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Coupling the PLANKTOM5.0 marine ecosystem model to the OCCAM 1º ocean general circulation model for investigation of the sensitivity of global biogeochemical cycles to variations in ecosystem complexity and physical environment
Full text linkThe earliest marine ecosystem models consisted of a simple representation of the main features of marine ecosystems, including, typically, variables for phytoplankton, zooplankton, nutrient and detritus (NPZD models). These have been incorporated into ocean general circulation models to give a basic representation of ecosystem function, providing predictions of bulk quantities such as global primary production, export and biomass which can be compared with available observations. A recent trend has been to increase the number of phytoplankton and zooplankton groups modelled, as analogues of different plankton groups observed to exist in the ocean, for example diatoms and cocolithophores (the so-called plankton functional type or PFT approach). It is usually assumed that the increase in complexity of the model will result in simulated ecosystems which more faithfully reproduce observations than NPZD models, but this has not been demonstrated systematically. The robustness of the PFT models to changes in model parameters and to changes to the physical environment in which it is embedded, have not been investigated. As a first step towards these goals, we incorporate a state-of-the-art PFT model, PLANKTOM5.0 into the OCCAM ocean general circulation model. A 6 year simulation is performed, covering the years 1989-1994 with identical parameter choices to an existing run of PLANKTOM5.0 coupled to the OPA general circulation model. This document describes the development of the coupled model and the 6 year simulation. Comparison with the OPA model and sensitivity of the solution to parameter choices will be described in a forthcoming journal paper
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Predictability of decadal variations in the thermohaline circulation climate
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A description of interdecadal time-scale propagating North Atlantic sea surface temperature anomalies and their effect on winter European climate, 1948-2002
No full textEastward-propagating interdecadal time-scale sea surface temperature (SST) winter anomalies have been shown to exist at the North Atlantic subpolar/subtropical gyre boundary. Heat flux and surface air temperature signatures of these anomalies are investigated using satellite- and ship-based SST observations and atmospheric reanalysis. Using bandpass filter analysis, retaining periods between 9 and 25 yr, a succession of coherent propagating SST anomalies is identified. The size, speed, propagation path, and decay characteristics of propagating anomalies detected during the period 1948–2002 are documented. The behavior of the propagations changes between the periods 1948–70 and 1970–2002. In the former period, SST anomalies propagated from the east coast of North America to the British Isles in 10 yr. The anomalies displayed a well-defined life cycle, growing in the western basin (west of 40°W) and decaying in the eastern basin. During the period 1970–2002, SST anomalies did not propagate deep into the eastern basin, but grew in the western basin and then ceased propagating. Oceanic anomalies have a comparable marked signature in surface sensible and latent heat fluxes and in surface air temperature. Winter surface heat flux anomalies act to amplify SST anomalies during the middle of their lifetimes, normally in the west-central Atlantic. At other times, heat flux anomalies are associated with decay of anomalies. Surface heat fluxes do not always act to cause propagation, and it is likely that other processes such as advection play a role in the propagation mechanism. North European winter surface air temperatures are raised or lowered by up to ±0.5°C over decadal time scales (1/3 of the total variation over the United Kingdom) when an SST anomaly reaches the eastern boundary. A variety of processes can cause SST variation on decadal time scales at the eastern boundary, but in the 1950s and 1960s the variability at these periods was the signature of features that had propagated across the Atlantic from the North American coast
Influence of bottom topography on integral constraints in zonal flows with parameterized potential vorticity fluxes
Full text linkAn integral constraint for eddy fluxes of potential vorticity (PV), corresponding to global momentum conservation, is applied to two-layer zonal quasi-geostrophic channel flow. This constraint must be satisfied for any type of parameterization of eddy PV fluxes. Bottom topography strongly influence the integral constraint compared to a flat bottom channel. An analytical solution for the mean flow solution has been found by using asymptotic expansion in a small parameter which is the ratio of the Rossby radius to the meridional extent of the channel. Applying the integral constraint to this solution, one can find restrictions for eddy PV transfer coefficients which relate the eddy fluxes of PV to the mean flow. These restrictions strongly deviate from restrictions for the channel with flat bottom topography
Impact on the ocean of extreme Greenland Sea heat loss in the HadCM3 coupled ocean atmosphere model
No full textThe ocean response to air-sea flux variability in the Greenland Sea is investigated using a 1000 year run of the coupled ocean-atmosphere model HadCM3. Evaluation of the density flux reveals that net heat flux anomalies have a greater impact on surface density changes than anomalies in both net evaporation and ice melt/formation. Averaged over the Greenland Sea, the annual mean density flux due to heat loss is 1.8 × 10?6 kg m?2 s?1, an order of magnitude greater than the net evaporation and the ice melt and formation terms, which are ?0.1 and ?0.2 × 10?6 kg m?2 s?1, respectively. Extreme winter heat loss events reach 250 W m?2 and are associated with reduced ice cover and anomalously strong northerly airflow over the Greenland Sea. They result in enhanced convection and modify the properties of deep water flowing south through the Denmark Strait. The deep water transport increases by about 30% when the strongest and weakest heat loss events are compared, and there is a corresponding reduction in temperature and salinity by up to 2.3°C and 0.38 psu. We also find significant correlations between deep western basin temperatures at 60°, 55°, and 49°N and the Greenland Sea heat flux anomalies which peak at lags of up to 4 years with the time delay increasing toward more southerly latitudes. Our results suggest that Greenland Sea heat flux variability is a key variable for understanding recent observations of significant interannual variability in Denmark Strait transport characteristics
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