摘要:

AbstractGlobal sea levels have been rising through the past century and are projected to rise at an accelerated rate throughout the 21st century. This has motivated a number of authors to search for already existing accelerations in observations, which would be, if present, vital for coastal protection planning purposes. No scientific consensus has been reached yet as to how a possible acceleration could be separated from intrinsic climate variability in sea level records. This has led to an intensive debate on its existence and, if absent, also on the general validity of current future projections. Here we shed light on the controversial discussion from a methodological point of view. To do so, we provide a comprehensive review of trend methods used in the community so far. This resulted in an overview of 30 methods, each having its individual mathematical formulation, flexibilities, and characteristics. We illustrate that varying trend approaches may lead to contradictory acceleration‐deceleration inferences. As for statistics‐oriented trend methods, we argue that checks on model assumptions and model selection techniques yield a way out. However, since these selection methods all have implicit assumptions, we show that good modeling practices are of importance too. We conclude at this point that (i) several differently characterized methods should be applied and discussed simultaneously, (ii) uncertainties should be taken into account to prevent biased or wrong conclusions, and (iii) removing internally generated climate variability by incorporating atmospheric or oceanographic information helps to uncover externally forced climate change sign

ISSN:0148-0227    

DOI:10.1002/2015JC010716

年代:2015

数据来源: WILEY

摘要:

AbstractArctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234‐thorium based estimates of the export flux of POC to onboard incubation‐based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth:ThE‐ratio) showed large variability (0.09 ± 0.19–1.3 ± 0.3). The highestThE‐ratio (1.3 ± 0.3) was recorded in a mono‐specific bloom ofPhaeocystis pouchetiilocated in the ice edge. Blooming diatom dominated areas also had highThE‐ratios (0.1 ± 0.1–0.5 ± 0.2), while mixed and/or prebloom communities showed lowerThE‐ratios (0.10 ± 0.03–0.19 ± 0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralized in the upper mesopelagic (100 m) at a similar rate as the material sinking from diatom blooms i

ISSN:0148-0227    

DOI:10.1002/2015JC010700

年代:2015

数据来源: WILEY

摘要:

AbstractField studies suggested that the biogeochemical settings and community structures are substantial different between the central Northern South China Sea (NSCS) and the upstream Kuroshio Current (KC). In particular, the water column of KC is characterized by substantially lower nutrients and productivity but higherTrichodesmiumabundance and nitrogen fixation compared to the NSCS. The mechanism driving the difference of the two marine ecosystems, however, remains inadequately understood. Here, a one‐dimensional biogeochemical model was developed to simulate the long‐term variability of lower‐trophic planktonic ecosystem for two pelagic stations in the NSCS and the KC near the Luzon Strait. The physical model included the vertical mixing driven by air‐sea interaction and the Ekman pumping induced by wind stress curl. The biological model was constructed by modifying a nitrogen‐based NPZD model with the incorporation of phosphorus cycle and diazotroph nitrogen fixation. After validation by several field data sets, the model was used to study the impact of long‐term physical forcing on ecosystem variability in the two distinct stations. Our results suggested that nutrient transport above nitracline during summer was largely controlled by vertical turbulent mixing, while Ekman pumping was important for nutrient transport below the nitracline. Our results also indicated that diazotroph community structure and N2fixation in the NSCS and the KC could be strongly influenced by physical processes through the impacts on vertical nutrient fluxes. The disadvantage of diazotroph in the NSCS in compared to the KC during the summer could be attributed to its high nitrate fluxes from subsurface leading to outcompete of diazotrophs by faster growing nondiazotroph ph

ISSN:0148-0227    

DOI:10.1002/2014JC010609

年代:2015

数据来源: WILEY

摘要:

AbstractThermal infrared (IR) imagery is used to quantify the high spatial and temporal variability of dissipation due to wave breaking in the surf zone. The foam produced in an actively breaking crest, or wave roller, has a distinct signature in IR imagery. A retrieval algorithm is developed to detect breaking waves and extract wave roller length using measurements taken during the Surf Zone Optics 2010 experiment at Duck, NC. The remotely derived roller length and an in situ estimate of wave slope are used to estimate dissipation due to wave breaking by means of the wave‐resolving model by Duncan (1981). The wave energy dissipation rate estimates show a pattern of increased breaking during low tide over a sand bar, consistent with in situ turbulent kinetic energy dissipation rate estimates from fixed and drifting instruments over the bar. When integrated over the surf zone width, these dissipation rate estimates account for 40–69% of the incoming wave energy flux. The Duncan (1981) estimates agree with those from a dissipation parameterization by Janssen and Battjes (2007), a wave energy dissipation model commonly applied within nearshore circulation mod

ISSN:0148-0227    

DOI:10.1002/2014JC010561

年代:2015

数据来源: WILEY

摘要:

AbstractThe oligotrophic regions of the subtropical gyres cover a significant portion of the global ocean, and exhibit considerable but poorly understood intraseasonal, interannual, and longer‐term variations in spatial extent. Here using historical observations of surface ocean nitrate, wind, and currents, we have investigated how horizontal and vertical supplies of nitrate control seasonal changes in the size and shape of oligotrophic regions of the subtropical North Atlantic. In general, the oligotrophic region of the subtropical North Atlantic is associated with the region of weak vertical supply of nitrate. Though the total vertical supply of nitrate here is generally greater than the total horizontal supply, we find that seasonal expansion and contraction of the oligotrophic region is consistent with changes in horizontal supply of nitrate. In this dynamic periphery of the subtropical gyre, the seasonal variations in chlorophyll are linked to variations in horizontal nitrate supply that facilitate changes in intracellular pigment concentrations, and to a lesser extent, phytoplankton biomass. Our results suggest that horizontal transports of nutrient are crucial in setting seasonal cycles of chlorophyll in large expanses of the subtropical North Atlantic, and may play a key and underappreciated role in regulating interannual variations in these globally important marine ecosystem

ISSN:0148-0227    

DOI:10.1002/2015JC010787

年代:2015

数据来源: WILEY

摘要:

AbstractAlthough much of the focus on future sea level rise concerns the long‐term trend associated with anthropogenic warming, on shorter time scales, internal climate variability can contribute significantly to regional sea level. Such sea level variability should be taken into consideration when planning efforts to mitigate the effects of future sea level change. In this study, we quantify the contribution to regional sea level of the El Niño‐Southern Oscillation (ENSO). Through cyclostationary empirical orthogonal function analysis (CSEOF) of the long reconstructed sea level data set and of a set of U.S. tide gauges, two global modes dominated by Pacific Ocean variability are identified and related to ENSO and, by extension, the Pacific Decadal Oscillation. By estimating the combined contribution of these two modes to regional sea level, we find that ENSO can contribute significantly on short time scales, with contributions of up to 20 cm along the west coast of the U.S. The CSEOF decomposition of the long tide gauge records around the U.S. highlights the influence of ENSO on the U.S. east coast. Tandem analyses of both the reconstructed and tide gauge records also examine the utility of the sea level reconstructions for near‐coast s

ISSN:0148-0227    

DOI:10.1002/2014JC010602

年代:2015

数据来源: WILEY

摘要:

AbstractOn the basis of five oceanographic cruises carried out in the Eastern Tropical Pacific off Mexico, relationships between the larval fish habitats (areas inhabited by larval fish assemblages) and the environmental circulation scales (mesoscale, seasonal, and interannual) were examined. Analysis of in situ data over a grid of hydrographic stations and oblique zooplankton hauls with bongo net (505 µm) was combined with orthogonal robust functions decomposition applied to altimetry anomalies obtained from satellite. During both cool (March and June) and warm (August and November) periods, Bray‐Curtis dissimilarity Index defined three recurrent larval fish habitats which varied in species composition and extent as a function of the environmental scales. The variability of the Tropical larval fish habitat (characterized by high species richness, and dominated byVinciguerria lucetia, Diogenichthys laternatus, and Diaphus pacificus) was associated with the seasonal changes. The Transitional‐California Current larval fish habitat (dominated byV. lucetiaandD. laternatus, with lower mean abundance and lower species richness than in the Tropical habitat) and Coastal‐and‐Upwelling larval fish habitat (dominated byBregmaceros bathymaster) was associated mainly with mesoscale activity induced by eddies and with coastal upwelling. During February 2010, the Tropical larval fish habitat predominated offshore and the Transitional‐California Current larval fish habitat was not present, which we attribute to the effect of El Niño conditions. Thus, the mesoscale, seasonal, and interannual environmental scales affect the composition and extension of larval fi

ISSN:0148-0227    

DOI:10.1002/2014JC010289

年代:2015

数据来源: WILEY

摘要:

AbstractIn the western North Atlantic, warm and saline water is brought by the North Atlantic Current (NAC) from the subtropics into the subpolar gyre. Four inverted echo sounders with high precision pressure sensors (PIES) were moored between 47°40′N and 52°30′N to study the main pathways of the NAC from the western into the eastern basin. The array configuration that forms three segments (northern, central, and southern) allows partitioning of the NAC and some assessment of NAC flow paths through the different Mid‐Atlantic Ridge fracture zones. We exploit the correlation between the NAC transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of NAC transports to the period from 1992 to 2013. The mean NAC transport over the entire 21 years is 27 ± 5 Sv, consisting of 60% warm water of subtropical origin and 40% subpolar water. We did not find a significant trend in the total transport time series, but individual segments had opposing trends, leading to a more focused NAC in the central subsection and decreasing transports in the southern and northern segments. The spectral analysis exhibits several significant peaks. The two most prominent are around 120 days, identified as the time scale of meanders and eddies, and at 4–9 years, most likely related to the NAO. Transport composites for the years of highest and lowest NAO indices showed a significantly higher transport (+2.9 Sv) during strong NAO years, mainly in the so

ISSN:0148-0227    

DOI:10.1002/2014JC010662

年代:2015

数据来源: WILEY

摘要:

AbstractThe interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi‐3‐D nearshore community model, NearCoM‐TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semidiurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave‐current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward‐directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel‐shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primari

ISSN:0148-0227    

DOI:10.1002/2014JC010425

年代:2015

数据来源: WILEY

摘要:

AbstractMicrostructure and current velocity measurements were collected at a cross‐channel transect in the James River under spring and neap tidal conditions in May 2010 to study cross‐estuary variations in vertical mixing. Results showed that near‐surface mixing was related to lateral circulation during the ebb phase of a tidal cycle, and that the linkage was somewhat similar from neap to spring tides. During neap tides, near‐surface mixing was generated by the straining of lateral density gradients influenced by the advection of fresh, riverine water on the right side (looking seaward) of the transect. Spring tide results revealed similar findings on the right side of the cross section. However, on the left side, the straining by velocity shears acted in concert with density straining. Weak along‐estuary velocities over the left shoal were connected to faster velocities in the channel via a clockwise lateral circulation (looking seaward). These results provided evidence that in the absence of direct wind forcing, near‐surface vertical mixing can occur from mechanisms uncoupled from bott

ISSN:0148-0227    

DOI:10.1002/2014JC010679

年代:2015

数据来源: WILEY