Development of a Coupled Regional Climate Simulation Model for the Lake Victoria Basin

Abstract

A three dimensional dynamical model for Lake Victoria based on the Princeton Ocean Model (POM) model has been developed. In this formulation the standard version of the POM model is modified by replacing the open boundaries with a closed coastline and adopting the bathymetry of Lake Victoria. The lake model has 9 equal vertical sigma levels and the horizontal resolution is 20 km, The model's upper boundary conditions are based on momentum, sensible heat and radiative energy fluxes derived from double-nested simulations of the NCAR regional climate model (RcgCM2) or idealized surface boundary forcing. The idealized simulations are 360-day model simulations to investigate, (i) the sensitivity of the lake's circulation on variations in the shear and direction of the idealized upper boundary wind stress forcing, (ii) the response to abrupt change in the direction of surface wind stress forcing to assess the memory of the lake, and (iii) the sensitivity of the lake's circulation on its vertical temperature stratification. The RcgCM2 model output is used for constructing the upper boundary conditions of the lake model for the October-November-December seasonal ``short-rains{''} of eastern Africa. In these experiments we apply selective and systematic suppression of radiation, heat and momentum fluxes contributions to the upper boundary forcing to determine their relative importance. The results show that the bathymetry and geometry of the lake play a fundamental role in determining the climatology of Lake Victoria. The regions of upwelling or downwelling are determined by wind stress and nearshore bathymetry. The momentum memory of Lake Victoria is about two weeks long. There exists Kelvin wave like disturbances in the thermocline that are trapped along the coast and propagate clockwise around Lake Victoria with periodicity of about 30 days. The oscillations entirely disappear in the case of the isothermal conditions. The lake surface circulation is characterized by anti-clockwise circular motion in response to the predominantly easterly surface winds. The three-dimensional model produces a surface temperature pattern indicative of horizontal lake water mixing characterized by a horizontal spiral pattern in the temperature field. This is associated with the spreading of a pool of warm water across the northern section of the lake. This pattern is not present in the one-dimensional model. Comparison of the POM model simulation results with the one dimensional thermal diffusion lake model, used in the NCAR RegCM2 regional climate model, indicates that the former produces more realistic results. This work represents an important phase towards the development of a fully coupled regional climate simulation model for the Lake Victoria basin.