PNNL

The Science

The strong winds and cloud cover of the Madden-Julian Oscillation (MJO) induce significant sea surface cooling over the Maritime Continent region of Southeast Asia, which includes Indonesia, the Philippines, and Papua New Guinea. The induced cooling may persist beyond 30 days from the MJO crossing. Using a combination of observations and numerical model simulations, this study demonstrated that the sea surface cooling caused by prior MJOs can significantly influence tropical cyclone (TC) intensification over the Maritime Continent. After the MJO leaves the Maritime Continent, TCs forming over that region experience cooler sea surface temperatures (SSTs). Consequently, the enthalpy fluxes, or flows of heat energy, at the air-sea interface under storms are reduced and their intensification rates decrease substantially.

The Impact

The mechanism identified in this study affects the most TC-prone region of the Southern Hemisphere and the region of Australia where most intense TC landfalls have occurred. Previously, the MJO was known to influence TCs worldwide through the atmosphere. However, this study showed the MJO can also influence TCs through the ocean, suggesting that improvements in TC prediction could be gained through more realistic representations of air-sea coupled processes in models. Finally, the study points to possible impacts on TCs due to long-term changes in MJO characteristics and upper-ocean stratification over the Maritime Continent.

Summary

Using observations and numerical model simulations, a research team systematically demonstrated that MJO events substantially affect TCs over the Maritime Continent region through an oceanic pathway. Using satellite SST products, they first demonstrated that MJOs cause significant sea surface cooling, 0.35 ± 0.12 °C on average, which persists for over a month. A mixed layer heat budget analysis was then performed using an eddy-permitting ocean reanalysis to understand the processes responsible for SST cooling caused by the MJO. Both changes in surface fluxes along with enhanced mixing beneath the mixed layer contribute towards the SST cooling observed during MJOs. Researchers performed an along-track Lagrangian composite analysis for TCs to the northwest of Australia, where MJO-induced sea surface cooling is strongest. Along TC tracks, they computed various atmospheric parameters that play a key role in TC intensification, pre-storm SSTs, enthalpy fluxes at the air-sea interface, and TC intensification rates. On average, the TCs that encountered the MJO-induced SST cooling had significantly reduced enthalpy fluxes and 50% lower intensification rates. To further support these observation-based results, researchers conducted two sets of simulations of Cyclone Olga (March 2000). One was a control set in which Olga experienced the SST cooling induced by a prior MJO, and the second removed SST cooling. Olga remained a tropical storm in the control set and intensified to a Category 1 strength after SST cooling removal, which was in broad agreement with the observational results. These results suggest that subseasonal TC predictions could be improved with better MJO forecasting and enhanced representation of air-sea coupled processes in models.

PNNL Contact

Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Funding

This work was sponsored by the Office of Biological & Environmental Research’s Regional & Global Model Analysis Program area.