Congratulations to Ao juan Passing her Ph.D. Dissertations smoothly
Ao juan successfully passed through the doctoral dissertation defense on May 7th 2015 at the 101 conference room of the IAP scientific research building. Her doctoral thesis title is the main modes of winter precipitation over East Asia and its influencing factors and predictors.
Anomalous winter precipitation, occurring with intense snowfall and severe droughts, has huge
impacts on natural ecology and social economy. Especially, anomalous winter precipitation evens have increased in recent years, which highlight the importance of studying variability of winter precipitation
and its influencing factors and predictors. The dissertation conducts a series of researches focusing on
above scientific issues. First, the decadal variability of winter precipitation over Eastern China and
corresponding influencing factors are studiedin this article. Further, scopes of winter precipitation are
extended from Eastern China to East Asia in the research, and the relationship of boreal winter precipitation over East Asia and boreal autumn Sea Surface Temperature (SST) triple pattern over the South Pacific Ocean (SPO), boreal autumn SST dipole patternover the Indian Ocean (IO), and boreal November snow
cover a nomalies over Eastern Europe are investigated. Above analyses imply that reveal of these external forcing factors have contributions to prediction of boreal winter precipitation over East Asia. The main conclusions are summarized as follows:
1.The decadal variability of main mode of winter precipitation over Eastern China and corresponding
The primary mode of winter precipitation variability over Eastern China presents a homogeneous
change for recent fifty years, and it has a significant decadal change around the late 1980s. The factors
influencing winter precipitation variability over Eastern China change over different interdecadal periods.
Before the late 1980s, the Eurasian (EU) mode and North Pacific Oscillation (NPO) mode were the two
major atmospheric factors. After the late 1980s, the influence of the EU mode remained. However, the
impact of the NPO significantly weakened, and a new Rossby Wave (RW) pattern became a key factor.
Further analyses of both observations and numerical simulations indicate that the convective activity over
the western tropical Pacific strengthened significantly around the late 1980s; the intensive convection
encouraged the RW mode and ultimately contributed to the anomalous winter precipitation over Eastern
China after the late 1980s. The result implies that the prediction of winter precipitation should consider
different interdecadal backgrounds; otherwise, the changing factors could result in failure of the prediction over some decadal periods.
2.The impact of boreal autumn triple pattern of SSTAs over the SPO on winter precipitation over East
The second pattern of boreal winter precipitation over East Asia shows a dipole distribution, and it
closely relates to the variability of triple pattern of SSTAs over the SPO in boreal autumn. The physical
linkage between boreal autumn SPO SSTAs and East Asian winter precipitation of dipole pattern is mainly
shown to be the seasonal persistence of the SPO SSTAs themselves. The persistence of SPO’s SSTAs beyond boreal autumn means that the signal of the boreal autumn SSTAs is memorized and transported to
boreal winter; this stimulates a meridional teleconnection pattern from the Southern Hemisphere to the Northern Hemisphere, resulting in a meridional dipole pattern of atmospheric circulation over East Asia in
boreal winter. As a major influencing factor, this dipole pattern of atmospheric circulation can finally lead to dipole pattern of anomalous precipitation over East Asia in boreal winter. These observed physical
processes are further confirmed through numerical simulations. In this study, the revealing of triple pattern
of SSTAs over SPO in boreal autumn and its influence on winter precipitation dipole pattern over East Asia, not only deepens our understanding of the variability in East Asian winter precipitation, but also provides a useful predictor for simulating future precipitation in the region.
3.The impact of boreal autumn dipole pattern of SSTAs over the IO on winter precipitation over East
Asia and Southeast Asia
The second mode of boreal autumn SSTAs over the tropical Indian Ocean presents a dipole distribution
with a reverse change in east and west regions, and it can influence boreal winter precipitation over East
Asia and Southeast Asia. The physical mechanism is that SSTAs of Indian Ocean Basin mode (IOB) in
boreal winter can memory most signals of Indian Ocean dipole mode (IOD) of SSTAs in boreal autumn;
and anomalous high SSTAs over IO in boreal winter can enhance the zonal circulation in the tropical
ocean and motivate a teleconnection wave train from tropical region to midhigh latitude region in the
Northern Hemisphere, which causes horizontal winds anomalies of “anticyclone cyclone anticyclone” and vertical motion anomalies of “descend–ascend descend” over the region from Southeast Asia to Northern part of East Asia; meanwhile, this wave train could bring abundant water vapors from IO
and the Northwest Pacific to the middle region of East Asia, resulting in more precipitation occurring
in this area, while less precipitation occurring in Southeast Asia, Northeast China and Japan. Above
observed results are further confirmed through numerical simulations. In this study, the revealing of dipole pattern of SSTAs over IO in boreal autumn and its influencing processon winter precipitation
over East Asia and Southeast Asia deepens our understanding for variability of winter precipitation in this
region. Moreover, SSTAs in boreal winter remembering most signals of dipole pattern of SSTAs in boreal
autumn imply that the signal of SSTAs’ dipole pattern has potential value for winter precipitation prediction
in the region.
4.The connection of boreal November snow cover over the Eastern Europe and on winter precipitation
over East Asia
The collaborative spatial and temporal features of winter precipitation over East Asiaand November
snow cover over Eurasia are analyzed basing on the Singular Value Decomposition (SVD) method. The
results show that a strong consistent variation of winter precipitation occurs over East Asia, especially
in north of 35° N, which closely relates to snow cover change over the Eastern Europe. Mechanism a
nalysis indicates that the physical linkage processes between the November snow cover and East Asian
winter precipitation are different over December and January- February. In December, he good persistence of the snow cover anomalies means that it can keep the signal of November snow cover anomalies to December, and exert impacts on December circulation and precipitation over East Asia. In JanuaryFebruary, snow cover anomalies disappear, while the variabilityof polar vortex resulted from
the interaction of the tropospherestratosphere serves as a bridge connecting November snow cover
anomalie over Eastern Europe and January– February precipitation anomalies over East Asia. The revealing of influence of the November snow cover anomalies on East Asian winter precipitation is important and useful, not only for understanding theEast Asian winter precipitation variability but also for its prediction.