Abstract
In this study, observational and model datasets are used to analyze winter precipitation and its leading empirical orthogonal function (EOF1) mode over Southeast China. EOF1 displays a dominant monosign pattern during the last 60 years; however, its major impacting factors have a decadal transition near the mid-1990s. The first principal component (PC1) is related to El Niño–Southern Oscillation (ENSO) after the mid-1990s and to the quasi-biennial oscillation (QBO) before the mid-1990s. An enhanced ENSO–precipitation relationship is associated with stronger ENSO-induced tropical zonal circulation and the westward shift of ENSO-induced SST over the tropical Pacific after the mid-1990s compared to before the mid-1990s. Negative correlation coefficients between QBO and precipitation are evident before the mid-1990s but are no longer statistically significant after the mid-1990s. This change originates from the interaction between QBO’s subtropical influences and the Holton–Tan effect. The QBO’s subtropical influence and the Holton–Tan effect lead to a zonal pressure gradient and meridional wind anomalies over East Asia before the mid-1990s, which further influence the meridional transport of water vapor and precipitation over Southeast China. However, the Holton–Tan effect is enhanced after the mid-1990s. Downward stratospheric polar vortex signals and the QBO’s subtropical influence cause a meridional pressure gradient over East Asia, and thus the relevant moisture flux divergence lacks statistical significance. The above results indicate that the subtropical response to QBO and the Holton–Tan effect should be considered together when using the QBO signal to improve forecasts of winter precipitation over East Asia.
Significance Statement
Southeast China winter precipitation (SCWP) is often attributed to variation in lower-atmospheric dynamics and sea surface temperature, such as El Niño–Southern Oscillation (ENSO). Few studies focus on the role of the mid- to upper atmosphere. In this study, we diagnose the influence of the stratospheric quasi-biennial oscillation (QBO) on SCWP. Both observational and modeling analyses indicate a strong decadal change in the QBO–SCWP relationship, which limits the use of the QBO in seasonal forecasts of SCWP. This decadal change originates from the strength of the QBO’s modulation of the stratospheric polar vortex. Our results provide a new perspective on the use of the mid- to upper atmosphere in seasonal forecasts of SCWP.
Abstract
In this study, observational and model datasets are used to analyze winter precipitation and its leading empirical orthogonal function (EOF1) mode over Southeast China. EOF1 displays a dominant monosign pattern during the last 60 years; however, its major impacting factors have a decadal transition near the mid-1990s. The first principal component (PC1) is related to El Niño–Southern Oscillation (ENSO) after the mid-1990s and to the quasi-biennial oscillation (QBO) before the mid-1990s. An enhanced ENSO–precipitation relationship is associated with stronger ENSO-induced tropical zonal circulation and the westward shift of ENSO-induced SST over the tropical Pacific after the mid-1990s compared to before the mid-1990s. Negative correlation coefficients between QBO and precipitation are evident before the mid-1990s but are no longer statistically significant after the mid-1990s. This change originates from the interaction between QBO’s subtropical influences and the Holton–Tan effect. The QBO’s subtropical influence and the Holton–Tan effect lead to a zonal pressure gradient and meridional wind anomalies over East Asia before the mid-1990s, which further influence the meridional transport of water vapor and precipitation over Southeast China. However, the Holton–Tan effect is enhanced after the mid-1990s. Downward stratospheric polar vortex signals and the QBO’s subtropical influence cause a meridional pressure gradient over East Asia, and thus the relevant moisture flux divergence lacks statistical significance. The above results indicate that the subtropical response to QBO and the Holton–Tan effect should be considered together when using the QBO signal to improve forecasts of winter precipitation over East Asia.
Significance Statement
Southeast China winter precipitation (SCWP) is often attributed to variation in lower-atmospheric dynamics and sea surface temperature, such as El Niño–Southern Oscillation (ENSO). Few studies focus on the role of the mid- to upper atmosphere. In this study, we diagnose the influence of the stratospheric quasi-biennial oscillation (QBO) on SCWP. Both observational and modeling analyses indicate a strong decadal change in the QBO–SCWP relationship, which limits the use of the QBO in seasonal forecasts of SCWP. This decadal change originates from the strength of the QBO’s modulation of the stratospheric polar vortex. Our results provide a new perspective on the use of the mid- to upper atmosphere in seasonal forecasts of SCWP.
Abstract
Axisymmetric theory of atmospheric circulation is extended for the case of concentrated equatorial cooling and annually averaged heating. The solutions are derived in a 1.5-layer shallow water model on the spherical Earth, which includes vertical mixing with diagnostic surface momentum. The axisymmetric solutions capture the sensitivity of the large-scale circulation to equatorial cooling seen in observations and in eddy-permitting models, namely, (i) weakening and widening of the meridional overturning circulation (MOC) and (ii) weakening and poleward shift of the subtropical jet. For sufficiently large equatorial cooling, a tropical anti-Hadley cell emerges that transports energy equatorward, balancing the equatorial energetic sink. The analytic solutions predict the critical cooling required for the emergence of the anti-Hadley cell and provide a simple mechanism for the response of the MOC to equatorial cooling. Specifically, equatorial cooling reduces net tropical heating, which weakens the circulation and shifts the edge of the rising branch poleward. This in turn reduces upper-level momentum which is set by surface momentum in the rising branch. The subtropical meridional temperature gradient decreases with upper-level momentum, requiring a widening of the circulation to close the energy budget. The subtropical jet therefore shifts poleward with the edge of the MOC and weakens due to the reduced upper-level angular momentum. The strong sensitivity to equatorial cooling seen in the axisymmetric system suggests that the above mechanism may have an important role in the sensitivity of the MOC to equatorial temperature anomalies on seasonal or longer time scales.
Significance Statement
Axisymmetric solutions for the response of the atmospheric circulation to concentrated equatorial cooling are derived, motivated by the equatorial cooling by the cold tongues of the Pacific and Atlantic. The solutions capture the response of the large-scale realistic atmosphere to equatorial cooling, namely a weakening and widening of the meridional overturning circulation and a weakening and poleward shift of the subtropical jet. In addition, for sufficiently strong cooling, a tropical anti-Hadley cell emerges. The solutions provide insight into the influence of biases in the representation of the cold tongues in climate models, the relation of interannual equatorial variability to the large-scale circulation, and changes in the large-scale atmospheric circulation on geological time scales.
Abstract
Axisymmetric theory of atmospheric circulation is extended for the case of concentrated equatorial cooling and annually averaged heating. The solutions are derived in a 1.5-layer shallow water model on the spherical Earth, which includes vertical mixing with diagnostic surface momentum. The axisymmetric solutions capture the sensitivity of the large-scale circulation to equatorial cooling seen in observations and in eddy-permitting models, namely, (i) weakening and widening of the meridional overturning circulation (MOC) and (ii) weakening and poleward shift of the subtropical jet. For sufficiently large equatorial cooling, a tropical anti-Hadley cell emerges that transports energy equatorward, balancing the equatorial energetic sink. The analytic solutions predict the critical cooling required for the emergence of the anti-Hadley cell and provide a simple mechanism for the response of the MOC to equatorial cooling. Specifically, equatorial cooling reduces net tropical heating, which weakens the circulation and shifts the edge of the rising branch poleward. This in turn reduces upper-level momentum which is set by surface momentum in the rising branch. The subtropical meridional temperature gradient decreases with upper-level momentum, requiring a widening of the circulation to close the energy budget. The subtropical jet therefore shifts poleward with the edge of the MOC and weakens due to the reduced upper-level angular momentum. The strong sensitivity to equatorial cooling seen in the axisymmetric system suggests that the above mechanism may have an important role in the sensitivity of the MOC to equatorial temperature anomalies on seasonal or longer time scales.
Significance Statement
Axisymmetric solutions for the response of the atmospheric circulation to concentrated equatorial cooling are derived, motivated by the equatorial cooling by the cold tongues of the Pacific and Atlantic. The solutions capture the response of the large-scale realistic atmosphere to equatorial cooling, namely a weakening and widening of the meridional overturning circulation and a weakening and poleward shift of the subtropical jet. In addition, for sufficiently strong cooling, a tropical anti-Hadley cell emerges. The solutions provide insight into the influence of biases in the representation of the cold tongues in climate models, the relation of interannual equatorial variability to the large-scale circulation, and changes in the large-scale atmospheric circulation on geological time scales.
Abstract
This paper explores whether particles within uniformly-spaced generating cells falling at terminal velocity within observed 2-D wind fields and idealized deformation flow beneath cloud top can be reorganized consistent with the presence of single and multi-banded structures present on WSR-88D radars. In the first experiment, two-dimensional wind fields, calculated along cross-sections normal to the long-axis of snow bands observed during three Northeast U.S. winter storms, were taken from the initialization of the High Resolution Rapid Refresh model. This experiment demonstrated that the greater the residence time of the particles in each of the three storms, the greater particle reorganization occurred. For experiments with longer residence times, increases in particle concentrations were nearly or directly collocated with reflectivity bands. For experiments with shorter residence times, particle reorganization still conformed to the band features but with less concentration enhancement. This experiment demonstrates that the combination of long particle residence time and net convergent cross-sectional flow through the cloud depth is sufficient to re-organize particles into locations consistent with precipitation bands. Increased concentrations of ice particles can then contribute, along with any dynamic forcing, to the low-level reflectivity bands seen on WSR-88D radars. In a second experiment, the impact of flow deformation on the re-organization of falling ice particles was investigated using an idealized kinematic model with stretching deformation flow of different depths and magnitudes. These experiments showed that deformation flow provides for little particle reorganization given typical deformation layer depths and magnitudes within the comma head of such storms.
Abstract
This paper explores whether particles within uniformly-spaced generating cells falling at terminal velocity within observed 2-D wind fields and idealized deformation flow beneath cloud top can be reorganized consistent with the presence of single and multi-banded structures present on WSR-88D radars. In the first experiment, two-dimensional wind fields, calculated along cross-sections normal to the long-axis of snow bands observed during three Northeast U.S. winter storms, were taken from the initialization of the High Resolution Rapid Refresh model. This experiment demonstrated that the greater the residence time of the particles in each of the three storms, the greater particle reorganization occurred. For experiments with longer residence times, increases in particle concentrations were nearly or directly collocated with reflectivity bands. For experiments with shorter residence times, particle reorganization still conformed to the band features but with less concentration enhancement. This experiment demonstrates that the combination of long particle residence time and net convergent cross-sectional flow through the cloud depth is sufficient to re-organize particles into locations consistent with precipitation bands. Increased concentrations of ice particles can then contribute, along with any dynamic forcing, to the low-level reflectivity bands seen on WSR-88D radars. In a second experiment, the impact of flow deformation on the re-organization of falling ice particles was investigated using an idealized kinematic model with stretching deformation flow of different depths and magnitudes. These experiments showed that deformation flow provides for little particle reorganization given typical deformation layer depths and magnitudes within the comma head of such storms.
Abstract
El Niño–Southern Oscillation (ENSO) exhibits highly asymmetric temporal evolutions between its warm and cold phases. While El Niño events usually terminate rapidly after their mature phase and show an already established transition into the cold phase by the following summer, many La Niña events tend to persist throughout the second year and even reintensify in the ensuing winter. While many mechanisms were proposed, no consensus has been reached yet and the essential physical processes responsible for the multiyear behavior of La Niña remain to be illustrated. Here, we show that a unique ocean physical process operates during multiyear La Niña events. It is characterized by rapid double reversals of zonal ocean current anomalies in the equatorial Pacific and exhibits a fairly regular near-annual periodicity. Mixed-layer heat budget analyses reveal comparable contributions of the thermocline and zonal advective feedbacks to the SST anomaly growth in the first year of multiyear La Niña events; however, the zonal advective feedback plays a dominant role in the reintensification of La Niña events. Furthermore, the unique ocean process is identified to be closely associated with the preconditioning heat content state in the central to eastern equatorial Pacific before the first year of La Niña, which has been shown in previous studies to play an active role in setting the stage for the future reintensification of La Niña. Despite systematic underestimation, the above oceanic process can be broadly reproduced by state-of-the-art climate models, providing a potential additional source of predictability for the multiyear La Niña events.
Abstract
El Niño–Southern Oscillation (ENSO) exhibits highly asymmetric temporal evolutions between its warm and cold phases. While El Niño events usually terminate rapidly after their mature phase and show an already established transition into the cold phase by the following summer, many La Niña events tend to persist throughout the second year and even reintensify in the ensuing winter. While many mechanisms were proposed, no consensus has been reached yet and the essential physical processes responsible for the multiyear behavior of La Niña remain to be illustrated. Here, we show that a unique ocean physical process operates during multiyear La Niña events. It is characterized by rapid double reversals of zonal ocean current anomalies in the equatorial Pacific and exhibits a fairly regular near-annual periodicity. Mixed-layer heat budget analyses reveal comparable contributions of the thermocline and zonal advective feedbacks to the SST anomaly growth in the first year of multiyear La Niña events; however, the zonal advective feedback plays a dominant role in the reintensification of La Niña events. Furthermore, the unique ocean process is identified to be closely associated with the preconditioning heat content state in the central to eastern equatorial Pacific before the first year of La Niña, which has been shown in previous studies to play an active role in setting the stage for the future reintensification of La Niña. Despite systematic underestimation, the above oceanic process can be broadly reproduced by state-of-the-art climate models, providing a potential additional source of predictability for the multiyear La Niña events.
Abstract
This study investigates the influence of the Atlantic multidecadal oscillation (AMO) on the multidecadal variability of winter surface air temperature in arid central Asia (ACASAT). Apart from a long-term warming trend, the observational analysis shows that the winter ACASAT exhibits a significant multidecadal variability, which is characterized by antiphase fluctuations with the AMO. The mechanism for this negative correlation between the AMO and the winter ACASAT is explored from the aspect of wave teleconnection. The AMO provides energy for the Scandinavian teleconnection pattern at middle and low altitudes by regulating the high-altitude wave train over the middle and high latitudes of Eurasia, and thus has an impact on the remote climate in arid central Asia. Results from the linear baroclinic model (LBM) provide evidence for the linkage between the AMO and the Scandinavian teleconnection pattern. When the AMO is in its warm periods, the Scandinavian teleconnection pattern is in a positive phase, which further makes the cold air from the northeast strengthen, leading to the anomalously colder surface air temperature in arid central Asia. Based on the relationship that the North Atlantic Oscillation (NAO) leads the AMO by 15–20 years, it is further found that there is a leading relationship between the NAO and the winter ACASAT via the AMO. On this basis, an empirical model using the NAO as a predictor was established to predict the ACASAT, and the empirical model shows good hindcast performance. Results from the model show that the winter ACASAT will continue to rise in the next 10 years and decline after 2030.
Abstract
This study investigates the influence of the Atlantic multidecadal oscillation (AMO) on the multidecadal variability of winter surface air temperature in arid central Asia (ACASAT). Apart from a long-term warming trend, the observational analysis shows that the winter ACASAT exhibits a significant multidecadal variability, which is characterized by antiphase fluctuations with the AMO. The mechanism for this negative correlation between the AMO and the winter ACASAT is explored from the aspect of wave teleconnection. The AMO provides energy for the Scandinavian teleconnection pattern at middle and low altitudes by regulating the high-altitude wave train over the middle and high latitudes of Eurasia, and thus has an impact on the remote climate in arid central Asia. Results from the linear baroclinic model (LBM) provide evidence for the linkage between the AMO and the Scandinavian teleconnection pattern. When the AMO is in its warm periods, the Scandinavian teleconnection pattern is in a positive phase, which further makes the cold air from the northeast strengthen, leading to the anomalously colder surface air temperature in arid central Asia. Based on the relationship that the North Atlantic Oscillation (NAO) leads the AMO by 15–20 years, it is further found that there is a leading relationship between the NAO and the winter ACASAT via the AMO. On this basis, an empirical model using the NAO as a predictor was established to predict the ACASAT, and the empirical model shows good hindcast performance. Results from the model show that the winter ACASAT will continue to rise in the next 10 years and decline after 2030.
Abstract
In this study, we analyze drivers of non–El Niño–Southern Oscillation (ENSO) precipitation variability in the Southwest United States (SWUS) and the influence of the atmospheric basic state, using atmosphere-only and ocean–atmosphere coupled simulations from the Community Earth System Model version 2 (CESM2) large ensemble. A cluster analysis identifies three main wave trains associated with non-ENSO SWUS precipitation in the experiments: a meridional ENSO-type wave train, an arching Pacific–North American-type (PNA) wave train, and a circumglobal zonal wave train. The zonal wave train cluster frequency differs between models and ENSO phase, with decreased frequency during El Niño and the coupled runs, and increased frequency during La Niña and the atmosphere-only runs. This is consistent with an El Niño–like bias of the atmospheric circulation in the coupled model, with strengthened subtropical westerlies in the central and eastern North Pacific that cause a retraction of the waveguide in the midlatitude eastern North Pacific. As such, zonal wave trains from the East Asian jet stream (EAJS) are more likely to be diverted southward in the east Pacific in the coupled large ensemble, with a consequently smaller role in driving SWUS precipitation variability. This study illustrates the need to reduce model biases in the background flow, particularly relating to the jet stream, in order to accurately capture the role of large-scale teleconnections in driving SWUS precipitation variability and improve future forecasting capabilities.
Abstract
In this study, we analyze drivers of non–El Niño–Southern Oscillation (ENSO) precipitation variability in the Southwest United States (SWUS) and the influence of the atmospheric basic state, using atmosphere-only and ocean–atmosphere coupled simulations from the Community Earth System Model version 2 (CESM2) large ensemble. A cluster analysis identifies three main wave trains associated with non-ENSO SWUS precipitation in the experiments: a meridional ENSO-type wave train, an arching Pacific–North American-type (PNA) wave train, and a circumglobal zonal wave train. The zonal wave train cluster frequency differs between models and ENSO phase, with decreased frequency during El Niño and the coupled runs, and increased frequency during La Niña and the atmosphere-only runs. This is consistent with an El Niño–like bias of the atmospheric circulation in the coupled model, with strengthened subtropical westerlies in the central and eastern North Pacific that cause a retraction of the waveguide in the midlatitude eastern North Pacific. As such, zonal wave trains from the East Asian jet stream (EAJS) are more likely to be diverted southward in the east Pacific in the coupled large ensemble, with a consequently smaller role in driving SWUS precipitation variability. This study illustrates the need to reduce model biases in the background flow, particularly relating to the jet stream, in order to accurately capture the role of large-scale teleconnections in driving SWUS precipitation variability and improve future forecasting capabilities.
