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Abstract
Summer rainfall in the central United States has singular interannual variations of a 3–6-yr period. Identifying the causes of these variations assures improvement in predictions of summer rainfall in the region.
A review of previous studies revealed a puzzling situation: the outstanding interannual variations of the summer rainfall in the central United States showed no persistent correlations with known influential interannual variations in the Northern Hemisphere and the El Niño–Southern Oscillation (ENSO). This study was undertaken to identify the cause of this situation and ultimately explain the causes of the observed interannual summer rainfall variations. Its results showed a teleconnection of the ENSO with the summer rainfall in the central United States. The intensity of which has varied over the last 125 years. The teleconnection was active in two epochs, 1871–1916 and 1948–78, and absent in the two epochs 1917–47 and 1979–present. This variation was associated with a multidecadal variation in both sea surface temperature and sea level pressure in the mid- and high-latitude North Pacific. In the epochs of active teleconnection, the circulation in the warm phase of ENSO favored a deformation field in the lower troposphere in the central United States causing wet summers and a reversed circulation in cold phase of ENSO yielding dry summers, a process that partially explains the interannual summer rainfall variations.
The result also showed that the variations of the teleconnection were “in phase” with the variation in the average surface temperature of the Northern Hemisphere. When the “abrupt warming” of the surface temperature developed in 1917–47 and the most recent two decades, the teleconnection broke down. Because of the limitation in data record length, this observed relationship and the persistence of the variation in the teleconnection need further investigations when additional data are available.
Abstract
Summer rainfall in the central United States has singular interannual variations of a 3–6-yr period. Identifying the causes of these variations assures improvement in predictions of summer rainfall in the region.
A review of previous studies revealed a puzzling situation: the outstanding interannual variations of the summer rainfall in the central United States showed no persistent correlations with known influential interannual variations in the Northern Hemisphere and the El Niño–Southern Oscillation (ENSO). This study was undertaken to identify the cause of this situation and ultimately explain the causes of the observed interannual summer rainfall variations. Its results showed a teleconnection of the ENSO with the summer rainfall in the central United States. The intensity of which has varied over the last 125 years. The teleconnection was active in two epochs, 1871–1916 and 1948–78, and absent in the two epochs 1917–47 and 1979–present. This variation was associated with a multidecadal variation in both sea surface temperature and sea level pressure in the mid- and high-latitude North Pacific. In the epochs of active teleconnection, the circulation in the warm phase of ENSO favored a deformation field in the lower troposphere in the central United States causing wet summers and a reversed circulation in cold phase of ENSO yielding dry summers, a process that partially explains the interannual summer rainfall variations.
The result also showed that the variations of the teleconnection were “in phase” with the variation in the average surface temperature of the Northern Hemisphere. When the “abrupt warming” of the surface temperature developed in 1917–47 and the most recent two decades, the teleconnection broke down. Because of the limitation in data record length, this observed relationship and the persistence of the variation in the teleconnection need further investigations when additional data are available.
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.
Abstract
Previous studies have identified several major causes for summer rainfall variations over the southwest United States, for example, land memory (i.e., relationships between antecedent winter season precipitation and snow cover anomalies and subsequent summer rainfall anomalies over the southwest United States; these anomalies are likely most important in the northwest United States, although antecedent anomalies in the southwest United States also may be important in determining summer rainfall variations) and sea surface temperature (SST) anomalies in the North Pacific. Atmospheric responses to these “boundary forces” interact with moisture flows from the Gulf of Mexico and from the Gulf of California to influence the rainfall in the Southwest. The land memory and the SST effects were further found to be “naturally separated,” in the sense that they each played a dominant role influencing the monsoon rainfall variation during different periods of the last century. This separation was also manifested by different dominant low-level moisture transport anomalies in those periods. Several new questions have arisen from these findings: How have the land memory and the SST effects been “separated,” so as to affect the monsoon rainfall variations during different periods, or “regimes”? And, what are the corresponding changes of low-level flows, and hence moisture transports into the southwest United States that help achieve the land memory or the SST effects on the rainfall variations during these different regimes? These questions, and related issues, are addressed using a numerical model of regional climate. The model was used to simulate 14 individual warm seasons (April–October) in each of the postulated regimes. Analyses of the simulation results showed systematic and significant changes in atmospheric circulation anomalies between the two regimes. In the early regime (1961–90), when the land memory effect was strong, the average geopotential height was lower and storm activity was more intense over the central and western United States than in the more recent regime (from 1990 on), indicating reduced eddy energy and momentum exchanges between high and low latitudes in the western United States. The effects of these changes on the monsoon rainfall were achieved by very different low-level flow and moisture transport anomalies. In the earlier regime, low-level flow and moisture transport anomalies in the southwest United States were primarily due to easterlies and southeasterlies into the Southwest for its wet monsoon conditions, with reversed anomalies for dry conditions. In the recent regime, these anomalies changed, with primarily southerlies and southwesterlies from the Gulf of California into the Southwest during its wet monsoon conditions, and reversed flow anomalies for dry conditions. These changes indicate that different physical processes, including those responsible for the planetary-scale atmospheric circulation, led to monsoon rainfall variations during each of these regimes.
Abstract
Previous studies have identified several major causes for summer rainfall variations over the southwest United States, for example, land memory (i.e., relationships between antecedent winter season precipitation and snow cover anomalies and subsequent summer rainfall anomalies over the southwest United States; these anomalies are likely most important in the northwest United States, although antecedent anomalies in the southwest United States also may be important in determining summer rainfall variations) and sea surface temperature (SST) anomalies in the North Pacific. Atmospheric responses to these “boundary forces” interact with moisture flows from the Gulf of Mexico and from the Gulf of California to influence the rainfall in the Southwest. The land memory and the SST effects were further found to be “naturally separated,” in the sense that they each played a dominant role influencing the monsoon rainfall variation during different periods of the last century. This separation was also manifested by different dominant low-level moisture transport anomalies in those periods. Several new questions have arisen from these findings: How have the land memory and the SST effects been “separated,” so as to affect the monsoon rainfall variations during different periods, or “regimes”? And, what are the corresponding changes of low-level flows, and hence moisture transports into the southwest United States that help achieve the land memory or the SST effects on the rainfall variations during these different regimes? These questions, and related issues, are addressed using a numerical model of regional climate. The model was used to simulate 14 individual warm seasons (April–October) in each of the postulated regimes. Analyses of the simulation results showed systematic and significant changes in atmospheric circulation anomalies between the two regimes. In the early regime (1961–90), when the land memory effect was strong, the average geopotential height was lower and storm activity was more intense over the central and western United States than in the more recent regime (from 1990 on), indicating reduced eddy energy and momentum exchanges between high and low latitudes in the western United States. The effects of these changes on the monsoon rainfall were achieved by very different low-level flow and moisture transport anomalies. In the earlier regime, low-level flow and moisture transport anomalies in the southwest United States were primarily due to easterlies and southeasterlies into the Southwest for its wet monsoon conditions, with reversed anomalies for dry conditions. In the recent regime, these anomalies changed, with primarily southerlies and southwesterlies from the Gulf of California into the Southwest during its wet monsoon conditions, and reversed flow anomalies for dry conditions. These changes indicate that different physical processes, including those responsible for the planetary-scale atmospheric circulation, led to monsoon rainfall variations during each of these regimes.
Abstract
The arid and semiarid region in central Asia is sensitive and vulnerable to climate variations. However, the sparse and highly unevenly distributed meteorological stations in the region provide limited data for understanding of the region’s climate variations. In this study, the near-surface air temperature change in central Asia from 1979 to 2011 was examined using observations from 81 meteorological stations, three local observation validated reanalysis datasets of relatively high spatial resolutions, and the Climate Research Unit (CRU) dataset. Major results suggested that the three reanalysis datasets match well with most of the local climate records, especially in the low-lying plain areas. The consensus of the multiple datasets showed significant regional surface air temperature increases of 0.36°–0.42°C decade−1 in the past 33 years. No significant contributions from declining irrigation and urbanization to temperature change were found. The rate is larger in recent years than in the early years in the study period. Additionally, unlike in many regions in the world, the temperature in winter showed no increase in central Asia in the last three decades, a noticeable departure from the global trend in the twentieth century. The largest increase in surface temperature was occurring in the spring season. Analyses further showed a warming center in the middle of the central Asian states and weakened temperature variability along the northwest–southeast temperature gradient from the northern Kazakhstan to southern Xinjiang. The reanalysis datasets also showed significant negative correlations between temperature increase rate and elevation in this complex terrain region.
Abstract
The arid and semiarid region in central Asia is sensitive and vulnerable to climate variations. However, the sparse and highly unevenly distributed meteorological stations in the region provide limited data for understanding of the region’s climate variations. In this study, the near-surface air temperature change in central Asia from 1979 to 2011 was examined using observations from 81 meteorological stations, three local observation validated reanalysis datasets of relatively high spatial resolutions, and the Climate Research Unit (CRU) dataset. Major results suggested that the three reanalysis datasets match well with most of the local climate records, especially in the low-lying plain areas. The consensus of the multiple datasets showed significant regional surface air temperature increases of 0.36°–0.42°C decade−1 in the past 33 years. No significant contributions from declining irrigation and urbanization to temperature change were found. The rate is larger in recent years than in the early years in the study period. Additionally, unlike in many regions in the world, the temperature in winter showed no increase in central Asia in the last three decades, a noticeable departure from the global trend in the twentieth century. The largest increase in surface temperature was occurring in the spring season. Analyses further showed a warming center in the middle of the central Asian states and weakened temperature variability along the northwest–southeast temperature gradient from the northern Kazakhstan to southern Xinjiang. The reanalysis datasets also showed significant negative correlations between temperature increase rate and elevation in this complex terrain region.
Abstract
In 1991, the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) established its Soil Moisture–Soil Temperature (SM–ST) Pilot Network consisting of 21 stations in 19 states in the contiguous United States. At each station, soil temperatures were measured at up to six different depths from 5.08 to 203.20 cm (or 2–80 in.) below the surface. Before 1997, the observations were made every 6 h, and they increased to hourly beginning in 1997. The goal of this network is to provide near–real time soil temperature and soil moisture observations in different regions across the United States for agricultural and water use management as well as for climate research. To improve the usefulness and increase the value of both the data and this network, a quality-control method for the soil temperature data was developed. The method used a soil heat diffusion model and its solution at individual sites to screen and distinguish erroneous soil temperature data and to provide their estimates. Evaluation of the quality-control method showed its accuracy and reliability, particularly when it was applied to hourly data. Application of this method to the data has yielded a high-quality, high-resolution soil temperature database from 1994 to 1999 for the network, which is accessible at the USDA National Water and Climate Center's Web site.
Abstract
In 1991, the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) established its Soil Moisture–Soil Temperature (SM–ST) Pilot Network consisting of 21 stations in 19 states in the contiguous United States. At each station, soil temperatures were measured at up to six different depths from 5.08 to 203.20 cm (or 2–80 in.) below the surface. Before 1997, the observations were made every 6 h, and they increased to hourly beginning in 1997. The goal of this network is to provide near–real time soil temperature and soil moisture observations in different regions across the United States for agricultural and water use management as well as for climate research. To improve the usefulness and increase the value of both the data and this network, a quality-control method for the soil temperature data was developed. The method used a soil heat diffusion model and its solution at individual sites to screen and distinguish erroneous soil temperature data and to provide their estimates. Evaluation of the quality-control method showed its accuracy and reliability, particularly when it was applied to hourly data. Application of this method to the data has yielded a high-quality, high-resolution soil temperature database from 1994 to 1999 for the network, which is accessible at the USDA National Water and Climate Center's Web site.
Abstract
Although eastward propagation has long been considered one of the essential features of the Madden-Julian waves, recent observations have revealed a stationary or quasi-stationary component in the oscillations, particularly in measures of the diabatic heating rate. Wave-CISK theories of the low-frequency oscillations have struggled to explain the observed period and vertical structure of the waves. On the other hand, theoretical and numerical studies have shown that low-frequency waves strongly resembling the observed oscillations can be excited by specified low-frequency oscillations of the convective heating. A problem with the latter set of theories is that the cause of the oscillatory heating has not been satisfactorily explained. It is proposed here that the observed low-frequency wave motions are the response to forcing by an essentially stationary, self-excited oscillating heat source that is produced by nonlinear interactions among radiation, cumulus convection, and the surface fluxes of sensible heat and moisture. Feedback of the large-scale motions on the latent heating is not required. Results from two very different one-dimensional models are presented to support this hypothesis. The physical processes included in the models are essentially the same, that is, radiation, cumulus convection, and the surface fluxes of sensible heat and moisture; the first model is highly simplified, however, while the second includes relatively sophisticated parameterizations of all the relevant physical processes. Results from both models show low-frequency oscillations of the latent heating, temperature, and moisture. Experiments show that the oscillations are favored by a warm sea surface and weak surface wind speeds, consistent with the observed conditions over the Indian Ocean and the tropical western Pacific Ocean.
Abstract
Although eastward propagation has long been considered one of the essential features of the Madden-Julian waves, recent observations have revealed a stationary or quasi-stationary component in the oscillations, particularly in measures of the diabatic heating rate. Wave-CISK theories of the low-frequency oscillations have struggled to explain the observed period and vertical structure of the waves. On the other hand, theoretical and numerical studies have shown that low-frequency waves strongly resembling the observed oscillations can be excited by specified low-frequency oscillations of the convective heating. A problem with the latter set of theories is that the cause of the oscillatory heating has not been satisfactorily explained. It is proposed here that the observed low-frequency wave motions are the response to forcing by an essentially stationary, self-excited oscillating heat source that is produced by nonlinear interactions among radiation, cumulus convection, and the surface fluxes of sensible heat and moisture. Feedback of the large-scale motions on the latent heating is not required. Results from two very different one-dimensional models are presented to support this hypothesis. The physical processes included in the models are essentially the same, that is, radiation, cumulus convection, and the surface fluxes of sensible heat and moisture; the first model is highly simplified, however, while the second includes relatively sophisticated parameterizations of all the relevant physical processes. Results from both models show low-frequency oscillations of the latent heating, temperature, and moisture. Experiments show that the oscillations are favored by a warm sea surface and weak surface wind speeds, consistent with the observed conditions over the Indian Ocean and the tropical western Pacific Ocean.
Abstract
A simple model is used to examine the hypothesis that nonlinear interactions among atmospheric radiation, cumulus convection, and the surface moisture flux can result in a stationary, low-frequency (30–60 day period) oscillating heat source in the tropical atmosphere. The model produces low-frequency oscillations of temperature, moisture, and precipitation. The mechanism that produces these oscillations is identified through analyses of the model and its results. The relevance of this mechanism to understanding the observed Madden-Julian oscillation in the tropical atmosphere over the Indian and western Pacific Ocean is discussed.
Abstract
A simple model is used to examine the hypothesis that nonlinear interactions among atmospheric radiation, cumulus convection, and the surface moisture flux can result in a stationary, low-frequency (30–60 day period) oscillating heat source in the tropical atmosphere. The model produces low-frequency oscillations of temperature, moisture, and precipitation. The mechanism that produces these oscillations is identified through analyses of the model and its results. The relevance of this mechanism to understanding the observed Madden-Julian oscillation in the tropical atmosphere over the Indian and western Pacific Ocean is discussed.