摘要:

AbstractMeasurements of Kuroshio Current velocity at the entrance to Luzon Strait along 18.75°N were made with an array of six moorings during June 2012 to June 2013. Strong positive relative vorticity of the order of the planetary vorticityfwas observed on the western flank of the Kuroshio in the upper 150 m. On the eastern flank, the negative vorticity observed was about an order of magnitude smaller thanf. Kuroshio transport near its origin is computed from direct measurements for the first time. Kuroshio transport has an annual mean of 15 Sv with a standard deviation of 3 Sv. It is modulated strongly by impinging westward propagating eddies, which are identified by an improved eddy detection method and tracked back to the interior ocean. Eight Kuroshio transport anomalies>5 Sv are identified; seven are explained by the westward propagating eddies. Cyclonic (anticyclonic) eddies decrease (increase) the zonal sea level anomaly (SLA) slope and reduce (enhance) Kuroshio transport. Large transport anomalies of>10 Sv within O(10 days) are associated with the pairs of cyclonic and anticyclonic eddies. The observed Kuroshio transport was strongly correlated with the SLA slope (correlation = 0.9). Analysis of SLA slope data at the entrance to Luzon Strait over the period 1992–2013 reveals a seasonal cycle with a positive anomaly (i.e., an enhanced Kuroshio transport) in winter and spring and a negative anomaly in summer and fall. Eddy induced vorticity near the Kuroshio has a similar seasonal cycle, suggesting that seasonal variation of the Kuroshio transport near its origin is modulated by the seasonal variation of the impinging mesoscale

ISSN:0148-0227    

DOI:10.1002/2013JC009548

年代:2014

数据来源: WILEY

摘要:

AbstractThe transports of water, heat, and salt between the northwestern shelf and deep interior of the Black Sea are investigated using a high‐resolution three‐dimensional primitive equation model. From April to August 2005, both onshore and offshore cross‐shelf break transports in the top 20 m were 0.24 Sv on average, which is equivalent to the replacement of 60% of the volume of surface shelf waters (0–20 m) per month. Two main exchange mechanisms are studied: Ekman transport, and transport by mesoscale eddies and associated meanders of the Rim Current. The Ekman drift causes nearly uniform onshore or offshore flow over a large section of the shelf break, but it is confined to the upper layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are restricted laterally. During the strong wind events of 15–22 April and 1–4 July, some 0.66 × 1012and 0.44 × 1012m3of water were removed from the northwestern shelf, respectively. In comparison, the single long‐lived Sevastopol Eddy generated a much larger offshore transfer of 2.84 × 1012m3over the period 23 April to 30 June, which is equivalent to 102% of the volume of northwestern shelf waters. Over the study period, salt exchanges increased the average density of the shelf waters by 0.67 kg m−3and reduced the density contrast between the shelf and deep sea, while lateral heat exchanges reduced the density of the shelf waters by 0.16 kg m−3and sharpened t

ISSN:0148-0227    

DOI:10.1002/2013JC009484

年代:2014

数据来源: WILEY

摘要:

AbstractWe investigate the role of deep‐ocean topography in scattering energy from the large spatial scales of the low‐mode internal tide to the smaller spatial scales of higher modes. The complete Green function method, which is not subject to the restrictions of the WKB approximation, is used for the first time to study the two‐dimensional scattering of a mode‐1 internal tide incident on subcritical and supercritical topography of any form in arbitrary stratifications. For an isolated Gaussian ridge in a uniform stratification, large amplitude critical topography is the most efficient at mode‐1 scattering and small amplitude topography scatters with an efficiency on the order of 5–10%. In a nonuniform stratification with a pycnocline, the results are qualitatively the same as for a constant stratification, albeit with the key features shifted to larger height ratios. Having validated these results by direct comparison with the results of nonlinear numerical simulations, and in the process demonstrated that WKB results are not appropriate for reasonable ocean predictions, we proceed to use the Green function approach to quantify the role of topographic scattering for the region of the Pacific Ocean surrounding the Hawaiian Islands chain. To the south, the Line Islands ridge is found to scatter ∼40% of a mode‐1 internal tide coming from the Hawaiian Ridge. To the north, realistic, small‐amplitude, rough topography scatters ∼5–10% of the energy out of mode 1 for transects of length 1000–3000 km. A significant finding is that compared to large extents of small‐amplitude, rough topography a single large topographic feature along the path of a mode‐1 internal tide plays the dominant role in s

ISSN:0148-0227    

DOI:10.1002/2013JC009152

年代:2014

数据来源: WILEY

摘要:

AbstractAcquisition of high resolution time series of water column and bottom dissolved oxygen (DO) concentrations inform the dynamics of oxygen depletion in St Helena Bay in the southern Benguela upwelling system at several scales of variability. The bay is characterized by seasonally recurrent hypoxia (<1.42 ml l−1) associated with a deep pool of oxygen‐depleted water and episodic anoxia (<0.02 ml l−1) driven by the nearshore (<20 m isobath) decay of red tide. Coastal wind forcing influences DO concentrations in the nearshore through its influence on bay productivity and the development of red tides; through shoreward advection of the bottom pool of oxygen‐depleted water as determined by the upwelling‐downwelling cycle; and through its control of water column stratification and mixing. A seasonal decline in bottom DO concentrations of ∼1.2 ml l−1occurs with a concurrent expansion of the bottom pool of oxygen depleted water in St Helena Bay. Upwelling of this water into the nearshore causes severe drops in DO concentration (<0.2 ml l−1), particularly during end‐of‐season upwelling, resulting in a significant narrowing of the habitable zone. Episodic anoxia through the entire water column is caused by localized degradation of red tides within the confines of the shallow nearshore environment. Oxygenation of the nearshore is achieved by ventilation of the water column particularly with the onset of winter mixing. No notable changes are evident in comparing recent measures of bottom DO concentrations in St Helena Bay to data collected in the late 19

ISSN:0148-0227    

DOI:10.1002/2013JC009443

年代:2014

数据来源: WILEY

摘要:

AbstractTsunami waveforms can be observed at offshore locations such as sea‐bottom pressure gauges or GPS‐mounted buoys. Recent work has focused on using these observations to make near‐field tsunami forecasts in real time. However, existing forecasting methods are limited in that they do not provide uncertainty estimates. This study develops a near‐field tsunami forecasting method with uncertainty estimates. The method embeds a conditional autoregressive model in a hierarchical Bayesian inverse model. Since we sample from the posterior distribution of interest using a Markov Chain Monte Carlo algorithm, not only the mean but also the variance for forecasts can be readily obtained. The proposed method is validated through simulation‐based experiments for four historical earthquakes in the Nankai Trou

ISSN:0148-0227    

DOI:10.1002/2013JC009334

年代:2014

数据来源: WILEY

摘要:

AbstractA comparison study between 10 Mediterranean lagoons has been carried out by means of the 3‐D numerical model SHYFEM. The investigated basins are the Venice and Marano‐Grado lagoons in the Northern Adriatic Sea, the Lesina and Varano lagoons in the Southern Adriatic Sea, the Taranto basin in the Ionian Sea, the Cabras Lagoon in Sardinia, the Ganzirri and Faro lagoons in Sicily, the Mar Menor in Spain, and the Nador Lagoon in Morocco. This study has been focused on hydrodynamics in terms of exchange rates, transport time scale, and mixing. Water exchange depends mainly on the inlet shape and tidal range, but also on the wind regimes in the case of multi‐inlet lagoons. Water renewal time, which is mostly determined by the exchange rate, is a powerful concept that allows lagoons to be characterized with a time scale. In the case of the studied lagoons, the renewal time ranged from few days in the Marano‐Grado Lagoon up to 1 year in the case of the Mar Menor. The analysis of the renewal time frequency distribution allows identifying subbasins. The numerical study proved to be a useful tool for the intercomparison and classification of the lagoons. These environments range from a leaky type to a choked type of lagoons and give a representative picture of the lagoons situated around the Mediterranean basin. Mixing efficiency turns out to be a function of the morphological complexity, but also of the forcings acting on the

ISSN:0148-0227    

DOI:10.1002/2013JC009512

年代:2014

数据来源: WILEY

摘要:

AbstractThe Crozet Archipelago, in the Indian sector of the Southern Ocean, constitutes one of the few physical barriers to the Antarctic Circumpolar Current. Interaction of the currents with the sediments deposited on the margins of these islands contributes to the supply of chemical elements—including iron and other micro‐nutrients—to offshore high‐nutrient, low‐chlorophyll (HNLC) waters. This natural fertilization sustains a phytoplankton bloom that was studied in the framework of the KEOPS‐2 project. In this work, we investigated the time scales of the surface water transport between the Crozet Island shelves and the offshore waters, a transport that contributes iron to the phytoplankton bloom. We report shelf‐water contact ages determined using geochemical tracers (radium isotopes) and physical data based on in situ drifter data and outputs of a model based on altimetric Lagrangian surface currents. The apparent ages of surface waters determined using the three independent methods are in relatively good agreement with each other. Our results provide constraints on the time scales of the transport between the shelf and offshore waters near the Crozet Islands and highlight the key role played by horizontal transport in natural iron fertilization and in defining the extension of the chlorophyll plume in this HNLC region of the S

ISSN:0148-0227    

DOI:10.1002/2013JC009305

年代:2014

数据来源: WILEY

摘要:

AbstractThe overturning circulation in the Red Sea exhibits a distinct seasonally reversing pattern and is studied using high‐resolution MIT general circulation model simulations. In the first part of this study, the vertical and horizontal structure of the summer overturning circulation and its dynamical mechanisms are presented from the model results. The seasonal water exchange in the Strait of Bab el Mandeb is successfully simulated, and the structures of the intruding subsurface Gulf of Aden intermediate water are in good agreement with summer observations in 2011. The model results suggest that the summer overturning circulation is driven by the combined effect of the shoaling of the thermocline in the Gulf of Aden resulting from remote winds in the Arabian Sea and an upward surface slope from the Red Sea to the Gulf of Aden set up by local surface winds in the Red Sea. In addition, during late summer two processes associated, respectively, with latitudinally differential heating and increased salinity in the southern Red Sea act together to cause the reversal of the contrast of the vertical density structure and the cessation of the summer overturning circulation. Dynamically, the subsurface northward pressure gradient force is mainly balanced by vertical viscosity resulting from the vertical shear and boundary friction in the Strait of Bab el Mandeb. Unlike some previous studies, the three‐layer summer exchange flows in the Strait of Bab el Mandeb do not appear to be hydraulically control

ISSN:0148-0227    

DOI:10.1002/2013JC009004

年代:2014

数据来源: WILEY

摘要:

AbstractThe shallow winter overturning circulation in the Red Sea is studied using a 50 year high‐resolution MITgcm (MIT general circulation model) simulation with realistic atmospheric forcing. The overturning circulation for a typical year, represented by 1980, and the climatological mean are analyzed using model output to delineate the three‐dimensional structure and to investigate the underlying dynamical mechanisms. The horizontal model circulation in the winter of 1980 is dominated by energetic eddies. The climatological model mean results suggest that the surface inflow intensifies in a western boundary current in the southern Red Sea that switches to an eastern boundary current north of 24°N. The overturning is accomplished through a cyclonic recirculation and a cross‐basin overturning circulation in the northern Red Sea, with major sinking occurring along a narrow band of width about 20 km along the eastern boundary and weaker upwelling along the western boundary. The northward pressure gradient force, strong vertical mixing, and horizontal mixing near the boundary are the essential dynamical components in the model's winter overturning circulation. The simulated water exchange is not hydraulically controlled in the Strait of Bab el Mandeb; instead, the exchange is limited by bottom and lateral boundary friction and, to a lesser extent, by interfacial friction due to the vertical viscosity at the interface between the inflow and the o

ISSN:0148-0227    

DOI:10.1002/2013JC009331

年代:2014

数据来源: WILEY

摘要:

AbstractThe Solomon Sea is an area of high level of eddy kinetic energy (EKE), and represents a transit area for the low‐latitude western boundary currents (LLWBCs) connecting the subtropics to the equatorial Pacific and playing a major role in ENSO dynamics. This study aims at documenting the surface mesoscale activity in the Solomon Sea for the first time. Our analysis is based on the joint analysis of altimetric data and outputs from a 1/12° model simulation. The highest surface EKE is observed in the northern part of the basin and extends southward to the central basin. An eddy tracking algorithm is used to document the characteristics and trajectories of coherent mesoscale vortices. Cyclonic eddies, generated in the south basin, are advected to the north by the LLWBCs before merging with stationary mesoscale structures present in the mean circulation. Anticyclonic eddies are less numerous. They are generated in the southeastern basin, propagate westward, reach the LLWBCs, and dissipate. The seasonal and interannual modulations of the mesoscale activity are well marked. At seasonal time scale, maximum (minimum) activity is in May–June (September). At interannual time scale, the mesoscale activity is particularly enhanced during La Niña conditions. If instabilities of the regional circulations seem to explain the generation of mesoscale features, the modulation of the mesoscale activity seems to be rather related with the intrusion at Solomon Strait of the surface South Equatorial Current, rather than to the LLWBCs, by modulating the horizontal and vertical shears suitable for instabil

ISSN:0148-0227    

DOI:10.1002/2013JC009614

年代:2014

数据来源: WILEY