Past Research I

My thesis work combined observational and numerical approaches to examine the Northern Hemisphere (NH) tropical-extratropical interaction during boreal summer as most previous work had focused on wintertime. I found a new type of teleconnection pattern – the so-called Circumglobal Teleconnection<1>. This pattern dominates the NH circulation variability over a broad range of timescales in the last 50 years, from intraseasonal<2>,<3> to interdecadal<4> and it exerts a significant influence on NH rainfall and surface temperature variability. The diabatic heating of the Indian summer monsoon, the impact of ENSO, and the energy propagation and barotropic instability of the basic-state flow are all important in shaping and maintaining this teleconnection pattern<5>. Thus, the Circumglobal Teleconnection acts as the leading pattern to link tropical forcing and extratropical climate variability in the summertime. This pattern has become a hot topic, with subsequent investigations focusing on the meteorological aspects, climatic aspects and practical forecast applications of NH circulation variability.

Past Research II

As a postdoctoral researcher at University of Washington, I have focused my attention on the long-term trend of Southern Hemisphere (SH) circulation and the recent significant changes of the Antarctic cryosphere. Published results regarding recent changes of SH climate have mostly focused on the trend in austral summer, which has been widely believed to be anthropogenic in origin. Climate variability in the SH and the Antarctica in other seasons has received much less attention. To make progress, my coauthors and I used observational data and a climate model to understand the cause of the warming trend in West Antarctica<6> and the Antarctic Peninsula<7> in recent decades during the non-summer seasons, as well as the accelerated melting rate of Pine Island Glacier since 1980s<8>,<9>. We found that the low frequency SST variability in the central tropical Pacific play a key role in warming the Pacific sector of Antarctica through generating a Rossby wave train.

A further look at the Southern Annular Mode (SAM), the leading mode of SH circulation variability, indicates an important impact of the tropical forcing on the SAM on both interannual and long term variability<10>. Thus, we suggest that the SAM should be redefined as a combination of an extratropical circulation response to the external tropical forcing and an intrinsic mode of the high-latitude basic state. We also suggest that besides the anthropogenic forcing, the tropical SST variability also exerts a significant impact on the recent trend of SH circulation.

The second project has been to understand the multi-decadal and centennial variability of the stable isotope ratios of water (?D and ?18O) recorded in the Antarctic ice-core data<11>,<12>. I used an isotope-enabled global climate model to reproduce ?D and ?18O in Antarctica. The experiment with observed tropical SSTs as the boundary condition can accurately simulate the observed ?18O over West Antarctica in the last 100 years, supporting our early work that the tropical SST variability is essential to drive low-frequency climate variability in West Antarctica.

Using the same model, my coauthors and I have performed and analyzed experiments to illuminate the impact of precessional forcing and glacial forcing on the climate and the isotopic composition of precipitation. We have also initiated a series of experiments to illuminate the impact of the evolution of the continental geometry and orography of Southern Asia on the regional and global climate and the isotopic composition of precipitation. These experiments feature realistic and idealized scenarios covering the past 50 million years, and are designed to isolate the impacts of evolving orography and continental geometry. We are collaborating with several geology groups around the country and this is an exciting extension of my research approach to more general problems in Earth System Science.

My current research also includes an ongoing project to understand the significant warming and sea-ice reduction in the Arctic in recent decades, especially after 1990s. We mainly examine this problem with a special emphasis on the connection between the circulation change in the Arctic and the remote SST forcing. We have made some progress and the result will be reported soon<13>.

Future Research

In my future research, I will use isotope-enabled coupled global climate models, instrumental data and paleoclimate proxy data to build knowledge and understand how the global atmospheric circulation and polar climates are interlinked and how they evolve together on interannual, interdecadal, centennial, and millennial time scales as well as in the future, especially due to changing natural forcing and increasing greenhouse gases and ozone change.