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Lejiang Yu, Shiyuan Zhong, Mingyu Zhou, Donald H. Lenschow, and Bo Sun

Abstract

The sharp decline of Arctic sea ice in recent decades has captured the attention of the climate science community. A majority of climate analyses performed to date have used monthly or seasonal data. Here, however, we analyze daily sea ice data for 1979–2016 using the self-organizing map (SOM) method to further examine and quantify the contributions of atmospheric circulation changes to the melt-season Arctic sea ice variability. Our results reveal two main variability modes: the Pacific sector mode and the Barents and Kara Seas mode, which together explain about two-thirds of the melt-season Arctic sea ice variability and more than 40% of its trend for the study period. The change in the frequencies of the two modes appears to be associated with the phase shift of the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO). The PDO and AMO trigger anomalous atmospheric circulations, in particular, the Greenland high and the North Atlantic Oscillation and anomalous warm and cold air advections into the Arctic Ocean. The changes in surface air temperature, lower-atmosphere moisture, and downwelling longwave radiation associated with the advection are consistent with the melt-season sea ice anomalies observed in various regions of the Arctic Ocean. These results help better understand the predictability of Arctic sea ice on multiple (synoptic, intraseasonal, and interannual) time scales.

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Xubin Zeng, Michael A. Brunke, Mingyu Zhou, Chris Fairall, Nicholas A. Bond, and Donald H. Lenschow

Abstract

The atmospheric boundary layer (ABL) height (h) is a crucial parameter for the treatment of the ABL in weather and climate models. About 1000 soundings from 11 cruises between 1995 and 2001 over the eastern Pacific have been analyzed to document the large meridional, zonal, seasonal, and interannual variations of h. In particular, its latitudinal distribution in August has three minima: near the equator, in the intertropical convergence zone (ITCZ), and over the subtropical stratus/stratocumulus region near the west coast of California and Mexico. The seasonal peak of h in the ITCZ zone (between 5.6° and 11.2°N) occurs in the spring (February or April), while it occurs in August between the equator and 5.6°N.

Comparison of these data with the 10-yr monthly output of the Community Climate System Model (CCSM2) reveals that overall the model underestimates h, particularly north of 20°N in August and September. Directly applying the radiosonde data to the CCSM2 formulation for computing h shows that, at the original vertical resolution (with the lowest five layers below 2.1 km), the CCSM2 formulation would significantly underestimate h. In particular, the correlation coefficient between the computed and observed h values is only 0.06 for cloudy cases. If the model resolution were doubled below 2.1 km, however, the performance of the model formulation would be significantly improved with a correlation coefficient of 0.78 for cloudy cases.

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Lejiang Yu, Shiyuan Zhong, Mingyu Zhou, Bo Sun, and Donald H. Lenschow

Abstract

The potential mechanisms underlying the observed increasing trend in Antarctic summertime sea ice cover for the 1979–2017 period have been investigated using a relatively novel method called the self-organizing map (SOM). Among the nine nodes generated to explain the variability of Antarctic sea ice cover, two (nodes 3 and 7) exhibit a statistically significant linear trend in the time series of the frequency of the SOM pattern occurrence that together explain 40% of the total trend in the sea ice cover. These two nodes have completely opposite spatial patterns and directions of trend and are associated with different atmospheric circulation patterns. Node 3, which represents an increase in sea ice over the Weddell Sea and the eastern Ross Sea and a decrease over the other coastal seas of West Antarctica, appears to be related to the positive phase of the southern annular mode (SAM) linked with the La Niña pattern of sea surface temperature (SST) over the tropical Pacific Ocean. The opposite spatial pattern and trend represented by node 7 is associated with a wave train originating over northern Australia. The anomalous wind field, surface downward longwave radiation, and surface air temperature generated by these circulation patterns are consistent with the spatial pattern and overall trends in the Antarctic sea ice cover.

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Xuhui Lee, Zhiqiu Gao, Chaolin Zhang, Fei Chen, Yinqiao Hu, Weimei Jiang, Shuhua Liu, Longhua Lu, Jielun Sun, Jiemin Wang, Zhihua Zeng, Qiang Zhang, Ming Zhao, and Mingyu Zhou
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Benzhi Zhou, Lianhong Gu, Yihui Ding, Lan Shao, Zhongmin Wu, Xiaosheng Yang, Changzhu Li, Zhengcai Li, Xiaoming Wang, Yonghui Cao, Bingshan Zeng, Mukui Yu, Mingyu Wang, Shengkun Wang, Honggang Sun, Aiguo Duan, Yanfei An, Xu Wang, and Weijian Kong

Abstract

Extreme events often expose vulnerabilities of socioeconomic infrastructures and point to directions of much-needed policy change. Integrated impact assessment of such events can lead to finding of sustainability principles. Southern and central China has for decades been undergoing a breakneck pace of socioeconomic development. In early 2008, a massive ice storm struck this region, immobilizing millions of people. The storm was a consequence of sustained convergence between tropical maritime and continental polar air masses, caused by an anomalously stable atmospheric general circulation pattern in both low and high latitudes. Successive waves of freezing rain occurred during a month period, coating southern and central China with a layer of ice 50–160 mm in thickness. We conducted an integrated impact assessment of this event to determine whether and how the context of socioeconomic and human-disturbed natural systems may affect the transition of natural events into human disasters. We found that 1) without contingency plans, advanced technologies dependent on interrelated energy supplies can create worse problems during extreme events, 2) the weakest link in disaster response lies between science and decision making, 3) biodiversity is a form of long-term insurance for sustainable forestry against extreme events, 4) sustainable extraction of nontimber goods and services is essential to risk planning for extreme events in forest resources use, 5) extreme events can cause food shortage directly by destroying crops and indirectly by disrupting food distribution channels, 6) concentrated economic development increases societal vulnerability to extreme events, and 7) formalized institutional mechanisms are needed to ensure that unexpected opportunities to learn lessons from weather disasters are not lost in distracting circumstances.

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