摘要:

Dense shelf water (DSW) is formed in East Antarctica from enhanced sea‐ice production driven by air‐sea interaction in coastal polynyas. Cross‐shelf export and downslope mixing of this DSW produces Antarctic Bottom Water, contributing to the lower limb of the global overturning circulation. We present biogeochemical observations from the Mertz Polynya region in summer 2007/2008, with additional observations from spring 2001 and winter 1996. The seasonal changes in mixed‐layer carbonate chemistry are driven by a combination of air‐sea CO2exchange, biological activity and the formation and melt of sea‐ice. The air‐sea fluxes in 2008 were ∼15 mmol C m−2d−1, and net community production, estimated from the summertime surface dissolved inorganic carbon (DIC) deficit, ranged from 0.9 to 1.2 mol C m−2yr−1. We show that biological modification of carbonate chemistry over the shelf in summer preconditions the DSW outflows from the Adélie Depression. This process appears to supply both organic material, and water depleted in CO2, and with enhanced carbonate saturation state (relative to inflowing water), to coral communities on the slope. We combined model‐based transports of exported DSW with the natural and anthropogenic (Cant) carbon concentrations and estimate that the annual outflows of DIC and Cantfrom the Mertz Polynya range from 320 to 560 Tg C yr−1, and from 3 to 6 Tg Cantyr−1, respectively. The formation and export of dense water from this region, and by extension all similar polynyas around Antarctica, is an effective mechanism for the transfer of anthropoge

ISSN:0148-0227    

DOI:10.1002/2013JC009286

年代:2014

数据来源: WILEY

摘要:

This study investigates the sea surface salinity (SSS) and barrier layer variability in the equatorial Pacific using recently available Aquarius and Argo data. Comparison between the two data sets indicates that Aquarius is able to capture most of the SSS features identified by Argo. Despite some discrepancies in the mean value, the SSS from the two data sets shows essentially the same seasonal cycle in both magnitude and phase. For the period of observation between August 2011 and July 2013 Aquarius nicely resolved the zonal displacement of the SSS front along the equator, showing its observing capacity of the western Pacific warm pool. Analysis of the Argo data provides further information on surface stratification. A thick barrier layer is present on the western side of the SSS front during all the period of observation, moving back and forth along the equator with its correlation with the Southern Oscillation Index exceeding 0.80. Generally, the thick barrier layer moves eastward during El Niño and westward during La Niña. The mechanisms responsible for this zonal displacement are discusse

ISSN:0148-0227    

DOI:10.1002/2013JC009375

年代:2014

数据来源: WILEY

摘要:

The impact of the surface wave field (sea state) on the wind stress over the ocean is investigated with fetch‐dependent seas under uniform wind and with complex seas under idealized tropical cyclone winds. Two different approaches are employed to calculate the wind stress and the mean wind profile. The near‐peak frequency range of the surface wave field is simulated using the WAVEWATCH III model. The high‐frequency part of the surface wave field is empirically determined using a range of different tail levels. The results suggest that the drag coefficient magnitude is very sensitive to the spectral tail level but is not as sensitive to the drag coefficient calculation methods. The drag coefficients at 40 m/s vary from1×10−3to4×10−3depending on the saturation level. The misalignment angle between the wind stress vector and the wind vector is sensitive to the stress calculation method used. In particular, if the cross‐wind swell is allowed to contribute to the wind stress, it tends to increase the misalignment angle. Our results predict enhanced sea state dependence of the drag coefficient for a fast moving tropical cyclone than for a slow moving storm or for simple fetch‐dependent seas. This may be attributed to swell that is significantly misaligned

ISSN:0148-0227    

DOI:10.1002/2013JC009289

年代:2014

数据来源: WILEY

摘要:

We use four current meter moorings and quasi‐synoptic hydrographic observations in conjunction with a one‐dimensional quasi‐geostrophic linear stability model to investigate downstream changes in the Brazil Current (BC) System between 22°S and 28°S. The data set depict the downstream thickening of the BC. Its vertical extension increases from 350 m at 22.7°S to 850 m at 27.9°S. Most of this deepening occurs between 25.5°S and 27.9°S and is linked to the bifurcation of the South Equatorial Current at intermediate depths (Santos bifurcation), which adds the Antarctic Intermediate Water flow to the BC. Geostrophic estimates suggest that the BC transport is increased by at least 4.3 Sv (∼70%) to the south of that bifurcation. Moreover, the Santos bifurcation is associated with a substantial increase in the barotropic component of the BC System. On average, the water column average kinetic energy (IKE) is 70% baroclinic to the north and 54% barotropic to the south of the bifurcation. Additionally, the BC shows conspicuous mesoscale activity off southeast Brazil. The water column average eddy kinetic energy accounts for 30–60% of the IKE. Instabilities of the mean flow may give rise to these mesoscale fluctuations. Indeed, the linear stability analysis suggests that the BC System is baroclinically unstable between 22°S and 28°S. In particular, the model predicts southwestward‐propagating fastest growing waves (∼190 km) from 25.5°S to 27.9°S and quasi‐standing most unstable modes (∼230 km) at 22.7°S. These modes have vertical structures roughly consistent

ISSN:0148-0227    

DOI:10.1002/2013JC009143

年代:2014

数据来源: WILEY

摘要:

A three‐dimensional wave‐current model is used to investigate wave‐induced circulations in a small estuarine bay and its impact on freshwater exchanges with the inner shelf, related to stratified river plume dispersion. Modeled salinity fields exhibit a lower salinity surface layer due to river outflows, with typical depth of 1 m inside the bay. The asymmetric wave forcing on the bay circulation, related to the local bathymetry, significantly impacts the river plumes. It is found that the transport initiated in the surf zone by the longshore current can oppose and thus reduce the primary outflow of freshwater through the bay inlets. Using the model to examine a high river runoff event occurring during a high‐energy wave episode, waves are found to induce a 24 h delay in freshwater evacuation. At the end of the runoff event, waves have reduced the freshwater flux to the ocean by a factor 5, and the total freshwater volume inside the bay is increased by 40%. According to the model, and for this event, the effect of the surf zone current on the bay flushing is larger than that of the wind. The freshwater balance is sensitive to incident wave conditions. Maximum freshwater retention is found for intermediate offshore wave heights 1 m < Hs < 2 m. For higher‐energy waves, the increase in the longshore current reduces the retention, which is two times lower forHs =

ISSN:0148-0227    

DOI:10.1002/2013JC009466

年代:2014

数据来源: WILEY

摘要:

Tides play an important role in ice sheet dynamics by modulating ice stream velocity, fracturing, and moving ice shelves and mixing water beneath them. Any changes in ice shelf extent or thickness will alter the tidal dynamics through modification of water column thickness and coastal topography but these will in turn feed back onto the overall ice shelf stability. Here, we show that removal or reduction in extent and/or thickness of the Ross and Ronne‐Filchner ice shelves would have a significant impact on the tides around Antarctica. The Ronne‐Filchner appears particularly vulnerable, with an increase inM2amplitude of over 0.5 m beneath much of the ice shelf potentially leading to tidally induced feedbacks on ice shelf/sheet dynamics. These results highlight the importance of understanding tidal feedbacks on ice shelves/streams due to their influence on ice sheet dynam

ISSN:0148-0227    

DOI:10.1002/2013JC009240

年代:2014

数据来源: WILEY

摘要:

The regional upper‐ocean dynamical and thermodynamical responses to two consecutive, strong tropical cyclones (TCs)—04B (15–19 October) and 05B (25 October to 3 November) in 1999 (hereafter, TC1 and TC2) in the Bay of Bengal (BoB) and the associated oceanic processes are investigated using an eddy‐permitting Hybrid Coordinate Ocean Model. The TC winds induce positive sea surface height anomalies (SSHA) along the northern BoB coastline and Andaman Sea due to onshore convergence, and negative SSHA along the TC tracks due to Ekman divergence, which in turn induce geostrophic flow. The TC‐associated radiation and precipitation have negligible effects on the northwestern BoB top 30 m averaged temperature (T0–30 m), while the strong TC winds significantly enhance turbulent heat flux causing T0–30 mdecrease. Due to the existence of the barrier layer and subsurface warm advection in the northwestern BoB, vertical mixing may induce near‐surface warming by entraining warm water from below. As a result, the proportion of the T0–30 mcooling caused by turbulent heat flux is likely elevated in the 6° × 6° footprints of the TCs. Both TC wind‐induced vertical mixing and upwelling significantly cool T0–30 min TC1's wake, while upwelling dominates the maximum cooling region in TC2's wake, likely due to the preceding deepening of the mixed layer by TC1. The near‐surface cooling for both TCs has rightward bias because of the higher winds and the resonant response on the right, and Ekman divergence extends the cooling areas outward. TC1 and TC2's sizes and prestorm oceanic conditions are found to be the most influential factor

ISSN:0148-0227    

DOI:10.1002/2013JC008965

年代:2014

数据来源: WILEY

摘要:

The noise present in infrared satellite measurements of sea surface temperature (SST) hampers the use of surface quasi‐geostrophic (SQG) equations to diagnose ocean dynamics at high resolutions. Here we propose a methodology to reduce the contribution of noise when diagnosing surface vorticity, divergence, and vertical velocity from SST able to retain the dynamics at scales of a few kilometers. It is based on the use of denoising techniques with curvelets as basis functions and the application of a selective low‐pass filters to improve the reconstruction of surface upwelling/downwelling patterns. First, it is tested using direct numerical simulations of SQG turbulence and then applied to diagnose low‐frequency vertical velocity patterns from real MODIS (Moderate Resolution Imaging Spectroradiometer) images. The methodology here presented, which is not tied to the validity of SQG equations nor to the use of SST, is quite general and can be applied to a wide range of measurements and dynamical frame

ISSN:0148-0227    

DOI:10.1002/2013JC009176

年代:2014

数据来源: WILEY

摘要:

A cross‐shore model of tidal flat heat and mass fluxes is developed to understand the heat exchange between the sediment bed and the water column. A convective heat‐transfer coefficient is used to model sediment‐water heat fluxes which are as great as 20% of the incoming solar shortwave radiation. The model results match well with observations and are used to assess processes across tidal to seasonal time scales. During the summer, tidal flat sediments store incoming shortwave radiation during exposure and act effectively as a net source of heat to the water column. This pattern changes in the winter, when the flats cool during exposure and act effectively as a net sink of heat. Additionally, during the summer water temperatures at the edge of the flooding front are elevated 5°C above the surface sediment temperatures. Model results replicate this process only when water column light extinction coefficients are high, consistent with visual observations of high turbidity (and thus high light absorption) at the leading edge of the flooding

ISSN:0148-0227    

DOI:10.1002/2013JC009225

年代:2014

数据来源: WILEY

摘要:

Previous studies have noted the presence of interannual to multidecadal variability in tide gauge sea level records which is correlated with meteorological variability and which can overwhelm the signal associated with global sea level rise. This study examines the usefulness of using a set of seven ocean reanalysis and synthesis products in studies of sea level variability by comparing the tide gauges and reanalysis products at a representative set of 87 tide gauge station locations. The comparison is carried out for both a half‐century base period and a century long‐extended period. Treating the set of products as an ensemble of realizations obtained using different techniques, the results show generally good agreement for the half‐century period with ensemble average correlations of 0.57 and RMS differences of 2.2 cm, reducing to a correlation of 0.5 for the extended period. A significant fraction of the difference between tide gauge sea level and product sea level is associated with meteorological forcing. These results support the conclusion that much of the interannual to multidecadal variability that appears in the tide gauge records is meteorologically driven. This suggests that ocean products have potential to be used to isolate this variability from the signal associated with the underlying global sea level

ISSN:0148-0227    

DOI:10.1002/2013JC009365

年代:2014

数据来源: WILEY