Pub Date : 2024-06-03DOI: 10.1175/jcli-d-23-0335.1
Antonio Navarra, Joe Tribbia, Stefan Klus, Paula Lorenzo-Sánchez
Abstract The majority of dynamical systems arising from applications show a chaotic character. This is especially true for climate and weather applications. We present here an application of Koopman operator theory to tropical and global SST that yields an approximation to the continuous spectrum typical of these situations. We also show that the Koopman modes yield a decomposition of the data sets that can be used to categorize the variability. Most relevant modes emerge naturally and they can be identified easily. A difference with other analysis methods such as EOF or Fourier expansion is that the Koopman modes have a dynamical interpretation thanks to their connection to the Koopman operator and they are not constrained in their shape by special requirements such as orthogonality (as it is the case for EOF) or pure periodicity (as in the case of Fourier expansions). The pure periodic modes emerge naturally and they form a subspace that can be interpreted as the limiting subspace for the variability. The stationary states therefore are the scaffolding around which the dynamics takes place. The modes can also be traced to the NINO variability and in the case of the global SST to the PDO.
{"title":"Variability of SST through Koopman modes","authors":"Antonio Navarra, Joe Tribbia, Stefan Klus, Paula Lorenzo-Sánchez","doi":"10.1175/jcli-d-23-0335.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0335.1","url":null,"abstract":"Abstract The majority of dynamical systems arising from applications show a chaotic character. This is especially true for climate and weather applications. We present here an application of Koopman operator theory to tropical and global SST that yields an approximation to the continuous spectrum typical of these situations. We also show that the Koopman modes yield a decomposition of the data sets that can be used to categorize the variability. Most relevant modes emerge naturally and they can be identified easily. A difference with other analysis methods such as EOF or Fourier expansion is that the Koopman modes have a dynamical interpretation thanks to their connection to the Koopman operator and they are not constrained in their shape by special requirements such as orthogonality (as it is the case for EOF) or pure periodicity (as in the case of Fourier expansions). The pure periodic modes emerge naturally and they form a subspace that can be interpreted as the limiting subspace for the variability. The stationary states therefore are the scaffolding around which the dynamics takes place. The modes can also be traced to the NINO variability and in the case of the global SST to the PDO.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141259345","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-06-03DOI: 10.1175/jcli-d-23-0365.1
Guido Vettoretti, Roman Nuterman, Markus Jochum
Abstract Riverine outflow between the land surface/cryosphere and ocean undergoes intricate physical and biogeochemical transformations in estuaries before it finally merges with oceanic waters. To enhance our understanding of these transformations, Estuary Box Models (EBMs) are being incorporated into comprehensive Earth System Models. These models aim to refine our knowledge of both physical and biogeochemical processes. In our study, we conducted simulations using the Community Earth System Model Version 2, both with and without the inclusion of an EBM that was jointly developed by the University of Connecticut and the National Center for Atmospheric Research, and by default included in the climate model. The objective was to examine the influence of these modifications on global climate patterns. We performed these simulations under fixed atmospheric and runoff conditions, using a standalone version of the ocean/sea-ice components of the model. Additionally, we conducted a fully coupled Earth System Model simulation at a two-degree atmosphere and one-degree ocean resolution. The implementation of the EBM into the ocean component of the model resulted in regional variations and noticeable improvements in the salinity distribution on the Siberian shelves and at the Amazon outflow. Interestingly, our findings revealed that the tropical Atlantic Ocean plays a significant role in controlling the global salinity distribution. Due to the Tropical Atlantic circulation, which redirects thermocline water southward while allowing surface waters to continue northward, the improved vertical mixing in the EBM leads to an accumulation of salt in the North Atlantic and a freshening of other ocean basins. This shift subsequently results in an intensification of the Atlantic Meridional Overturning Circulation and a northward shift of tropical precipitation patterns.
{"title":"Impacts of Parameterizing Estuary Mixing on the Large-Scale Circulations in the Community Earth System Model","authors":"Guido Vettoretti, Roman Nuterman, Markus Jochum","doi":"10.1175/jcli-d-23-0365.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0365.1","url":null,"abstract":"Abstract Riverine outflow between the land surface/cryosphere and ocean undergoes intricate physical and biogeochemical transformations in estuaries before it finally merges with oceanic waters. To enhance our understanding of these transformations, Estuary Box Models (EBMs) are being incorporated into comprehensive Earth System Models. These models aim to refine our knowledge of both physical and biogeochemical processes. In our study, we conducted simulations using the Community Earth System Model Version 2, both with and without the inclusion of an EBM that was jointly developed by the University of Connecticut and the National Center for Atmospheric Research, and by default included in the climate model. The objective was to examine the influence of these modifications on global climate patterns. We performed these simulations under fixed atmospheric and runoff conditions, using a standalone version of the ocean/sea-ice components of the model. Additionally, we conducted a fully coupled Earth System Model simulation at a two-degree atmosphere and one-degree ocean resolution. The implementation of the EBM into the ocean component of the model resulted in regional variations and noticeable improvements in the salinity distribution on the Siberian shelves and at the Amazon outflow. Interestingly, our findings revealed that the tropical Atlantic Ocean plays a significant role in controlling the global salinity distribution. Due to the Tropical Atlantic circulation, which redirects thermocline water southward while allowing surface waters to continue northward, the improved vertical mixing in the EBM leads to an accumulation of salt in the North Atlantic and a freshening of other ocean basins. This shift subsequently results in an intensification of the Atlantic Meridional Overturning Circulation and a northward shift of tropical precipitation patterns.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141259346","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract This study investigates skill enhancement in operational seasonal forecasts of Beijing Climate Center’s Climate System Model through regional Climate-Weather Research and Forecasting (CWRF) downscaling and improved land initialization in China. The downscaling mitigates regional climate biases, enhancing precipitation pattern correlations by 0.29 in spring and 0.21 in summer. It also strengthens predictive capabilities for interannual anomalies, expanding skillful temperature forecast areas by 6% in spring and 12% in summer. Remarkably, during seven of ten years with relative high predictability, the downscaling increases average seasonal precipitation anomaly correlations by 0.22 and 0.25. Additionally, substitution of initial land conditions via a Common Land Model integration reduces snow cover and cold biases across the Tibetan Plateau and Mongolia-Northeast China, consistently contributing to CWRF’s overall enhanced forecasting capabilities. Improved downscaling predictive skill is attributed to CWRF’s enhanced physics representation, accurately capturing intricate regional interactions and associated teleconnections across China, especially linked to the Tibetan Plateau’s blocking and thermal effects. In summer, CWRF predicts an intensified South Asian High alongside a strengthened East Asian Jet compared to CSM, amplifying cold air advection and warm moisture transport over central to northeast regions. Consequently, rainfall distributions and interannual anomalies over these areas experience substantial improvements. Similar enhanced circulation processes elucidate skill improvement from land initialization, where accurate specification of initial snow cover and soil temperature within sensitive regions persists in influencing local and remote circulations extending beyond two seasons. Our findings emphasize the potential of improving physics representation and surface initialization to markedly enhance regional climate predictions.
{"title":"CWRF downscaling with improved land surface initialization enhances spring-summer seasonal climate prediction skill in China","authors":"Han Zhang, Xin-Zhong Liang, Yongjiu Dai, Lianchun Song, Qingquan Li, Fang Wang, Shulei Zhang","doi":"10.1175/jcli-d-23-0565.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0565.1","url":null,"abstract":"Abstract This study investigates skill enhancement in operational seasonal forecasts of Beijing Climate Center’s Climate System Model through regional Climate-Weather Research and Forecasting (CWRF) downscaling and improved land initialization in China. The downscaling mitigates regional climate biases, enhancing precipitation pattern correlations by 0.29 in spring and 0.21 in summer. It also strengthens predictive capabilities for interannual anomalies, expanding skillful temperature forecast areas by 6% in spring and 12% in summer. Remarkably, during seven of ten years with relative high predictability, the downscaling increases average seasonal precipitation anomaly correlations by 0.22 and 0.25. Additionally, substitution of initial land conditions via a Common Land Model integration reduces snow cover and cold biases across the Tibetan Plateau and Mongolia-Northeast China, consistently contributing to CWRF’s overall enhanced forecasting capabilities. Improved downscaling predictive skill is attributed to CWRF’s enhanced physics representation, accurately capturing intricate regional interactions and associated teleconnections across China, especially linked to the Tibetan Plateau’s blocking and thermal effects. In summer, CWRF predicts an intensified South Asian High alongside a strengthened East Asian Jet compared to CSM, amplifying cold air advection and warm moisture transport over central to northeast regions. Consequently, rainfall distributions and interannual anomalies over these areas experience substantial improvements. Similar enhanced circulation processes elucidate skill improvement from land initialization, where accurate specification of initial snow cover and soil temperature within sensitive regions persists in influencing local and remote circulations extending beyond two seasons. Our findings emphasize the potential of improving physics representation and surface initialization to markedly enhance regional climate predictions.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141194061","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-05-30DOI: 10.1175/jcli-d-22-0955.1
Suqin Q. Duan, Karen A. McKinnon, Isla R. Simpson
Abstract Climate change projections show amplified warming associated with dry conditions over tropical land. We compare two perspectives explaining this amplified warming: one based on tropical atmospheric dynamics, and the other focusing on soil moisture and surface fluxes. We first compare the full spatiotemporal distribution of changes in key variables in the two perspectives under a quadrupling of CO2 using daily output from the CMIP6 simulations. Both perspectives center around the partitioning of the total energy/energy flux into the temperature and humidity components. We examine the contribution of this temperature/humidity partitioning in the base climate and its change under warming to rising temperatures by deriving a diagnostic linearized perturbation model that relates the magnitude of warming to (1) changes in the total energy/energy flux, (2) the base-climate temperature/humidity partitioning, and (3) changes in the partitioning under warming. We show that the spatiotemporal structure of warming in CMIP6 models is well predicted by the inverse of the base-climate partition factor, which we term the base-climate sensitivity: conditions that are drier in the base climate have a higher base-climate sensitivity and experience more warming. On top of this relationship, changes in the partition factor under intermediate (between wet and dry) surface conditions further enhance or dampen the warming. We discuss the mechanistic link between the two perspectives by illustrating the strong relationships between lower tropospheric temperature lapse rates, a key variable for the atmospheric perspective, and surfaces fluxes, a key component of the land surface perspective.
{"title":"Two perspectives on amplified warming over tropical land examined in CMIP6 models","authors":"Suqin Q. Duan, Karen A. McKinnon, Isla R. Simpson","doi":"10.1175/jcli-d-22-0955.1","DOIUrl":"https://doi.org/10.1175/jcli-d-22-0955.1","url":null,"abstract":"Abstract Climate change projections show amplified warming associated with dry conditions over tropical land. We compare two perspectives explaining this amplified warming: one based on tropical atmospheric dynamics, and the other focusing on soil moisture and surface fluxes. We first compare the full spatiotemporal distribution of changes in key variables in the two perspectives under a quadrupling of CO2 using daily output from the CMIP6 simulations. Both perspectives center around the partitioning of the total energy/energy flux into the temperature and humidity components. We examine the contribution of this temperature/humidity partitioning in the base climate and its change under warming to rising temperatures by deriving a diagnostic linearized perturbation model that relates the magnitude of warming to (1) changes in the total energy/energy flux, (2) the base-climate temperature/humidity partitioning, and (3) changes in the partitioning under warming. We show that the spatiotemporal structure of warming in CMIP6 models is well predicted by the inverse of the base-climate partition factor, which we term the base-climate sensitivity: conditions that are drier in the base climate have a higher base-climate sensitivity and experience more warming. On top of this relationship, changes in the partition factor under intermediate (between wet and dry) surface conditions further enhance or dampen the warming. We discuss the mechanistic link between the two perspectives by illustrating the strong relationships between lower tropospheric temperature lapse rates, a key variable for the atmospheric perspective, and surfaces fluxes, a key component of the land surface perspective.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141194199","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-05-27DOI: 10.1175/jcli-d-23-0437.1
Martin Jucker, Chris Lucas, Deepashree Dutta
Abstract The amount of water vapor injected into the stratosphere after the eruption of Hunga Tonga-Hunga Ha’apai (HTHH) was unprecedented, and it is therefore unclear what it might mean for surface climate. We use chemistry climate model simulations to assess the long-term surface impacts of stratospheric water vapor (SWV) anomalies similar to those caused by HTHH, but neglect the relatively minor aerosol loading from the eruption. The simulations show that the SWV anomalies lead to strong and persistent warming of Northern Hemisphere landmasses in boreal winter, and austral winter cooling over Australia, years after eruption, demonstrating that large SWV forcing can have surface impacts on a decadal timescale. We also emphasize that the surface response to SWV anomalies is more complex than simple warming due to greenhouse forcing and is influenced by factors such as regional circulation patterns and cloud feedbacks. Further research is needed to fully understand the multi-year effects of SWV anomalies and their relationship with climate phenomena like El Nino Southern Oscillation.
{"title":"Long-term climate impacts of large stratospheric water vapor perturbations","authors":"Martin Jucker, Chris Lucas, Deepashree Dutta","doi":"10.1175/jcli-d-23-0437.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0437.1","url":null,"abstract":"Abstract The amount of water vapor injected into the stratosphere after the eruption of Hunga Tonga-Hunga Ha’apai (HTHH) was unprecedented, and it is therefore unclear what it might mean for surface climate. We use chemistry climate model simulations to assess the long-term surface impacts of stratospheric water vapor (SWV) anomalies similar to those caused by HTHH, but neglect the relatively minor aerosol loading from the eruption. The simulations show that the SWV anomalies lead to strong and persistent warming of Northern Hemisphere landmasses in boreal winter, and austral winter cooling over Australia, years after eruption, demonstrating that large SWV forcing can have surface impacts on a decadal timescale. We also emphasize that the surface response to SWV anomalies is more complex than simple warming due to greenhouse forcing and is influenced by factors such as regional circulation patterns and cloud feedbacks. Further research is needed to fully understand the multi-year effects of SWV anomalies and their relationship with climate phenomena like El Nino Southern Oscillation.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141170864","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract The spring Pacific meridional mode (PMM) is an important precursor of El Niño–Southern Oscillation (ENSO). However, recent studies reported that only about half of the spring PMM events were followed by ENSO events. This study examines the role of internal climate variability in modulating the impact of PMM on ENSO using 100-member ensemble simulations of the Max Planck Institute Earth System Model (MPI-ESM). The relationship between spring PMM and following winter ENSO shows a large spread among the 100 members. The variation of spring Aleutian low (AL) intensity is identified to be an important factor modulating the PMM–ENSO relation. The spring AL affects the PMM–ENSO relationship by modifying PMM-generated low-level zonal wind anomalies over the tropical western Pacific. The strengthening of the spring AL is accompanied by westerly wind anomalies over the midlatitude northwestern Pacific, leading to sea surface temperature (SST) cooling there via an enhancement of upward surface heat flux. This results in increased meridional SST gradient and leads to northerly wind anomalies over the subtropical northwestern Pacific, which turn to surface westerly wind anomalies after reaching the equatorial western Pacific due to the conservation of potential vorticity. Thus, the low-level westerly (easterly) wind anomalies over the tropical western Pacific associated with the positive (negative) spring PMM were strengthened (weakened), which further contributes to an enhanced (a weakened) PMM–ENSO relation. The mechanism for the modulation of the AL on the spring PMM–ENSO relationship is verified by a set of AGCM simulations. This study suggests that the condition of the spring AL should be considered when predicting ENSO on the basis of the PMM. Significance Statement Spring Pacific meridional mode (PMM) is a predictor of ENSO, but not all spring PMM events are accompanied by the occurrence of ENSO events. This study aims to explore the influence of internal climate variability on the relationship between spring PMM and following ENSO. It is revealed that the Aleutian low exerts a crucial modulation on the spring PMM–ENSO relationship. The underlying physical mechanisms for the impact of the Aleutian low on the relationship between spring PMM and ENSO are further examined. The results of this study have important implications for improving the prediction of ENSO.
{"title":"The Role of the Aleutian Low in the Relationship between Spring Pacific Meridional Mode and Following ENSO","authors":"Yuqiong Zheng, Shangfeng Chen, Wen Chen, Renguang Wu, Zhibiao Wang, Bin Yu, Peng Hu, Jinling Piao","doi":"10.1175/jcli-d-23-0440.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0440.1","url":null,"abstract":"Abstract The spring Pacific meridional mode (PMM) is an important precursor of El Niño–Southern Oscillation (ENSO). However, recent studies reported that only about half of the spring PMM events were followed by ENSO events. This study examines the role of internal climate variability in modulating the impact of PMM on ENSO using 100-member ensemble simulations of the Max Planck Institute Earth System Model (MPI-ESM). The relationship between spring PMM and following winter ENSO shows a large spread among the 100 members. The variation of spring Aleutian low (AL) intensity is identified to be an important factor modulating the PMM–ENSO relation. The spring AL affects the PMM–ENSO relationship by modifying PMM-generated low-level zonal wind anomalies over the tropical western Pacific. The strengthening of the spring AL is accompanied by westerly wind anomalies over the midlatitude northwestern Pacific, leading to sea surface temperature (SST) cooling there via an enhancement of upward surface heat flux. This results in increased meridional SST gradient and leads to northerly wind anomalies over the subtropical northwestern Pacific, which turn to surface westerly wind anomalies after reaching the equatorial western Pacific due to the conservation of potential vorticity. Thus, the low-level westerly (easterly) wind anomalies over the tropical western Pacific associated with the positive (negative) spring PMM were strengthened (weakened), which further contributes to an enhanced (a weakened) PMM–ENSO relation. The mechanism for the modulation of the AL on the spring PMM–ENSO relationship is verified by a set of AGCM simulations. This study suggests that the condition of the spring AL should be considered when predicting ENSO on the basis of the PMM. Significance Statement Spring Pacific meridional mode (PMM) is a predictor of ENSO, but not all spring PMM events are accompanied by the occurrence of ENSO events. This study aims to explore the influence of internal climate variability on the relationship between spring PMM and following ENSO. It is revealed that the Aleutian low exerts a crucial modulation on the spring PMM–ENSO relationship. The underlying physical mechanisms for the impact of the Aleutian low on the relationship between spring PMM and ENSO are further examined. The results of this study have important implications for improving the prediction of ENSO.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141170861","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract Previous studies have pointed out that the tropical easterly jet (TEJ) core varies longitudinally or latitudinally. Whether there is a linkage between longitudinal and latitudinal variations of the TEJ core remains unclear. We found that, on the interannual time scale, the northward (southward) movement of the TEJ core is typically accompanied by a westward (eastward) shift, characterized by a noticeable northwest–southeast (NW–SE) displacement. This NW–SE shift is most evident in July. A locational index is defined to capture this shift by the difference of area-averaged 200-hPa zonal winds between the western Arabian Sea (AS) and the southern tip of the Indian Peninsula. Observations and numerical simulations demonstrated that the northwestward-shifted (southeastward-shifted) TEJ core is caused by the joint and individual influences from the enhanced (suppressed) convective activities over the eastern AS and suppressed (enhanced) convective activities over the northern Bay of Bengal–South China Sea (BOB–SCS). Enhanced (suppressed) convective activities over the eastern AS can induce upper-tropospheric divergence (convergence) and anticyclonic (cyclonic) circulations to the northwest of the convection, leading to anomalous easterly (westerly) over the western AS. The suppressed (enhanced) convective activities over the northern BOB–SCS can further facilitate the northwestward (southeastward) shift through inducing anomalous cyclonic (anticyclonic) circulation centering at the BOB and the associated anomalous westerly (easterly) over the southern tip of the Indian Peninsula. The NW–SE shift of the TEJ core may have an implication for the change in the area of the intense rainfall in South Asia. Significance Statement The purpose of this study is to explore the linkage between the zonal and meridional variations of the core of the tropical easterly jet (TEJ) and its underlying mechanisms. We found that the TEJ core features a pronounced northwest–southeast shift and this phenomenon only occurs in July. Thus, we defined a locational index to depict this unique characteristic and reveal its relationship with the anomalous convective activities over the eastern Arabian Sea and the northern Bay of Bengal–South China Sea. These results may help improve our understanding of the characteristics and mechanisms of the variations of the TEJ core.
摘要 以前的研究指出，热带东风喷流（TEJ）核心在经度或纬度上都有变化。TEJ核心的经度和纬度变化之间是否存在联系仍不清楚。我们发现，在年际时间尺度上，TEJ 核心向北（向南）移动通常伴随着向西（向东）移动，其特征是明显的西北-东南（NW-SE）位移。这种西北-东南位移在 7 月份最为明显。通过阿拉伯海西部（AS）和印度半岛南端之间的 200 hPa 区域平均地带风的差异，定义了一个定位指数来捕捉这种移动。观测和数值模拟表明，TEJ 核心向西北偏移（向东南偏移）是由阿拉伯海东部对流活动增强（抑制）和孟加拉湾-南海北部对流活动抑制（增强）共同和单独影响造成的。AS 东部上空增强（抑制）的对流活动会诱发对流层上层辐合（辐合）和对流西北部的反气旋（气旋）环流，导致 AS 西部上空出现异常偏东（偏西）气流。BOB-SCS北部上空被抑制（增强）的对流活动可通过诱发以BOB为中心的异常气旋（反气旋）环流和印度半岛南端上空的相关异常西风（东风），进一步促进西北（东南）偏移。TEJ 核心的西北-东南移动可能会对南亚强降雨区域的变化产生影响。意义说明 本研究的目的是探讨热带偏东气流（TEJ）核心的地带性和经向变化之间的联系及其内在机制。我们发现，TEJ核心具有明显的西北-东南偏移特征，而且这种现象只出现在7月份。因此，我们定义了一个位置指数来描述这一独特特征，并揭示了它与阿拉伯海东部和孟加拉湾-南海北部异常对流活动的关系。这些结果可能有助于提高我们对 TEJ 核心变化特征和机制的认识。
{"title":"Characteristics and Mechanisms of the Interannual Variability of the Northwest–Southeast Shift of the Tropical Easterly Jet’s Core in July","authors":"Shihua Liu, Sihua Huang, Yanke Tan, Zhiping Wen, Xiaodan Chen, Yuanyuan Guo","doi":"10.1175/jcli-d-23-0291.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0291.1","url":null,"abstract":"Abstract Previous studies have pointed out that the tropical easterly jet (TEJ) core varies longitudinally or latitudinally. Whether there is a linkage between longitudinal and latitudinal variations of the TEJ core remains unclear. We found that, on the interannual time scale, the northward (southward) movement of the TEJ core is typically accompanied by a westward (eastward) shift, characterized by a noticeable northwest–southeast (NW–SE) displacement. This NW–SE shift is most evident in July. A locational index is defined to capture this shift by the difference of area-averaged 200-hPa zonal winds between the western Arabian Sea (AS) and the southern tip of the Indian Peninsula. Observations and numerical simulations demonstrated that the northwestward-shifted (southeastward-shifted) TEJ core is caused by the joint and individual influences from the enhanced (suppressed) convective activities over the eastern AS and suppressed (enhanced) convective activities over the northern Bay of Bengal–South China Sea (BOB–SCS). Enhanced (suppressed) convective activities over the eastern AS can induce upper-tropospheric divergence (convergence) and anticyclonic (cyclonic) circulations to the northwest of the convection, leading to anomalous easterly (westerly) over the western AS. The suppressed (enhanced) convective activities over the northern BOB–SCS can further facilitate the northwestward (southeastward) shift through inducing anomalous cyclonic (anticyclonic) circulation centering at the BOB and the associated anomalous westerly (easterly) over the southern tip of the Indian Peninsula. The NW–SE shift of the TEJ core may have an implication for the change in the area of the intense rainfall in South Asia. Significance Statement The purpose of this study is to explore the linkage between the zonal and meridional variations of the core of the tropical easterly jet (TEJ) and its underlying mechanisms. We found that the TEJ core features a pronounced northwest–southeast shift and this phenomenon only occurs in July. Thus, we defined a locational index to depict this unique characteristic and reveal its relationship with the anomalous convective activities over the eastern Arabian Sea and the northern Bay of Bengal–South China Sea. These results may help improve our understanding of the characteristics and mechanisms of the variations of the TEJ core.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141148920","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-05-22DOI: 10.1175/jcli-d-23-0749.1
Alain T. Tamoffo, T. Weber, W. Cabos, P. Monerie, Kerry H. Cook, Dmitry V. Sein, A. Dosio, N. Klutse, A. A. Akinsanola, D. Jacob
This study explores the added value (AV) of a regional earth system model (ESM) compared to an atmosphere-only regional climate model (RCM) in simulating West African Monsoon (WAM) rainfall. The primary goals are to foster discussions on the suitability of coupled RCMs for WAM projections and deepen our understanding of ocean-atmosphere coupling’s influence on the WAM system. The study employs results from dynamical downscaling of the ERA-Interim reanalysis and Max Plank Institute ESM (MPI-ESM-LR) by two RCMs, REMO (atmosphere-only) and ROM (REMO coupled with Max Planck Institute Ocean Model; MPIOM), at ∼25-km horizontal resolution. Results show that in regions distant from coupling domain boundaries such as West Africa (WA), constraint conditions from ERA-Interim are more beneficial than coupling effects. REMO, reliant on oceanic sea surface temperatures (SSTs) from observations and influenced by ERA-Interim, is biased under coupling conditions, although coupling offers potential advantages in representing heat and mass fluxes. Contrastingly, as intended, coupling improves SSTs-monsoon fluxes’ relationships under ESM-forced conditions. In this latter case, coupling features a dipole-like spatial structure of AV, improving precipitation over the Guinean coast but degrading precipitation over half of the Sahel. Our extensive examination of physical processes and mechanisms underpinning the WAM system supports the plausibility of AV. Additionally, we found that the monsoonal dynamics over the ocean respond to convective activity, with the Sahara-Sahel surface temperature gradient serving as the maintenance mechanism. While further efforts are needed to enhance the coupled RCM, we advocate for its use in the context of WAM rainfall forecasts and projections.
{"title":"West African Monsoon System’s Responses to Global Ocean-Regional Atmosphere Coupling","authors":"Alain T. Tamoffo, T. Weber, W. Cabos, P. Monerie, Kerry H. Cook, Dmitry V. Sein, A. Dosio, N. Klutse, A. A. Akinsanola, D. Jacob","doi":"10.1175/jcli-d-23-0749.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0749.1","url":null,"abstract":"\u0000This study explores the added value (AV) of a regional earth system model (ESM) compared to an atmosphere-only regional climate model (RCM) in simulating West African Monsoon (WAM) rainfall. The primary goals are to foster discussions on the suitability of coupled RCMs for WAM projections and deepen our understanding of ocean-atmosphere coupling’s influence on the WAM system. The study employs results from dynamical downscaling of the ERA-Interim reanalysis and Max Plank Institute ESM (MPI-ESM-LR) by two RCMs, REMO (atmosphere-only) and ROM (REMO coupled with Max Planck Institute Ocean Model; MPIOM), at ∼25-km horizontal resolution. Results show that in regions distant from coupling domain boundaries such as West Africa (WA), constraint conditions from ERA-Interim are more beneficial than coupling effects. REMO, reliant on oceanic sea surface temperatures (SSTs) from observations and influenced by ERA-Interim, is biased under coupling conditions, although coupling offers potential advantages in representing heat and mass fluxes. Contrastingly, as intended, coupling improves SSTs-monsoon fluxes’ relationships under ESM-forced conditions. In this latter case, coupling features a dipole-like spatial structure of AV, improving precipitation over the Guinean coast but degrading precipitation over half of the Sahel. Our extensive examination of physical processes and mechanisms underpinning the WAM system supports the plausibility of AV. Additionally, we found that the monsoonal dynamics over the ocean respond to convective activity, with the Sahara-Sahel surface temperature gradient serving as the maintenance mechanism. While further efforts are needed to enhance the coupled RCM, we advocate for its use in the context of WAM rainfall forecasts and projections.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141111709","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Tibetan Plateau’s (TP) topography has long been recognized for its impact on climate. However, recognition of the influence of the TP on global weather variability remains insufficient. Therefore, this study used numerical simulations to demonstrate the influences of the TP and its mechanical and thermal forcing on global high-frequency temperature variability and eddy kinetic energy (EKE). Despite local influences, the TP influenced the high-frequency temperature variability in far-flung regions like North America. In summer, the TP’s influence on high-frequency temperature variability showed dipole patterns in Eurasia and tripole patterns in North America, which were mainly induced by TP thermal forcing. In winter, the TP’s influence on high-frequency temperature variability was dominated by mechanical forcing and was less significant for remote regions than in summer. Mechanical forcing dominated EKE in both summer and winter. Furthermore, the horizontal temperature advection dominated the TP’s influence on high-frequency temperature variability for both its thermal effect in summer and its mechanical effect in winter, wherein EKE, as the dynamical factor, determined the horizontal temperature advection rather than the thermodynamical factor, the temperature gradient. Our findings suggest that the TP, via its mechanical and thermal forcing, may have an impact on temperature-related weather extremes around the world.
{"title":"Impact of the Tibetan Plateau on Global High-frequency Temperature Variability","authors":"Zifan Su, Yongkun Xie, Jianping Huang, Guoxiong Wu, Yuzhi Liu, X. Guan","doi":"10.1175/jcli-d-23-0271.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0271.1","url":null,"abstract":"\u0000The Tibetan Plateau’s (TP) topography has long been recognized for its impact on climate. However, recognition of the influence of the TP on global weather variability remains insufficient. Therefore, this study used numerical simulations to demonstrate the influences of the TP and its mechanical and thermal forcing on global high-frequency temperature variability and eddy kinetic energy (EKE). Despite local influences, the TP influenced the high-frequency temperature variability in far-flung regions like North America. In summer, the TP’s influence on high-frequency temperature variability showed dipole patterns in Eurasia and tripole patterns in North America, which were mainly induced by TP thermal forcing. In winter, the TP’s influence on high-frequency temperature variability was dominated by mechanical forcing and was less significant for remote regions than in summer. Mechanical forcing dominated EKE in both summer and winter. Furthermore, the horizontal temperature advection dominated the TP’s influence on high-frequency temperature variability for both its thermal effect in summer and its mechanical effect in winter, wherein EKE, as the dynamical factor, determined the horizontal temperature advection rather than the thermodynamical factor, the temperature gradient. Our findings suggest that the TP, via its mechanical and thermal forcing, may have an impact on temperature-related weather extremes around the world.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141115348","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-05-21DOI: 10.1175/jcli-d-23-0429.1
Chen Liu, Lei Chen, Stefan Liess
The features of large-scale atmospheric circulations, storm tracks, and the mean flow-eddy interaction during winter Pacific-North American (PNA) events are investigated using National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data at subseasonal timescale from 1979 to 2022. The day-to-day variations of storm-track activity and stream function reveal that storm-track activity varies along the evolution of mean flow. To better understand storm track variability with the mean flow-eddy interaction, further exploration is made by analyzing local energy energetics. The changes in horizontal and vertical baroclinic energy conversions from background flow correspond to the storm track anomalies over the North Pacific, indicating that the anomalies in storm tracks are due to the anomalous mean flow associated with PNA patterns impacting energy conversion through mean flow-eddy interaction. Eddy feedback driven by vorticity and heat fluxes is analyzed. This provides a concrete illustration of how eddy feedback serves as a positive factor for the upper-tropospheric circulation anomalies associated with the PNA pattern.
{"title":"Deciphering Mean Flow-Eddy Interaction in Pacific-North American Teleconnection Linked to Storm Tracks at Subseasonal Timescale","authors":"Chen Liu, Lei Chen, Stefan Liess","doi":"10.1175/jcli-d-23-0429.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0429.1","url":null,"abstract":"\u0000The features of large-scale atmospheric circulations, storm tracks, and the mean flow-eddy interaction during winter Pacific-North American (PNA) events are investigated using National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data at subseasonal timescale from 1979 to 2022. The day-to-day variations of storm-track activity and stream function reveal that storm-track activity varies along the evolution of mean flow. To better understand storm track variability with the mean flow-eddy interaction, further exploration is made by analyzing local energy energetics. The changes in horizontal and vertical baroclinic energy conversions from background flow correspond to the storm track anomalies over the North Pacific, indicating that the anomalies in storm tracks are due to the anomalous mean flow associated with PNA patterns impacting energy conversion through mean flow-eddy interaction. Eddy feedback driven by vorticity and heat fluxes is analyzed. This provides a concrete illustration of how eddy feedback serves as a positive factor for the upper-tropospheric circulation anomalies associated with the PNA pattern.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141113796","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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