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Climate teleconnections and indicators of coastal systems response
This article surveys secular urban sandy beaches erosion – accretion and its relationship with climate teleconnections, e.g., El Niño Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), and extreme events, e.g., storm surges, great rivers' floods, and heavy rains in adjacent basins. The paper aims to discuss these issues and the expected coastal retreat as a consequence of manmade climate changes, e.g., sea level rise (SLR) and increased storminess in the coming decades. Several beaches (Buceo, Malvín, Pocitos, Ramírez) and two tidal creek sandy mouths (Carrasco and Pando), with different characteristics but all constrained by coastal linear infrastructure were studied. These sites are located along the urbanised coast of the middle region of Río de la Plata microtidal river estuary. All of them show a more or less strong retreat trend with alternated fluctuations, e.g., weak retreat likely due to sea level rise, significant erosion very likely due to storm surges, and processes of loss of sediment stock, as well as episodes of sand recovery. Therefore, these beaches require interventions to preserve their beach prism and dry sand surface. In search for answers to better understand why and under what conditions the process of advance and retreat of the coastline occur, we have analysed different teleconnections and carried out reanalyses for wind anomalies during ENSO events from 1951 to 2010. Both weak and moderate erosion–accretion periods are likely related to atmospheric anomalies, e.g., wind direction changes and the consequent swell and littoral drift changes, related to El Niño and La Niña events. The former associated with accretion and the latter with erosion. In the past most interventions have been reactive. Increased knowledge of climate and weather relationship with the sedimentary balance provides an approach that would allow developing beach risk-management, pro-active strategies and climate adaption measures focused on the generation and recovery planning based on the analysis of the occurrence and prediction of El Niño/La Niña events.
Evaluation of oceanic transport parameters using transient tracers from observations and model output
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Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales?
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Identifying Lagrangian coherent vortices in a mesoscale ocean model
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Variability of bottom carbonate chemistry over the deep coral reefs in the Florida Straits and the impacts of mesoscale processes
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Hydrodynamic responses of estuarine bays along the Texas-Louisiana coast during Hurricane Harvey
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Modelling twentieth century global ocean circulation and iceberg flux at 48°N: implications for west Greenland iceberg discharge
We have used a coupled ocean–iceberg model to study the variation in global ocean circulation and North Atlantic iceberg flux from 1900 to 2008. The latter component of the study focused particularly on Greenland icebergs feeding into the Labrador Current and past Newfoundland. The model was forced with daily heat, freshwater and wind fluxes from the Twentieth Century Reanalysis. The reanalysis heat fluxes were shown to be offset from the, shorter, NCEP reanalysis and a grid-point correction was applied to this component of the forcing. The model produces a generally realistic ocean circulation, although with an enhanced Atlantic Meridional Overturning largely due to the forcing. The modelled iceberg flux at 48°N is well correlated with the long-term observed flux when using a modelled iceberg discharge that varies in a similar fashion to the highly variable observed flux at 48°N. From this model we infer changes in the spatial and temporal variability of iceberg calving from western Greenland. During the first third of the twentieth century the majority of modelled icebergs reaching 48°N derive from southern Greenland, while only after 1930 is the traditional perspective of a majority of such icebergs originating from Baffin Bay consistent with model results. Decadal-scale changes in the dominant regional sources are found, with oscillations between western Greenland and northern Baffin Bay. The latter origin was modelled to be most important in the last third of the twentieth century, although west Greenland sources have increased in importance in recent years. The model correctly reproduces the pronounced late spring peak in flux at 48°N for southern Greenland icebergs, but has an approximately six month offset for icebergs from Baffin Bay, most likely due to resolution issues leading to model icebergs not being delayed in shallow coastal waters, whereas in reality they may be grounded for some time or trapped in coastal sea-ice.
Plankton energy flows using a global size-structured and trait-based model
Plankton community models are critical tools for understanding the processes that shape marine plankton communities, how plankton communities impact biogeochemical cycles, and the feedbacks between community structure and function. Here, using the flexible Marine Biogeochemistry Library (MARBL), we present the Size-based Plankton ECological TRAits (MARBL-SPECTRA) model, which is designed to represent a diverse plankton community while remaining computationally tractable. MARBL-SPECTRA is composed of nine phytoplankton and six zooplankton size classes represented using allometric scaling relationships for physiological traits and interactions within multiple functional types. MARBL-SPECTRA is embedded within the global ocean component of the Community Earth System Model (CESM) and simulates large-scale, emergent patterns in phytoplankton growth limitation, plankton phenology, plankton generation time, and trophic transfer efficiency. The model qualitatively reproduces observed global patterns of surface nutrients, chlorophyll biomass, net primary production, and the biogeographies of a range of plankton size classes. In addition, the model simulates how predator:prey dynamics and trophic efficiency vary across gradients in total ecosystem productivity. Shorter food chains that export proportionally more carbon from the surface to the ocean interior occur in productive, eutrophic regions, whereas in oligotrophic regions, the food chains are relatively long and export less organic matter from the surface. The union of functional type modeling with size-resolved, trait-based modeling approaches allows MARBL-SPECTRA to capture both large-scale elemental cycles and the structure of planktonic food webs affecting trophic transfer efficiency.
Process-Specific Contributions to Anomalous Java Mixed Layer Cooling During Positive IOD Events
Negative sea surface temperature (SST) anomalies associated with positive Indian Ocean Dipole (pIOD) events first appear in the seasonal upwelling zone along the southern coast of Java during May–July. The evolution of anomalous SSTs in this coastal region is analyzed by computing a temperature budget using output from a strongly eddy-active ocean general circulation model. The seasonal cooling south of Java in May–July is driven by a reduction in incoming shortwave radiation and by vertical mixing, consistent with earlier studies in the region; however, the model budget also shows an advective contribution that drives anomalous cooling at the onset of pIOD events. To identify which process(es) are responsible for the anomalous advective cooling during pIOD events, a novel process index regression method is used to estimate the contributions of wind stress, equatorial Kelvin waves, mesoscale eddies, and Lombok Strait flow to anomalous cooling south of Java. Using this method, wind stress forcing along the west coast of Sumatra is found to make the most substantial contribution to anomalous cooling south of Java, with lesser contributions from equatorially sourced Kelvin waves and local wind stress. Mesoscale eddies redistribute heat from the Lombok Strait outflow, and have an anomalous warming effect on the eastern side of the upwelling region. The process-specific temperature budget south of Java highlights the importance of wind stress forcing west of Sumatra relative to equatorial and local forcing, and explains most of the mixed layer temperature anomaly evolution associated with advection during pIOD events.
Last Century Warming Over the Canadian Atlantic Shelves Linked to Weak Atlantic Meridional Overturning Circulation
We report the Atlantic meridional overturning circulation (AMOC) is a key component of the global climate system. Recent studies suggested a twentieth-century weakening of the AMOC of unprecedented amplitude (~15%) over the last millennium. Here we present a record of δ 18 O in benthic foraminifera from sediment cores retrieved from the Laurentian Channel and demonstrate that the δ 18 O trend is linked to the strength of the AMOC. In this 100-year record, the AMOC signal decreased steadily to reach its minimum value in the late 1970s, where the weakest AMOC signal then remains constant until 2000. We also present a longer δ 18 O record of 1,500 years and highlight the uniqueness of the last century δ 18 O trend. Moreover, the Little Ice Age period is characterized by statistically heavier δ 18 O, suggesting a relatively weak AMOC. Lastly, implications for understanding the mechanisms driving the intensity of AMOC under global warming and high-latitude freshwater input are discussed.
Seasonal Cycle and Annual Reversal of the Somali Current in an Eddy-Resolving Global Ocean Model
The annual cycle of the Somali Current is studied using a forced strongly eddy-active global ocean general circulation model. The annual cycle and timing of the reversal of the Somali Current differ depending on the location along the coast of northeastern Africa. A momentum budget analysis calculated from the model output shows that both annual Rossby waves and the life cycle of the Great Whirl impact the northern part of the Somali Current (roughly 5°N to 10°N). In particular, the nonlinear term is of leading order importance when the Great Whirl is present. This is in contrast to the leading-order geostrophic balance between 2°N and 5°N. South of 2°N, equatorial dynamics prevail, and the nonlinear term is again important, and its reversal is strongly affected by the northward flowing East African Coastal Current. Alongshore wind forcing is influential at all latitudes, but only the transition in flow direction during boreal spring between 2°N and 5°N directly follows that of the wind.
Argo Observations of the Deep Mixing Band in the Southern Ocean: A Salinity Modeling Challenge
The Southern Ocean plays an important role in mediating oceanic uptake of CO 2 and heat due to a strong meridional overturning circulation. Gridded Argo float data for 2004–2017 were used to evaluate subsurface processes at the mixed layer depth (MLD) that occur in a narrow deep mixing band. Shifts in the value of the Turner Angle at the MLD indicate that early in the season the MLD deepens slowly as it encounters and is stabilized by a subsurface salt maximum. By September mixing has penetrated this salinity feature and the rate of deepening is faster once the MLD is deeper than the depth where the maximum salinity occurs (~150–200 m). This distinctive salinity layer is the result of surface Ekman transport of fresh water from the south and subsurface advection of high-salinity water from the north. Two configurations of the Community Earth System Model (CESM) ocean-ice forced hindcast experiments—one with 1° and the other with 0.1° horizontal resolution (Parallel Ocean Program low and high resolutions [POP-LR and POP-HR], respectively)—are compared with the Argo data for 2005–2009. POP-LR has a shallow MLD bias common to many Fifth Coupled Models Intercomparison Project (CMIP5) models, while POP-HR has a mix of deep and shallow MLD biases. While both models were able to replicate the large-scale processes leading to formation of a high-salinity layer, the salinity feature in POP-HR is too strong and deep. Neither model was able to replicate the vertical mixing processes leading to penetration of the subsurface salt maximum.
Oceanic Heat Delivery to the Antarctic Continental Shelf: Large-Scale, Low-Frequency Variability
According to the article, onshore penetration of oceanic water across the Antarctic continental slope (ACS) plays a major role in global sea level rise by delivering heat to the Antarctic marginal seas, thus contributing to the basal melting of ice shelves. Here the time-mean (Φ mean ) and eddy (Φ eddy ) components of the heat transport (Φ) across the 1,000-m isobath along the entire ACS are investigated using a 0.1° global coupled ocean/sea ice simulation based on the Los Alamos Parallel Ocean Program (POP) and sea ice (Community Ice CodE) models. In this work, comparison with in situ hydrography shows that the model successfully represents the basic water mass structure, with a warm bias in the Circumpolar Deep Water layer. Segments of on-shelf Φ, with lengths of O(100–1,000 km), are found along the ACS. The circumpolar integral of the annually averaged Φ is O(20 TW), with Φ eddy always on-shelf, while Φ mean fluctuates between on-shelf and off-shelf. Stirring along isoneutral surfaces is often the dominant process by which eddies transport heat across the ACS, but advection of heat by both mean flow-topography interactions and eddies can also be significant depending on the along- and across-slope location. The seasonal and interannual variability of the circumpolarly integrated Φ mean is controlled by convergence of Ekman transport within the ACS. Prominent warming features at the bottom of the continental shelf (consistent with observed temperature trends) are found both during high-Southern Annular Mode and high-Niño 3.4 periods, suggesting that climate modes can modulate the heat transfer from the Southern Ocean to the ACS across the entire Antarctic margin.
The Effect of Internal Variability on Ocean Temperature Adjustment in a Low-Resolution CESM Initial Condition Ensemble
Due to its large heat capacity and circulation, the ocean contributes significantly to global heat uptake, global heat transport, spatial temperature patterns, and variability. Quantifying ocean heat uptake across different temporal and spatial scales is important to quantify Earth's climate response to anthropogenic warming. Here we evaluate ocean adjustment time scales from two different fully coupled climate model ensembles using the Community Earth System Model. Both ensembles use the same model version, anthropogenic and natural forcings, and coupling configurations, but we initialize the ensembles in two different ways: (1) sampling joint internal variability of the ocean–atmosphere system (unique atmosphere and ocean conditions) and (2) sampling the internal variability of the atmosphere only (unique atmosphere, identical ocean conditions). Uncertainty due to internal variability is used as a proxy to quantify the time scales of ocean temperature adjustment at different depths and basins in Community Earth System Model. Time scales of equilibration are longer in the deep ocean than the upper ocean, highlighting the vertical structure of dynamic adjustment. The Atlantic equilibrates on shorter time scales (82 years above 1,000 m, 140 years below 1,000 m) relative to the Pacific (106 years above 1,000 m, 444 years below 1,000 m) in Community Earth System Model due to the large North Atlantic Deep Water formation and strong overturning circulation in the Atlantic. These results have broad implications for analyzing internal climate variability, ocean adjustment, and drift in global coupled model experiments and intercomparisons.