Instability in boundary layer between the North Equatorial Current and underlying zonal jets based on mooring observations

Two Flavors of Intraseasonal Variability and Their Dynamics in the North Equatorial Current/Undercurrent Region

“…To investigate the contribution of baroclinic instability in generating this type of ISV quantitatively, we adopted a 2.5-layer linearized reduced gravity model, which assumed that the ocean is composed of two active upper layers and an inert, infinitely deep abyssal layer. The general framework of this model is similar to the one used by past studies (e.g., Qiu, 1999;Liu et al, 2003;Qiu et al, 2014;Wang et al, 2020) to explore the dynamics of the seasonal eddy field modulation in the WTP. Under the quasi-geostrophic approximation, the governing equations of the perturbation potential vorticity in the two active layers are given as follow:…”

“…Generally speaking, the surfaceintensified ISV observed by the ADCP mooring and satellite altimeter is suggested to be induced by baroclinic instability of the NEC system. Using the same 2.5-layer model, previous studies have proposed that the NEC in the zonal band between 10 • and 18 • N is baroclinic stable on basin-scale, based on a qualitative stability analysis by dividing the upper 300 m ocean above the main thermocline into two sublayers of 150 m-thick (e.g., Qiu, 1999;Wang et al, 2020). It is worth noting that the surface-intensified ISV signals in the NEC region unveiled by the mooring measurement at 130 • E concentrate in the upper 100 m. Consequently, the simplification in this study is different from theirs, which leads to the conclusion in this study that the surfaceintensified ISV signals derived from mooring measurement are related to the baroclinic instability of the vertically sheared upper part of the NEC.…”