Search Results

You are looking at 1 - 10 of 37 items for :

  • Author or Editor: Siegfried D. Schubert x
  • Journal of Climate x
  • All content x
Clear All Modify Search
Michael A. Alexander and Siegfried D. Schubert

Abstract

The column budget technique described by Oort and Vonder Haar (1976) is used to assess the physical consistency and accuracy of estimates of the earth-atmosphere energy balance. Regional estimates of the atmospheric budget terms, the net radiation at the top of the atmosphere, and the time tendency and flux divergence of energy are calculated for the Special Observing Periods of the FGGE year. The data are assimilated by the Goddard Laboratory for the Atmospheres (GLA) four-dimensional analysis system. Ocean heat storage is obtained from marine temperature records while the energy flux through the surface and ocean heat flux divergence are computed as residuals.

During winter the midlatitude oceans supply large quantities of energy to the overlying atmosphere which then transports the energy to the continental heat sinks, the energy flows in the opposite direction during summer. The energy exchange between continental and oceanic regions is much stronger in the Northern Hemisphere where land coverage and land-sea differences are greater.

The uncertainties in the energy balance calculations are assessed by examining the errors in the observations, the data assimilation system including the GLA general circulation model, and the energy budget procedures. Sensitivity tests, error analyses and comparison with other studies indicate that the uncertainties in the continental-scale atmospheric energy flux divergence and the surface energy flux are approximately 20 W m−2 and 30 W m−2, respectively. We conclude that at present it is not possible to estimate accurately the ocean heat divergence and transport using the column budget technique.

Full access
Chung-Kyu Park and Siegfried D. Schubert

Abstract

East Asian countries experienced record-breaking heat waves and drought conditions during the summer monsoon season of 1994. This study documents the large-scale circulation associated with the drought and suggests a forcing mechanism responsible for the anomalous evolution of the East Asian monsoon. The results, based on Goddard Earth Observing System (GEOS) global assimilated data for 1985–94, indicate that the absence of monsoon rainfall during July 1994 over central China and the southern parts of Korea and Japan is due to the unusually early development of the climatological upper-level anticyclonic flow east of the Tibetan Plateau. The anomalous July anticyclonic circulation over the East Asian–northwestern Pacific region and the cyclonic circulation over the subtropical western Pacific, which are more typical of August, acted to reduce the moisture supply from the western Pacific and the Indian Ocean leading to suppressed rainfall over East Asia. The similarity of the July 1994 East Asian circulation anomalies to the climatological July to August change in these quantities suggests that the anomalies may be viewed as an acceleration of the seasonal cycle in which the circulation transitions to August conditions earlier than normal.

Neither tropical nor middle latitude SST anomalies provide a viable forcing mechanism for the 1994 East Asian circulation anomalies: the tropical anomalies are weak and the middle latitude anomalies, while stronger, appear to be primarily a response to atmospheric forcing, though they may feed back to reinforce the atmospheric anomalies. It is suggested, instead, that the anomalous circulation is primarily the result of an orographic forcing associated with zonal wind changes over Tibet. The zonal wind change, characterized by an anomalous northward shift of the East Asian jet is, in turn, tied to unusually persistent stationary waves extending from northern Europe, which developed prior to the onset of the East Asian anticyclone. Several other occurrences of atmospheric anomalies similar in structure (though weaker in amplitude) to the July 1994 anomalies are found in the previous nine summers, suggesting the operative mechanism is not unique to 1994. Such a mechanism appears to operate both for the climatological development of the ridge and for the occurrences of similar anomalies in previous summers: in the former the northward shift of the jet over Tibet is a reflection of climatological seasonal change in the zonal wind, while in the latter, the shift is the result of anomalies similar in structure to the 1994 European–Asian wave pattern.

The indirect role of the Eurasian waves in the development of the East Asian circulation anomalies suggests that useful monthly and longer predictions of the monsoon rests, not only on our ability to predict the occurrence of these waves, but also on our ability to properly model their interaction with orography.

Full access
H. Mark Helfand and Siegfried D. Schubert

Abstract

The Great Plains region of the United States is characterized by some of the most frequent and regular occurrences of a nocturnal low-level jet (LLJ). While the LLJ is generally confined to the lowest Kilometer of the atmosphere, it may cover a substantial region of the Great Plains, and typically reaches maximum amplitudes of more than 20 m s−1.

A two-month, springtime simulation with the Goddard Earth Observing System (GEOS-1) atmospheric general circulation model (AGCM) has produced a Great Plains LLJ with a vertical and temporal structure, directionality, and climatological distribution that compare favorably with observations. The diurnal cycle of the low-level flow is dramatic and coherent over a subcontinental area that includes much of the western United States and northern Mexico. This cycle can be interpreted as the nightly intrusion of the anticyclonic, subtropical gyre (associated with the Bermuda high) into the North American continent as surface friction decreases. The AGCM also simulates a pair of northerly LLJ maxima off the California coast, which seem to correspond to observations of a so-called “Baja Jet.” Other apparently related diurnal variations extending well into the upper troposphere are documented and compared with observations.

The time-averaged climatological picture of the low-level flow is dominated over land by the nocturnal phase of the diurnal cycle, in which surface friction is minimal and wind speeds are strongest. This pattern, with its zones of strong convergence, is characteristic of an unsteady, strongly forced flow. Over the open ocean, the mean low-level flow is more reminiscent of a smooth, climatological pattern.

Analysis of the simulated moisture budget for the continental United States reveals a horizontally confined region of strong southerly moisture transport with a strong diurnal cycle in the region of the Great Plains LLJ, as has been found in observations of water vapor transport. The LLJ plays a key role in that budget by transporting almost one-third of all the moisture that enters the continental United States with most of the influx from the LLJ (slightly less than two-thirds of it) entering during the 12 nighttime hours. However, it is the mean flow pattern and not covariances associated with the diurnal cycle that contribute most significantly to the total time-mean moisture transport. Covariances on the synoptic and longer timescales contribute only about one-fifth of the total time-mean transport of moisture in the jet region, and covariances on the diurnal timescale are negative and negligible despite the strong diurnal signal in the wind.

Full access
Siegfried D. Schubert and Man Li Wu

Abstract

The predictability of the 1997 and 1998 south Asian summer monsoon winds is examined from an ensemble of 10 atmospheric general circulation model simulations with prescribed sea surface temperatures (SSTs) and soil moisture. The simulations have no memory of atmospheric initial conditions for the periods of interest.

The model simulations show that the 1998 monsoon is considerably more predictable than the 1997 monsoon. During May and June of 1998 the predictability of the low-level wind anomalies is largely associated with a local response to anomalously warm Indian Ocean SSTs. Predictability increases late in the season (July and August) as a result of the strengthening of the anomalous Walker circulation and the associated development of easterly low-level wind anomalies that extend westward across India and the Arabian Sea. During these months the model is also the most skillful, with the analyses showing a similar late-season westward extension of the easterly wind anomalies.

The model shows little predictability or skill in the monthly mean low-level winds over Southeast Asia during 1997. Predictable wind anomalies do occur over the western Indian Ocean and Indonesia; however, over the Indian Ocean the predictability is artificial, because the model is responding to SST anomalies that were wind driven. The reduced predictability in the low-level winds during 1997 appears to be the result of a weaker (as compared with 1998) simulated anomalous Walker circulation, and the reduced skill is associated with pronounced intraseasonal activity that is not captured well by the model. It is remarkable that the model does produce an ensemble mean Madden–Julian oscillation (MJO) response, though it is approximately in quadrature with, and much weaker than, the observed MJO anomalies during 1997.

Full access
Randal D. Koster, Yehui Chang, Hailan Wang, and Siegfried D. Schubert

Abstract

A series of stationary wave model (SWM) experiments are performed in which the boreal summer atmosphere is forced, over a number of locations in the continental United States, with an idealized diabatic heating anomaly that mimics the atmospheric heating associated with a dry land surface. For localized heating within a large portion of the continental interior, regardless of the specific location of this heating, the spatial pattern of the forced atmospheric circulation anomaly (in terms of 250-hPa eddy streamfunction) is largely the same: a high anomaly forms over west-central North America and a low anomaly forms to the east. In supplemental atmospheric general circulation model (AGCM) experiments, similar results are found; imposing soil moisture dryness in the AGCM in different locations within the U.S. interior tends to produce the aforementioned pattern, along with an associated near-surface warming and precipitation deficit in the center of the continent. The SWM-based and AGCM-based patterns generally agree with composites generated using reanalysis and precipitation gauge data. The AGCM experiments also suggest that dry anomalies imposed in the lower Mississippi River valley have remote surface impacts of particularly large spatial extent, and a region along the eastern half of the U.S.–Canadian border is particularly sensitive to dry anomalies in a number of remote areas. Overall, the SWM and AGCM experiments support the idea of a positive feedback loop operating over the continent: dry surface conditions in many interior locations lead to changes in atmospheric circulation that act to enhance further the overall dryness of the continental interior.

Full access
Randal D. Koster, Yehui Chang, and Siegfried D. Schubert

Abstract

While the ability of land surface conditions to influence the atmosphere has been demonstrated in various modeling and observational studies, the precise mechanisms by which land–atmosphere feedback occurs are still largely unknown: particularly the mechanisms that allow land moisture state in one region to affect atmospheric conditions in another. Such remote impacts are examined here in the context of atmospheric general circulation model (AGCM) simulations, leading to the identification of one potential mechanism: the phase locking and amplification of a planetary wave through the imposition of a spatial pattern of soil moisture at the land surface. This mechanism, shown here to be relevant in the AGCM, apparently also operates in nature, as suggested by supporting evidence found in reanalysis data.

Full access
Hailan Wang, Siegfried D. Schubert, Randal D. Koster, and Yehui Chang

Abstract

Past modeling simulations, supported by observational composites, indicate that during boreal summer, dry soil moisture anomalies in very different locations within the U.S. continental interior tend to induce the same upper-tropospheric circulation pattern: a high anomaly forms over west-central North America and a low anomaly forms to the east. The present study investigates the causes of this apparent phase locking of the upper-level circulation response and extends the investigation to other land regions in the Northern Hemisphere. The phase locking over North America is found to be induced by zonal asymmetries in the local basic state originating from North American orography. Specifically, orography-induced zonal variations of air temperature, those in the lower troposphere in particular, and surface pressure play a dominant role in placing the soil moisture–forced negative Rossby wave source (dominated by upper-level divergence anomalies) over the eastern leeside of the Western Cordillera, which subsequently produces an upper-level high anomaly over west-central North America, with the downstream anomalous circulation responses phase locked by continuity. The zonal variations of the local climatological atmospheric circulation, manifested as a climatological high over central North America, help shape the spatial pattern of the upper-level circulation responses. Considering the rest of the Northern Hemisphere, the northern Middle East exhibits similar phase locking, also induced by local orography. The Middle Eastern phase locking, however, is not as pronounced as that over North America; North America is where soil moisture anomalies have the greatest impact on the upper-tropospheric circulation.

Full access
Michael G. Bosilovich, Siegfried D. Schubert, and Gregory K. Walker

Abstract

In this study, numerical simulations of the twentieth-century climate are evaluated, focusing on the changes in the intensity of the global water cycle. A new model diagnostic of atmospheric water vapor cycling rate is developed and employed that relies on constituent tracers predicted at the model time step. This diagnostic is compared to a simplified traditional calculation of cycling rate, based on monthly averages of precipitation and total water content. The mean sensitivity of both diagnostics to variations in climate forcing is comparable. However, the new diagnostic produces systematically larger values with more variability.

Climate simulations were performed using SSTs of the early (1902–21) and late (1979–98) twentieth century along with the appropriate CO2 forcing. In general, the increase of global precipitation with the increases in SST that occurred between the early and late twentieth century is small. However, an increase of atmospheric temperature leads to a systematic increase in total precipitable water. As a result, the residence time of water in the atmosphere increased, indicating a reduction of the global cycling rate. This result was explored further using a number of 50-yr climate simulations from different models forced with observed SST. The anomalies and trends in the cycling rate and hydrologic variables of different GCMs are remarkably similar. The global annual anomalies of precipitation show a significant upward trend related to the upward trend of surface temperature, during the latter half of the twentieth century. While this implies an increase in the simulated hydrologic cycle intensity, a concomitant increase of total precipitable water again leads to a decrease in the calculated global cycling rate. An analysis of the land/sea differences shows that the simulated precipitation over land has a decreasing trend, while the oceanic precipitation has an upward trend consistent with previous studies and the available observations. The decreasing continental trend in precipitation is located primarily over tropical land regions, with some other regions, such as North America, experiencing an increasing trend. Precipitation trends are diagnosed further using the water tracers to delineate the precipitation that occurs because of continental evaporation, as opposed to oceanic evaporation. These model diagnostics show that over global land areas, the recycling of continental moisture is decreasing in time. However, the recycling changes are not spatially uniform so that some regions, most notably over the United States, experience continental recycling of water that increases in time.

Full access
Siegfried D. Schubert, Max J. Suarez, Yehui Chang, and Grant Branstator

Abstract

This study examines the variability in forecasts of the January–February–March (JFM) mean extratropical circulation and how that variability is modulated by the El Niño–Southern Oscillation. The analysis is based on ensembles of seasonal simulations made with an atmospheric general circulation model (AGCM) forced with sea surface temperatures observed during the 1983 El Niño and 1989 La Niña events. The AGCM produces pronounced interannual differences in the magnitude of the extratropical seasonal mean noise (intraensemble JFM variability). The North Pacific, in particular, shows extensive regions in which the 1989 seasonal mean noise kinetic energy (SKE), which is dominated by a “Pacific–North American (PNA)–like” spatial structure, is more than 2 times that of the 1983 forecasts. The larger SKE in 1989 is associated with a larger-than-normal barotropic conversion of kinetic energy from the mean Pacific jet to the seasonal mean noise. The generation of SKE by submonthly transients also shows substantial interannual differences, though these are much smaller than the differences in the mean flow conversions. An analysis of the generation of monthly mean noise kinetic energy and its variability suggests that the seasonal mean noise is predominantly a statistical residue of variability resulting from dynamical processes operating on monthly and shorter timescales.

A stochastically forced barotropic model (linearized about the AGCM's 1983 and 1989 seasonal and ensemble mean states) is used to further assess the role of the basic state, submonthly transients, and tropical forcing in modulating the uncertainties in the seasonal AGCM forecasts. When forced globally with spatially white noise, the linear model generates much larger variance for the 1989 basic state, consistent with the AGCM results. The extratropical variability for the 1989 basic state is dominated by a single eigenmode and is strongly coupled with forcing over the tropical western Pacific and the Indian Ocean. Linear calculations that include forcing from the AGCM variance of the tropical forcing and submonthly transients show a small impact on the variability over the PNA region as compared with that of the basic-state differences.

Full access
Young-Kwon Lim, Siegfried D. Schubert, Yehui Chang, and Hailan Wang

Abstract

This study examines the within-season monthly variation of the El Niño response over North America during December–March using the NASA/GEOS model. In agreement with previous studies, the skill of 1-month-lead GEOS coupled model forecasts of precipitation over North America is largest (smallest) for February (January), with similar results in uncoupled mode. A key finding is that the relatively poor January skill is the result of the model placing the main circulation anomaly over the northeast Pacific slightly to the west of the observed, resulting in precipitation anomalies that lie off the coast instead of over land as observed. In contrast, during February the observed circulation anomaly over the northeast Pacific shifts westward, lining up with the predicted anomaly, which is essentially unchanged from January, resulting in both the observed and predicted precipitation anomalies remaining off the coast. Furthermore, the largest precipitation anomalies occur along the southern tier of states associated with an eastward extended jet—something that the models capture reasonably well. Simulations with a stationary wave model indicate that the placement of January El Niño response to the west of the observed over the northeast Pacific is the result of biases in the January climatological stationary waves, rather than errors in the tropical Pacific El Niño heating anomalies in January. Furthermore, evidence is provided that the relatively poor simulation of the observed January climatology, characterized by a strengthened North Pacific jet and enhanced ridge over western North America, can be traced back to biases in the January climatology heating over the Tibet region and the tropical western Pacific.

Restricted access