By Matt Smith
Bjorn, 40, and U.S. Open champion McIlroy began the day 13 under after rattling in a succession of birdies in near-perfect conditions on the first two rounds.
Saturday was a different story, however, as offshore gusts buffeted the Majlis course and temperatures topped 30 degrees Celsius, making it tough for the leading trio as they teed off under a searing midday sun.
By the turn, Bjorn was one under for the round, having squandered a two-foot birdie chance on the ninth green, while McIlroy, 22, was on par.
It could have been worse for the Ulsterman. Normally so strong on this long game, the world number two missed the fairway from the tee on four of the opening nine holes, most calamitously on the seventh when his drive cleared the lake but then rolled back into the water for a penalty shot.
He made partial amends, staying calm to sink a 20-foot putt down the slope to escape with a bogey.
But Bjorn, who had bogeyed the fifth, took full advantage, tapping in a five-foot birdie putt for a two-shot swing to reclaim the lead.
McIlroy's solitary birdie of the opening nine came on the second hole, sinking a 10-foot putt on the short par four and all the while Westwood, 38, was gaining ground.
The world number three had sunk successive birdies on the final three holes of Friday's round to start the day three behind at 10 under and he reached 13 under before a bogey on the sixth hole stalled his charge.
(Editing by Alison Wildey)