PNNL

The Science 

To understand the predictability of rainfall in the Indo-Pacific maritime continent region, the team analyzed precipitation and surface-temperature observations, MERRA-2 reanalysis, and several ensembles of simulations using version 3 of the Energy Exascale Earth System Model to isolate the role of remote land surface temperature. One experiment suppressed interannual land variability while retaining the same prescribed ocean conditions. A 50-member ensemble tested how initial land states affected Maritime Continent rainfall. Two additional 25-member ensembles differed only by an imposed ±1 K Congo Basin land-temperature perturbation, allowing the researchers to isolate the modeled atmospheric response to Congo surface temperature. These experiments established Congo Basin land surface temperature as a source of interannual predictability of Indo-Pacific maritime continent rainfall through its modulation of the zonal position of the rainfall.

The Impact 

Rainfall over the Maritime Continent supports water supplies, agriculture, energy systems, and disaster preparedness. The Madden–Julian Oscillation also affects weather far beyond the tropical Indo-Pacific. This study identified Congo Basin surface temperature as an additional influence on both rainfall and MJO activity over the Indo-Pacific Maritime continent region.

When the Congo was anomalously warm, rainfall decreased around the Maritime Continent and increased over the western Pacific. MJO activity also became less frequent over the Maritime Continent and more frequent or stronger farther east. These results suggested that forecasts based only on ocean conditions could omit a relevant source of variability. Monitoring and better initializing African land conditions could therefore help improve seasonal-to-interannual predictions of tropical rainfall and MJO behavior.

Summary 

The researchers found that land-surface variability contributed to the east–west contrast in Indo-Pacific precipitation. In the E3SMv3 control simulation, which allowed land conditions to evolve, Maritime Continent precipitation decreased while precipitation increased farther east. When interannual land variability and trends were suppressed, the Maritime Continent instead showed an increasing precipitation trend more similar to adjacent ocean regions.

Across the 50-member land-initial-condition ensemble, Congo Basin surface temperature showed the strongest relationship with subsequent Maritime Continent rainfall. Warmer Congo initial conditions were associated with reduced rainfall around the Maritime Continent and increased rainfall over the western Pacific. Satellite observations and reanalysis produced a similar spatial pattern.

The idealized ±1 K Congo experiments isolated the modeled mechanism. Warmer Congo conditions generated low- to mid-tropospheric heating and altered low-level winds. The resulting Kelvin-like atmospheric response produced convergence over the western Indian Ocean, divergence and reduced convection over the Maritime Continent, and compensating convergence and increased rainfall over the western Pacific. Consistent with this mean rainfall shift, warm Congo conditions produced statistically significant changes in MJO occurrence, with reduced activity over the Maritime Continent and increased activity in western Pacific phases.

The experiments prescribed ocean surface conditions and used a model resolution of approximately 100 kilometers. The study therefore did not represent two-way ocean feedbacks or fully resolve island-scale processes across the Maritime Continent.

Contact 

Renu Joseph, Earth and Environmental System Modeling (EESM) Program, Renu.Joseph@science.doe.gov 

L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov

Funding 

The U.S. Department of Energy Office of Science, Biological and Environmental Research supported this work through the Regional and Global Model Analysis program area and the Water Cycle: Modeling of Circulation, Convection, and Earth System Mechanisms Scientific Focus Area. The research used computing resources at the National Energy Research Scientific Computing Center, a DOE Office of Science user facility.

This text was initially generated using artificial intelligence, and subsequently refined and validated for accuracy, tone, and context by experts at Pacific Northwest National Laboratory.