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The Coupled Model Project was established at the National Meteorological Center (NMC) in January 1991 to develop a multiseason forecast system based on coupled ocean-atmosphere general circulation models. This provided a focus to combine expertise in near real-time ocean modeling and analyses situated in the Climate Analysis Center (CAC) with expertise in atmospheric modeling and data assimilation in the Development Division. Since the inception of the project, considerable progress has been made toward establishing a coupled forecast system. A T40 version of NMC's operational global medium-range forecast model (MRF) has been modified so as to have improved response to boundary forcing from the Tropics. In extended simulations, which are forced with observed historical global sea surface temperature (SST) fields, the model reproduces much of the observed tropical Pacific and North American rainfall and temperature variability. An ocean reanalysis has been performed for the Pacific basin starting from July 1982 to present and uses a dynamical model-based assimilation system. This also provides the ocean initial conditions for coupled forecast experiments. The current coupled forecast model consists of an active Pacific Ocean model coupled to the T40 version of the NMC's MRF. In the future, a global ocean model will be used to include climate information from the other ocean basins. The initial experiments focused on forecasting Northern Hemisphere winter SST anomalies in the tropical Pacific with a lead time of two seasons. The coupled model showed considerable skill during these experiments. Work is currently under way to quantify the skill in predicting climatic variability over North America.
The Coupled Model Project was established at the National Meteorological Center (NMC) in January 1991 to develop a multiseason forecast system based on coupled ocean-atmosphere general circulation models. This provided a focus to combine expertise in near real-time ocean modeling and analyses situated in the Climate Analysis Center (CAC) with expertise in atmospheric modeling and data assimilation in the Development Division. Since the inception of the project, considerable progress has been made toward establishing a coupled forecast system. A T40 version of NMC's operational global medium-range forecast model (MRF) has been modified so as to have improved response to boundary forcing from the Tropics. In extended simulations, which are forced with observed historical global sea surface temperature (SST) fields, the model reproduces much of the observed tropical Pacific and North American rainfall and temperature variability. An ocean reanalysis has been performed for the Pacific basin starting from July 1982 to present and uses a dynamical model-based assimilation system. This also provides the ocean initial conditions for coupled forecast experiments. The current coupled forecast model consists of an active Pacific Ocean model coupled to the T40 version of the NMC's MRF. In the future, a global ocean model will be used to include climate information from the other ocean basins. The initial experiments focused on forecasting Northern Hemisphere winter SST anomalies in the tropical Pacific with a lead time of two seasons. The coupled model showed considerable skill during these experiments. Work is currently under way to quantify the skill in predicting climatic variability over North America.
Abstract
In this study, the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) has been evaluated as a means of enhancing the depiction of regional details beyond that which is capable in low-resolution global models. Three-month-long simulations driven by the NCEP–National Center for Atmospheric Research 40-yr reanalysis data are conducted with a horizontal resolution of about 50 km over the United States, for the two winters and summers. The selected winter cases are December–February (DJF) 1991/92 (warm eastern Pacific SST anomalies) and DJF 1992/93 (normal eastern Pacific SST anomalies). Summer cases are May–July (MJJ) 1988 (a drought in the Great Plains) and MJJ 1993 (a flooding).
Overall, the results from the model are very satisfactory in terms of the precipitation distribution for different seasons as well as the representation of large-scale features. Evaluation of simulated large-scale features reveals that the model does not exhibit a discernible synoptic-scale drift during the 3-month integration period, irrespective of the seasons. Surprisingly, the model simulation is found to correct some biases in the large-scale fields that exist in the reanalysis data. This bias reduction is attributed to the improved depiction of physical processes within the RSM. This finding indicates that one should take special care in the interpretation and validation of simulated results against the analyzed data.
Evaluation of the RSM simulated precipitation for the winter and summer cases generally agrees with results obtained from previous studies. For instance, the skill for simulated precipitation in the winter cases exceeds that of the summer cases by a factor of 2. Comparison of simulated precipitation with observations reveals the 3-month-long RSM simulated precipitation to be more skillful than that obtained from the reanalysis data (the 6-h forecast from the data assimilation system). In addition to seasonal variations in precipitation, daily variation in the simulated precipitation is quite good. However, detailed analysis points to the need for further RSM development, particularly in physics. In the summer cases the grid-resolvable precipitation physics simulate excessive precipitation over the northern United States. A more serious problem is found in the diurnal cycle of the simulation precipitation, in that the model initiates convection too early. Despite these deficiencies, it is concluded that the NCEP RSM is a very useful tool for regional climate studies.
Abstract
In this study, the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) has been evaluated as a means of enhancing the depiction of regional details beyond that which is capable in low-resolution global models. Three-month-long simulations driven by the NCEP–National Center for Atmospheric Research 40-yr reanalysis data are conducted with a horizontal resolution of about 50 km over the United States, for the two winters and summers. The selected winter cases are December–February (DJF) 1991/92 (warm eastern Pacific SST anomalies) and DJF 1992/93 (normal eastern Pacific SST anomalies). Summer cases are May–July (MJJ) 1988 (a drought in the Great Plains) and MJJ 1993 (a flooding).
Overall, the results from the model are very satisfactory in terms of the precipitation distribution for different seasons as well as the representation of large-scale features. Evaluation of simulated large-scale features reveals that the model does not exhibit a discernible synoptic-scale drift during the 3-month integration period, irrespective of the seasons. Surprisingly, the model simulation is found to correct some biases in the large-scale fields that exist in the reanalysis data. This bias reduction is attributed to the improved depiction of physical processes within the RSM. This finding indicates that one should take special care in the interpretation and validation of simulated results against the analyzed data.
Evaluation of the RSM simulated precipitation for the winter and summer cases generally agrees with results obtained from previous studies. For instance, the skill for simulated precipitation in the winter cases exceeds that of the summer cases by a factor of 2. Comparison of simulated precipitation with observations reveals the 3-month-long RSM simulated precipitation to be more skillful than that obtained from the reanalysis data (the 6-h forecast from the data assimilation system). In addition to seasonal variations in precipitation, daily variation in the simulated precipitation is quite good. However, detailed analysis points to the need for further RSM development, particularly in physics. In the summer cases the grid-resolvable precipitation physics simulate excessive precipitation over the northern United States. A more serious problem is found in the diurnal cycle of the simulation precipitation, in that the model initiates convection too early. Despite these deficiencies, it is concluded that the NCEP RSM is a very useful tool for regional climate studies.
Abstract
A section along 110°W in the eastern Pacific from about 6°N to 6°S was occupied in March and June of 1981. Measurements consisted of absolute velocity profiles and CTD cuts. The large-scale structure of the subsurface zonal flow remained relatively invariant between these cruises. The Equatorial Undercurrent and North and South Equatorial Undercurrents appear as strong eastward flows, separated by westward currents. Away from the equator, comparison of currents estimated geostrophically with the direct observations indicate that the two techniques are in agreement within estimated errors except close to the surface. In the vicinity of the equator the geostrophic technique in general fails and the directly measured currents must be used. During March, within 3° of the equator from the surface to 700 m, the flow was more eastward by about 0.15 m s−1; than in June. In March, the flow and temperature fields were relatively symmetric about the equator. By June, strong asymmetries had developed. In the top 100 m, eastward flow extended from the Undercurrent to about 3°S. A strong, shallow westward flow was situated over and to the north of the Undercurrent. A shallow southward flow developed from 4°N to 2°S. Order-of-magnitude estimates suggest that this can advect westward momentum onto the equator in the top 50 m and modify the Undercurrent. Asymmetry also developed in the near-surface thermal field. In June, upwelling was primarily located south of the equator. This resulted in a cold band lying south of the equator at the core of which the flow was predominantly eastward. A strong meridional temperature gradient at the equator separated the colder water from warmer water to the north. Thee asymmetries develop presumably in response to the seasonal increase from March to June of the winds. Computations of zonal transports in various σ t -classes in the near-surface layers suggest that the bulk of the Undercurrent water does not return west on the same density surfaces, but does so in the surface layers.
Abstract
A section along 110°W in the eastern Pacific from about 6°N to 6°S was occupied in March and June of 1981. Measurements consisted of absolute velocity profiles and CTD cuts. The large-scale structure of the subsurface zonal flow remained relatively invariant between these cruises. The Equatorial Undercurrent and North and South Equatorial Undercurrents appear as strong eastward flows, separated by westward currents. Away from the equator, comparison of currents estimated geostrophically with the direct observations indicate that the two techniques are in agreement within estimated errors except close to the surface. In the vicinity of the equator the geostrophic technique in general fails and the directly measured currents must be used. During March, within 3° of the equator from the surface to 700 m, the flow was more eastward by about 0.15 m s−1; than in June. In March, the flow and temperature fields were relatively symmetric about the equator. By June, strong asymmetries had developed. In the top 100 m, eastward flow extended from the Undercurrent to about 3°S. A strong, shallow westward flow was situated over and to the north of the Undercurrent. A shallow southward flow developed from 4°N to 2°S. Order-of-magnitude estimates suggest that this can advect westward momentum onto the equator in the top 50 m and modify the Undercurrent. Asymmetry also developed in the near-surface thermal field. In June, upwelling was primarily located south of the equator. This resulted in a cold band lying south of the equator at the core of which the flow was predominantly eastward. A strong meridional temperature gradient at the equator separated the colder water from warmer water to the north. Thee asymmetries develop presumably in response to the seasonal increase from March to June of the winds. Computations of zonal transports in various σ t -classes in the near-surface layers suggest that the bulk of the Undercurrent water does not return west on the same density surfaces, but does so in the surface layers.
Abstract
During March–April 1980, a velocity and CTD transect was made in the Pacific along the equator from 110 to 180°W. The horizontal baroclinic pressure gradient was observed to be primary confined between 160 and 130°W. Direct velocity profiles between 125 and 159°W showed the equatorial undercurrent to be a continuous feature. Maximum eastward transport (per unit width) in the undercurrent was 2.5 × 108 cm2 s−1 at 150°W and decreased both westward and eastward to about 1.5 × 108 cm2 s−1 at 159°W and at 125°W. Despite these variations the maximum speeds along the transect remained ∼150 cm s−1.
Beneath the undercurrent, the velocities decreased and were of the order of 25 cm s−1. They exhibited small-scale variation in the vertical as has recently been observed at other equatorial locations. Above 1600 m, the vertical wavelength of these variations in the zonal component was ∼300 m. Small-scale features in these zonal velocities were identifiable over 10° of longitude (1000 km). The small vertical and large horizontal scale suggests that these features might be Kelvin or long Rossby waves. The meridional velocities were primarily confined to the top 1000 m and their structure differed in profiles taken 5° apart.
Abstract
During March–April 1980, a velocity and CTD transect was made in the Pacific along the equator from 110 to 180°W. The horizontal baroclinic pressure gradient was observed to be primary confined between 160 and 130°W. Direct velocity profiles between 125 and 159°W showed the equatorial undercurrent to be a continuous feature. Maximum eastward transport (per unit width) in the undercurrent was 2.5 × 108 cm2 s−1 at 150°W and decreased both westward and eastward to about 1.5 × 108 cm2 s−1 at 159°W and at 125°W. Despite these variations the maximum speeds along the transect remained ∼150 cm s−1.
Beneath the undercurrent, the velocities decreased and were of the order of 25 cm s−1. They exhibited small-scale variation in the vertical as has recently been observed at other equatorial locations. Above 1600 m, the vertical wavelength of these variations in the zonal component was ∼300 m. Small-scale features in these zonal velocities were identifiable over 10° of longitude (1000 km). The small vertical and large horizontal scale suggests that these features might be Kelvin or long Rossby waves. The meridional velocities were primarily confined to the top 1000 m and their structure differed in profiles taken 5° apart.
Abstract
No abstract available.
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No abstract available.
Abstract
A linear model is developed for the near-equatorial zone to estimate wind-driven convergences in the near-surface viscous boundary layer. Using the winds observed during EASTROPAC, an attempt is made to relate these convergences to the measured displacements of the tropical thermocline. Between 4° and 15°N, the sign of the displacements is predicted; however, the amplitude is generally underestimated. At the equator, extremely large values of the vertical eddy coefficients are necessary in order to obtain agreement between predicted and observed changes. This probably indicates that some essential physics has been neglected.
Abstract
A linear model is developed for the near-equatorial zone to estimate wind-driven convergences in the near-surface viscous boundary layer. Using the winds observed during EASTROPAC, an attempt is made to relate these convergences to the measured displacements of the tropical thermocline. Between 4° and 15°N, the sign of the displacements is predicted; however, the amplitude is generally underestimated. At the equator, extremely large values of the vertical eddy coefficients are necessary in order to obtain agreement between predicted and observed changes. This probably indicates that some essential physics has been neglected.
Abstract
An improved forecast system has been developed for El Niño–Southern Oscillation (ENSO) prediction at the National Centers for Environmental Prediction. Improvements have been made both to the ocean data assimilation system and to the coupled ocean–atmosphere forecast model. In Part I of a two-part paper the authors describe the new assimilation system. The important changes are 1) the incorporation of vertical variation in the first-guess error variance that concentrates temperature corrections in the thermocline and 2) the overall reduction in the magnitude of the estimated first-guess error. The new system was used to produce a set of retrospective ocean analyses for 1980–95. The new analyses are less noisy than their earlier counterparts and compare more favorably with independent measurements of temperature, currents, and sea surface height variability. Part II of this work presents the results of using these analyses to initialize the coupled forecast model for ENSO prediction.
Abstract
An improved forecast system has been developed for El Niño–Southern Oscillation (ENSO) prediction at the National Centers for Environmental Prediction. Improvements have been made both to the ocean data assimilation system and to the coupled ocean–atmosphere forecast model. In Part I of a two-part paper the authors describe the new assimilation system. The important changes are 1) the incorporation of vertical variation in the first-guess error variance that concentrates temperature corrections in the thermocline and 2) the overall reduction in the magnitude of the estimated first-guess error. The new system was used to produce a set of retrospective ocean analyses for 1980–95. The new analyses are less noisy than their earlier counterparts and compare more favorably with independent measurements of temperature, currents, and sea surface height variability. Part II of this work presents the results of using these analyses to initialize the coupled forecast model for ENSO prediction.
Abstract
An improved forecast system has been developed and implemented for ENSO prediction at the National Centers for Environmental Prediction (NCEP). This system consists of a new ocean data assimilation system and an improved coupled ocean–atmosphere forecast model (CMP12) for ENSO prediction. The new ocean data assimilation system is described in Part I of this two-part paper.
The new coupled forecast model (CMP12) is a variation of the standard NCEP coupled model (CMP10). Major changes in the new coupled model are improved vertical mixing for the ocean model; relaxation of the model’s surface salinity to the climatological annual cycle; and incorporation of an anomalous freshwater flux forcing. Also, the domain in which the oceanic SST couples to the atmosphere is limited to the tropical Pacific.
Evaluation of ENSO prediction results show that the new coupled model, using the more accurate ocean initial conditions, achieves higher prediction skill. However, two sets of hindcasting experiments (one using the more accurate ocean initial conditions but the old coupled model, the other using the new coupled model but the less accurate ocean initial conditions), result in no improvement in prediction skill. These results indicate that future improvement in ENSO prediction skill requires systematically improving both the coupled model and the ocean analysis system. The authors’ results also suggest that for the purpose of initializing the coupled model for ENSO prediction, care should be taken to give sufficient weight to the model dynamics during the ocean data assimilation. This can reduce the danger of aliasing large-scale model biases into the low-frequency variability in the ocean initial conditions, and also reduce the introduction of small-scale noise into the initial conditions caused by overfitting the model to sparse observations.
Abstract
An improved forecast system has been developed and implemented for ENSO prediction at the National Centers for Environmental Prediction (NCEP). This system consists of a new ocean data assimilation system and an improved coupled ocean–atmosphere forecast model (CMP12) for ENSO prediction. The new ocean data assimilation system is described in Part I of this two-part paper.
The new coupled forecast model (CMP12) is a variation of the standard NCEP coupled model (CMP10). Major changes in the new coupled model are improved vertical mixing for the ocean model; relaxation of the model’s surface salinity to the climatological annual cycle; and incorporation of an anomalous freshwater flux forcing. Also, the domain in which the oceanic SST couples to the atmosphere is limited to the tropical Pacific.
Evaluation of ENSO prediction results show that the new coupled model, using the more accurate ocean initial conditions, achieves higher prediction skill. However, two sets of hindcasting experiments (one using the more accurate ocean initial conditions but the old coupled model, the other using the new coupled model but the less accurate ocean initial conditions), result in no improvement in prediction skill. These results indicate that future improvement in ENSO prediction skill requires systematically improving both the coupled model and the ocean analysis system. The authors’ results also suggest that for the purpose of initializing the coupled model for ENSO prediction, care should be taken to give sufficient weight to the model dynamics during the ocean data assimilation. This can reduce the danger of aliasing large-scale model biases into the low-frequency variability in the ocean initial conditions, and also reduce the introduction of small-scale noise into the initial conditions caused by overfitting the model to sparse observations.
Abstract
In this paper, the authors discuss observed climatic variability from 1982 to early 1995 and emphasize the contrasts between the period of strong interannual variability during the 1980s and the period of more persistent features beginning in 1990. Three versions of the NCEP coupled forecast model, which were developed to predict interannual sea surface temperature variability in the equatorial Pacific, are described and their performance compared for those two periods.
Climatic variability during 1982–1992 in the tropical Pacific was dominated by strong low-frequency interannual variations characterized by three warm and two cold El Niño episodes. However, beginning in 1990, the climate state has been characterized by a pattern of persistent positive SST anomalies in the tropical Pacific, especially in the central Pacific near the date line, and weaker than normal trade winds. Superimposed on this were several occurrences of short-lived, generally small-amplitude warmings in the eastern equatorial Pacific. Some of the short-lived warmings amplified into mature warm episodes, such as in spring 1993 and in late 1994.
The NCEP coupled models showed useful skill in predicting low-frequency SST variability associated with warm episodes in the tropical Pacific during the 1982–1992 period. However, the short-lived warmings in spring 1993 and fall/winter 1994/95 were not well predicted by the NCEP coupled models. Neither were they predicted by most of the other dynamic or statistical forecast models. If these short-lived warmings truly represent a different behavior of the coupled ocean-atmosphere system on intraseasonal timescales, the skill levels that were developed for predicting the strong low-frequency SST variability of the 1980s are probably not relevant. The lead times for skillful forecasts of short-lived episodes such as those observed in recent years will no doubt be only a few months.
Abstract
In this paper, the authors discuss observed climatic variability from 1982 to early 1995 and emphasize the contrasts between the period of strong interannual variability during the 1980s and the period of more persistent features beginning in 1990. Three versions of the NCEP coupled forecast model, which were developed to predict interannual sea surface temperature variability in the equatorial Pacific, are described and their performance compared for those two periods.
Climatic variability during 1982–1992 in the tropical Pacific was dominated by strong low-frequency interannual variations characterized by three warm and two cold El Niño episodes. However, beginning in 1990, the climate state has been characterized by a pattern of persistent positive SST anomalies in the tropical Pacific, especially in the central Pacific near the date line, and weaker than normal trade winds. Superimposed on this were several occurrences of short-lived, generally small-amplitude warmings in the eastern equatorial Pacific. Some of the short-lived warmings amplified into mature warm episodes, such as in spring 1993 and in late 1994.
The NCEP coupled models showed useful skill in predicting low-frequency SST variability associated with warm episodes in the tropical Pacific during the 1982–1992 period. However, the short-lived warmings in spring 1993 and fall/winter 1994/95 were not well predicted by the NCEP coupled models. Neither were they predicted by most of the other dynamic or statistical forecast models. If these short-lived warmings truly represent a different behavior of the coupled ocean-atmosphere system on intraseasonal timescales, the skill levels that were developed for predicting the strong low-frequency SST variability of the 1980s are probably not relevant. The lead times for skillful forecasts of short-lived episodes such as those observed in recent years will no doubt be only a few months.
Abstract
Equatorial Pacific current and temperature fields were simulated with and without assimilation of subsurface temperature measurements for April 1992–March 1995 and compared with moored buoy and research vessel current measurements. Data assimilation intensified the mean east–west slope of the thermocline along the equator in the eastern Pacific, shifted eastward the longitude of the mean Equatorial Undercurrent (EUC) maximum speed 800 km to 125°W, and produced a 25% stronger mean EUC core speed in the eastern Pacific. In the eastern Pacific the mean EUC core speed simulated with data assimilation was slightly more representative of observations compared to that computed without data assimilated; in the western Pacific the data assimilation had no impact on mean EUC simulations.
Data assimilation intensified the north–south slope of the thermocline south of the equator in the western Pacific to produce a thicker and more intense westward-flowing South Equatorial Current (SEC) in the western Pacific. In the western Pacific the mean SEC transport per unit width simulated with data assimilation was more representative of observations compared to that computed without data assimilation. However, large differences remained between the observed SEC transport per unit width and that simulated with data assimilation. In the eastern Pacific, the data assimilation had no impact on mean SEC simulations.
The temporal variability of monthly mean EUC core speeds and SEC transports per unit width were increased significantly by data assimilation. It also increased the representativeness of monthly mean SEC transports per unit width to the observations. However, the data representativeness of monthly mean EUC core speeds was decreased. Results could be explained by the coupling between zonal gradient of temperature and EUC and between meridional gradient of temperature and SEC. Longitudinal variations along the Pacific equator of the impact of data assimilation on the EUC and SEC precludes the choice of a single site to evaluate the effectiveness of data assimilation schemes.
Abstract
Equatorial Pacific current and temperature fields were simulated with and without assimilation of subsurface temperature measurements for April 1992–March 1995 and compared with moored buoy and research vessel current measurements. Data assimilation intensified the mean east–west slope of the thermocline along the equator in the eastern Pacific, shifted eastward the longitude of the mean Equatorial Undercurrent (EUC) maximum speed 800 km to 125°W, and produced a 25% stronger mean EUC core speed in the eastern Pacific. In the eastern Pacific the mean EUC core speed simulated with data assimilation was slightly more representative of observations compared to that computed without data assimilated; in the western Pacific the data assimilation had no impact on mean EUC simulations.
Data assimilation intensified the north–south slope of the thermocline south of the equator in the western Pacific to produce a thicker and more intense westward-flowing South Equatorial Current (SEC) in the western Pacific. In the western Pacific the mean SEC transport per unit width simulated with data assimilation was more representative of observations compared to that computed without data assimilation. However, large differences remained between the observed SEC transport per unit width and that simulated with data assimilation. In the eastern Pacific, the data assimilation had no impact on mean SEC simulations.
The temporal variability of monthly mean EUC core speeds and SEC transports per unit width were increased significantly by data assimilation. It also increased the representativeness of monthly mean SEC transports per unit width to the observations. However, the data representativeness of monthly mean EUC core speeds was decreased. Results could be explained by the coupling between zonal gradient of temperature and EUC and between meridional gradient of temperature and SEC. Longitudinal variations along the Pacific equator of the impact of data assimilation on the EUC and SEC precludes the choice of a single site to evaluate the effectiveness of data assimilation schemes.
