Pub Date : 2024-04-05DOI: 10.1175/jcli-d-23-0739.1
Valentina Ortiz-Guzmán, Martin Jucker, Steven Sherwood
Abstract The Southern Hemisphere climate and weather are affected by several modes of variability and climate phenomena across different time and spatial scales. An additional key component of the atmosphere dynamics that greatly influences weather is quasi-stationary Rossby waves, which attract particular interest as they are often associated with synoptic scale extreme events. In the Southern Hemisphere extratropical circulation, the most prominent quasi-stationary Rossby wave pattern is the zonal wavenumber 3 (ZW3), which has been shown to have impacts on meridional heat and momentum transport in mid to high-latitudes, and on Antarctic sea-ice extent. However, little is known about its impacts outside of polar regions. In this work, we use ERA5 reanalysis data on monthly time scales to explore the influence of phase and amplitude of ZW3 on temperature and precipitation across the Southern Hemisphere midlatitudes. Our results show significant impact in various regions for all seasons. One of the most substantial effects is observed in precipitation over southeastern Brazil during austral summer, where different phases of the ZW3 force opposite anomalies. When using ZW3 phase and amplitude as prior information, the probability of occurrence of precipitation extremes in this region increases up to three times. Additionally, we find that this ZW3 weather signature is largely independent of the zonally symmetric Southern Annular Mode (SAM); neither does it seem to be linked to El Niño Southern Oscillation (ENSO) or Indian Ocean Dipole (IOD) signal.
{"title":"Zonal Wavenumber 3 forces extreme precipitation in South America","authors":"Valentina Ortiz-Guzmán, Martin Jucker, Steven Sherwood","doi":"10.1175/jcli-d-23-0739.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0739.1","url":null,"abstract":"Abstract The Southern Hemisphere climate and weather are affected by several modes of variability and climate phenomena across different time and spatial scales. An additional key component of the atmosphere dynamics that greatly influences weather is quasi-stationary Rossby waves, which attract particular interest as they are often associated with synoptic scale extreme events. In the Southern Hemisphere extratropical circulation, the most prominent quasi-stationary Rossby wave pattern is the zonal wavenumber 3 (ZW3), which has been shown to have impacts on meridional heat and momentum transport in mid to high-latitudes, and on Antarctic sea-ice extent. However, little is known about its impacts outside of polar regions. In this work, we use ERA5 reanalysis data on monthly time scales to explore the influence of phase and amplitude of ZW3 on temperature and precipitation across the Southern Hemisphere midlatitudes. Our results show significant impact in various regions for all seasons. One of the most substantial effects is observed in precipitation over southeastern Brazil during austral summer, where different phases of the ZW3 force opposite anomalies. When using ZW3 phase and amplitude as prior information, the probability of occurrence of precipitation extremes in this region increases up to three times. Additionally, we find that this ZW3 weather signature is largely independent of the zonally symmetric Southern Annular Mode (SAM); neither does it seem to be linked to El Niño Southern Oscillation (ENSO) or Indian Ocean Dipole (IOD) signal.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"8 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140572036","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 : 2024-04-04DOI: 10.1175/jcli-d-23-0672.1
Shaohua Chen, Haikun Zhao, Philip J. Klotzbach, Jian Cao, Jia Liang, Weican Zhou, Liguang Wu
Abstract On inter-annual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave train (SSW) relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG-TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of the large-scale circulation and tropical cyclones.
{"title":"Western North Pacific monsoon vorticity distribution as a potential driver of interannual meridional migration of the boreal summer synoptic scale wave train","authors":"Shaohua Chen, Haikun Zhao, Philip J. Klotzbach, Jian Cao, Jia Liang, Weican Zhou, Liguang Wu","doi":"10.1175/jcli-d-23-0672.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0672.1","url":null,"abstract":"Abstract On inter-annual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave train (SSW) relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG-TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of the large-scale circulation and tropical cyclones.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"21 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140572011","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 : 2024-04-03DOI: 10.1175/jcli-d-23-0462.1
Yinxing Liu, Zhiwei Zhang, Qingguo Yuan, Wei Zhao
Abstract Meridional heat transport induced by oceanic mesoscale eddies (EHT) plays a significant role in the heat budget of Southern Ocean (SO) but the decadal trends in EHT and its associated mechanisms are still obscure. Here, this scientific issue is investigated by combining concurrent satellite observations and Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) reanalysis data over the 24 years between 1993–2016. The results reveal that the surface EHT from both satellite and ECCO2 data consistently show decadal poleward increasing trends in the SO, particularly in the latitude band of Antarctic Circumpolar Current (ACC). In terms of average in the ACC band, the ECCO2-derived EHT over the upper 1000 m has a linear trend of 1.1×10−2 PW per decade or 16% per decade compared with its time-mean value of 0.07 PW. Diagnostic analysis based on “mixing length” theory suggests that the decadal strengthening eddy kinetic energy (EKE) is the dominant mechanism for the increase of EHT in the SO. By performing energy budget analysis, we further find that the decadal increase of EKE is mainly caused by the strengthened baroclinic instability of large-scale circulation that converts more available potential energy to EKE. For the strengthened baroclinic instability in the SO, it is attributed to the increasing large-scale wind stress work on the large-scale circulation corresponding to the positive phase of Southern Annular Mode between 1993–2016. The decadal trends in EHT identified here may help understand decadal variations of heat storage and sea-ice extent in the SO.
{"title":"Decadal trends in the Southern Ocean meridional eddy heat transport","authors":"Yinxing Liu, Zhiwei Zhang, Qingguo Yuan, Wei Zhao","doi":"10.1175/jcli-d-23-0462.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0462.1","url":null,"abstract":"Abstract Meridional heat transport induced by oceanic mesoscale eddies (EHT) plays a significant role in the heat budget of Southern Ocean (SO) but the decadal trends in EHT and its associated mechanisms are still obscure. Here, this scientific issue is investigated by combining concurrent satellite observations and Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) reanalysis data over the 24 years between 1993–2016. The results reveal that the surface EHT from both satellite and ECCO2 data consistently show decadal poleward increasing trends in the SO, particularly in the latitude band of Antarctic Circumpolar Current (ACC). In terms of average in the ACC band, the ECCO2-derived EHT over the upper 1000 m has a linear trend of 1.1×10−2 PW per decade or 16% per decade compared with its time-mean value of 0.07 PW. Diagnostic analysis based on “mixing length” theory suggests that the decadal strengthening eddy kinetic energy (EKE) is the dominant mechanism for the increase of EHT in the SO. By performing energy budget analysis, we further find that the decadal increase of EKE is mainly caused by the strengthened baroclinic instability of large-scale circulation that converts more available potential energy to EKE. For the strengthened baroclinic instability in the SO, it is attributed to the increasing large-scale wind stress work on the large-scale circulation corresponding to the positive phase of Southern Annular Mode between 1993–2016. The decadal trends in EHT identified here may help understand decadal variations of heat storage and sea-ice extent in the SO.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"233 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140571922","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 : 2024-04-01DOI: 10.1175/jcli-d-23-0431.1
Mengyu Wei, Jun Yang, Yongyun Hu, Yonggang Liu, Shineng Hu, Xiang Li, Jiawenjing Lan, Jiaqi Guo, Shuai Yuan, Ji Nie
Abstract Both observations and simulations show that under global warming there exists warming deficit in the North Atlantic, known as the North Atlantic warming hole (NAWH). Here we show that similar warming hole occurs in the sub-polar Pacific ocean of paleo-climate simulations. As solar constant is increased, local surface becomes substantially cooler rather than warmer in the sub-polar paleo-Pacific ocean under the land-sea configurations of 70, 90, and 150 million years ago (Ma). The warming hole has a magnitude of ≈3 °C and locates in the Northern Hemisphere in 70Ma and 90Ma. The warming hole in 150Ma has a magnitude of ≈1 °C and locates in the Southern Hemisphere. Both atmospheric and oceanic processes contribute to trigger the warming hole. For 70Ma and 90Ma experiments, atmospheric teleconnection along a great circle from tropics to extratropics intensifies surface winds over sub-polar ocean and thereby increases relatively cool seawater transport from high to low latitudes. Meanwhile, global meridional overturning circulation (GMOC) becomes weaker, causing a divergence of the meridional ocean heat transport in the warming hole region. An increasing of regional cloud shortwave cooling effect acts to further enhance the warming hole. For 150Ma experiments, the warming hole is related to the meridional shift of mid-latitude jet stream and the weakening of GMOC in the Southern Hemisphere. The strength and phase of the atmospheric teleconnection and the response of GMOC strongly depend on land-sea configuration, resulting to the paleo-Pacific warming hole to occur in special periods only.
{"title":"Simulated Warming Hole in Paleo-Pacific Oceans","authors":"Mengyu Wei, Jun Yang, Yongyun Hu, Yonggang Liu, Shineng Hu, Xiang Li, Jiawenjing Lan, Jiaqi Guo, Shuai Yuan, Ji Nie","doi":"10.1175/jcli-d-23-0431.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0431.1","url":null,"abstract":"Abstract Both observations and simulations show that under global warming there exists warming deficit in the North Atlantic, known as the North Atlantic warming hole (NAWH). Here we show that similar warming hole occurs in the sub-polar Pacific ocean of paleo-climate simulations. As solar constant is increased, local surface becomes substantially cooler rather than warmer in the sub-polar paleo-Pacific ocean under the land-sea configurations of 70, 90, and 150 million years ago (Ma). The warming hole has a magnitude of ≈3 °C and locates in the Northern Hemisphere in 70Ma and 90Ma. The warming hole in 150Ma has a magnitude of ≈1 °C and locates in the Southern Hemisphere. Both atmospheric and oceanic processes contribute to trigger the warming hole. For 70Ma and 90Ma experiments, atmospheric teleconnection along a great circle from tropics to extratropics intensifies surface winds over sub-polar ocean and thereby increases relatively cool seawater transport from high to low latitudes. Meanwhile, global meridional overturning circulation (GMOC) becomes weaker, causing a divergence of the meridional ocean heat transport in the warming hole region. An increasing of regional cloud shortwave cooling effect acts to further enhance the warming hole. For 150Ma experiments, the warming hole is related to the meridional shift of mid-latitude jet stream and the weakening of GMOC in the Southern Hemisphere. The strength and phase of the atmospheric teleconnection and the response of GMOC strongly depend on land-sea configuration, resulting to the paleo-Pacific warming hole to occur in special periods only.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"1 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140571923","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 mean vertical advection of anomalous vertical temperature gradient is considered as the dominant generation mechanism of positive sea surface temperature (SST) anomalies associated with the canonical El Niño. However, most past studies had a residual term in their heat budget analysis and/or did not quantify the role of vertical mixing even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific. To quantitatively assess the importance of vertical mixing, a mixed layer heat budget analysis is performed using a hindcast simulation forced by daily mean atmospheric reanalysis data. It is found that when the mixed layer depth is defined as the depth at which potential density increases by 0.125 kg m−3 from the sea surface, the development of positive SST anomalies is predominantly governed by reductions in the cooling by vertical mixing, and their magnitude is much larger than those by vertical advection. The anomalous warming by vertical mixing may be partly explained by an anomalous deepening of the thermocline that leads to a decrease in the vertical temperature gradient, giving rise to suppression of the climatological cooling by vertical mixing. Also, an anomalously thick mixed layer reduces sensitivity to cooling by the mean vertical mixing and contributes to the anomalous SST warming. On the other hand, the dominant negative feedbacks are attributed to both anomalous surface heat loss and anomalous deepening of the mixed layer that weakens warming by the mean surface heat flux.
摘要 异常垂直温度梯度的平均垂直平流被认为是与典型厄尔尼诺现象相关的正海面温度（SST）异常的主要生成机制。然而，尽管在东赤道太平洋观测到了上层海洋活跃的垂直湍流混合，但过去的大多数研究在热预算分析中都有一个残差项，并且/或者没有量化垂直混合的作用。为了定量评估垂直混合的重要性，利用大气再分析日均值数据的后报模拟进行了混合层热量收支分析。结果发现，当混合层深度被定义为潜在密度从海面增加 0.125 kg m-3 的深度时，正 SST 异常的发展主要受垂直混合冷却减少的影响，其幅度远远大于垂直平流的影响。垂直混合异常增温的部分原因可能是热跃层异常加深，导致垂直温度梯度减小，从而抑制了气候学上的垂直混合冷却。此外，异常厚的混合层降低了对平均垂直混合冷却的敏感性，也是海温异常变暖的原因之一。另一方面，主要的负反馈归因于异常地表热损失和异常混合层加深，这削弱了平均地表热通量的增暖作用。
{"title":"Generation mechanisms of SST anomalies associated with the canonical El Niño focusing on vertical mixing","authors":"Kouya Nakamura, Shoichiro Kido, Takashi Ijichi, Tomoki Tozuka","doi":"10.1175/jcli-d-23-0288.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0288.1","url":null,"abstract":"Abstract The mean vertical advection of anomalous vertical temperature gradient is considered as the dominant generation mechanism of positive sea surface temperature (SST) anomalies associated with the canonical El Niño. However, most past studies had a residual term in their heat budget analysis and/or did not quantify the role of vertical mixing even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific. To quantitatively assess the importance of vertical mixing, a mixed layer heat budget analysis is performed using a hindcast simulation forced by daily mean atmospheric reanalysis data. It is found that when the mixed layer depth is defined as the depth at which potential density increases by 0.125 kg m−3 from the sea surface, the development of positive SST anomalies is predominantly governed by reductions in the cooling by vertical mixing, and their magnitude is much larger than those by vertical advection. The anomalous warming by vertical mixing may be partly explained by an anomalous deepening of the thermocline that leads to a decrease in the vertical temperature gradient, giving rise to suppression of the climatological cooling by vertical mixing. Also, an anomalously thick mixed layer reduces sensitivity to cooling by the mean vertical mixing and contributes to the anomalous SST warming. On the other hand, the dominant negative feedbacks are attributed to both anomalous surface heat loss and anomalous deepening of the mixed layer that weakens warming by the mean surface heat flux.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"9 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140314577","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 : 2024-03-27DOI: 10.1175/jcli-d-23-0747.1
Mingmei Xie, Bo Wu, Jia-Zhen Wang, Chunzai Wang, Xiubao Sun
Abstract On decadal timescales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls.
{"title":"Formation mechanisms of the decadal Indian Ocean dipole driven by remote forcing from the tropical Pacific","authors":"Mingmei Xie, Bo Wu, Jia-Zhen Wang, Chunzai Wang, Xiubao Sun","doi":"10.1175/jcli-d-23-0747.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0747.1","url":null,"abstract":"Abstract On decadal timescales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"51 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-03-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140314661","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 : 2024-03-26DOI: 10.1175/jcli-d-23-0280.1
Zili Shen, Anmin Duan, Wen Zhou, Yuzhuo Peng, Jinxiao Li
Abstract Two large ensemble simulations are adopted to investigate the relative contribution of external forcing and internal variability to Arctic sea ice variability on different timescales since 1960 by correcting the response error of models to external forcing using observational datasets. Our study suggests that previous approaches might overestimate the real impact of internal variability on Arctic sea ice change especially on long time scales. Our results indicate that in both March and September, internal variability plays a dominant role on all time scales over the 20th century, while the anthropogenic signal on sea ice change can be steadily and consistently detected on a time scale of more than 20 year after 2000s. We also reveal that the dominant mode of internal variability in March shows consistency across different time scales. On the contrary, the pattern of internal variability in September is highly nonuniform over the Arctic and varies across different timescales, indicating that sea ice internal variability in September at different time scales is driven by different factors.
{"title":"Reconciling Roles of External Forcing and Internal Variability in Arctic Sea Ice Change on Different Timescales","authors":"Zili Shen, Anmin Duan, Wen Zhou, Yuzhuo Peng, Jinxiao Li","doi":"10.1175/jcli-d-23-0280.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0280.1","url":null,"abstract":"Abstract Two large ensemble simulations are adopted to investigate the relative contribution of external forcing and internal variability to Arctic sea ice variability on different timescales since 1960 by correcting the response error of models to external forcing using observational datasets. Our study suggests that previous approaches might overestimate the real impact of internal variability on Arctic sea ice change especially on long time scales. Our results indicate that in both March and September, internal variability plays a dominant role on all time scales over the 20th century, while the anthropogenic signal on sea ice change can be steadily and consistently detected on a time scale of more than 20 year after 2000s. We also reveal that the dominant mode of internal variability in March shows consistency across different time scales. On the contrary, the pattern of internal variability in September is highly nonuniform over the Arctic and varies across different timescales, indicating that sea ice internal variability in September at different time scales is driven by different factors.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"24 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140301649","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 : 2024-03-26DOI: 10.1175/jcli-d-23-0449.1
Jie Wang, Dake Chen, Tao Lian, Baosheng Li, Xiang Han, Ting Liu
Abstract The sudden halting of the extreme 2014/15 El Niño expected by many was attributed to the absence of westerly wind bursts (WWBs) in late spring and early summer 2014 in previous works, yet the cause of the lack of WWBs was overlooked. Using the ERA5 reanalysis and IBTrACS dataset, as well as a set of coupled model experiments, we showed that the absence of WWBs in May efficiently downgraded the intensity of the 2014/15 El Niño from a moderate to a weak event, and was closely associated with a strong suppressive MJO originating from the central tropical Indian Ocean in mid-April 2014. The suppressive MJO underwent two pathways once passing through the Maritime Continent in early May. Along the eastward pathway, the strong suppressive MJO prevailed over the western-central equatorial Pacific, directly prohibiting the occurrence of WWBs at the equator via inducing equatorial easterly anomaly. Along the northeastward pathway, the downward motions with relative dry air and strong vertical zonal wind shear associated with the suppressive MJO suppressed the activity of the tropical cyclones in the northwestern tropical Pacific, another source of WWBs. Our results indicate that the contributions of MJO to the development of El Niño from both the direct and indirect ways should be taken into account for improving El Niño prediction.
{"title":"Suppressive MJO in April 2014 downgraded the 2014/15 El Niño","authors":"Jie Wang, Dake Chen, Tao Lian, Baosheng Li, Xiang Han, Ting Liu","doi":"10.1175/jcli-d-23-0449.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0449.1","url":null,"abstract":"Abstract The sudden halting of the extreme 2014/15 El Niño expected by many was attributed to the absence of westerly wind bursts (WWBs) in late spring and early summer 2014 in previous works, yet the cause of the lack of WWBs was overlooked. Using the ERA5 reanalysis and IBTrACS dataset, as well as a set of coupled model experiments, we showed that the absence of WWBs in May efficiently downgraded the intensity of the 2014/15 El Niño from a moderate to a weak event, and was closely associated with a strong suppressive MJO originating from the central tropical Indian Ocean in mid-April 2014. The suppressive MJO underwent two pathways once passing through the Maritime Continent in early May. Along the eastward pathway, the strong suppressive MJO prevailed over the western-central equatorial Pacific, directly prohibiting the occurrence of WWBs at the equator via inducing equatorial easterly anomaly. Along the northeastward pathway, the downward motions with relative dry air and strong vertical zonal wind shear associated with the suppressive MJO suppressed the activity of the tropical cyclones in the northwestern tropical Pacific, another source of WWBs. Our results indicate that the contributions of MJO to the development of El Niño from both the direct and indirect ways should be taken into account for improving El Niño prediction.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"16 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140314913","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 : 2024-03-26DOI: 10.1175/jcli-d-23-0582.1
Qi Sun, Haikun Zhao, Philip J. Klotzbach, Xiang Han, Jun Gao, Jin Wu, Zhanhong Ma
Abstract There has been increased focus in recent years on the impact of the Pacific Meridional Mode (PMM) and the Atlantic Meridional Mode (AMM) on weather and climate events. This study shows an increased synergistic impact of both the PMM and AMM on eastern North Pacific (ENP) extended boreal summer (June-November) tropical cyclone frequency (TCF) since the 1990s. This increase in the combined impact of both the PMM and AMM on ENP TCF is mainly due to a stronger modulation of the AMM on TCF since the early 1990s and of a stronger modulation of the PMM on TCF since the late 1990s. A budget analysis of the genesis potential index highlights the important contribution of changes in vertical wind shear to the recent strengthened AMM-TCF relationship, while potential intensity and vertical wind shear are the two most important drivers of the recent increase in the PMM-TCF relationship. This intensified association is largely explained by changes in the mean state of sea surface temperatures in the tropical Atlantic associated with the Atlantic Muit-decadal Oscillation (AMO) and trade wind magnitude in the subtropical Pacific Ocean associated with the Pacific Decadal Oscillation (PDO). This study highlights an asymmetric effect of the AMO and PDO on these two meridional modes and ENP TC genesis frequency and provides a better understanding of ENP TC activity on interannual-to-decadal time scales.
{"title":"Strengthened Combined Impact of the Pacific and Atlantic Meridional Modes on Eastern North Pacific Tropical Cyclones since the 1990s","authors":"Qi Sun, Haikun Zhao, Philip J. Klotzbach, Xiang Han, Jun Gao, Jin Wu, Zhanhong Ma","doi":"10.1175/jcli-d-23-0582.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0582.1","url":null,"abstract":"Abstract There has been increased focus in recent years on the impact of the Pacific Meridional Mode (PMM) and the Atlantic Meridional Mode (AMM) on weather and climate events. This study shows an increased synergistic impact of both the PMM and AMM on eastern North Pacific (ENP) extended boreal summer (June-November) tropical cyclone frequency (TCF) since the 1990s. This increase in the combined impact of both the PMM and AMM on ENP TCF is mainly due to a stronger modulation of the AMM on TCF since the early 1990s and of a stronger modulation of the PMM on TCF since the late 1990s. A budget analysis of the genesis potential index highlights the important contribution of changes in vertical wind shear to the recent strengthened AMM-TCF relationship, while potential intensity and vertical wind shear are the two most important drivers of the recent increase in the PMM-TCF relationship. This intensified association is largely explained by changes in the mean state of sea surface temperatures in the tropical Atlantic associated with the Atlantic Muit-decadal Oscillation (AMO) and trade wind magnitude in the subtropical Pacific Ocean associated with the Pacific Decadal Oscillation (PDO). This study highlights an asymmetric effect of the AMO and PDO on these two meridional modes and ENP TC genesis frequency and provides a better understanding of ENP TC activity on interannual-to-decadal time scales.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"2018 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140301848","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 : 2024-03-25DOI: 10.1175/jcli-d-23-0696.1
Siegfried D. Schubert, Yehui Chang, Anthony M. DeAngelis, Young-Kwon Lim, Natalie P. Thomas, Randal D. Koster, Michael G. Bosilovich, Andrea M. Molod, Allison Collow, Amin Dezfuli
Abstract In late December of 2022 and the first half of January 2023 an unprecedented series of atmospheric rivers (ARs) produced near record heavy rains and flooding over much of California. Here we employ the NASA GEOS AGCM run in a “replay” mode, together with more idealized simulations with a stationary wave model, to identify the remote forcing regions, mechanisms and underlying predictability of this flooding event. In particular, the study addresses the underlying causes of a persistent positive Pacific/North American (PNA) - like circulation pattern that facilitated the development of the ARs. We show that that pattern developed in late December as a result of vorticity forcing in the North Pacific jet exit region. We further provide evidence that this vorticity forcing was the result of a chain of events initiated in mid-December with the development of a Rossby wave (as a result of forcing linked to the MJO) that propagated from the northern Indian Ocean into the North Pacific. As such, both the initiation of the event and the eventual development of the PNA depended critically on internally-generated Rossby wave forcings, with the North Pacific jet playing a key role. This, combined with contemporaneous SST (La Niña) forcing that produced a circulation response in the AGCM that was essentially opposite to the positive PNA, underscores the fundamental lack of predictability of the event at seasonal time scales. Forecasts produced with the GEOS coupled model suggests that useful skill in predicting the PNA and extreme precipitation over California was in fact limited to lead times shorter than about 3 weeks.
{"title":"Insights into the Causes and Predictability of the 2022/23 California Flooding","authors":"Siegfried D. Schubert, Yehui Chang, Anthony M. DeAngelis, Young-Kwon Lim, Natalie P. Thomas, Randal D. Koster, Michael G. Bosilovich, Andrea M. Molod, Allison Collow, Amin Dezfuli","doi":"10.1175/jcli-d-23-0696.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0696.1","url":null,"abstract":"Abstract In late December of 2022 and the first half of January 2023 an unprecedented series of atmospheric rivers (ARs) produced near record heavy rains and flooding over much of California. Here we employ the NASA GEOS AGCM run in a “replay” mode, together with more idealized simulations with a stationary wave model, to identify the remote forcing regions, mechanisms and underlying predictability of this flooding event. In particular, the study addresses the underlying causes of a persistent positive Pacific/North American (PNA) - like circulation pattern that facilitated the development of the ARs. We show that that pattern developed in late December as a result of vorticity forcing in the North Pacific jet exit region. We further provide evidence that this vorticity forcing was the result of a chain of events initiated in mid-December with the development of a Rossby wave (as a result of forcing linked to the MJO) that propagated from the northern Indian Ocean into the North Pacific. As such, both the initiation of the event and the eventual development of the PNA depended critically on internally-generated Rossby wave forcings, with the North Pacific jet playing a key role. This, combined with contemporaneous SST (La Niña) forcing that produced a circulation response in the AGCM that was essentially opposite to the positive PNA, underscores the fundamental lack of predictability of the event at seasonal time scales. Forecasts produced with the GEOS coupled model suggests that useful skill in predicting the PNA and extreme precipitation over California was in fact limited to lead times shorter than about 3 weeks.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":"24 1","pages":""},"PeriodicalIF":4.9,"publicationDate":"2024-03-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140301847","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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