Projected response of Arabian sea Oxygen minimum zone to climate change: Insights from a set of downscaled experiments

2020 ◽  
Author(s):  
Parvathi Vallivattathillam ◽  
Zouhair Lachkar ◽  
Marina Levy ◽  
Shafer Smith
Keyword(s):  
Climate Change ◽  
Boundary Conditions ◽  
Arabian Sea ◽  
Ocean Modeling ◽  
Depth Range ◽  
Lateral Boundary ◽  
Regional Ocean Modeling System ◽  
Control Simulation ◽  
Lateral Boundary Conditions ◽  
Oxygen Minimum

<p>The land-locked northern boundary and seasonal high productivity in the Arabian sea (AS) leads to the formation and the maintenance of one of the most intense and thickest open ocean oxygen minimum zones (OMZ) there. Earlier studies based on both observation and model sensitivity experiments have reported that this perennial OMZ is highly sensitive to the strength of the monsoonal circulation and surface heating. Model simulations from the fifth phase of Coupled Model Intercomparison project (CMIP5) indicate significant changes in the Indian monsoonal circulation and the atmospheric heat fluxes under climate change. However, the future projection of AS OMZ under climate change remains largely uncertain and ill-understood. This is mainly due to a poor representation of the AS OMZ in the CMIP5 simulations and an important spread in their future oxygen projections for the region. Here we explore how downscaling CMIP5 global simulations with a high-resolution configuration of the Regional Ocean Modeling System (ROMS) model coupled to a nitrogen-based NPZD ecosystem model can help improving the representation of the AS OMZ and reduce the spread in CMIP5 projections. To this end, we performed a climatological “reference” simulation, i.e., the control simulation, where ROMS is forced with observed atmospheric and lateral boundary conditions, and a set of corresponding downscaled sensitivity experiment where ROMS is forced with atmospheric and lateral boundary conditions derived from global CMIP5 simulations. For the downscaling experiment, we chose two best performing models from the CMIP5 database based on their skill in simulating the present day (historical) climatology. The control simulation has been extensively validated against the observations for its skill in simulating the physical and biogeochemical variables. We explore the sensitivity of the downscaled oxygen distribution and OMZ to the regional model setup by varying the model resolution from 1/3deg to 1/12deg and expanding the model domain from a small AS-limited domain to one encompassing the full Indian Ocean. We show that the downscaled experiments improve the representation of different classes of oxygen (Oxic - O2 > 60mmol/l; Hypoxic - 60mmol/l >= O2 > 4mmol/l; and the Suboxic  - 4 mmol/l > O2 > 0 mmol/l) within the 0-1500m depth range. In particular, the downscaled experiments simulate a much smaller fraction of suboxic waters relative to hypoxic and oxic fractions, in agreement with observations.</p><p> </p>

Arabian sea Oxygen Minimum Zone projections under climate change: sensitivity to forcing and model resolution

2021 ◽  
Author(s):  
Parvathi Vallivattathillam ◽  
Zouhair Lachkar ◽  
Marina Lévy ◽  
Shafer Smith
Keyword(s):  
Climate Change ◽  
Arabian Sea ◽  
Heat Fluxes ◽  
Ocean Modeling ◽  
Future Climate Change ◽  
Depth Range ◽  
Model Resolution ◽  
Regional Ocean Modeling System ◽  
Control Simulation ◽  
Oxygen Minimum

<p>The Arabian sea (AS) hosts one of the most intense oxygen minimum zones (OMZ) in the open ocean. This OMZ is formed and maintained by the peculiar geography and the associated monsoonal productivity in the AS and  is highly sensitive to the strength of monsoonal circulation and surface heating. Model projection from the fifth phase of Coupled Model Intercomparison project (CMIP5) indicate significant changes in both the Indian monsoonal circulation, atmospheric heat fluxes and primary productivity under climate change, but the response of the AS OMZ to these changes remain largely ill-understood. The poor representation of the AS OMZ and lack of oxygen diagnostics in the CMIP5 simulations pose major limitations in exploring the response of AS OMZ to future climate change. In this study, we use a set of regional downscaled experiments with a high-resolution configuration of the Regional Ocean Modeling System (ROMS) model coupled to a nitrogen-based NPZD ecosystem model to examine the sensitivity of the AS OMZ response to a range of CMIP5 forcing anomalies and to model resolution. Our downscaled set of experiments are based on a climatological control simulation forced with observed climatological atmospheric and lateral boundary conditions, to which climate change anomalies derived from CMIP5 simulations are added to construct climate change forcing fields. The control simulation has been extensively validated against observations. We explore the sensitivity of the downscaled oxygen distribution and OMZ to the regional model setup by varying the model horizontal resolution from 1/3 - 1/12 degree. In agreement with the set of available coarse resolution CMIP5 projections, our downscaled experiments show a future increase in the oxygen levels within the core of AS OMZ. The downscaled experiments improve the realistic representation of different classes of water (Oxic - O2 > 60mmol/l; Hypoxic - 60mmol/l >= O2 > 4mmol/l; and the Suboxic - 4 mmol/l > O2 > 0 mmol/l) within the 0-1500m depth range. We find that the projected oxygen changes in the AS OMZ are largely driven by the Apparent Oxygen Utilisation (AOU), which vary with forcing and model resolution, leading to a wide spread in the AS OMZ response to climate change.</p>

scholarly journals Dynamical Downscaling of Future Hydrographic Changes over the Northwest Atlantic Ocean

2020 ◽  
Vol 33 (7) ◽  
pp. 2871-2890 ◽  
Author(s):  
Sang-Ik Shin ◽  
Michael A. Alexander
Keyword(s):  
Climate Change ◽  
Boundary Conditions ◽  
Ocean Circulation ◽  
Radiative Forcing ◽  
Climate Model ◽  
Gulf Of Maine ◽  
Global Climate Model ◽  
Ocean Modeling ◽  
Regional Ocean Modeling System ◽  
The U.S

AbstractProjected climate changes along the U.S. East and Gulf Coasts were examined using the eddy-resolving Regional Ocean Modeling System (ROMS). First, a control (CTRL) ROMS simulation was performed using boundary conditions derived from observations. Then climate change signals, obtained as mean seasonal cycle differences between the recent past (1976–2005) and future (2070–99) periods in a coupled global climate model under the RCP8.5 greenhouse gas trajectory, were added to the initial and boundary conditions of the CTRL in a second (RCP85) ROMS simulation. The differences between the RCP85 and CTRL simulations were used to investigate the regional effects of climate change. Relative to the coarse-resolution coupled climate model, the downscaled projection shows that SST changes become more pronounced near the U.S. East Coast, and the Gulf Stream is further reduced in speed and shifted southward. Moreover, the downscaled projection shows enhanced warming of ocean bottom temperatures along the U.S. East and Gulf Coasts, particularly in the Gulf of Maine and the Gulf of Saint Lawrence. The enhanced warming was related to an improved representation of the ocean circulation, including topographically trapped coastal ocean currents and slope water intrusion through the Northeast Channel into the Gulf of Maine. In response to increased radiative forcing, much warmer than present-day Labrador Subarctic Slope Waters entered the Gulf of Maine through the Northeast Channel, warming the deeper portions of the gulf by more than 4°C.

scholarly journals Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity

2018 ◽  
Vol 15 (1) ◽  
pp. 159-186 ◽  
Author(s):  
Zouhair Lachkar ◽  
Marina Lévy ◽  
Shafer Smith
Keyword(s):  
Arabian Sea ◽  
Large Scale ◽  
Indian Monsoon ◽  
Ocean Modeling ◽  
Climate Projections ◽  
Regional Ocean Modeling System ◽  
Marine Habitats ◽  
Monsoon Wind ◽  
Wind Intensity ◽  
Oxygen Minimum

Abstract. The decline in oxygen supply to the ocean associated with global warming is expected to expand oxygen minimum zones (OMZs). This global trend can be attenuated or amplified by regional processes. In the Arabian Sea, the world's thickest OMZ is highly vulnerable to changes in the Indian monsoon wind. Evidence from paleo-records and future climate projections indicates strong variations of the Indian monsoon wind intensity over climatic timescales. Yet, the response of the OMZ to these wind changes remains poorly understood and its amplitude and timescale unexplored. Here, we investigate the impacts of perturbations in Indian monsoon wind intensity (from −50 to +50 %) on the size and intensity of the Arabian Sea OMZ, and examine the biogeochemical and ecological implications of these changes. To this end, we conducted a series of eddy-resolving simulations of the Arabian Sea using the Regional Ocean Modeling System (ROMS) coupled to a nitrogen-based nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model that includes a representation of the O2 cycle. We show that the Arabian Sea productivity increases and its OMZ expands and deepens in response to monsoon wind intensification. These responses are dominated by the perturbation of the summer monsoon wind, whereas the changes in the winter monsoon wind play a secondary role. While the productivity responds quickly and nearly linearly to wind increase (i.e., on a timescale of years), the OMZ response is much slower (i.e., a timescale of decades). Our analysis reveals that the OMZ expansion at depth is driven by increased oxygen biological consumption, whereas its surface weakening is induced by increased ventilation. The enhanced ventilation favors episodic intrusions of oxic waters in the lower epipelagic zone (100–200 m) of the western and central Arabian Sea, leading to intermittent expansions of marine habitats and a more frequent alternation of hypoxic and oxic conditions there. The increased productivity and deepening of the OMZ also lead to a strong intensification of denitrification at depth, resulting in a substantial amplification of fixed nitrogen depletion in the Arabian Sea. We conclude that changes in the Indian monsoon can affect, on longer timescales, the large-scale biogeochemical cycles of nitrogen and carbon, with a positive feedback on climate change in the case of stronger winds. Additional potential changes in large-scale ocean ventilation and stratification may affect the sensitivity of the Arabian Sea OMZ to monsoon intensification.

scholarly journals Changes in U.S. East Coast Cyclone Dynamics with Climate Change

2015 ◽  
Vol 28 (2) ◽  
pp. 468-484 ◽  
Author(s):  
Christopher G. Marciano ◽  
Gary M. Lackmann ◽  
Walter A. Robinson
Keyword(s):  
Climate Change ◽  
Boundary Conditions ◽  
Potential Vorticity ◽  
East Coast ◽  
The United States ◽  
Sea Level Pressure ◽  
Cyclone Intensity ◽  
Field Amplification ◽  
Lateral Boundary Conditions ◽  
Relative Potential

Abstract Previous studies investigating the impacts of climate change on extratropical cyclones have primarily focused on changes in the frequency, intensity, and distribution of these events. Fewer studies have directly investigated changes in the storm-scale dynamics of individual cyclones. Precipitation associated with these events is projected to increase with warming owing to increased atmospheric water vapor content. This presents the potential for enhancement of cyclone intensity through increased lower-tropospheric diabatic potential vorticity generation. This hypothesis is tested using the Weather Research and Forecasting Model to simulate individual wintertime extratropical cyclone events along the United States East Coast in present-day and future thermodynamic environments. Thermodynamic changes derived from an ensemble of GCMs for the IPCC Fourth Assessment Report (AR4) A2 emissions scenario are applied to analyzed initial and lateral boundary conditions of observed strongly developing cyclone events, holding relative humidity constant. The perturbed boundary conditions are then used to drive future simulations of these strongly developing events. Present-to-future changes in the storm-scale dynamics are assessed using Earth-relative and storm-relative compositing. Precipitation increases at a rate slightly less than that dictated by the Clausius–Clapeyron relation with warming. Increases in cyclone intensity are seen in the form of minimum sea level pressure decreases and a strengthened 10-m wind field. Amplification of the low-level jet occurs because of the enhancement of latent heating. Storm-relative potential vorticity diagnostics indicate a strengthening of diabatic potential vorticity near the cyclone center, thus supporting the hypothesis that enhanced latent heat release is responsible for this regional increase in future cyclone intensity.

scholarly journals Advanced error diagnostics of the CMAQ and Chimere modelling systems within the AQMEII3 model evaluation framework

2017 ◽  
Author(s):  
Efisio Solazzo ◽  
Christian Hogrefe ◽  
Augustin Colette ◽  
Marta Garcia-Vivanco ◽  
Stefano Galmarini
Keyword(s):  
Wind Speed ◽  
North America ◽  
Boundary Conditions ◽  
Time Scale ◽  
Model Evaluation ◽  
Evaluation Framework ◽  
Diagnostic Methods ◽  
Base Case ◽  
Lateral Boundary ◽  
Lateral Boundary Conditions

Abstract. The work here complements the overview analysis of the modelling systems participating in the third phase of the Air Quality Model Evaluation International Initiative (AQMEII3) by focusing on the performance for hourly surface ozone by two modelling systems, Chimere for Europe and CMAQ for North America. The evaluation strategy outlined in the course of the three phases of the AQMEII activity, aimed to build up a diagnostic methodology for model evaluation, is pursued here and novel diagnostic methods are proposed. In addition to evaluating the base case simulation in which all model components are configured in their standard mode, the analysis also makes use of sensitivity simulations in which the models have been applied by altering and/or zeroing lateral boundary conditions, emissions of anthropogenic precursors, and ozone dry deposition. To help understand of the causes of model deficiencies, the error components (bias, variance, and covariance) of the base case and of the sensitivity runs are analysed in conjunction with time-scale considerations and error modelling using the available error fields of temperature, wind speed, and NOx concentration. The results reveal the effectiveness and diagnostic power of the methods devised (which remains the main scope of this study), allowing the detection of the time scale and the fields that the two models are most sensitive to. The representation of planetary boundary layers (PBL) dynamics is pivotal to both models. In particular: i) The fluctuations slower than −1.5 days account for 70–85 % of the total ozone quadratic error; ii) A recursive, systematic error with daily periodicity is detected, responsible for 10–20 % of the quadratic total error; iii) Errors in representing the timing of the daily transition between stability regimes in the PBL are responsible for a covariance error as large as 9 ppb (as much as the standard deviation of the network-average ozone observations in summer in both Europe and North America); iv) The CMAQ ozone error has a weak/negligible dependence on the errors in NO2 and wind speed, while the error in NO2 significantly impacts the ozone error produced by Chimere; v) On a continent wide monitoring network-average, a zeroing out of anthropogenic emissions produces an error increase of 45 % (25 %) during summer and of 56 % (null) during winter for Chimere (CMAQ), while a zeroing out of lateral boundary conditions results in an ozone error increase of 30 % during summer and of 180 % during winter (CMAQ).

scholarly journals Lateral boundary conditions for a local anisotropic ice-flow model

2002 ◽  
Vol 35 ◽  
pp. 503-509 ◽  
Author(s):  
Olivier Gagliardini ◽  
Jacques Meyssonnier
Keyword(s):  
Boundary Conditions ◽  
Flow Model ◽  
Ice Sheet ◽  
Lateral Boundary ◽  
Local Flow ◽  
Ice Flow ◽  
Lateral Boundary Conditions ◽  
Potential Applications ◽  
Two Dimensional Flow ◽  
Spatial Domains

AbstractA local two-dimensional flow model which accounts for the anisotropic behaviour of polar ice and the evolution of its strain-induced anisotropy is briefly reviewed. Due to its complexity, it is not yet possible to use this model to simulate the flow of a whole ice sheet, and its potential applications are presently restricted to limited spatial domains around existing drilling sites. In order to calculate the local flow of ice, boundary conditions must be applied on the lateral edges of the studied domain. Since these limits correspond to fictitious sections of the ice sheet, the type of boundary condition to adopt is not obvious. In the present paper, different kinds of boundary conditions of the Dirichlet type, applied at the lateral boundary of an idealized ice sheet of simplified geometry, are discussed. This will serve as a first step towards the coupling of the local flow model with a global ice-sheet flow model.

Sign in / Sign up

Export Citation Format

Share Document