Glacial Climates and Monsoon Dynamics in Proxies and Models

Monsoons are critical features of the global hydrological cycle, but our understanding of their past and future variability remains incomplete. In this talk, I use proxy indicators of past monsoonal climates and general circulation model simulations to explore the processes that regulate the long-term evolution of these circulations. In particular, I focus on the North American Monsoon (NAM), an iconic feature of the Southwest climate that is the dominant source of rainfall for northwest Mexico and Arizona. Despite its importance, the NAM remains one of the least understood monsoon systems. Novel measurements of the isotopic composition of leaf waxes indicate a regional decrease in monsoon rainfall during the Last Glacial Maximum (21 ka BP), and show that the deglacial trajectory of the NAM closely tracks North American ice cover. GCM simulations reproduce this link between monsoon strength and ice volume, largely as a result of ice-sheet induced changes in the subtropical jet that `ventilate' the monsoon by favoring the mixing of cold, dry air into the NAM region. This work coheres with a growing body of literature that highlights the role of mid-latitude circulations in altering the energetic environment for monsoon convection. It also shows that comparisons of the sensitivity of regional hydroclimates to large-scale forcings across proxies and models can provide unique insights into the dynamical drivers of climate change.