The University of Â鶹´«Ã½ at Mānoa has identified a large-scale tropical disturbance called the Madden¨CJulian Oscillation (MJO) as a significant driver of the islands’ climate, including extreme events, such as the extraordinary rainfall Â鶹´«Ã½ experienced in March and April. This weather pattern travels eastward through the tropics every 30–60 days and, , significantly boosts rainfall during its active phases, particularly on windward slopes.
This research advances scientific knowledge of the processes that influence ±á²¹·É²¹¾±ʻ¾±¡¯²õ climate and can help improve forecasts one to three months in advance.
“Understanding how the MJO affects ±á²¹·É²¹¾±ʻ¾±¡¯²õ climate helps explain rainfall variability on timescales of weeks to months,” said Audrey Nash, lead author of the study and doctoral candidate in the in UH ²Ñā²Ô´Ç²¹¡¯²õ . “The MJO evolves slowly and can be monitored in real time. Understanding its influence can help scientists and forecasters better anticipate periods of heavy rainfall, drought conditions, and shifts in weather patterns across the islands.”
High-resolution data reveals the pattern
While the MJO was known to influence weather patterns across the tropics, its impact on Â鶹´«Ã½ had not previously been examined in detail at timescales of one to three months.
Nash and Giuseppe Torri, associate professor of atmospheric sciences, analyzed long-term, high-resolution atmospheric and rainfall datasets covering Â鶹´«Ã½ and the surrounding Pacific Ocean, including data from the Â鶹´«Ã½ Climate Data Portal. By compositing rainfall, temperature and atmospheric variables across different phases of the MJO, they identified consistent patterns showing how the MJO modulates rainfall and climate conditions across the Hawaiian Islands.
“We expected a small impact, but it was surprising how consistently rainfall across the islands responds to active and suppressed phases of the MJO,” said Nash.
Active phases of the MJO are also associated with cooler temperatures, higher humidity and stronger northeasterly winds across the islands. The authors note that these patterns appear to be linked to large-scale atmospheric responses to the MJO, including slow moving Rossby waves in the central North Pacific and strengthening of the local Hadley Circulation, a major feature of global atmospheric movement that cools the tropics and warms the poles.
“Improving our understanding of rainfall variability is critical for water management, agriculture, and hazard preparedness,” said Nash. “This work reflects the University of ±á²¹·É²¹¾±ʻ¾±¡¯²õ mission to study the unique environmental systems that shape life in the islands and to provide science that benefits local communities.”