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[1] Frequency and along-track wave number spectra of zonal (u) and meridional (v) geostrophic surface velocity anomalies, retrieved from the Jason-1–TOPEX/Poseidon (JTP) tandem mission, are presented as global, basin, latitudinal, and regional averages. Over the entire extratropical ocean, frequency spectra show slightly higher energy levels for the v component in the frequency range below 100 days relative to the u component, a lower energy for periods longer than 180 days. Furthermore, quasi-universal frequency slopes of close to f−2 are found for the high-frequency part of all extratropical spectra representing regions with various different eddy energy levels. Differences in wave number spectra between u and v velocity components both computed along the satellite track result primarily from the sampling of the flow field on the JTP-track with a meridionally changing track orientation. Taking this into account, nearly similar spectral shapes are found for the wave number spectra of both velocity components over extratropical regions.

[1] Ocean evaporation (E) and precipitation (P) are the fundamental components of the global water cycle. They are also the freshwater flux forcing (i.e., E-P) for the open ocean salinity. The apparent connection between ocean salinity and the global water cycle leads to the proposition of using the oceans as a rain gauge. However, the exact relationship between E-P and salinity is governed by complex upper ocean dynamics, which may complicate the inference of the water cycle from salinity observations. To gain a better understanding of the ocean rain gauge concept, here we address a fundamental issue as to how E-P and salinity are related on the seasonal timescales. A global map that outlines the dominant process for the mixed-layer salinity (MLS) in different regions is thus derived, using a lower-order MLS dynamics that allows key balance terms (i.e., E-P, the Ekman and geostrophic advection, vertical entrainment, and horizontal diffusion) to be computed from satellite-derived data sets and a salinity climatology. Major E-P control on seasonal MLS variability is found in two regions: the tropical convergence zones featuring heavy rainfall and the western North Pacific and Atlantic under the influence of high evaporation. Within this regime, E-P accounts for 40–70% MLS variance with peak correlations occurring at 2–4 month lead time. Outside of the tropics, the MLS variations are governed predominantly by the Ekman advection, and then vertical entrainment. The study suggests that the E-P regime could serve as a window of opportunity for testing the ocean rain gauge concept once satellite salinity observations are available.

First published online : 23.09.11; International audience; The rate of generation of internal gravity waves in the lee of small length scale topography by geostrophic flow in the World Ocean was estimated using linear theory with corrections for finite amplitude topography. Several global data sets were combined for the calculation including an ocean circulation model for the near-bottom geostrophic flow statistics, over 500 abyssal current meter records, historical climatological data for the buoyancy frequency, and two independent estimates of the small scale topographic statistical properties. The first topography estimate was based on an empirically-derived relationship between paleo-spreading rates and abyssal hill roughness, with corrections for sedimentation. The second estimate was based on small-scale (

surface to 1000 m and back took 5.6 h and covered 5.3 km, resulting in a horizontal speed of 0.26 m s −1 . SeaSoar undulated between the surface and 400 m, completing a cycle in 11 min while covering 2.6 km, for a speed of 3.9 m s −1 . Adjacent profiles of temperature and salinity are compared between the two platforms to prove that each is accurate. Spray and SeaSoar data are compared through sections, isopycnal spatial series, and wave number spectra. The relative slowness of the glider results in the projection of high‐ frequency oceanic variability, such as internal waves, onto spatial structure. The projection is caused by Doppler smearing because of finite speed and aliasing due to discrete sampling. The projected variability is apparent in properties measured on depth surfaces or in isopycnal depth. No projected variability is seen in observations of properties on constant density surfaces because internal waves are intrinsically filtered. Wave number spectra suggest that projected variability affects properties at constant depth at wavelengths shorter than 30 km. These results imply that isobaric quantities, like geostrophic shear, are valid at wavelengths longer than 30 km, while isopycnal quantities, like spice, may be analyzed for scales as small as a glider measures.

[1] The circulation over the continental shelf break of the western and southwestern Gulf of Mexico is inferred from the analysis of drifter trajectories and 12–19 months of continuous current measurements at seven different locations. The interpretation of the data is backed up by satellite altimetry, coastal sea level from tide gauges and wind model outputs. In accordance with previous numerical results, subinertial surface currents are driven by the wind along the shelves of the states of Tamaulipas and Veracruz. Northern wind regimes would force southward currents, whereas southern wind regimes would force northward currents at the surface but southward near the bottom, through a process involving Ekman drift and geostrophic balance. Our results show, however, that alongshore current variations are not correlated with the wind over the Western Campeche Bank. In addition, we identify other sources of current forcing. The transient eddies that collapse along the continental shelf can force strong alongshore currents and overwhelm the influence of established wind regimes. Their erratic occurrence is likely to be a major factor of interannual variability of the alongshore currents. Also, we point out the existence of coastally trapped waves generated by the wind in the northern shelf of Tamaulipas and propagating down to the Western Campeche Bank. The period of these waves ranges between 6 and 10 days, with phase speeds in the 4 m/s range.

[1] The North Brazil Current (NBC) connects the North and South Atlantic and is the major pathway for the surface return flow of the Atlantic meridional overturning circulation (AMOC). Here, we calculate the NBC geostrophic transport time series based on 5 decades of observations near the western boundary off the coast of Brazil. Results reveal a multidecadal NBC variability that lags Labrador Sea deep convection by a few years. The NBC transport time series is coherent with the Atlantic Multidecadal Oscillation in sea surface temperature, which also has been widely linked to AMOC fluctuations in previous modeling studies. Our results thus suggest that the observed multidecadal NBC transport variability is a useful indicator for AMOC variations. The suggested connection between the NBC and AMOC is assessed in a 700 year control simulation of the Geophysical Fluid Dynamics Laboratory's CM2.1 coupled climate model. The model results are in agreement with observations and further demonstrate that the variability of NBC transport is a good index for tracking AMOC variations. Concerning the debate about whether a slowdown of AMOC has already occurred under global warming, the observed NBC transport time series suggests strong multidecadal variability but no significant trend.

[1] A large-eddy simulation (LES) code is coupled with a land surface model to investigate the diurnal variation of the atmospheric boundary layer (ABL). The diurnal evolution of the ABL is driven by a time-varying incoming solar radiation. The results show that the domain average surface fluxes of sensible heat, water vapor, and carbon dioxide are smooth functions of time but the fluxes at any given surface grid point show random variations, especially the sensible heat flux. At the ABL top, the LES-resolved entrainment fluxes of these scalars also evolve with time and are not fixed fractions of their respective surface fluxes. Entrainment efficiency (the ratio of entrainment flux at zi to weδϕ, where zi is the ABL height, we is entrainment velocity, and δϕ is the jump of scalar across the entrainment zone) is highest for CO2 and lowest for sensible heat. The first-order jump condition model is very good approximation to simulated entrainment fluxes which are largely controlled by the vertical gradients of the scalars across the capping inversion. Our results suggest that over the range of geostrophic winds considered (0–5 m s−1), neither the surface nor the entrainment flux reveals sensitivity to the geostrophic wind speed variations.

[1] A dust storm that originated over the Black Rock Desert (BRD) of northwestern Nevada is investigated. Our primary goal is to more clearly understand the sequence of dynamical processes that generate surface winds responsible for entraining dust from this desert. In addition to reliance on conventional surface and upper-air observations, we make use of reanalysis data sets (NCAR/NCEP and NARR)—blends of primitive equation model forecasts and observations. From these data sets, we obtain the evolution of vertical motion patterns and ageostrophic motions associated with the event. In contrast to earlier studies that have emphasized the importance of indirect transverse circulations about an upper-level jet streak, our results indicate that in this case the transition from an indirect to a direct circulation pattern across the exit region of upper-level jet streak is central to creation of low-level winds that ablate dust from the desert. It is further argued that the transition of vertical circulation patterns is in response to adjustments to geostrophic imbalance—an adjustment time scale of 6–9 h. Although unproven, we suggest that antecedent rainfall over the alkali desert 2 weeks prior to the event was instrumental in lowering the bulk density of sediments and thereby improved the chances for dust ablation by the atmospheric disturbance. We comprehensively compare/contrast our results with those of earlier investigators, and we present an alternative view of key dynamical signatures in atmospheric flow that portend the likelihood of dust storms over the western United States.

[1] A prototypical ocean circulation model, and associated 4-D variational data assimilation system, is configured in support of the Southern Ocean Gas Exchange Experiment (Southern Ocean GasEx). The regional circulation model has 4 km horizontal resolution and 5 m vertical resolution in the upper ocean. In situ temperature and salinity observations and remotely sensed, gridded sea surface temperature and sea surface height observations are combined with the circulation model between 6 March and 4 April 2008. The benefits of customized data assimilation in support of a single oceanographic cruise are illustrated using Southern Ocean GasEx as a case study. The assimilated fields fit the observations within their respective uncertainties and fit 4–6 times better than the initial guess solution before assimilation. The fit of the assimilated fields is up to 100 times closer to observations than various global state estimate products using a cost function variance metric. The space-time variability in mixed layer depth is consistent with the Southern Ocean GasEx observations and shows a deepening through March 2008. This variability is correlated with sea level height anomalies, sea surface temperature, and air temperature, during the assimilation window. The mixed layer depth estimate between 21 March and 4 April 2008 at the location of the Southern Ocean GasEx deliberate tracer release is 41–54 ± 6 m. The average zonal volume flux estimate is 16 ± 1.9 Sv (1 Sv is 106 m3 s−1) for latitudes 50.7°S–51.8°S, longitude 38.5°W. The zonal volume flux approximately halves during 6–16 March; temporal changes in volume flux are associated with geostrophic circulation but not with the Ekman circulation.

[1] The Chukchi Sea (CS) circulation reconstructed for September 1990 to October 1991 from sea ice and ocean data is presented and analyzed. The core of the observational data used in this study comprises the records from 12 moorings deployed in 1990 and 1991 in U.S. and Russian waters and two hydrographic surveys conducted in the region in the fall of 1990 and 1991. The observations are processed by a two-step data assimilation procedure involving the Pan-Arctic Ice-Ocean Modeling and Assimilation System (employing a nudging algorithm for sea ice data assimilation) and the Semi-implicit Ocean Model [utilizing a conventional four-dimensional variational (4D-var) assimilation technique]. The reconstructed CS circulation is studied to identify pathways and assess residence times of Pacific water in the region; quantify the balances of volume, freshwater, and heat content; and determine the leading dynamical factors configuring the CS circulation. It is found that in 1990–1991 (high AO index and a cyclonic circulation regime) Pacific water transiting the CS toward the Canada basin followed two major pathways, namely via Herald Canyon (Herald branch of circulation, 0.23 Sv) and between Herald Shoal and Cape Lisburne (central branch of circulation and Alaskan Coastal Current, 0.32 Sv). The annual mean flow through Long Strait was negligible (0.01 Sv). Typical residence time of Pacific water in the region varied between 150 days for waters entering the CS in September and 270 days for waters entering in February/March. Momentum balance analysis reveals that geostrophic balance between barotropic pressure gradient and Coriolis force dominated for most of the year. Baroclinic effects were important for circulation only in the regions with large horizontal salinity gradients associated with the fresh Alaskan and Siberian coastal currents and the Cape Lisburne and Great Siberian polynyas. In the polynyas, the baroclinic effects were due to strong salinification and convection processes associated with sea ice formation.

[1] Numerical experiments of rotating stratified turbulence are conducted in order to investigate the mesoscale energy cascade process in the free atmosphere. We examine the downscale energy cascade process originated by energy injection at large scales. The energy spectrum created by this injection follows a power law, and its slope depends on planetary rotation rate and static stability. While the spectral slope tends to be steeper with increased static stability in cases without planetary rotation, it becomes insensitive to the planetary rotation rate and static stability and converges to the range from −1.9 to −2.1 in cases with an increased rotation rate. The roles of geostrophic and gravity modes in the cascade processes are discussed. It is shown that nonlinear interactions between geostrophic and gravity modes play a crucial role in the downscale energy cascades. In particular, the effect of the planetary rotation mainly influences the triad interaction composed of one geostrophic and two gravity modes in the energy cascade process. Energy flux to smaller scales is intensified by this type of triad interaction with increased planetary rotation rate.

[1] Alongshore wind speed and sea surface temperature (SST) from coastal National Data Buoy Center buoys are used to study the variability of wind-driven coastal upwelling from 1982 to 2008. A long-term increase in upwelling is observed in central California (35°N–39°N) with stronger upwelling-favorable winds, colder water, and more frequent occurrences of upwelling days during the upwelling season (March–July). Further, a longer upwelling season is observed in the same region, starting earlier in the spring and persisting later in the fall. These interannual changes in upwelling strength and seasonal duration are investigated in this study. Changes in alongshore wind (forcing of upwelling) are poorly correlated with the El Nino–Southern Oscillation or the Pacific Decadal Oscillation, but the Northern Oscillation and the North Pacific Gyre Oscillation correlate with the geostrophic upwelling-favorable winds in the region. However, changes in SST (an upwelling response) are correlated with both changes in wind (upwelling forcing) and the climate indices. Although this short record cannot differentiate between multidecadal cycles and persistent trends, this data-based result does corroborate model-based projections of increased upwelling in this region due to global climate change. This increase in upwelling is understood to be a response to the strengthening of large-scale pressure gradient fields partially due to global-scale climate change. Farther north and farther south in California, other processes also have a significant influence on coastal conditions, such that the tendency for increased upwelling is not evident in the same way.

[1] Geostrophic surface velocity anomalies are used to analyze the annual variations of the large-scale geostrophic currents and of the eddy kinetic energy (EKE) field of the ocean circulation. The underlying geostrophic currents were estimated from the Jason-1-TOPEX/Poseidon (JTP) tandem altimetric sea surface height data using the “parallel track approach” with a 10 km along-track resolution; however, because of the given separation of the tracks of the two satellites only large mesoscale eddies are resolved by the tandem measurements. The analysis covers the entire 3 year period of the tandem mission (109 repeat cycles) from September 2002 to September 2005. The analysis of the seasonal flow changes reveals annual changes of all major current systems, but especially of the zonal flow field in low latitudes, leading to zonal jets on the annual cycle in the southern Pacific, Atlantic, and Indian oceans. In middle and high latitudes, indications of a seasonally modulated strength of the Sverdrup circulation emerge from the analysis. The EKE field also shows changes in its amplitude on the annual period. In low latitudes, those can be rationalized as resulting from seasonally modulated currents. In middle and high latitudes, changes in the wind-driven barotropic circulation loom large, which are not represented in other altimetric velocity products. Results shown suggest that velocity time series of the JTP tandem mission should be continued through similar constellations, e.g., of Jason-1 and Jason-2.

[1] Vertical exchange between deep, nutrient-rich and nutrient-depleted surface waters controls the physical supply of nutrients into the euphotic zone in the oligotrophic waters of the subtropical North Pacific Ocean. Sea surface temperature and ocean color satellite images are used in this study to show that submesoscale chlorophyll patches are locally intensified. These patches are associated with surface temperature fronts and regions of strong stretching of the eddying flow. The latter are identified by calculating the finite-size Lyapunov exponents of the geostrophic flow derived from the sea surface height field from altimetry. Surface frontogenesis and nonlinear Ekman pumping (NLEP) arise as natural candidates for the sporadic generation of large vertical velocities that could favor the formation of these features. A high-resolution regional numerical model simulation confirms that large vertical velocities are indeed associated with strong density fronts and gradients of relative vorticity. Using the Q vector divergence as a proxy for frontogenetically generated vertical velocities yields good agreement with the model vertical velocities, particularly in regions where they are large. The pattern and magnitude of NLEP suggests that it is also contributing to the strong vertical motions, although NLEP and model vertical velocities are not necessarily in phase. Surface frontogenesis and NLEP provide a possible explanation for the episodic injection of nutrients at the submesoscale, which are then horizontally stirred and modulated by the unstable manifolds of the flow field. Given that these surface-intensified processes would be biogeochemically more effective when the pycnocline is shallow, they could also serve as one possible explanation for the late summer chlorophyll blooms that are known to occur in the region.

Dynamics along the continental slopes are difficult to observe given the wide spectrum of temporal and spatial variability of physical processes which occur (coastal currents, meanders, eddies, etc.). Studying such complex dynamics requires the development of synergic approaches that use integrated observing systems. In this context, we present the results of an observational program conducted in the Balearic Sea combining coastal gliders and altimetry. The objectives of this experiment are to study regional dynamics using new technologies, such as gliders, in synergy with satellite altimetry and to investigate the limitations and potential improvement to altimetric data sets in the coastal zone. In this regard, new methodologies have been developed to compute consistent altimetric and glider velocities, and a novel technique to estimate absolute glider velocities, combining surface glider geostrophic velocities with integrated currents estimated from the glider GPS positioning, has been applied. In addition, the altimetric velocity computation has been improved, especially in the coastal zone, using high-frequency along-track sampling associated with new filtering and editing techniques. This approach proves efficient for homogenizing the physical contents of altimetry and glider surface currents (percentage of standard deviation explained is >40) and characterizing regional dynamics in the Balearic Sea through a combined analysis of a high-resolution observing system, such as the appearance of anomalous intense mesoscale features missing in the classical circulation scheme of the Balearic Sea., This work has been partially funded by the CICYT‐funded projects COOL (CTM2006‐12072/MAR) and ECOOP (CTM 2007‐31006‐E) and by EU‐funded projects FP6 ECOOP and FP7 MyOcean.

The North Atlantic meridional overturning 6 circulation (MOC) is believed to play an important role in regulating the Earth’s climate. Yet, there is still much uncertainty regarding the dynamics of the MOC and its variability. It is well established, however, that, through geostrophy, the zonally integrated meridional transport at a particular latitude and depth can be determined from the east-west bottom pressure difference across the basin. Therefore, rather than consider the MOC as a large-scale system, this paper focuses on the dynamics of this geostrophic relationship in two numerical ocean models at a single latitude (50◦14 N) in the sub-polar Atlantic. First, it is shown that the bottom pressure on the western boundary is sufficient to recover, with high fidelity, the interannual meridional transport variability at 50◦17 N over a one hundred year period in the climate model HadCM3. It is found that the variability of western boundary pressure is closely associated with density changes over the continental slope. These changes lead to a large zonal gradient in potential energy and an unfeasible, depth-mean velocity over the slope. The western boundary pressure, from which the meridional transport can be recovered, is generated as a compensation to this, and limits the depth-mean flow. This demonstrates that in numerical ocean models, at least, meridional transport variability is generated as a local response to density changes on the western slope. Whether this is a true representation of actual ocean variability is uncertain, but if it were, then meridional transport variability could largely be determined using only the density field on the western slope.

[1] The speed factor and deflection angle of the wind-driven surface flow were estimated for the Tsushima Strait from long (≥3.5 years) records of surface velocities measured with HF radar, geostrophic velocities estimated from sea level differences, and analyzed wind velocities. Large seasonal variations in both the speed factor and deflection angle were found. The speed factor, which is defined here as the ratio of the wind-driven surface flow to the (waterside) friction velocity, is 7.7–9.5 in winter and 11.4–14.3 in summer, while the deflection angle is 17.7°–27.3° in winter and increases to 48.5°–67.3° in summer. A likely cause of these seasonal variations is the seasonal variation of the eddy viscosity profile. It was also found that the speed factor in winter increases with the friction velocity. This dependence is possibly explained by deepening of the wind-driven surface flow with increasing friction velocity: as the friction velocity increases, HF radar (whose measurement depth is fixed) tends to measure a shallower part of the wind-driven surface flow whose speed factor is larger.

[1] In this paper, we examine the seasonal and interannual to decadal variability of oceanic downwelling in the Beaufort Sea. The surface wind stress is the primary driver for variability in the upper Arctic Ocean and sea ice. The seasonal variability of the surface wind over the western Arctic is strongly influenced by a high sea level pressure center that emerges in the fall and diminishes in the summer. The wind stress and sea ice velocity are both anticyclonic from fall to spring and thus force an upwelling along the Alaskan and Canadian coast and downwelling in the interior Beaufort Sea. The upwelling and downwelling varied significantly on the interannual to decadal time scales from 1979 to 2006. There was no significant correlation between the upwelling/downwelling rate in the Beaufort Sea and the Arctic Oscillation index over this 28 year period. The coastal upwelling and interior downwelling in the Beaufort Sea had gradually intensified from 1979 to 2006. This change was almost entirely due to the increase in sea ice velocity according to three additional sensitivity calculations. The anticyclonic ice velocity over the western Arctic Ocean accelerated in the 28 year period, and the acceleration was not driven solely by the wind stress. The geostrophic wind condition was actually similar between 1979–1986 and 1997–2004. However, the ice velocity was much greater in the latter period. We hypothesize that the change in ice dynamics (thinner and less areal coverage) was responsible for the change of ice velocity.

[1] We analyze sea surface temperature (SST) and chlorophyll a (Chl a) satellite data to study the seasonal variability of the upwelling off central Chile. Data from an oceanographic cruise are used to illustrate the vertical characteristics of the upwelling front and the geostrophic flow. The mean offshore extension of the SST front in summer is ∼110 km. There, SST changes from ∼13.5°C near the coast to 14.5°–15°C offshore. Hydrographic data show that the thermocline becomes progressively shallower toward the coast but that at a certain distance it abruptly bends to form a horizontal front that extends until ∼50 m depth. Satellite data show that the Chl a decays exponentially offshore, and a concentration of ∼1 mg m−3 indicates the offshore limit of the frontal region. The SST front observed most of the year is absent in winter, when the coastal water is only slightly colder than the oceanic one; however, relatively higher Chl a values remain present near the coast during winter. Northwest of Punta Lavapie (37°15′S), the frontal region is located progressively farther offshore, suggesting that a branch of an equatorward surface jet separates from the coast to form a meander, which is associated with a filament of cold water and an anticyclonic eddy observed frequently in this zone during summer. Hydrographic data show that isotherm and current disturbances related to mesoscale eddies can extend several hundreds of meters below the surface. In contrast, the coastal jet is quite shallow, extending until ∼150 m depth.

[1] The mixed layer salinity budget in the northeastern subarctic Pacific is evaluated using 5 years (2003–2007) of Argo profiling float data, precipitation and evaporation, geostrophic velocity data, and wind stress observations. In this region the mixed layer salinity has a strong seasonal cycle, driven by seasonality in precipitation, evaporation, Ekman advection, and entrainment. Geostrophic advection effects show relatively little seasonal variability. Precipitation and Ekman effects in this area generally result in net decreases in salinity, while the evaporation, geostrophic advection, and entrainment terms yield increases. Within an annual cycle, the salinity tendency is positive during winter and fall, balanced by surface fluxes (evaporation and precipitation), entrainment, and geostrophic advection. The salinity tendency is negative during spring and summer. During these two seasons, it appears that salinity is controlled by precipitation, evaporation, and geostrophic advection. Overall, the precipitation term makes the largest contribution to the seasonal salinity budget, and the entrainment is especially important in autumn and winter.

[1] A 2-D divergence-free interpolator is presented which is demonstrated to give more realistic values between sparse velocity data than interpolating velocity components independently. The interpolator enforces physical dependence between the velocity components by ensuring mass is conserved. Tests using data for a synthetic eddy show that independent interpolation of components can give unrealistic velocities between widely spaced data. However, divergence-free interpolation can reproduce the eddy almost as well with sparse data as with dense data. In the synthetic tests the divergence-free radial basis function (RBF) interpolator performed as well as, or better than, optimal interpolation. A significant advantage of the “greedy fit” RBF interpolator implemented here is the small number of coefficients required to describe the interpolating surface. For large data sets this gives the interpolator a significant computational advantage over techniques, like Optimal Interpolation, which require one coefficient per data point. The RBF's economy of parameters and imposing physics with the divergence-free constraint makes RBF interpolation of sparse data much more resistant to following the noise. The 2-D divergence-free interpolator can be used to interpolate geostrophic velocities or the horizontal transport of high-frequency flows such as tidal currents. Tests with data from the local dynamics experiment moorings and from moving vessel acoustic Doppler current profiler measurements of tidal flow show the divergence-free interpolator is better able to reproduce data left out of the interpolation than interpolating velocity components independently.

[1] Numerical simulations based on realistic seafloor topography are used to examine near-bottom currents in the region of the Endeavour segment of Juan de Fuca Ridge (Endeavour Ridge) in the northeast Pacific. Results support earlier findings that hydrothermal venting within the ∼2200-m deep axial valley, rather than topographically modified, basin-scale cross-ridge geostrophic flow, is responsible for the near-steady northward currents observed within the confines of the valley. Although it does not generate deep flow within the valley, the basin-scale circulation determines the horizontal redistribution of the hydrothermal plumes once they rise above the ridge crest. Simulations of the near-bottom temperature and salinity fields reveal that model runs that incorporate a deep westward background flow most closely reproduce the observed plume anomaly distributions above the ridge, indicating that bottom currents in the region are predominantly westward, counter to the prevailing southeasterly flow of the wind-driven California Current in the upper half of the water column.

[1] This research examines the potential of ocean velocity observations generated from the maximum cross-correlation technique (MCC) applied to thermal infrared imagery to capture the total current at a high sampling rate, especially in the nearshore region, allowing for a novel view of the California Current. Comparison of 12 years of weekly ocean velocity fields derived from the MCC method with velocity observations from drifting buoys reveals strong correspondence between the data sets. The MCC method, however, is able to produce over ∼ 10 times the number of observations. Comparison of MCC velocities with geostrophic velocities from altimetry demonstrates differences that suggest ageostrophic currents in the MCC observations that are likely wind driven. The time-averaged mean velocity field from MCC observations reveals strong offshore jet-like features that extend off of coastline promontories. Eddy statistics from the MCC velocity observations indicate dramatically different dynamics in the nearshore and offshore regions. Differences in spatial statistics derived from the MCC observations compared to previous studies of the region are attributed to wind-driven currents in the observations that can dominate the variability of the current in the nearshore region.

[1] Cyclonic circulation dominates flow in the Bering Sea basin. The eastward flowing Aleutian North Slope Current (ANSC) flows along the north slope of the Aleutian Islands, turning northwestward in the southeast corner of the basin to form the Bering Slope Current (BSC). During the period 1997 to 2007, a pair of hydrographic lines was occupied 14 times in the southeastern portion of the basin. One transect was across the ANSC, and the second was across the BSC. In addition, a series of five yearlong moorings was deployed in a water depth of 1000 m in the ANSC, and a single yearlong mooring was deployed to the northeast in a water depth of 2200 m. At the primary mooring site, strong variability in temperature and salinity occurred at fortnightly and annual periods, while strong variability in the currents occurred at fortnightly and semiannual periods. The mean geostrophic flow relative to 1500 m, calculated from the 14 occupations of the hydrographic lines, was 3.1 × 106 m3 s−1 in the ANSC and 3.3 × 106 m3 s−1 in the BSC. A significant barotropic component, measured by the current meters, adds ∼3 × 106 m3 s−1 to the transport.

[1] The present study combines simple geostrophic considerations and observational data to examine the along-slope evolution of the barotropic transport following the continental slope in the Nordic Seas and Arctic Ocean. Following geostrophic theory, the transport evolution is calculated from bottom densities in five different transects within the Nordic Seas. This theoretically predicted transport evolution is compared with transports estimated directly from velocity measurements. Between the Faroe-Shetland Channel and the Svinoy section off southern Norway, across the Fram Strait (below 800 m), and between the Fram Strait and the Greenland Sea the agreement is generally good between geostrophic theory and observations. This indicates that many aspects of the barotropic flow in these regions are captured by geostrophic dynamics. Between the eastern and western Fram Strait above 800 m and between Svinoy and the eastern Fram Strait the observed flow fields differ to a large extent from the flow field predicted by geostrophic dynamics, indicating that ageostrophic dynamics play a more important role in these regions. The barotropic transport differences between the eastern and western Fram Strait contain information about the barotropic transport evolution along the entire Arctic continental slope. The good agreement between observations and theory across the strait (below 800 m) therefore indicates that the flow along the deeper part of the Arctic continental slope is well described by geostrophic dynamics. According to our results, the noticeably more baroclinic nature of the flow in the western Fram Strait, compared with the eastern strait, can then be ascribed to water mass modifications along the Arctic continental slope.

[1] This paper (1) compares the bottom flows of three existing high-resolution global simulations of the oceanic general circulation to near-bottom flows in a current meter database and (2) estimates, from the simulations, the global energy dissipation rate of the general circulation by quadratic bottom boundary layer drag. The study utilizes a data-assimilative run of the Naval Research Laboratory Layered Ocean Model (NLOM), a nonassimilative run of NLOM, and a nonassimilative run of the Parallel Ocean Program z-level ocean model. Generally speaking, the simulations have some difficulty matching the flows in individual current meter records. However, averages of model values of (the time average of the cube of bottom velocity, which is proportional to the dissipation rate) computed over all the current meter sites agree to within a factor of 2.7 or better with averages computed from the current meters, at least in certain depth ranges. The models therefore likely provide reasonable order-of-magnitude estimates of areally integrated dissipation by bottom drag. Global dissipation rates range from 0.14 to 0.65 TW, suggesting that bottom drag represents a substantial sink of the ∼1 TW wind-power transformed into geostrophic motions.

[1] A coupled boundary layer–Aeolian dust model of the Martian atmosphere is presented. This model was developed to determine how radiation (scattering, absorption, and emission) by dust affects the boundary layer and, in turn, how this affects the dust distribution in the atmosphere. This was achieved by coupling a planetary boundary layer (PBL) model with the 2007 dust model of P. A. Taylor et al. The principle motivation here is to determine whether it will be possible to use the lidar on board the Phoenix lander to detect the depth of the Martian boundary layer from the dust distribution. Runs were conducted for different dust profiles, roughness lengths, and geostrophic winds. These indicate that there should be a distinct horizon in dust concentration which will be detectable by the Phoenix lidar. The model is also applied to the 1977B dust storm optical data of Viking Lander 1, and our analysis indicates a significant improvement over previous 1-D studies of dust storm decay.

[1] The interpretation of surface altimetric signals in terms of Rossby waves is revisited. Rather than make the long-wave approximation, the horizontal scale of the waves is adjusted to optimally fit the phase speed predicted by linear theory to that observed by altimetry, assuming a first baroclinic mode vertical structure. It is found that in the tropical band the observations can be fit if the wavelength of the waves is assumed to be large, of order 600 km or so. However poleward of ±30°, it is more difficult to fit linear theory to the observations, and the fit is less good than at lower latitudes: the required scale of the waves must be reduced to about 100 km, somewhat larger than the local deformation wavelength. It is argued that these results can be interpreted in terms of Rossby wave, baroclinic instability, and turbulence theory. At low latitudes there is an overlap between geostrophic turbulence and Rossby wave timescales, and so, an upscale energy transfer from baroclinic instability at the deformation scale produces waves. At high latitudes there is no such overlap and waves are not produced by upscale energy transfer. These ideas are tested by using surface drifter data to infer turbulent velocities and timescales that are compared to those of linear Rossby waves. A transition from a field dominated by waves to one dominated by turbulence occurs at about ±30°, broadly consistent with the transition that is required to fit linear theory to altimetric observations.

[1] The magnitude, location, and extent of the California Undercurrent (CUC) off southern California is investigated using cruises of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) from 1993 to 2003 which provide hydrographic, biochemical, and acoustic Doppler current profiler (ADCP) velocity data on a quarterly basis. This study is the first use of the decade-long ADCP time series; it improves on prior geostrophic calculations by providing an absolute velocity reference for estimates of currents and transport. The long-term mean reveals two undercurrent cores in the region south of Point Conception and north of Baja, California: one in the region of the continental slope within the Southern California Bight (SCB) and a second off the Santa Rosa Ridge (SRR). A single core is observed off Point Conception. Spiciness is found to be a good indicator of the presence of the CUC; however, direct velocity observations or a deep reference level are required to resolve the full strength of the CUC cores. In particular, the core off the SRR would be almost entirely missed by geostrophic calculations relative to 500 m, the maximum depth sampled by CalCOFI. The undercurrent transport off Point Conception is estimated to be about 1.7 ± 0.1 Sv, slightly less than the sum of that estimated for the SCB (1.0 ± 0.1 Sv) and that estimated off the SRR (1.1 ± 0.1 Sv) and consistent with some of the flow turning offshore at Point Conception. The CUC in the SCB is strongest in summer, while that off the SRR is strongest in fall. The CUC off Point Conception is strong in both summer and fall, reflecting the confluence of these two branches. Interannual variability is also present, and the velocity and spiciness of the CUC appear to peak during El Nino periods.

[1] Atmospheric dust disturbances ranging in size from dust devils to planet-encircling dust storms are ubiquitous on Mars. After dust devils, the most common disturbances are local- or regional-scale disturbances. The origin of some of these mesoscale systems has been previously investigated and found to be linked to lifting along frontal systems or cap edge circulations. Very little attention has been given to whether the lifted dust in these systems result in radiative forcing that might modulate the local system dynamics with an amplitude large enough to affect local dust-lifting processes. Idealized numerical modeling results presented herein show that a positive feedback process between local dynamics and radiative forcing of lifted dust can occur under some conditions. The feedback process is distinctly different than an enhancement of the general circulation by increasing atmospheric dust loading because the dynamical effects of this feedback process occur locally, within the disturbance itself. Optimal conditions for growth of initial atmospheric dust perturbations include (1) subtropical latitudes associated with relatively large solar insolation and moderate coriolis force; (2) modest dust-lifting thresholds and dust-lifting efficiencies; (3) relatively large initial dust perturbations; (4) steep background lapse rates; and (5) a barotropic environment. The positive feedback process is explained by a combination of geostrophic adjustment theory and a Carnot engine-like mechanism related to the Wind-Induced Sensible Heat Exchange hypothesis for tropical cyclones on Earth.

[1] Present methods used to retrieve altimeter data do not provide reliable estimates of sea surface height (SSH) in the nearshore region, resulting in a measurement gap of 25–50 km next to the coast. In the present work, gridded SSH fields produced by Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) in the offshore region are combined with coastal tide gauge time series of SSH to improve estimation in that gap along the west coast of the United States in the northern California Current System between 40 and 45N and 123.8 and 126W. To assess the increase in skill provided by this procedure, the geostrophic alongshore currents, calculated from the new SSH fields in the gap region, are compared to three in situ, nearshore current measurements, resulting in correlation coefficients of 0.73–0.83 and standard deviations of the differences of 11.6–12.6 cm/s, substantially improved from the AVISO-only results. When the Ekman current components are estimated and added to the geostrophic currents, comparisons to the 10 m deep acoustic Doppler current profiler velocities are only slightly improved. The Ekman components make a more significant contribution when compared to HF radar surface current measurements, providing correlations of 0.94 and standard deviations of the differences of 6.4–9.5 cm/s. These results represent a dramatic improvement in the quality of the SSH fields and estimated alongshore currents when additional, realistic SSH data from the coastal region are added. Here we use coastal tide gauges to provide the additional SSH data but also discuss more general approaches for altimeter SSH retrievals in coastal regions where tide gauge data are not available.

[1] The distribution of wind-induced internal wave energy in the world's oceans is investigated using a full three-dimensional primitive equation model. Special attention is directed to the global energy input to the surface near-inertial motions and the subsequent downward energy propagation into the deep ocean. We find that the model results for near-inertial energy in the oceanic mixed layer, depth-integrated horizontal energy fluxes, and vertical structures of WKB-scaled kinetic energy are all consistent with the available observations in the regions of significant wind energy input and that the annual mean of the global wind energy input becomes ∼0.4 TW. It is also found that most of the wind-induced energy resides in high vertical modes, 75–85% of which is dissipated in the surface 150 m. The present study therefore predicts that the total wind-induced near-inertial energy available for deep-ocean mixing is limited to, at most, 0.1 TW, which is an order of magnitude smaller than previously estimated. Adding the energy flux from tide-topography interactions of ∼0.9 TW, we can conclude that the total energy available for deep-ocean mixing is ∼1.0 TW, obviously falling short of the required power to sustain the global overturning circulation. This might suggest the existence of other important energy sources, such as the one through geostrophic adjustment processes, and/or additional mechanisms sustaining the global overturning circulation, such as effects of Ekman upwelling in the Southern Ocean. Another possibility is that previous estimates of the volume transport of the global overturning circulation might be too large.

[1] The relationship between changes in sea-surface height, bottom pressure, and overturning is explored using isopycnal model experiments for the North Atlantic. Changes in high-latitude forcing are communicated rapidly over the basin through boundary wave propagation along the continental slope, involving a hybrid mixture of Kelvin and topographic Rossby waves, as well as spreading more slowly through advection along the western boundary. This wave communication leads to coherent signals in sea-surface height and bottom pressure variability extending for several thousand kilometers along the continental slope. The model results are in broad agreement with altimetric diagnostics, and the patterns only alter in detail with the realism of the topography. The adjustment in bottom pressure is directly linked to a change in overturning since west-east contrasts in bottom pressure are associated with a zonal integral in the meridional geostrophic flow. Correlation patterns reveal that temporal changes in overturning are primarily connected to the vertical contrast in bottom pressure, across the shelf and continental slope, along the western boundary.

A time series of the net geostrophic transport through the Tasman Sea (representing the flow of the East Australian Current (EAC) Extension) is determined from a full-depth CTD section, 15 years of high-density XBT transects, and satellite altimetry data. A section between Sydney and Wellington (PX34) has been occupied four times per year since 1991 with high resolution XBT sampling. Two methods to infer baroclinic transport from proxy data along the section are presented. The first uses shallow XBT transects to derive geostrophic transport relative to a deep (2000 m) reference level. In the second approach (SynTS) the subsurface temperature and salinity structure are inferred from satellite surface height and temperature fields using a model developed from historical in situ observations. The baroclinic transport is then computed in the usual manner. The methods are validated using both a full-depth CTD occupation of the PX34 section and further transects crossing the EAC in the northern Tasman Sea. There is close agreement between the 49 XBT and SynTS PX34 transport estimates obtained between 1992 and 2006. The time series of transport through the Sydney–Wellington section shows a range of temporal signals from eddyscale, seasonal, interannnual to decadal. In particular, we note that the net EAC flow ranges from 5 Sv in 1995 to a maximum of 16 Sv in 2000/2001. This decadal variation confirms the EAC response to a spin-up of the South Pacific circulation forced by changes in the basin-wide winds and matches the changes in oceanic properties observed in the Tasman Sea.

[1] In late May of 2008, the NASA/JPL Phoenix spacecraft will touch down near its targeted landing site on Mars (68.2°N, 126.6°W). Entry, descent, and landing (EDL) occurs in the late afternoon (∼1630 hours local solar time (LST)) during late northern spring (Ls ∼ 78°). Using a mesoscale and a large-eddy simulation (LES) model, we have investigated the range of conditions that might be encountered in the lower atmosphere during EDL. High-resolution (∼18 km) results from the Oregon State University Mars MM5 (OSU MMM5) are used to understand the hazards from the transient circulations prominent during this season. Poleward of ∼80°N these storms produce strong winds (∼35 m s−1) near the ground; however, owing to the synoptic structure of these storms, and the deep convective mixed layer equatorward of the seasonal cap boundary during EDL, our modeling suggests the spacecraft would not be in winds stronger than ∼20 m s−1 at parachute separation. The storm-driven variability is much weaker at Phoenix latitudes than it is poleward of the seasonal cap edge (result from an extensive sensitivity study). The OSU MLES model is used to explicitly simulate the hazards of convection and atmospheric turbulence at very high resolution (100 m). This modeling suggests that an upper bound for the maximum expected horizontal-mean atmospheric turbulent kinetic energy (TKE) is ∼12 m2 s−2, seen ∼3 km above the ground at ∼1430 hours LST. TKE amplitudes are greatest when the horizontal mean wind is large (shear production) and/or the surface albedo is low (a lower albedo enhances buoyancy production, mimicking decreased atmospheric stability after a storm advects colder air into the region). LES simulations predict deep mixed layers (∼6–7 km), ∼1.5 km deeper than the mesoscale model (∼5 km). Mesoscale modeling suggests that the actual landing site differs meteorologically from other longitudes (larger-amplitude diurnal wind cycle), a consequence of the strong thermal circulations that are excited by the very large regional topography. The OSU MLES model was modified for this work to utilize time- and height-dependent geostrophic wind forcing (constructed from OSU MMM5 results). With this forcing, the OSU MLES provides a site-specific simulation, where the time/height variability of the horizontal mean LES wind field is in good agreement with the OSU MMM5. On the basis of some statistical analysis, we have good confidence that the “full-spectrum” wind field is within engineering guidelines for Phoenix EDL.

[1] A diagnostic circulation model and water mass analyses are used to examine variability in the structure and circulation of the Juan de Fuca Eddy, a highly productive region at the northern end of the California Current. Results from three years of field studies demonstrate that the eddy increases in spatial extent from early to late summer as the vertically averaged contribution of California Undercurrent source water grows from ∼60% in June to ∼80% in September. Typical near-surface eddy radii range from ∼15 km in the early summer to ∼30 km in September and increase with depth. Below 100 m, eddy radii are ∼40 km. Fresher water, associated with the estuarine outflow from the Juan de Fuca Strait, is advected around the eddy margin. During southward wind conditions, the combination of cyclonic geostrophic flow and wind-driven currents in the surface Ekman layer cause the eddy to be “leaky” on its southern perimeter. Eddy surface circulation becomes more retentive (up to ∼32 d observed) during periods of weak winds or frequent northward reversals. The presence of the eddy facilitates large inputs of dissolved inorganic nutrients into the region through two mechanisms: doming of California Undercurrent water within the eddy and enhanced cross-shelf advection of Juan de Fuca Strait outflow. The combination of these sources results in a persistent, broad (100 km offshore) region of elevated macronutrients. The retentive circulation patterns combined with persistent nutrient supply may favor the development of toxigenic diatom blooms of Pseudo-nitzschia species in this region.

[1] In April 2001 four hydrographic sections perpendicular to the southern East Madagascar Current were surveyed as part of the Agulhas Current Sources Experiment. Observations with a vessel mounted and a lowered ADCP produced information on the current field while temperature, salinity, oxygen and nutrient data obtained with a CTD-Rosette system, gave information on the water mass structure of the currents southeast of Madagascar. The peak velocity in the pole-ward East Madagascar Current through these four sections had a typical magnitude of ∼110 cm/s, while the width of this current was of the order of 120 km. The mean pole-ward volume transport rate of this current during the survey above the 5°C isotherm was estimated to be 37 ± 10 Sv. On all four sections an undercurrent was observed at intermediate depths below the East Madagascar Current. Its equator-ward transport rate amounted to 2.8 ± 1.4 Sv. Offshore of the East Madagascar Current the shallow South Indian Ocean Countercurrent was observed. This eastward frontal jet coincided with the barotropic and thermohaline front that separates the saline Subtropical Surface Water from the fresher Tropical Surface Water in the East Madagascar Current. The near-surface geostrophic flow of the East Madagascar Current, derived from satellite altimetry data from 1992 to 2005, suggests a strong variability of this transport due to eddy variability and interannual changes. The long-term pole-ward mean transport of the East Madagascar Current, roughly estimated from those altimetry data amounts to 32 Sv. The upper-ocean water mass of the East Madagascar Current was very saline in 2001, compared to WOCE surveys from 1995. Comparison of our undercurrent data with those of the WOCE surveys in 1995 confirms that the undercurrent is a recurrent feature. Its water mass properties are relatively saline, due to the presence of water originating from the Red Sea outflow at intermediate levels. The saline water was advected from the Mozambique Channel to the eastern slope of Madagascar.

[1] The present paper describes the physical characteristics of an upwelling filament off Cape St. Vincent (southern Iberia) in October 2004, based on remote sensing, hydrographic, ADCP and wind data. It was associated with a meandering jet similar to eastern Pacific or Canary filaments, though its dynamical features were weaker as a result of the surface-trapped (top 100 m) and weak horizontal density gradients. The filament transported 0.9 Sv of coastal water offshore, of which 8% was due to Ekman transport; it formed following the destabilization of an alongshore equatorward upwelling jet close to the cape. The filament was structurally asymmetric, with anticyclonic geostrophic vorticity to the north and more strongly cyclonic to the south, as a result of the meanders and existing eddies. The filament comprised a sequence of cold, submesoscale (∼30 km diameter) cyclones. Vertical velocities of ±15 m·d−1 were associated with meanders of the jet. The surface and subsurface circulation was coupled. At middepths (90–150 m) anticyclonic recirculation of northern water seemed associated with a separated, older upwelling jet. Mediterranean Water (MW) was present below 350 dbar as a topographically steered undercurrent and a meddy-cyclone dipole. These undercurrents contributed to the formation of the October filament through baroclinic instability associated with the enhanced vertical shear.

[1] Satellite-derived mean sea level anomalies were blended with mean dynamic topography to obtain seasonal climatologies of the eastern South Pacific Ocean's surface circulation with an emphasis on the Humboldt Current. The Humboldt Current is confined to a narrow northward flowing jet-like stream of about 350 km associated with maximum geostrophic velocities of more than 15 cm s−1 from 35° to 23°S. The current streams closer to the coast in the south (within 500 km), but is found 500 km offshore in its northern section. A bifurcation takes place at 23°S, with a more coastal branch heading toward the coast of southern Peru (15°S) and an oceanic branch continuing in a north-northwesterly direction. The Humboldt Current intensifies by 3 cm s−1 in austral summer as compared to winter. In contrast, the Humboldt Current's coastal branch is stronger in austral winter by the same order of magnitude. We attribute these distinct seasonal flow fields to the meridional movement of the South Pacific Anticyclone, which includes a seasonal relocation of a strong wind stress curl nucleus from offshore central-southern (austral summer) to central-northern Chile (austral winter), thus driving the Humboldt Current by means of seasonal Sverdrup dynamics. The geostrophic flow field in the eastern South Pacific communicates colder and fresher Sub-Antarctic Surface Water toward the tropics along the eastern rim of the subtropical gyre.

[1] Sea level of the Japan/East Sea observed by the TOPEX/Poseidon (T/P) satellite altimeter is analyzed using a 1/4°-resolution ocean general circulation model. A significant fraction of the Japan/East Sea sea level variability is found to be spatially uniform with periods ranging from 20 d to a year. The model simulation is consistent with T/P records in terms of the basin-wide sea level fluctuation's spectral energy and coherence. The simulation indicates that the changes are barotropic in nature and controlled, notably at high frequencies, by the net mass transport through the straits of the Japan/East Sea driven by winds in the vicinity of the Korea/Tsushima and Soya Straits. A series of barotropic simulations suggest that the sea level fluctuations are the result of a dynamic balance at the straits among near-strait winds, friction, and geostrophic control. The basin-wide sea level response is a linear superposition of changes due to winds near the individual straits. In particular, a basin-wide sea level response can be established by winds near either one of the straits alone. For the specific geometry and winds, winds near the Soya Strait have a larger impact on the Japan/East Sea mean sea level than those near the Korea/Tsushima Strait.

[1] The bulk heat transfer coefficient in the ice–upper ocean system (Kb) in the ice melt season is estimated by a new method at 18 areas that cover much of the Antarctic seasonal ice zone. The method is based on a model in which ice melting is caused only by heat input through open water and is treated in a bulk fashion in the ice–upper ocean system. Kb is estimated by fitting a convergent curve derived from the model to an observed ice concentration–temperature plot (CT-plot). Estimated Kb is 1.15 ± 0.72 × 10−4 m s−1 on average. If Kb can be expressed by the product of the heat transfer coefficient (ch) and the friction velocity (uτ), ch is 0.0113 ± 0.0055. This value is about two times larger than that estimated at the ice bottom. The relationship between Kb and the geostrophic wind speed (Uw), which is roughly proportional to uτ, shows a significant positive correlation, as expected. Further, Kb seems more likely to be proportional to the square or cube of Uw rather than a linear relationship. Since Kb estimated from our method is associated with ice melting in a bulk fashion in the ice–upper ocean system, this relationship likely indicates both the mixing process of heat in the upper ocean (proportional to uτ3) and the local heat transfer process at the ice–ocean interface (proportional to uτ).

[1] Numerical simulations with the Regional Ocean Modeling System are used to study the generation of the cyclonic Juan de Fuca Eddy located off the entrance of Juan de Fuca Strait in summer. An initial simulation forced with average summer upwelling favorable winds, tides, and buoyancy boundary conditions that maintain an estuarine flow in Juan de Fuca Strait produces an eddy and currents that are in reasonable agreement with observations. Sensitivity studies are then carried out to explore the importance of these three forcing mechanisms. The relative proximity of dense water in the bottom estuarine flow entering the strait is shown to lead to enhanced upwelling off Cape Flattery when either wind or tidal forcing is applied. This upwelled water then mixes with the estuarine outflow and is advected offshore. The tidal upwelling arises through three mechanisms: M2 vertical excursions of nearly 20 m at 50 m depth west of the cape on the flood tide; strong tidally rectified vertical velocities west of the cape; and the spilling of denser bottom water over the western wall of Juan de Fuca Canyon on the ebb tide. The cyclonic eddy is a consequence of geostrophic adjustment to the doming isopycnals that arise from the upwelling. These model simulations refute an earlier hypothesis that the eddy is generated when California Undercurrent water is drawn up Tully Canyon and onto the Vancouver Island shelf, suggesting instead that these canyon dynamics play only a secondary role in maintaining the eddy once it is formed.

A nested-grid ocean circulation modeling system is used to study the response of Lunenburg Bay in Nova Scotia, Canada, to local wind-forcing, tides, remotely generated waves, and buoyancy forcing in the summer and fall of 2003. Quantitative comparisons between observations and model results demonstrate that the modeling system reproduces reasonably well the observed sea level, temperature, salinity, and currents in the bay. Numerical results reveal that the spatial and temporal variability of temperature and salinity in the bay during the study period is mainly forced by the local wind stress and surface heat/freshwater fluxes, with some contribution from tidal circulation. In particular, the local heat balance on the monthly timescale is dominated by cooling due to vertical advection and warming due to horizontal advection and net surface heat flux, while high-frequency variations (timescales of 1–30 days) are mainly associated with vertical advection, i.e., wind-induced upwelling and downwelling. There is also a strong baroclinic throughflow over the deep water region outside Lunenburg Bay that is strongly influenced by wind-forcing. The vertically integrated momentum balance analysis indicates a modified geostrophic balance on the monthly timescale and longer, and is dominated by the pressure term and wind minus bottom stress in the high-frequency band.

[1] Using GPS radio occultation data during 2001–2005, we studied the climatological behavior of atmospheric gravity waves in the polar stratosphere. We calculated temperature fluctuations with vertical wavelengths shorter than 7 km and then determined the wave potential energy, Ep, every month in a longitude-latitude cell of 20° × 10° between 12 km and 33 km. In the Arctic region (50–90°N), Ep shows an annual variation with maximum in winter, consistent with the zonal mean horizontal wind, V, and the Eliassen-Palm (E-P) flux, Fz. The large Fz values indicate higher planetary wave activity, resulting in distortion of the polar vortex. The unbalanced flow can then excite gravity waves through geostrophic adjustment. In the Antarctic region (50–90°S), Ep gradually increases during winter and reaches its maximum in spring before decreasing rapidly. The time derivative of V coincides with the Ep peak and the horizontal distribution of Ep has a similar structure to V, suggesting that the Ep enhancement is closely related to the decay of the polar vortex. During major warming events over the Arctic, the divergence of E-P flux, ΔF, was enhanced, coinciding with large Ep. In the Antarctic, ΔF strongly correlates with Ep in spring. Gravity waves seem to be effectively generated through planetary wave transience and/or breaking. Orographic generation of gravity waves seems to be important in limited areas only, such as Scandinavia and the Antarctic Peninsula, showing that it is less important than the polar night jet in determining the climatological behavior of gravity waves.

We use QuikSCAT scatterometer data, together with geostrophic surface currents calculated from a combination of satellite altimetry, gravity and drifter data, to investigate the rate of work done on the geostrophic circulation by wind stress. In particular, we test the suggestion that accounting for ocean currents in the calculation of stress from 10 m winds can result in a reduction of 20–35% in the wind work, compared with an approximate calculation in which currents are not accounted for. We calculate the predicted effect of accounting for ocean currents to be a reduction in power of about 0.19 TW, and find a total power input from observations which include this effect to be 0.76 TW, smaller than earlier estimates by about the right amount. By recalculating the power input using smoothed wind stresses or currents, we demonstrate that the effect of ocean currents is visible in the midlatitude data, and close to the predicted value. Proof that the data are adequate to resolve the effect in the tropics, however, is lacking, suggesting that additional processes may also be important in this region.

[1] The mean heat and freshwater transports in the North Pacific subtropical gyre during 1998–2002 are estimated. High-resolution expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) transects (PX-40, Honolulu to Japan; PX-37, San Francisco to Honolulu; PX-10, Honolulu to Guam; PX-44, Guam to Taiwan/Hong Kong) are used to calculate geostrophic transport across each of the ship tracks. Ekman transport is estimated from satellite-scatterometer wind stress. The mean heat and freshwater transport convergences into the northern box bounded by the PX-40/37 transects and the Tsushima and Bering Straits are 0.26 ± 0.16 pW (pW = 1015 W) and −0.26 ± 0.11 Sv (Sv = 106 m3/s), respectively. Heat and freshwater transport convergences into the western box bounded by the PX-40/10/44 transects and the Tsushima Strait are estimated to be 0.32 ± 0.17 pW and 0.08 ± 0.07 Sv, respectively. In both boxes, warmer waters transported inward by the Ekman flow and by the Kuroshio are compensated by the export of waters at cooler temperatures, whose peaks are found in the temperatures of the mode waters formed in the North Pacific. The salt budget is also described to consider the mechanisms of freshwater transport. Since the western box includes the region with the strongest heat loss to the atmosphere and is possibly a key region for climatic decadal variation, it is necessary to continue the high-resolution XBT/XCTD measurement and to make an effort at improving the estimation of heat and freshwater transports in order to contribute to advancing climate studies.

[1] The East Greenland Current (EGC) dynamically connects the Arctic Ocean to the North Atlantic on the western side of the Nordic Seas. Observations show that the speed of the EGC considerably varies along the East Greenland Slope (EGS). Here it is shown, using current meter data reported in the literature and climatological hydrographic fields, that velocity and transport variations along the EGS are supported by the cross-isobath component of the density-dependent geostrophic flow relative to the bottom. The relative flow impinging on (leaving) the EGS in a northern (southern) limb of the cyclonic circulation in the Nordic Seas strengthens (weakens) the along-isobath bottom geostrophic flow. Variations of the latter are clearly associated with along-isobath bottom density gradients. Current observations indicate an increase of the along-isobath bottom velocity from 79°N to 75°N equal to about 9 and 10 cm s -1 on the upper (1000 m isobath) and lower (2000 m isobath) EGS, respectively. Corresponding estimates based on bottom density distribution along the 1000 and 2000 m isobaths are grossly consistent with the observations given above though we obtain a higher increase (13 cm s -1 ) at 1000 m and lower increase (6 cm s -1 ) at 2000 m. Considering the variability of the system and the poor resolution of the observations we find this to be a very convincing result, demonstrating the power of the geostrophic approximation for such estimates.

[1] This paper presents simulations of the Adriatic Sea response to two distinct bora wind events, one in summer (11–20 August 2001), when the water is stratified, and the other in winter (11–20 February 2003), when it is vertically homogeneous. The simulations employ the DieCAST model on a 1.2-min grid (about 2-km resolution) and resolve the mesoscale variability because the grid size falls below the first baroclinic deformation radius (about 5–10 km) and the model has very low horizontal dissipation. The model is initialized with seasonally averaged temperature and salinity data and spun up with climatological winds. The summer of August 2001 event leads to the generation of a coastal current directed paradoxically to the left of the wind and identified with the summertime Istrian Coastal Countercurrent. Analysis of the physics simulated by the model leads to the conclusion that this current is caused by baroclinic geostrophic adjustment of the Istrian coastal waters following a rapid but strong wind impulse. According to both satellite observations and model simulations, the current persists for more than a week after the bora event. The winter event of February 2003 generates a slightly less complicated situation because the shallow northern Adriatic is then vertically homogeneous, but in situ observations at the time of this particular event permit a comparison of model results with observations and thus an evaluation of the model performance. In particular, the model simulates correctly a series of currents, bifurcations, and confluences of the wind-driven currents across the northern Adriatic basin. A lesson learned is that a bora event, though generally strong, especially in winter, leaves a legacy that does not obliterate completely what existed prior to the event. In other words, the state of the northern Adriatic basin is fashioned by sequential events, and one bora event may not be viewed in isolation but must be considered as one episode in an unfolding succession of events.

[1] The evolution of freshwater plumes and the associated salinity fronts in the northern Bay of Bengal ( henceforth the bay) is studied using rotated empirical orthogonal function (REOF) analysis and extended associate pattern analysis (EAPA). The results show that sea surface salinity distribution is featured by eastern-bay and western-bay plumes in the northern bay during different seasons. The western-bay plume begins in early July, peaks in late August, and then turns into a bay-shaped plume with the two plumes in either side of the bay, which peaks in late October. The southward extension of the western-bay plume can be explained by the southwestward geostrophic flow associated with the cyclonic gyre in the northern bay, which counters the northeastward Ekman drift driven by wind stress. The offshore expansion of the western-bay plume is induced by the offshore Ekman drift which also produces a salinity front near the east coast of India. The bay-shaped plume appears when the cyclonic gyre shifts westward and a weak anticyclonic gyre occupies the northeastern bay. As the season advances, the western part of the bay-shaped plume decays while the eastern part persists until the following June, which is believed to be associated with the anticyclonic gyre in the northern bay. The evolution of the plumes except the eastern part of the bay-shaped plume in fall can be partly explained by the seasonal variation of mass transport associated with the Sverdrup balance. The fact that the western-bay (eastern-bay) plume appears when surface freshwater flux in the northeastern bay increases ( decreases) dramatically suggests that the plumes are not produced directly by surface freshwater flux. River discharge seems to be the freshwater source for the plumes and has little to do with the evolution of the plumes.

The Hatteras Front is a persistent mesoscale cross-shelf oriented front off Cape Hatteras, North Carolina. It is the boundary between relatively cool, fresh Mid-Atlantic Bight shelf waters and warmer, saltier shelf waters of the South Atlantic Bight, which both converge along-shelf upon Cape Hatteras year round. The Frontal Interaction Near Cape Hatteras (FINCH) project was conducted in 2004-2005 to intensively sample the Hatteras Front with shipboard ADCP and undulating towed CTD. This paper documents velocity and density structures associated with the cross-shelf oriented zone of Hatteras Front during the August 2004 field season. Property gradients across the Hatteras Front are large, with temperature (T) and salinity (S) differences of ∼4-6°C, 2-5 psu, respectively over distances of 1-2 km. The T and S are not completely compensating, and a strong density (p) gradient also exists, with Ap of∼2 kg/m 3 across a gentler 10 km wide front. The density gradient results in a steric sea-level height gradient of ∼ 1-2 cm across the Front, which is in approximate geostrophic balance with a surface intensified jet, directed shoreward along the cross-shelf oriented Front. The velocity is sheared with depth at 3.0 × 10 -2 to 5.0 x 10- 2 s -1 in the upper 5 m of the jet; a rate consistent with the density gradient according to the thermal wind relationship. Shoreward transport of ∼4.8 x 104 m 3 /s results from the surface intensified jet. The structure of the velocity field associated with the Hatteras Front resembles that of a slope-controlled buoyant plume, as described by Lentz and Helfrich (2002). Velocity and density structures are similar during both advancing (southwestward) and retreating (northeastward) motion of the Front.