AMOC Trends From 1850 to 2100 At Interannual To Multi-Decadal Time Scales Corroborated By Changes In Salinity Budget

The Atlantic Meridional Overturning Circulation (AMOC) is a key player in the global coupled ocean-atmosphere climate system. To characterise the potential of an AMOC slowdown, a past and future trend probability analysis is applied using 16 models from the Coupled Model Intercomparison Project Phase 5. We determine the probability of AMOC annual to multidecadal trends under the historical period and two future climate scenarios (`business-as-usual’ scenario - RCP8.5 and `stabilisation’ scenario - RCP4.5). We show that the probability of a AMOC decline in model data shifts outside its range of intrinsic variability (determined from the pre-industrial control runs) for sustained 5-year trend or longer. This suggests that interannual AMOC events are not significantly affected by future climate scenario, and so potentially neither by anthropogenic forcing. Furthermore, under the ‘business-as-usual’ scenario the probability of a 20-year decline remains high (87\%) until 2100, however in a ‘stabilisation’ scenario the trend probability recovers its pre-industrial values by 2100. A 20-year unique event is identified from 1995 to 2015, marked by simultaneous unique features in the AMOC and salinity transport that are not replicated over any other 20-year period within the 250 years studied. These features include the maximum probability and magnitude of an `intense’ AMOC decline, and a sustained 20-year decline in subpolar salinity transport caused by internal oceanic processes (as opposed to external atmospheric forcing). This work therefore highlights the potential use of direct salinity transport observations, and ensemble mean numerical models to represent and understand changes in past, present, and future AMOC.

Threat Posed by Future Sea-Level Rise to Freshwater Resources in the Upper Pearl River Estuary

The degradation of densely populated river delta environments due to the accelerating rise in sea level can affect the availability of freshwater for municipal supplies, irrigation, and industrial use. A fully calibrated three-dimensional numerical model is used in this study to evaluate the threat posed by the sea-level rise, which predicted to occur by 2100, to freshwater resources in the upper tributaries of Pearl River Estuary. The results indicate that both the intensity and duration of dry-season saltwater intrusion greatly increase as the sea level rises, making the water at drinking-water intake stations for the four waterworks no longer suitable for municipal supply. Flow modulation is performed to identify the threshold at which saltwater intrusion could be effectively suppressed in response to both sea-level rise and dry season hydrodynamics. The number of days for which water meets the drinking-water standard decreases as the sea level rises, but increases with increased river flow. The combined effect of future drought and sea-level rise would further limit the availability of freshwater in the upper tributaries. Stronger upstream salinity transport during flood tide are found in the sea-level rise case. The increased flood tidal salinity transport would have great impact on the tidal freshwater wetlands.

Effective Diahaline Diffusivities in Estuaries

<p>The present study aims to estimate effective diahaline turbulent salinity fluxes and diffusivities in numerical model simulations of estuarine scenarios. The underlying method is based on a quantification of salinity mixing per salinity class, which is shown to be twice the turbulent salinity transport across the respective isohaline. Using this relation, the recently derived universal law of estuarine mixing, predicting that average mixing per salinity class is twice the respective salinity times the river run‐off, can be directly derived. The turbulent salinity transport is accurately decomposed into physical (due to the turbulence closure) and numerical (due to truncation errors of the salinity advection scheme) contributions. The effective diahaline diffusivity representative for a salinity class and an estuarine region results as the ratio of the diahaline turbulent salinity transport and the respective (negative) salinity gradient, both integrated over the isohaline area in that region and averaged over a specified period. With this approach, the physical (or numerical) diffusivities are calculated as half of the product of physical (or numerical) mixing and the isohaline volume, divided by the square of the isohaline area. The method for accurately calculating physical and numerical diahaline diffusivities is tested and demonstrated for a three‐dimensional idealized exponential estuary. As a major product of this study, maps of the spatial distribution of the effective diahaline diffusivities are shown for the model estuary.</p>

Influence of the Ocean Mesoscale Eddy–Atmosphere Thermal Feedback on the Upper-Ocean Haline Stratification

The ocean mesoscale eddy–atmosphere (OME-A) interaction through the eddy-induced sea surface temperature anomaly can feedback on ocean dynamics in various ways (referred to as the OME-A thermal feedback). In this study, the influence of the OME-A thermal feedback on the upper-ocean haline structure is analyzed based on high-resolution coupled simulations. In the Oyashio Extension where pronounced surface temperature and salinity fronts are collocated, the haline stratification in the upper 200 m is significantly enhanced by the OME-A thermal feedback. This enhancement is mainly attributed to the weakening of the upward eddy salinity transport in response to the OME-A thermal feedback. The OME-A thermal feedback influences the vertical eddy salinity transport through its differed impacts on the mesoscale buoyancy and temperature anomaly variances. As temperature and salinity in the Oyashio Extension are strongly compensated for their effects on buoyancy, the dissipation of the mesoscale buoyancy anomaly variance b′2 by the OME-A thermal feedback is considerably weaker than that estimated from the mesoscale temperature anomaly alone, i.e., (gαT′)2, with g the gravity acceleration and α the thermal expansion coefficient. Correspondingly, the vertical eddy buoyancy transport (w′b′) is weakened by the OME-A thermal feedback to a lesser extent than its thermal component (gαw′T′). The different responses of w′b′ and gαw′T′ to the OME-A thermal feedback are reconciled by the reduced vertical eddy salinity transport.

Impacts of Storm Surges on Hydrodynamics and Salinity of Sabine Lake and Sabine River Diversion Canal

In this study, a hydrodynamic and salinity transport model was developed for simulations of Sabine Lake water system located on the Texas-Louisiana border. The target simulation area ranges from Sabine River near Deweyville, TX as the north boundary to the Gulf of Mexico as the south boundary, and from Neches River near Beaumont, TX as the west boundary to part of Gulf Intracoastal Waterway (GIWW) and Sabine River Diversion Canal (SRDC) as the east boundary. The entire area includes several major water bodies, such as Sabine Lake, Sabine River, Sabine Pass, Sabine Neches Canal (Ship Channel), and part of GIWW and SRDC. The SRDC supplies fresh water to the area industry, mainly petrochemical. High salinity in SRDC could significantly affect the daily production of the industry. The major purposes of this study is to use the validated hydrodynamic and salinity transport model to assess and predict the salinity in SRDC under severe weather conditions such as hurricane storm surges. Measurement data from NOAA and USGS were used to calibrate the boundary conditions as well as to validate the model. Two different levels of storm surges each lasting for 24 hours were simulated, 0.5 and 1 meter, respectively, and the salinity in SRDC was monitored and compared to analyze the storm surge threats on SDRC water quality. The result shows that it took about 2 days for the salinity reaching SRDC under the 1m storm surge condition and about 3 days under 0.5m surge condition and the salinity value could reach as high as 5 to 10 ppt.

Calibration and Validation of Hydrodynamics and Salinity Transport Model for Sabine Lake Water System

In this study, a hydrodynamic and a salinity transport models were developed for simulations of Sabine Lake water system located on the Texas-Louisiana border. The target simulation area includes several major water bodies, such as Sabine Lake, Sabine River, Sabine Pass, Sabine Neches Canal (Ship Channel), and part of Gulf Intracoastal Waterway (GIWW) and Sabine River Diversion Canal (SRDC). The SRDC supplies fresh water to the area industry, mainly petrochemical. High salinity in SRDC could significantly affect the daily production of the industry. Two-dimensional (2D) depth-averaged shallow water equation set and 2D depth-averaged salinity transport equation were used for developing the hydrodynamic and salinity transport numerical models in order to carry out the simulation. The major purposes of this study are to calibrate and validate hydrodynamic and salinity transport models in order to assess and predict the salinity in SRDC under severe weather conditions such as hurricane storm surges in future study. Measurement data from National Oceanic and Atmospheric Administration (NOAA) and United States Geological Survey (USGS) were used to calibrate the boundary conditions as well as to validate the model. Boundary conditions were calibrated at locations in Sabine Pass and in the north edge of the lake by using water–surface elevation data. Hydrodynamic model was validated at the USGS location using water–surface elevation data. Then, the simulation estimations of water surface level and salinity were compared at three locations, and the results show the accuracy of the validated model. Parallel computing was conducted in this study as well, and computational efficiency was compared.