When ENSO happens, the present theory and observation suggest that the depth of the mixed layer in the eastern and western Pacific Ocean change and the upwelling will therefore be weakened or strengthened. But how about the mixing processes at the bottom of the mixed layer? Maybe we could use some TOGA data to do some research on that.
Recently, there is paper on the variation of mixing with tropical instability waves and it could be a good reference for this possible work.
[1] J. Moum, R. Lien, A. Perlin, J. Nash, M. Gregg, and P. Wiles. Sea surface cooling at the Equator by subsurface mixing in tropical instability waves. Nature Geoscience, 266:761–765, 2009.
