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A Simple Model for Stratified Shelf Flow Fields with Bottom Friction
No full textThe effect of bottom friction on the subinertial frequency motion of stratified shelf flow fields is studied in a two-layer ƒ-plane model with idealized shelf and slope bottom topography. Coastal-trapped fire waves and motion forced by the alongshore component of the wind stress at the coast are considered. Vertical turbulent-diffusion effects are assumed to be present in thin surface and bottom-boundary layers, but not at the density interface. Simplifications are achieved by assuming that typical alongshore scales are larger than the offshore scales given by the internal Rossby radius of deformation δᵣ and the shelf-slope width, that the upper-layer depth is small compared with the lower-layer depth, and that the topography of the continental margin may be represented by a linear bottom slope of small magnitude. Some results are not dependent on the presence of variable bottom topography; these are obtained first with a flat-bottom ocean adjacent to a vertical coast. A characteristic feature of free and forced motion with alongshore gradients is a decrease of lower-layer velocity and a resultant concentration of flow in the upper layer as the frequency approaches zero. For internal Kelvin waves of frequency ω, this change in velocity structure occurs for ω/α ≪ 1, where α⁻¹ is barotropic spin-down time, and is accompanied by a decrease in frictional decay as ω/α → 0. As a result, coastal internal Kelvin waves may be able to participate with relatively small damping by bottom friction in low-frequency phenomena such as El Niño. For motion forced at frequency σ and alongshore wavenumber l, this change in structure occurs for σ/α ≪ l and σ(lδᵣ)⁻¹ ≪ 1. Concurrently, the magnitude of the barotropic, forced-shelf-wave component of the flow goes to zero as σ → 0. Thus, the “arrested topographic wave” is absent and plays no role in the steady solutions. Qualitatively similar behavior is found on the Oregon shelf in the summer where monthly mean alongshore current at midshelf have substantial vertical shear, but corresponding fluctuations on the several-day time scale are nearly depth-independent. Generalized first-order wave equations are derived to describe the alongshore (y) and time (t) dependence of the lowest-order baroclinic and barotropic components. The response to a wind strees with Heaviside-unit-function behavior in both y and t clearly illustrates how the effects of stratification liberate the “arrested topographic wave” and how a steady state is achieved where the current to the upper layer and to a region near the coast with offshore scale of O(δᵣ)
A Model of Wind- and Buoyancy-Driven Ocean Circulation
No full textA layered model of steady geostrophic ocean circulation driven by wind stress and buoyancy flux at the surface is derived. Potential vorticity, or thickness, of the two near-surface layers is driven by Ekman pumping and buoyancy pumping. The latter is represented as a flow of mass proportional to the modified buoyancy flux, across the first submerged layer interface. This mass flux is modified by the advection of buoyancy in the wind-driven Ekman layer. Though diffusive diapycnal buoyancy flux across deeper layers is neglected at lowest order, it is essential for the global balance of the buoyancy budget. The global buoyancy balance requirement determines such parameters as the midocean outcrop latitudes of layers that outcrop in the subtropical gyre, and the depths of interfaces at the eastern boundary of layers that do not. These parameters control the mean thicknesses of the layers and, with the diapycnal diffusivity, the mean diffusive flux of buoyancy through each active layer. In this way the area-mean stratification is determined by the wind-driven circulation and the surface buoyancy flux.
Model solutions were computed for two idealized runs differing only by the amplitude of buoyancy forcing In run A, the surface buoyancy flux was chown to give a meridional buoyancy transport equivalent to 0.15 PW (1 PW = 1 petawatt) across the subtropical-subarctic gyre boundary. In run B, the buoyancy forcing was adjusted to give an intergyre meridional buoyancy transport equivalent to 0.51 PW. In both runs diapycnal diffusivities in the layers were held at O(10⁻⁴ m² s⁻¹). These two runs gave density contrasts over the active layers of 8 kg m⁻³ (run A) and 18 kg m⁻³ (run B). The latter is an extremely large figure compared to the maximum density contrast across the ocean pycnocline observed in nature. The author concludes that the ocean cannot accomplish meridional buoyancy transport equivalent to O(1 PW), while diapycnal diffusivities are O(10⁻⁴ m² s⁻¹) and density gradients across the pycnocline are O(4 kg m⁻³/1000 m). It is necessary for global buoyancy and heat balance that there are regions in the oceans with far larger diapycnal diffusivities than O(10⁻⁴ m² s⁻¹). Likely candidates for such regions are the upper layers of the ocean, where extremely powerful mixing can be driven by surface wind stirring and convection, and the high-energy zones of the western boundary currents
Alongshelf Variability of Inner-Shelf Circulation along the Central Oregon Coast during Summer
Get PDFThe spatial and temporal variability of inner-shelf circulation along the central Oregon coast during the
2004 upwelling season is described using a 70-km-long array of moorings along the 15-m isobath. Circulation
at three stations located onshore of a submarine bank differed from that of a station north of the bank,
despite the relatively uniform wind forcing and inner-shelf bathymetry present. During upwelling-favorable
winds, strong southward alongshelf flow occurred north of the bank, no alongshelf flow occurred onshore of
the northern part of the bank, and increasing southward flow occurred onshore of the southern part of the
bank. During downwelling-favorable winds, strong northward flow occurred in the inner shelf onshore of the
bank while weak flow occurred north of the bank. These alongshelf differences in inner-shelf circulation were
due to the effects of the bank, which isolated the inner shelf onshore of the bank from the regional upwelling
circulation that was evident at the northernmost station. As a result, circulation onshore of the bank was
driven primarily by local wind forcing, while flow north of the bank was only partially driven by local winds.A
secondary mode of variability, attributed to the movement of the regional upwelling jet due to remote
forcings, contributed the bulk of the variability observed north of the bank. With the time-dependent wind
forcing present, acceleration was an important term in the depth-averaged alongshelf momentum equation at
all stations. During upwelling, bottom stress and acceleration opposed the wind stress north of the bank,
while bottom stress was weaker onshore of the bank where the across-shelf momentum flux and the
alongshelf pressure gradient balanced the residual of the acceleration and stresses. During downwelling,
waters onshore of the bank surged northward at magnitudes much larger than that found north of the bank.
These spatial variations developed as the season progressed and the regional upwelling circulation intensified,
explaining known variations in growth and recruitment of nearshore invertebrate species
Accretion of interplanetary dust in polar ice
Get PDFMeasurements of helium isotopes in particles separated from polar ice demonstrate that extraterrestrial ³He dominates the ³He flux at the GISP2 (Greenland) and Vostok (Antarctica) ice core sites. Replicate measurements of late Holocene ice samples yield ³He fluxes of 0.62±0.27×10⁻¹² cm³ STP cm⁻² ka⁻¹ (GISP2) and 0.77 ± 0.25 × 10⁻¹² cm³ STP cm⁻² ka⁻¹ (Vostok), similar to results from marine sediments. These are the first detailed measurements of ³He in particles from ice core samples, and they demonstrate the utility of the ice core record for evaluating the temporal history of the extraterrestrial dust flux. Results from Vostok samples from 1096–1403 m depth (75–97 ka B.P.) are similar to the late Holocene data, with the exception of two highly anomalous results from 1307 m. The latter probably indicate the presence of rare, large or gas‐rich extraterrestrial particles
Further Evidence for Coastal Trapped Waves along the Peru Coast
No full textTime series of coastal sea level during 1976–77, from 2°12′S to 17°S along the west coast of South America, show that low-frequency, ω < 0.25 cycles per day (cpd), fluctuations propagate poleward with the phase speed of baroclinic Kelvin waves (2–3 m s⁻¹). The alongshore coherence is highest in the frequency band 0.1–0.2 cpd. Computing the frequency-domain empirical orthogonal functions (EOF) for alongshore current, from an army of current meters extending from 5°S to 15°S during March-May 1977, gives 70% of the variance in the 0.1–0.2 cpd frequency band to an EOF mode with poleward phase propagation at 2.75 m s⁻¹. The vertical structure of the alongshore current fluctuations (0.1–0.2 cpd) over the continental slope at 5°S and 15°S is consistent with a first-mode baroclinic Kelvin wave. The current and sea-level fluctuations are coherent and propagate poleward through latitudes where their frequency equals the local inertial frequency. The fluctuations are not significantly coherent with coastal winds from 4°S to 15°S and am therefore presumed to have an equatorial origin. Intermittent sea-level data at the Galapagos Islands during the period provide tenuous evidence that these fluctuations, propagating poleward as coastally trapped waves, previously traveled in the equatorial wave guide
Comparisons of GCM and Observed Surface Wind Fields over the Tropical Indian and Pacific Oceans
No full textMany of the processes that have important effects on both the climatological distribution and interannual variability of sea surface temperatures (SSTs) in the tropical oceans are greatly affected by the surface wind field. For this reason accurate simulation of the surface wind is a key factor governing the success of coupled tropical ocean-atmosphere models. This paper presents the results of two analyses that investigate the quality of wind fields produced by three general circulation models (GCMs) over the tropical Indian and Pacific oceans.
The first analysis concerns the annual cycles of the tropical wind fields simulated by versions of the GCM at the Oregon State University (OSU), European Centre for Medium Range Forecasts (ECMWF), and National Center for Atmospheric Research (NCAR). These models have similar horizontal resolutions but vary widely in vertical resolution. The results show that although there are substantial differences in model performance, apparently related to differences in vertical resolution, there are also clear similarities in their behavior. Each GCM did best in major trade wind regions and somewhat poorly in convectively active areas with light winds. This finding suggests that the formulations governing the interactions between persistent convection and the circulation may limit model performance.
A second analysis examines the response of the NCAR GCM, in terms of tropical Pacific wind stress, to prescribed SST anomalies over the period 1961–1972. It was found that the model response to SST anomalies associated with the El Niño/Southern Oscillation(ENSO) was distinct and in some respects resembled that of the real atmosphere. However, there were important discrepancies in the spatial configuration of the GCM field and in the amplitude of response of the GCM to the SST anomalies. An analysis of these discrepancies suggests that while the trapped equatorial Kelvin wave response of an ocean model coupled to this GCM would be qualitatively correct, differences in the GCM and observed forcing fields would result in large errors away from the equator. Tests with the Florida State University model of the tropical Pacific, to be reported in a later paper, support this conclusion.
Taken together, these findings suggest that while GCMs are capable of reproducing correctly many features of the tropical surface wind fields, discrepancies remain with respect to both the annual cycle and the response to the anomalous SST patterns associated with ENSO. These discrepancies appear to be related, at least in part, to interactions between organized convection and the circulation. To what degree these differences would affect the oceanic component of a coupled model is currently under study
Mapping the Martian polar ice caps: Applications of terrestrial optical remote sensing methods
Get PDFWith improvements in both instrumentation and algorithms, methods for mapping terrestrial snow cover using optical remote sensing data have progressed significantly over the past decade. Multispectral data can now be used to determine not only the presence or absence of snow but the fraction of snow cover in a pixel. Radiative transfer models have been used to quantify the nonlinear relationship between surface reflectance and grain size thereby providing the basis for mapping snow grain size from surface reflectance images. Model-derived characterization of the bidirectional reflectance distribution function provides the means for converting measured bidirectional reflectance to directional-hemispherical albedo. In recent work, this approach has allowed climatologists to examine the large scale seasonal variability of albedo on the Greenland ice sheet. This seasonal albedo variability results from increases in snow grain size and exposure of the underlying ice cap as the seasonal snow cover ablates away. With the current Mars Global Surveyor and future missions to Mars, it will soon be possible to apply some of these terrestrial mapping methods to learn more about Martian ice properties, extent, and variability. Distinct differences exist between Mars and Earth ice mapping conditions, including surface temperature, ice type, ice-mineral mixtures, and atmospheric properties, so a direct application of terrestrial snow and ice mapping methods may not be possible. However, expertise in mapping and interpreting terrestrial snow and ice will contribute to the inventory of techniques for mapping planetary ices. Furthermore, adaptation of terrestrial methods will provide a basis for comparison of terrestrial and planetary cryospheric components
The Accuracies of Smoothed Sea Surface Height Fields Constructed from Tandem Satellite Altimeter Datasets
Get PDFA technique previously developed for assessing the effects of sampling errors on sea surface height (SSH) fields constructed from satellite altimeter data is extended to include measurement errors, thus providing estimates of the total mean-squared error of the SSH fields. The measurement error contribution becomes an important consideration with the greater sampling density of a coordinated tandem satellite mission. Mean-squared errors are calculated for a variety of tandem altimeter sampling patterns. The resolution capability of each sampling pattern is assessed from a subjectively chosen but consistent set of criteria for the mean value and the spatial and temporal inhomogeneity of the root-mean-squared errors computed over a representative large collection of estimation times and locations.
For a mean mapping error threshold tolerance criterion of 25% of the signal standard deviation, the filter cutoff wavelength and period defining the resolution capability of SSH fields constructed from a tandem TOPEX/Poseidon (T/P) and Jason satellite sampling pattern with evenly spaced ground tracks are about 2.2° by 20 days. This can be compared with the resolution capability of about 6° by 20 days that can be obtained from a single altimeter in the T/P orbit. A tandem T/P–Jason mission with 0.75° spacing between simultaneously sampled parallel tracks that has been suggested for estimating geostrophic velocity yields an SSH mapping resolution capability of about 3.7° by 20 days. For the anticipated factor-of-2 larger orbit errors for ENVISAT compared with Jason, the resolution capability of a tandem Jason–ENVISAT scenario is about 3° by 20 days.
For mapping the SSH field, the tandem T/P–Jason sampling patterns with evenly spaced, interleaved ground tracks and either a 5-day or a 0-day offset is far better than the other tandem altimeter mission scenarios considered here. For the highest-resolution mapping, the 5-day offset is preferable to the 0-day offset. The scientific benefits of such a tandem mission are discussed in the context of two specific examples: Rossby wave dispersion and investigation of eddy–mean flow interaction
Microscale Quantification of the Absorption by Dissolved and Particulate Material in Coastal Waters with an ac-9
Get PDFMeasuring coastal and oceanic absorption coefficients of dissolved and particulate matter in the visible domain usually requires a methodology for amplifying the natural signal because conventional spectrophotometers lack the necessary sensitivity. The WET Labs ac-9 is a recently developed in situ absorption and attenuation meter with a precision better than ±0.001 m⁻¹ in the raw signal, which is sufficient to make these measurements in pristine samples. Whereas the superior sensitivity of the ac-9 has been well documented, the accuracy of in situ measurements for bio-optical applications has not been rigorously evaluated.
Obtaining accurate results with an ac-9 requires careful attention to calibration procedures because baselines drift as a result of the changing optical properties of several ac-9 components. To correct in situ measurements for instrument drift, a pressurized flow procedure was developed for calibrating an ac-9 with optically clean water. In situ, micro- (cm) to fine- (m) scale vertical profiles of spectral total absorption, a[subscript]t(λ), and spectral absorption of dissolved materials, a[subscript]g(λ), were then measured concurrently using multiple meters, corrected for drift, temperature, salinity, and scattering errors and subsequently compared. Particulate absorption, a[subscript]p(λ), was obtained from a[subscript]t(λ) − a[subscript]g(λ). CTD microstructure was simultaneously recorded. Vertical profiles of a[subscript]g(λ), a[subscript]t(λ), and a[subscript]p(λ) were replicated with different meters within ±0.005 m⁻¹, and spectral relationships compared well with laboratory measurements and hydrographic structur