Paleoclimate evidence shows that rainfall over heavily-populated regions of East Asia is susceptible to change. Recent cave speleothem records from China going back a quarter of a million years convincingly show that the climate over East Asia underwent large and abrupt swings during this time period. These oscillations occur at multiple timescales, from a few hundred to tens of thousands of years, and they are tied to distinct and varied forcings on the global climate system during Earth's climate history. The standard interpretation of the speleothem records is that the East Asian summer monsoon intensified or diminished during these oscillations. However, this description is qualitative, and does not factor in the complex behavior of today's East Asian climate which exhibiting both abrupt temporal and spatial shifts as it progresses through the season, in particular from Spring to Meiyu rains, and from Meiyu to Summer rains.
This research explores the hypothesis that changes to the north-south position jet stream impinging on the Tibetan Plateau are the underlying cause of the abrupt climate changes in the past. Emerging dynamical studies of the modern East Asian climate suggest that it is the latitudinal position of the jet relative to the Plateau that determines the rainfall seasonality -- when the jet is South of the Plateau, it is in a Spring rainfall regime; when it goes just North, then Meiyu; and when the jet goes well North of the Plateau, Summer. The hypothesis provides specific predictions that can be tested for different past climate scenarios, in particular during the so-called Dansgaard-Oeschger events recorded in ice core records over Greenland; and changes to insolation from changes to the Earth's orbital configuration.
The jet transition hypothesis will be tested using three complementary approaches: i) Examination of the association between jet streams and East Asian rainfall changes in state-of-the art simulations of two particular past climate scenarios (mid-Holocene and Last Glacial Maximum) by various modeling groups around the world; ii) Idealized climate model simulations using a high-resolution atmospheric general circulation model necessary for resolving the fine-scale features of East Asian climate; iii) Investigating the modern climate data and high-resolution simulations of 20th century climate to see if similar jet-rainfall relationships hold for climate changes over this historical period.
This project involves international collaborations with scientists from Taiwan and China, and supports a graduate student at UC Berkeley. Public lectures on paleoclimate changes (with focus on East Asia) will be offered at the Lawrence Hall of Science, using the "Science on a Sphere" (SOS) setup for climate data visualization.
