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Shi Z.,CAS Institute of Earth Environment | Shi Z.,Chinese Academy of Sciences | Shi Z.,Joint Center for Global Change Studies | Sha Y.,CAS Institute of Earth Environment | Liu X.,CAS Institute of Earth Environment
Journal of Climate | Year: 2017

Topographic insulation is one of the primary origins for the influence of the Tibetan Plateau (TP) on Asian climate. The Yunnan-Guizhou (YG) Plateau, at the southeastern margin of the TP, is known to block the northern branch of the Indian monsoon circulation in summer. However, it is an open question whether this blocking feeds back to the monsoon. In this study, the effect of the YG topography on the Indian monsoon and its comparison with that of the TP were evaluated using general circulation model experiments. The results showed that the TP strengthens the monsoon precipitation, especially during the onset. However, the YG topography significantly weakens the monsoon. With the YG topography, strengthened low-level airflow around the YG Plateau induces anomalous anticyclonic winds to the southwest, and the changes remodulate the whole circulation structure over Asia. As a result, the Indian monsoon becomes weakened from the Bay of Bengal to the Indian subcontinent and Arabian Sea, as does the associated precipitation. In addition, the YG topography affects the anomalous warming center over the TP and the precipitation during the monsoon onset. The YG-reduced summer precipitation occupied approximately one-third of the total increment compared to the entire TP. The Indian monsoon weakened by YG topography distinctly opposes the traditional paleoclimatic viewpoint that all of the TP topography contributes to the monsoon strengthening. In fact, the climatic effect of the TP depends closely upon both its central and marginal topography, and the topography of its subterrains does not necessarily play a similar role. © 2017 American Meteorological Society.

Wu X.,CAS Institute of Atmospheric Physics | Wu X.,University of Chinese Academy of Sciences | Mao J.,CAS Institute of Atmospheric Physics | Mao J.,Joint Center for Global Change Studies
International Journal of Climatology | Year: 2017

The interdecadal variability of early summer South China (SC) monsoon rainfall (SCMR) is investigated based on several long-term rainfall data sets from 1926 to 2013. Both correlation and composite analyses suggest a significant out-of-phase relationship between interdecadal fluctuations of SCMR and the preceding wintertime Pacific Decadal Oscillation (PDO), with negative (positive) PDO epochs favouring above-normal (below-normal) SCMR. The physical mechanism relating the wintertime PDO to the interdecadal variability of SCMR appears to take place via atmospheric and oceanic teleconnections. The below-normal SCMR is characterized by a meridionally arranged vortex (MAV) pattern across the East Asia–western North Pacific (WNP) sector, with an anomalous cyclone, an anomalous anticyclone and an anomalous cyclone occurring alternately over the South China Sea (SCS)–WNP, Yangtze Basin and Lake Baikal in the early summer. This may respond to the PDO-related positive sea surface temperature (SST) anomalies locally over the SCS–WNP and remotely over the tropical eastern Pacific, with the latter forcing anomalous Hadley and Walker circulations. SC is located directly under the divergent moisture environment of anomalous northeasterlies between the anomalous cyclone over the SCS–WNP and the anticyclone over the Yangtze Basin. Again, the below-normal SCMR is preceded by negative (positive) SST anomalies in the midlatitude North Pacific (tropical eastern Pacific) associated with positive PDO in wintertime. Such an anomalous SST pattern subsequently induces a strengthened and northward-shifted upper-tropospheric jet in early summer, leading to significant upper-tropospheric convergence over SC. The out-of-phase relationship between SCMR and wintertime PDO as well as related physical processes are validated by coupled model simulations. © 2016 Royal Meteorological Society

Sun C.,Beijing Normal University | Sun C.,CAS Institute of Atmospheric Physics | Li J.,Beijing Normal University | Li J.,Joint Center for Global Change Studies | Jin F.-F.,University of Hawaii at Manoa
Climate Dynamics | Year: 2015

Wavelet analysis of the annual North Atlantic Oscillation (NAO) index back to 1659 reveals a significant frequency band at about 60 years. Recent NAO decadal variations, including the increasing trend during 1960–1990 and decreasing trend since the mid-1990s, can be well explained by the approximate 60-year cycle. This quasi 60-year oscillation of the NAO is realistically reproduced in a long-term control simulation with version 4 of the Community Climate System Model, and the possible mechanisms are further investigated. The positive NAO forces the strengthening of the Atlantic meridional overturning circulation (AMOC) and induces a basin-wide uniform sea surface temperature (SST) warming that corresponds to the Atlantic multidecadal oscillation (AMO). The SST field exhibits a delayed response to the preceding enhanced AMOC, and shows a pattern similar to the North Atlantic tripole (NAT), with SST warming in the northern North Atlantic and cooling in the southern part. This SST pattern (negative NAT phase) may lead to an atmospheric response that resembles the negative NAO phase, and subsequently the oscillation proceeds, but in the opposite sense. Based on these mechanisms, a simple delayed oscillator model is established to explain the quasi-periodic multidecadal variability of the NAO. The magnitude of the NAO forcing of the AMOC/AMO and the time delay of the AMOC/AMO feedback are two key parameters of the delayed oscillator. For a given set of parameters, the quasi 60-year cycle of the NAO can be well predicted. This delayed oscillator model is useful for understanding of the oscillatory mechanism of the NAO, which has significant potential for decadal predictions as well as the interpretation of proxy data records. © 2015 Springer-Verlag Berlin Heidelberg

Li X.,Tsinghua University | Gong P.,Tsinghua University | Gong P.,Joint Center for Global Change Studies
Remote Sensing of Environment | Year: 2016

Satellite based human settlement extraction at medium resolution (30 m) with supervised classification has been widely carried out. However, adequate training sample collection and mapping accuracy are two hindering factors over large regions. Here we propose a new framework for efficient human settlement extraction from Landsat images over large areas. First, an inventory-based training set is adopted to obtain some statistical parameters required to build a non-settlement mask. The mask can not only reduce unnecessary computation but also reduce the impact of background noise. Thereafter, for the un-masked areas we calculate the similarity of each image pixel to pre-collected sample points, and only those within certain threshold are treated as the settlement class. This approach is very fast and has been applied to three rapidly developing regions in China. Accuracy assessment indicates that the mean overall accuracies are 87%, 89% and 89% for Jing-Jin-Ji region, Yangtze River Delta and Pearl River Delta, respectively. This work may be applied to human settlement extraction at even broader spatial scales. © 2016 Elsevier Inc.

Tian F.,Tsinghua University | Tian F.,Joint Center for Global Change Studies
Earth and Planetary Science Letters | Year: 2015

It is recently proposed that early stellar luminosity evolution of M dwarfs leads to severe water loss and the buildup of massive O2 atmospheres on rocky exoplanets in the habitable zone of these stars if interactions of such O2 atmospheres with planetary surfaces are inefficient. Here we show that even without considering atmosphere-surface interactions, the existence of a massive O2 atmosphere on such exoplanets is not an unavoidable consequence around M0-M3 stars and depends on stellar XUV properties, the mass of the exoplanets, and most importantly the initial planetary water inventories. In the case of inefficient atmosphere-surface interactions, the distribution of atmospheric O2 contents on these exoplanets should be bi-modal and such a distribution could be verified by future surveys of rocky exoplanets. © 2015 Elsevier B.V.

Huang X.M.,Tsinghua University | Wang W.C.,Joint Center for Global Change Studies | Fu H.H.,Joint Center for Global Change Studies | Yang G.W.,Joint Center for Global Change Studies | And 2 more authors.
Geoscientific Model Development | Year: 2014

We describe the design and implementation of climate fast input/output (CFIO), a fast input/output (I/O) library for high-resolution climate models. CFIO provides a simple method for modelers to overlap the I/O phase with the computing phase automatically, so as to shorten the running time of numerical simulations. To minimize the code modifications required for porting, CFIO provides similar interfaces and features to parallel Network Common Data Form (PnetCDF), which is one of the most widely used I/O libraries in climate models. We deployed CFIO in three high-resolution climate models, including two ocean models (POP and LICOM) and one sea ice model (CICE). The experimental results show that CFIO improves the performance of climate models significantly versus the original serial I/O approach. When running with CFIO at 0.1 resolution with about 1000 CPU cores, we managed to reduce the running time by factors of 7.9, 4.6 and 2.0 for POP, CICE, and LICOM, respectively. We also compared the performance of CFIO against two existing libraries, PnetCDF and parallel I/O (PIO), in different scenarios. For scenarios with both data output and computations, CFIO decreases the I/O overhead compared to PnetCDF and PIO. © Author(s) 2014.

Tian F.,Tsinghua University | Tian F.,Joint Center for Global Change Studies
Icarus | Year: 2015

It is recently proposed that planets in the habitable zones (HZ) of pre-main-sequence (PMS) M dwarfs are good targets for the detection of habitable environments. In this note we show that future ground-based telescopes will be able to observe planets in time-evolving HZ of PMS M dwarfs with duration 10-100. Myrs. Based on X-ray measurements, there are >18 M0-M4 PMS stars within 10. pc, the characterization of potentially habitable exoplanets around which could provide highly valuable information regarding the evolution of habitable environments. There are tens of M dwarfs within 10. pc with X-ray to total luminosity ratios similar to that of the young Sun, the observations of potential planets around which could significantly improve our understanding of the physical states of early Solar System rocky planets. © 2015 Elsevier Inc.

Li W.,Tsinghua University | Gong P.,Tsinghua University | Gong P.,Joint Center for Global Change Studies
Remote Sensing of Environment | Year: 2016

Continuous monitoring of coastline dynamics is of crucial importance to the understanding of relative contributions of various potential driving factors behind the long-term coastline change. While a large number of efforts have been made to extract coastline and detect coastline change with remotely sensed data, the temporal frequency and spatial resolution of coastline datasets obtained are generally not fine enough to reflect the detailed process of coastline retreat and/or advance, particularly in coastlines with subtle variability. To overcome these limitations, we developed a method to continuously monitor the dynamics of a muddy coastline with subtle variability in western Florida at annual and subpixel scales using time-series Landsat data (1984-2013). First, robust indicators were used to indicate the annual "average" location of the dynamic coastline. Due to the complexity of muddy-coast morphology, the annual average location is represented not by the coast "line", but by the fractional inundated "area" of coastline pixels (pixels where the coastline is located), namely annually inundated area. Second, the annually inundated area of coastline pixels was estimated with a model proposed in this study, and the uncertainty was estimated with the Monte Carlo method. The retrievals were validated at 10 sites with aerial imagery, and the overall RMSE (root mean square error) is 11.48%. Third, the long-term trend for the time series of annually inundated area was derived with a statistical model. The results indicate that the muddy coast in western Florida continues to shrink with an average rate of 0.42 ± 0.05 km2/year during the three decades. This study demonstrates the feasibility of time-series Landsat data in continuous monitoring of coastline dynamics. © 2016 Elsevier Inc..

Hu G.,CAS Institute of Remote Sensing | Jia L.,CAS Institute of Remote Sensing | Jia L.,Joint Center for Global Change Studies
Remote Sensing | Year: 2015

As a typical inland river basin, Heihe River basin has been experiencing severe water resource competition between different land cover types, especially in the middle stream and downstream areas. Terrestrial actual evapotranspiration (ETa), including evaporation from soil and water surfaces, evaporation of rainfall interception, transpiration of vegetation canopy and sublimation of snow and glaciers, is an important component of the water cycle in the Heihe River basin. We developed a hybrid remotely sensed ETa estimation model named ETMonitor to estimate the daily actual evapotranspiration of the Heihe River basin for the years 2009-2011 at a spatial resolution of 1 km. The model was forced by a variety of biophysical parameters derived from microwave and optical remote sensing observations. The estimated ETa was evaluated using eddy covariance (EC) flux observations at local scale and compared with the annual precipitation and the MODIS ETa product (MOD16) at regional scale. The spatial distribution and the seasonal variation of the estimated ETa were analyzed. The results indicate that the estimated ETa shows reasonable spatial and temporal patterns with respect to the diverse cold and arid landscapes in the upstream, middle stream and downstream regions, and is useful for various applications to improve the rational allocation of water resources in the Heihe River basin. © 2015 by the authors.

Shi Z.,CAS Institute of Earth Environment | Shi Z.,Chinese Academy of Sciences | Shi Z.,Joint Center for Global Change Studies
Quaternary Science Reviews | Year: 2016

The responses of Asian summer monsoon and associated precipitation to orbital forcing have been intensively explored during the past 30 years, but debate still exists regarding whether or not the Asian monsoon is controlled by northern or southern summer insolation on the precessional timescale. Various modeling studies have been conducted that support the potential roles played by the insolation in both hemispheres. Among these previous studies, however, the main emphasis has been on the Asian monsoon intensity, with the response of monsoon duration having received little consideration. In the present study, the response of the rainy season duration over different monsoon areas to orbital forcing and its contribution to total annual precipitation are evaluated using an atmospheric general circulation model. The results show that the durations of the rainy seasons, especially their withdrawal, in northern East Asia and the India-Bay of Bengal region, are sensitive to precession change under interglacial-like conditions. Compared to those during stronger boreal summer insolation, the Asian monsoon-associated rainy seasons at weaker insolation last longer, although the peak intensity is smaller. This longer duration of rainfall, which results from the change in land-ocean thermal contrast associated with atmospheric diabatic heating, can counterbalance the weakened intensity in certain places and induce an opposite response of total annual precipitation. However, the duration effect of Asian monsoon is limited under glacial-like conditions. Nevertheless, monsoon duration is a factor that can dominate the orbital-scale variability of Asian monsoon, alongside the intensity, and it should therefore receive greater attention when attempting to explain orbital-scale monsoon change. © 2016 Elsevier Ltd.

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