Mark brings up a good point via the NHC discussion on Olga -- it's not just the surface temperatures that are important, although having that surface heating (and moisture) does help a lot. Instead, the key factor is the difference in temperature between the ocean's surface and the top of the troposphere, or the tropopause. If you have cooler than ideal SSTs, say 5C below normal, you can "make up for it" with cooler temperatures at the tropopause, also 5C cooler than what you might normally see in the tropics.
This all works because we view a tropical cyclone as a heat engine. As with all machines, a heat engine has a specified efficiency -- you aren't going to get it working perfectly, in many cases. With a tropical system, the efficiency is generally about 30%; this efficiency is specified by the difference in temperatures noted above. This helps to drive a lot of the midlatitude "tropically transitioned" tropical systems and was a key factor in the development of Vince.
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