The temperature of the ocean surrounding Papua New Guinea has a strong influence on average monthly air temperatures. Changes in the temperature from season to season are small but more marked around Port Moresby than further to the north.
Papua New Guinea has a wet season from November to April and a dry season from May to October, but these seasons are only noticeably different in Port Moresby, where about 78% of the yearly average rainfall
comes in the wet season. Due to their location in the West Pacific Warm Pool, islands in the north of Papua New Guinea experience rain throughout the year. As a result, Kavieng’s average annual rainfall (3150 mm) is much higher than Port Moresby’s (1190 mm).
Most of the rainfall in Port Moresby comes from the West Pacific Monsoon. Large differences in temperature between the land and the ocean drive the monsoon, and its seasonal arrival usually brings a switch from very dry to very wet conditions. In the north of the country rainfall is more consistent year-round, although the peak in rainfall corresponds to the monsoon season.
Rainfall in the north of Papua New Guinea is also affected by the Intertropical Convergence Zone and, to a lesser extent, the South Pacific Convergence Zone. These bands of heavy rainfall are caused by air
rising over warm water where winds converge, resulting in thunderstorm activity. The South Pacific Convergence Zone extends across the South Pacific Ocean from the Solomon Islands to east of the Cook Islands,
whilst the Intertropical Convergence Zone lies across the Pacific just north of the equator.
Papua New Guinea’s climate varies considerably from year to year due to the El Niño-Southern Oscillation. This is a natural climate pattern that occurs across the tropical Pacific Ocean and affects weather around the world. There are two extreme phases of the El Niño-Southern Oscillation: El Niño and La Niña. There is also a neutral phase. Generally in Papua New Guinea El Niño years are usually drier than normal while La Niña events are usually wetter. La Niña-associated prolonged rainfall has led to flooding and landslides, whilst El Niño-associated droughts have also taken their toll on Papua New Guinea. During El Niño events the monsoon season also starts later. The dry season at Port Moresby is cooler than normal in El Niño years and warmer than normal in La Niña years, while the wet season tends to be warmer and drier than normal during an El Niño event.