Tropical Pacific basin‐wide adjustment and oceanic waves

Abstract

Abstract.This study examines the basin-wide adjustment process associated with E1 Nifio-Southern Oscillation (ENSO) in terms of oceanic waves using the Cane-Zebiak model. The Rossby waves induce not only the meridional mass divergence in the ocean interior but also the incoming zonal mass flux at the western boundary, which generally overcompensates the interior meridional divergence. Therefore, the Rossby waves alone cannot make a negative feedback. However, the outgoing zonal mass flux associated with the reflected Kelvin wave at the western boundary reduces the incoming boundary mass flux associated with the Rossby waves, resulting in the tendency of equatorial basinwide heat content varying in phase with the meridional divergence. Also shown is that the nonequilibrium adjustment of the tropical Pacific Ocean is mainly due to the free Kelvin waves reflected at the western boundary.The ENSO has been recognized as a self-sustained oscillating phenomena whose origin is an ocean-atmosphere coupled instability and whose oscillatory nature is attributed to the slow oceanic adjustment. The slow oceanic adjustment has been explained by either 'delay oscillator' the- Two theories commonly emphasize the slow ocean adjustment but explain it in a different manner.The delay oscillator theory attributes the negative feedback mechanism to the free Kelvin waves generated by a reflection of Rossby waves at the western boundary. In the recharge oscillator mechanism, on the other hand, the negative feedback mechanism is achieved by the slow adjustment of the total heat content/mass due to the meridional The recharge oscillator appears not to consider the western boundary reflection. However, the reflection affects the equatorial heat content by modifying the zonal fluxes, particularly at the western boundary, such that the net boundary flux is reduced by the increase of reflection at the western boundary. This is because at the boundary, the zonal

Wind Stress Curl and ENSO Discharge/Recharge in the Equatorial Pacific

“…Previous theoretical (e.g., Zebiak 1989;An and Kang 2001) and observational (Meinen and McPhaden 2001) work has suggested that this western boundary layer transport and, therefore, TЈ W contribute significantly. However TЈ W could not be estimated as directly as the other terms in (2.15) because the geostrophic ocean current estimates at 156°E, generously provided by Christopher Meinen (Meinen and McPhaden 2001;Meinen 2005), were only available at 3.5°S, 0°, 3.5°, and 6.5°N.…”

Role of nonlinear ocean dynamic response to wind on the asymmetrical transition of El Niño and La Niña

“…In BH89 (i.e., equation ), ⟨ h ⟩ is the reflected Kelvin wave, but we use the zonal mean thermocline depth over the equatorial band (5°S–5°N). This approximation (i.e., ⟨ h ⟩ ≈ h reflected Kelvin ) is quite reasonable because the forced Kelvin waves mainly over the eastern Pacific and the forced Rossby waves mainly over the western Pacific almost cancel each other by averaging over an equatorial band [ An and Kang , ]. As shown in Figure c, the maximum correlation is observed when ⟨ τ x ⟩ leads ⟨ h ⟩ by 12 months ( r = −0.447; α < 0.01).…”

Feedback process responsible for intermodel diversity of ENSO variability

“…The β h is basically affected by two factors: ocean stratification and wind stress pattern/intensity (i.e., its zonal and meridional shape) [e.g., An and Kang, 2001;Chen et al, 2015;Im et al, 2015;An and Kim, 2017]. The β h is basically affected by two factors: ocean stratification and wind stress pattern/intensity (i.e., its zonal and meridional shape) [e.g., An and Kang, 2001;Chen et al, 2015;Im et al, 2015;An and Kim, 2017].…”

“…Under a simple dynamical framework, we have β h ≈ L 2ρ o g 0 H [Hirst, 1986;An and Kang, 2001;An and Bong, 2016], where L is the ocean basin length, H is the mean thermocline depth, ρ ο is the ocean water density, and g 0 is the reduced gravity. An inverse relationship between β h and the mean thermocline depth might be due to that most wind stress momentum is trapped above the thermocline [An and Bong, 2016;An and Kim, 2017].…”