Search Results
You are looking at 11 - 20 of 69 items for
- Author or Editor: Kingtse C. Mo x
- Refine by Access: All Content x
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
Pattern correlations between daily anomalies have been used to study the persistence of the Southern Hemisphere circulations. The dataset consists of daily Australian analyses of 500 mb heights and sea level pressure for the period from 1972 to 1983. Compared to the Northern Hemisphere, the pattern correlations are much lower and more variable in the Southern Hemisphere. The mean one-day lag autocorrelation is only 0.57, compared to 0.81 in the Northern Hemisphere. The correlations increase significantly for the filtered anomalies, which consist of the planetary wavenumbers from 0 to 4.
Subjective criteria based on the pattern correlations are used to select quasi-stationary events. A series of 5 or more daily maps is defined to be quasi-stationary if the pattern correlations between all pairs of five consecutive maps in this time series are larger than or equal to 0.5. In winter, quasi-stationary events can be classified in terms of wavenumbers. Waves 3 and 4 are by far the dominant waves. More than half of the events have wave 3 amplitude with geographically fixed orientations.
Abstract
Pattern correlations between daily anomalies have been used to study the persistence of the Southern Hemisphere circulations. The dataset consists of daily Australian analyses of 500 mb heights and sea level pressure for the period from 1972 to 1983. Compared to the Northern Hemisphere, the pattern correlations are much lower and more variable in the Southern Hemisphere. The mean one-day lag autocorrelation is only 0.57, compared to 0.81 in the Northern Hemisphere. The correlations increase significantly for the filtered anomalies, which consist of the planetary wavenumbers from 0 to 4.
Subjective criteria based on the pattern correlations are used to select quasi-stationary events. A series of 5 or more daily maps is defined to be quasi-stationary if the pattern correlations between all pairs of five consecutive maps in this time series are larger than or equal to 0.5. In winter, quasi-stationary events can be classified in terms of wavenumbers. Waves 3 and 4 are by far the dominant waves. More than half of the events have wave 3 amplitude with geographically fixed orientations.
Abstract
Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation.
A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.
Abstract
Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation.
A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.
Abstract
Flash drought refers to relatively short periods of warm surface temperature and anomalously low and rapid decreasing soil moisture (SM). Based on the physical mechanisms associated with flash droughts, these events are classified into two categories: heat wave and precipitation P deficit flash droughts. In previous work, the authors have defined heat wave flash droughts as resulting from the confluence of severe warm air temperature T air, which increases evapotranspiration (ET), and anomalously low and decreasing SM. Here, a second type of flash drought caused by precipitation deficits is explored. The authors term these events P-deficit flash droughts, which they associate with lack of P. Precipitation deficits cause ET to decrease and temperature to increase. The P-deficit flash droughts are analyzed based on observations of P, T air, and SM and ET reconstructed using land surface models for the period 1916–2013. The authors find that P-deficit flash droughts are more common than heat wave flash droughts. They are about twice as likely to occur as heat wave flash droughts over the conterminous United States. They are most prevalent over the southern United States with maxima over the southern Great Plains and the Southwest, in contrast to heat wave flash droughts that are mostly likely to occur over the Midwest and the Pacific Northwest, where the vegetation cover is dense.
Abstract
Flash drought refers to relatively short periods of warm surface temperature and anomalously low and rapid decreasing soil moisture (SM). Based on the physical mechanisms associated with flash droughts, these events are classified into two categories: heat wave and precipitation P deficit flash droughts. In previous work, the authors have defined heat wave flash droughts as resulting from the confluence of severe warm air temperature T air, which increases evapotranspiration (ET), and anomalously low and decreasing SM. Here, a second type of flash drought caused by precipitation deficits is explored. The authors term these events P-deficit flash droughts, which they associate with lack of P. Precipitation deficits cause ET to decrease and temperature to increase. The P-deficit flash droughts are analyzed based on observations of P, T air, and SM and ET reconstructed using land surface models for the period 1916–2013. The authors find that P-deficit flash droughts are more common than heat wave flash droughts. They are about twice as likely to occur as heat wave flash droughts over the conterminous United States. They are most prevalent over the southern United States with maxima over the southern Great Plains and the Southwest, in contrast to heat wave flash droughts that are mostly likely to occur over the Midwest and the Pacific Northwest, where the vegetation cover is dense.
Abstract
Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.
This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.
In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.
Abstract
Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.
This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.
In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.
Abstract
Observations of monthly mean sea level pressure, surface air temperature, and 500 mb and 300 mb geopotential heights and temperatures are used to study trends in the Southern Hemisphere from 1951–81.
The winter mean sea level pressure fell over the Indian/Atlantic half of the hemisphere from the 1950s to the 1960s, and rose over the other half. Generally, these trends reversed from the 1960s to the 1970s. The trends are equivalent barotropic.
The trends of temperatures are often regionally dependent. There was a significant warming over Antarctica from the 1960s to 1970s at all upper levels except for a small area on the Indian Ocean side.
Abstract
Observations of monthly mean sea level pressure, surface air temperature, and 500 mb and 300 mb geopotential heights and temperatures are used to study trends in the Southern Hemisphere from 1951–81.
The winter mean sea level pressure fell over the Indian/Atlantic half of the hemisphere from the 1950s to the 1960s, and rose over the other half. Generally, these trends reversed from the 1960s to the 1970s. The trends are equivalent barotropic.
The trends of temperatures are often regionally dependent. There was a significant warming over Antarctica from the 1960s to 1970s at all upper levels except for a small area on the Indian Ocean side.
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.
