Water vapour source impacts on oxygen isotope variability in tropical precipitation during Heinrich events

Abstract

Abstract.Water isotope records such as speleothems provide extensive evidence of past tropical hydrological changes. During Heinrich events, isotopic changes in monsoon regions have been interpreted as implying a widespread drying through the Northern Hemisphere tropics and an antiphased precipitation response in the south. Here, we examine the sources of this variability using a water isotope-enabled general circulation model, Goddard Institute for Space Studies ModelE. We incorporate a new suite of vapour source distribution tracers to help constrain the impact of precipitation source region changes on the isotopic composition of precipitation and to identify nonlocal amount effects. We simulate a collapse of the North Atlantic meridional overturning circulation with a large freshwater input to the region as an idealised analogue to iceberg discharge during Heinrich events. An increase in monsoon intensity, defined by vertical wind shear, is modelled over the South American domain, with small decreases simulated over Asia. Simulated isotopic anomalies agree well with proxy climate records, with lighter isotopic values simulated over South America and enriched values across East Asia. For this particular abrupt climate event, we identify which climatic change is most likely linked to water isotope change -changes in local precipitation amount, monsoon intensity, water vapour source distributions or precipitation seasonality. We categorise individual sites according to the climate variability that water isotope changes are most closely associated with, and find that the dominant isotopic controls are not consistent across the tropics -simple local explanations, in particular, fall short of explaining water isotope variability at all sites. Instead, the best interpretations appear to be site specific and often regional in scale.

Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania)

“…We therefore suggest that an ENSO-like impact over southern Tanzania and hence the major portion of the Rufiji catchment area was not the main mechanism for the H1 drought. Results from climate model studies suggest a north-south antiphase relation in African annual precipitation in response to North Atlantic cooling, consistent with latitudinal migrations of the ITCZ's annual mean position (e.g., Lewis et al, 2010;Kageyama et al, 2013). In line with this hypothesis, the arid phase recorded in our data during H1 has (within age model uncertainties) a pronounced wet counterpart in the Zambezi region (Schefuß et al, 2011;Otto-Bliesner et al, 2014).…”

The influence of volcanic eruptions on the climate of tropical South America during the last millennium in an isotope-enabled general circulation model

“…At each phase change (including precipitation, evaporation, ice formation or melting) an appropriate fractionation factor is applied and all freshwater fluxes are tagged isotopically. Stable isotope results from the lineage of GISS models have a long history of being tested against observations and proxy records (e.g., Schmidt et al, 2007;Schmidt, 2008, 2009;Lewis et al, 2010Lewis et al, , 2013Lewis et al, , 2014Field et al, 2014).…”

Glacial–interglacial changes in H<sub>2</sub><sup>18</sup>O, HDO and deuterium excess – results from the fully coupled ECHAM5/MPI-OM Earth system model

“…LeGrande and Schmidt (2009) expanded these analyses by performing eight Holocene time slice simulations, each ∼ 1000 years apart. Lewis et al (2010) used the same GISS-E model for simulating the consequences of a large freshwater input into the North Atlantic as an idealized analogue to iceberg discharge during Heinrich events. As a second fully coupled GCM, the HadCM3 model has been enhanced by a stable water isotope diagnostics module by Tindall et al (2009) for analyses of the present-day isotopic signature of El Niño-Southern Oscillation and the tropical amount effect.…”

“…speleothems, some studies have suggested that the amount of precipitation could be mainly responsible for determining the water isotope concentration (Fleitmann et al, 2003;Wang et al, 2001) -this is called the amount effect (Dansgaard, 1964;Rozanski et al, 1992). Furthermore, in these regions δ 18 O and δD might also reflect convective activity along a moisture trajectory (Vimeux et al, 2005;Yao et al, 2012), changes to regional moisture sources, and the intensity or provenance of atmospheric transport pathways (LeGrande and Schmidt, 2009;Dayem et al, 2010;Lewis et al, 2010;Maher and Thompson, 2012;Caley et al, 2014a;Tan, 2014). High-resolution and well-dated records of δ 18 O of calcite in tropical speleothems in Asia or South America have therefore been interpreted in terms of past monsoon dynamics (Cruz et al, 2005;Wang et al, 2008).…”