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":"74 1","pages":""},"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":"39 1","pages":""},"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":"81 1","pages":""},"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":"6 1","pages":""},"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":"16 1","pages":""},"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":"66 1","pages":""},"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":"162 1","pages":""},"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 : 2024-05-13DOI: 10.1175/jcli-d-23-0562.1
Dingyu Ju, Jianqi Sun, Haixu Hong, Mengqi Zhang
Abstract Summer drought over northern Asia (NA) seriously threatens the local fragile ecological environment and social economy development. In this study, using the standardized precipitation evapotranspiration index (SPEI), we firstly identify the dominant modes of interannual variability in summer drought condition over NA, and then explore the atmospheric patterns responsible for the formation of the modes. The results show that the first empirical orthogonal function mode (EOF1) of summer SPEI over NA exhibits a meridional dipole pattern, which is influenced primarily by the Polar–Eurasian teleconnection (POL) and Circumglobal teleconnection (CGT) patterns. Under the influence of negative POL and positive CGT patterns, there is an anomalous anticyclone (cyclone) over northwestern Siberia (Lake Baikal to Northeast China). Such atmospheric circulations lead to meridional dipole patterns in air temperature, moisture condition, vertical motion, and cloud cover over NA, favoring decreased (increased) precipitation and increased (decreased) potential evapotranspiration over northern (southern) NA, finally contributing to the formation of EOF1. The EOF2 shows an approximate zonal dipole pattern, which is influenced by the British-Baikal Corridor (BBC) and Scandinavia teleconnection (SCA) patterns. The positive BBC and SCA patterns can lead to an anomalous anticyclone over the Ural Mountains and cyclone over the Lake Baikal. Such atmospheric circulations result in a zonal dipole pattern in precipitation and potential evapotranspiration over NA through changing the local moisture condition, air temperature, and radiation, consequently favoring the formation of EOF2. Fitting analysis indicates that the aforementioned atmospheric factors can explain 76% (55%) of the interannual variability of EOF1 (EOF2).
{"title":"Atmospheric teleconnections responsible for the dominant patterns of interannual variability in summer drought over northern Asia","authors":"Dingyu Ju, Jianqi Sun, Haixu Hong, Mengqi Zhang","doi":"10.1175/jcli-d-23-0562.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0562.1","url":null,"abstract":"Abstract Summer drought over northern Asia (NA) seriously threatens the local fragile ecological environment and social economy development. In this study, using the standardized precipitation evapotranspiration index (SPEI), we firstly identify the dominant modes of interannual variability in summer drought condition over NA, and then explore the atmospheric patterns responsible for the formation of the modes. The results show that the first empirical orthogonal function mode (EOF1) of summer SPEI over NA exhibits a meridional dipole pattern, which is influenced primarily by the Polar–Eurasian teleconnection (POL) and Circumglobal teleconnection (CGT) patterns. Under the influence of negative POL and positive CGT patterns, there is an anomalous anticyclone (cyclone) over northwestern Siberia (Lake Baikal to Northeast China). Such atmospheric circulations lead to meridional dipole patterns in air temperature, moisture condition, vertical motion, and cloud cover over NA, favoring decreased (increased) precipitation and increased (decreased) potential evapotranspiration over northern (southern) NA, finally contributing to the formation of EOF1. The EOF2 shows an approximate zonal dipole pattern, which is influenced by the British-Baikal Corridor (BBC) and Scandinavia teleconnection (SCA) patterns. The positive BBC and SCA patterns can lead to an anomalous anticyclone over the Ural Mountains and cyclone over the Lake Baikal. Such atmospheric circulations result in a zonal dipole pattern in precipitation and potential evapotranspiration over NA through changing the local moisture condition, air temperature, and radiation, consequently favoring the formation of EOF2. Fitting analysis indicates that the aforementioned atmospheric factors can explain 76% (55%) of the interannual variability of EOF1 (EOF2).","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"67 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-05-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140939653","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 Interdecadal variations of the global land monsoon have been previously attributed to internal fluctuations of the climate system, but the role of natural external forcings was under-explored. Here, we investigate this issue by using the Community Earth System Model ensemble simulations over the last millennium (950-1850 A.D.). Our analysis reveals that the surface temperature, with two dominant structures (global cooling/warming and longitudinal sea-surface temperature gradient in the tropical Pacific, which affects the Walker circulation), predominantly shapes the leading forced mode of the global land monsoon. This mode, representing 19% of the total variance, manifests as consistent features across South Asia, the southern part of East Asia, North Australia, South America, and western South Africa, contrasting with other monsoon regions. Under global cooling conditions, the monsoon intensity is enhanced in the northern parts of the East Asian and eastern parts of the North and South African monsoons, but it decreases in the other monsoon regions. Under weak Walker circulation conditions, changes in atmospheric circulation in response to the sea surface temperature gradient in the tropical Pacific are associated with a substantial attenuation of almost all land monsoon regions. It was further shown that the global mean surface temperature and the tropical Pacific temperature gradient jointly account for 75% of the total variance in the leading mode of the global land monsoon, with 29% and 46% as their respective contribution. Furthermore, our results suggest that volcanic eruptions are the dominant external forcing for these variations. These findings provide valuable insights for future research on global monsoon dynamics.
{"title":"Influence of Natural External Forcings on Interdecadal Variation of Global Land Monsoon over the last millennium in CESM-LME","authors":"Zhiyuan Wang, Laurent Z. X. Li, Xiaoyi Shi, Jianglin Wang, Jia Jia","doi":"10.1175/jcli-d-23-0443.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0443.1","url":null,"abstract":"Abstract Interdecadal variations of the global land monsoon have been previously attributed to internal fluctuations of the climate system, but the role of natural external forcings was under-explored. Here, we investigate this issue by using the Community Earth System Model ensemble simulations over the last millennium (950-1850 A.D.). Our analysis reveals that the surface temperature, with two dominant structures (global cooling/warming and longitudinal sea-surface temperature gradient in the tropical Pacific, which affects the Walker circulation), predominantly shapes the leading forced mode of the global land monsoon. This mode, representing 19% of the total variance, manifests as consistent features across South Asia, the southern part of East Asia, North Australia, South America, and western South Africa, contrasting with other monsoon regions. Under global cooling conditions, the monsoon intensity is enhanced in the northern parts of the East Asian and eastern parts of the North and South African monsoons, but it decreases in the other monsoon regions. Under weak Walker circulation conditions, changes in atmospheric circulation in response to the sea surface temperature gradient in the tropical Pacific are associated with a substantial attenuation of almost all land monsoon regions. It was further shown that the global mean surface temperature and the tropical Pacific temperature gradient jointly account for 75% of the total variance in the leading mode of the global land monsoon, with 29% and 46% as their respective contribution. Furthermore, our results suggest that volcanic eruptions are the dominant external forcing for these variations. These findings provide valuable insights for future research on global monsoon dynamics.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"25 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140939645","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 Coupled data assimilation (CDA) uses coupled model dynamics and physics to extract observational information from measured data in multiple Earth system domains to reconstruct historical states of the Earth system, forming a reanalysis of climate variability. Due to imperfect numerical schemes in modeling dynamics and physics, models are usually biased from the real world. Such model bias is a critical obstacle in the reconstruction of historical variability by combining model and observations, and, to some degree, causes divergence of CDA results because of individual model behavior in each system. Here, based on a multitimescale high-efficiency filtering algorithm which includes a deep ocean bias relaxing scheme, we first develop a high-efficiency online CDA system with the Community Earth System Model (CESM-MSHea-CDA). Then, together with the other previously-established CDA system (CM2-MSHea-CDA) within the Coupled Model version 2.1 model that is developed by Geophysical Fluid Dynamics Laboratory, we conduct climate reanalysis for the past four decades (1978–2018). Evaluations show that due to improved representation for multiscale background statistics and effective deep ocean model bias relaxing, both CDA systems produce convergent estimation of variability for major climate signals such as variability of basin-scale ocean heat content, ENSO, PDO, etc. Particularly, both CDA systems generate similar time-mean of global and Atlantic meridional overturning circulations that converge to the geostrophic velocity estimate from climatological temperature and salinity data. The CDA-estimated mass transport at typical measurement sections is mostly consistent with the observations.
{"title":"An Assessment of Convergence of Climate Reanalyses from Two Coupled Data Assimilation Systems with Identical High-Efficiently Filtering","authors":"Yingjing Jiang, Lv Lu, Shaoqing Zhang, Chenyu Zhu, Yang Gao, Zikuan Lin, Lingfeng Wan, Mingkui Li, Xiaolin Yu, Lixin Wu, Xiaopei Lin","doi":"10.1175/jcli-d-23-0423.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0423.1","url":null,"abstract":"Abstract Coupled data assimilation (CDA) uses coupled model dynamics and physics to extract observational information from measured data in multiple Earth system domains to reconstruct historical states of the Earth system, forming a reanalysis of climate variability. Due to imperfect numerical schemes in modeling dynamics and physics, models are usually biased from the real world. Such model bias is a critical obstacle in the reconstruction of historical variability by combining model and observations, and, to some degree, causes divergence of CDA results because of individual model behavior in each system. Here, based on a multitimescale high-efficiency filtering algorithm which includes a deep ocean bias relaxing scheme, we first develop a high-efficiency online CDA system with the Community Earth System Model (CESM-MSHea-CDA). Then, together with the other previously-established CDA system (CM2-MSHea-CDA) within the Coupled Model version 2.1 model that is developed by Geophysical Fluid Dynamics Laboratory, we conduct climate reanalysis for the past four decades (1978–2018). Evaluations show that due to improved representation for multiscale background statistics and effective deep ocean model bias relaxing, both CDA systems produce convergent estimation of variability for major climate signals such as variability of basin-scale ocean heat content, ENSO, PDO, etc. Particularly, both CDA systems generate similar time-mean of global and Atlantic meridional overturning circulations that converge to the geostrophic velocity estimate from climatological temperature and salinity data. The CDA-estimated mass transport at typical measurement sections is mostly consistent with the observations.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"24 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-05-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140939563","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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