162
5 3D Level Modelling
In some years, warmer than average sea temperatures in the eastern equatorial
Indian Ocean and western Pacific Ocean trigger a local disturbance of the atmospheric circulation, called the Walker circulation. As a result of this, a tropical
cyclone pair develops in the western equatorial Pacific Ocean and the Trade Winds
in this region are substituted by westerly (that is, eastward blowing) winds. The
modified atmospheric circulation, in turn, creates disturbances in the ocean’s density field via localised upwelling or downwelling. This triggers an equatorial Kelvin
wave propagating eastward to the other side of the Pacific where is suppresses the
semipermanent coastal upwelling of cold and nutrient-enriched water off the Peruvian and Californian coasts and leads to widespread kills of plankton, fish, and sea
birds. These occasions are called El Ni˜ no (Spanish for “the child”) events for they
occur usually around the Christmas season.
5.12 Exercise 25: Simulation of an El-Ni ˜
no Event
5.12.1 Aim
The aim of this exercise is the simulate disturbances in the equatorial western Pacific
Ocean leading to an El-Ni˜ no event on the eastern side of the Pacific Ocean.
5.12.2 Task Description
The model domain has a horizontal extent of 2,000 × 2,000 km, resolved by lateral
grid spacings of Δx = Δy = 20 km (Fig. 5.28). Coasts are located in the west and in
the east. The northern and southern limits of the model domain are treated as open
boundaries. Total water depth is set to 250 m to allow for relatively long numerical
time steps noting that the free-surface version of the model is being used. Vertical
grid 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 column is set to 1,027 kg/m
3 . Underneath, the density is 1,028 kg/m
3 . The phase speed
of 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 . The
equator 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 lateral
grid spacing chosen. The equatorial inertial period (take Eq. 5.34) is approximately
2.7 days.
5 3D Level Modelling
In some years, warmer than average sea temperatures in the eastern equatorial
Indian Ocean and western Pacific Ocean trigger a local disturbance of the atmospheric circulation, called the Walker circulation. As a result of this, a tropical
cyclone pair develops in the western equatorial Pacific Ocean and the Trade Winds
in this region are substituted by westerly (that is, eastward blowing) winds. The
modified atmospheric circulation, in turn, creates disturbances in the ocean’s density field via localised upwelling or downwelling. This triggers an equatorial Kelvin
wave propagating eastward to the other side of the Pacific where is suppresses the
semipermanent coastal upwelling of cold and nutrient-enriched water off the Peruvian and Californian coasts and leads to widespread kills of plankton, fish, and sea
birds. These occasions are called El Ni˜ no (Spanish for “the child”) events for they
occur usually around the Christmas season.
5.12 Exercise 25: Simulation of an El-Ni ˜
no Event
5.12.1 Aim
The aim of this exercise is the simulate disturbances in the equatorial western Pacific
Ocean leading to an El-Ni˜ no event on the eastern side of the Pacific Ocean.
5.12.2 Task Description
The model domain has a horizontal extent of 2,000 × 2,000 km, resolved by lateral
grid spacings of Δx = Δy = 20 km (Fig. 5.28). Coasts are located in the west and in
the east. The northern and southern limits of the model domain are treated as open
boundaries. Total water depth is set to 250 m to allow for relatively long numerical
time steps noting that the free-surface version of the model is being used. Vertical
grid 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 column is set to 1,027 kg/m
3 . Underneath, the density is 1,028 kg/m
3 . The phase speed
of 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 . The
equator 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 lateral
grid spacing chosen. The equatorial inertial period (take Eq. 5.34) is approximately
2.7 days.