Question about "water roll over"

[JL]

Request someone explain the term "water roll over" ? I am guessing it what we have experienced over the last week from Dana Point to Long beach with Water temps going down at least 3 - 5 degrees and in some cases very dirty water. Fishing just outside the harbors this week has been extremely tough. Not even mackerel, or any hits at all. First time in 7 years i had 3 outings that generated 1 lone sheep head.

How often does rollover happen?

thanks for you help