolar band consisting of mean streamlines passing through Drake Passage. Its mean path exhibits a globally meandering pattern. The calculation of zonal heat transport shows that the ACC warms along its equatorward segments (South Atlantic and Indian Ocean) and cools along its poleward segment (South Pacific). The primary heat sources for the ACC system are two western boundary currents, the Brazil Current and the Agulhas Current. The mean baroclinic flow relative to 3000 dbar carries 0.14 PW poleward heat flux across 56°S and 0.08 PW across 60°S.
SST‐forced surface wind variability in the tropical Atlantic: An empirical model
Authors: Chung Chul , Nigam Sumant , Carton James
Published: October 2002

Simple dynamical models developed for the tropical Pacific surface winds may not be directly applicable in the Atlantic in view of the basin's different geometry and size and the potential impact of deep heating from over the adjacent African and South American continents. As simple dynamical models generate surface winds from sea‐surface temperature (SST), it is worth ascertaining the extent to which monthly surface winds over the tropical (30°S–30°N) Atlantic are driven by this sector's own SST variations. A procedure is developed to separate this internally driven (i.e., by tropical Atlantic basin SSTs) component of the surface wind field from those induced by changes external to this tropical basin. Our analysis shows the internally driven component to dominate wind variability only within the equatorial (10°S–10°N) Atlantic. In contrast, Pacific surface winds driven by the tropical Pacific basin SSTs dominate variability across the tropical Pacific. The vertical structure of atmospheric diabatic heating linked with the leading modes of internally driven variability in the two basins (e.g., Pacific El Niño and the Atlantic Niño) is compared to assess the suitability of hypotheses underlying simple dynamical models. The potential limitations of these models, particularly, in the tropical Atlantic, motivated the development of an empirical model for Atlantic surface‐wind variability. The model is based on rotated principal component analysis (RPCA) of the internally driven component of interannual variability in the tropical Atlantic basin. The empirical model is constructed from RPCA analysis of combined SST and surface wind anomalies of individual calendar months (from COADS data set) and outperforms the simple dynamical models. The model's performance is also compared to an empirical model based on the singular value decomposition analysis of SST and surface winds.
Analysis for BrO in zenith‐sky spectra: An intercomparison exercise for analysis improvement
Authors: Aliwell S. R. , Van Roozendael M. , Johnston P. V. , Richter A. , Wagner T. , Arlander D. W. , Burrows J. P. , Fish D. J. , Jones R. L. , Tørnkvist K. K. , Lambert J.‐C. , Pfeilsticker K. , Pundt I.
Published: October 2002

The analysis for BrO using the technique of differential optical absorption spectroscopy as applied to spectra of light scattered from the zenith sky has historically presented something of a challenge, leading to uncertainty about the accuracy of measurements. This has largely been due to the large sensitivity of the measurement to many analysis parameters and due to the small size of the absorption features being measured. BrO differential slant columns have been measured by six different groups taking part in an intercomparison exercise at Observatoire de Haute‐Provence in France from 23 to 27 June 1996. The data are analyzed in a collaborative attempt to improve the overall analysis for BrO through investigation of a series of sources of errors in the instrumentation, calibration, input to the analysis, and the spectral analysis itself. The study included comprehensive sensitivity tests performed using both actual measurements and synthetic data. The latter proved invaluable for assessing several aspects of the spectral analysis without the limitations of spectral quality and instrument variability. The most significant sources of error are identified as the wavelength calibration of several of the absorption cross sections fitted and of the measured spectra themselves, the wavelength region of the fitting, the temperature dependence of the O3 absorption cross sections, failure to adequately account for the so‐called I0 effect, inadequate offset correction, and inadequate measurement of the individual instrument slit functions. Recommendations for optimal analysis settings are presented, and comparing the results from the analysis of the campaign data shows BrO differential slant column observations from the various groups to be in agreement to within 4% on average between 87° and 90° solar zenith angle, with a scatter of 16%.
Reactive uptake of NO3 by liquid and frozen organics
Authors: Moise T. , Talukdar R. K. , Frost G. J. , Fox R. W. , Rudich Y.
Published: October 2002

The reactive uptake of the NO3 radical by liquid and frozen organics was studied in a rotating wall flow tube coupled to a White cell. The organic liquids used included alkanes, alkenes, an alcohol, and carboxylic acids with conjugated and nonconjugated unsaturated bonds. The reactive uptake coefficients, γ, of NO3 on n‐hexadecane, 1‐octadecene, 1‐hexadecene, cis + trans 7‐tetradecene, n‐octanoic acid, 2,2,4,4,6,8,8 heptamethyl nonane, 1‐octanol, cis, trans 9,11 and 10,12 octadecadienoic acid, cis‐9, cis‐12 octadecadienoic acid were determined. The reactive uptake coefficients measured with the organic liquids varied from 1.4 × 10−3 to 1.5 × 10−2. The uptake coefficients of NO3 by n‐hexadecane and n‐octanoic acid decreased by a factor of ∼5 upon freezing. This behavior is explained by reaction occurring in the bulk of the organic liquid as well as on the surface. For the rest of the compounds the change in values of γ upon freezing of the liquids was within the experimental uncertainty. This is attributed to predominant uptake of NO3 by the top few molecular surface layers of the organic substrate and continuous replenishment of the surface layer by evaporation and/or mobility of the surface. These conclusions are corroborated by estimation of the diffuso‐reactive length and solubility constant of NO3 in these liquids. The reactivity of NO3 with the organic surfaces is shown to correlate well with the known gas‐phase chemistry of NO3. The effect on the atmospheric chemistry of the NO3 radical due to its interaction with organic aerosols is studied using an atmospheric box model applying realistic atmospheric scenarios. The inclusion of NO3 uptake on organic aerosol can decrease the NO3 lifetime by 10% or more.