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Scholarly Works (9 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2020)
An intensification of East Asian rainfall usually occurs in summers following El Niño. We propose that this teleconnection is mediated via the westerlies impinging on the Tibetan Plateau, through El Niño's control on tropical tropospheric temperature. This is distinct from previous studies that attribute the El Niño's influence to changes in the western Pacific subtropical anticyclone. The warming in the eastern equatorial Pacific leads to uniform warming of the entire tropical troposphere, which sharpens the temperature gradient between the tropics and the subtropics and shifts the westerlies southward. The westerlies impinge over the Tibetan Plateau for longer and, through interaction with the topography, induce intense and persistent extratropical northerlies downstream of the plateau that in turn intensifies the East Asian rainband. The rainband has previously been shown to intensify in a similar manner in a warming climate, suggesting that the El Niño response provides an analog for future changes.
    Cover page: Southward Shift of Westerlies Intensifies the East Asian Early Summer Rainband Following El Niño
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    This study explores how the termination of the mei-yu is dynamically linked to the westerlies impinging on the Tibetan Plateau. It is found that the mei-yu stage terminates when the maximum upper-tropospheric westerlies shift beyond the northern edge of the plateau, around 408N. This termination is accompanied by the disappearance of tropospheric northerlies over northeastern China. The link between the transit of the jet axis across the northern edge of the plateau, the disappearance of northerlies, and termination of the mei-yu holds on a range of time scales from interannual through seasonal and pentad. Diagnostic analysis indicates that the weakening of the meridional moisture contrast and meridional wind convergence, mainly resulting from the disappearance of northerlies, causes the demise of the mei-yu front. The authors propose that the westerlies migrating north of the plateau and consequent weakening of the extratropical northerlies triggers the mei-yu termination. Model simulations are employed to test the causality between the jet and the orographic downstream northerlies by repositioning the northern edge of the plateau. As the plateau edge extends northward, orographic forcing on the westerlies strengthens, leading to persistent strong downstream northerlies and a prolonged mei-yu. Idealized simulations with a dry dynamical core further demonstrate the dynamical link between the weakening of orographically forced downstream northerlies with the positioning of the jet from south to north of the plateau. Changes in the magnitude of orographically forced stationary waves are proposed to explain why the downstream northerlies disappear when the jet axis migrates beyond the northern edge of the plateau.
      Cover page: Interaction of the westerlies with the Tibetan Plateau in determining the mei-yu termination Interaction of the westerlies with the Tibetan Plateau in determining the mei-yu termination
      • Thesis
      • Peer Reviewed
      UC Berkeley Electronic Theses and Dissertations (2020)

      This dissertation explores the dynamical linkage between the orographic forcing of the Ti- betan Plateau on the westerlies, and the seasonal migration of the East Asian summer monsoonal rainfall.

      Chapter 2 focuses on evolution of the East Asian summer monsoon (EASM) from early to late Holocene. The Holocene EASM was previously characterized as a trend toward weaker monsoon intensity paced by orbital insolation. It is demonstrated here that this evolution is more accurately characterized as changes in the transition timing and duration of the EASM seasonal stages (spring, pre-mei-yu, mei-yu, midsummer), and tied to the north–south dis- placement of the westerlies relative to Tibet. To this end, time-slice simulations across the Holocene are employed using an atmospheric general circulation model. Self-organizing maps are used to objectively identify the transition timing and duration of the EASM seasonal stages. Compared to the late Holocene, an earlier onset of mei-yu and an earlier transition from mei-yu to midsummer in the early to mid-Holocene are found, resulting in a short- ened mei-yu and prolonged midsummer stage. These changes are accompanied by an earlier northward positioning of the westerlies relative to Tibet. Invoking changes to seasonal tran- sitions also provides a more satisfactory explanation for two key observations of Holocene East Asian climate: the “asynchronous Holocene optimum” and changes to dust emissions. A mechanism is proposed to explain the altered EASM seasonality in the simulated early to mid-Holocene. The insolation increase over the boreal summer reduces the pole–equator temperature gradient, leading to northward-shifted and weakened westerlies. The meridional position of the westerlies relative to the Tibetan Plateau determines the onset of mei-yu and possibly the onset of the midsummer stage. The northward shift in the westerlies triggers ear- lier seasonal rainfall transitions and, in particular, a shorter mei-yu and longer midsummer stage.

      Chapter 3 explores how the termination of the mei-yu is dynamically linked to the west- erlies impinging on the Tibetan Plateau. It is found that the mei-yu stage terminates when the maximum upper-tropospheric westerlies shift beyond the northern edge of the plateau, around 40◦ N. This termination is accompanied by the disappearance of tropospheric norther- lies over northeastern China. The link between the transit of the jet axis across the northern edge of the plateau, the disappearance of northerlies, and termination of the mei-yu holds on a range of time scales from interannual through seasonal and pentad. Diagnostic analysis indicates that the weakening of the meridional moisture contrast and meridional wind con- vergence, mainly resulting from the disappearance of northerlies, causes the demise of the mei-yu front. We propose that the westerlies migrating north of the plateau and consequent weakening of the extratropical northerlies triggers the mei-yu termination. Model simula- tions are employed to test the causality between the jet and the orographic downstream northerlies by repositioning the northern edge of the plateau. As the plateau edge extends northward, orographic forcing on the westerlies strengthens, leading to persistent strong downstream northerlies and a prolonged mei-yu. Idealized simulations with a dry dynamical core further demonstrate the dynamical link between the weakening of orographically forced downstream northerlies with the positioning of the jet from south to north of the plateau. Changes in the magnitude of orographically forced stationary waves are proposed to explain why the downstream northerlies disappear when the jet axis migrates beyond the northern edge of the plateau.

      Chapter 4 proposes an interpretation on the teleconnection between El Niño and East Asian early summer rainband. An intensification of East Asian rainfall usually occurs sum- mers following El Niño. We propose that this teleconnection is mediated via the westerlies impinging on the Tibetan Plateau, through El Niño’s control on tropical tropospheric tem- perature. This is distinct from previous studies that attribute the El Niño’s influence to changes in the Western Pacific subtropical anticyclone. The warming in the eastern equato- rial Pacific leads to uniform warming of the entire tropical troposphere, which sharpens the temperature gradient between the tropics and the subtropics and shifts the westerlies south- ward. The westerlies impinge over the Tibetan Plateau for longer and, through interaction with the topography, induce intense and persistent extratropical northerlies downstream of the plateau that in turn intensifies the East Asian rainband. The rainband has previously been shown to intensify in a similar manner in a warming climate, suggesting that the El Niño response provides an analog for future changes.

        Cover page: Westerly Jet and Seasonal Transitions of the East Asian Summer Monsoon
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2023)
        Abstract Understanding the roles of land surface conditions and atmospheric circulation on continental daily temperature variance is key to improving predictions of temperature extremes. Evaporative resistance (rs, hereafter), a function of the land cover type, reflects the ease with which water can be evaporated or transpired and is a strong control on land–atmosphere interactions. This study explores the effects of rs perturbations on summer daily temperature variance using the Simple Land Interface Model (SLIM) by mimicking, for rs only, a global land cover conversion from forest to crop/grassland. Decreasing rs causes a global cooling. The cooling is larger in wetter areas and weaker in drier areas, and primarily results from perturbations in shortwave radiation (SW) and latent heat flux (LH). Decreasing rs enhances cloud cover due to greater land surface evaporation and thus reduces incoming SW over most land areas. When rs decreases, wetter areas experience strong evaporative cooling, while drier areas become more moisture-limited and thus experience less cooling. Thermal advection further shapes the temperature response by damping the combined impacts of SW and LH. Temperature variance increases in drier areas and decreases in wetter areas as rs decreases. The temperature variance changes can be largely explained from changes in the combined variance of SW and LH, including an important contribution of changes in the covariance of SW and LH. In contrast, the effects of changes in thermal advection variance mainly affect the Northern Hemisphere midlatitudes. Significance Statement This study aims to better understand processes governing daily near-surface air temperature variance over land. We use an idealized modeling framework to explore the effects of land surface evaporative resistance (a parameter that controls how hard it is to evaporate water from the surface) on summer daily temperature variance. We find that a uniform decrease of evaporative resistance across the global land surface causes changes in the temperature variance that can be predicted from changes in the combined variance of shortwave radiation and latent heat flux. The variance of horizontal advection is important in altering the temperature variance in the Northern Hemisphere midlatitudes. Our findings shed light on predicting the characteristics of temperature variability as a function of surface conditions.
          Cover page: Understanding Responses of Summer Continental Daily Temperature Variance to Perturbations in the Land Surface Evaporative ResistanceCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          The Holocene East Asian summer monsoon (EASM) was previously characterized as a trend toward weaker monsoon intensity paced by orbital insolation. It is demonstrated here that this evolution is more accurately characterized as changes in the transition timing and duration of the EASM seasonal stages (spring, pre-mei-yu, mei-yu, midsummer), and tied to the north-south displacement of the westerlies relative to Tibet. To this end, time-slice simulations across the Holocene are employed using an atmospheric general circulation model. Self-organizing maps are used to objectively identify the transition timing and duration of the EASM seasonal stages. Compared to the late Holocene, an earlier onset of mei-yu and an earlier transition from mei-yu to midsummer in the early to mid-Holocene are found, resulting in a shortened mei-yu and prolonged midsummer stage. These changes are accompanied by an earlier northward positioning of the westerlies relative to Tibet. Invoking changes to seasonal transitions also provides a more satisfactory explanation for two key observations of Holocene East Asian climate: the "asynchronous Holocene optimum" and changes to dust emissions. A mechanism is proposed to explain the altered EASM seasonality in the simulated early to mid-Holocene. The insolation increase over the boreal summer reduces the pole-equator temperature gradient, leading to northward-shifted and weakened westerlies. The meridional position of the westerlies relative to the Tibetan Plateau determines the onset of mei-yu and possibly the onset of the midsummer stage. The northward shift in the westerlies triggers earlier seasonal rainfall transitions and, in particular, a shorter mei-yu and longer midsummer stage.
            Cover page: Seasonal Transitions and the Westerly Jet in the Holocene East Asian Summer Monsoon
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2019)
            Increased atmospheric moisture content and changing land-ocean thermal contrasts are usually implicated in the East Asian summer rainfall response to global warming. We highlight an additional influence whereby increased meridional stationary eddy circulation downstream of the Tibetan Plateau drives a stronger moisture flux convergence over southeastern China, intensifying the early summer rainband. This effect occurs robustly across the Community Earth System Model large ensemble Representative Control Pathway 8.5 simulations, and only for its pre-Meiyu stage (mid-May through late June). The westerly jet impinging on the Plateau is also enhanced, suggesting an orographic influence on the westerlies as the root cause. The increased westerlies result from warmer tropical tropospheric temperatures that enhance the meridional temperature gradient at the latitudes of the Plateau. Our results highlight the potential role of the westerlies in altering the East Asian early summer monsoon rainfall in the late 21st century.
              Cover page: Intensification of the Pre‐Meiyu Rainband in the Late 21st Century
              • Article
              • Peer Reviewed
              UC Riverside Previously Published Works (2021)
              In Arabidopsis thaliana, each ribosomal protein (RP) is encoded by a small gene family consisting of two or more highly homologous paralogues, which results in ribosome heterogeneity. It is largely unknown that how genes from multiple member containing RP families are regulated at transcriptional level to accommodate the needs of different plant organs and developmental stages. In this study, we investigated the transcript accumulation profiles of RP genes and found that the expression levels of RP genes are varied dramatically in different organs and developmental stages. Although most RP genes are found to be ubiquitously transcribed, some are obviously transcribed with spatiotemporal specificity. The hierarchical clustering trees of transcript accumulation intensity of RP genes revealed that different organs and developmental stages have different population of RP gene transcripts. By interrogating of the expression fluctuation trend of RP genes, we found that in spite of the fact that most groups of paralogous RP genes are transcribed in concerted manners, some RPs gene have contrasting expression patterns. When transcripts of paralogous RP genes from the same family are considered together, the expression level of most RP genes are well-matched but some are obviously higher or lower, therefore we speculate that some superfluous RPs may act outside the ribosome and a portion of ribosomes may lack one or even more RP(s). Altogether, our analysis results suggested that functional divergence may exist among heterogeneous ribosomes that resulted from different combination of RP paralogues, and substoichiometry of several RP gene families may lead to another layer of heterogeneous ribosomes which also have divergent functions in plants.
                Cover page: High resolution RNA‐seq profiling of genes encoding ribosomal proteins across different organs and developmental stages in Arabidopsis thaliana
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2020)
                The sea surface temperature (SST) contrast between the Northern Hemisphere (NH) and Southern Hemisphere (SH) influences the location of the intertropical convergence zone (ITCZ) and the intensity of the monsoon systems. This study examines the contributions of external forcing and unforced internal variability to the interhemispheric SST contrast in HadSST3 and ERSSTv5 observations, and 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) from 1881 to 2012. Using multimodel mean fingerprints, a significant influence of anthropogenic, but not natural, forcing is detected in the interhemispheric SST contrast, with the observed response larger than that of the model mean in ERSSTv5. The forced response consists of asymmetric NH–SH SST cooling from the mid-twentieth century to around 1980, followed by opposite NH–SH SST warming. The remaining best-estimate residual or unforced component is marked by NH–SH SST maxima in the 1930s and mid-1960s, and a rapid NH–SH SST decrease around 1970. Examination of decadal shifts in the observed interhemispheric SST contrast highlights the shift around 1970 as the most prominent from 1881 to 2012. Both NH and SH SST variability contributed to the shift, which appears not to be attributable to external forcings. Most models examined fail to capture such large-magnitude shifts in their control simulations, although some models with high interhemispheric SST variability are able to produce them. Large-magnitude shifts produced by the control simulations feature disparate spatial SST patterns, some of which are consistent with changes typically associated with the Atlantic meridional overturning circulation (AMOC).
                  Cover page: Forced and unforced decadal behavior of the interhemispheric SST contrast during the instrumental period (1881–2012): contextualizing the abrupt shift around 1970 Forced and unforced decadal behavior of the interhemispheric SST contrast during the instrumental period (1881–2012): contextualizing the abrupt shift around 1970
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2018)
                  Speleothem oxygen isotope records have revolutionized our understanding of the paleo East Asian monsoon, yet there is fundamental disagreement on what they represent in terms of the hydroclimate changes. We report a multiproxy speleothem record of monsoon evolution during the last deglaciation from the middle Yangtze region, which indicates a wetter central eastern China during North Atlantic cooling episodes, despite the oxygen isotopic record suggesting a weaker monsoon. We show that this apparent contradiction can be resolved if the changes are interpreted as a lengthening of the Meiyu rains and shortened post-Meiyu stage, in accordance with a recent hypothesis. Model simulations support this interpretation and further reveal the role of the westerlies in communicating the North Atlantic influence to the East Asian climate.
                    Cover page: East Asian hydroclimate modulated by the position of the westerlies during Termination I
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