One factor that may play a role is that hot air is less dense, so assuming equal wind speed the hotter it gets the less force the wind exerts against the turbine blades. I live in Denver, and being at high altitude we have lots of experience with how lower air density affects the efficiency of things like computer cooling fans. Though the reverse process of a wind turbine, the principle is the same. A computer that has adequate cooling at sea level can easily overheat in Denver unless you manually increase the RPM of the fans because for each revolution of the fan blades it is moving fewer air molecules.
To further illustrate the wind turbine issue, just imagine one in a vacuum. How much electricity would that one generate?
Hot air rises . Slowly rises. In fact, all of the US south of the Mason-Dixon line is in the same latitude region known as the Doldrums off of the west coast of Africa. Ships were marooned there for days and weeks with no winds.
The same thing happens on the east coast. When the Bermuda high spreads out the wind goes light and the temps go up. Happens most of the summer.