Earth has three convection cells per hemisphere. These are the Polar cell, Ferrel cell and Hadley cell. These create the trade winds, westerlies and polar easterlies which control many aspects of climate, such as where rain shadows & dry zones are located and where ocean currents flow.
However, if you were to slow down the rotation rate of a planet, you could turn those three convection cells into a single, gigantic convection cell. Assume a roughly Earth-like planet in all characteristics (sans rotation rate, which is slowed down in order to turn the three convection cells into one - and apparently axial tilt might have an effect, I'm not sure). Because climate is a combination of multiple factors, there are several questions to consider:
- How would the movement of oceanic currents be affected?
- In what direction would the wind blow at different latitudes of the planet's surface?
- Where would the dry zones of the planet be? The wet zones?
- Where would the hot zones of the planet be? The cold zones?
- How would this affect violent weather phenomena such as hurricanes and tornadoes?