Advanced Ocean Modelling: Using Open-Source Software

5.11 The El-Niño Southern Oscillation 161Fig. 5.27 Illustration of the horizontal flow field associated with centres of high pressure (elevatedsea level) or low pressure (lowered sea level) at the equator. Both patterns create flows that triggereastward propagation of the disturbanceThe other branch of equatorially trapped waves follows the dispersion relation:βkω ≈−k 2 + (2n + 1)/Req2(5.34)These waves, called equatorial planetary waves or equatorial Rossby waves,travel westward at slow speeds and, hence, opposite to equatorial Kelvin waves.The special case n = 0 is characterised by the dispersion relation:ωT eq − 1ωT eq= kR eq (5.35)This wave exhibits a mixed behavior between planetary and inertia-gravity wavesand is therefore called mixed planetary-inertia-gravity wave. Interestingly, unlikeequatorial Kelvin waves, the mixed wave propagates westward along the equator.Although all these distinct equatorial wave types exist in theory, they appearin the real ocean only if being excited by some external mechanism. For instance,dense water flows passing the equator can trigger equatorial inertial oscillations (seeprevious exercise) and disturbances of the larger-scale equatorial wind system cantrigger equatorial Rossby waves (see next exercise). Spatial and temporal scales ofthe initial perturbation determine hereby the wave type and wave mode that, in theend, appears magnified in the ocean.5.11 The El-Niño Southern Oscillation5.11.1 BackgroundThe El-Niño Southern Oscillation (ENSO) is a synonym of a pronounced climatevariation that occurs in the equatorial and tropical Pacific region on time scales of2–10 years. This oscillation is caused by a large-scale dynamic and thermodynamicinteraction between the ocean and the atmosphere. Only a brief description of thisinteraction is given here.