The Similan Islands, a Thai archipelago in the Andaman Sea located near the shelf break, are subjected to frequent (up to several events per hour) and abrupt changes in physicochemical conditions, particularly during the dry season (NE monsoon, January through April) and to an intense monsoon season with strong surface wave action (May to October). The exposed west slopes of the islands feature more coral species, but lack a carbonate reef framework. By contrast, the sheltered east sides show a complex reef framework dominated by massive Porites. Our results suggest that the sudden changes in temperature, pH and nutrients (drops of up to 10°C and 0.6 U and increases of up to 9.4 μmol NOx l−1, respectively) due to pulsed upwelling events may rival the importance of surface waves and storms in shaping coral distribution and reef development.