Northern Sumatra Surf Seasons & Weather
Nias Island is just below the equator and the northern tip of Sumatra is about 5° north of the equator, so these islands are generally wetter and more lush than the islands east of Bali, with a weather pattern similar to the Mentawai.
The Indonesian archipelago covers three time zones, stretched along the equator between the hot, dry Australian landmass to the south, and the hot, wet Asian landmass to the north. With the equator running through it, Indonesia’s climate is tropical and there is little difference in the length of a day year around, but because of those land masses lurking above and below, Indonesia’s seasons divide in two: wet and dry.
The wet season begins in October and lasts until March, with peak rainfall in January and February when high pressure flowing south from the Asian mainland combines with humid air from the Indian Ocean to bring rain throughout the archipelago – although the northern and western islands get more rain than those in the south and east.
The wet season tapers off in March when high pressure over Australia pushes air north. The dry season runs April through September and peaks in July and August, which is also in the middle of the prime surf season.
Prime season for swell in Indonesia is the southern hemisphere winter, May through September, when low-pressure systems in the Southern Ocean and Indian Ocean pulse ground swells toward the equator. These ground swells travel for thousands of miles, allowing them time to “unwrap”, organize and march onto the reefs of Indonesia perfectly groomed.
Winds moving off continents are a factor, but so are local winds. From March to May the surf is smaller, but there is less wind and days are frequently sunny and glassy. From June to August there is more swell but the wind is 80% southeast tradewinds which are favorable at most spots, but not all. In September and October there is less southeast wind and less swell again, which can be very good for the rights.
Prime surf season is also prime weather season, which is why you have seen so many surf movies showing surfers going berserk in perfect surf, under blue sunny skies, with offshore winds adding that little bit extra.
Although most of the rain falls during the wet season, rainstorms can occur at any time of year. Air temperatures range between 75° and 85° year around and water temperatures are always in the high 70s to low 80s.
When you see people wearing wetsuits in Indonesia, it’s for reef and sun protection, not warmth.