Advanced Ocean Modelling: Using Open-Source Software

162 5 3D Level ModellingIn some years, warmer than average sea temperatures in the eastern equatorialIndian Ocean and western Pacific Ocean trigger a local disturbance of the atmosphericcirculation, called the Walker circulation. As a result of this, a tropicalcyclone pair develops in the western equatorial Pacific Ocean and the Trade Windsin this region are substituted by westerly (that is, eastward blowing) winds. Themodified atmospheric circulation, in turn, creates disturbances in the ocean’s densityfield via localised upwelling or downwelling. This triggers an equatorial Kelvinwave propagating eastward to the other side of the Pacific where is suppresses thesemipermanent coastal upwelling of cold and nutrient-enriched water off the Peruvianand Californian coasts and leads to widespread kills of plankton, fish, and seabirds. These occasions are called El Niño (Spanish for “the child”) events for theyoccur usually around the Christmas season.5.12 Exercise 25: Simulation of an El-Niño Event5.12.1 AimThe aim of this exercise is the simulate disturbances in the equatorial western PacificOcean leading to an El-Niño event on the eastern side of the Pacific Ocean.5.12.2 Task DescriptionThe model domain has a horizontal extent of 2,000 × 2,000 km, resolved by lateralgrid spacings of Δx = Δy = 20 km (Fig. 5.28). Coasts are located in the west and inthe east. The northern and southern limits of the model domain are treated as openboundaries. Total water depth is set to 250 m to allow for relatively long numericaltime steps noting that the free-surface version of the model is being used. Verticalgrid spacing is set to Δz = 50 m, and the numerical time step is set to 120 secs.Initially, the ocean is at rest. Seawater density in the upper 100 m of the water columnis set to 1,027 kg/m 3 . Underneath, the density is 1,028 kg/m 3 . The phase speedof internal gravity waves associated with this density configuration is c = 0.75 m/s,using Eq. (3.63). The equatorial beta-plane approximation is employed with:f = βy (5.36)where y is the meridional coordinate and β is set to β = 2.5 × 10 −11 m −1 s −1 .Theequator at y = 0 cuts the model domain in half. The equatorial radius of deformation(take Eq. 5.27) is about 174 km, which is reasonably well resolved by the lateralgrid spacing chosen. The equatorial inertial period (take Eq. 5.34) is approximately2.7 days.