bluuebunny/crossref_metadata_2025 huggingface dataset:
Motion of the Earth's bow shock in the deep‐tail flank
Authors: Tsubouchi Ken , Terasawa T. , Mukai T. , Saito Y. , Yamamoto T. , Kokubun S. , Kojima H. , Matsumoto H.
Published: February 2004

The motional properties of the Earth's bow shock are analyzed using Geotail observations from beyond 70 RE behind the Earth. It is verified, with the help of an MHD model, that the shock motion mechanism can be explained as a consequence of the interaction between an upstream Alfvénic disturbance and the shock. From 68 samples of bow shock crossings observed in the flank region, the motion was found to be described by a large‐scale flutter, accompanied by two‐ or three‐dimensional motions, rather than as a simple one‐dimensional oscillation. We observed that the local shock normal direction is distributed ∼ ±20° around the direction of its static shape. The surface velocity of the bow shock, Vshock, increases with increasing deviation of the shock normal direction, n, from its equilibrium direction. We have further shown that there is a linear relationship between Vshock and n. Upon a directional change of the solar wind magnetic field, the bow shock moves so that the incident solar wind flux in the shock rest frame is kept approximately constant. Results from MHD model calculations correlate well with such observations, where the bow shock minimizes the thermodynamic variations in the magnetosheath in response to upstream perturbations. Some extraordinary observations that the shock normal vector has an antisunward component can also be accounted for in this context.
Simple analytical solutions for propagating diffusive barriers and application to the 1974 minicycle
Authors: Wibberenz G. , Cane H. V.
Published: February 2004

The cosmic ray minicycle of 1974 is a medium‐term modulation event where a clear anticorrelation is observed between the cosmic ray intensity and the heliospheric magnetic field magnitude B. It occurs during a period of minimum solar activity in an A > 0 magnetic polarity epoch. Cosmic ray modulation during the event is studied with an analytical, spherically symmetric approximation for a propagating diffusive barrier. Under the assumption that the cosmic ray radial diffusion coefficient K scales inversely with B, the temporal development of the disturbance can be obtained from the observed variation of B. A recovery process is described by a simple exponential, with a constant recovery time throughout the event. The profile of the intensity decrease as predicted by the model varies with the temporal development of the disturbance and with the duration of the disturbance in relation to the recovery timescale. The model predicts a maximum depression which is about a factor of 2.5 too small compared with neutron monitor observations. A replacement of the inverse relation K ∝ 1/B by the more general case K ∝ B−n and a value of n ∼ 2.5 leads to a good agreement between model and observations. It is remarkable how closely the high‐energy cosmic ray intensity follows the temporal structure of the magnetic field magnitude, suggesting that the magnetic field enhancement plays an important role for modulation during the minicycle. For the period under study, radial diffusion is essentially determined by diffusion parallel to the average field. Based on quasilinear theory a variation K‖∝B−2 is suggested, consistent with the value of n = 2.5 ± 1.0 derived in the paper.
Ionospheric electrical conductances produced by auroral proton precipitation
Authors: Galand Marina , Richmond Arthur D.
Published: February 2004

Prom incoherent scatter radar observations and space‐borne particle detector data, it appears that energetic proton precipitation can sometimes, for some locations, be a major source of ionization in the auroral ionosphere and contribute significantly to the electrical conductances. Here we propose a simple parameterization for the Pedersen and Hall conductances produced by proton precipitation. The derivation is based on a proton transport code for computing the electron production rate and on an effective recombination coefficient for deducing the electron density. The atmospheric neutral densities and temperatures and the geomagnetic‐field strength are obtained from standard models. The incident protons are assumed to have a Maxwellian distribution in energy with a mean energy 〈E〉 in the 2–40 keV range and an energy flux Q0. The parameterized Pedersen and Hall conductances are functions of 〈E〉 and Q0, as well as of the geomagnetic‐field strength. The dependence on these quantities is compared with those obtained for electron precipitation and for solar EUV radiation. To add the contribution of proton precipitation to the total conductances for electrodynamic studies in auroral regions, the conductances produced by electron and proton precipitations can be combined by applying a root‐sum‐square approximation.
DE 2 observations of morningside and eveningside plasma density depletions in the equatorial ionosphere
Authors: Palmroth M. , Laakso H. , Fejer B. G. , Pfaff R. F.
Published: February 2004

The occurrence of equatorial density depletions in the nightside F region ionosphere has been investigated by using observations gathered by the polar‐orbiting Dynamics Explorer 2 satellite from August 1981 to February 1983. A variety of electric field/plasma drift patterns were observed within these depletions, including updrafting, downdrafting, bifurcating, converging, subsonic, and supersonic flows. The depletions, 116 events in total, are distributed over two groups: group I (eveningside depletions) consists of the events in the 1900–2300 MLT sector, and group II (morningside depletions) are the events in the 2300–0600 MLT sector. A statistical analysis reveals clear differences in the density depletion occurrence rates between the two groups. Magnetic activity appears to suppress the generation of eveningside depletions with a delay of 2–3 hours. In the morningside the probability of observing depletions increases instantly with increasing magnetic activity; yet the best correlation is found with a 2‐hour delay. This indicates that in the premidnight sector the substorm‐induced dynamo and prompt penetration electric fields induce westward electric fields in the equatorial ionosphere 2 hours after the onset in the auroral region. In the postmidnight sector, high‐latitude ionospheric disturbances induce instantly equatorial eastward electric fields that move the F layer to higher altitudes, where it can become unstable to the Rayleigh‐Taylor instability. Eveningside depletions were observed at all longitudes except those over the Pacific Ocean, while the morningside depletions occurred mostly over the Pacific and Atlantic‐African sectors.
Modeling anisotropy and plate‐driven flow in the Tonga subduction zone back arc
Authors: Fischer Karen M. , Parmentier E. M. , Stine Alexander R. , Wolf Elizabeth R.
Published: February 2004

The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two‐dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orientation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5–1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (∼4% for shear strains of 1.5, with a maximum value of ∼6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt‐filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.
Observation of ocean tides below the Filchner and Ronne Ice Shelves, Antarctica, using synthetic aperture radar interferometry: Comparison with tide model predictions
Authors: Rignot E. , Padman L. , MacAyeal D. R. , Schmeltz M.
Published: February 2004

Tides near and under floating glacial ice, such as ice shelves and glacier termini in fjords, can influence heat transport into the subice cavity, mixing of the under‐ice water column, and the calving and subsequent drift of icebergs. Free‐surface displacement patterns associated with ocean variability below glacial ice can be observed by differencing two synthetic aperture radar (SAR) interferograms, each of which represents the combination of the displacement patterns associated with the time‐varying vertical motion and the time‐independent lateral ice flow. We present the pattern of net free‐surface displacement for the iceberg calving regions of the Ronne and Filchner Ice Shelves in the southern Weddell Sea. By comparing SAR‐based displacement fields with ocean tidal models, the free‐surface displacement variability for these regions is found to be dominated by ocean tides. The inverse barometer effect, i.e., the ocean's isostatic response to changing atmospheric pressure, also contributes to the observed vertical displacement. The principal value of using SAR interferometry in this manner lies in the very high lateral resolution (tens of meters) obtained over the large region covered by each SAR image. Small features that are not well resolved by the typical grid spacing of ocean tidal models may contribute to such processes as iceberg calving and cross‐frontal ventilation of the ocean cavity under the ice shelf.
A dendrochronological analysis of driftwood in the Northern Dvina delta and on northern Novaya Zemlya
Authors: Johansen Stein
Published: February 2004

Analysis of the wood anatomy of modern driftwood logs deposited by the Northern Dvina River, Russia, reveals nearly identical amounts of Picea and Pinus in contrast to the driftwood deposits in the European Arctic, which are dominated by Pinus. Two Picea and two Pinus driftwood chronologies that were constructed could be cross‐dated with several chronologies available from the drainage area of the Northern Dvina as well as with driftwood chronologies from Jan Mayen in the Greenland Sea or the Barents Sea coast of north Norway. The degree of cross dating indicates that two major source areas are represented among the driftwood logs examined, i.e., the lower part of the Pinega drainage basin and the eastern central part of the province of Archangel. The origin suggested corresponds with the development of the important logging areas. Driftwood pine and spruce logs from northern Novaya Zemlya cross‐dated with chronologies from northwest Russia and the Lower Angara region reveal a transport of ice‐rafted wood between the Barents Sea and the northern Kara Sea. Two dispersal routes of driftwood from northwest Russia to locations in the Barents Sea and the Norwegian and Greenland Sea are proposed, via a counter‐clockwise circulation in the Barents Sea and via export of drift ice to the Arctic Ocean north of Novaya Zemlya.
Sea ice transports in the Weddell Sea
Authors: Harms Sabine , Fahrbach Eberhard , Strass Volker H.
Published: February 2004

Time series of sea ice draft in the Weddell Sea are evaluated together with hydrographic observations, satellite passive microwave data, and ice drift for estimation of the freshwater fluxes into and out of the Weddell Sea. Ice draft is measured with moored upward looking sonars since 1990 along two transects across the Weddell Gyre. One transect, extending from the tip of the Antarctic Peninsula to Kapp Norvegia, was sampled between 1990 and 1994 and covers the flow into and out of the southern Weddell Sea. The other transect, sampled since 1996 and extending from the Antarctic continent northward along the Greenwich meridian, covers the exchange of water masses between the eastern and the western Weddell Sea. In order to relate results obtained during the different time periods, empirical relationships are established between the length of the sea ice season, derived from the satellite passive microwave data and defined as the number of days per year with the sea ice concentration exceeding 15%, and (1) the annual mean ice draft and (2) the annual mean ice volume transport. By using these empirical relationships, estimates of annual mean ice drafts and ice volume transports are derived at all mooring sites for the period February 1979 through February 1999. Wind and current force a westward ice transport in the coastal areas of the eastern Weddell Sea and a northward ice transport in the west. During the 2‐year period 1991/1992 the mean ice volume export from the Weddell Sea is (50 ± 19) × 103 m3 s−1. This freshwater export is representative for a longer‐term (20‐year) mean and exceeds the average amount of freshwater gained by precipitation and ice shelf melt by about 19×103 m3 s−1, yielding an upper bound for the formation rate of newly ventilated bottom water in the Weddell Sea of 2.6 Sv.
Global seasonal climatologies of ocean chlorophyll: Blending in situ and satellite data for the Coastal Zone Color Scanner era
Authors: Gregg Watson W. , Conkright Margarita E.
Published: February 2004

The historical archives of in situ (National Oceanographic Data Center) and satellite (Coastal Zone Color Scanner (CZCS)) chlorophyll data were combined using the blended analysis method of Reynolds [1988] in an attempt to construct an improved climatological seasonal representation of global chlorophyll distributions. The results of the blended analysis differed dramatically from the CZCS representation: Global chlorophyll estimates increased 8–35% in the blended analysis depending upon season. Regional differences were even larger, up to 140% in the equatorial Indian Ocean in summer (during the southwest monsoon). Tropical Pacific chlorophyll values increased 25–41%. The results suggested that the CZCS generally underestimates chlorophyll. Regional and seasonal differences in the blended analysis were sufficiently large as to produce a different representation of global chlorophyll distributions than otherwise inferred from CZCS data alone. Analyses of primary production and biogeochemical cycles may be substantially impacted by these results.
On the validity of downgradient eddy closures in ocean models
Authors: Roberts Malcolm J. , Marshall David P.
Published: February 2004

Results are presented from an eddy‐resolving primitive equation ocean model to test the hypothesis that geostrophic eddies can be parameterized through a variety of downgradient closures. Time‐mean eddy fluxes of temperature, isopycnic thickness, and quasi‐geostrophic potential vorticity are diagnosed on level surfaces from 5 years of model data and are spatially correlated with the corresponding time‐mean gradients. We find no discernible correlation between the absolute eddy fluxes and mean gradients. However, after decomposing the eddy fluxes into “rotational” and “divergent” components we find a positive correlation between the divergent eddy fluxes and the mean gradients, although the overall correlation coefficients remain small, typically O(0.2). The correlation between the eddy temperature fluxes and mean temperature gradients is positive only over the upper 2 km and is negative at depth, suggesting that the abyssal temperature fluxes are upgradient. In contrast, the correlations between eddy fluxes and mean gradients of isopycnic thickness and potential vorticity are positive over the most of the fluid column. The overall correlations are similar for both thickness and potential vorticity closures. We further decompose the divergent component of the eddy fluxes into components directed perpendicular and parallel to mean contours. We find that both the perpendicular and the parallel components are similar in magnitude.
Determination of high‐frequency wind variability from observations and application to North Atlantic wave modeling
Authors: Bauer Eva , Weisse Ralf
Published: February 2004

The influence of quasi‐realistic high‐frequency wind variability for the probability distribution of surface waves and for the skill of the predictions with the wave model WAM is investigated. So far, the sensitivity of the surface wave field to high‐frequency wind variability has been demonstrated only for idealized wave conditions or particular events using rather simplified high‐frequency wind forcings. In this study the problem is addressed more generally by estimating the high‐frequency wind variability (for periods shorter than 6 hours) and by assessing its relevance for a particular North Atlantic winter. Wind observations with 20 min time resolution are used to build a statistical wind generator that reconstructs (in a statistical sense) that fraction of the wind variability that is missing in the analyzed model wind fields. These quasi‐realistic wind fluctuations, superimposed on analyzed 6 hourly wind fields, produced an increase of the air‐sea momentum flux and resulted in a moderate but systematic increase of the average wave heights and in their short‐term variability. While the results are qualitatively consistent with the findings of earlier studies under simplified conditions, they differ quantitatively. The change of the distribution of the wave data is found to be less pronounced for young sea states in the storm track region than for older sea states in the low latitudes. The response yields everywhere a significant amplification of the spectral variance for periods below 12 hours and an increase of the probability of extreme wave heights. Thus the increased air‐sea momentum flux is seen to be effective for predictions of the probability distribution of wave data and may also influence predictions of the ocean circulation.
Interannual variability of upper ocean vorticity balances in the Gulf of Alaska
Authors: Murray Colin P. , Morey Steven L. , O'Brien James J.
Published: February 2004

A high‐resolution numerical ocean model is used to examine the interannual variability of the upper ocean vorticity budget in the Gulf of Alaska. A circulation equation is derived for a layer representative of the upper ocean. The equation is a balance of the area integral of vorticity over the domain with the time‐ and area‐integrated vorticity flux through the layer top, bottom, and lateral boundaries and dissipation. Time series of each component of the equation are constructed and examined for interannual variability. Other native and derived model variables are analyzed to facilitate explanation of observed interannual variability. The model data show that interannual fluctuations in the time rate of change of the circulation in the Gulf of Alaska are small compared to the rate at which the dominant sources and sinks feed and drain vorticity from the domain. Variability on interannual timescales is predominantly due to atmospheric and oceanic teleconnections originating with the El Niño‐Southern Oscillation. Direct forcing of the wind on the ocean is the strongest driving mechanism for interannual variability of the gyre circulation. However, large fluctuations in the wind forcing are partially balanced by other processes. El Niño events excite downwelling coastal Kelvin waves that propagate northward along the eastern continental margin of the northeast Pacific Ocean. Arrival of the Kelvin waves at the peak of the annual cycle of the Alaskan Current triggers baroclinic instabilities that result in the formation of a coast‐wide train of large anticyclonic eddies. The more southerly of these eddies tend to propagate to the southwest, advecting negative vorticity out of the Gulf of Alaska. This net gain of vorticity partially balances the contemporaneous reduction of vorticity input to the region by the wind stress curl. Excess vorticity is advected out of the region in the Alaskan Stream to maintain the circulation over longer timescales.
What is the mean seasonal cycle of surface heat flux in the equatorial Pacific?
Authors: Wang Weimin , McPhaden Michael J.
Published: February 2004

The mean seasonal cycles of six state‐of‐the‐art surface heat flux products (three based on widely available data and three based on numerical model reanalysis fields) are compared in the equatorial Pacific with heat fluxes computed from Tropical Atmosphere‐Ocean (TAO) buoy data. Net surface heat flux and individual flux components derived from these products exhibit large deviations from TAO. We find that a significant contribution to these differences, which are often 50 W m−2 or more for net flux, is the use of systematically biased bulk variables in the computation of turbulent surface heat fluxes. We also find that for some products, compensating errors in bulk variables lead to fortuitous agreement with turbulent heat fluxes estimated from TAO. Finally, comparisons of TAO‐derived fluxes with tuned and untuned heat flux estimates from the Comprehensive Ocean Atmosphere Data Set indicate better agreement with untuned fluxes, suggesting that commonly used ad hoc strategies to close the global ocean heat budget are not strictly valid.
Seasonal and nonseasonal variability of satellite‐derived chlorophyll and colored dissolved organic matter concentration in the California Current
Authors: Kahru Mati , Mitchell B. Greg
Published: February 2004

Time series of surface chlorophyll a concentration (Chl) and colored dissolved organic matter (CDOM) derived from the Ocean Color and Temperature Sensor and Sea‐Viewing Wide Field‐of‐View Sensor were evaluated for the California Current area using regional algorithms. Satellite data composited for 8‐day periods provide the ability to describe large‐scale changes in surface parameters. These changes are difficult to detect based on in situ observations alone that suffer from undersampling the large temporal and spatial variability, especially in Chl. We detected no significant bias in satellite Chl estimates compared with ship‐based measurements. The variability in CDOM concentration was significantly smaller than that in Chl, both spatially and temporally. While being subject to large interannual and short‐term variations, offshore waters (100–1000 km from the shore) have an annual cycle of Chl and CDOM with a maximum in winter‐spring (December‐March) and a minimum in late summer. For inshore waters the maximum is more likely in spring (April‐May). We detect significant increase in both Chl and CDOM off central and southern California during the La Niña year of 1999. The trend of increasing Chl and CDOM from October 1996 to June 2000 is statistically significant in many areas.
Chloropigment distribution and transport on the inner shelf off Duck, North Carolina
Authors: D'Sa Eurico J. , Lohrenz Steven E. , Churchill James H. , Asper Vernon L. , Largier John L. , Williams Albert J.
Published: February 2004

The distribution and movement of chloropigments (chlorophylls and associated degradation products) in the bottom boundary layer near Duck, North Carolina, were examined during July and August 1994. Time series of chloropigment fluorescence, current velocity, and surface wave properties were acquired from instruments mounted on a bottom tripod set at 20 m depth. These data were combined with moored current meter measurements, meteorological data, and shipboard surveys in a comparative assessment of physical processes and chloropigment distribution over a wide range of temporal and spatial scales. Two dominant scales of chloropigment variation were observed. On numerous occasions, small‐scale (order m) structure in the near‐bottom fluorescence field was observed, even in the absence of identifiable structure in the temperature and salinity fields. Over larger time‐scales and space scales, variations in fluorescence were related to changes in water mass properties that could be attributed to alternating events of upwelling and downwelling. This view was reinforced by shipboard measurements that revealed correlations between fluorescence and hydrographic fields, both of which were modified by wind‐forced upwelling and downwelling and by the advection of low‐salinity water from Chesapeake Bay. Local resuspension of sediments did not contribute appreciably to the near‐bottom pigment load seen at the tripod, because of low bottom stress. Estimates of chloropigment flux indicated a net shoreward transport of chloropigments in the lower boundary layer. However, the rapid fluctuations of currents and pigment concentrations gave rise to large and frequent variations in chloropigment fluxes, generating uncertainty in extrapolations of this finding to longer timescales.
The effect of a storm on the 1992 summer sea ice cover of the Beaufort, Chukchi, and East Siberian Seas
Authors: Holt Benjamin , Martin Seelye
Published: February 2004

For the summer Beaufort, Chukchi, and East Siberian Seas a variety of active and passive microwave satellite data are used to determine the response of the ice edge and interior to a storm. Specifically, the study concentrates on a low‐pressure system that passed over the region between August 14 and 20, 1992, with peak geostrophic winds of about 18 m s−1. Through the use of the ERS‐1 imaging radar and the Special Sensor Microwave/Imager the ice response is examined at two locations: at the ice edge for a nearly stationary location in the Chukchi Sea and in the ice interior along three swaths in the Beaufort, Chukchi, and East Siberian Seas. The ice edge observations show that the storm fractures the large floes into small floes, some of which are advected into the adjacent warm water. The ice interior observations show that the storm caused an increase in the open water amount and a shift in the floe size distribution toward smaller floes. In the ice interior, application of the cumulative number distribution N(d), where N is the number of floes per unit area that are no smaller than some floe diameter d, shows that for d>1 km, N behaves like d−α, where α lies in the range 1.8–2.9. Our analysis also shows that this slope is not affected by the storm and is slightly larger near the ice edge.
Variational Assimilation of Tropical Atmosphere‐Ocean and expendable bathythermograph data in the Hamburg Ocean Primitive Equation ocean general circulation model, adjusting the surface fluxes in the tropical ocean
Authors: Bonekamp Hans , van Oldenborgh Geert Jan , Burgers Gerrit
Published: February 2004

A four‐dimensional variational method has been developed that assimilates expandable bathythermograph and Tropical Atmosphere‐Ocean subsurface temperature data into the Hamburg Ocean Primitive Equation model. The method decreases the misfit between model and observed ocean temperatures by adjusting the surface forcing. The main goal of the assimilation scheme is to improve ocean analyses in the tropical Pacific. As a first study, only wind stress is adjusted in the assimilation. In two identical twin experiments it is demonstrated that the scheme works well in the equatorial Pacific. The scheme is capable of reducing errors in the ocean analysis that originate either from the wind stress forcing or the initial state. The impact of model errors on the data assimilation is investigated in a experiment with real observations. In this experiment the temperature innovations near the equator are comparable to those of an optimal interpolation data assimilation scheme.
A simple model of interannual displacements of the Gulf Stream
Authors: Taylor Arnold H. , Gangopadhyay Avijit
Published: February 2004

Interannual variations in the latitude of the north wall of the Gulf Stream 1966–1999 can be hindcast by a simple model forced solely by monthly values of the North Atlantic Oscillation (NAO) index. The model is a modified version of the one‐dimensional model developed by D. Behringer et al. and utilizes the observed tendency for the centers of the Azores high and the Iceland low to move north or south as the NAO strengthens or weakens. Only one parameter, the active upper layer thickness, is changed from the original values. Although the model incorporates the thermal feedback on the wind stress used by Behringer et al., the results are almost unchanged without it. In generating the Gulf Stream position the model averages the NAO index over several months and introduces a time lag of at least a year. These delays are associated with the time taken for the lateral advection of heat. The model results are also compared with estimates of the latitude of the Gulf Stream made by A. Gangopadhyay et al. 1977–1988 and by T.M. Joyce et al. 1955–1997, and with the series of Gulf Stream transports 1954–1997 constructed by R.G. Curry and M.S. McCartney. The model reproduces the observed weakness of the seasonal variations in comparison to interannual variations. The NAO data are used to hindcast the annual latitude of the Gulf Stream for the period 1825–1999. The results show that since 1970 the Gulf Stream may have been more consistently farther south than during any period of the past 170 years.
Mean and time‐varying meridional transport of heat at the tropical/subtropical boundary of the North Pacific Ocean
Authors: Roemmich Dean , Gilson John , Cornuelle Bruce , Weller Robert
Published: February 2004

Ocean heat transport near the tropical/subtropical boundary of the North Pacific during 1993–1999 is described, including its mean and time variability. Twenty‐eight trans‐Pacific high‐resolution expendable bathythermograph (XBT)/expendable conductivity‐temperature‐depth (XCTD) transects are used together with directly measured and operational wind estimates to calculate the geostrophic and Ekman transports. The mean heat transport across the XBT transect was 0.83±0.12 pW during the 7 year period. The large number of transects enables a stable estimate of the mean field to be made, with error bars based on the known variability. The North Pacific heat engine is a shallow meridional overturning circulation that includes warm Ekman and western boundary current components flowing northward, balanced by a southward flow of cool thermocline waters (including Subtropical Mode Waters). A near‐balance of geostrophic and Ekman transports holds in an interannual sense as well as for the time mean. Interannual variability in geostrophic transport is strikingly similar to the pattern of central North Pacific sea level pressure variability (the North Pacific Index). The interannual range in heat transport was more than 0.4 pW during 1993–1999, with maximum northward values about 1 pW in early 1994 and early 1997. The ocean heat transport time series is similar to that of European Centre for Medium‐Range Weather Forecasts air‐sea heat flux integrated over the Pacific north of the XBT line. The repeating nature of the XBT/XCTD transects, with direct wind measurements, allows a substantial improvement over previous heat transport estimates based on one‐time transects. A global system is envisioned for observing the time‐varying ocean heat transport and its role in the Earth's heat budget and climate system.
Hydrographic structure and transport of the Oyashio south of Hokkaido and the formation of North Pacific Intermediate Water
Authors: Yasuda Ichiro , Hiroe Yutaka , Komatsu Kosei , Kawasaki Kiyoshi , Joyce Terrence M. , Bahr Frank , Kawasaki Yasuhiro
Published: February 2004

Hydrographic structure and transport of the Oyashio south of Hokkaido were described with conductivity‐temperature‐depth and lowered acoustic Doppler current profiler (LADCP) survey performed in June 1998. The southwestward Oyashio transport just off the Hokkaido coast was 10.1 Sv in the density of 26.6–27.5σθ, in which 2.5 Sv was from the Okhotsk Sea and 7.6 Sv was from the Western Subarctic Gyre (WSAG). The Oyashio northeastward countercurrent was 4.9 Sv. The cross‐gyre Oyashio transport in the area from the east coast of Hokkaido to the Subarctic Front was estimated to be 5.2 Sv; 2.4 Sv in 26.6–27.0σθ was mainly composed of low potential vorticity Okhotsk Sea water and 2.8 Sv in 27.0–27.5σθ mostly from WSAG, suggesting that the Okhotsk Sea water (WSAG water) would contribute to the formation of the upper (lower) part of the North Pacific Intermediate Water (NPIW). The Oyashio water was lower in oxygen than in the subtropical areas south of the Subarctic Front in the density of 26.9–27.6σθ, possibly because of the absence of the Antarctic Intermediate Water (AAIW) influence with the relatively high oxygen water that might be transported along the western boundary of the North Pacific Ocean and along the Kuroshio Extension, increasing the oxygen in the areas south of the Subarctic Front. Just south of the Subarctic Front, a cold‐ and fresh‐core anticyclonic eddy was observed with a salinity minimum in the core, suggesting one possible formation process of NPIW. The density of the potential vorticity minimum in the southwestward Oyashio near Hokkaido was at around 26.65σθ, which was lower than 26.8–26.9σθ observed in the early 1990s. This is possibly because the potential vorticity vertical profiles in the Okhotsk Sea and WSAG significantly changed corresponding to the water mass regime shift occurred in the Subarctic Pacific in the mid‐1990s [Kawasaki, 1999].
Variability in sea surface height: A qualitative measure for the meridional overturning in the North Atlantic
Authors: Häkkinen Sirpa
Published: February 2004

Sea surface height (SSH) from altimeter observations (1992–2000) and modeling results are investigated to determine the modes of variability and the linkages to the state of oceanic circulation in the North Atlantic. Three different model experiments are used with climatological surface forcing appended by wind stress and/or buoyancy flux anomalies. The simulated SSH and gyre circulation and altimeter data are analyzed using the empirical orthogonal function analysis. It is shown that decadal variability in the leading SSH and gyre circulation mode originates from the basin‐scale thermal forcing component, not from wind stress driving. This means that low‐frequency variations of SSH along the Gulf Stream reflect predominantly overturning changes. The horizontal gyre circulation changes are also related to the overturning due to the topography which couples baroclinic and barotropic flows. SSH variability outside the western boundary current region, on the eastern side of the North Atlantic basin, is determined by local and remote (Rossby waves) wind stress forcing. The model simulations and altimeter data suggest that there was a large SSH change in 1995–1996 over the Gulf Stream which was associated with an abrupt weakening of the overturning.
Effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position
Authors: van Enckevort I. M. J. , Ruessink B. G.
Published: February 2004

Remote sensing of wave breaker patterns, clearly visible as high‐intensity bands in time exposure video images, has become a powerful tool to obtain large‐scale (kilometers) and long‐term (years) time series of nearshore sandbar position. However, intensity‐based bar crest positions xi differ from directly measured positions xb by a time‐varying distance Δx, which is of O(10 m) and depends on the offshore wave height H0, the water level η0, and the bathymetry itself. The effect of these parameters on Δx was investigated from simultaneous video observations and bathymetric surveys, obtained in the double‐barred system at Egmond aan Zee, Netherlands, and from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsaturated, xi was observed and predicted to move offshore as either η0 decreased or H0 increased. Under saturated conditions, Δx only responded to changes in η0. Additional model investigations showed that an increase in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the Δx variability at the outer bar and increased the Δx variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined.
Influence of atmospheric circulation on the maximum ice extent in the Baltic Sea
Authors: Omstedt Anders , Chen Deliang
Published: February 2004

This work analyzes long‐term changes in the annual maximum ice extent in the Baltic Sea and Skagerrak between 1720 and 1997. It focuses on the sensitivity of the ice extent to changes in air temperature and on the relationships between the ice extent and large‐scale atmospheric circulation. A significant regime shift in 1877 explains the decreasing trend in the ice extent. The regime shift indicates a change from a relatively cold climate regime to a relatively warm one, which is likely a result of changed atmospheric circulation. In addition, the analysis shows that a colder climate is associated with higher variability in the ice extent and with higher sensitivity of the ice extent to changes in winter air temperature. Moreover, the ice extent is fairly well correlated with the North Atlantic Oscillation (NAO) index during winter, which supports the results of earlier studies. However, the moving correlation analysis shows that the relationship between the NAO index and the ice extent is not stationary over time. A statistical model was established that links the ice extent and a set of circulation indices. It not only confirms the importance of the zonal flow but also implies the impact of meridional wind and vorticity. The usefulness of the statistical model is demonstrated by comparing its performance with that of a numerical model and with independent observations. The statistical model achieves a skill close to that of the numerical model. We conclude that this model can be a useful tool in estimating the mean conditions of the ice extent from monthly pressures, allowing for the use of the general circulation model output for predictions of mean ice extent.
Initiation of the spring phytoplankton increase in the Antarctic Polar Front zone at 170°W
Authors: Landry Michael R. , Brown Susan L. , Selph Karen E. , Abbott M. R. , Letelier R. M. , Christensen Stephanie , Bidigare Robert R. , Casciotti K.
Published: February 2004

During austral summer 1997, satellite imagery revealed enhanced chlorophyll associated with the Antarctic Polar Front at 170°W. Phytoplankton growth conditions during the early stages of the spring increase were investigated on the Antarctic Environment and Southern Ocean Process Study Survey I cruise using flow cytometry (FCM) and microscopy to characterize community biomass, composition and biological stratification and dilution experiments to estimate growth and grazing rates. Physical and biological measures showed a general shoaling of mixed layer depth from ∼200 to <100 m from late October to early November. Plankton assemblages on the southern side of the frontal jet (∼0°C waters) differed from those on the northern side (∼2°C) in enhanced relative importance of larger (>20 μm) cells, greater contributions of diatoms and ciliates, and a twofold higher ratio of protistan grazers to photoautotrophs. Phytoplankton community growth rates from incubations at 10 and 23% of surface incident light showed good agreement between high‐performance liquid chromatography estimates of chlorophyll a (Chl a) (0.20 d−1) and FCM cell‐based (0.21 d−1) results. Fucoxanthin‐based estimates for diatoms were 0.24 d−1. Mean estimates of microzooplankton grazing from the three phytoplankton measures were 0.16, 0.12, and 0.11 d−1, respectively. Heterotrophs typically consumed 40–100% of their body carbon per day and thus presumably grew at rates similar to phytoplankton. The low net rates of Chl a increase in shipboard bottle incubations (0.04 d−1) were consistent with the slow downstream accumulation of phytoplankton biomass (0.03 d−1) as measured with instrumented Lagrangian drifters through the month of November. Both were slightly less than the net rate estimates from SeaSoar surveys (0.05 d−1) because of the effects of pigment photoadaptation (bleaching) during this time of increasing light level and water column stratification.
Mesoscale physical and bio‐optical structure of the Antarctic Polar Front near 170°W during austral spring
Authors: Barth John A. , Cowles Timothy J. , Pierce Stephen D.
Published: February 2004

As part of the U.S. Joint Global Ocean Flux Study Southern Ocean program, high‐resolution surveys of the Antarctic Polar Front near 170°W were conducted during October‐November 1997 with a towed undulating system equipped with conductivity‐temperature‐depth and bio‐optical sensors. Transects along 170°W and two successive mapping surveys revealed zonal bands with sharp meridional gradients in east‐west velocity. The Polar Front (PF) was characterized by a sea surface temperature drop from 1.6° to −1.6°C between 60.35° and 61.10°S, with eastward velocities of 0.4–0.5 m s−1 in the core of the PF jet. Deep mixed layers (>200 m) were found within and north of the PF, but mixed layers shoaled to 100–125 m south of the PF to the edge of loose ice at 62.3°S. Highest mixed layer chlorophyll concentrations (0.35 mg m−3) in late October along 170°W were to the south of the PF and associated with cold, fresh water. A large meander of the PF was observed with an alongfront wavelength of 175 km, a cross‐front peak‐to‐peak amplitude of 100 km, and an eastward phase propagation of 0.05–0.08 m s−1, all of which are consistent with its formation via hydrodynamic instability of the PF jet. Highest‐phytoplankton biomass was located just poleward of the center of the PF jet. A high‐chlorophyll (up to 1.1 mg m−3) 50 by 50 km region was found downstream of the cyclonic bend associated with the meander. A survey 7.5 days later revealed growth of this high biomass region so that chlorophyll was in excess of 0.8 mg m−3 over an 80 km cross front by (at least) 80 km alongfront region. High biomass was observed to grow in place with respect to the meander rather than being displaced far downstream as would be expected from advection. This pattern is consistent with meander‐driven upwelling of nutrients and/or trace metals, which in turn stimulates phytoplankton growth. Detailed cross sections of the PF reveal narrow 10–20 km wide bands or filaments of phytoplankton biomass that have temperature/salinity properties distinct from surrounding water and are coherent for at least 120 km alongfront.
Unusual larval abundance of Scyllarides nodifer and Albunea sp. during an intrusion of low‐salinity Mississippi flood water in the Florida Keys in September 1993: Insight into larval transport from upstream
Authors: Yeung Cynthia , Criales Maria M. , Lee Thomas N.
Published: February 2004

A massive intrusion of low‐salinity water (salinity = 31–35) from the Mississippi River to the Florida Keys in September 1993 coincided with an unusual abundance of the phyllosoma larvae (stages IV–VII) of a slipper lobster, Scyllarides nodifer, and the zoeae (stages III–V) of a sand crab, Albunea sp. These species are abundant in the northeastern Gulf of Mexico, but their adults and early stage larvae are not common in the Florida Keys. The influx of S. nodifer and Albunea sp. larvae into the Florida Keys is believed to have originated on the northeastern Gulf of Mexico shelf. The southward transport of the larvae within a low‐salinity plume from the Mississippi River was apparently caused by anomalous eastward winds moving shelf waters into the Loop Current, which had extended to within ∼170 km of the Mississippi Delta. During normal‐salinity conditions (salinity > 36) the spiny lobster Panulirus argus is the most numerous species of phyllosoma larvae in the coastal waters of the Florida Keys. A wide range of stages of P. argus is present year‐round in the Florida Keys, suggesting multiple larval sources upstream possibly in the Yucatan Peninsula and the Caribbean. In contrast, the source of S. nodifer and Albunea sp. recruits for the Florida Keys may lie principally in the northeastern gulf. The recruitment success of larvae of gulf origin to the Florida Keys may depend partly on the coincidence of specific wind and Loop Current transport conditions with an availability of larvae for expatriation at the source.
Annual cycle of primary production in the Cariaco Basin: Response to upwelling and implications for vertical export
Authors: Muller‐Karger Frank , Varela Ramon , Thunell Robert , Scranton Mary , Bohrer Richard , Taylor Gordon , Capelo Juan , Astor Yrene , Tappa Eric , Ho Tung‐Yuan , Walsh John J.
Published: February 2004

Monthly hydrographic, primary production, bacterial production, and settling particulate carbon flux observations were collected between November 1995 and December 1997 at 10.5°N, 64.67°W within the Cariaco Basin, off Venezuela. Upwelling of Subtropical Underwater (SUW) started around October and lasted through approximately May of the following year. Wind speeds >7 m s−1 were observed between January and June, with weaker winds (<5 m s−1) between July and December. The upwelling cycle was therefore out of phase with that of the trade winds by 2–3 months. A seasonal cycle punctuated by transient extremes associated with subsurface ventilation events was observed in primary production. High bacterial activity and organic carbon recycling rates were observed near the oxic‐anoxic interface. Integrated primary production was 690 gC m−2 yr−1 in 1996 and 540 gC m−2 yr−1 in 1997. Settling carbon flux measured with sediment traps was about 5.6% of integrated primary production at 275 m and about 1.7% at 1225 m, with no seasonality in the proportion of vertical flux to primary production. In total, between 10 and 11 gC m−2 yr−1 were delivered to the bottom sediment of Cariaco, which suggests that between 4×105 and 1×106 t of C yr−1 were delivered to sediments within the upwelling area of the Cariaco Basin. This represents permanent sequestration of carbon previously entrained in the North Atlantic gyre in the area of formation of SUW. Results suggests that upwelled inorganic nitrogen, rather than nitrogen fixation, is responsible for the large productivity and particulate carbon settling flux in the Cariaco Basin.
Role of late winter mesoscale events in the biogeochemical variability of the upper water column of the North Pacific Subtropical Gyre
Authors: Letelier R. M. , Karl D. M. , Abbott M. R. , Flament P. , Freilich M. , Lukas R. , Strub T.
Published: February 2004

Continuous records of upper water column (0–150 m) temperature profiles, spectral distribution of down welling irradiance, and phytoplankton solar‐induced fluorescence at 25 m depth were obtained during the inaugural deployment of the Hawaii Air‐sea Logging Experiment, A Long‐term Oligotrophic Habitat Assessment (HALE ALOHA) mooring, near the Hawaii Ocean Time‐series (HOT) Station ALOHA (22°45′N; 158°00′W). The temperature record showed a strong upwelling event in March–April 1997, displacing the thermocline by 120 m. Remote sensing satellite (NSCAT and TOPEX/ERS 2) analyses suggest that the observed upwelling was a result of strong wind divergence and the passage of a cyclonic eddy through the HOT program study area. At the onset of the upwelling event increases in colored dissolved organic matter (CDOM) and chlorophyll fluorescence efficiency in the upper water column were detected by changes in the spectral distribution of the down welling irradiance. The 0–25 m mean chlorophyll a (chl a) concentration increased threefold toward the end of the upwelling period. Water column samples collected during the monthly HOT cruises also indicate that the relative contribution of diatoms to total chl a increased twofold inside the eddy. The long‐term temporal variability in frequency and intensity of these poorly resolved mesoscale events might be key factors determining the structure of the pelagic ecosystem in the North Pacific Subtropical Gyre. Integrating multi‐year remote sensing satellite, moored, and vessel‐based time series records permits a quantification of the spatial and temporal scale of upper water column perturbations and the characterization of the pelagic ecosystem response at various timescales.