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Dehai Luo, Yiqing Xiao, Yao Yao, Aiguo Dai, Ian Simmonds, and Christian L. E. Franzke

Abstract

In Part I of this study, the impact of Ural blocking (UB) on the warm Arctic–cold Eurasian (WACE) pattern associated with the winter (DJF) arctic sea ice loss during 1979–2013 is examined by dividing the arctic sea ice reduction region into two dominant subregions: the Barents and Kara Seas (BKS) and the North American high-latitude (NAH) region (Baffin and Hudson Bay, Davis Strait, and Labrador Sea). It is found that atmospheric response to arctic sea ice loss resembles a negative Arctic response oscillation with a dominant positive height anomaly over the Eurasian subarctic region. Regression analyses of the two subregions further show that the sea ice loss over the BKS corresponds to the UB pattern together with a positive North Atlantic Oscillation (NAO+) and is followed by a WACE anomaly, while the sea ice reduction in the NAH region corresponds to a negative NAO (NAO) pattern with a cold anomaly over northern Eurasia.

Further analyses reveal that the UB pattern is more persistent during the period 2000–13 (P2) than 1979–99 (P1) because of the reduced middle-to-high-latitude mean westerly winds over Eurasia associated with the intense BKS warming. During P2 the establishment of the UB becomes a slow process because of the role of the BKS warming, while its decay is slightly rapid. In the presence of the long-lived UB that often occurs with the NAO+, the BKS-warming-induced DJF-mean anticyclonic anomaly is intensified and widened and then expands southward during P2 to amplify the WACE pattern and induce the southward displacement of its cold anomaly and the further loss of the BKS sea ice. Thus, midlatitude Eurasian cold events should be more frequent as the sea ice loss continues over the BKS.

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Yijia Hu, Yimin Zhu, Zhong Zhong, and Yao Ha

Abstract

The prediction of mei-yu onset date (MOD) in the middle and lower reaches of the Yangtze River valley (MLYRV) is an important and challenging task for those making seasonal climate predictions in China. In this paper, the atmospheric and oceanic conditions in the preceding winter and spring related to MOD are analyzed. It is found that the MOD is associated with the intensity of the Ural high and the East Asian trough in high latitudes, with the intensity of the upper-level westerly jet in middle latitudes, and with the contrast of land–sea temperature and pressure in the preceding winter and spring, which are proxies for the intensity of the East Asian winter monsoon (EAWM). It is suggested that the intensity of the EAWM is the most crucial factor affecting the MOD. Years with an early MOD usually correspond to strong EAWMs in the preceding winter, and vice versa. The EAWM can affect the MOD by influencing the East Asian summer monsoon (EASM) through tropical ocean–atmosphere and tropical–extratropical interactions. Based on the above analysis, a physics-based statistical forecast model is established using multivariable linear regression techniques. The hindcast of MOD during the 13 yr from 1998 to 2010 is carried out to evaluate the performance of this forecast model. The MOD can be predicted successfully in 8 out of the 13 yr. The forecast model predicts the MOD in the years with strong mei-yu intensity more accurately than in those with weak mei-yu intensity, especially for cases of extreme flooding. This is useful in the prevention of flooding disasters.

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Yuan Sun, Lan Yi, Zhong Zhong, and Yao Ha

Abstract

The latest version of the Weather Research and Forecasting model (WRFV3.5) is used to evaluate the performance of the Grell and Freitas (GF13) cumulus parameterization scheme on the model convergence in simulations of a tropical cyclone (TC) at gray-zone resolutions. The simulated TC intensity converges to a finite limit as the grid spacing varies from 7.5 to 1 km. The reasons for the model convergence are investigated from perspectives of subgrid-scale processes and thermodynamic and dynamic structures. It is found that the impacts of above factors are notably different with varying model resolutions. The convective heating and drying increase as the grid spacing decreases, which inhibits the explicit microphysical parameterization preventing the simulated TC from overly intensifying. As the grid spacing decreases from 7.5 to 5 km, the TC intensity increases because of a stronger secondary circulation, a larger magnitude and proportion of strong eyewall updraft, and a greater amount of latent heating in the eyewall. As the grid spacing decreases from 5 to 3 km, the radius of maximum wind (RMW) decreases and the radial pressure gradient increases leading to an increase in TC intensity. The simulated TC intensity changes slightly as the grid spacing decreases from 3 to 1 km since the RMW and the storm structure both change little. The slight changes in the simulated TC intensity at such high resolutions indicate a great model convergence. Therefore, the GF13 presents an appropriate option that increases the model convergence in the TC intensity simulation at gray-zone resolution.

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Ruibo Lei, Zhijun Li, Yanfeng Cheng, Xin Wang, and Yao Chen

Abstract

High-precision ice thickness observations are required to gain a better understanding of ocean–ice–atmosphere interactions and to validate numerical sea ice models. A new apparatus for monitoring sea ice and snow thickness has been developed, based on the magnetostrictive-delay-line (MDL) principle for positioning sensors. This system is suited for monitoring fixed measurement sites on undeformed ice. The apparatus presented herein has been tested on landfast ice near Zhongshan Station, East Antarctica, for about 6 months during the austral autumn and winter of 2006; valid data records from the deployment are available for more than 90% of the deployment’s duration. The apparatus’s precision has been estimated to be ±0.002 m for the deployment. Therefore, it is possible that this apparatus may become a standard for sea ice/snow thickness monitoring.

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Jie Cao, Ping Yao, Lin Wang, and Kui Liu

Abstract

Based on reanalysis and observational datasets, this study proposes a reasonable mechanism for summer rainfall variations over the low-latitude highlands (LLH) of China, in which a subtropical Indian Ocean dipole (SIOD)-like pattern is the key external thermal forcing. In summers with a positive SIOD-like pattern, sea surface temperature (SST) anomalies may lead to lower-tropospheric divergence over the tropical Indian Ocean and convergence over the subtropical southwestern Indian Ocean and Arabian Sea. The convergence over the Arabian Sea can induce easterly anomalies of the divergent wind component off the eastern coast of the Bay of Bengal (BOB), while the divergence over the tropical Indian Ocean can change the interhemispheric vertical circulation and produce a descending motion over the same area and cyclonic anomalies in the rotational wind component over the Indian peninsula. The combined effect of the divergent and rotational wind anomalies and enhanced interhemispheric vertical circulation facilitates easterly anomalies and weakens climatological water vapor flux to the northern BOB. Therefore, anomalous water vapor divergence and less precipitation are observed over the LLH. In summers with a negative SIOD-like pattern, the situation is approximately the same but with opposite polarity and a weaker role of the divergent wind component. Further analyses indicate that the summertime SIOD-like pattern can be traced to preceding seasons, especially in positive SIOD-like years. The SST–wind–evaporation feedback mechanism could account for maintenance of the SIOD-like pattern. These results provide efficient prediction potential for summer rainfall variations over the LLH.

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T. Yao, Howard J. Freeland, and Lawrence A. Mysak

Abstract

Six current-meter mooring were deployed in a line approximately 600 km in length along the continental shelf of British Columbia. Analysis of the low frequency (periods exceeding a day) fluctuations in current for the winter 1981–82 period is discussed. Alongshore currents off Vancouver Island are mutually correlated with time lag less than a day. The region of mutual correlation does not extend north of Vancouver Island, across Queen Charlotte Sound. Coherence is observed between currents south and north of Queen Charlotte Sound only in a frequency band where there is mutual coherence with local wind. A comparison is made between observation and free coastal-trapped wave theory. Off northern Vancouver Island, where the shelf is narrower than off southern Vancouver Island, there is increased vertical shear, a feature of the second coastal-trapped wave mode. A consistency test is applied using the cross spectral matrix of alongshore components of velocity. In the dominant energy-containing frequency bands (periods ≳10 days), the structure of alongshore currents off Vancouver Island is consistent with the two lowest free coastal-trapped wave modes locked in phase.

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Yao Ha, Zhong Zhong, Yijia Hu, and Xiuqun Yang

Abstract

This study investigates the influences of ENSO on tropical cyclone (TC) kinetic energy and its meridional transport in the western North Pacific (WNP) using the TC wind field obtained after a method for removing TC vortices from reanalysis data is applied. Results show that ENSO strongly modulates TC kinetic energy and its meridional transport in the WNP, but their effects and regions differ. The TC kinetic energy is positively correlated with the Niño-3.4 index in the entire WNP, and its poleward transport is positively (negatively) correlated with the Niño-3.4 index in the eastern WNP (the western WNP and the South China Sea); these correlations are statistically significant. The maximum TC kinetic energy is located around 25°N, 135°E (25°N, 125°E) in the warm (cold) year, showing an east–west pattern during different ENSO phases. The meridional transport of TC kinetic energy exhibits a dipole pattern over the WNP, with the poleward (equatorward) transport in the eastern (western) WNP. Both poleward and equatorward transports strengthen (weaken) and shift eastward (westward) in El Niño (La Niña) years. Therefore, El Niño has strong influences on TC kinetic energy and its meridional transport.

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Dong-Peng Guo, Ren-Tai Yao, and Dan Fan

Abstract

This paper introduces a wind tunnel experiment to study the effect of the cooling tower of a nuclear power plant on the flow and the characteristics of visible plume regions. The relevant characteristics of the flow field near the cooling tower, such as the plume rise and the visible plume region, are compared with the results of previous experimental data from Électricité de France (EDF) and the Briggs formulas. The results show that the wind tunnel experiment can simulate the top backflow of the cooling tower and the rear cavity regions among others. In the near-wake region, including the recirculation cavity, mean velocity decreases and turbulence intensity increases significantly. The maximum turbulence intensity observed is 0.5. In addition, the disturbed flow extent of the cooling tower top reaches 1.5 times the cooling tower height. Analysis of the visible plume region shows that the wind tunnel experiment can simulate the variation of a visible plume region. The results are consistent with the wind tunnel experiment of EDF. Moreover, the plume rise analysis shows that the wind tunnel experiment data are in agreement with the Briggs formulas for 50–200 m. As a whole, the proposed wind tunnel experiment can simulate the flow field variation of the visible plume region and the plume rise around the buildings with reasonable accuracy.

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Yao Ha, Zhong Zhong, Xiuqun Yang, and Yuan Sun

Abstract

This study focuses on statistical analysis of anomalous tropical cyclone (TC) activities and the physical mechanisms behind these anomalies. Different patterns of decaying of the warm sea surface temperature anomaly (SSTA) over the equatorial central-eastern Pacific are categorized into three types: eastern Pacific warming decaying to La Niña (EPWDL), eastern Pacific warming decaying to a neutral phase (EPWDN), and a central Pacific warming decaying year (CPWD). Differences in TC activity over the western North Pacific (WNP) corresponding to the above three types are discussed, and possible mechanisms are proposed. For EPWDL, TC genesis shows a significant positive (negative) anomaly over the northwestern (southeastern) WNP and more TCs move westward and make landfall over the southern East Asian coast. This is attributed primarily to the combined modulation of La Niña and the warm equatorial east Indian Ocean SSTA. For EPWDN, enhanced TC genesis is observed over the northeastern WNP, and suppressed TC activity is located mainly in the zonal region extending from the Philippine Sea to the eastern WNP, close to 160°E. Most of the TCs formed over the eastern WNP experience early recurvature east of 140°E, then move northeastward; hence, fewer TCs move northwestward to make landfall over the East Asian coast. For CPWD, the enhanced TC activity appears over the western WNP. This is due to the weak anomalous cyclonic circulation over the Philippines, primarily caused by the weaker, more westward-shifting warm SSTA compared to that in the previous warming year over the central Pacific.

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R. W. Higgins, K. C. Mo, and Y. Yao

Abstract

Relationships between the interannual variability of the U.S. summer precipitation regime and the intensification, weakening, or changes in position of the climatological-mean circulation features that organize this regime are examined. The focus is on the atmospheric conditions over the conterminous United States relative to wet and dry monsoons over the southwestern United States. The onset of the monsoon in this region, which typically begins in early July, is determined using an index based on daily observed precipitation for a 32-yr (1963–94) period. Composites of observed precipitation and various fields from the National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis for wet and dry monsoons are used to show that the interannual variability of the summer precipitation regime closely mimics the seasonal changes associated with the development of the North American monsoon system.

The warm season precipitation regime is characterized by a continental-scale precipitation pattern consisting of an out-of-phase relationship between the Southwest and the Great Plains/Northern Tier and an in-phase relationship between the Southwest and the East Coast. This pattern is preserved for both wet and dry monsoons, but the Southwest is relatively wetter and the Great Plains are relatively drier during wet monsoons. Wet (dry) monsoons are also associated with a stronger (weaker) upper-tropospheric monsoon anticyclone over the western United States, consistent with changes in the upper-tropospheric divergence, midtropospheric vertical motion, and precipitation patterns. The intensity of the monsoon anticyclone over the western United States appears to be one of the most fundamental controls on summertime precipitation downstream over the Great Plains.

Evidence is presented that the interannual variability of the U.S. warm season precipitation regime is linked to the season-to-reason “memory” of the coupled atmosphere–ocean system over the eastern tropical Pacific. In particular, it is shown that SST anomalies in the eastern Pacific cold tongue and precipitation anomalies in the intertropical convergence zone, present during the winter and spring preceding the monsoon, are linked via an anomalous local Hadley circulation to the warm season precipitation regime over the United States and Mexico. Wet (dry) summer monsoons tend to follow winters characterized by dry (wet) conditions in the Southwest and wet (dry) conditions in the Pacific Northwest. This association is attributed, in part, to the memory imparted to the atmosphere by the accompanying Pacific SST anomalies.

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