Pub Date : 2024-06-12DOI: 10.1175/jcli-d-23-0494.1
Pep Cos, Raül Marcos-Matamoros, Markus Donat, Rashed Mahmood, F. Doblas-Reyes
There are several methods to constrain multi-model projections of future climate. This study assesses the quality of four constraining methods in representing the near-term summer temperature projections of the Mediterranean region. Three are based on phasing in ocean surface temperature variations based on observations or decadal predictions, and method is based on measuring performance and independence of the individual simulations. The comparison has been carried out with a new framework inspired by the forecast quality assessment of decadal predictions. The framework led to quality estimates of the constrained projection approaches obtained by producing 20-year temperature estimates every year from 1970 to 2000 and computing quality metrics against observational references. The evaluation results show some differences between constraining approaches. The improvement or deterioration against quality measures of the full, unconstrained, CMIP6 ensemble show strong spatial heterogeneity. From the analysis of the selection approaches it is found that the constraints based on sea surface temperature (SST) fields are affected not only by the variability but also by the warming trend. The weighting method generally shows small quality differences with respect to the full CMIP6 ensemble. Despite caveats of the different methods there is potential to improve the near-term climate projections as some significant quality enhancements were found in some approaches according to the evaluation metrics used. This study suggests a good understanding of the constraining methods and their forecast quality is required before using them to take informed decisions. Our study opens the door to optimising these methods for the Mediterranean climate and highlights the need for evaluating the constraints through retrospective assessments against observational references.
{"title":"Near-term Mediterranean summer temperature climate projections: a comparison of constraining methods","authors":"Pep Cos, Raül Marcos-Matamoros, Markus Donat, Rashed Mahmood, F. Doblas-Reyes","doi":"10.1175/jcli-d-23-0494.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0494.1","url":null,"abstract":"\u0000 There are several methods to constrain multi-model projections of future climate. This study assesses the quality of four constraining methods in representing the near-term summer temperature projections of the Mediterranean region. Three are based on phasing in ocean surface temperature variations based on observations or decadal predictions, and method is based on measuring performance and independence of the individual simulations. The comparison has been carried out with a new framework inspired by the forecast quality assessment of decadal predictions. The framework led to quality estimates of the constrained projection approaches obtained by producing 20-year temperature estimates every year from 1970 to 2000 and computing quality metrics against observational references.\u0000The evaluation results show some differences between constraining approaches. The improvement or deterioration against quality measures of the full, unconstrained, CMIP6 ensemble show strong spatial heterogeneity. From the analysis of the selection approaches it is found that the constraints based on sea surface temperature (SST) fields are affected not only by the variability but also by the warming trend. The weighting method generally shows small quality differences with respect to the full CMIP6 ensemble. Despite caveats of the different methods there is potential to improve the near-term climate projections as some significant quality enhancements were found in some approaches according to the evaluation metrics used. This study suggests a good understanding of the constraining methods and their forecast quality is required before using them to take informed decisions. Our study opens the door to optimising these methods for the Mediterranean climate and highlights the need for evaluating the constraints through retrospective assessments against observational references.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141354180","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 : 2024-06-11DOI: 10.1175/jcli-d-23-0736.1
Yuanyuan Huang, Daehyun Kim, Tian Zhou, Xiaoming Shi
Cloud-radiative interaction (CRI) is a fundamental process that modulates tropical circulation and intraseasonal variability, including the Madden-Julian Oscillation (MJO). In this study, we investigate how the mean state of the tropical atmosphere and MJO respond to CRI intensity changes and provide insights into the underlying mechanisms, using the aquaplanet configuration in CESM2. By enhancing CRI through tuning the DCS parameter (an auto-conversion threshold size in Morrison and Gettelman (2008) cloud microphysics scheme), we demonstrate that DCS-induced CRI intensification is linked to a warmer troposphere, increased tropical moisture, strengthened Hadley Cell (HC), stronger trade winds, and a stronger equatorward intertropical convergence zone (ITCZ) with more clouds and precipitation, reflecting stronger cloud-radiation-circulation feedback. The intensified CRI also leads to the intensification and slower propagation of the simulated MJO-like mode despite the MJO-like signals becoming less distinguishable from the background due to the influence of other waves. The MJO intensification is likely associated with the mean state changes that support the development of deep convection. Moreover, the CRI itself, especially the interaction with the longwave radiation, also directly influences the MJO’s maintenance and propagation, more contributing to the maintenance of column moist static energy (MSE) and deceleration of its eastward propagation on intraseasonal timescales.
{"title":"The Role of Cloud-Radiative Interaction in Tropical Circulation and the Madden-Julian Oscillation","authors":"Yuanyuan Huang, Daehyun Kim, Tian Zhou, Xiaoming Shi","doi":"10.1175/jcli-d-23-0736.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0736.1","url":null,"abstract":"\u0000Cloud-radiative interaction (CRI) is a fundamental process that modulates tropical circulation and intraseasonal variability, including the Madden-Julian Oscillation (MJO). In this study, we investigate how the mean state of the tropical atmosphere and MJO respond to CRI intensity changes and provide insights into the underlying mechanisms, using the aquaplanet configuration in CESM2. By enhancing CRI through tuning the DCS parameter (an auto-conversion threshold size in Morrison and Gettelman (2008) cloud microphysics scheme), we demonstrate that DCS-induced CRI intensification is linked to a warmer troposphere, increased tropical moisture, strengthened Hadley Cell (HC), stronger trade winds, and a stronger equatorward intertropical convergence zone (ITCZ) with more clouds and precipitation, reflecting stronger cloud-radiation-circulation feedback. The intensified CRI also leads to the intensification and slower propagation of the simulated MJO-like mode despite the MJO-like signals becoming less distinguishable from the background due to the influence of other waves. The MJO intensification is likely associated with the mean state changes that support the development of deep convection. Moreover, the CRI itself, especially the interaction with the longwave radiation, also directly influences the MJO’s maintenance and propagation, more contributing to the maintenance of column moist static energy (MSE) and deceleration of its eastward propagation on intraseasonal timescales.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141358387","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 : 2024-06-10DOI: 10.1175/jcli-d-23-0379.1
Leilane Passos, H. Langehaug, M. Årthun, F. Straneo
Decadal thermohaline anomalies carried northwards by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern Subpolar gyre using the reanalyses EN4.2.2 for the ocean and ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the Subpolar North Atlantic and the Norwegian Sea. We use observation-based data sets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply Complex Empirical Orthogonal functions. Our results show that when a warm anomaly enters the eastern Subpolar gyre, more SFWMT occurs in light-density classes (27.0-27.2 kg m−3). In contrast, when a cold anomaly enters the eastern Subpolar gyre, more SFWMT occurs in denser classes (27.4-27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm-salty and cold-fresh subsurface anomalies, repeating throughout the 74-year-long record with 4 warm-salt and cold-fresh periods after the 50s. The cold-fresh anomaly periods happen simultaneously with the Great salinity anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the SPNA than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic Meridional Overturning Circulation and predictability in the North Atlantic region.
{"title":"On the relation between thermohaline anomalies and water mass transformation in the Eastern Subpolar North Atlantic","authors":"Leilane Passos, H. Langehaug, M. Årthun, F. Straneo","doi":"10.1175/jcli-d-23-0379.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0379.1","url":null,"abstract":"\u0000Decadal thermohaline anomalies carried northwards by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern Subpolar gyre using the reanalyses EN4.2.2 for the ocean and ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the Subpolar North Atlantic and the Norwegian Sea. We use observation-based data sets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply Complex Empirical Orthogonal functions. Our results show that when a warm anomaly enters the eastern Subpolar gyre, more SFWMT occurs in light-density classes (27.0-27.2 kg m−3). In contrast, when a cold anomaly enters the eastern Subpolar gyre, more SFWMT occurs in denser classes (27.4-27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm-salty and cold-fresh subsurface anomalies, repeating throughout the 74-year-long record with 4 warm-salt and cold-fresh periods after the 50s. The cold-fresh anomaly periods happen simultaneously with the Great salinity anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the SPNA than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic Meridional Overturning Circulation and predictability in the North Atlantic region.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141365683","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 : 2024-06-07DOI: 10.1175/jcli-d-23-0444.1
Ziming Chen, Tianjun Zhou, Xiaolong Chen, Lixia Zhang, Yun Qian, Zeyi Wang, Linqiang He, L. R. Leung
Understanding global monsoon (GM) variability and projecting its future changes relies heavily on climate models. However, climate models generally show pronounced biases in GM simulations, and the reasons for this remain unclear. Here, we evaluate the performance of 20 pairs of climate models that participated in both the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6), and identify the sources of their GM simulation biases from an energy transport perspective. The multimodel mean improvement in CMIP6 compared to CMIP5 is demonstrated by the increasing skill scores for various GM metrics from 0.20~0.79 to 0.48~0.83. More specifically, the dry biases in the Northern Hemispheric Summer Monsoon (NHSM) precipitation in CMIP5 (root mean square error, RMSE: 1.85 mm/day) are reduced in CMIP6 (RMSE: 1.66 mm/day). This higher simulation skill is associated with higher skill in simulating the precipitation-solstitial mode, monsoon intensity, and monsoon domains. The improvement in the NHSM precipitation simulation results from that in the meridional transport of atmospheric energy. Atmospheric energy budget analysis shows that the negative biases in downward surface longwave radiation and northward energy transport are smaller in CMIP6 than in CMIP5 in the boreal summer, resulting in a more realistic interhemispheric thermal contrast and meridional gradient of moist static energy. However, a major weakness of the CMIP6 models is found in the Southern Hemisphere Summer Monsoon precipitation simulation due to the positive bias in the top-of-atmosphere downward longwave radiation. This study shows that reasonably reproducing the meridional global atmospheric energy transportation is necessary for skillful GM simulation.
{"title":"Understanding the biases in global monsoon simulations from the perspective of atmospheric energy transport","authors":"Ziming Chen, Tianjun Zhou, Xiaolong Chen, Lixia Zhang, Yun Qian, Zeyi Wang, Linqiang He, L. R. Leung","doi":"10.1175/jcli-d-23-0444.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0444.1","url":null,"abstract":"\u0000Understanding global monsoon (GM) variability and projecting its future changes relies heavily on climate models. However, climate models generally show pronounced biases in GM simulations, and the reasons for this remain unclear. Here, we evaluate the performance of 20 pairs of climate models that participated in both the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6), and identify the sources of their GM simulation biases from an energy transport perspective. The multimodel mean improvement in CMIP6 compared to CMIP5 is demonstrated by the increasing skill scores for various GM metrics from 0.20~0.79 to 0.48~0.83. More specifically, the dry biases in the Northern Hemispheric Summer Monsoon (NHSM) precipitation in CMIP5 (root mean square error, RMSE: 1.85 mm/day) are reduced in CMIP6 (RMSE: 1.66 mm/day). This higher simulation skill is associated with higher skill in simulating the precipitation-solstitial mode, monsoon intensity, and monsoon domains. The improvement in the NHSM precipitation simulation results from that in the meridional transport of atmospheric energy. Atmospheric energy budget analysis shows that the negative biases in downward surface longwave radiation and northward energy transport are smaller in CMIP6 than in CMIP5 in the boreal summer, resulting in a more realistic interhemispheric thermal contrast and meridional gradient of moist static energy. However, a major weakness of the CMIP6 models is found in the Southern Hemisphere Summer Monsoon precipitation simulation due to the positive bias in the top-of-atmosphere downward longwave radiation. This study shows that reasonably reproducing the meridional global atmospheric energy transportation is necessary for skillful GM simulation.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141374348","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 : 2024-06-06DOI: 10.1175/jcli-d-23-0371.1
V. Narinesingh, Huan Guo, Stephen T. Garner, Yi Ming
Coupled ocean and prescribed sea surface temperature (SST) experiments are performed to investigate the drivers of Northern Hemisphere (NH) midlatitude winter circulation and blocking changes in warmer climates. In coupled experiments, a historical simulation is compared to a simulation following an end of the 21st century SSP5-8.5 emissions scenario. The SSP5-8.5 simulation yields poleward shifted jets and an enhanced stationary wave pattern compared to the historical simulation. In terms of blocking, a reduction is found across North America and over the Pacific Ocean with suggestion of more blocking over parts of Eurasia. Separately, prescribed SST experiments are performed decomposing the SSP5-8.5 SST response into a uniform warming component plus a spatially dependent change in SST pattern. SSP5-8.5 changes in circulation are primarily driven by a uniform warming of SST. Uniform warming is also found to account for most of the SSP5-8.5 blocking reduction over North America and the Pacific Ocean, but not over Eurasia. El Niño like changes to the SST pattern also yield less blocking over the Pacific and North America. However, adding the responses of uniform and pattern experiments yields a non-linear overreduction of blocking compared to the SSP5-8.5 experiment. Regional analyses of block energetics suggest that much of the reductions in blocking in warming simulations are driven by decreased baroclinic conversion in some regions and enhanced dissipation from diabatic sources in others.
{"title":"Uniform SST Warming Explains Most of the NH Winter Circulation and Blocking Response in a Warmer Climate","authors":"V. Narinesingh, Huan Guo, Stephen T. Garner, Yi Ming","doi":"10.1175/jcli-d-23-0371.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0371.1","url":null,"abstract":"\u0000Coupled ocean and prescribed sea surface temperature (SST) experiments are performed to investigate the drivers of Northern Hemisphere (NH) midlatitude winter circulation and blocking changes in warmer climates. In coupled experiments, a historical simulation is compared to a simulation following an end of the 21st century SSP5-8.5 emissions scenario. The SSP5-8.5 simulation yields poleward shifted jets and an enhanced stationary wave pattern compared to the historical simulation. In terms of blocking, a reduction is found across North America and over the Pacific Ocean with suggestion of more blocking over parts of Eurasia. Separately, prescribed SST experiments are performed decomposing the SSP5-8.5 SST response into a uniform warming component plus a spatially dependent change in SST pattern. SSP5-8.5 changes in circulation are primarily driven by a uniform warming of SST. Uniform warming is also found to account for most of the SSP5-8.5 blocking reduction over North America and the Pacific Ocean, but not over Eurasia. El Niño like changes to the SST pattern also yield less blocking over the Pacific and North America. However, adding the responses of uniform and pattern experiments yields a non-linear overreduction of blocking compared to the SSP5-8.5 experiment. Regional analyses of block energetics suggest that much of the reductions in blocking in warming simulations are driven by decreased baroclinic conversion in some regions and enhanced dissipation from diabatic sources in others.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141376809","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 : 2024-06-06DOI: 10.1175/jcli-d-23-0367.1
Jian-Hua Qian, B. Viner, Stephen Noble, David Werth, Joseph Wermter, Steven Chiswell, Cuihua Li
Observed precipitation changes in the Southeast U.S. (SEUS) are spatially heterogeneous. Most of the inland SEUS and eastern Gulf coast become drier and the east coast north of Charleston SC and southern Florida become wetter from the old 30-year period of 1961-1990 to the recent period of 1991-2020. The observed climate change is examined from the perspective of daily weather types (WTs). A k-means clustering analysis has been conducted using daily 850 hPa circulation for 1948-2021. The obtained ten weather types (WTs) peak in different seasons, respectively. The frequencies and precipitation intensity of the WTs have been analyzed. A winter WT characterized by a Western Appalachian trough and a summer WT featuring North Atlantic Subtropical High (NASH) have a rising trend of annual frequency from 1948 to 2021. An Appalachian High in the autumn has a decreasing frequency but become drier and stronger. Some precipitation intensity change and small location shift have also been observed. The drying up on the eastern Gulf coast and the inland area of the SEUS is mainly caused by the weakened southwesterly low-level jet on the western flank of the NASH that reduces rain in the spring, the less frequent but stronger and drier Appalachian High in the summer and autumn, and the weaker and more western located Plains trough in the winter, spring, and autumn. The precipitation increase on the east coast and southern Florida is majorly due to more frequent, stronger, and rainier troughs along the western Appalachian as well as the east coast.
{"title":"Understanding Observed Precipitation Change and the New Climate Normal from the Perspective of Daily Weather Types in the Southeast U.S.","authors":"Jian-Hua Qian, B. Viner, Stephen Noble, David Werth, Joseph Wermter, Steven Chiswell, Cuihua Li","doi":"10.1175/jcli-d-23-0367.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0367.1","url":null,"abstract":"\u0000Observed precipitation changes in the Southeast U.S. (SEUS) are spatially heterogeneous. Most of the inland SEUS and eastern Gulf coast become drier and the east coast north of Charleston SC and southern Florida become wetter from the old 30-year period of 1961-1990 to the recent period of 1991-2020. The observed climate change is examined from the perspective of daily weather types (WTs). A k-means clustering analysis has been conducted using daily 850 hPa circulation for 1948-2021. The obtained ten weather types (WTs) peak in different seasons, respectively. The frequencies and precipitation intensity of the WTs have been analyzed. A winter WT characterized by a Western Appalachian trough and a summer WT featuring North Atlantic Subtropical High (NASH) have a rising trend of annual frequency from 1948 to 2021. An Appalachian High in the autumn has a decreasing frequency but become drier and stronger. Some precipitation intensity change and small location shift have also been observed. The drying up on the eastern Gulf coast and the inland area of the SEUS is mainly caused by the weakened southwesterly low-level jet on the western flank of the NASH that reduces rain in the spring, the less frequent but stronger and drier Appalachian High in the summer and autumn, and the weaker and more western located Plains trough in the winter, spring, and autumn. The precipitation increase on the east coast and southern Florida is majorly due to more frequent, stronger, and rainier troughs along the western Appalachian as well as the east coast.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141376470","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 : 2024-06-04DOI: 10.1175/jcli-d-23-0654.1
Yifei Fan, Duo Chan, Pengfei Zhang, Laifang Li
Abstract Despite global warming, sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob (North Atlantic cold blob), signifies a unique role of the subpolar North Atlantic in uptaking heat and hence impacts downstream weather and climate. However, a lack of observational records and its constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from the Coupled Model Intercomparison Project Phase 6, we assess the primary processes driving the North Atlantic cold blob within individual models and the consistency of mechanisms across models. We show that 11 out of 32 models, which we call “Cold Bold” models, simulate subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. Whereas four out of the 11 Cold Blob models indicate decreased Oceanic Heat Transport Convergence (OHTC) as the key mechanism, another four models suggest changes in radiative processes making predominant contributions. The contribution of OHTC and radiative processes are comparable in the remaining three models. Such a model spread in the mechanism of cold blob formation may be associated with distinct base-state Atlantic Meridional Overturning Circulation (AMOC) strength, which explains about 39% of the inter-model spread in the contribution of OHTC to the simulated cold blob. Models with a stronger base-state AMOC suggest a greater role of OHTC, whereas those with a weaker base-state AMOC indicate radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single models should be interpreted with caution.
{"title":"Disagreement on the North Atlantic Cold Blob Formation Mechanisms among Climate Models","authors":"Yifei Fan, Duo Chan, Pengfei Zhang, Laifang Li","doi":"10.1175/jcli-d-23-0654.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0654.1","url":null,"abstract":"Abstract Despite global warming, sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob (North Atlantic cold blob), signifies a unique role of the subpolar North Atlantic in uptaking heat and hence impacts downstream weather and climate. However, a lack of observational records and its constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from the Coupled Model Intercomparison Project Phase 6, we assess the primary processes driving the North Atlantic cold blob within individual models and the consistency of mechanisms across models. We show that 11 out of 32 models, which we call “Cold Bold” models, simulate subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. Whereas four out of the 11 Cold Blob models indicate decreased Oceanic Heat Transport Convergence (OHTC) as the key mechanism, another four models suggest changes in radiative processes making predominant contributions. The contribution of OHTC and radiative processes are comparable in the remaining three models. Such a model spread in the mechanism of cold blob formation may be associated with distinct base-state Atlantic Meridional Overturning Circulation (AMOC) strength, which explains about 39% of the inter-model spread in the contribution of OHTC to the simulated cold blob. Models with a stronger base-state AMOC suggest a greater role of OHTC, whereas those with a weaker base-state AMOC indicate radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single models should be interpreted with caution.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141259335","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 precipitation changes in arid and semi-arid Central Asia have great impacts on the local fragile ecosystem. The summer precipitation in Central Asia shows obvious interannual variations, but the corresponding crucial moisture transporting processes remain unclear. Therefore, this study employs the Lagrangian model FLEXPART to achieve this goal. Results show that the moisture of climatological summer precipitation in Central Asia is mainly from the local regions, the surrounding Western and Northern Eurasian regions. The contribution of local evaporation from Western Central Asia is about two times of that from Eastern Central Asia. At the interannual timescale, the moisture variations are mainly influenced by the local regions and the Western Eurasia, while the local evaporation is mainly from western Central Asia. Totally, the Eurasian evaporation plays a dominant role in the interannual variations of Central Asian summer precipitation by contributing more than 90% of the total moisture. The moisture transports associated with Central Asian summer precipitation interannual variations are impacted by the anomalous cyclones over western and northeastern part of Central Asia during the wet years, which enhance the moisture convergence and hence increase the summer precipitation in Central Asia. The anomalous cyclone over western part of Central Asia is correlated with the changes in intensity of Eurasian Summer Subtropical Westerly Jet (ESSWJ), while the anomalous cyclone over northeastern part of Central Asia is correlated with both ESSWJ and the British–Baikal Corridor pattern teleconnection in association with the polar front jet.
{"title":"Dominant role of Eurasian evaporation on the moisture sources of the interannual variations in Central Asian summer precipitation","authors":"Dongdong Peng, Tianjun Zhou, Xin Huang, Chao He, Lixia Zhang","doi":"10.1175/jcli-d-23-0515.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0515.1","url":null,"abstract":"Abstract The precipitation changes in arid and semi-arid Central Asia have great impacts on the local fragile ecosystem. The summer precipitation in Central Asia shows obvious interannual variations, but the corresponding crucial moisture transporting processes remain unclear. Therefore, this study employs the Lagrangian model FLEXPART to achieve this goal. Results show that the moisture of climatological summer precipitation in Central Asia is mainly from the local regions, the surrounding Western and Northern Eurasian regions. The contribution of local evaporation from Western Central Asia is about two times of that from Eastern Central Asia. At the interannual timescale, the moisture variations are mainly influenced by the local regions and the Western Eurasia, while the local evaporation is mainly from western Central Asia. Totally, the Eurasian evaporation plays a dominant role in the interannual variations of Central Asian summer precipitation by contributing more than 90% of the total moisture. The moisture transports associated with Central Asian summer precipitation interannual variations are impacted by the anomalous cyclones over western and northeastern part of Central Asia during the wet years, which enhance the moisture convergence and hence increase the summer precipitation in Central Asia. The anomalous cyclone over western part of Central Asia is correlated with the changes in intensity of Eurasian Summer Subtropical Westerly Jet (ESSWJ), while the anomalous cyclone over northeastern part of Central Asia is correlated with both ESSWJ and the British–Baikal Corridor pattern teleconnection in association with the polar front jet.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141259330","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 : 2024-06-04DOI: 10.1175/jcli-d-23-0475.1
Dazhi Xi, Ning Lin, Renzhi Jing, Patrick Harr, Michael Oppenheimer
Abstract North Atlantic tropical cyclone (TC) activity under a high-emission scenario is projected using a statistical synthetic storm model coupled with nine Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models. The ensemble projection shows that the annual frequency of TCs generated in the basin will decrease from 15.91 (1979-2014) to 12.16 (2075-2100), and TC activity will shift poleward and coast-ward. The mean of lifetime maximum intensity will increase from 66.50 knots to 75.04 knots. Large discrepancies in TC frequency and intensity projections are found among the nine CMIP6 climate models. The uncertainty in the projection of wind shear is the leading cause of the discrepancies in the TC climatology projection, dominating the uncertainties in the projection of thermodynamic parameters such as potential intensity and saturation deficit. The uncertainty in the projection of wind shear may be related to the different projections of horizontal gradient of vertically integrated temperature in the climate models, which can be induced by different parameterizations of physical processes including surface process, sea ice, and cloud feedback. Informed by the uncertainty analysis, a surrogate model is developed to provide the first-order estimation of TC activity in climate models based on large-scale environmental features.
{"title":"Uncertainties Inherent from Large-Scale Climate Projections in the Statistical Downscaling Projection of North Atlantic Tropical Cyclone Activity","authors":"Dazhi Xi, Ning Lin, Renzhi Jing, Patrick Harr, Michael Oppenheimer","doi":"10.1175/jcli-d-23-0475.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0475.1","url":null,"abstract":"Abstract North Atlantic tropical cyclone (TC) activity under a high-emission scenario is projected using a statistical synthetic storm model coupled with nine Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models. The ensemble projection shows that the annual frequency of TCs generated in the basin will decrease from 15.91 (1979-2014) to 12.16 (2075-2100), and TC activity will shift poleward and coast-ward. The mean of lifetime maximum intensity will increase from 66.50 knots to 75.04 knots. Large discrepancies in TC frequency and intensity projections are found among the nine CMIP6 climate models. The uncertainty in the projection of wind shear is the leading cause of the discrepancies in the TC climatology projection, dominating the uncertainties in the projection of thermodynamic parameters such as potential intensity and saturation deficit. The uncertainty in the projection of wind shear may be related to the different projections of horizontal gradient of vertically integrated temperature in the climate models, which can be induced by different parameterizations of physical processes including surface process, sea ice, and cloud feedback. Informed by the uncertainty analysis, a surrogate model is developed to provide the first-order estimation of TC activity in climate models based on large-scale environmental features.","PeriodicalId":15472,"journal":{"name":"Journal of Climate","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141259331","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 the interannual variability of Marine Heatwaves (MHWs) in the Bay of Bengal (BOB) associated with the Indian Ocean Dipole (IOD) from 1982 to 2021. The results revealed a significant positive correlation at the 95% confidence level between the IOD and MHW days in the central bay at the peak of the IOD in autumn. During positive IOD (pIOD) events, the central bay experienced more MHW days in autumn, with an average increase of 7.4 days. The increased MHW days in the central bay could be primarily attributed to the enhanced net heat flux (TQ), which is 9.7 times the contribution of ocean dynamic processes (horizontal advection + entrainment). The reduced latent heat flux loss and enhanced shortwave radiation due to the anomalous atmospheric low-level high pressure associated with the pIOD account for 63% and 50%, respectively, of the anomalous enhanced TQ, while the longwave radiation and sensible heat flux make smaller contributions of −20% and 7%. In addition, thermocline deepening in the southwestern bay, caused by this anomalous high pressure and associated anticyclonic wind anomalies, favors the occurrence and persistence of MHWs by reducing the mixed-layer cooling rate. In addition to the influence of the IOD, the El Niño-Southern Oscillation mainly affects MHWs from winter to the following summer, which confirms the result of a previous study.
{"title":"An increase in autumn marine heatwaves caused by the Indian Ocean Dipole in the Bay of Bengal","authors":"Kunming Liang, Yun Qiu, Xinyu Lin, Wenshu Lin, Xutao Ni, Yijun He","doi":"10.1175/jcli-d-23-0541.1","DOIUrl":"https://doi.org/10.1175/jcli-d-23-0541.1","url":null,"abstract":"Abstract This study investigates the interannual variability of Marine Heatwaves (MHWs) in the Bay of Bengal (BOB) associated with the Indian Ocean Dipole (IOD) from 1982 to 2021. The results revealed a significant positive correlation at the 95% confidence level between the IOD and MHW days in the central bay at the peak of the IOD in autumn. During positive IOD (pIOD) events, the central bay experienced more MHW days in autumn, with an average increase of 7.4 days. The increased MHW days in the central bay could be primarily attributed to the enhanced net heat flux (TQ), which is 9.7 times the contribution of ocean dynamic processes (horizontal advection + entrainment). The reduced latent heat flux loss and enhanced shortwave radiation due to the anomalous atmospheric low-level high pressure associated with the pIOD account for 63% and 50%, respectively, of the anomalous enhanced TQ, while the longwave radiation and sensible heat flux make smaller contributions of −20% and 7%. In addition, thermocline deepening in the southwestern bay, caused by this anomalous high pressure and associated anticyclonic wind anomalies, favors the occurrence and persistence of MHWs by reducing the mixed-layer cooling rate. In addition to the influence of the IOD, the El Niño-Southern Oscillation mainly affects MHWs from winter to the following summer, which confirms the result of a previous study.","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":"141259202","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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