The Indonesian archipelago straddles the equator and lies in the area influenced by the position of the Intertropical Convergence Zone. As such it has a tropical monsoon climate with typified by cloudiness and precipitation, warm temperatures, and high humidifies, that is characterised by two monsoons. Maximum day time temperatures close to 30 degrees or above year-round, water temperatures in the mid to high 20’s, make Indo weather ideal for surfing for at least 6 months of the year. Transition periods and actual onset times for these monsoons vary from one end of the island groups to the other, but the mean months divided into the Wet Season (November – April) and Dry Season (May – October).
Northeast Monsoon (Wet Season) (Nov – April)
During these months the subtropical ridge is at its farthest south and the thermal high over Australia has been replaced by a thermal low. This combination pulls the monsoon trough (trade wind convergence zone) into its farthest south position of the year lying across Java by December and south of the islands in January. With most of the main surfing locations in the southern hemisphere, you can expect the wettest weather over the famed surfing breaks during this period. This wet weather is brought in by the monsoonal winds from the northwest and their convergence with southeast trade winds. The transition to the wet season begins in October in Sumatra and Java and in early November further east and it is on everywhere by the end of November. The number of rain days peaks with the passing of the trough and varies from across the archipelago. Java has the most rain days in November to January at 15+, and further east over Bali, Lombok and Sumba have the most rain days in January through April at 12 to 15. Mean high temperatures are between 29C and 31C. The mean lows are from 23C to 25C.
Southwest Monsoon (Dry Season) (May – Oct)
The subtropical ridge is at its farthest north position by mid-June, and drags the monsoon trough north of the area and allows southeast trade flow to cover most of the archipelago by May and continues until October. This creates clean conditions for surfing at many of the better-known surf breaks from Macaroni’s in the Mentawis to Uluwatu in Bali. This is also the time that more intense low-pressure systems start to form through the Indian and Southern Oceans. Large, long-period groundswell can travel 1000’s of kilometres once generated by these winter storms, reaching the exposed southwest coastlines of Indonesia with great power and size. With drier weather also occurring at this time of year, it is generally considered the golden time to go. The east islands are into the dry season as much as a couple of months earlier than Sumatra. The most rain in this season occurs in May and early June on Java and Sumatra with 6 days with rain. By July and August at most locations this drops to virtually 0. Mean high temperatures are between 29C and 31C. The mean lows are from 23C to 25C.
