Beschreibung:
Abstract The different types of instability in the equatorial β-plane approximation are analyzed by means of a 2D linear stability problem. The double-diffusive (DD) and diffusive/baroclinic (2D baroclinic and McIntyre) instabilities are shown not to develop if contours of the mean salinity/density have a parabolic, symmetrical-relative-to-the-equator shape. Using modeling results, an illustrative scheme of Equatorial Undercurrent (EUC) regions where different types of instability can develop is presented and subsequently applied to understand the driving forces of the intrusions observed in a closed spaced CTD section, located between the equator and 1°N. Long coherence intrusions are situated within two isopycnal layers, aligned to 25 (layer 1) and 26.3 (layer 2) σT, where the vertical shear is low. It was shown from the model that the layer-1 intrusions being observed in the midlayer of the EUC where the mean horizontal gradient of salinity is approximately constant are likely generated by a combined effect of DD instability and instability due to linear horizontal shear. The layer-2 intrusions being observed in the lower part of EUC where the mean salinity contours have a parabolic shape likely arise because of linear horizontal shear only, while double diffusion can be considered as an effect that increases the growth rate of unstable modes. Special attention is focused on two different parts of the EUC in the mixing of the thermocline. It is noted that the EUC only makes the mass transfer by long coherence intrusions in certain layers where the vertical shear is small. Conversely, the EUC contributes to the growth rate of unstable modes due to the horizontal linear shear.