Bronwen L. Konecky

Coherent changes of southeastern equatorial and northern African rainfall during the last deglaciation

During the last deglaciation, wetter conditions developed abruptly ~14,700 years ago in southeastern equatorial and northern Africa and continued into the Holocene. Explaining the abrupt onset and hemispheric coherence of this early African Humid Period is challenging due to opposing seasonal insolation patterns. In this work, we use a transient simulation with a climate model that provides a mechanistic understanding of deglacial tropical African precipitation changes. Our results show that meltwater-induced reduction in the Atlantic meridional overturning circulation (AMOC) during the early deglaciation suppressed precipitation in both regions. Once the AMOC reestablished, wetter conditions developed north of the equator in response to high summer insolation and increasing greenhouse gas (GHG) concentrations, whereas wetter conditions south of the equator were a response primarily to the GHG increase. Increasing greenhouse gas concentrations caused rainfall changes in equatorial Africa at the onset of the African Humid Period. Greenhouse gases drove African rainfall Much of equatorial Africa suddenly became much wetter ∼14,700 years ago, ushering in an “African Humid Period” that continued well into the Holocene. Why? Otto-Bliesner et al. use a climate model to show that a reduction in the Atlantic Meridional Overturning Circulation (AMOC) at the beginning of the last deglaciation caused a reduction in precipitation in northern and southeastern equatorial Africa. When the AMOC became stronger again, wetter conditions developed in response to a combination of increasing greenhouse gas concentrations and strong summer sun. As atmospheric greenhouse gas concentrations continue to increase, these results may have implications for the future of African hydroclimate, water resources, and agriculture. Science, this issue p. 1223

Glacial forcing of central Indonesian hydroclimate since 60,000 y B.P.

Significance Climate variability in the tropical western Pacific exerts enormous influence on global climate, yet its history remains poorly constrained. We present the region’s first continuous terrestrial sedimentary record of surface hydrology and vegetation spanning the last 60,000 y based upon geochemical data from Lake Towuti, Indonesia. Our data demonstrate that wet conditions and rainforest ecosystems present during the Holocene and during marine isotope stage 3 were interrupted by severe drying between ∼33,000 and 16,000 y B.P., when high-latitude ice sheets expanded and global temperatures cooled. These findings indicate an important role for glacial boundary conditions in pacing tropical western Pacific climate change, and highlight the potential for the western Pacific to amplify global climate change during glacial–interglacial cycles. The Indo-Pacific warm pool houses the largest zone of deep atmospheric convection on Earth and plays a critical role in global climate variations. Despite the region’s importance, changes in Indo-Pacific hydroclimate on orbital timescales remain poorly constrained. Here we present high-resolution geochemical records of surface runoff and vegetation from sediment cores from Lake Towuti, on the island of Sulawesi in central Indonesia, that continuously span the past 60,000 y. We show that wet conditions and rainforest ecosystems on Sulawesi present during marine isotope stage 3 (MIS3) and the Holocene were interrupted by severe drying between ∼33,000 and 16,000 y B.P. when Northern Hemisphere ice sheets expanded and global temperatures cooled. Our record reveals little direct influence of precessional orbital forcing on regional climate, and the similarity between MIS3 and Holocene climates observed in Lake Towuti suggests that exposure of the Sunda Shelf has a weaker influence on regional hydroclimate and terrestrial ecosystems than suggested previously. We infer that hydrological variability in this part of Indonesia varies strongly in response to high-latitude climate forcing, likely through reorganizations of the monsoons and the position of the intertropical convergence zone. These findings suggest an important role for the tropical western Pacific in amplifying glacial–interglacial climate variability.

Paired stable isotopologues in precipitation and vapor: A case study of the amount effect within western tropical Pacific storms

Understanding controls on the stable isotopic composition of precipitation and vapor in the West Pacific Warm Pool is vital for accurate representation of convective processes in models and correct interpretation of isotope-based paleoclimate proxies, yet a lack of direct observational evidence precludes the utility of these isotopic tracers. Results from a measurement campaign at Manus Island, Papua New Guinea from 28 April to 8 May 2013 demonstrate variability in the stable isotopic composition (δD and δ18O) of precipitation and vapor in individual precipitation events and over a 10 day period. Isotope ratios in water vapor and precipitation progressively increased throughout the period of measurement, coincident with a transition from high to low regional convective activity. Vapor isotope ratios approached equilibrium with seawater during the quiescent period and likely reflected downwind advection of distilled vapor and re-evaporation of rainfall during the period of regional convection. On a 5 min timescale across individual storms, isotope ratios in precipitation were strongly correlated with isotope ratios in surface vapor. However, individual precipitation isotope ratios were not strongly correlated with surface meteorological data, including precipitation rate, in all storms. Yet across all events, precipitation deuterium excess was negatively correlated with surface temperature, sea level pressure, and cloud base height and positively correlated with precipitation rate and relative humidity. Paired surface precipitation and vapor isotope ratios indicate condensation at boundary layer temperatures. The ratio of these paired values decreased with increasing precipitation rate during some precipitation events, suggesting rain re-evaporation and precipitation in equilibrium with an isotopically distinct upper level moisture source. Results from the short campaign support the interpretation that isotope ratios in precipitation and vapor in the western tropical Pacific are indicators of regional convective intensity at the timescale of days to weeks. However, a nonstationary relationship between rain rate and stable isotope ratios in precipitation during individual convective events suggests that condensation, rain evaporation, moisture recycling, and regional moisture convergence do not always yield an amount effect relationship on intraevent timescales.