The response of stable oxygen isotopes in South American Summer Monsoon rainfall to variability of the local Hadley circulation and projections of subsidence-zone expansion

Abstract

Anthropogenic emissions are driving a weakening and expansion of the tropical circulation in the zonal mean state, though this zonal averaging masks regional variability. Ultimately, it is critically important to understand the regional perspective in order to assess the local hydroclimate impacts of such changes. In South American, the regional overturning circulation comprises the principle dynamic structure of the summertime monsoon circulation. The influences of circulation changes on the monsoon hydrologic variability are best traced by oxygen isotopes in precipitation and water vapor, which integrate phase changes from evaporation in the tropical Atlantic to rainout across the monsoon domain, into the tropical Andes, and as far south as southeastern Brazil and northern Argentina. Yet to what extent the changes in circulation are reflected by δ¹⁸O_p and δ¹⁸O_v changes has not yet been tested. Novel isotope-enabled simulations extending to the end of the 21st century illustrate the enrichment of heavy isotopes in precipitation and tropical drying that accompanies weakening of the regional overturning circulation. This work confirms that projected future trends of Hadley circulation weakening and expansion also apply on a regional scale over South America. Furthermore, this study demonstrates the capacity of utilizing oxygen isotope ratios as a proxy for atmospheric circulation in the past to reconstruct historic regional Hadley circulation variability.

Key interpretations

Haldey circulation change in South America during the austral summer, corresponding to the South American Summer Monsoon mature season, is measured by a poleward expansion of the subsidence zone in an isotope-enabled global climate model ensemble simulation (CESM1.2 model coupled to CAM6).

Latitude – pressure level cross-sections of the DJF mass flux with divergent winds (v_φ, ω_φ) across South America (averaged over 80--35°W). a) Beginning of the 21st century climatology (2010 – 2038 CE), b) End of the 21st century climatology (2070 – 2098 CE). c) End -- Beginning of 21st century difference. In the difference plot, zero contours are highlighted in black, stippling indicates statistically significant differences for at least 3 out of 4 ensemble members (p < 0.05), and only significant vectors are plotted (p < 0.05). Scale for panel a) and b) is on the left while scale for panel c) is on the right.

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This work is currently being revised for journal submission.