El Niño's impact on California precipitation: seasonality, regionality, and El Niño intensity

California has experienced severe drought in recent years posing great challenges to agricultural production, water resources, and land management. El Niño, as the prime source of seasonal to interannual climate predictability, offers the potential of amelioration of drought in California. Here El Niño's impacts on California winter precipitation are examined, focusing on variations by season, region, and the strength of El Niño using observational data for the period 1901–2010. The El Niño influence on California precipitation strengthens from early to late winter and is stronger in the south than the north. Eight of ten moderate-to-strong El Niños in the late winter put southern California in the wettest tercile and none of these ten events put northern California in the driest tercile. The early to late winter strengthening of the El Niño impact on precipitation occurs even as El Niño weakens and is associated with a strengthening and eastward extending tropical deep convection anomaly allowed by the late winter warming of the climatological mean sea surface temperature over the tropical eastern Pacific

Seasonality and Regionality of ENSO Teleconnections and Impacts on North America

The El Niño – Southern Oscillation (ENSO) has far-reaching impacts across the globe and provides the most reliable source of seasonal to interannual climate prediction over North America. Though numerous studies have discussed the impacts of ENSO teleconnections on North America during boreal winter, it is becoming more and more apparent that the regional impacts of ENSO teleconnections are highly sensitive to the seasonal evolution of ENSO events. Also, the significant impacts of ENSO are not limited to the boreal winter seasons. To address these knowledge gaps, this thesis examines the seasonal dependence of ENSO teleconnections and impacts on North American surface climate, focusing on two examples. Chapter 1 examines the relationship between El Niño – California winter precipitation. Results show that the probability of the anomalous statewide-wetness increases as El Niño intensity increases. Also, the influences of El Niño on California winter precipitation are statistically significant in late winter (Feb-Apr), but not in early winter even though that is when El Niño usually reaches its peak intensity. Chapter 2 further investigates why the strong 2015/16 El Niño failed to bring above normal winter precipitation to California, focusing on the role of westward shifted equatorial Pacific sea surface temperature anomalies (SSTAs) based on two reasons: the maximum equatorial Pacific SSTAs was located westward during the 2015/16 winter compared to those during the 1982/83 and 1997/98 winters, both of which brought extremely wet late winters to California. Also, the North American Multi-Model Ensemble (NMME) forecasts overestimated the eastern tropical Pacific SSTAs and California precipitation in the 2015/16 late winter, compared to observations. The Atmospheric General Circulation Model (AGCM) experiments suggested that the SST forecast error in NMME contributed partially to the wet bias in California precipitation forecast in the 2015/16 late winter. However, the atmospheric internal variability could have also played a large role in the dry California winter during the event. ENSO also exerts significant impacts on agricultural production over the Midwest during boreal summer. Chapter 3 examines the physical processes of the ENSO summer teleconnection, focusing on the summer when a La Niña is either transitioning from an earlier El Niño winter or persisting from an existing La Niña winter. The results demonstrate that the impacts are most significant during the summer when El Niño is transitioning to La Niña compared to that when La Niña is persisting, even though both can loosely be defined as developing La Niña summer. During the transitioning summer, both the decaying El Niño and the developing La Niña induce suppressed deep convection over the tropical Pacific and thereby the corresponding Rossby wave propagations toward North America, resulting in a statistically significant anomalous anticyclone over northeastern North America and, therefore, a robust warming signal over the Midwest. These features are unique to the developing La Niña transitioning from El Niño, but not the persistent La Niña. In Chapter 4, we further evaluate the performance of NCAR CAM5 forced with historical SSTA in terms of the La Niña summer teleconnections. Though the model ensemble mean well reproduces the features in the preceding El Niño/La Niña winters, the model ensemble mean has very limited skill in simulating the tropical convection and extratropical teleconnections during both the transitioning and persisting summers. The weak responses in the model ensemble mean are attributed to large variability in both the tropical precipitation, especially over the western Pacific, and atmospheric circulation during summer season. This thesis synthesizes the physical processes and assessments of climate models in different seasons to establish the sensitivity of regional climate to the seasonal dependence of ENSO teleconnections. We demonstrate that the strongest impacts of ENSO on North American regional climate might not be necessarily simultaneous with maximum tropical Pacific SST anomalies. We also emphasize the importance of the multi-year ENSO evolutions when addressing the seasonal impacts on North American summertime climate. The findings in this thesis could benefit the improvement of seasonal hydroclimate forecasting skills in the future

Assessing ENSO summer teleconnections, impacts, and predictability in North America

During the summer when an El Niño event is transitioning to a La Niña event, the extratropical teleconnections exert robust warming anomalies over the U.S. Midwest threatening agricultural production. This study assesses the performance of current climate models in capturing the prominent observed extratropical responses over North America during the transitioning La Niña summer, based on atmospheric general circulation model experiments and coupled models from the North American Multimodel Ensemble (NMME). The ensemble mean of the SST-forced experiments across the transitioning La Niña summers does not capture the robust warming in the Midwest. The SST-forced experiments do not produce consistent subtropical western Pacific (WP) negative precipitation anomalies and this leads to the poor simulations of extratropical teleconnections over North America. In the NMME models, with active air–sea interaction, the negative WP precipitation anomalies show better agreement across the models and with observations. However, the downstream wave train pattern and the resulting extratropical responses over North America exhibit large disagreement across the models and are consistently weaker than in observations. Furthermore, in these climate models, an anomalous anticyclone does not robustly translate into a warm anomaly over the Midwest, in disagreement with observations. This work suggests that, during the El Niño to La Niña transitioning summer, active air–sea interaction is important in simulating tropical precipitation over the WP. Nevertheless, skillful representations of the Rossby wave propagation and land–atmosphere processes in climate models are also essential for skillful simulations of extratropical responses over North America

ENSO Teleconnections and Impacts on U.S. Summertime Temperature during a Multiyear La Niña Life Cycle

El Niño–Southern Oscillation (ENSO) teleconnections have been recognized as possible negative influences on crop yields in the United States during the summer growing season, especially in a developing La Niña summer. This study examines the physical processes of the ENSO summer teleconnections and remote impacts on the United States during a multiyear La Niña life cycle. Since 1950, a developing La Niña summer is either when an El Niño is transitioning to a La Niña or when a La Niña is persisting. Due to the distinct prior ENSO conditions, the oceanic and atmospheric characteristics in the tropics are dissimilar in these two different La Niña summers, leading to different teleconnection patterns. During the transitioning summer, the decaying El Niño and the developing La Niña induce suppressed deep convection over both the subtropical western Pacific (WP) and the tropical central Pacific (CP). Both of these two suppressed convection regions induce Rossby wave propagation extending toward North America, resulting in a statistically significant anomalous anticyclone over northeastern North America and, therefore, a robust warming signal over the Midwest. In contrast, during the persisting summer, only one suppressed convection region is present over the tropical CP induced by the La Niña SST forcing, resulting in a weak and insignificant extratropical teleconnection. Experiments from a stationary wave model confirm that the suppressed convection over the subtropical WP during the transitioning summer not only contributes substantially to the robust warming over the Midwest but also causes the teleconnections to be different from those in the persisting summer

Harmonic chirp imaging method for ultrasound contrast agent

Coded excitation is currently used in medical ultrasound to increase signal-to-noise ratio (SNR) and penetration depth. We propose a chirp excitation method\ud for contrast agents using the second harmonic component of the response. This method is based on a compression filter that selectively compresses and extracts the second harmonic component from the received echo signal. Simulations have shown a clear increase in response for chirp excitation\ud over pulse excitation with the same peak amplitude. This was confirmed by two-dimensional (2-D) optical observations of bubble response with a fast framing camera. To evaluate the harmonic compression method, we applied it to\ud simulated bubble echoes, to measured propagation harmonics, and to B-mode scans of a flow phantom and compared it to regular pulse excitation imaging. An increase of approximately 10 dB in SNR was found for chirp excitation. The\ud compression method was found to perform well in terms of resolution. Axial resolution was in all cases within 10% of the axial resolution from pulse excitation. Range side-lobe levels were 30 dB below the main lobe for the simulated bubble echoes and measured propagation harmonics. However,\ud side-lobes were visible in the B-mode contrast images

Role of the equatorial Pacific SST forecast error in the late winter California precipitation forecast for the 2015-16 El Niño

During the strong 2015/16 El Niño, only normal to below-average precipitation fell across California in the late winter. This disagrees with both predictions by the ensemble mean of forecast models and expectations for strong El Niños. The authors examine one of the possible reasons why this event did not bring expected precipitation to California in the late winter. The maximum equatorial Pacific sea surface temperature anomalies (SSTAs) were located, compared to the 1982/83 and 1997/98 strong El Niños, farther to the west in the 2015/16 winter, which possibly caused less convection in the eastern tropical Pacific and shifted the teleconnection patterns westward in the North Pacific, thus weakening the influences on California. The SSTA and precipitation forecast for February–April 2016, based on the North American Multimodel Ensemble, showed large discrepancies from observations, with the ensemble mean of most of the models overestimating SSTAs in the eastern tropical Pacific and California precipitation. Atmospheric general circulation model (AGCM) experiments were conducted to test the hypothesis that the warmer eastern tropical Pacific SSTA forecast may have caused the wetter forecast in California in 2015/16 compared to observations. The AGCM experiments suggest it is difficult to assert that the eastern tropical Pacific SSTAs caused the too-wet California precipitation forecast, especially in Southern California, given that the models disagree. Results indicate forecast error can be influenced by atmosphere-model sensitivity to forecast SSTs, but they also indicate atmospheric internal variability may have been responsible for the combination of a strong El Niño and near-normal California precipitation

An Adaptive Load Balance and Handoff Management Strategy for Hierarchical Infrastructure Networks

Hierarchical cellular networks that employ microcells with overlaying macrocells have been proposed to increase the traffic-carrying capacity and circuit quality. Variations in the traffic loads among cells will lessen the traffic-carrying capacity. Moreover, the handoff procedure usually takes place when the call crosses the cell boundary. An ineffective management will increase the system overheads, such as channel switch, data switch, and even network switch. The invetigation proposes an effective load balance and handoff management strategy. This strategy are implemented to solve traffic-adaption problem that can enhance the traffic-carrying capacity for variations in traffic. For the management of handoff procedure, our strategy considers the mobility of mobile hosts and the bandwidth utilization in macrocells. It can descrease the number of handoffs and, accordingly, lessen the system overhead. Furthermore, the simulation results are presented to confirm the efficiency of the proposed strategy

Aqueous Extract of Shi-Liu-Wei-Liu-Qi-Yin Induces G2/M Phase Arrest and Apoptosis in Human Bladder Carcinoma Cells via Fas and Mitochondrial Pathway

Shi-Liu-Wei-Liu-Qi-Yin (SLWLQY) was traditionally used to treat cancers. However, scientific evidence of the anticancer effects still remains undefined. In this study, we aimed to clarify the possible mechanisms of SLWLQY in treating cancer. We evaluated the effects of SLWLQY on apoptosis-related experiments inducing in TSGH-8301 cells by (i) 3-(4,5-dimethylthiazol-zyl)-2,5-diphenylterazolium bromide (MTT) for cytotoxicity; (ii) cell-cycle analysis and (iii) western blot analysis of the G2/M-phase and apoptosis regulatory proteins. Human bladder carcinoma TSGH-8301 cells were transplanted into BALB/c nude mice as a tumor model for evaluating the antitumor effect of SLWLQY. Treatment of SLWLQY resulted in the G2/M phase arrest and apoptotic death in a dose-dependent manner, accompanied by a decrease in cyclin-dependent kinases (cdc2) and cyclins (cyclin B1). SLWLQY stimulated increases in the protein expression of Fas and FasL, and induced the cleavage of caspase-3, caspase-9 and caspase-8. The ratio of Bax/Bcl2 was increased by SLWLQY treatment. SLWLQY markedly reduced tumor size in TSGH-8301 cells-xenografted tumor tissues. In the tissue specimen, SLWLQY up-regulated the expression of Fas, FasL and Bax proteins, and down-regulated Bcl2 as well as in in vitro assay. Our results showed that SLWLQY reduced tumor growth, caused cell-cycle arrest and apoptosis in TSGH-8301 cells via the Fas and mitochondrial pathway

G band Raman double resonance in twisted bilayer graphene: an evidence of band splitting and folding

The stacking faults (deviates from Bernal) will break the translational symmetry of multilayer graphenes and modify their electronic and optical behaviors to the extent depending on the interlayer coupling strength. This paper addresses the stacking-induced band splitting and folding effect on the electronic band structure of twisted bilayer graphene. Based on the first-principles density functional theory study, we predict that the band folding effect of graphene layers may enable the G band Raman double resonance in the visible excitation range. Such prediction is confirmed experimentally with our Raman observation that the resonant energies of the resonant G mode are strongly dependent on the stacking geometry of graphene layers.Comment: 16 pages, 4 figures, Accepted by Phys. Rev.