bluuebunny/crossref_metadata_2025 huggingface dataset:
Partial anhysteretic remanent magnetization in magnetite 2. Reciprocity
Authors: Yu Yongjae , Dunlop David J. , Özdemir Özden
Published: November 2002

One necessary condition for successful determination of relative paleomagnetic field intensity using anhysteretic remanent magnetization (ARM) methods is reciprocity: a partial ARM, produced by a steady field H applied over a narrow interval (2, 1) of alternating field (AF), must demagnetize over the same interval (2, 1). Experimentally, we find that partial ARMs of single‐domain (SD) and pseudosingle‐domain (PSD) grains demagnetize mainly between 2 and 1, whereas >50% of partial ARMs of large PSD and multidomain (MD) grains are erased below 1, giving a low‐field tail in the coercivity distribution. Natural pumices, granites, and oceanic basalts violated reciprocity, but lake sediments, gabbros, andesite, and red scoria had relatively small low‐coercivity tails and are better candidates for paleointensity work. Using total ARM to simulate natural remanence, we carried out pseudo‐Thellier paleointensity determinations for coarse PSD and MD grains. ARM demagnetization outweighed partial ARM acquisition at the same AF step, resulting in convex‐down curves of ARM remaining versus partial ARM gained (pseudo‐Arai plot). Pseudo‐Arai plots predicted from experimentally determined distributions of blocking and unblocking fields agreed well with measured pseudo‐Thellier results, in particular explaining convex‐down MD curves.
Subcritical and supercritical regimes in epidemic models of earthquake aftershocks
Authors: Helmstetter Agnès , Sornette Didier
Published: November 2002

We present an analytical solution and numerical tests of the epidemic‐type aftershock (ETAS) model for aftershocks, which describes foreshocks, aftershocks, and main shocks on the same footing. In this model, each earthquake of magnitude m triggers aftershocks with a rate proportional to 10αm. The occurrence rate of direct aftershocks triggered by a single main shock decreases with the time from the main shock according to the “local” modified Omori law K/(t + c)p with p = 1 + θ. Contrary to the usual definition, the ETAS model does not impose an aftershock to have a magnitude smaller than the main shock. Starting with a main shock at time t = 0 that triggers aftershocks according to the local Omori law, which in turn trigger their own aftershocks and so on, we study the seismicity rate of the global aftershock sequence composed of all the secondary and subsequent aftershock sequences. The effective branching parameter n, defined as the mean aftershock number triggered per event, controls the transition between a subcritical regime n < 1 and a supercritical regime n > 1. A characteristic time t*, function of all the ETAS parameters, marks the transition from the early time behavior to the large time behavior. In the subcritical regime, we recover and document the crossover from an Omori exponent 1 − θ for t < t* to 1 + θ for t > t* found previously in the work of Sornette and Sornette for a special case of the ETAS model. In the supercritical regime n > 1 and θ > 0, we find a novel transition from an Omori decay law with exponent 1 − θ for t < t* to an explosive exponential increase of the seismicity rate for t > t*. The case θ < 0 yields an infinite n‐value. In this case, we find another characteristic time τ controlling the crossover from an Omori law with exponent 1 − |θ| for t < τ, similar to the local law, to an exponential increase at large times. These results can rationalize many of the stylized facts reported for aftershock and foreshock sequences, such as (1) the suggestion that a small p‐value may be a precursor of a large earthquake, (2) the relative seismic quiescence sometimes observed before large aftershocks, (3) the positive correlation between b and p values, (4) the observation that great earthquakes are sometimes preceded by a decrease of b‐value, and (5) the acceleration of the seismicity preceding great earthquakes.
Concordant paleolatitudes for Neoproterozoic ophiolitic rocks of the Trinity Complex, Klamath Mountains, California
Authors: Mankinen Edward A. , Lindsley‐Griffin Nancy , Griffin John R.
Published: November 2002

New paleomagnetic results from the eastern Klamath Mountains of northern California show that Neoproterozoic rocks of the Trinity ophiolitic complex and overlying Middle Devonian volcanic rocks are latitudinally concordant with cratonal North America. Combining paleomagnetic data with regional geologic and faunal evidence suggests that the Trinity Complex and related terranes of the eastern Klamath plate were linked in some fashion to the North American craton throughout that time, but that distance between them may have varied considerably. A possible model that is consistent with our paleomagnetic results and the geologic evidence is that the Trinity Complex formed and migrated parallel to paleolatitude in the basin between Laurasia and Australia–East Antarctica as the Rodinian supercontinent began to break up. It then continued to move parallel to paleolatitude at least through Middle Devonian time. Although the eastern Klamath plate served as a nucleus against which more western components of the Klamath Mountains province amalgamated, the Klamath superterrane was not accreted to North America until Early Cretaceous time.
A 6 year record of baroclinic transport variability of the Antarctic Circumpolar Current at 140°E derived from expendable bathythermograph and altimeter measurements
Authors: Rintoul Stephen R. , Sokolov Serguei , Church John
Published: November 2002

Repeat hydrographic sections across the Antarctic Circumpolar Current are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function. Cross validation shows this relationship can be used to infer baroclinic transport (above and relative to 2500 dbar) from temperature measurements with an error of a few per cent. The mean transport distribution derived from 31 austral summer expendable bathythermograph (XBT) sections over a 6 year period consists of westward flow immediately south of Tasmania, a broad band of strong eastward flow between 50° and 55°S, and three cores of eastward flow south of 55°S. By defining a second empirical relationship between surface dynamic height and cumulative transport a continuous time series of baroclinic transport is derived from altimeter measurements of sea surface height. Transports derived from altimetry in this way agree well with simultaneous in situ estimates (root mean square error in net transport is 4 × 106 m3 s−1), suggesting sea level anomalies largely reflect baroclinic changes above 2500 dbar. The 10 day sampling of the altimeter transport time series shows the irregular XBT sampling aliases variability at unresolved timescales. The standard deviation of net transport above and relative to 2500 m is 4.3 × 106 m3 s−1. The variability in net transport is largest (2.7 × 106 m3 s−1) in the quasi‐annual band (periods of 4 months to 1.5 years), slightly smaller (2.3 × 106 m3 s−1) in the mesoscale band (<4 months), and smallest in the interannual band (>1.5 years, 1.5 × 106 m3 s−1). Changes in transport are correlated with local changes in both wind stress and wind stress curl in the quasi‐annual and interannual bands, but the transport time series is too short to draw significant conclusions.
Harmonic analysis of climatological sea surface salinity
Authors: Boyer Tim P. , Levitus Sydney
Published: November 2002

A Fourier decomposition of the World Ocean Atlas 1998 (WOA98) fields of climatological monthly mean salinity is used to describe the annual cycle of sea surface salinity (SSS). Global data sets of river runoff and evaporation minus precipitation (E−P) are used to put the annual cycle of SSS in the context of the global climate system. Most of the world ocean is shown to have an annual cycle of less than 0.3 on the practical salinity scale (PSS). Areas with an annual cycle larger than 0.3 include the northern Indian Ocean, the tropical Pacific and Atlantic, and the northern North Atlantic.
On the potential impact of sea surface salinity observations on ENSO predictions
Authors: Ballabrera‐Poy J. , Murtugudde R. , Busalacchi A. J.
Published: November 2002

Multiple regression analysis is used here to construct statistical prediction models for the El Niño/Southern Oscillation (ENSO) to explore the potential impact of monitoring Pacific Ocean sea surface salinity (SSS) on prediction of equatorial Pacific sea surface temperature (SST). This study, one of the firsts focusing on the direct role of SSS in ENSO predictions, is motivated by proposed missions for remote sensing of SSS. A forward stepwise method is used to extract significant predictors of the Niño‐3 SST index from observed monthly anomalies of tropical SST, SSS, sea level, freshwater flux, and components of the wind stress. The results indicate that SSS monitoring would have small impact on the statistical nowcast (reconstruction) of ENSO but a potential role in the 6–12 month forecasts. Correlation maps show two regions of high correlation: an equatorial region (between 170°E and 160°W) and an off‐equatorial region (between 170°E and 140°W and 5°S and 20°S). Short lag correlations display the negative relationship between the warm phase of ENSO and the negative equatorial SSS anomalies related with the increase of local rainfall. Such an equatorial negative correlation coexists with an area of positive correlations off the equator. The region with positive correlations moves eastward as the lag increases, reaching the geographical limit of the SSS observations at 6 months lag. The region of negative correlation moves northward and becomes weaker as the lag increases (it is nonsignificant for 9 months lag). For lags longer than 9 months, significant positive correlations are found south of the equator (5°S–10°S). At these lags, positive salinity anomalies have the potential to modify the subsurface stratification of the western Pacific as they are subducted westward. Thus, the availability of continuous remotely sensed SSS data might add considerably to ENSO predictions at longer lead times as a result of SSS‐induced changes in the subsurface density field.
North Atlantic Oscillation–induced changes of the upper layer circulation in the northern North Atlantic Ocean
Authors: Bersch Manfred
Published: November 2002

Hydrographic data of the 1990s along World Ocean Circulation Experiment sections A1E (Greenland‐Ireland) and A2 (Newfoundland‐France) indicate a redistribution of cold, low‐saline subarctic waters and warm, saline subtropical waters in the upper layer of the northern North Atlantic within about 2 years after the North Atlantic Oscillation had turned from a period with strong westerlies until 1995 to a period with weak westerlies in 1996 and 1997. In the latter period, subarctic waters spread preferably southward along the continental slope east of Newfoundland rather than eastward with the North Atlantic Current (NAC) into the Irminger, Iceland, and West European Basins. As a consequence, the Subarctic Front and the associated NAC shifted eastward in the Newfoundland Basin, where subarctic waters accumulate, and westward in the Iceland Basin, where a large, warm, and saline anomaly was found in 1996 and 1997, indicating a contraction of the subpolar gyre. In the Irminger Basin the anomaly occurred in 1998 and 1999. Whereas the cyclonic circulation in the Irminger and Iceland Basins weakened, it intensified in the Newfoundland Basin, where a strengthened Labrador Current retroflection joined the NAC. This and the increased anticyclonic recirculation of subtropical waters in the Newfoundland and West European Basins caused a considerable reduction of the northward heat transport across 47°N in the upper layer in 1997. The anticyclonic circulation occurring in the West European Basin suggested a northward expansion of the subtropical gyre.
Uncertainties in altimetry‐based velocity estimates
Authors: Leeuwenburgh Olwijn , Stammer Detlef
Published: November 2002

Three methods to estimate ocean geostrophic surface currents from satellite altimetry measurements are evaluated for several single‐ and multiple‐satellite configurations, with specific emphasis on resulting uncertainties. Altimetric sea surface height measurements are simulated by sampling along satellite ground tracks the surface pressure output from the 1/10° North Atlantic run of the Los Alamos Parallel Ocean Program model and by subsequently adding realistic instrument and orbit errors. The effects of both sampling and data errors on the velocity estimates are discussed. The satellite orbit configurations considered represent current missions or candidates for future coordinated tandem missions. Data error budgets are based on those of existing missions and on estimates for new altimetric technology currently under development. In midlatitude regions characterized by strong variability, such as the Gulf Stream region, velocities estimated at crossovers of interleaved tracks, and along a virtual ground track between two parallel tracks with a 0.75° zonal offset, are found to be comparable in accuracy and more accurate than velocities estimated from optimally interpolated sea surface height maps. Error variances as low as 15–25% of the local signal variance can be obtained from all three methods near the Gulf Stream core. Larger relative errors are found almost everywhere else with the exact details of the error in the two velocity components depending on data error, orbit configuration, latitude, estimation method, and smoothing. Several scientific applications of the configurations and methods are discussed, including the estimation of Reynolds stresses, momentum fluxes, velocity spectra, and covariance functions. Accuracy and applicability suggest that the newly proposed parallel track configuration is a viable option for future tandem missions.
Microwave backscatter from directional seas
Authors: McDaniel Suzanne T.
Published: November 2002

Rough surface scattering theory is applied to predict the directional asymmetry of microwave backscatter from the sea for assumed forms of the surface directional wave number spectrum. Several wave spectral models exist that describe the directional distribution of gravity waves near their spectral peak. These waves are shown to govern the directionality of microwave backscatter at low incidence angles for light winds. Under such conditions the scattering asymmetry predicted using these models exceeds that observed at microwave frequencies of 5.3 and 14 GHz. Effects of the spatial and temporal variability of directional seas and the proximity of land are insufficient to account for this difference. The assumption of similarity that is common to all of the gravity wave spectral models appears to be a more likely source of the disparity between measured and predicted scattering asymmetry. Directional wave data acquired using oceanographic buoys are found also to exhibit a wind speed dependence inconsistent with the similarity principle.
Iron, nutrient, and phytoplankton distributions in Oregon coastal waters
Authors: Chase Zanna , van Geen Alexander , Kosro P. Michael , Marra John , Wheeler Patricia A.
Published: November 2002

The relationship between iron and nitrate concentrations was examined off the coast of Oregon during the upwelling season. Surface Fe and N (nitrate + nitrite) concentrations measured underway by flow injection analysis ranged from <0.3 to 20 nmol L−1 and <0.1 to 30 μmol L−1, respectively. Total dissolvable Fe concentrations, measured in unfiltered, acidified samples in surface waters and in vertical profiles, ranged from <0.3 to 300 nmol L−1. Surface water Fe and N concentrations were highly variable and uncoupled. Our observations indicate two dominant sources of Fe to Oregon coastal waters: Slope or shelf sediments and the Columbia River. Sedimentary iron, probably largely in the particulate form, appears to be added to surface waters through wind‐induced vertical mixing during strong winds, through thickening of the bottom mixed layer during relaxation or downwelling favorable wind conditions, and through outcropping of shelf bottom waters during upwelling events. The existence of multiple iron sources and the generally high iron concentrations may explain why the distribution of phytoplankton, measured both remotely (by Sea‐viewing Wide Field‐of‐view Sensor) and underway (by in vivo fluorescence), appeared to be driven primarily by physical dynamics and was not strongly linked to the distribution of iron. Nevertheless, at some offshore stations where underway Fe concentrations were <0.3 nmol L−1, underway measurements of the physiological state of phytoplankton by fast repetition rate fluorometry were consistent with mild iron stress, and cross‐shelf nutrient distributions were consistent with iron regulation of the magnitude of phytoplankton blooms.
Estimation of air‐sea heat flux from ocean measurements: An ill‐posed problem
Authors: Zhu Jiang , Kamachi Masafumi , Wang Dongxiao
Published: November 2002

In this paper we addressed that the estimation of ocean surface heat flux from ocean temperature data is an ill‐posed inverse problem, just like a well‐known ill‐posed problem: differentiation of noisy data. We reviewed engineering literature on such problems. Using a Mellor‐Yamada level 2.5 closure model and simulated temperature data, we conducted numerical experiments of retrieving heat fluxes using variational data assimilation. This study shows that estimated nonsolar heat fluxes with high time resolution have large errors even if the observation errors are small. We also discussed the ill‐posedness based on sensitivity coefficients. However, by applying some regularization methods and a longer assimilation window, it is feasible to estimate heat fluxes with reasonable accuracies, at least at some lower temporal resolution. On the basis of our model and experiment configurations, the high nonlinearity of the ocean mixed layer model also can cause difficulty in optimization when the assimilation window is long (e.g., 7 days). We proposed a modified an adjoint method approach and yielded good results.
Structure of the Mediterranean Undercurrent and Mediterranean Water spreading around the southwestern Iberian Peninsula
Authors: Bower Amy S. , Serra Nuno , Ambar Isabel
Published: November 2002

RAFOS float and hydrographic data are analyzed to investigate the pathways, rates, and mechanisms of Mediterranean Water (MW) spreading into the North Atlantic from the Gulf of Cadiz, with an emphasis on processes other than meddies. We find that the transition of the Mediterranean outflow from a density current to an intermediate‐depth jet occurs as it enters Portimão Canyon south of Portugal due to rapidly steepening topography and associated turbulent entrainment. This feature was not seen in recent plume model simulations due to their use of smoothed topography. In addition to the traditional lower salinity maximum (or core), a denser, deeper continuous vein of MW was also traceable from Portimão Canyon to Cape St. Vincent. The mean velocity structure of the Mediterranean Undercurrent changes from a ∼30‐km‐wide jet with peak mean westward speed of ∼0.2 m s−1 along the southern Iberian slope, to a broader, much weaker northward flow with maximum mean speed of only ∼0.06 m s−1 along the western Iberian slope, due in part to changes in the bottom slope. Lower core MW spread mainly (1) southward into the interior Gulf of Cadiz, (2) northward along the western Iberian slope, and (3) westward into the deep Tagus Abyssal Plain. The average spreading rate was 0.01–0.02 m s−1 toward the west–northwest direction. Meddies forming near the slope were found to have an indirect but profound impact on MW spreading, often diverting MW approaching from upstream into the interior. The results are compared to recent hydrography‐based studies of MW spreading pathways.
Sheet flow dynamics under monochromatic nonbreaking waves
Authors: Dohmen‐Janssen C. Marjolein , Hanes Daniel M.
Published: November 2002

For the first time, detailed measurements of sediment concentrations and grain velocities inside the sheet flow layer under prototype surface gravity waves have been carried out in combination with measurements of suspension processes above the sheet flow layer. Experiments were performed in a large‐scale wave flume using natural sand. Sand transport under high waves in shallow water is mainly contained within the so‐called “sheet flow layer,” a thin layer (10–60 grain diameters) in which the volume concentration of sand decreases by an order of magnitude from a value near 0.6 at the stationary bed. The thickness of the layer varies over a wave cycle and the maximum thickness increases with increasing peak Shields stress. The concentrations within the sheet flow layer vary approximately synchronously with the orbital velocity measured by an Acoustic Doppler Velocimeter (ADV) located 0.1 m above the bed, with typical phase lags of 0–π/5. In contrast, the suspended sediment concentrations a few centimeters and higher above the bed exhibit larger phase lags. Grain velocities were successfully measured in the middle and upper portions of the sheet flow layer around the time of their maximums. These velocities increased weakly with elevation from approximately 50% to 70% of the velocity outside the wave boundary layer. The observations are compared to previous experimental work and are found to be mainly consistent with observations in steady unidirectional flows and in oscillating water tunnels (OWTs), although differences in the suspended sediment concentration and the total sediment transport rate are apparent. Observations are also compared to two very different models: a 1DV suspension model for oscillatory flow with enhanced boundary roughness and a two‐phase collisional grain flow model for steady unidirectional flow. While the suspension model describes the velocity profile fairly well and the collisional model describes the concentration profile well, neither model accurately predicts both the velocity and the concentration and therefore the sediment flux over the full vertical extent of the sheet flow.
Interannual sea level changes and associated mass transports in the tropical Pacific from TOPEX/Poseidon data and linear model results (1964–1999)
Authors: Alory Gaël , Delcroix Thierry
Published: November 2002

The main modes of interannual climate variability in the tropical Pacific are identified using satellite‐derived wind (ERS) and sea level (TOPEX/Poseidon) data for the 1993–1999 period and output from a linear model forced by observed Florida State University (FSU) wind for the 1964–1999 period. During the 1993–1999 period a dominant El Niño‐Southern Oscillation (ENSO) signal showed up in the equatorial band as a zonal seesaw pattern in sea level around a fulcrum at the dateline, associated with an equatorial patch of zonal wind centered near the dateline. This oscillation is described in terms of long equatorial waves. A secondary ocean‐atmosphere coupled mode of variability, mainly linked to the strong 1997–1998 El Niño event, is identified, consisting of a meridional seesaw pattern in sea level with its fulcrum at 5°N, associated with opposite zonal wind anomalies in the northwestern and southeastern parts of the tropical Pacific. Both modes of variability are well reproduced in the linear model forced by either the ERS wind during the 1993–1999 period or the FSU wind during the 1964–1999 period. The second mode of interannual variability, particularly active during the very strong 1982–1983 and 1997–1998 El Niño events, is reminiscent of the recharge oscillator theory. However, it shows no equatorial symmetry, and its temporal function seems to include a decadal component. Sensitivity studies enabled us to identify idealized zonal wind patterns as being responsible for the observed and modeled sea level zonal and meridional seesaw patterns. The role of horizontal (geostrophic and Ekman) mass advection on the sea level variations is then quantified in three selected boxes based on the sea level zonal and meridional seesaw patterns (the 15°–5°S, 156°E–80°W southern box, the 5°S–5°N, 136°E–80°W equatorial box, and the 5°–15°N, 136°E–80°W northern box). For both modes of variability, there is little mass advection across 15°N and 15°S. At the ENSO timescale (first mode of variability) the equatorial box fills or empties mainly through zonal geostrophic transport across its western boundary, while meridional transport partially counterbalances this mass budget. The second mode of variability accounts in large part for the buildup and depletion of the equatorial band, mainly through changes in the 5°N meridional geostrophic transport. Its modulation at decadal timescale could explain the shift from balanced El Niño/La Niña conditions before the 1982–1983 major El Niño event to prevailing El Niño conditions after.
Breaking criteria and energy losses for three‐dimensional wave breaking
Authors: Wu Chin H. , Nepf H. M.
Published: November 2002

Laboratory experiments were used to explore the influence of spatial focusing and diffraction on the evolution of unsteady, three‐dimensional, deep water wave packets with a constant‐steepness spectrum. The wave packets were generated by 13 independently programmed paddles and evolved to breaking near the midpoint of a 4 m × 11 m test section. Detailed measurements of surface displacements were made across the entire test section and were used to examine energy losses and breaking criteria. Three forms of breaking criteria were considered: (1) geometric criteria based on local and global wave steepness, (2) a kinematic criterion based on particle and phase velocities, and (3) a dynamic criterion based on higher harmonic energy evolution. The results indicate that directionality of the waves can either increase (focusing waves) or decrease (diffracting waves) the geometric breaking criterion as well as breaking severity. In contrast, the directionality of waves had little effect on the kinematic criterion. At breaking, the ratio of local particle velocity and phase velocity was shown to be larger than unity for both focusing and diffracting waves. Indeed, the robustness and simplicity of the kinematic criterion make it an excellent choice for field application. Finally, the directionality of waves did not alter the up‐frequency energy transfer associated with wave steepening. The three‐dimensional, spatially focusing and diffracting wave packets lost 34% and 18% of their energy, respectively, as a result of plunging breakers and lost 12% and 9%, respectively, as a result of spilling breakers. Comparable two‐dimensional breakers with the same spectral shape lost 16% for plunging and 12% for spilling.
Role of the overturning circulation in determining the potential vorticity over the abyssal ocean
Authors: Williams Richard G. , Day Kate , Roussenov Vassil , Wood Richard
Published: November 2002

The evolution of the potential vorticity (PV) in the deep waters is diagnosed from output of the Hadley Centre Climate Model over a 1000 year integration. Extensive regions of low PV are formed whenever there is an inflow of dense fluid across the equator into a relatively isolated basin. Initially, the model reveals low contrasts in PV in the deep waters over the North Pacific and higher contrasts over the Southern Hemisphere and North Atlantic, which are consistent with climatological diagnostics. However, a drift in the climate model leads to the northward influx of bottom water weakening in the Pacific but strengthening in the Atlantic. This change in overturning circulation leads to the PV along deep neutral surfaces acquiring a weaker contrast over the North Atlantic compared with the Southern Hemisphere and the North Pacific. Transient tracer experiments suggest that these regions of low PV are useful in identifying abyssal layers that are strongly ventilated from across the equator.
Internal wave generation by tidal flow over a sill in a rotating channel
Authors: Katsumata Katsurou , Hibiya Toshiyuki
Published: November 2002

Many straits are not sufficiently narrow to justify neglecting the effect of the Earth's rotation. In the present study, analytic solutions are derived for the internal waves generated by strong tide‐sill interaction in a rotating channel, and the effect of the Earth's rotation on the internal wave generation is clarified. Solutions include the Poincaré waves with frequencies higher than the frequency of tidal forcing so that even when the forcing frequency is subinertial, there exist freely propagating Poincaré waves. The Kelvin waves as well as the Poincaré waves are generated. The amplitude of the Kelvin waves turns out to be much larger than that of the Poincaré waves so that most of the excited energy propagates away from the sill in the form of the Kelvin waves.
Subinertial variability in the flow through the Strait of Gibraltar
Authors: García Lafuente J. , Álvarez Fanjul E. , Vargas J. M. , Ratsimandresy A. W.
Published: November 2002

Current meter observations collected within Canary Azores Gibraltar Observations (CANIGO) project have been analyzed in order to study subinertial flows through the Strait of Gibraltar. Estimated net flow has been compared with hindcasts provided by Nivmar Prediction System‐Hamburg Shelf Ocean Model (HAMSOM) circulation model forced by wind stress and atmospheric pressure applied to the North Atlantic Ocean and the Mediterranean Sea. The model was first run‐forced by atmospheric pressure and then by atmospheric pressure and wind stress in order to assess the relative importance of each external agent on the subinertial flow. The main driving force is the atmospheric pressure over the Mediterranean Sea, although wind stress in the Atlantic side of the strait may contribute appreciably to subinertial net flow. Inflow variations account for 60% of the subinertial variability approximately. The interface depth correlates well with the net flow fluctuations, sinking or rising under positive (toward the Mediterranean) or negative fluctuations, respectively, with an average gain of around −60 m/Sv. These results have been interpreted in the scope of the hydraulic two‐layer theory to conclude that the exchange is submaximal rather than maximal. Salinity on the interface increases (decreases) for positive (negative) net flow fluctuations. This is explained in terms of increased (decreased) recirculation of water from the passive Mediterranean layer driven by the velocity changes that atmospheric forcing induces in the active Atlantic layer. The contribution of the recirculated water to the fluctuations of the net flow is of secondary importance (around 5% on average), but the layer that recirculates may be thicker than 50 m, what could have important biological implications.
Evidence in hydrography and density fine structure for enhanced vertical mixing over the Mid‐Atlantic Ridge in the western Atlantic
Authors: Mauritzen C. , Polzin K. L. , McCartney M. S. , Millard R. C. , West‐Mack D. E.
Published: November 2002

Anomalous conditions exist in the salinity, oxygen, and nutrient fields over the western flank of the Northern Hemisphere Mid‐Atlantic Ridge. We examine possible advective sources for this anomaly, but determine that vertical mixing is the most likely cause. We proceed to use knowledge gained from the Brazil Basin Tracer Release Experiment in the South Atlantic (where microstructure and fine structure were obtained to explore the intensity, spatial distribution, and mechanisms of mixing in the deep ocean) to interpret density fine structure from common conductivity‐temperature‐depth data in the North Atlantic. These data support the hypothesis that the anomalous hydrographic conditions are associated with enhanced levels of vertical mixing. The inferred levels of vertical diffusivity over the Northern Hemisphere Mid‐Atlantic Ridge are as high as in the South Atlantic: 1–10 × 10−4 m2/s.
Continuity preserving modified maximum cross‐correlation technique
Authors: Zavialov Peter O. , Grigorieva Julia V. , Möller Osmar O. , Kostianoy Andrey G. , Gregoire Marilaure
Published: November 2002

The maximum cross‐correlation (MCC) method reconstructs the surface advective velocity fields from the displacements of spatial patterns in pairs of sequential satellite (normally infrared) images. However, the performance of the conventional MCC method is not always satisfactory. One of the main reasons for this is the fact that the method can correctly estimate only the velocity component parallel to the gradient of the property depicted in the images, while any small displacement perpendicular to the gradient (i.e., directed along the isolines) essentially maps the spatial pattern onto itself and therefore can not be detected using the conventional MCC technique. In the present work we propose a modification of the MCC method that allows circumventing this basic deficiency and improving the performance of the MCC technique. In this approach, the “cross‐isoline” components of the velocity field are obtained as in the conventional MCC scheme; however, the “along‐isoline” components derived from the MCC are disregarded as unreliable. Instead, the “true” along‐isoline components are then reconstructed from the given cross‐isoline velocity field based on the continuity requirement and on the condition of no normal flow at solid boundaries. This inverse problem is solved by constructing the two‐dimensional stream function in the curvilinear coordinate frame associated with the image isolines. The method is illustrated using AVHRR images from the southwestern Atlantic Ocean and the Black Sea. The results are compared with some direct drifter and current meter measurements and geostrophic estimates.
Snowball Earth: Ice thickness on the tropical ocean
Authors: Warren Stephen G. , Brandt Richard E. , Grenfell Thomas C. , McKay Christopher P.
Published: November 2002

On the tropical oceans of a neo‐Proterozoic Snowball Earth, snow‐free ice would have existed in regions of net sublimation. Photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear. The steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates: the heat flux from the ocean to the ice base, the latent heat of freezing to the ice base, and the solar energy absorbed within the ice. A preliminary study, using a broadband model for solar radiation and assuming a large freezing rate, had indicated that tropical ice might be only a few meters thick. Here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight. We use a spectral model for solar absorption, computing radiative transfer at 60 wavelengths, considering absorption by the ice, and scattering by bubbles. With the spectral model, the computed ice thickness is much greater. For a solar flux of 320 W m−2 at the equatorial surface and expected albedo of 0.5 for bare sea ice, we find that surface temperatures below −12°C generate ice layers too thick for photosynthesis (>100 m). If the albedo were as low as 0.4, thick ice would occur only for surface temperatures below −25°C, but such low temperatures would be difficult to maintain with such low albedo. For surface temperatures warmer than these limits, the ice becomes thin (<1 m) and is unlikely to represent a coherent ice layer. However, glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice‐free patches.
Numerical simulations of a storm‐generated island‐trapped wave event at the Hawaiian Islands
Authors: Merrifield M. A. , Yang L. , Luther D. S.
Published: November 2002

The structure and generation of subinertial trapped waves at the Hawaiian Islands are investigated using a primitive equation model with realistic bathymetry, stratification, and wind forcing. The strongest wave event measured on the island of Hawaii during the 1980s is selected as a case study. An extratropical cyclone traveling to the east provided the forcing. Three model runs are considered: a single island and a chain of islands forced by synoptic‐scale winds and a chain of islands with enhanced winds to simulate features of the mesoscale forcing. Each model run documents the establishment of high and low surface elevation anomalies (0.05 m amplitudes) on opposite sides of the island of Hawaii due to convergences and divergences in the surface wind‐driven current. Predicted along‐shelf current amplitudes exceed 0.4 m/s, with the strongest currents near the surface and weak amplitudes (<0.1 m/s) below 100 m depth. Following the storm, a gravest mode island‐trapped wave with a 59 hour period progresses around the island for at least five wave cycles. Although the storm itself lasts only 2 days, the rotation of the wind field in time reinforces the gravest mode wave at the island of Hawaii. A similar forced pattern is established at the other major islands; however, the free wave response is much weaker than at Hawaii because of a mismatch in the forcing and resonant wave mode frequencies. In comparison to tide gauge measurements and a current meter record from the northwest coast of Hawaii, the model shows similar time dependence, although amplitudes are underpredicted by approximately a factor of 2. Increasing the wind speeds near Hawaii yields closer agreement with the observations. Exchange of wave energy between the islands is weak (<10%) but noticeable, particularly in determining the decay rate of the Hawaii island‐trapped wave.
Barotropic waves generated by storms moving rapidly over shallow water
Authors: Mercer Doug , Sheng Jinyu , Greatbatch Richard J. , Bobanović Joško
Published: November 2002

In the falls of both 1999 and 2000, waves with characteristics similar to tsunami hit the southeast coast of Newfoundland, Canada. The waves were large enough to cause local flooding, damage to docks, and other destruction. There is, however, no evidence of seismic events, underwater landslides, or slumping events on either occasion. Other explanations, such as storm surge, also appear unlikely, and local weather conditions at the coast were not exceptional at the time. On both occasions, tropical storms moved rapidly across the Grand Banks of Newfoundland from southwest to northeast, with a translation speed of ≈30 m s−1. A significant, nonisostatic response to atmospheric pressure forcing can be expected over the shallow water of the banks since the translation speed of the storms is comparable to the local shallow water gravity wave speed. We speculate that the atmospheric pressure forcing associated with the storms generated a barotropic wake, and we use a numerical model to argue that as the storm moved back over the deep ocean, the wake was refracted and/or reflected by the variable bathymetry at the edge of the banks and that it was the refraction of the wake toward the coast that led to the unusual sea level events in southeastern Newfoundland. The numerical model results are in general agreement with the eye witness reports. The model‐computed wave activity hits the southeast coast of Newfoundland at about the right time and in the right areas for both events, although for the 1999 event the model response is weaker than is observed at Port Rexton in Trinity Bay. The reason for the poorer model performance in the 1999 case is not known, although we do find that the model results are sensitive to uncertainty in the exact track taken by the storm across the banks. The model results demonstrate that the period and wavelength of the gravity waves comprising the wake are, in general, proportional to the length scale of the pressure forcing, an exception being the model response in Conception Bay, Newfoundland, where a resonant seiche response is found to dominate.
Evaluation of the NCEP/NCAR reanalysis in the NE Pacific and the Bering Sea
Authors: Ladd Carol , Bond Nicholas A.
Published: November 2002

The quality of the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis surface forcing fields is evaluated for the Bering Sea and the NE Pacific using a series of buoy measurements from 1995 to 2000. Because the mooring data have not been assimilated into the reanalysis, they provide an independent measure of the accuracy of the reanalysis in these regions. Emphasis is placed on 10 m winds and shortwave radiation, as they are important parameters for forcing ocean models. Except close to the coast where the reanalysis does not adequately account for the blocking influence of the topography, wind directions were accurately estimated by the reanalysis. Comparisons with the reanalysis suggest a small bias with NCEP 10 m winds greater than observed by ∼5%. In the Bering Sea, downward shortwave radiation in the reanalysis is overestimated by roughly 70–80 W m−2 averaged over the summer data. In the NE Pacific, the reanalysis overestimates downward shortwave radiation by approximately 20 W m−2 in both summer and winter. Inaccurate representation of clouds in the reanalysis model is the most likely reason for the overestimate. Analysis of weather patterns during periods of particularly large overestimation of shortwave radiation suggests that the reanalysis better reproduces cloudiness during synoptic‐scale cyclonic disturbances and is less able to accurately reproduce low clouds and sea fog in fair weather conditions.
Aerosol correction for remotely sensed sea surface temperatures from the National Oceanic and Atmospheric Administration advanced very high resolution radiometer
Authors: Nalli Nicholas R. , Stowe Larry L.
Published: November 2002

This research presents the first‐phase derivation and implementation of daytime aerosol correction algorithms for remotely sensed sea surface temperature (SST) from the advanced very high resolution radiometer (AVHRR) instrument flown onboard NOAA polar orbiting satellites. To accomplish this, a long‐term (1990–1998), global AVHRR‐buoy match‐up database was created by merging the NOAA/NASA Pathfinder Atmospheres and Pathfinder Oceans data sets. The merged data set is unique in that it includes daytime estimates of aerosol optical depth (AOD) derived from AVHRR channel 1 (0.63 μm) under global conditions of significant aerosol loading. Histograms of retrieved AOD reveal monomodal, lognormal distributions for both tropospheric and stratospheric aerosol modes. It is then shown empirically that the SST depression caused under each aerosol mode can be expressed as a linear function in two predictors, these being the slant path AOD retrieved from AVHRR channel 1 along with the ratio of channels 1 and 2 normalized reflectances. On the basis of these relationships, parametric equations are derived to provide an aerosol correction for retrievals from the daytime NOAA operational multichannel and nonlinear SST algorithms. Separate sets of coefficients are utilized for two aerosol modes: tropospheric (i.e., dust, smoke, haze) and stratospheric/tropospheric (i.e., following a major volcanic eruption). The equations are shown to significantly reduce retrieved SST bias using an independent set of match‐ups. Eliminating aerosol‐induced bias in both real‐time and retrospective processing will enhance the utility of the AVHRR SST for the general user community and in climate research.
Carbonaceous aerosol over the Indian Ocean: OC/EC fractions and selected specifications from size‐segregated onboard samples
Authors: Neusüß C. , Gnauk T. , Plewka A. , Herrmann H. , Quinn P. K.
Published: November 2002

Concentrations and mass fractions of organic carbon, elemental carbon, and numerous organic species have been determined for submicrometer and supermicrometer particles in the Indian Ocean on the cruise of the R/V Ron Brown during INDOEX in February/March 1999. The total carbon (TC) concentration was low for air masses originating from the southern hemisphere of the Indian Ocean, where only small amounts of organic carbon (OC) could be detected in supermicrometer (sea salt) particles. The OC concentration was typically more than one order of magnitude higher in the northern Indian Ocean, where continental air masses were encountered, both for submicrometer and supermicrometer particles. Elemental carbon (EC) was mostly found in submicrometer particles, increasing up to more than two orders of magnitude in the northern part compared to the southern part of the Indian Ocean. The mass fraction of carbonaceous material was in the range of 6–15% and 2–12% for submicrometer and supermicrometer particles, respectively, with the higher values for more polluted air masses. A high degree of mixing with light scattering material is the main reason for high‐absorption coefficients. The absorption efficiency is higher for lower mass fractions of EC. Various short‐chain dicarboxylic and hydroxylated dicarboxylic acids have been determined. Dicarboxylic acids were mostly found in supermicrometer particles, whereas hydroxylated dicarboxylic acids showed up mainly in submicrometer particles. Alkanes and polycyclic aromatic hydrocarbons have been found in small amounts, leading to the conclusion that an important part of the organic material is of secondary origin. This is confirmed by the good correlation of the sum of carboxylic acids with OC, nitrate with a less pronounced correlation with sulfate. However, low OC/EC ratios indicate the existence of primary OC.
Soil emissions of N2O, NO, and CO2 in Brazilian Savannas: Effects of vegetation type, seasonality, and prescribed fires
Authors: Pinto Alexandre de Siqueira , Bustamante Mercedes M. C. , Kisselle Keith , Burke Roger , Zepp Richard , Viana Laura T. , Varella Renato F. , Molina Marirosa
Published: November 2002

Using closed chamber techniques, soil fluxes of NO, N2O, and CO2 were measured from September 1999 to November 2000 in savanna areas of central Brazil (cerrado) subjected to prescribed fires. Our studies focused on two vegetation types, cerrado stricto sensu (20–50% canopy cover) and campo sujo (open, grass dominated), which were either burned every 2 years or protected from fire. Soil moisture and vegetation type were more important in controlling NO and CO2 fluxes than fire regime (early dry season, middle dry season or late dry season burning). N2O fluxes, however, were very low and below detection limit in any of the vegetation‐fire treatments. NO emissions increased after burning (1.0 ng NO–N cm−2 h−1), but flux returned quickly to prefire levels and even lower. In comparison, NO emissions increased 100‐fold (to 10.5 ng NO–N cm−2 h−1) during a water‐addition experiment in unburned campo sujo, and to 1.0 ng NO–N cm−2 h−1 in unburned cerrado and 1.9 ng NO–N cm−2 h−1 in burned cerrado with the first rains. Low NO and N2O emissions, low nitrification rates, and the majority of inorganic N in the form of NH4+ all indicate a conservative N cycle in the cerrado. CO2 fluxes increased with the onset of the rainy season and after artificial water addition. The highest CO2 measured in the wet season was 6.3 μmol CO2 m−2 s−1 in burned campo sujo. During the dry season, soil respiration in burned and unburned treatments were similar (average flux = 1.6–2.3 μmol CO2 m−2 s−1). Differences between fire treatments of cerrado and campo sujo CO2 fluxes are attributed to differences in relative litter production and root activity.