Precipitation changes in the western tropical Pacific over the past millennium

An intense band of thunderstorms circles the globe near the equator where the northeast and southeast trade winds come together, known as the Intertropical Convergence Zone (or ITCZ) (Figure 1). The most prominent rainfall feature on the planet, the ITCZ moves northward in the Northern Hemisphere summer, and southward during the Southern Hemisphere summer. As a result, many locations in the tropics to experience distinct rainy seasons and dry seasons. In addition to migrating north and south on a seasonal basis, it is hypothesized that the ITCZ shifts its mean position on longer timescales (e.g., decades to centuries) due to changes in the amount of solar radiation reaching the Earth’s surface and other external forcing factors. Such shifts would cause a redistribution of rainfall across the tropics and have profound socioeconomic and ecological impacts in regions dependent on predictable rainfall patterns.

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Researchers look to the paleoclimate record to see whether the ITCZ shifted significantly during warm and cool intervals in the past. Understanding the spatial extent and magnitude of rainfall changes during past climate events that occurred prior to our observations from rain gauges or satellites is important to improve the accuracy of predictions about the reorganization of rainfall patterns under different climate scenarios.

The Little Ice Age (LIA) was a cool period lasting from 1400 to 1850 AD when glacier advances occurred in the Northern Hemisphere and significant changes in rainfall and water availability occurred throughout the globe. Changes in the distribution of heat and precipitation in the tropical Pacific region can have far-reaching consequences, including impacts on droughts and weather patterns in North America. Understanding how the tropical Pacific responded to a climate event such as the LIA will help researchers better understand the drivers of some of the observed patterns in North America. Scientists from the USGS and University of Washington set out to investigate the shifts in the tropical Pacific rainfall patterns during the LIA using sediment records of the last 1,000 years collected from lakes in Palau. Palau is an island nation situated in the western tropical Pacific Ocean, at the northern extent of the modern ITCZ. The scientists used geochemical indicators to generate a proxy record of relative rainfall variability in Palau during the past millennium.

The rainfall reconstruction is based on the hydrogen isotopic composition of a molecule called dinosterol in lake sediments. Dinosterol is produced by phytoplankton (algae) living on the surface of brackish lakes, and its hydrogen isotopic composition effectively tracks mean annual moisture balance (evaporation minus precipitation). The study found that there was a shift toward drier conditions in Palau at the beginning of the LIA (around 1400 AD). Conditions in Palau became progressively drier until the mid-19^th century, when a rapid return to wetter conditions coincided with rapid warming of the northern Hemisphere after the LIA. This Palau record, combined with other rainfall reconstructions from elsewhere in the tropical Pacific, suggests that Northern Hemisphere cooling during the Little Ice Age resulted in a southward shift of the ITCZ by as much 5º latitude.

Combined with studies from other sites in the tropics, this research is helping researchers map the magnitude of ITCZ fluctuations during different climate regimes in the past. This information aims to improve the ability of models to accurately predict the magnitude and spatial extent of fluctuations in ITCZ fluctuations in future climate scenarios.

The paper, "Precipitation changes in the western tropical Pacific over the past millennium," was published in Geology.

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