As early as 1964, Dansgaard carried out the pioneering work on the precipitation “amount effect” whereby a negative correlation between stable isotope ratios (δ18O) in precipitation and corresponding precipitation amount was documented for tropical regions (Dansgaard, 1964). This finding provided the fundamental basis for paleohydroclimate reconstructions using δ18O records preserved in ice cores, speleothems and tree rings obtained from the low latitudes.
However, Dr. Pradeep Aggarwal (the International Atomic Energy Agency) proposed a new theory in Nature Geoscience in 2016, which suggests that precipitationδ18Oin the tropical regions can reflect the variability of stratiform fraction/rain type proportions (hereafter called “stratiform fraction” theory). This is because that precipitation δ18O in the tropical regions showed a significant negative correlation with the stratiform fraction (the proportion of stratiform rain to total precipitation), independent of the “amount effect” (Aggarwal et al., 2016). This new theory has been widely accepted and cited. However, whether the new theory faithfully reflects the true signals of precipitation δ18O in the tropics remains unclear.
Recently, Prof. Wusheng Yu, PhD. Rong Guo (the Institute of Tibetan Plateau Research, Chinese Academy of Sciences) and coauthors revisited the “stratiform fraction” theory. They found that the precipitation δ18O in the tropical regions can reflect the convective intensity but cannot reflect the stratification fraction or rain type proportions. Their study was published in Science Advances on August 14, 2024.
Prof. Yu introduced that, based on data from more observation stations in the pantropics (35°N-35°S), they revealed the relationships between precipitation δ18O and stratiform fraction and convective intensity on different (daily, monthly, and annual) time scales. They found that, contrary to the “stratiform fraction” theory, the negative correlation between δ18O and stratiform fraction is very weak, while the positive correlation with convective intensity remains significant on every time scale.
The key theoretical basis of the “stratiform fraction” theory is that the δ18O values of stratiform precipitation are lower, while the δ18O values of convective precipitation are higher. Aggarwal et al. (2016) tried to prove this based on cloud physical processes. However, Yu et al. (2024) provided a lot of evidences that refute the views of Aggarwal et al. (2016). First, many observed results in the pantropics show that both low and high δ18O values can be found in stratiform precipitation. Second, in the pantropics, the convective system is very strong, and more convective clouds appear than stratiform clouds, which cause that more convective precipitation occurs in the pantropics than stratiform precipitation. More importantly, stratiform precipitation is inseparable from deep convection as the convection-generated cumulonimbus clouds are not only the most important cloud in the pantropics, but also act as a genitus mother cloud to the precipitating stratiform cloud. Therefore, changes in the strength of convective activity have a strong “imprinting effect” on the δ18O of stratiform precipitation. In addition, the δ18O values in stratiform precipitation are also closely correlated with the precipitation formation processes in the stratiform region. These processes include the Wegener-Bergeron-Findeisen (WBF) condensation process that occurs above the melt layer, and the re-evaporation process that occurs below the melt layer. These processes can further enrich isotope ratios in stratiform precipitation (the “re-enrichment effect”). Finally, they further emphasize that even though precipitation isotope ratios are sensitive to proportions of convective and stratiform precipitation in some regions, the rationale behind the phenomenon is still driven by changes in convection intensity.
The “stratiform fraction” theory has many serious weaknesses. Yu et al. (2024) findings decode the true signal of stable isotopes of precipitation in the pantropics. They also suggested that new theories need to be validated at more locations on different time scales before gaining widespread acceptance.
References:
Dansgaard W. Stable isotopes in precipitation. Tellus 16, 436–468 (1964).
Aggarwal P. K., Romatschke U., Araguas-Araguas L., et al. Proportions of convective and stratiform precipitation revealed in water isotope ratios. Nature Geoscience 9, 624–629 (2016).
Yu W., Guo R., Thompson L.G., et al. Water isotope ratios reflect convection intensity rather than rain type proportions in the pantropics. Science Advances 10, DOI: 10.1126/sciadv.ado3258 (2024).