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- Author or Editor: Qi Hu x
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Abstract
Recent studies have identified two sources alternating their dominant roles in the interannual summer rainfall variations in the central United States. One is the ENSO cycle in the tropical Pacific, and the other is the interannual variability in the intensity of the southerly flow from the Gulf of Mexico. The ENSO cycle affected the rainfall variation through an atmospheric teleconnection, which was particularly strong in 1871–1916 and 1948–78. When the teleconnection weakened in 1917–47 and 1979–2002, the southerly flow from the Gulf of Mexico invigorated its effect on the interannual rainfall variation. Because the effect of the two sources could result in different rainfall processes in the central United States, their alternation should have built similar variation into the region’s summer season diurnal rainfall pattern.
An hourly rainfall dataset was used to examine this hypothesis. Results showed a multidecadal variation in the diurnal rainfall pattern. In the decades when the southerly flow dominated the rainfall variation, the diurnal pattern had large rainfall in late night/morning hours with a sharp rainfall peak in the midnight hour. In the decades when the southerly flow effect weakened, a different diurnal pattern emerged, with small late night/morning hour rainfall and a broad plateau of rainfall in the late night/early morning hours. This diurnal pattern change was happening simultaneously with variations of the southerly moisture flux and moisture convergence in the central United States. These coherent variations show that the summer season diurnal rainfall pattern varied at a multidecadal scale consistent with the alternation frequency of the two sources. In addition to showing the variation in the diurnal rainfall pattern, results of this study provide the knowledge for understanding the climate of extreme events, particularly heavy rainfall and floods, in the central United States.
Abstract
Recent studies have identified two sources alternating their dominant roles in the interannual summer rainfall variations in the central United States. One is the ENSO cycle in the tropical Pacific, and the other is the interannual variability in the intensity of the southerly flow from the Gulf of Mexico. The ENSO cycle affected the rainfall variation through an atmospheric teleconnection, which was particularly strong in 1871–1916 and 1948–78. When the teleconnection weakened in 1917–47 and 1979–2002, the southerly flow from the Gulf of Mexico invigorated its effect on the interannual rainfall variation. Because the effect of the two sources could result in different rainfall processes in the central United States, their alternation should have built similar variation into the region’s summer season diurnal rainfall pattern.
An hourly rainfall dataset was used to examine this hypothesis. Results showed a multidecadal variation in the diurnal rainfall pattern. In the decades when the southerly flow dominated the rainfall variation, the diurnal pattern had large rainfall in late night/morning hours with a sharp rainfall peak in the midnight hour. In the decades when the southerly flow effect weakened, a different diurnal pattern emerged, with small late night/morning hour rainfall and a broad plateau of rainfall in the late night/early morning hours. This diurnal pattern change was happening simultaneously with variations of the southerly moisture flux and moisture convergence in the central United States. These coherent variations show that the summer season diurnal rainfall pattern varied at a multidecadal scale consistent with the alternation frequency of the two sources. In addition to showing the variation in the diurnal rainfall pattern, results of this study provide the knowledge for understanding the climate of extreme events, particularly heavy rainfall and floods, in the central United States.
Abstract
The author presents a cumulus parameterization that uses a cloud model that describes atmospheric convection as consisting of a sequence of intermittently rising thermals. The total mass of thermals in a convection event is determined by the amount of convective available potential energy in local soundings. In the cloud model, it is assumed that a thermal entrains environmental air only at a thin layer around the top frontier of its rising body. The entrained air mass mixes with the thermal’s air and produces “mixtures” that then detach themselves from the thermal. This limited mixing prevents deep erosion to the thermal’s buoyancy by entrainment and mixing processes. The remainder of the thermal continues rising to higher levels and forming more mixtures on its way to its own level of neutral buoyancy. The mixtures also rise or sink from the levels where they form to their level of neutral buoyancy.
Evaluation of this scheme using Global Atmospheric Research Program Atlantic Tropical Experiment data shows that the parameterized convective heating and drying rates are consistent with observations. The calculated convective precipitation also shows a distribution similar to the observed total precipitation, except at the trough of the easterly waves where calculated precipitation is smaller than observed. The capability of this scheme in describing cumulus convection is further tested in a fully prognostic one-dimensional climate model. Results from this evaluation show reasonable climatological temperature and relative humidity profiles in the troposphere.
Abstract
The author presents a cumulus parameterization that uses a cloud model that describes atmospheric convection as consisting of a sequence of intermittently rising thermals. The total mass of thermals in a convection event is determined by the amount of convective available potential energy in local soundings. In the cloud model, it is assumed that a thermal entrains environmental air only at a thin layer around the top frontier of its rising body. The entrained air mass mixes with the thermal’s air and produces “mixtures” that then detach themselves from the thermal. This limited mixing prevents deep erosion to the thermal’s buoyancy by entrainment and mixing processes. The remainder of the thermal continues rising to higher levels and forming more mixtures on its way to its own level of neutral buoyancy. The mixtures also rise or sink from the levels where they form to their level of neutral buoyancy.
Evaluation of this scheme using Global Atmospheric Research Program Atlantic Tropical Experiment data shows that the parameterized convective heating and drying rates are consistent with observations. The calculated convective precipitation also shows a distribution similar to the observed total precipitation, except at the trough of the easterly waves where calculated precipitation is smaller than observed. The capability of this scheme in describing cumulus convection is further tested in a fully prognostic one-dimensional climate model. Results from this evaluation show reasonable climatological temperature and relative humidity profiles in the troposphere.
Abstract
The author revisits the singular value decomposition (SVD) method and shows that the nonuniqueness of the left and right singular vectors related to SVD posts limitations on applications of the method. Caution should be observed when the heterogeneous and homogeneous correlation maps are applied to interpret the relationship between two meteorological data series.
Abstract
The author revisits the singular value decomposition (SVD) method and shows that the nonuniqueness of the left and right singular vectors related to SVD posts limitations on applications of the method. Caution should be observed when the heterogeneous and homogeneous correlation maps are applied to interpret the relationship between two meteorological data series.
Abstract
Recent studies have identified a connection between the summer monsoon rainfall in the southwest United States and anomalies of the antecedent winter precipitation and snowpack in the northwest United States. This connection shows a seasonal-scale predictability of the precipitation and indicates a seasonal predictability of the land–atmosphere system (the “land memory”) in the western United States. Although some efforts have been devoted to understanding this predictability, the physical processes constituting it remain unexplained. In this empirical study, a potential source, the soil enthalpy, and its role in land memory are examined for the recent epoch of a strong land memory (1961–90). The rationale is that the soil enthalpy variation has magnitudes comparable to the atmospheric enthalpy changes at various time scales, and the soil enthalpy anomaly in the top 20–50-cm soil column can persist for 2–3 months. As shown by the major results of this study, a persistent negative anomaly of the soil enthalpy in the northwest United States is related to negative anomalies of the surface and the lower-troposphere temperature in that region. Subsequently, the lower-troposphere temperature and related higher-atmospheric pressure anomalies in the northwest United States during late spring and the early summer months encourage a northward position of the lower-troposphere monsoon ridge in the western United States and, therefore, create a circulation that favors an above-average monsoon rainfall in the southwest United States. A weaker summer monsoon occurs when a sequence of opposite anomalies develops after a warm and dry winter in the northwest United States. In this regard, the soil enthalpy variations may serve to “record” the winter precipitation and temperature anomalies and “release” their effects on summer monsoon rainfall through interactions of soil enthalpy with the surface and lower-troposphere temperatures.
Abstract
Recent studies have identified a connection between the summer monsoon rainfall in the southwest United States and anomalies of the antecedent winter precipitation and snowpack in the northwest United States. This connection shows a seasonal-scale predictability of the precipitation and indicates a seasonal predictability of the land–atmosphere system (the “land memory”) in the western United States. Although some efforts have been devoted to understanding this predictability, the physical processes constituting it remain unexplained. In this empirical study, a potential source, the soil enthalpy, and its role in land memory are examined for the recent epoch of a strong land memory (1961–90). The rationale is that the soil enthalpy variation has magnitudes comparable to the atmospheric enthalpy changes at various time scales, and the soil enthalpy anomaly in the top 20–50-cm soil column can persist for 2–3 months. As shown by the major results of this study, a persistent negative anomaly of the soil enthalpy in the northwest United States is related to negative anomalies of the surface and the lower-troposphere temperature in that region. Subsequently, the lower-troposphere temperature and related higher-atmospheric pressure anomalies in the northwest United States during late spring and the early summer months encourage a northward position of the lower-troposphere monsoon ridge in the western United States and, therefore, create a circulation that favors an above-average monsoon rainfall in the southwest United States. A weaker summer monsoon occurs when a sequence of opposite anomalies develops after a warm and dry winter in the northwest United States. In this regard, the soil enthalpy variations may serve to “record” the winter precipitation and temperature anomalies and “release” their effects on summer monsoon rainfall through interactions of soil enthalpy with the surface and lower-troposphere temperatures.
Abstract
The “land memory” refers to an interseasonal predictability of the summer monsoon rainfall in the southwestern United States, describing a relationship of the summer monsoon rainfall anomaly with anomalies in the antecedent winter season snow and land surface conditions in the western United States. This relationship has varied, however, showing a peculiar on-and-off feature in the last century. It is important to understand this variation so that the relationship can be used to assist making predictions of the monsoon rainfall for that region. This note offers the evidence and shows that the change of the land memory may have been a reflection of an irregular variation in the persistence of the sea surface temperature anomaly (SSTA) in the North Pacific Ocean; in epochs when the SSTA persisted from winter through summer, the SSTA and related anomalies in atmospheric circulation could have dominated the summer monsoon variation, whereas in epochs when the persistence collapsed the SSTA effect weakened and the effect of the land processes on the summer monsoon rainfall became prominent.
Abstract
The “land memory” refers to an interseasonal predictability of the summer monsoon rainfall in the southwestern United States, describing a relationship of the summer monsoon rainfall anomaly with anomalies in the antecedent winter season snow and land surface conditions in the western United States. This relationship has varied, however, showing a peculiar on-and-off feature in the last century. It is important to understand this variation so that the relationship can be used to assist making predictions of the monsoon rainfall for that region. This note offers the evidence and shows that the change of the land memory may have been a reflection of an irregular variation in the persistence of the sea surface temperature anomaly (SSTA) in the North Pacific Ocean; in epochs when the SSTA persisted from winter through summer, the SSTA and related anomalies in atmospheric circulation could have dominated the summer monsoon variation, whereas in epochs when the persistence collapsed the SSTA effect weakened and the effect of the land processes on the summer monsoon rainfall became prominent.
Abstract
Observational studies have created a dilemma on how El Niño–Southern Oscillation (ENSO) may have affected interannual variations of summer rainfall in northern China; some suggested a consistent effect while others showed a complete lack of effect. This dilemma is resolved in this study, which shows that ENSO has affected the summer rainfall in northern China and the effect has varied at multidecadal scales. The question of how the ENSO teleconnection with northern China rainfall variation was established is addressed, and an answer pointing to the Indian summer monsoon as a “facilitator” connecting ENSO and northern China rainfall variation is examined. The Indian monsoon circulation interacted with the regional circulations in northern China in some epochs and such interaction was interrupted in other epochs. When the interaction was active, the Indian monsoon variations originating from ENSO, during El Niño or La Niña, was extended to affect the rainfall variation in northern China, creating a teleconnection of ENSO with northern China rainfall. When the interaction weakened or was inactive, the ENSO effect languished. Additional analyses were done to address the related question of why the interactions have alternated. The alternation was suggested to result from variations of the large-scale circulation in the Eurasian continent. The circulation anomalies showed lowering (rising) 500-hPa geopotential height centered at Mongolia and western China in some epochs, enhancing cyclonic (anticyclonic) rotation in mid- and low-level winds and creating (disrupting) a moisture convoy from the Indian monsoon region to northern China and synergetic convergence/divergence anomalies in the monsoon region and in northern China. Results of this study contribute to the understanding of interannual and multidecadal variations of the summer rainfall in the semiarid region of northern China.
Abstract
Observational studies have created a dilemma on how El Niño–Southern Oscillation (ENSO) may have affected interannual variations of summer rainfall in northern China; some suggested a consistent effect while others showed a complete lack of effect. This dilemma is resolved in this study, which shows that ENSO has affected the summer rainfall in northern China and the effect has varied at multidecadal scales. The question of how the ENSO teleconnection with northern China rainfall variation was established is addressed, and an answer pointing to the Indian summer monsoon as a “facilitator” connecting ENSO and northern China rainfall variation is examined. The Indian monsoon circulation interacted with the regional circulations in northern China in some epochs and such interaction was interrupted in other epochs. When the interaction was active, the Indian monsoon variations originating from ENSO, during El Niño or La Niña, was extended to affect the rainfall variation in northern China, creating a teleconnection of ENSO with northern China rainfall. When the interaction weakened or was inactive, the ENSO effect languished. Additional analyses were done to address the related question of why the interactions have alternated. The alternation was suggested to result from variations of the large-scale circulation in the Eurasian continent. The circulation anomalies showed lowering (rising) 500-hPa geopotential height centered at Mongolia and western China in some epochs, enhancing cyclonic (anticyclonic) rotation in mid- and low-level winds and creating (disrupting) a moisture convoy from the Indian monsoon region to northern China and synergetic convergence/divergence anomalies in the monsoon region and in northern China. Results of this study contribute to the understanding of interannual and multidecadal variations of the summer rainfall in the semiarid region of northern China.
Abstract
Several studies of the established warm season climate records for eastern China (1470–1997) showed alternating dry and wet periods at centennial scales. The spatial patterns show that when a dry condition or drought was observed in southern China, a wet or flood situation was found in the northern part of eastern China and vice versa. These patterns suggest a meridional variation of the centennial-scale wet/dry anomalies.
This study analyzed the same data and showed that the dry and wet anomalies initially appeared in the northern part of eastern China and then migrated southward to affect the low latitudes. An extension of this analysis to the United States revealed a similar southward migration of dry/wet anomalies that first developed in the high latitudes in the western part of the country. The average speed of the migrations in both areas is about 3.0° of latitude per 10 years.
The results suggest that mechanisms in mid- and high latitudes may play critical roles in the development of drought in high- as well as subtropical-latitude regions. The findings also indicate key areas to monitor for prediction of extended periods of frequent droughts or floods in “downstream” regions in the migration of the centennial-scale anomalies.
Abstract
Several studies of the established warm season climate records for eastern China (1470–1997) showed alternating dry and wet periods at centennial scales. The spatial patterns show that when a dry condition or drought was observed in southern China, a wet or flood situation was found in the northern part of eastern China and vice versa. These patterns suggest a meridional variation of the centennial-scale wet/dry anomalies.
This study analyzed the same data and showed that the dry and wet anomalies initially appeared in the northern part of eastern China and then migrated southward to affect the low latitudes. An extension of this analysis to the United States revealed a similar southward migration of dry/wet anomalies that first developed in the high latitudes in the western part of the country. The average speed of the migrations in both areas is about 3.0° of latitude per 10 years.
The results suggest that mechanisms in mid- and high latitudes may play critical roles in the development of drought in high- as well as subtropical-latitude regions. The findings also indicate key areas to monitor for prediction of extended periods of frequent droughts or floods in “downstream” regions in the migration of the centennial-scale anomalies.
Abstract
This study continues the investigation of causes of the interannual variations in summer rainfall in the central United States. A previous study by the authors showed that the ENSO teleconnection significantly affected the interannual variations in summer rainfall in the central United States in two epochs, 1871–1916 and 1949–78. The teleconnection effect weakened in the epochs 1917–48 and 1979–97. The current study partially answers the question: What affected the interannual summer rainfall variations in the two epochs when the ENSO teleconnection weakened? Its results showed that the low-level southerly flow from the Gulf of Mexico was another source of interannual summer rainfall variations. The southerly flow possessed significant interannual variations independent of the ENSO variation. In the epochs when the ENSO teleconnection broke down, the variations of the southerly flow amplified. In the meantime, the circulation anomalies in the lower troposphere in the central United States favored a convergence and an unstable thermal profile. They helped to engage the variations in the southerly flow and the summer rainfall variation in the central United States and to maintain the interannual summer rainfall variation. A coherent variation of this source and ENSO teleconnection in different epochs sustained the observed interannual variations of the summer rainfall in the central United States. The coherent variation of the role of these two different sources was in accord with the multidecadal variation in SST in the mid- and high-latitude North Pacific Ocean, supporting the notion that the multidecadal variation in the SST may have facilitated the coherent variation.
Abstract
This study continues the investigation of causes of the interannual variations in summer rainfall in the central United States. A previous study by the authors showed that the ENSO teleconnection significantly affected the interannual variations in summer rainfall in the central United States in two epochs, 1871–1916 and 1949–78. The teleconnection effect weakened in the epochs 1917–48 and 1979–97. The current study partially answers the question: What affected the interannual summer rainfall variations in the two epochs when the ENSO teleconnection weakened? Its results showed that the low-level southerly flow from the Gulf of Mexico was another source of interannual summer rainfall variations. The southerly flow possessed significant interannual variations independent of the ENSO variation. In the epochs when the ENSO teleconnection broke down, the variations of the southerly flow amplified. In the meantime, the circulation anomalies in the lower troposphere in the central United States favored a convergence and an unstable thermal profile. They helped to engage the variations in the southerly flow and the summer rainfall variation in the central United States and to maintain the interannual summer rainfall variation. A coherent variation of this source and ENSO teleconnection in different epochs sustained the observed interannual variations of the summer rainfall in the central United States. The coherent variation of the role of these two different sources was in accord with the multidecadal variation in SST in the mid- and high-latitude North Pacific Ocean, supporting the notion that the multidecadal variation in the SST may have facilitated the coherent variation.
Abstract
The following questions are addressed in this study using an array of data and statistical methods: 1) does the North American monsoon region have a single dominant monsoon system; 2) if it has more than one, what are they; and 3) what are major causes of interannual monsoon rainfall variations in these systems? Results showed two dominant summer monsoon systems in the region: one in south-central Mexico, south of the 26°N, and the other in the southwestern United States and northwestern Mexico. Monsoon rainfall variations in these regions are usually opposite to each other and have different causes. The interannual variations in monsoon rainfall in south-central Mexico were highly affected by interannual variations in the intertropical convergence zone (ITCZ) in the eastern tropical Pacific. A northern (southern) position of the ITCZ, often related to cooler (warmer) than normal sea surface temperatures in the eastern tropical Pacific Ocean, corresponded to strong (weak) monsoon.
The “land memory effect” was evident in interannual variations of monsoon rainfall in the southwestern United States, shown by strong correlations of the summer rainfall variation versus antecedent winter precipitation anomalies in the western United States. However, the effect was not robust but varied fairly regularly. It was strong from approximately 1920 to 1930 and disappeared from 1931 to 1960. It regained its strength from 1961 to 1990 but has weakened again since 1990. The forcing of this variation was identified as a multidecadal variation in atmosphere circulations in the North Pacific–North American sector and the land memory effect was part of this variation. This multidecadal variation has to be included in prediction methods in order for them to correctly describe seasonal and interannual variations in summer rainfall in the North American monsoon region.
Abstract
The following questions are addressed in this study using an array of data and statistical methods: 1) does the North American monsoon region have a single dominant monsoon system; 2) if it has more than one, what are they; and 3) what are major causes of interannual monsoon rainfall variations in these systems? Results showed two dominant summer monsoon systems in the region: one in south-central Mexico, south of the 26°N, and the other in the southwestern United States and northwestern Mexico. Monsoon rainfall variations in these regions are usually opposite to each other and have different causes. The interannual variations in monsoon rainfall in south-central Mexico were highly affected by interannual variations in the intertropical convergence zone (ITCZ) in the eastern tropical Pacific. A northern (southern) position of the ITCZ, often related to cooler (warmer) than normal sea surface temperatures in the eastern tropical Pacific Ocean, corresponded to strong (weak) monsoon.
The “land memory effect” was evident in interannual variations of monsoon rainfall in the southwestern United States, shown by strong correlations of the summer rainfall variation versus antecedent winter precipitation anomalies in the western United States. However, the effect was not robust but varied fairly regularly. It was strong from approximately 1920 to 1930 and disappeared from 1931 to 1960. It regained its strength from 1961 to 1990 but has weakened again since 1990. The forcing of this variation was identified as a multidecadal variation in atmosphere circulations in the North Pacific–North American sector and the land memory effect was part of this variation. This multidecadal variation has to be included in prediction methods in order for them to correctly describe seasonal and interannual variations in summer rainfall in the North American monsoon region.
Abstract
The North American summer monsoon holds the key to understanding warm season rainfall variations in the region from northern Mexico to the Southwest and the central United States. Studies of the monsoon have pictured mosaic submonsoonal regions and different processes influencing monsoon variations. Among the influencing processes is the “land memory,” showing primarily the influence of the antecedent winter season precipitation (snow) anomalies in the Northwest on summer rainfall anomalies in the Southwest. More intriguingly, the land memory has been found to vary at the multidecadal time scale. This memory change may actually reflect multidecadal variations of the atmospheric circulation in the North American monsoon region. This notion is examined in this study by first establishing the North American monsoon regimes from relationships of summer rainfall variations in central and western North America, and then quantifying their variations at the multidecadal scale in the twentieth century. Results of these analyses show two monsoon regimes: one featured with consistent variations in summer rainfall in west Mexico and the Southwest and an opposite variation pattern in the central United States, and the other with consistent rainfall variations in west Mexico and the central United States but different from the variations in the southwest United States. These regimes have alternated at multidecadal scales in the twentieth century.
This alternation of the regimes is found to be in phase with the North Atlantic Multidecadal Oscillation (AMO). In warm and cold phases of the AMO, distinctive circulation anomalies are found in central and western North America, where lower than average pressure prevailed in the warm phase and the opposite anomaly in the cold phase. Associated wind anomalies configured different patterns for moisture transport and may have contributed to the development and variation of the monsoon regimes. These results indicate that investigations of the effects of AMO and its interaction with the North Pacific circulations could lead to a better understanding of the North American monsoon variations.
Abstract
The North American summer monsoon holds the key to understanding warm season rainfall variations in the region from northern Mexico to the Southwest and the central United States. Studies of the monsoon have pictured mosaic submonsoonal regions and different processes influencing monsoon variations. Among the influencing processes is the “land memory,” showing primarily the influence of the antecedent winter season precipitation (snow) anomalies in the Northwest on summer rainfall anomalies in the Southwest. More intriguingly, the land memory has been found to vary at the multidecadal time scale. This memory change may actually reflect multidecadal variations of the atmospheric circulation in the North American monsoon region. This notion is examined in this study by first establishing the North American monsoon regimes from relationships of summer rainfall variations in central and western North America, and then quantifying their variations at the multidecadal scale in the twentieth century. Results of these analyses show two monsoon regimes: one featured with consistent variations in summer rainfall in west Mexico and the Southwest and an opposite variation pattern in the central United States, and the other with consistent rainfall variations in west Mexico and the central United States but different from the variations in the southwest United States. These regimes have alternated at multidecadal scales in the twentieth century.
This alternation of the regimes is found to be in phase with the North Atlantic Multidecadal Oscillation (AMO). In warm and cold phases of the AMO, distinctive circulation anomalies are found in central and western North America, where lower than average pressure prevailed in the warm phase and the opposite anomaly in the cold phase. Associated wind anomalies configured different patterns for moisture transport and may have contributed to the development and variation of the monsoon regimes. These results indicate that investigations of the effects of AMO and its interaction with the North Pacific circulations could lead to a better understanding of the North American monsoon variations.
