On the Dynamical Relationship between Equatorial Pacific Surface Currents, Zonally Averaged Equatorial Sea Level, and El Niño Prediction

Abstract

Previous work has shown that for large zonal scales and low frequencies, wind-forced sea level, even near the equator, can be described by wind-forced long Rossby waves. In the eastern equatorial Pacific where the interannual wind forcing is small, these waves are essentially locally unforced and propagate westward from the boundary. At the boundary the waves’ sea level is in phase because of geostrophy and no normal flow to the boundary. However, because the waves propagate more slowly with increasing latitude, west of the boundary lag increases as latitude increases. Consequently a northward sea level gradient is like a time derivative, and the zonal geostrophic flow is like a time derivative of the sea level. This implies that the equatorial flow should lead the equatorial sea level by about 9 months on El Niño time scales. However, analysis shows that when dissipation of the large-scale flow is taken into account, this lead is reduced to about 3 months. This lead time is approximately the dissipation time scale of the second vertical mode, which dominates the zonal surface flow. Since the eastern equatorial Pacific sea level ηE is proportional to eastern equatorial thermocline displacement and El Niño, the zonal equatorial flow leads El Niño indices. Analysis also shows that the zonally averaged equatorial Pacific sea level leads El Niño and that this lead is associated with the geostrophic zonal velocity and the long Rossby wave physics.

On the physical interpretation of the lead relation between Warm Water Volume and the El Niño Southern Oscillation

“…We retain the first five baroclinic modes but will show that equatorial Pacific OHC interannual variations obtained from the first two modes reproduce most of the observed variations, in agreement with earlier studies (e.g. Chen et al 1995;Boulanger and Menkes 2001;Shu and Clarke 2002;Izumo et al 2016;Zhang and Clarke 2017).…”

“…Many past studies (e.g. Chen et al 1995;Boulanger and Menkes 2001;Shu and Clarke 2002;Izumo et al 2016;Zhang and Clarke 2017) have shown the importance of the second baroclinic mode for ENSO-related equatorial thermocline depth variations. Including the 2 nd baroclinic mode in our model increases the dominance of K f , since K f,2 is fast enough to be rather in-phase with K f,1 and amplifies it, while the slower R1 f,2 does not, being less in phase with R1 f,1 (not shown).…”

“…This slower feedback has been related to the Sverdrup balance (Jin 1997a,b) but also to Rossby wave dynamics (Bosc and Delcroix 2008;Zhang and Clarke 2017;Clarke et al 2007). In this paradigm, easterly wind anomalies during La Niña induce a slow recharge in the equatorial Pacific through meridional transport converging in the equatorial band.…”

On the Physics of the Warm Water Volume and El Niño/La Niña Predictability

“…The lower-correlation equatorial region between about 1658E and 1708W is influenced by an interannual shallow (50-70 m deep) freshwater jet that is fundamental to ENSO dynamics (Zhang and Clarke 2015). Because of the jet, the monthly anomalous sea level in this region has a contribution not only from thermocline variability, but also from the dynamic height z TF associated with the low-salinity jet.…”

“…Although there have been several theories relating El Niño/La Niña to the WWV (Jin 1997a,b;Clarke et al 2007;Fedorov 2010;Clarke 2010;Thual et al 2013;Neske and McGregor 2018), only recently (Zhang and Clarke 2017) has the direct linkage between the zonal equatorial flow and WWV begun to be appreciated. In this paper we will apply that relationship to understand the physics of why the WWV is a useful predictor of El Niño beyond the spring persistence barrier.…”