Several large rivers enter Winam Gulf, a large shallow semi-enclosed embayment of Lake Victoria. Given the vast spatial scale of this embayment (1400 km 2 ), horizontal transport of waterborne material plays a major role in determining its biochemical properties. To identify the dominant hydrodynamic characteristics of this horizontal transport, a combination of field data and three-dimensional numerical modeling were employed to investigate effects due to variable wind forcing, riverine inflows, and interactions with the rest of Lake Victoria including astronomical tides and a complex bathymetry. Currents in the Gulf were oscillatory due to periodic wind and tidal forcing leading to surface seiching, with a residual circulation component. In the western region of the Gulf near the connection with Lake Victoria, substantial vertical and horizontal shear in the currents resulted in efficient horizontal dispersion with rates of up to 160 m 2 s 21 simulated using numerical tracers. In the eastern region, however, rates of horizontal dispersion were reduced to around 12 m 2 s 21 . This spatial variability in hydrodynamic conditions underpins regional differences in ecological properties of Winam Gulf.

Horizontal dispersion mechanisms must be well defined in order to determine the origin and fate of heterogeneous distributions of biogeochemical material. Here data from an intensive field campaign in Valle de Bravo Reservoir, Mexico, was used to run a three-dimensional hydrodynamic model, and the simulated flow field was used to characterize the horizontal motions and investigate the dispersal of waterborne particles driven by the dominant circulation patterns. The surface layer horizontal motions included an oscillatory component associated with two basin-scale internal wave modes and a spatially and temporally complex flow field associated mostly with topographic circulation. Modeling of particle advection by the horizontal motions led to areal dispersal rates of O(1) to O(10) m 2 s 21 , comparable with bulk dispersion rates observed in other surface layer studies. The aperiodic topographic component of the horizontal flow field, not the oscillatory motion or a combination of both, sustained most of the particle dispersal. In particular, local flow features, such as stagnation points, were shown to be a significant driver for horizontal dispersion and should be taken into consideration when analyzing horizontal distributions of waterborne material.

Several processes associated with spatial variations in buoyancy flux and mixing set up local and lake-wide horizontal temperature gradients, that in turn drive slow gravitational currents. These motions can dominate the horizontal transport and re-distribution of biological and chemical material. Here intrusions, indicative of horizontal transport, are identified in field data from a small drinking water reservoir, and the origin and character of the flows investigated using a 3-dimensional (3D) hydrodynamic model. It is shown that a horizontal temperature gradient is set up along the surface layer, due to upwelling shifting the metalimnion closer to the surface towards the upwind region, leading to a spatial variation in entrainment. The flows driven by these gradients form significant mass flux paths, enhancing exchange with the boundaries and controlling the fate of upwelled fluid. Further, the interaction of these currents with other hydrodynamic conditions is explored; namely the interaction with surface wind-driven currents, and the influence of different internal seiches generated by alternative lake bathymetries.

The Australian Community Climate and Earth System Simulator (ACCESS) has been developed at the Centre for Australian Weather and Climate Research. It is a coupled modeling system consisting of ocean, atmosphere and land surface. The ACCESS atmospheric component is the UK Met Office Unified Model (UM). The initial results from the ACCESS coupled model had significant errors in the sea surface temperature (SST). It has been identified that the SST bias is largely due to errors in the representation of clouds. We have found that the use of the homogenous cloud distribution within model grid-boxes produced an underestimation of solar radiation reaching the surface, causing a cooling effect. The model cloud scheme PC2 does not produce enough high cloud cover, which also led to a cooling effect. These two deficiencies have been largely remedied by the implementation of the triple-cloud scheme and a modification to the ice cloud fraction parameterisation in the PC2 cloud scheme. We have also modified the air-sea flux exchange scheme to improve the simulation of ocean currents. These modifications have led to significant improvements in the simulation of SST in ACCESS.

Horizontal dispersion mechanisms must be well defined in order to determine the origin and fate of heterogeneous distributions of biogeochemical material. Here data from an intensive field campaign in Valle de Bravo Reservoir, Mexico, was used to run a three‐dimensional hydrodynamic model, and the simulated flow field was used to characterize the horizontal motions and investigate the dispersal of waterborne particles driven by the dominant circulation patterns. The surface layer horizontal motions included an oscillatory component associated with two basin‐scale internal wave modes and a spatially and temporally complex flow field associated mostly with topographic circulation. Modeling of particle advection by the horizontal motions led to areal dispersal rates of O(1) to O(10) m2 s−1, comparable with bulk dispersion rates observed in other surface layer studies. The aperiodic topographic component of the horizontal flow field, not the oscillatory motion or a combination of both, sustained most of the particle dispersal. In particular, local flow features, such as stagnation points, were shown to be a significant driver for horizontal dispersion and should be taken into consideration when analyzing horizontal distributions of waterborne material.