Search Results
You are looking at 41 - 50 of 69 items for
- Author or Editor: Kingtse C. Mo x
- Refine by Access: All Content x
Abstract
Monthly mean teleconnections during the northern winter between proxies for tropical heating (OLR and SST data) and Northern Hemisphere 700 mb circulation patterns (PNA, TNH, and WPO) are examined, principally with correlation analysis. In particular it is found that positive projections on all three patterns are highly probable during certain strong ENSO winters but the means to predict their relative strengths was not discovered, although the absolute strength of the TNH pattern is directly related to SST anomalies in the central Pacific. Other ENSO winters also have a tendency for positive PNA and WPO projections, but for a negative TNH projection. For other winters the importance of an area north of the equator and 25 degrees to the west of the date line is confirmed as a probable source region for the PNA pattern. Another area about 25 degrees to the east of the date line is singled out as a possible tropical response to the PNA pattern. Implications for current and future GCM experiments and long-range prediction are discussed.
Abstract
Monthly mean teleconnections during the northern winter between proxies for tropical heating (OLR and SST data) and Northern Hemisphere 700 mb circulation patterns (PNA, TNH, and WPO) are examined, principally with correlation analysis. In particular it is found that positive projections on all three patterns are highly probable during certain strong ENSO winters but the means to predict their relative strengths was not discovered, although the absolute strength of the TNH pattern is directly related to SST anomalies in the central Pacific. Other ENSO winters also have a tendency for positive PNA and WPO projections, but for a negative TNH projection. For other winters the importance of an area north of the equator and 25 degrees to the west of the date line is confirmed as a probable source region for the PNA pattern. Another area about 25 degrees to the east of the date line is singled out as a possible tropical response to the PNA pattern. Implications for current and future GCM experiments and long-range prediction are discussed.
Abstract
Simultaneous and lagged correlation statistics have been calculated between time series of seasonal height anomalies at selected stations and extratropical grid-point anomalies in both hemispheres. The tropical stations in two major tropical precipitation zones, the Indo-China maritime continent and Africa, are well correlated with each other. These stations are also correlated with stations in the North Pacific and Australia, but the coefficients are smaller. The correlations between height anomalies at any of these stations and Northern Hemisphere height anomalies show a well-defined global pattern. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical Northern Hemisphere (TNH) pattern or a mixed pattern having both elements. All three patterns, PNA, TNH and WPO (Western Pacific Oscillation), have been linked to tropical variations. The correlations between height anomalies at these well-correlated stations and the Southern Hemisphere height anomalies at the 500 mb level give the summer teleconnection pattern of Mo and White (1985). The vertical structure of patterns indicate that they are approximately equivalent barotropic.
The TNH pattern tends to be associated more with tropical variability for time scales longer than one season, while the PNA pattern is present in both high- and low-pass filtered analyses, although weakly in the former. Moreover, its low frequency connection to the tropics appears to be confined to ENSO years.
During ENSO years both patterns appear in both simultaneous and lagged maps, but in non-ENSO years, the TNH is weak in simultaneous charts.
Abstract
Simultaneous and lagged correlation statistics have been calculated between time series of seasonal height anomalies at selected stations and extratropical grid-point anomalies in both hemispheres. The tropical stations in two major tropical precipitation zones, the Indo-China maritime continent and Africa, are well correlated with each other. These stations are also correlated with stations in the North Pacific and Australia, but the coefficients are smaller. The correlations between height anomalies at any of these stations and Northern Hemisphere height anomalies show a well-defined global pattern. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical Northern Hemisphere (TNH) pattern or a mixed pattern having both elements. All three patterns, PNA, TNH and WPO (Western Pacific Oscillation), have been linked to tropical variations. The correlations between height anomalies at these well-correlated stations and the Southern Hemisphere height anomalies at the 500 mb level give the summer teleconnection pattern of Mo and White (1985). The vertical structure of patterns indicate that they are approximately equivalent barotropic.
The TNH pattern tends to be associated more with tropical variability for time scales longer than one season, while the PNA pattern is present in both high- and low-pass filtered analyses, although weakly in the former. Moreover, its low frequency connection to the tropics appears to be confined to ENSO years.
During ENSO years both patterns appear in both simultaneous and lagged maps, but in non-ENSO years, the TNH is weak in simultaneous charts.
Abstract
Teleconnections are calculated from monthly mean anomalies of sea level pressure and 500 mb geopotential height for the Southern Hemisphere (10–90°S) for five-month winter and summer seasons. The monthly means were calculated from Australian analyses for the period from June 1972 to November 1980.
Zonally averaged anomalies at both sea level and 500 mb display an out-of-phase relation between low and high latitudes and in midlatitudes are negatively correlated with anomalies in the subtropics and polar regions. In winter a striking zonal wavenumber 3 pattern is found over the Southern Ocean. Anomalies in 500 mb geopotential heights at (50°S, 95°E), (58°S, 150°W) and (38°S, 15°W) exhibit strong positive correlations while showing weaker negative correlations with heights over Antarctica and in low latitudes. A similar pattern appears in sea level pressure.
In summer anomalies in 500 mb geopotential height over the three subtropical continents appear to occur out of phase with anomalies over the subtropical oceans and in a zonal wavenumber 3 pattern over the Southern Ocean near 55°S. Much of the pattern appears largely associated with low-frequency variability. A similar pattern with weaker correlations appears in sea level pressure in summer. The strongest teleconnection pattern in summertime sea level pressure features a strong negative correlation between the eastern Indian Ocean northwest of Australia and the subtropical eastern Pacific near Tahiti, which strongly resembles the pattern associated with the Southern Oscillation.
The above features are well-reproduced in both halves of the period and in station data. The features found at 500 mb also appear in patterns obtained from eigenvector analysis.
Abstract
Teleconnections are calculated from monthly mean anomalies of sea level pressure and 500 mb geopotential height for the Southern Hemisphere (10–90°S) for five-month winter and summer seasons. The monthly means were calculated from Australian analyses for the period from June 1972 to November 1980.
Zonally averaged anomalies at both sea level and 500 mb display an out-of-phase relation between low and high latitudes and in midlatitudes are negatively correlated with anomalies in the subtropics and polar regions. In winter a striking zonal wavenumber 3 pattern is found over the Southern Ocean. Anomalies in 500 mb geopotential heights at (50°S, 95°E), (58°S, 150°W) and (38°S, 15°W) exhibit strong positive correlations while showing weaker negative correlations with heights over Antarctica and in low latitudes. A similar pattern appears in sea level pressure.
In summer anomalies in 500 mb geopotential height over the three subtropical continents appear to occur out of phase with anomalies over the subtropical oceans and in a zonal wavenumber 3 pattern over the Southern Ocean near 55°S. Much of the pattern appears largely associated with low-frequency variability. A similar pattern with weaker correlations appears in sea level pressure in summer. The strongest teleconnection pattern in summertime sea level pressure features a strong negative correlation between the eastern Indian Ocean northwest of Australia and the subtropical eastern Pacific near Tahiti, which strongly resembles the pattern associated with the Southern Oscillation.
The above features are well-reproduced in both halves of the period and in station data. The features found at 500 mb also appear in patterns obtained from eigenvector analysis.
Abstract
Precipitation forecasts from six climate models in the North American Multi-Model Ensemble (NMME) are combined with observed precipitation data to generate forecasts of the standardized precipitation index (SPI) for global land areas, and their skill was evaluated over the period 1982–2010. The skill of monthly precipitation forecasts from the NMME is also assessed. The value-added utility in using the NMME models to predict the SPI is identified by comparing the skill of its forecasts with a baseline skill based solely on the inherent persistence characteristics of the SPI itself. As expected, skill of the NMME-generated SPI forecasts depends on the season, location, and specific index considered (the 3- and 6-month SPI were evaluated). In virtually all locations and seasons, statistically significant skill is found at lead times of 1–2 months, although the skill comes largely from initial conditions. Added skill from the NMME is primarily in regions exhibiting El Niño–Southern Oscillation (ENSO) teleconnections. Knowledge of the initial drought state is critical in SPI prediction, and there are considerable differences in observed SPI values between different datasets. Root-mean-square differences between datasets can exceed typical thresholds for drought, particularly in the tropics. This is particularly problematic for precipitation products available in near–real time. Thus, in the near term, the largest advances in the global prediction of meteorological drought are obtainable from improvements in near-real-time precipitation observations for the globe. In the longer term, improvements in precipitation forecast skill from dynamical models will be essential in this effort.
Abstract
Precipitation forecasts from six climate models in the North American Multi-Model Ensemble (NMME) are combined with observed precipitation data to generate forecasts of the standardized precipitation index (SPI) for global land areas, and their skill was evaluated over the period 1982–2010. The skill of monthly precipitation forecasts from the NMME is also assessed. The value-added utility in using the NMME models to predict the SPI is identified by comparing the skill of its forecasts with a baseline skill based solely on the inherent persistence characteristics of the SPI itself. As expected, skill of the NMME-generated SPI forecasts depends on the season, location, and specific index considered (the 3- and 6-month SPI were evaluated). In virtually all locations and seasons, statistically significant skill is found at lead times of 1–2 months, although the skill comes largely from initial conditions. Added skill from the NMME is primarily in regions exhibiting El Niño–Southern Oscillation (ENSO) teleconnections. Knowledge of the initial drought state is critical in SPI prediction, and there are considerable differences in observed SPI values between different datasets. Root-mean-square differences between datasets can exceed typical thresholds for drought, particularly in the tropics. This is particularly problematic for precipitation products available in near–real time. Thus, in the near term, the largest advances in the global prediction of meteorological drought are obtainable from improvements in near-real-time precipitation observations for the globe. In the longer term, improvements in precipitation forecast skill from dynamical models will be essential in this effort.
Abstract
Ten years of forecasts during Northern Hemisphere winter produced by the NCEP–NCAR reanalysis project are analyzed and their errors are documented with a focus on forecasts over South America. Previous studies have documented a seesaw pattern in the South Atlantic Convergence Zone (SACZ). Events associated with a strong SACZ are periods when the subtropical plains of South America exhibit precipitation deficits. When the SACZ weakens, precipitation in the plains is abundant. The forecast errors during these periods are examined separately. An error climatology is also obtained based on all available forecasts for January and February.
It is found that cases with weak SACZ are characterized by relatively smaller forecast errors in the Americas and neighboring oceans during the first three days than are strong SACZ cases. The error growth for these weak SACZ cases is larger, and the forecast errors exceed those in the strong SACZ cases after about 5 days. Upper-tropospheric divergence is systematically underpredicted over tropical and subtropical South America in the 117 studied forecasts, and it is overpredicted over the Caribbean. Similar features are also found in the 6-h forecast precipitation verified against global precipitation index estimates. The 3–8-day model forecasts retain some characteristics of the weak and strong SACZ events, but underpredict their amplitudes. Eight-day predicted upper-tropospheric zonal winds have substantial errors over North America. Integrations with a simple global numerical model suggest that the predicted wind error over North America is due to latent heating errors associated with precipitation forecast errors located nearby. Central and western Pacific errors do not appear to be as important for wind forecast errors in this region. Weak SACZ events are characterized by stronger low-level jets and more poleward moisture transport east of the Andes than are strong SACZ events in the NCEP–NCAR reanalysis. This distinction is substantially weakened in the 8-day forecasts. Integrations with a global model suggest that inadequate simulation of the distribution of latent heating and of radiative heating may have contributed to the inability of the NCEP forecast model to adequately distinguish low-level flow structures during positive and negative events
Abstract
Ten years of forecasts during Northern Hemisphere winter produced by the NCEP–NCAR reanalysis project are analyzed and their errors are documented with a focus on forecasts over South America. Previous studies have documented a seesaw pattern in the South Atlantic Convergence Zone (SACZ). Events associated with a strong SACZ are periods when the subtropical plains of South America exhibit precipitation deficits. When the SACZ weakens, precipitation in the plains is abundant. The forecast errors during these periods are examined separately. An error climatology is also obtained based on all available forecasts for January and February.
It is found that cases with weak SACZ are characterized by relatively smaller forecast errors in the Americas and neighboring oceans during the first three days than are strong SACZ cases. The error growth for these weak SACZ cases is larger, and the forecast errors exceed those in the strong SACZ cases after about 5 days. Upper-tropospheric divergence is systematically underpredicted over tropical and subtropical South America in the 117 studied forecasts, and it is overpredicted over the Caribbean. Similar features are also found in the 6-h forecast precipitation verified against global precipitation index estimates. The 3–8-day model forecasts retain some characteristics of the weak and strong SACZ events, but underpredict their amplitudes. Eight-day predicted upper-tropospheric zonal winds have substantial errors over North America. Integrations with a simple global numerical model suggest that the predicted wind error over North America is due to latent heating errors associated with precipitation forecast errors located nearby. Central and western Pacific errors do not appear to be as important for wind forecast errors in this region. Weak SACZ events are characterized by stronger low-level jets and more poleward moisture transport east of the Andes than are strong SACZ events in the NCEP–NCAR reanalysis. This distinction is substantially weakened in the 8-day forecasts. Integrations with a global model suggest that inadequate simulation of the distribution of latent heating and of radiative heating may have contributed to the inability of the NCEP forecast model to adequately distinguish low-level flow structures during positive and negative events
Abstract
Regional summertime atmospheric conditions of 1993 are analyzed with the University of Utah Local Area Model (ULAM) by nudging boundary values and large internal scales of the local model toward values produced by the Nested Grid Model (NCEP/NOAA) initial analyses and forecasts archived at 6-h intervals. The approach allows the local ULAM to develop finer-scale structures in the precipitation and circulation forecasts than those resolved by the NGM. The study focuses on the influence of surface evaporation upon rainfall and low-level flow in regional simulations. Much of the rainfall simulated in the control experiment occurred from the late afternoon to early morning hours, with a pronounced midday minimum over the flood region.
The moisture flux from the south due to the low-level jet (LLJ) provides much of the moisture source for the precipitation, and it is shown that the net moisture influx is significantly larger than the rainfall rate over the flood region. As a consequence, modifications of surface evaporation apparently are relatively more important in changing the buoyancy and resulting LLJ strength than they are in providing additional moisture to the already plentiful moisture influx from the Gulf of Mexico. This suggests that accurate surface evaporation in the Great Plains is necessary for accurate simulation of dynamic support for rainfall.
The LLJ and especially its diurnal oscillation increase for drier surface conditions in the vicinity of the jet core, providing more effective convergence patterns to support rainfall in these cases than in cases of stronger surface evaporation. This appears to be a more important mechanism for rainfall release over the Mississippi River basin than moistening through local evapotranspiration, although the latter also contributes to more rainfall when this moistening occurs downwind of the jet core.
Abstract
Regional summertime atmospheric conditions of 1993 are analyzed with the University of Utah Local Area Model (ULAM) by nudging boundary values and large internal scales of the local model toward values produced by the Nested Grid Model (NCEP/NOAA) initial analyses and forecasts archived at 6-h intervals. The approach allows the local ULAM to develop finer-scale structures in the precipitation and circulation forecasts than those resolved by the NGM. The study focuses on the influence of surface evaporation upon rainfall and low-level flow in regional simulations. Much of the rainfall simulated in the control experiment occurred from the late afternoon to early morning hours, with a pronounced midday minimum over the flood region.
The moisture flux from the south due to the low-level jet (LLJ) provides much of the moisture source for the precipitation, and it is shown that the net moisture influx is significantly larger than the rainfall rate over the flood region. As a consequence, modifications of surface evaporation apparently are relatively more important in changing the buoyancy and resulting LLJ strength than they are in providing additional moisture to the already plentiful moisture influx from the Gulf of Mexico. This suggests that accurate surface evaporation in the Great Plains is necessary for accurate simulation of dynamic support for rainfall.
The LLJ and especially its diurnal oscillation increase for drier surface conditions in the vicinity of the jet core, providing more effective convergence patterns to support rainfall in these cases than in cases of stronger surface evaporation. This appears to be a more important mechanism for rainfall release over the Mississippi River basin than moistening through local evapotranspiration, although the latter also contributes to more rainfall when this moistening occurs downwind of the jet core.
Abstract
General Circulation Model (GCM) experiments have been performed to determine mechanisms that maintained the blocking episode in the Australian-New Zealand region during the period 8–22 June 1982. A control forecast reproduces the persistent ridge. Several mechanistic experiments lead to the following conclusions. (i) The block was not due to orographic forcing, which has only a small local influence on the winter atmospheric circulation in the Southern Hemisphere. (ii) The block was not produced by the sea surface temperature anomalies (SST). By comparing the relative location of low-level atmospheric vorticity and SST anomalies, we are able to show that during June 1982 the atmospheric blocking was the cause of the SST anomalies in the Pacific. (iii) The block was not a response to tropical heating or the Asian Monsoon. There are only weak effects on the block when the tropical heating or heating in the Pacific region is suppressed. (iv) The most important boundary forcing maintaining this blocking ridge is heating associated with the land-sea contrast. The height fields are more zonally symmetric when the land-sea contrast is suppressed. The local land-sea contrast in the Australian region also contributed to maintain the stationary blocking ridge. The sensible heat release in the subantarctic region is an important mechanism that maintains the block. (v) Finally, the daily spectral energetics of the control experiment suggests that the baroclinic amplification of planetary-scale waves forced by synoptic-scale disturbances played an important role in the evolution of this blocking process.
Abstract
General Circulation Model (GCM) experiments have been performed to determine mechanisms that maintained the blocking episode in the Australian-New Zealand region during the period 8–22 June 1982. A control forecast reproduces the persistent ridge. Several mechanistic experiments lead to the following conclusions. (i) The block was not due to orographic forcing, which has only a small local influence on the winter atmospheric circulation in the Southern Hemisphere. (ii) The block was not produced by the sea surface temperature anomalies (SST). By comparing the relative location of low-level atmospheric vorticity and SST anomalies, we are able to show that during June 1982 the atmospheric blocking was the cause of the SST anomalies in the Pacific. (iii) The block was not a response to tropical heating or the Asian Monsoon. There are only weak effects on the block when the tropical heating or heating in the Pacific region is suppressed. (iv) The most important boundary forcing maintaining this blocking ridge is heating associated with the land-sea contrast. The height fields are more zonally symmetric when the land-sea contrast is suppressed. The local land-sea contrast in the Australian region also contributed to maintain the stationary blocking ridge. The sensible heat release in the subantarctic region is an important mechanism that maintains the block. (v) Finally, the daily spectral energetics of the control experiment suggests that the baroclinic amplification of planetary-scale waves forced by synoptic-scale disturbances played an important role in the evolution of this blocking process.
Abstract
Studies by van Loon and Jenne, van Loon et al., Trenberth and others indicate that stationary waves in the Southern Hemisphere are dominated by planetary scales. Kalnay and Halem reported the presence of large amplitude, short-scale stationary waves during the month of January 1979 in the lee of South America and their disappearance in February 1979. In this paper we present further observational evidence of the January waves.
We also perform two 15-day forecast experiments with the GLAS Fourth-Order General Circulation Model, and initial conditions corresponding to 5 January and 4 February 1979. These factors reproduce reasonably well the presence of the January waves and their absence in February. Several mechanistic experiments to determine the origin of the waves are then performed.
The principal conclusions are
a) Large amplitude stationary Rossby waves with zonal wavenumber ≈7 were present between 20° and 40°S both in the South Pacific and east of South America during January 1979. They appear in satellite observations as enhanced bands of high clouds associated with the South Pacific Convergence Zone (SPCZ) and the Amazon. Examination of satellite observations during 1974–79 indicates a correlation between the intensity of stationary cloud bands in the two regions.
b) The stationary waves in the lee of South America are not of orographic origin since they are associated with a ridge rather than a trough east of the Andes. A “no Andes” forecast experiment confirms this argument.
c) The waves could not be produced by a CISK mechanism suggested by Kalnay and Halem, because of their rather barotropic vertical structure. Sea surface temperature (SST) anomalies in the South Atlantic were of the same scale as the waves, but stronger at the end of January. This, and strong correlation between low level atmospheric cyclonic vorticity and cold SST anomalies indicate that the atmospheric stationary waves were the cause of the ocean temperature anomalies, which in turn provided a negative feedback to the atmosphere.
d) Several experiments modifying the coefficient of latent heat lead to the conclusion that tropical heating is important in the maintenance of the waves. Furthermore, the convection in the subtropical waves themselves is important in sustaining their amplitude and phase, and the Walker type of circulation associated with the SPCZ is also a contributor to the maintenance of the South American waves. These results confirm the existence of a relationship between the occurrence of a strong South Pacific Convergence Zone, somewhat eastward from its climatological position, and the strong “South Atlantic Convergence Zone” observed in outgoing longwave radiation maps.
Abstract
Studies by van Loon and Jenne, van Loon et al., Trenberth and others indicate that stationary waves in the Southern Hemisphere are dominated by planetary scales. Kalnay and Halem reported the presence of large amplitude, short-scale stationary waves during the month of January 1979 in the lee of South America and their disappearance in February 1979. In this paper we present further observational evidence of the January waves.
We also perform two 15-day forecast experiments with the GLAS Fourth-Order General Circulation Model, and initial conditions corresponding to 5 January and 4 February 1979. These factors reproduce reasonably well the presence of the January waves and their absence in February. Several mechanistic experiments to determine the origin of the waves are then performed.
The principal conclusions are
a) Large amplitude stationary Rossby waves with zonal wavenumber ≈7 were present between 20° and 40°S both in the South Pacific and east of South America during January 1979. They appear in satellite observations as enhanced bands of high clouds associated with the South Pacific Convergence Zone (SPCZ) and the Amazon. Examination of satellite observations during 1974–79 indicates a correlation between the intensity of stationary cloud bands in the two regions.
b) The stationary waves in the lee of South America are not of orographic origin since they are associated with a ridge rather than a trough east of the Andes. A “no Andes” forecast experiment confirms this argument.
c) The waves could not be produced by a CISK mechanism suggested by Kalnay and Halem, because of their rather barotropic vertical structure. Sea surface temperature (SST) anomalies in the South Atlantic were of the same scale as the waves, but stronger at the end of January. This, and strong correlation between low level atmospheric cyclonic vorticity and cold SST anomalies indicate that the atmospheric stationary waves were the cause of the ocean temperature anomalies, which in turn provided a negative feedback to the atmosphere.
d) Several experiments modifying the coefficient of latent heat lead to the conclusion that tropical heating is important in the maintenance of the waves. Furthermore, the convection in the subtropical waves themselves is important in sustaining their amplitude and phase, and the Walker type of circulation associated with the SPCZ is also a contributor to the maintenance of the South American waves. These results confirm the existence of a relationship between the occurrence of a strong South Pacific Convergence Zone, somewhat eastward from its climatological position, and the strong “South Atlantic Convergence Zone” observed in outgoing longwave radiation maps.
Abstract
Low-frequency variability of large-scale atmospheric dynamics can be represented schematically by a Markov chain of multiple flow regimes. This Markov chain contains useful information for the long-range forecaster, provided that the statistical significance of the associated transition matrix can be reliably tested. Monte Carlo simulation yields a very reliable significance test for the elements of this matrix. The results of this test agree with previously used empirical formulae when each cluster of maps identified as a distinct flow regime is sufficiently large and when they all contain a comparable number of maps. Monte Carlo simulation provides a more reliable way to test the statistical significance of transitions to and from small clusters. It can determine the most likely transitions, as well as the most unlikely ones, with a prescribed level of statistical significance.
Abstract
Low-frequency variability of large-scale atmospheric dynamics can be represented schematically by a Markov chain of multiple flow regimes. This Markov chain contains useful information for the long-range forecaster, provided that the statistical significance of the associated transition matrix can be reliably tested. Monte Carlo simulation yields a very reliable significance test for the elements of this matrix. The results of this test agree with previously used empirical formulae when each cluster of maps identified as a distinct flow regime is sufficiently large and when they all contain a comparable number of maps. Monte Carlo simulation provides a more reliable way to test the statistical significance of transitions to and from small clusters. It can determine the most likely transitions, as well as the most unlikely ones, with a prescribed level of statistical significance.
Abstract
The physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation model linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient eddy activity in late May and June. The feedback of eddies in the time mean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the Rocky Mountains in the presence of a strong westerly mean flow maintained by the eddies upstream.
Composites from cases similar to that of the 1993 summer exhibit strong low-level southerly flow cast of the Rockies and suggest that the low-level jet may be an important mechanism to sustain the anomalous rainfall.
It is concluded that the effect of the eddies in maintaining a strong upper-level zonal flow, the role of the Rockies in sustaining a lee trough, and an associated low-level jet that brings in tropical moisture are essential ingredients in developing and maintaining floodlike conditions over the central United States.
Abstract
The physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation model linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient eddy activity in late May and June. The feedback of eddies in the time mean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the Rocky Mountains in the presence of a strong westerly mean flow maintained by the eddies upstream.
Composites from cases similar to that of the 1993 summer exhibit strong low-level southerly flow cast of the Rockies and suggest that the low-level jet may be an important mechanism to sustain the anomalous rainfall.
It is concluded that the effect of the eddies in maintaining a strong upper-level zonal flow, the role of the Rockies in sustaining a lee trough, and an associated low-level jet that brings in tropical moisture are essential ingredients in developing and maintaining floodlike conditions over the central United States.
