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- Author or Editor: Kingtse C. Mo x
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Abstract
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Abstract
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Abstract
Long-term trends and interannual variations of circulation anomalies in the Southern Hemisphere are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis from 1949 to 1998. The changes in planetary circulation regimes are linked to global sea surface temperature anomalies (SSTAs).
Empirical orthogonal function (EOF) analysis was performed on 500-hPa height anomalies. The leading mode EOF1 shows a strong zonal symmetry with a phase reversal between height anomalies in high and midlatitudes. Apart from zonal symmetry, a zonal wavenumber 3 is evident with three centers located in three southern oceans. In the low-frequency band with fluctuations longer than 60 months, EOF1 is associated with the second rotated EOF mode of SSTAs with positive loadings over three southern oceans and negative loadings in the North Pacific and the North Atlantic.
The next two modes are the Pacific–South American (PSA) patterns. They depict wave-3 patterns in quadrature with each other and a well-defined wave train from the tropical Pacific to Argentina with large amplitudes in the Pacific–South American sector. On decadal timescales, the abrupt warming over the central and eastern Pacific is related to the strengthening of PSA1. In the interannual band, PSA1 is associated with the low-frequency part of El Niño–Southern Oscillation (ENSO) variability with the dominant period of 40–48 months. PSA2 is associated with the quasi-biennial component of ENSO variability with a period of 26 months.
Abstract
Long-term trends and interannual variations of circulation anomalies in the Southern Hemisphere are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis from 1949 to 1998. The changes in planetary circulation regimes are linked to global sea surface temperature anomalies (SSTAs).
Empirical orthogonal function (EOF) analysis was performed on 500-hPa height anomalies. The leading mode EOF1 shows a strong zonal symmetry with a phase reversal between height anomalies in high and midlatitudes. Apart from zonal symmetry, a zonal wavenumber 3 is evident with three centers located in three southern oceans. In the low-frequency band with fluctuations longer than 60 months, EOF1 is associated with the second rotated EOF mode of SSTAs with positive loadings over three southern oceans and negative loadings in the North Pacific and the North Atlantic.
The next two modes are the Pacific–South American (PSA) patterns. They depict wave-3 patterns in quadrature with each other and a well-defined wave train from the tropical Pacific to Argentina with large amplitudes in the Pacific–South American sector. On decadal timescales, the abrupt warming over the central and eastern Pacific is related to the strengthening of PSA1. In the interannual band, PSA1 is associated with the low-frequency part of El Niño–Southern Oscillation (ENSO) variability with the dominant period of 40–48 months. PSA2 is associated with the quasi-biennial component of ENSO variability with a period of 26 months.
Abstract
The ensemble canonical correlation (ECC) prediction method is used to predict summer (July–September) and winter (January–March) seasonal mean surface temperature (T surf) over the United States. The predictors are the global sea surface temperature (SST), sea level pressure over the Northern Hemisphere T surf, and soil moisture over the United States from one to two seasons lead, as well as the model outputs from the NCEP seasonal forecast model. The canonical correlation analysis (CCA) prediction is performed for each variable separately. The predicted T surf fields form an ensemble. The ensemble forecast is the weighted average of its members. Both the simple ensemble forecast and the superensemble forecast are tested. The simple ensemble mean is the equally weighted average of its members. The weighting function for the superensemble forecast is determined by linear regression analysis.
Overall, both ensemble forecasts improve skill. On average, the superensemble gives the best performance. For summer, both ensemble forecasts improve skill substantially in comparison with the CCA forecasts based on the SST alone. Different variables recognize different forcing. They have forecast skills over different regions of the United States. Therefore, the ensemble forecasts are skillful.
For summer, the leading SST modes that contribute to the sources of skill are associated with the long-term decadal trends, ENSO, and variability in the North Atlantic. In addition to SSTs, soil moisture in March–May also plays an important role in forecasting T surf in summer. For winter, SSTs in the tropical Pacific associated with the decadal and ENSO variability dominate the contribution.
Abstract
The ensemble canonical correlation (ECC) prediction method is used to predict summer (July–September) and winter (January–March) seasonal mean surface temperature (T surf) over the United States. The predictors are the global sea surface temperature (SST), sea level pressure over the Northern Hemisphere T surf, and soil moisture over the United States from one to two seasons lead, as well as the model outputs from the NCEP seasonal forecast model. The canonical correlation analysis (CCA) prediction is performed for each variable separately. The predicted T surf fields form an ensemble. The ensemble forecast is the weighted average of its members. Both the simple ensemble forecast and the superensemble forecast are tested. The simple ensemble mean is the equally weighted average of its members. The weighting function for the superensemble forecast is determined by linear regression analysis.
Overall, both ensemble forecasts improve skill. On average, the superensemble gives the best performance. For summer, both ensemble forecasts improve skill substantially in comparison with the CCA forecasts based on the SST alone. Different variables recognize different forcing. They have forecast skills over different regions of the United States. Therefore, the ensemble forecasts are skillful.
For summer, the leading SST modes that contribute to the sources of skill are associated with the long-term decadal trends, ENSO, and variability in the North Atlantic. In addition to SSTs, soil moisture in March–May also plays an important role in forecasting T surf in summer. For winter, SSTs in the tropical Pacific associated with the decadal and ENSO variability dominate the contribution.
Abstract
Data from observations and the Intergovernmental Panel on Climate Change (IPCC) twentieth-century climate change model [phase 3 of the Coupled Model Intercomparison Project (CMIP3)] simulations were analyzed to examine the decadal changes of the impact of ENSO on air temperature T air and precipitation P over the United States. The comparison of composites for the early period (1915–60) and the recent period (1962–2006) indicates that cooling (warming) over the south and warming (cooling) over the north during ENSO warm (cold) winters have been weakening. The ENSO influence on winter P over the Southwest is strengthening, while the impact on P over the Ohio Valley is weakening for the recent decades. These differences are not due to the long-term trends in T air or P; they are attributed to the occurrence of the central Pacific (CPAC) ENSO events in the recent years. The CPAC ENSO differs from the canonical eastern Pacific (EPAC) ENSO. The EPAC ENSO has a sea surface temperature anomaly (SSTA) maximum in the eastern Pacific. Enhanced convection extends from the date line to the eastern Pacific, with negative anomalies in the western Pacific. The atmospheric responses resemble a tropical Northern Hemisphere pattern. The wave train is consistent with the north–south T air contrast over North America during the EPAC ENSO winters. The CPAC ENSO has enhanced convection in the central Pacific. The atmospheric responses show a Pacific–North American pattern. It is consistent with west–east contrast in T air and more rainfall over the Southwest during the CPAC ENSO winters.
Abstract
Data from observations and the Intergovernmental Panel on Climate Change (IPCC) twentieth-century climate change model [phase 3 of the Coupled Model Intercomparison Project (CMIP3)] simulations were analyzed to examine the decadal changes of the impact of ENSO on air temperature T air and precipitation P over the United States. The comparison of composites for the early period (1915–60) and the recent period (1962–2006) indicates that cooling (warming) over the south and warming (cooling) over the north during ENSO warm (cold) winters have been weakening. The ENSO influence on winter P over the Southwest is strengthening, while the impact on P over the Ohio Valley is weakening for the recent decades. These differences are not due to the long-term trends in T air or P; they are attributed to the occurrence of the central Pacific (CPAC) ENSO events in the recent years. The CPAC ENSO differs from the canonical eastern Pacific (EPAC) ENSO. The EPAC ENSO has a sea surface temperature anomaly (SSTA) maximum in the eastern Pacific. Enhanced convection extends from the date line to the eastern Pacific, with negative anomalies in the western Pacific. The atmospheric responses resemble a tropical Northern Hemisphere pattern. The wave train is consistent with the north–south T air contrast over North America during the EPAC ENSO winters. The CPAC ENSO has enhanced convection in the central Pacific. The atmospheric responses show a Pacific–North American pattern. It is consistent with west–east contrast in T air and more rainfall over the Southwest during the CPAC ENSO winters.
Abstract
No abstract available.
Abstract
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Abstract
Two sets of experiments were performed. The first set, denoted SSTA, consisted of 90-day forecasts with sea surface temperature anomalies updated with observed values daily during the entire integration. For the summers 1987 and 1988, three SSTA experiments were made using three different initial conditions centered on 1 June of that year, separated by 1 day. The second set of experiments, denoted CSST, used the same initial conditions as the first set, but the integrations were performed using climatological SSTs. All numerical experiments were done using the NMC T80 spectral model of 1990, which is the same model used in making operational medium-range forecasts.
Simulated seasonal ensemble-mean rainfall was compared with satellite estimates of precipitation and observed station rainfall data. Overall agreement between them is good. Two centers of maximum rainfall, over the Arabian Sea and the Bay of Bengal, are captured by the model, but it fails to capture the movement of the rainfall associated with the Indian monsoon. The model is able to simulate the interannual variability of rain in India and over the Sahel, although the simulated convection in the central Pacific associated with the 1987 warm episode is not realistic.
When the model is able to simulate the convection associated with the SSTAs, then the updated SSTs have a large positive impact on tropical impact seasonal forecasts. The impact on the extratropical forecasts is, in general, positive but small.
Abstract
Two sets of experiments were performed. The first set, denoted SSTA, consisted of 90-day forecasts with sea surface temperature anomalies updated with observed values daily during the entire integration. For the summers 1987 and 1988, three SSTA experiments were made using three different initial conditions centered on 1 June of that year, separated by 1 day. The second set of experiments, denoted CSST, used the same initial conditions as the first set, but the integrations were performed using climatological SSTs. All numerical experiments were done using the NMC T80 spectral model of 1990, which is the same model used in making operational medium-range forecasts.
Simulated seasonal ensemble-mean rainfall was compared with satellite estimates of precipitation and observed station rainfall data. Overall agreement between them is good. Two centers of maximum rainfall, over the Arabian Sea and the Bay of Bengal, are captured by the model, but it fails to capture the movement of the rainfall associated with the Indian monsoon. The model is able to simulate the interannual variability of rain in India and over the Sahel, although the simulated convection in the central Pacific associated with the 1987 warm episode is not realistic.
When the model is able to simulate the convection associated with the SSTAs, then the updated SSTs have a large positive impact on tropical impact seasonal forecasts. The impact on the extratropical forecasts is, in general, positive but small.
Abstract
A composite analysis of multiyear (1985–93) global reanalyses produced by the NCEP/NCAR and the NASA/DAO is used to show that the development of persistent North Pacific (PNP) circulation anomalies during NH winter is linked to tropical intraseasonal oscillations. The development is initiated over the tropical west Pacific by anomalous convection (characterized by an east–west dipole structure) one to two weeks prior to the extratropical onset time in both reanalyses. As tropical heating moves eastward toward the central Pacific, anomalous divergent outflow associated with the local Hadley circulation generates an anomalous Rossby wave sink (source) in the subtropics, consistent with the retraction (extension) of the Pacific jet. Prior to onset the signature of the forced anomalies is a pair of cyclonic (anticyclonic) circulation anomalies centered near the node of the tropical heating dipole. Wave trains extending from the region of anomalous convection into the extratropics set the stage for the subsequent rapid development of the PNP anomalies. After onset, the mature PNP anomalies extend equatorward to feed back (through modifications to the moisture transport) on the tropical precipitation anomalies. Throughout the evolution, the tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations in both reanalyses.
Abstract
A composite analysis of multiyear (1985–93) global reanalyses produced by the NCEP/NCAR and the NASA/DAO is used to show that the development of persistent North Pacific (PNP) circulation anomalies during NH winter is linked to tropical intraseasonal oscillations. The development is initiated over the tropical west Pacific by anomalous convection (characterized by an east–west dipole structure) one to two weeks prior to the extratropical onset time in both reanalyses. As tropical heating moves eastward toward the central Pacific, anomalous divergent outflow associated with the local Hadley circulation generates an anomalous Rossby wave sink (source) in the subtropics, consistent with the retraction (extension) of the Pacific jet. Prior to onset the signature of the forced anomalies is a pair of cyclonic (anticyclonic) circulation anomalies centered near the node of the tropical heating dipole. Wave trains extending from the region of anomalous convection into the extratropics set the stage for the subsequent rapid development of the PNP anomalies. After onset, the mature PNP anomalies extend equatorward to feed back (through modifications to the moisture transport) on the tropical precipitation anomalies. Throughout the evolution, the tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations in both reanalyses.
Abstract
Atmospheric circulation anomalies and hydrologic processes associated with California wet and dry events were examined during Northern Hemisphere winter. The precipitation anomaly over the west coast of North America shows a north–south three-cell pattern. Heavy precipitation in California is accompanied by dry conditions over Washington, British Columbia, and along the southeastern coast of Alaska and reduced precipitation over the subtropical eastern Pacific. The inverse relationship between California and the Pacific Northwest is supported by the transport of moisture flux. During wet events, the southern branch of moisture flux transport strengthens and brings moisture from the North Pacific to California, hence enhanced rainfall. Strengthened moisture flux transport northward to the area north of Washington is consistent with suppressed rainfall in California.
The local precipitation anomaly pattern in the eastern tropical Pacific just north of the equator has a large influence on precipitation events in California. The enhanced precipitation generates strong rising motion. The associated sinking motion is located over California. Strong sinking motion and strong upper-level convergence favor dry conditions in California. Conversely, suppressed rainfall in the eastern Pacific is associated with above-normal precipitation in California.
Precipitation in California is likely below normal during cold ENSO events. When convection in the central Pacific is enhanced, California has heavy precipitation if rainfall in the subtropical eastern Pacific is suppressed. In addition to ENSO, precipitation in California is also modulated by the tropical intraseasonal oscillation. Wet (dry) events are favored during the phase of the oscillation associated with enhanced convection near 150°E (120°E) in the tropical Pacific.
Abstract
Atmospheric circulation anomalies and hydrologic processes associated with California wet and dry events were examined during Northern Hemisphere winter. The precipitation anomaly over the west coast of North America shows a north–south three-cell pattern. Heavy precipitation in California is accompanied by dry conditions over Washington, British Columbia, and along the southeastern coast of Alaska and reduced precipitation over the subtropical eastern Pacific. The inverse relationship between California and the Pacific Northwest is supported by the transport of moisture flux. During wet events, the southern branch of moisture flux transport strengthens and brings moisture from the North Pacific to California, hence enhanced rainfall. Strengthened moisture flux transport northward to the area north of Washington is consistent with suppressed rainfall in California.
The local precipitation anomaly pattern in the eastern tropical Pacific just north of the equator has a large influence on precipitation events in California. The enhanced precipitation generates strong rising motion. The associated sinking motion is located over California. Strong sinking motion and strong upper-level convergence favor dry conditions in California. Conversely, suppressed rainfall in the eastern Pacific is associated with above-normal precipitation in California.
Precipitation in California is likely below normal during cold ENSO events. When convection in the central Pacific is enhanced, California has heavy precipitation if rainfall in the subtropical eastern Pacific is suppressed. In addition to ENSO, precipitation in California is also modulated by the tropical intraseasonal oscillation. Wet (dry) events are favored during the phase of the oscillation associated with enhanced convection near 150°E (120°E) in the tropical Pacific.
Abstract
Droughts and persistent wet spells over the United States and northwest Mexico have preferred regions of occurrence and persistence. Wet or dry conditions that persist more than 1 yr tend to occur over the interior United States west of 90°–95°W and northwest Mexico. In contrast, events over the eastern United States are less likely to occur and often last less than 6 months.
The long persistent drought and wet spells are often modulated by low-frequency sea surface temperature anomalies (SSTAs). The persistent dry or wet conditions over northwest Mexico and the Southwest are associated with decadal variability of SSTAs over the North Pacific. Persistent events over the northwestern mountains are associated with two decadal SSTA modes. One mode has loadings over three southern oceans and another one is an El Niño–Southern Oscillation (ENSO) like decadal mode. Wet and dry conditions over the Pacific Northwest and the Great Plains are often associated with ENSO.
The seasonal cycle of precipitation over the central-eastern United States, the East Coast, and the Ohio Valley is weak. Drought and wet spells over these regions are less persistent because the ENSO events have opposite impacts on precipitation for summer and winter.
Abstract
Droughts and persistent wet spells over the United States and northwest Mexico have preferred regions of occurrence and persistence. Wet or dry conditions that persist more than 1 yr tend to occur over the interior United States west of 90°–95°W and northwest Mexico. In contrast, events over the eastern United States are less likely to occur and often last less than 6 months.
The long persistent drought and wet spells are often modulated by low-frequency sea surface temperature anomalies (SSTAs). The persistent dry or wet conditions over northwest Mexico and the Southwest are associated with decadal variability of SSTAs over the North Pacific. Persistent events over the northwestern mountains are associated with two decadal SSTA modes. One mode has loadings over three southern oceans and another one is an El Niño–Southern Oscillation (ENSO) like decadal mode. Wet and dry conditions over the Pacific Northwest and the Great Plains are often associated with ENSO.
The seasonal cycle of precipitation over the central-eastern United States, the East Coast, and the Ohio Valley is weak. Drought and wet spells over these regions are less persistent because the ENSO events have opposite impacts on precipitation for summer and winter.
Abstract
The physical mechanism of summertime precipitation over Arizona and New Mexico (AZNM) is examined using regional model experiments. Two sets of regional model simulations with different physics packages produce very different precipitation (P) over the Southwest including AZNM. The better simulation that produces a wet monsoon similar to the observations has larger evaporation (E) over AZNM and stronger moisture flux from the Gulf of California into AZNM.
Diagnostics of the simulations suggested that the increase in precipitation is not due to the increase in evaporation locally but rather to the change in moisture flux. Regional model experiments were then designed to isolate the impact of local E and the large-scale flow. Both regional model experiments and diagnostics support the following physical mechanism: There is an increase in E in the realistic simulation due to the change in land surface physics. This increase in E is compensated by the decrease in sensible heat, which leads to the colder land surface. Associated with this cooling, the surface pressure raises and the Southwest heat low weakens due to the increase in the surface pressure. This alters the large-scale low-level circulation and increases the occurrence of the low-level moisture surge events from the Gulf of California into AZNM, and accordingly, increases P. The mechanism is also found in observations of day-to-day variation of precipitation over AZNM.
Abstract
The physical mechanism of summertime precipitation over Arizona and New Mexico (AZNM) is examined using regional model experiments. Two sets of regional model simulations with different physics packages produce very different precipitation (P) over the Southwest including AZNM. The better simulation that produces a wet monsoon similar to the observations has larger evaporation (E) over AZNM and stronger moisture flux from the Gulf of California into AZNM.
Diagnostics of the simulations suggested that the increase in precipitation is not due to the increase in evaporation locally but rather to the change in moisture flux. Regional model experiments were then designed to isolate the impact of local E and the large-scale flow. Both regional model experiments and diagnostics support the following physical mechanism: There is an increase in E in the realistic simulation due to the change in land surface physics. This increase in E is compensated by the decrease in sensible heat, which leads to the colder land surface. Associated with this cooling, the surface pressure raises and the Southwest heat low weakens due to the increase in the surface pressure. This alters the large-scale low-level circulation and increases the occurrence of the low-level moisture surge events from the Gulf of California into AZNM, and accordingly, increases P. The mechanism is also found in observations of day-to-day variation of precipitation over AZNM.
