The Atlantic's freshwater budget under climate change in the Community Earth System Model with strongly eddying oceans

We investigate the freshwater budget of the Atlantic and Arctic oceans in coupled climate change simulations with the Community Earth System Model and compare a strongly eddying setup with 0.1∘ ocean grid spacing to a non-eddying 1∘ configuration typical of Coupled Model Intercomparison Project phase 6 (CMIP6) models. Details of this budget are important to understand the evolution of the Atlantic Meridional Overturning Circulation (AMOC) under climate change. We find that the slowdown of the AMOC in the year 2100 under the increasing CO2 concentrations of the Representative Concentration Pathway 8.5 (RCP8.5) scenario is almost identical between both simulations. Also, the surface freshwater fluxes are similar in their mean and trend under climate change in both simulations. While the basin-scale total freshwater transport is similar between the simulations, significant local differences exist. The high-ocean-resolution simulation exhibits significantly reduced ocean state biases, notably in the salt distribution, due to an improved circulation. Mesoscale eddies contribute considerably to the freshwater and salt transport, in particular at the boundaries of the subtropical and subpolar gyres. Both simulations start in the single equilibrium AMOC regime according to a commonly used AMOC stability indicator and evolve towards the multiple equilibrium regime under climate change, but only the high-resolution simulation enters it due to the reduced biases in the freshwater budget.

Interannual Variability of Tropical Atlantic-to-Pacific Moisture Transport Linked to ENSO, Atlantic Niño, and Freshwater Budget in the Northwestern Tropical Atlantic

Moisture transport from the Atlantic to Pacific is important for basin-scale freshwater budget and the formation of meridional ocean circulation. Although the climatological tropical Atlantic-to-Pacific moisture transport (TAPMORT) has been well investigated, few studies have focused on its variability. Here we investigate the interannual variability of TAPMORT based on the atmospheric reanalysis data sets. The TAPMORT interannual variability is dominated by the variations of trans-basin winds across Central America, and peaks in late boreal summer and late boreal winter. 1) In late summer, a developing El Niño and a mature Atlantic Niña set up an interbasin sea-surface temperature (SST) gradient that strengthens the low-level jet across Central America and therefore TAPMORT (with weakened TAPMORT for opposite signed events). This process typically occurs from July to September, with a peak in August. 2) In late winter, the strengthened southern North American center of the Pacific-North American-like pattern intensifies the TAPMORT variations. Although atmospheric interannual variability dominates these variations, extreme El Niño events are also important for the teleconnections. This process shows a single peak in February, in contrast to the persistent peak in late summer. We further demonstrate that the persistent TAPMORT variability in late summer dominates the moisture divergence over the northwestern tropical Atlantic and modulates freshwater flux there. Thus, our study improves the understanding of how TAPMORT interannual variability and the related interbasin SST gradient regulate the northwestern tropical Atlantic freshwater budget and the related salinity variability.

Forced Changes in the Arctic Freshwater Budget Emerge in the Early 21st Century

<p>Arctic liquid freshwater (FW) storage has shown a large increase over the past decades, posing the question: Is the Arctic FW budget already showing clear signs of anthropogenic climate change, or are the observed changes the result of multi-decadal variability? Using large ensemble simulations from the Community Earth System model (CESM), we show that the observed change in liquid and solid Arctic FW storage is likely already driven by the changing climate. Generally, the emergence of forced changes in Arctic FW fluxes occurs earlier for oceanic fluxes than for atmospheric or land fluxes. Nares Strait liquid FW flux is the first to show emergence outside the range of background variability in the model, with this change potentially already occurring, followed by Davis Strait. Other FW fluxes have likely started to shift but have not   yet emerged into a completely different regime. By re-sampling the model simulations, we find that the already changing nature of many FW budget terms over the short (~maximum 25 years) observational period can delay detection of shift and emergence from observations. Future emissions reductions have the potential to avoid the emergence of some FW fluxes beyond the background variability, in particular for runoff and Fram Strait solid FW export. However, under both low and high warming scenarios, all FW fluxes show changes, just not always completely outside the background variability as simulated by the CESM. Overall, this study provides an example of how large ensembles can be used to diagnose forced changes in short observational timeseries.</p>

Significant contribution of small icebergs to the freshwater budget in Greenland fjords

Icebergs represent nearly half of the mass loss from the Greenland Ice Sheet and provide a distributed source of freshwater along fjords which can alter fjord circulation, nutrient levels, and ultimately the Meridional Overturning Circulation. Here we present analyses of high resolution optical satellite imagery using convolutional neural networks to accurately delineate iceberg edges in two East Greenland fjords. We find that a significant portion of icebergs in fjords are comprised of small icebergs that were not detected in previously-available coarser resolution satellite images. We show that the preponderance of small icebergs results in high freshwater delivery, as well as a short life span of icebergs in fjords. We conclude that an inability to identify small icebergs leads to inaccurate frequency-size distribution of icebergs in Greenland fjords, an underestimation of iceberg area (specifically for small icebergs), and an overestimation of iceberg life span.

The Atlantic's Freshwater Budget under Climate Change in the Community Earth System Model with Strongly Eddying Oceans

We investigate the freshwater and salinity budget of the Atlantic and Arctic oceans in a strongly eddying coupled climate change simulation with the Community Earth System Model (CESM) and compare it to a simulation with a coarse ocean resolution CESM configuration, typical of CMIP6 models. Details of these budgets are important to understand the evolution of the Atlantic Meridional Overturning Circulation (AMOC) under climate change. We find that the slowdown of the AMOC in 2100 under the increasing CO2 concentrations of the RCP8.5 scenario is almost identical between both simulations. Also, the surface freshwater fluxes are similar in their mean and trend under climate change in both simulations. While the basin-scale total freshwater transport is similar between the simulations, significant local differences exist. The high ocean resolution simulation exhibits significantly reduced ocean state biases, notably in the salt distribution, due to an improved circulation. Mesoscale eddies contribute considerably to the freshwater and salt transport, in particular at the boundary of the subtropical and subpolar gyres. Both simulations start in the single equilibrium AMOC regime according to a commonly used AMOC stability indicator and evolve towards the multiple equilibrium regime under climate change, but only the high resolution simulation enters it due to the reduced biases in the freshwater budget.

Forecasting Net Groundwater Depletion in Well Irrigation Areas with Long Short-term Memory Networks

<p>Due to the scarcity of available surface water, many irrigated areas in North China Plain (NCP) heavily rely on groundwater, which has resulted in groundwater overexploitation and massive environmental impacts, such as groundwater depression core and land subsidence. The net groundwater depletion, one of the groundwater indicators, means the actual groundwater consumption for human impact. This indicator is quite essential for the evaluation of the effects of agricultural activities in well irrigation areas. However, net depletion forecasts, which can help inform the management of well irrigation areas, are generally unavailable with easy methods. Therefore, this study explored machine learning models, Long Short-term Memory (LSTM) networks, to forecast net groundwater depletion in well irrigation counties, Hebei Province. Firstly, Luancheng county was selected to construct the forecasting model. The training dataset was prepared by collecting the measured precipitation, remote sensing evaporation and groundwater table from 2006-2017. Besides, an agro-hydrological model (Soil-Water-Atmosphere-Plant, SWAP) with an optimization tool (Parameter ESTimation, PEST) was used to calculate the net depletion, and an unsaturated-saturated zone water balance conceptual hydrological model was constructed to calculate the net groundwater use. Secondly, to determine the effect of training data type on model accuracy, freshwater budget (evaporation minus precipitation), change of groundwater table and net groundwater use were chosen as training inputs by analyzing related temporal variable characteristics of net groundwater depletion. The response time of training inputs with net groundwater depletion were also approximated with highest cross-correlation value (CCF). Then, by circular bootstrapping methods to enlarge the Luancheng datasets from 2006-2016, the annual and monthly model for forecasting the net depletion were respectively trained with enlarged Luancheng datasets. Additionally, to test the model’s ability to predict the net groundwater depletion in other well irrigation areas with the similar rule of groundwater depletion, the annual and monthly forecasting scenarios were also carried out in the adjacent county, Zhaoxian. The results showed that both of the monthly and annual models estimating the groundwater net depletion had good performance in Zhaoxian from 2006-2017, with NSE of 0.91 and 0.81, respectively. According to the modelling results, further analysis showed that groundwater depletion in research counties mainly occurred in spring (March to May) and winter (December to February). In addition, the major factor leading to groundwater depletion in spring and winter was freshwater budget; while in summer and autumn, soil moisture determined the depletion activity. These results demonstrate the feasible use of LSTM networks to create annual and monthly forecasts of net groundwater depletion in well irrigation areas with similar depletion rule, which can provide valuable suggestion to well irrigation management in NCP within a challenging environment.</p><p><strong>Keywords: net groundwater depletion; long short-term memory; well irrigation areas</strong></p>

Coastal Wetlands: Ecosystems Affected by Urbanization?

Coastal wetlands are ecosystems that provide multiple benefits to human settlements; nonetheless, they are seriously threatened due to both a lack of planning instruments and human activities associated mainly with urban growth. An understanding of their functioning and status is crucial for their protection and conservation. Two wetlands with different degrees of urbanization, Rocuant-Andalién (highly urbanized) and Tubul-Raqui (with little urbanization), were analyzed using temperature, salinity, dissolved oxygen, pH, turbidity, granulometry, fecal coliform, and macroinvertebrate assemblage variables in summer and winter. In both wetlands marked seasonality in salinity, temperature and sediment texture classification, regulated by oceanic influence and changes in the freshwater budget, was observed. In the Rocuant-Andalién wetland, the increases in pH, dissolved oxygen, gravel percentage, and coliform concentration were statistically significant. Urbanization generated negative impacts on macroinvertebrate assemblage structure that inhabit the wetlands; greater richness and abundance (8.5 times greater) were recorded in the Tubul-Raqui wetland than in the more urbanized wetland. The multivariate statistical analysis reflects the alteration of these complex systems.