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Abstract
The tropical cloud forest ecosystem in western equatorial Africa (WEA) is known to be sensitive to the presence of an extensive and persistent low-level stratiform cloud deck during the long dry season from June to September (JJAS). Here, we present a new climatology of the diurnal cycle of the low-level cloud cover from surface synoptic stations over WEA during JJAS 1971–2019. For the period JJAS 2008–19, we also utilized estimates of cloudiness from four satellite products, namely, the Satellite Application Facility on Support to Nowcasting and Very Short Range Forecasting (SAFNWC) cloud classification, the Day and Night Microphysical Schemes (DMS/NMS), and cross sections from CALIPSO and CloudSat (2B-GEOPROF-lidar). A comparison with surface stations reveals that the NMS at night together with SAFNWC at daytime yield the smallest biases. The climatological analysis reveals that low-level clouds persist during the day over the coastal plains and windward side of the low mountain ranges. Conversely, on their leeward sides, i.e., over the plateaus, a decrease of the low-level cloud frequency is observed in the afternoon, together with a change from stratocumulus to cumulus. At night, the low-level cloud deck reforms over this region with the largest cloud occurrence frequencies in the morning. Vertical profiles from 2B-GEOPROF-lidar reveal cloud tops below 3000 m even at daytime. The station data and the suitable satellite products form the basis to better understand the physical processes controlling the clouds and to evaluate cloudiness from reanalyses and models.
Abstract
The tropical cloud forest ecosystem in western equatorial Africa (WEA) is known to be sensitive to the presence of an extensive and persistent low-level stratiform cloud deck during the long dry season from June to September (JJAS). Here, we present a new climatology of the diurnal cycle of the low-level cloud cover from surface synoptic stations over WEA during JJAS 1971–2019. For the period JJAS 2008–19, we also utilized estimates of cloudiness from four satellite products, namely, the Satellite Application Facility on Support to Nowcasting and Very Short Range Forecasting (SAFNWC) cloud classification, the Day and Night Microphysical Schemes (DMS/NMS), and cross sections from CALIPSO and CloudSat (2B-GEOPROF-lidar). A comparison with surface stations reveals that the NMS at night together with SAFNWC at daytime yield the smallest biases. The climatological analysis reveals that low-level clouds persist during the day over the coastal plains and windward side of the low mountain ranges. Conversely, on their leeward sides, i.e., over the plateaus, a decrease of the low-level cloud frequency is observed in the afternoon, together with a change from stratocumulus to cumulus. At night, the low-level cloud deck reforms over this region with the largest cloud occurrence frequencies in the morning. Vertical profiles from 2B-GEOPROF-lidar reveal cloud tops below 3000 m even at daytime. The station data and the suitable satellite products form the basis to better understand the physical processes controlling the clouds and to evaluate cloudiness from reanalyses and models.
Abstract
We investigate the land–ocean warming contrast mechanisms, ϕ, defined as the land-mean surface air temperature (SAT) change divided by the ocean-mean SAT change, in a transient climate response (TCR) obtained from the Coupled Model Intercomparison Project phase 6 (CMIP6) 1% per year CO2 increase experiments (1pctCO2). The energy budget framework devised in Part I is applied to 15 CMIP6 1pctCO2 simulations, and the climate response in year 140 when the CO2 concentration was quadrupled was compared with a near-equilibrium climate response (NEQ), defined as the last 30-yr mean in the abrupt CO2 quadrupling (abrupt4×CO2) experiments. It is shown that ϕ is larger in TCR than in NEQ by approximately 4%, although the difference is not statistically significant. In TCR, effective radiative forcing is large over land compared to the ocean, and this is the main contributor to ϕ as in NEQ. The time evolution of ϕ in 1pctCO2 can be reconstructed by means of the fast and slow components of climate response in abrupt4×CO2, indicating that the essential mechanism for the land–ocean warming contrast shown in Part I applies to TCR. Furthermore, our analyses reveal a compensation between land-to-ocean atmospheric energy transport that decreases ϕ and ocean heat uptake that increases ϕ. Regardless of the time scale of the response, these two processes are linked by the change in atmospheric circulation, leading to the small combined effect. As a result, the multimodel mean ϕ in 1pctCO2 is roughly time invariant at approximately 1.5 despite the continuous increase in CO2.
Significance Statement
The land–ocean warming contrast, which indicates large land surface warming compared to ocean surface warming in response to an increase in atmospheric CO2 concentration, is a striking feature of human-induced global warming. This study focuses on temporal changes in the magnitude of the land–ocean warming contrast in transient climate change simulations and shows that the magnitude of the land–ocean warming contrast is nearly constant over time, maintaining a ratio of approximately 1.5, between land and ocean surface warming. This small temporal change is explained mainly by a compensation between land-to-ocean energy transport and ocean heat uptake, because both act in opposite directions to the land–ocean warming contrast.
Abstract
We investigate the land–ocean warming contrast mechanisms, ϕ, defined as the land-mean surface air temperature (SAT) change divided by the ocean-mean SAT change, in a transient climate response (TCR) obtained from the Coupled Model Intercomparison Project phase 6 (CMIP6) 1% per year CO2 increase experiments (1pctCO2). The energy budget framework devised in Part I is applied to 15 CMIP6 1pctCO2 simulations, and the climate response in year 140 when the CO2 concentration was quadrupled was compared with a near-equilibrium climate response (NEQ), defined as the last 30-yr mean in the abrupt CO2 quadrupling (abrupt4×CO2) experiments. It is shown that ϕ is larger in TCR than in NEQ by approximately 4%, although the difference is not statistically significant. In TCR, effective radiative forcing is large over land compared to the ocean, and this is the main contributor to ϕ as in NEQ. The time evolution of ϕ in 1pctCO2 can be reconstructed by means of the fast and slow components of climate response in abrupt4×CO2, indicating that the essential mechanism for the land–ocean warming contrast shown in Part I applies to TCR. Furthermore, our analyses reveal a compensation between land-to-ocean atmospheric energy transport that decreases ϕ and ocean heat uptake that increases ϕ. Regardless of the time scale of the response, these two processes are linked by the change in atmospheric circulation, leading to the small combined effect. As a result, the multimodel mean ϕ in 1pctCO2 is roughly time invariant at approximately 1.5 despite the continuous increase in CO2.
Significance Statement
The land–ocean warming contrast, which indicates large land surface warming compared to ocean surface warming in response to an increase in atmospheric CO2 concentration, is a striking feature of human-induced global warming. This study focuses on temporal changes in the magnitude of the land–ocean warming contrast in transient climate change simulations and shows that the magnitude of the land–ocean warming contrast is nearly constant over time, maintaining a ratio of approximately 1.5, between land and ocean surface warming. This small temporal change is explained mainly by a compensation between land-to-ocean energy transport and ocean heat uptake, because both act in opposite directions to the land–ocean warming contrast.
Abstract
The impact of North Indian atmospheric diabatic heating variation on summer rainfall over Central Asia (CA) at an interannual scale during 1960–2019 was investigated from thermal adaptation and water vapor transportation perspective. The results showed that more precipitation in southeastern CA is associated with the southward subtropical westerly jet (SWJ), caused by the ascending motion and weakened water vapor output on the south side. When the SWJ moves southward, the high-level water vapor transportation on the south side changes from outward (−1.9 × 106 kg s−1) to inward (0.6 × 106 kg s−1), and the positive anomalous relative vorticity advections by the basic westerly winds produce ascending anomalies over southeastern CA. The position change in the SWJ was mainly related to atmospheric diabatic heating over northern India (NI). The thermal vorticity adaptation caused by a weakened heating rate over NI leads to an anomalous upper-level cyclone over southeastern CA, and the associated cold temperature advection eventually cools the upper troposphere of southeastern CA and reduces the temperature gradient at mid-to-high latitudes, leading to the southward SWJ. Thermal adaptation of the circulation and temperature anomaly over southeastern CA to the NI thermal forcing were also verified by numerical experiments. Both the abnormal ascending motions and the weakened outward water vapor associated with the southward SWJ, caused by the weakened heating rate over NI, lead to more summer rainfall in southeastern CA. The changes in diabatic heating over NI are closely related to Indian Ocean SST. When the Indian Ocean SST is warmer, the south Asian summer monsoon weakens, causing less precipitation and, thus, a weakened heating rate over NI.
Significance Statement
This study established that the northern Indian atmospheric diabatic heating anomalies associated with Indian Ocean SST variation play an important role in influencing precipitation in central Asia (CA). The weakening of the atmospheric diabatic heating over the NI would not only cause an abnormal cyclone and cooling over southern CA through thermal adaptation but also lead to southward subtropical westerly jet (SWJ), ascending motions, and decreased outward water vapor on the south side in southeastern CA, eventually resulting in more precipitation in southeastern CA. The results emphasize the influence of tropical SST and atmospheric heat sources on midlatitude climate and are important for understanding summer precipitation change in southeastern CA.
Abstract
The impact of North Indian atmospheric diabatic heating variation on summer rainfall over Central Asia (CA) at an interannual scale during 1960–2019 was investigated from thermal adaptation and water vapor transportation perspective. The results showed that more precipitation in southeastern CA is associated with the southward subtropical westerly jet (SWJ), caused by the ascending motion and weakened water vapor output on the south side. When the SWJ moves southward, the high-level water vapor transportation on the south side changes from outward (−1.9 × 106 kg s−1) to inward (0.6 × 106 kg s−1), and the positive anomalous relative vorticity advections by the basic westerly winds produce ascending anomalies over southeastern CA. The position change in the SWJ was mainly related to atmospheric diabatic heating over northern India (NI). The thermal vorticity adaptation caused by a weakened heating rate over NI leads to an anomalous upper-level cyclone over southeastern CA, and the associated cold temperature advection eventually cools the upper troposphere of southeastern CA and reduces the temperature gradient at mid-to-high latitudes, leading to the southward SWJ. Thermal adaptation of the circulation and temperature anomaly over southeastern CA to the NI thermal forcing were also verified by numerical experiments. Both the abnormal ascending motions and the weakened outward water vapor associated with the southward SWJ, caused by the weakened heating rate over NI, lead to more summer rainfall in southeastern CA. The changes in diabatic heating over NI are closely related to Indian Ocean SST. When the Indian Ocean SST is warmer, the south Asian summer monsoon weakens, causing less precipitation and, thus, a weakened heating rate over NI.
Significance Statement
This study established that the northern Indian atmospheric diabatic heating anomalies associated with Indian Ocean SST variation play an important role in influencing precipitation in central Asia (CA). The weakening of the atmospheric diabatic heating over the NI would not only cause an abnormal cyclone and cooling over southern CA through thermal adaptation but also lead to southward subtropical westerly jet (SWJ), ascending motions, and decreased outward water vapor on the south side in southeastern CA, eventually resulting in more precipitation in southeastern CA. The results emphasize the influence of tropical SST and atmospheric heat sources on midlatitude climate and are important for understanding summer precipitation change in southeastern CA.
Abstract
The Pacific meridional modes (PMMs) are the leading ocean–atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both NPMM- and SPMM-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether oceanic heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM. Thus, the NPMM SST anomalies appear to instead exhibit a poleward shift, although still freely propagating westward from the preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind–evaporation–SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to determine due to the strong simulation bias of the SPMM evolution in the DOM.
Significance Statement
While it is known that both NPMM- and SPMM-associated SST anomalies are primarily driven by net surface heat flux variations during their evolution, whether ocean advections would play a role remains unknown. Here, we show that ocean advections play a role in the evolution of both PMMs. In particular, for the NPMM evolution, ocean advections play a damping role in the south of the NPMM center, causing a tendency for the NPMM to be displaced northward. The role of ocean advection challenges the prevailing notion that the NPMM simply evolves equatorward through the wind–evaporation–SST feedback.
Abstract
The Pacific meridional modes (PMMs) are the leading ocean–atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both NPMM- and SPMM-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether oceanic heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM. Thus, the NPMM SST anomalies appear to instead exhibit a poleward shift, although still freely propagating westward from the preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind–evaporation–SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to determine due to the strong simulation bias of the SPMM evolution in the DOM.
Significance Statement
While it is known that both NPMM- and SPMM-associated SST anomalies are primarily driven by net surface heat flux variations during their evolution, whether ocean advections would play a role remains unknown. Here, we show that ocean advections play a role in the evolution of both PMMs. In particular, for the NPMM evolution, ocean advections play a damping role in the south of the NPMM center, causing a tendency for the NPMM to be displaced northward. The role of ocean advection challenges the prevailing notion that the NPMM simply evolves equatorward through the wind–evaporation–SST feedback.
Abstract
The Southeast Tibetan Plateau (SETP) is a major region where many low-latitude glaciers are located, with spring precipitation being a major input of the glacier mass balance. This study shows that early spring precipitation has decreased significantly since 1999, which is attributed to declined moisture contribution from the far-field sources (west of 70°E) induced by the weakened subtropical westerlies. The possible physical mechanism underlying this change has also been revealed. It is found that snow-cover extent (SCE) in March reduced in midlatitude Eurasia after 1999; meanwhile, strong solar radiation during this month may have exacerbated snow melting through snow albedo–radiation interactions. These two processes led to warming and caused a strong anticyclone over midlatitude Eurasia that weakened the subtropical westerlies near 30°N. This decadal change in the subtropical westerlies led to a decrease in moisture transport upstream. As a result, the windward slopes of large terrain along the latitudinal belt near 30°N received less precipitation, and the decrease in SETP precipitation was part of this change. A further analysis shows that the positive correlation between the westerlies and precipitation has weakened since 1999.
Significance Statement
The purpose of this study is to reveal the decreased early spring precipitation and explore its possible physical mechanism in the Southeast Tibetan Plateau (SETP), which is crucial to understand the shrinkage of the local glacier. Our results indicate that the reduction of snow cover in midlatitude Eurasia since 1999 and the strong solar radiation in March contributed to the weakening subtropical westerlies, which further resulted in the decreasing precipitation in the SETP and other windward slopes of large terrain along the latitudinal 30°N belt in Eurasia.
Abstract
The Southeast Tibetan Plateau (SETP) is a major region where many low-latitude glaciers are located, with spring precipitation being a major input of the glacier mass balance. This study shows that early spring precipitation has decreased significantly since 1999, which is attributed to declined moisture contribution from the far-field sources (west of 70°E) induced by the weakened subtropical westerlies. The possible physical mechanism underlying this change has also been revealed. It is found that snow-cover extent (SCE) in March reduced in midlatitude Eurasia after 1999; meanwhile, strong solar radiation during this month may have exacerbated snow melting through snow albedo–radiation interactions. These two processes led to warming and caused a strong anticyclone over midlatitude Eurasia that weakened the subtropical westerlies near 30°N. This decadal change in the subtropical westerlies led to a decrease in moisture transport upstream. As a result, the windward slopes of large terrain along the latitudinal belt near 30°N received less precipitation, and the decrease in SETP precipitation was part of this change. A further analysis shows that the positive correlation between the westerlies and precipitation has weakened since 1999.
Significance Statement
The purpose of this study is to reveal the decreased early spring precipitation and explore its possible physical mechanism in the Southeast Tibetan Plateau (SETP), which is crucial to understand the shrinkage of the local glacier. Our results indicate that the reduction of snow cover in midlatitude Eurasia since 1999 and the strong solar radiation in March contributed to the weakening subtropical westerlies, which further resulted in the decreasing precipitation in the SETP and other windward slopes of large terrain along the latitudinal 30°N belt in Eurasia.
Abstract
Three levels of process-oriented model diagnostics are applied to evaluate the Global Ensemble Forecast System version 12 (GEFSv12) reforecasts. The level-1 diagnostics are focused on model systematic errors, which reveals that precipitation onset over tropical oceans occurs too early in terms of column water vapor accumulation. Since precipitation acts to deplete water vapor, this results in prevailing negative biases of precipitable water in the tropics. It is also associated with overtransport of moisture into the mid- and upper troposphere, leading to a dry bias in the lower troposphere and a wet bias in the mid–upper troposphere. The level-2 diagnostics evaluate some major predictability sources on the extended-range time scale: the Madden–Julian oscillation (MJO) and North American weather regimes. It is found that the GEFSv12 can skillfully forecast the MJO up to 16 days ahead in terms of the Real-time Multivariate MJO indices (bivariate correlation ≥ 0.6) and can reasonably represent the MJO propagation across the Maritime Continent. The weakened and less coherent MJO signals with increasing forecast lead times may be attributed to humidity biases over the Indo-Pacific warm pool region. It is also found that the weather regimes can be skillfully predicted up to 12 days ahead with persistence comparable to the observation. In the level-3 diagnostics, we examined some high-impact weather systems. The GEFSv12 shows reduced mean biases in tropical cyclone genesis distribution and improved performance in capturing tropical cyclone interannual variability, and midlatitude blocking climatology in the GEFSv12 also shows a better agreement with the observations than in the GEFSv10.
Significance Statement
The latest U.S. operational weather prediction model—Global Ensemble Forecast System version 12—is evaluated using a suite of physics-based diagnostic metrics from a climatic perspective. The foci of our study consist of three levels: 1) systematic biases in physical processes, 2) tropical and extratropical extended-range predictability sources, and 3) high-impact weather systems like hurricanes and blockings. Such process-oriented diagnostics help us link the model performance to the deficiencies of physics parameterization and thus provide useful information on future model improvement.
Abstract
Three levels of process-oriented model diagnostics are applied to evaluate the Global Ensemble Forecast System version 12 (GEFSv12) reforecasts. The level-1 diagnostics are focused on model systematic errors, which reveals that precipitation onset over tropical oceans occurs too early in terms of column water vapor accumulation. Since precipitation acts to deplete water vapor, this results in prevailing negative biases of precipitable water in the tropics. It is also associated with overtransport of moisture into the mid- and upper troposphere, leading to a dry bias in the lower troposphere and a wet bias in the mid–upper troposphere. The level-2 diagnostics evaluate some major predictability sources on the extended-range time scale: the Madden–Julian oscillation (MJO) and North American weather regimes. It is found that the GEFSv12 can skillfully forecast the MJO up to 16 days ahead in terms of the Real-time Multivariate MJO indices (bivariate correlation ≥ 0.6) and can reasonably represent the MJO propagation across the Maritime Continent. The weakened and less coherent MJO signals with increasing forecast lead times may be attributed to humidity biases over the Indo-Pacific warm pool region. It is also found that the weather regimes can be skillfully predicted up to 12 days ahead with persistence comparable to the observation. In the level-3 diagnostics, we examined some high-impact weather systems. The GEFSv12 shows reduced mean biases in tropical cyclone genesis distribution and improved performance in capturing tropical cyclone interannual variability, and midlatitude blocking climatology in the GEFSv12 also shows a better agreement with the observations than in the GEFSv10.
Significance Statement
The latest U.S. operational weather prediction model—Global Ensemble Forecast System version 12—is evaluated using a suite of physics-based diagnostic metrics from a climatic perspective. The foci of our study consist of three levels: 1) systematic biases in physical processes, 2) tropical and extratropical extended-range predictability sources, and 3) high-impact weather systems like hurricanes and blockings. Such process-oriented diagnostics help us link the model performance to the deficiencies of physics parameterization and thus provide useful information on future model improvement.
Abstract
This study identified that the Silk Road pattern (SRP), which is a teleconnection pattern along the Asian upper-tropospheric westerly jet, becomes significantly weakened in August after the mid-1990s. The SRP in August dominates the upper-tropospheric meridional wind variability over the Eurasian continent before the mid-1990s but does not afterward. Further results suggested that the summer North Atlantic Oscillation (SNAO) and the South Asian rainfall play a role in inducing this decadal weakening of SRP. Before the mid-1990s, the SNAO is stronger and its southern pole is located over northwestern Europe but is weakened and its southern pole shifts southwestward afterward, resulting in the decadal weakening of its contribution to the SRP. In addition, the relationship between the SRP and South Asian rainfall is substantially weakened after the mid-1990s, which also contributes to the weakening of SRP.
Abstract
This study identified that the Silk Road pattern (SRP), which is a teleconnection pattern along the Asian upper-tropospheric westerly jet, becomes significantly weakened in August after the mid-1990s. The SRP in August dominates the upper-tropospheric meridional wind variability over the Eurasian continent before the mid-1990s but does not afterward. Further results suggested that the summer North Atlantic Oscillation (SNAO) and the South Asian rainfall play a role in inducing this decadal weakening of SRP. Before the mid-1990s, the SNAO is stronger and its southern pole is located over northwestern Europe but is weakened and its southern pole shifts southwestward afterward, resulting in the decadal weakening of its contribution to the SRP. In addition, the relationship between the SRP and South Asian rainfall is substantially weakened after the mid-1990s, which also contributes to the weakening of SRP.
Abstract
The influence of mid–high-latitude intraseasonal variability (ISV) on the occurrence frequency of the Northeast China cold vortex (NCCV) in early summer was examined through statistical analysis and thermal–dynamic diagnostics. A multivariable empirical orthogonal function (MVEOF) was employed to extract the thermal–pressure coupled ISV mode. Our results show that the geopotential height and air temperature over the NCCV active region exhibit a statistically significant intraseasonal periodicity of 20–60 days. The dominant ISV mode features a westward-propagated zonal dipole pattern, which is generated over the Lake Baikal region and triggered by intraseasonal wave energy accumulation. By dividing the ISV cycle into eight phases, it is found that more NCCVs with a large scope occur in phases 5–8 than those in phases 1–4. The positive (negative) geopotential height and air temperature tendencies in phases 1–4 (5–8) act to suppress (facilitate) the NCCV activity. The thermodynamic tendency budget and scale decomposition reveal that when an anomalous intraseasonal cyclonic circulation propagates westward from Lake Baikal to the Ural Mountains, the anomalous southwesterly transports mean negative vorticity from the north side of the Tibetan Plateau to Northeast Asia and transports mean warm air temperature from low latitudes to high latitudes, leading to the positive geopotential height and air temperature tendencies and thereby restraining the NCCV activity. The opposite is also true for the facilitation of the NCCV modulated by the negative geopotential height and air temperature tendencies.
Significance Statement
The purpose of this study is to better understand the factors controlling the Northeast China cold vortex (NCCV) activity in early summer. It is important because influences of the subtropical monsoonal circulation are usually confined to southern China in this season and the anomalous atmospheric circulation from the mid–high latitudes plays a more important role in the generation of the NCCV. Our results provide a guide on how intraseasonal variability at mid–high latitudes controls the occurrence frequency of the NCCV and highlight the process of thermal and dynamical modulation in the NCCV.
Abstract
The influence of mid–high-latitude intraseasonal variability (ISV) on the occurrence frequency of the Northeast China cold vortex (NCCV) in early summer was examined through statistical analysis and thermal–dynamic diagnostics. A multivariable empirical orthogonal function (MVEOF) was employed to extract the thermal–pressure coupled ISV mode. Our results show that the geopotential height and air temperature over the NCCV active region exhibit a statistically significant intraseasonal periodicity of 20–60 days. The dominant ISV mode features a westward-propagated zonal dipole pattern, which is generated over the Lake Baikal region and triggered by intraseasonal wave energy accumulation. By dividing the ISV cycle into eight phases, it is found that more NCCVs with a large scope occur in phases 5–8 than those in phases 1–4. The positive (negative) geopotential height and air temperature tendencies in phases 1–4 (5–8) act to suppress (facilitate) the NCCV activity. The thermodynamic tendency budget and scale decomposition reveal that when an anomalous intraseasonal cyclonic circulation propagates westward from Lake Baikal to the Ural Mountains, the anomalous southwesterly transports mean negative vorticity from the north side of the Tibetan Plateau to Northeast Asia and transports mean warm air temperature from low latitudes to high latitudes, leading to the positive geopotential height and air temperature tendencies and thereby restraining the NCCV activity. The opposite is also true for the facilitation of the NCCV modulated by the negative geopotential height and air temperature tendencies.
Significance Statement
The purpose of this study is to better understand the factors controlling the Northeast China cold vortex (NCCV) activity in early summer. It is important because influences of the subtropical monsoonal circulation are usually confined to southern China in this season and the anomalous atmospheric circulation from the mid–high latitudes plays a more important role in the generation of the NCCV. Our results provide a guide on how intraseasonal variability at mid–high latitudes controls the occurrence frequency of the NCCV and highlight the process of thermal and dynamical modulation in the NCCV.
Abstract
The baroclinic annular mode (BAM) is the leading mode of variability in extratropical eddy activity characterized by its hemispheric-scale pulsing. Based on atmospheric reanalysis data for the Southern Hemisphere, this study reveals BAM-associated systematic modulations not only in fluxes associated with subweekly transient disturbances, as found by earlier studies, but also in their spatial structure involved in the dynamics of the BAM. Specifically, in the positive phase of the BAM characterized by enhanced activity of transient disturbances, their lower-tropospheric baroclinic structure becomes more distinct, and they tend to be more elongated meridionally in both the upper and lower troposphere. These BAM-associated structural modulations of the disturbances favor the more efficient baroclinic development via enhanced poleward heat transport and their downstream development, which can contribute to hemispheric-scale enhancement of kinetic energy associated with the disturbances. In addition, a tendency of the disturbances to exhibit horizontally tilting structure becomes more evident in the positive phase of the BAM, which is favorable for enhanced transport of westerly momentum from the subtropics to the midlatitude polar-front jet, or equivalently enhanced wave-activity propagation from the midlatitude storm track into the subtropics. This modulation lags the peak of anomalous kinetic energy of the disturbances, thus acting to contribute to the decay of the BAM signature. A set of numerical simulations suggests that the BAM-associated pulsing in storm-track activity and structural modulations are manifestations of atmospheric internal dynamics, which can be significantly amplified in the presence of a midlatitude oceanic frontal zone through the formation of more organized and coherent baroclinic wave packets.
Abstract
The baroclinic annular mode (BAM) is the leading mode of variability in extratropical eddy activity characterized by its hemispheric-scale pulsing. Based on atmospheric reanalysis data for the Southern Hemisphere, this study reveals BAM-associated systematic modulations not only in fluxes associated with subweekly transient disturbances, as found by earlier studies, but also in their spatial structure involved in the dynamics of the BAM. Specifically, in the positive phase of the BAM characterized by enhanced activity of transient disturbances, their lower-tropospheric baroclinic structure becomes more distinct, and they tend to be more elongated meridionally in both the upper and lower troposphere. These BAM-associated structural modulations of the disturbances favor the more efficient baroclinic development via enhanced poleward heat transport and their downstream development, which can contribute to hemispheric-scale enhancement of kinetic energy associated with the disturbances. In addition, a tendency of the disturbances to exhibit horizontally tilting structure becomes more evident in the positive phase of the BAM, which is favorable for enhanced transport of westerly momentum from the subtropics to the midlatitude polar-front jet, or equivalently enhanced wave-activity propagation from the midlatitude storm track into the subtropics. This modulation lags the peak of anomalous kinetic energy of the disturbances, thus acting to contribute to the decay of the BAM signature. A set of numerical simulations suggests that the BAM-associated pulsing in storm-track activity and structural modulations are manifestations of atmospheric internal dynamics, which can be significantly amplified in the presence of a midlatitude oceanic frontal zone through the formation of more organized and coherent baroclinic wave packets.
