FORTE 2.0: a fast, parallel and flexible coupled climate model

FORTE 2.0 is an intermediate-resolution coupled atmosphere–ocean general circulation model (AOGCM) consisting of the Intermediate General Circulation Model 4 (IGCM4), a T42 spectral atmosphere with 35σ layers, coupled to Modular Ocean Model – Array (MOMA), a 2∘ × 2∘ ocean with 15 z-layer depth levels. Sea ice is represented by a simple flux barrier. Both the atmosphere and ocean components are coded in Fortran. It is capable of producing a stable climate for long integrations without the need for flux adjustments. One flexibility afforded by the IGCM4 atmosphere is the ability to configure the atmosphere with either 35σ layers (troposphere and stratosphere) or 20σ layers (troposphere only). This enables experimental designs for exploring the roles of the troposphere and stratosphere, and the faster integration of the 20σ layer configuration enables longer duration studies on modest hardware. A description of FORTE 2.0 is given, followed by the analysis of two 2000-year control integrations, one using the 35σ configuration of IGCM4 and one using the 20σ configuration.

Parallel I∕O in Flexible Modelling System (FMS) and Modular Ocean Model 5 (MOM5)

We present an implementation of parallel I∕O in the Modular Ocean Model (MOM), a numerical ocean model used for climate forecasting, and determine its optimal performance over a range of tuning parameters. Our implementation uses the parallel API of the netCDF library, and we investigate the potential bottlenecks associated with the model configuration, netCDF implementation, the underpinning MPI-IO library/implementations and Lustre filesystem. We investigate the performance of a global 0.25∘ resolution model using 240 and 960 CPUs. The best performance is observed when we limit the number of contiguous I∕O domains on each compute node and assign one MPI rank to aggregate and to write the data from each node, while ensuring that all nodes participate in writing this data to our Lustre filesystem. These best-performance configurations are then applied to a higher 0.1∘ resolution global model using 720 and 1440 CPUs, where we observe even greater performance improvements. In all cases, the tuned parallel I∕O implementation achieves much faster write speeds relative to serial single-file I∕O, with write speeds up to 60 times faster at higher resolutions. Under the constraints outlined above, we observe that the performance scales as the number of compute nodes and I∕O aggregators are increased, ensuring the continued scalability of I∕O-intensive MOM5 model runs that will be used in our next-generation higher-resolution simulations.

Coupling the Parallel Ice Sheet Model with the Modular Ocean Model via an Antarctic ice-shelf cavity module

<p>Ocean-ice shelf interactions are the main drivers for the current mass loss from the Antarctic Ice Sheet. Recent studies have shown that increased continental meltwater input can enhance discharge through ice-ocean feedbacks. This raises the need for interactive modelling between ocean and ice-sheet systems to assess the consequences of additional freshwater input on the Antarctic region and beyond. While high-resolution simulations (1/4 degree or greater) can resolve detailed interactions between the ocean and ice shelf, the computational costs make them applicable only for regional studies or decadal to centennial time scales. In this study we present a framework for coupling a coarse resolution ocean model (MOM) to the Parallel Ice Sheet Model (PISM) via the Potsdam Ice-shelf Cavity mOdel (PICO). The intermediate model PICO approximates the overturning circulation in ice shelf cavities and includes ice-ocean boundary layer physics. We present this offline coupling approach and discuss the fluxes exchanged between the distinct model runs as well as energy and mass conservation. Using this flexible and computationally efficient framework, feedbacks between the ice and ocean can be analysed on a global spatial scale and paleoclimate time-scales.</p><p> </p>

A regional study of Bay of Bengal processes using radiation boundary condition in Modular Ocean Model

<p>The ocean exchanges heat and mass with the atmosphere in form of shortwave and longwave radiations, precipitation, and evaporation. The regional scale ocean processes governed by this exchange play a vital role in modulating the local dynamics of the Indian Ocean. For instance, the meso-scale eddies and waves control the ocean vertical temperature structure, mixed layer depth, and the thermocline. The Indian Ocean Observing System (IndOOS) recommends the need of proper understanding of heat budget in the Indian Ocean to resolve the mesoscale and submesoscale processes, which trigger large scale ocean circulation, cyclonic eddies, plumes etc. In a regional domain, the stability of ocean also depends on the local parameters, namely, wind pattern, precipitation, runoff and exchange of heat and mass fluxes near the domain boundary. The main objective of this study is to understand the effect of atmospheric wind and solar radiation on the ocean surface and sub-surface characteristics using Modular Ocean Model (MOM5).</p><p>A regional domain in the Bay of Bengal (BoB) is selected, which has unique features, such as, large amount of freshwater flux, seasonal wind reversal and high amount of solar radiation due the geographic location. The dynamics in BoB is important for understanding the Indian summer and winter monsoon seasons and associated weather patterns. A regional ocean modelling approach is adopted using MOM5 with horizontal grid resolution (0.25<sup>0</sup>) while maintaining the vertical grid-size as 1m near the surface region which increases with depth. For the regional domain, radiation open boundary condition (OBC) is implemented on three lateral boundaries of domain, based on the technique proposed by Orlanski (1976). The OBC at the lateral boundaries help in smooth exchange of current and tracers. K-profile parameterization (KPP) vertical mixing scheme is used that accounts for effects of shear, wave breaking, and double diffusion. The model is started from a state of rest and simulated for a period of 10 years using 6-hourly solar radiation (Japanese 25-year reanalysis (JRA-25)) and daily averaged wind stress (SODA reanalysis) dataset. After five years of model spin-up, the last five years of simulated output is considered to ensure consistency of model results. Heat budget calculation shows good agreement with WHOI OA Air-Sea Fluxes (OAFlux). Smooth exchange of mass and fluxes is observed near boundary, which confirms successful implementation of OBC. Implementation of KPP scheme enhances mixing in the upper ocean layers with more realistic thermocline formation and turbulent kinetic energy (TKE). The model is able to mimic the seasonal variability in the ocean currents enforced due to winds. The Sea Surface Temperature (SST) is in good agreement with SODA reanalysis data.</p><p>A plume like mesoscale feature in the SST plot is captured in the present study (that is also observed in microwave SST), but found to be missing in earlier BoB study with sponge boundary conditions. Finer scale resolution (0.125<sup>0</sup>) study is in progress, which is expected to show secondary mesoscale structures and their evolution. The results from this study would help in better understanding of the influence regional-scale processes on local ocean dynamics.</p>

MOMSO 1.0 – an eddying Southern Ocean model configuration with fairly equilibrated natural carbon

We present a new near-global coupled biogeochemical ocean-circulation model configuration. The configuration features a horizontal discretization with a grid spacing of less than 11 km in the Southern Ocean and gradually coarsens in meridional direction to more than 200 km at 64∘ N, where the model is bounded by a solid wall. The underlying code framework is the Geophysical Fluid Dynamics Laboratory (GFDL)'s Modular Ocean Model coupled to the Biogeochemistry with Light, Iron, Nutrients and Gases (BLING) ecosystem model of Galbraith et al. (2010). The configuration is unique in that it features both a relatively equilibrated oceanic carbon inventory and an eddying ocean circulation based on a realistic model geometry/bathymetry – a combination that has been precluded by prohibitive computational cost in the past. Results from a simulation with climatological forcing and a sensitivity experiment with increasing winds suggest that the configuration is sufficiently equilibrated to explore Southern Ocean carbon uptake dynamics on decadal timescales. The configuration is dubbed MOMSO, a Modular Ocean Model Southern Ocean configuration.

Parallel I/O in FMS and MOM5

We present an implementation of parallel I/O in the Modular Ocean Model (MOM), a numerical ocean model used for climate forecasting, and determine its optimal performance over a range of tuning parameters. Our implementation uses the parallel API of the netCDF library, and we investigate the potential bottlenecks associated with the model configuration, netCDF implementation, the underpinning MPI-IO library/implementations and Lustre filesystem. We investigate the performance of a global 0.25° resolution model using 240 and 960 CPUs. The best performance is observed when we limit the number of contiguous I/O domains on each compute node and assign one MPI rank to aggregate and write the data from each node, while ensuring that all nodes participate in writing this data to our Lustre filesystem. These best performance configurations are then applied to a higher 0.1° resolution global model using 720 and 1440 CPUs, where we observe even greater performance improvements. In all cases, the tuned parallel I/O implementation achieves much faster write speeds relative to serial single-file I/O, with write speeds up to 60 times faster at higher resolutions. Under the constraints outlined above, we observe that the performance scales as the number of compute nodes and I/O aggregators are increased, ensuring the continued scalability of I/O-intensive MOM5 model runs that will be used in our next generation higher resolution simulations.

MOMSO 1.0 - a near-global, coupled biogeochemical ocean-circulation model configuration with realistic eddy kinetic energy in the Southern Ocean

We present a new near-global coupled biogeochemical ocean-circulation model configuration. The configuration features a horizontal discretization with a grid spacing of less than 11 km in the Southern Ocean and gradually coarsens in meridional direction to more than 200 km at 64° N where the model is bounded by a solid wall. The underlying code framework is GFDL's Modular Ocean Model coupled to the Biology Light Iron Nutrients and Gasses (BLING) ecosystem model of Galbraith et al. (2010). The configuration is cutting-edge in that it features both a relatively equilibrated oceanic carbon inventory and a realistic representation of eddy kinetic energy – a combination that has, to-date, been precluded by prohibitive computational cost. Results from a simulation with climatological forcing and a sensitivity experiment with increasing winds suggest that the configuration is suited to explore Southern Ocean Carbon uptake dynamics on decadal timescales. Further, the fidelity of simulated bottom water temperatures off and on the Antarctic Shelf suggest that the configuration may be used to provide boundary conditions to ice-sheet models. The configuration is dubbed MOMSO a Modular Ocean Model Southern Ocean configuration.

A Modified Vertical Mixing Parameterization for Its Improved Ocean and Coupled Simulations in the Tropical Pacific

Climate models suffer from significant biases over the tropical Pacific Ocean, including a too-cold cold tongue and too-warm temperature at the depth of the thermocline. The emergence of model biases can be partly attributed to vertical mixing parameterizations, in which there are great uncertainties in selections of functional forms and empirical parameters. In this paper, the impacts of two different vertical mixing schemes on the tropical Pacific temperature simulations are investigated using version 5 of the Modular Ocean Model (MOM5). One vertical mixing scheme is the widely used K-profile parameterization (KPP) scheme, and the other is a hybrid mixing scheme (the Chen scheme) by combining a Kraus–Turner-type bulk mixed layer (ML) model with Peters et al.’s shear instability mixing model (PGT model). It is shown that the Chen scheme works better than the KPP scheme for SST simulation but produces an exaggerated subsurface warm bias simultaneously. The better SST simulation can be attributed to the employment of the PGT model, which produces lower levels of shear instability mixing than its counterpart in the KPP scheme. Furthermore, a modified KPP scheme is presented in which its shear instability mixing model and constant background diffusivity are replaced by the PGT model and the Argo-derived background diffusivity, respectively. This new scheme is then employed into MOM5-based ocean-only and coupled simulations, demonstrating substantial improvements in temperature simulations over the tropical Pacific. The modified KPP scheme can be easily employed into other ocean models, offering an effective way to improve ocean simulations.

Increased Surface Ocean Heating by Colored Detrital Matter (CDM) Linked to Greater Northern Hemisphere Ice Formation in the GFDL CM2Mc ESM

Recent observations of Arctic Ocean optical properties have found that colored dissolved organic matter (CDOM) is of primary importance in determining the nonwater absorption coefficient of light in this region. Although CDOM is an important optical constituent in the Arctic Ocean, it is not included in most of the current generation of Earth system models (ESMs). In this study, model runs were conducted with and without light attenuation by colored detrital matter (CDM), the combined optical contribution of CDOM and nonalgal particles. The fully coupled GFDL CM2 with Modular Ocean Model version 4p1 (MOM4p1) at coarse resolution (CM2Mc) ESM was used to examine the differences in heating and ice formation in the high northern latitudes. The annual cycle of sea surface temperature (SST) is amplified in the model run where the optical attenuation by CDM is included. Annually averaged integrated ice mass is 5% greater and total ice extent is 6% greater owing to colder wintertime SSTs. Differences in ocean heating (i.e., temperature tendency) between the two model runs are well represented by the combined changes in heating by penetrating shortwave radiation, mixing, and surface heat fluxes in the upper 100 m. Shortwave radiation is attenuated closer to the surface, which reduces heating below 10 m during summer months. Mixing entrains colder waters into the mixed layer during the autumn and winter months. Increased cloudiness and ice thickness in the model run with CDM reduces incoming shortwave radiation.

MOMBA 1.1 – a high-resolution Baltic Sea configuration of GFDL's Modular Ocean Model

We present a new coupled ocean-circulation–ice model configuration of the Baltic Sea. The model features, contrary to most existing configurations, a high horizontal resolution of ≈ 1 nautical mile (≈ 1.85 km), which is eddy-resolving over much of the domain. The vertical discretisation comprises a total of 47 vertical levels. Results from a 1987 to 1999 hindcast simulation show that the model's fidelity is competitive. As suggested by a comparison with sea surface temperatures observed from space, this applies especially to near-surface processes. Hence, the configuration is well suited to serve as a nucleus of a fully fledged coupled ocean-circulation–biogeochemical model (which is yet to be developed). A caveat is that the model fails to reproduce major inflow events. We trace this back to spurious vertical circulation patterns at the sills which may well be endemic to high-resolution models based on geopotential coordinates. Further, we present indications that – so far neglected – eddy/wind effects exert significant control on wind-induced up- and downwelling.