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 Intensification and deepening of the Arabian Sea Oxygen Minimum Zone in response to increase in Indian monsoon wind intensity

2017 ◽  
Author(s):  
Zouhair Lachkar ◽  
Marina Lévy ◽  
Shafer Smith
Keyword(s):  
Arabian Sea ◽  
Large Scale ◽  
Indian Monsoon ◽  
Ecosystem Model ◽  
Climate Projections ◽  
Marine Habitats ◽  
Epipelagic Zone ◽  
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 indicate 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 Oceanic 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.

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>

Indian monsoon wind variability since ∼ 11 kyr in the northwestern and northeastern Arabian Sea

2021 ◽  
pp. 104882
Author(s):  
Muthusamy Ravichandran ◽  
Anil K. Gupta ◽  
Kuppusamy Mohan ◽  
Chokkalingam Lakshumanan
Keyword(s):  
Arabian Sea ◽  
Indian Monsoon ◽  
Wind Variability ◽  
Monsoon Wind

Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations

2020 ◽  
Author(s):  
Takeshi Izumo ◽  
Maratt Satheesan Swathi ◽  
Matthieu Lengaigne ◽  
Jérôme Vialard ◽  
Dr Ramesh Kumar
Keyword(s):  
Indian Ocean ◽  
Indian Summer Monsoon ◽  
Arabian Sea ◽  
Large Scale ◽  
Indian Monsoon ◽  
Westerly Wind ◽  
Low Level ◽  
Intraseasonal Variations ◽  
The Indian Ocean ◽  
Low Level Jet

<p>A strong Low-Level Jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations  only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmosphericdeep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific / North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.</p><p> </p><p>Related reference: Swathi M.S, Takeshi Izumo, Matthieu Lengaigne, Jérôme Vialard and M.R. Ramesh Kumar:Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, accepted in Climate Dynamics.</p>

scholarly journals Rossby Wave Instability and Apparent Phase Speeds in Large Ocean Basins

2007 ◽  
Vol 37 (5) ◽  
pp. 1177-1191 ◽  
Author(s):  
P. E. Isachsen ◽  
J. H. LaCasce ◽  
J. Pedlosky
Keyword(s):  
Large Scale ◽  
Initial Conditions ◽  
Water System ◽  
Local Deformation ◽  
Equation Model ◽  
Ocean Modeling ◽  
High Latitudes ◽  
Regional Ocean Modeling System ◽  
Latitude Range ◽  
The Stability

Abstract The stability of baroclinic Rossby waves in large ocean basins is examined, and the quasigeostrophic (QG) results of LaCasce and Pedlosky are generalized. First, stability equations are derived for perturbations on large-scale waves, using the two-layer shallow-water system. These equations resemble the QG stability equations, except that they retain the variation of the internal deformation radius with latitude. The equations are solved numerically for different initial conditions through eigenmode calculations and time stepping. The fastest-growing eigenmodes are intensified at high latitudes, and the slower-growing modes are intensified at lower latitudes. All of the modes have meridional scales and growth times that are comparable to the deformation radius in the latitude range where the eigenmode is intensified. This is what one would expect if one had applied QG theory in latitude bands. The evolution of large-scale waves was then simulated using the Regional Ocean Modeling System primitive equation model. The results are consistent with the theoretical predictions, with deformation-scale perturbations growing at rates inversely proportional to the local deformation radius. The waves succumb to the perturbations at the mid- to high latitudes, but are able to cross the basin at low latitudes before doing so. Also, the barotropic waves produced by the instability propagate faster than the baroclinic long-wave speed, which may explain the discrepancy in speeds noted by Chelton and Schlax.

scholarly journals Causes of uncertainties in the representation of the Arabian Sea oxygen minimum zone in CMIP5 models

2021 ◽  
Author(s):  
Henrike Schmidt ◽  
Julia Getzlaff ◽  
Ulrike Löptien ◽  
Andreas Oschlies
Keyword(s):  
Southern Ocean ◽  
Arabian Sea ◽  
Climate Models ◽  
Water Mass ◽  
Sea Water ◽  
Coupled Model ◽  
Detailed Comparison ◽  
Cmip5 Models ◽  
Marine Habitats ◽  
Oxygen Minimum

Abstract. Open ocean oxygen minimum zones (OMZs) occur in regions with high biological productivity and weak ventilation. They restrict marine habitats and alter biogeochemical cycles. Global models show generally a large model-data misfit with regard to oxygen. Reliable statements about their future development and the quantification of their interaction with climate change are currently not possible. One of the most intense OMZs is located in the Arabian Sea (AS). We give an overview of the main model deficiencies with a detailed comparison of the historical state of ten climate models from the 5th coupled model intercomparison project (CMIP5) that present our present-day understanding of physical and biogeochemical processes. Considering a threshold of 60 μmol l−1, we find a general underestimation of the OMZ volume in the AS compared to observations, that is caused by a too shallow layer of oxygen-poor water in the models. The deviation of oxygen values in the deep AS is the result of subduction of higher oxygenated waters in the Southern Ocean in the models compared to observations. In addition, model deficiencies related to the coarse resolution of the abyssal ocean, are identified in the deep water mass transport from the Southern Ocean northward into the AS. Differences in simulated water mass properties and ventilation rates of Red Sea Water and Persian Gulf Water cause different mixing in the AS and thus influence the intensity of the OMZ. These differences also point towards variations in the parametrisations of the overflow from the marginal seas among the models. The results of this study are intended to foster future model improvements regarding the OMZ in the AS.

scholarly journals The Seasonality of Convective Events in the Labrador Sea

2014 ◽  
Vol 27 (17) ◽  
pp. 6456-6471 ◽  
Author(s):  
Hao Luo ◽  
Annalisa Bracco ◽  
Fan Zhang
Keyword(s):  
Large Scale ◽  
Current System ◽  
Horizontal Resolution ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Ocean Modeling ◽  
Labrador Sea ◽  
Boundary Current ◽  
Regional Ocean Modeling System ◽  
Surface Heat Fluxes

Abstract Modeling deep convection is a key challenge for climate science. Here two simulations of the Labrador Sea circulation obtained with the Regional Ocean Modeling System (ROMS) run at a horizontal resolution of 7.5 km are used to characterize the response of convection to atmospheric forcing and its seasonal variability over the period 1980–2009. The integrations compare well with the sparse observations available. The modeled convection varies in three key aspects over the 30 years considered. First, its magnitude changes greatly at decadal scales. This aspect is supported by the in situ observations. Second, the initiation and peak of convection (i.e., initiation and maximum) shift by 2–3 weeks between strong and weak convective years. Third, the duration of convection varies by approximately one month between strong and weak years. The last two changes are associated with the variability of the time-integrated surface heat fluxes over the Labrador Sea during winter and spring, while the first results from changes in both atmospheric heat fluxes and oceanic conditions through the lateral inflow of warm Irminger Water from the boundary current system to the basin interior. Changes in surface heat fluxes over the convective region are linked to large-scale modes of variability, the North Atlantic Oscillation and Arctic Oscillation. Implications for modeling the climate variability of the Labrador basin are discussed.

Sign in / Sign up

Export Citation Format

Share Document