The TIW may be there for El Nino, it is just more obvious with La Nina.
I talked to one presenter at AGU about the sharpness of the front
Over a short lateral distance the vortex front shows a sharp change in temperature. The vortices are large in diameter and regularly spaced like traveling waves
Warner, S. J., Holmes, R. M., M. Hawkins, E. H., S. Hoecker-Martínez, M., Savage, A. C., & Moum, J. N. (2018). Buoyant Gravity Currents Released from Tropical Instability Waves. Journal of Physical Oceanography, 48(2), 361–382. doi:10.1175/jpo-d-17-0144.1