Intraseasonal Variability in Diurnal Rainfall over New Guinea and the Surrounding Oceans during Austral Summer

Abstract

Abstract High-resolution Tropical Rainfall Measuring Mission (TRMM) rainfall data for six wet seasons (December–March) were used to investigate the time and space structure of the diurnal cycle of rainfall over and around New Guinea, a major island of the Maritime Continent. The diurnal cycle shows a systematic modulation associated with intraseasonal variability in the large-scale circulation pattern, with regimes associated with low-level easterlies or westerlies over the island. Lower-tropospheric easterly (westerly) wind components correspond to periods of inactive (active) convection over the islands that are associated with the passage of intraseasonal atmospheric disturbances such as the Madden–Julian oscillation (MJO). A striking feature is the diurnal rainfall that develops over the central mountain ranges in the evening and propagates toward the southwest (northeast) of the island with an inferred phase speed of about 2–3 m s−1 under low-level easterly (westerly) flow. In the case of the easterly regime, diurnal rainfall is strongly concentrated over the southwestern part of the island, inhibited from spreading offshore southwest of New Guinea. Under the westerly regime, in contrast, the rainfall area spread far and wide along the low-level westerlies from the island toward the Pacific Ocean. Significant offshore rainfall propagation extending from the island appears during the night over the north-northeastern coast and moves with a phase speed of about 7–8 m s−1, reaching the open ocean the following day. Possible processes for controlling the variability in diurnal rainfall through the interaction between large-scale circulation and previously denoted complex local circulation over the island are discussed.