Thermocline - The layer of maximum vertical temperature gradient in the ocean, lying between the surface ocean and the abyssal ocean. (climatehotmap.org)
As regards Katrina, although were it not for AGW a hurricane as damaging as Katrina might have occurred if one were to wait long enough, surely Katrina itself was made more damaging by AGW as that was a factor contributing to Caribbean SSTs and the local tropospheric vertical temperature gradients at that point in time? (realclimate.org)
Nevertheless, they are quite adequate to create and maintain strong vertical temperature gradients and major temperature differences across the globe. (wattsupwiththat.com)