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W. M. Washington and S. M. Daggupaty

Abstract

A global circulation model (GCM) developed at the National Center for Atmospheric Research (NCAR) has been used to simulate the large-scale features of the Asian-African summer monsoon. The model has 6 vertical layers of 3-km thickness with a 2½° horizontal latitude-longitude grid. The physical processes incorporated are solar and infrared radiation, with cloudiness explicitly calculated from a model-generated relative humidity distribution. The latent heat released from precipitation is derived from stable lifting and cumulus convection. Also included in the model are subgrid-scale vertical and horizontal transports of momentum, sensible heat, and latent heat.

We compare the computed sea-level pressure, wind, cloudiness, and precipitation patterns with. observed data and, in particular, concentrate on the strong low-level monsoon jet near eastern Kenya and Somalia. The model correctly simulates this jet in position; however, the wind maxima are weaker than observed. Because of the relatively coarse model resolution, we fail to obtain the important monsoon depressions which form in the Bay of Bengal or near Bombay. In nature, these depressions account for a large part of the precipitation in India and its surrounding regions.

This study demonstrates that a global circulation model is capable of simulating many of the features observed in the Asian-African summer monsoon.

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S. K. Kao, C. N. Chi, and W. M. Washington

Abstract

An analysis of the three-dimensional, large-scale movement of air particles for the winter months with the NCAR general circulation model indicates that the horizontal movement of particles in the upper troposphere is greatly affected by wave motion in mid- and high latitudes, by the field of horizontal convergence and divergence, and by mean meridional circulation in the tropics. The mean center of mass of particles in both hemispheres generally moves toward respective poles and the mean squire of the meridional component of the particle distances generally decreases with increasing time, indicating the effect of horizontal convergence on particle movement near the subtropics. The vertical movement of the particles is affected by upward motion near the thermal equator and downward motion near the subtropical region in the Northern and Southern Hemispheres. The vertical dispersion is most intense in the tropics and decreases toward the poles. There are two maxima of particle accumulation, one occurring near 15°N, the other near 30°S, and a minimum accumulation of particles appears near the thermal equator, indicating the effects of the divergence field and meridional circulation between the thermal equator and the subtropics.

The mean squares of zonal, meridional and vertical components of the distance for dusty” of particles released at the equator and 45°N appear to consist of two components, a monotonicaly increasing component due essentially to the effect of turbulent diffusion, and a periodic component due primarily to the horizontal velocity convergence and divergence of mean motion.

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Jill Williams, R. G. Barry, and W. M. Washington

Abstract

The NCAR global circulation model has been used to simulate global atmospheric conditions using boundary conditions representing those of the present day and those of the Würm/Wisconsin glacial maximum at about 20,000 years ago, for January and July cases.

The mean zonal wind strength in the July ice age case in the middle latitudes of the Northern Hemisphere was comparable with present winter conditions. Also in the ice age cases, the upper westerlies were not apparently displaced south of the Laurentide ice sheet. The Icelandic and Aleutian lows in January were displaced 10° southward, the Siberian high remained unchanged from the control situation, and a new low center was found over eastern Europe and the European USSR. In July high pressure developed over most of Asia. Maps of cyclone frequency in a 30-day period showed the influence of major ice sheets and sea ice in displacing zones of cyclone activity southward in January. Frequent cyclones occurred over central North America and there was a continuation of cyclone activity in the North Atlantic and from eastern Europe into Asia. There was virtually no cyclonic activity near the Laurentide ice sheet in July.

Cloud cover and precipitation were also analyzed. Changes in precipitation for the glacial maximum cases are mainly quantitative rather than affecting its spatial distribution. The zonal averages show small changes for the Southern Hemisphere. In the Northern Hemisphere precipitation was decreased slightly in winter with most pronounced effects between 0–10N and 55–70N. The summer shows a dramatic reduction of precipitation north of 10N.

There is broad agreement between these paleo-climatological reconstructions and those of other studies using different models.

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Gerald A. Meehl, Grant W. Branstator, and Warren M. Washington

Abstract

In this paper, an attempt is made to estimate possible sensitivities of El Niño-Southern Oscillation (ENSO)-related effects in a climate with increased carbon dioxide (CO2). To illustrate this sensitivity, results are shown from two different interactive ocean-atmosphere model configurations and an atmospheric model with prescribed heating anomalies. In the first, an atmospheric general circulation model (GCM) is coupled to a global coarse-grid dynamical ocean GCM (coupled model). In the second, the same atmospheric model is coupled to a simple nondynamic slab-ocean mixed-layer model (mixed-layer model). In the third, an atmospheric model is run in perpetual January mode with observed sea surface temperatures (SSTs) and prescribed tropical tropospheric heating anomalies (prescribed-heating model). Results from the coupled model show that interannual SST variability (with warm and cold events relative to the mean SST) continues to occur in the tropics with a doubling of CO2. This variability is superimposed on mean SSTs in the tropical eastern Pacific that are higher by about 1°. The pattern of precipitation and soil-moisture anomalies in the tropics is similar in model warm events with present amounts of CO2 (1 × CO2) and in warm events with instantaneously doubled CO2 (2 × CO2). When a warm-event SST anomaly is superimposed, the rise in mean SST in the tropical eastern Pacific from the doubling of CO2 leads to increased evaporation and low-level moisture convergence, greater precipitation over the SST anomaly, and an intensification of atmospheric anomalies in the tropics involved with the anomalous large-scale east-west (Walker) circulation. Consequently, differences of precipitation and soil moisture between 1 × CO2 and 2 × CO2 warm events show that most anomalously dry areas become drier (implying risk of increased drought in those regions in 2 × CO2 Warm events) and anomalously wet areas wetter in the coupled model. In the extratropics, the increased CO2 causes a large change in the midlatitude atmospheric circulation. This is associated with an alteration of extratropical teleconnections in 2 × CO warm events compared to 1 × CO2 warm events in a relative sense, with more zonally symmetric anomalies in sea level pressure and 200- mb height. Similar results in the tropics and extratropics are obtained for the mixed-layer model with warm-event SST anomalies in the tropical Pacific prescribed for 1 × CO2 and 2 × CO2 mean climates, and from the prescribed-heating model with anomalous heat sources in the tropical troposphere analogous to those in 1 × CO2 and 2 × CO2 warm events. This study is a precursor to future higher-resolution model studies that could also address possible changes in ENSO but with better representation of coupled mechanisms thought to contribute to ENSO.

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Aiguo Dai, A. Hu, G. A. Meehl, W. M. Washington, and W. G. Strand

Abstract

A 1200-yr unforced control run and future climate change simulations using the Parallel Climate Model (PCM), a coupled atmosphere–ocean–land–sea ice global model with no flux adjustments and relatively high resolution (∼2.8° for the atmosphere and 2/3° for the oceans) are analyzed for changes in Atlantic Ocean circulations. For the forced simulations, historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used. The Atlantic thermohaline circulation (THC) shows large multidecadal (15–40 yr) variations with mean-peak amplitudes of 1.5–3.0 Sv (1 Sv ≡ 106 m3 s−1) and a sharp peak of power around a 24-yr period in the control run. Associated with the THC oscillations, there are large variations in North Atlantic Ocean heat transport, sea surface temperature (SST) and salinity (SSS), sea ice fraction, and net surface water and energy fluxes, which all lag the variations in THC strength by 2–3 yr. However, the net effect of the SST and SSS variations on upper-ocean density in the midlatitude North Atlantic leads the THC variations by about 6 yr, which results in the 24-yr period. The simulated SST and sea ice spatial patterns associated with the THC oscillations resemble those in observed SST and sea ice concentrations that are associated with the North Atlantic Oscillation (NAO). The results suggest a dominant role of the advective mechanism and strong coupling between the THC and the NAO, whose index also shows a sharp peak around the 24-yr time scale in the control run. In the forced simulations, the THC weakens by ∼12% in the twenty-first century and continues to weaken by an additional ∼10% in the twenty-second century if CO2 keeps rising, but the THC stabilizes if CO2 levels off. The THC weakening results from stabilizing temperature increases that are larger in the upper and northern Atlantic Ocean than in the deep and southern parts of the basin. In both the control and forced simulations, as the THC gains (loses) strength and depth, the separated Gulf Stream (GS) moves southward (northward) while the subpolar gyre centered at the Labrador Sea contracts from (expands to) the east with the North Atlantic Current (NAC) being shifted westward (eastward). These horizontal circulation changes, which are dynamically linked to the THC changes, induce large temperature and salinity variations around the GS and NAC paths.

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Gerald A. Meehl, Warren M. Washington, Julie M. Arblaster, Thomas W. Bettge, and Warren G. Strand Jr.

Abstract

A methodology is formulated to evaluate the possible changes in decadal-timescale (10–20-yr period) surface temperature variability and associated low-frequency fluctuations of anthropogenic forcing and changes in climate base state due to the forcing in simulations of twentieth- and twenty-first-century climate in a global coupled climate model without flux adjustment. The two climate change experiments both start in the year 1900. The first uses greenhouse gas radiative forcing (represented by equivalent CO2) observed during the twentieth century, and extends greenhouse gas forcing to the year 2035 by increasing CO2 1% yr−1 compound after 1990 (CO2-only experiment). The second includes the same greenhouse gas forcing as the first, but adds the effects of time-varying geographic distributions of monthly sulfate aerosol radiative forcing represented by a change in surface albedo (CO2 + sulfates experiment). The climate change experiments are compared with a 135-yr control experiment with no change in external forcing. Climate system responses in the CO2-only and CO2 + sulfates experiments in this particular model are marked not only by greater warming at high latitudes in the winter hemisphere, but also by a global El Niño–like pattern in surface temperature, precipitation, and sea level pressure. This pattern is characterized by a relatively greater increase of SST in the central and eastern equatorial Pacific in comparison with the west, a shift of precipitation maxima from the western Pacific to the central Pacific, mostly decreases of Asian–Australian monsoon strength, lower pressure over the eastern tropical Pacific, deeper midlatitude troughs in the North and South Pacific, and higher pressure over Australasia. Time series analysis of globally averaged temperature and an EOF analysis of surface temperature are consistent with previous results in that enhanced low-frequency variability with periods greater than around 20 yr is introduced into the model coupled climate system with a comparable timescale to the forcing. To examine the possible effects of the associated changes in base state on decadal timescale variability (10–20-yr periods), the surface temperature time series are filtered to retain only variability on that timescale. The El Niño–like pattern of decadal variability seen in the observations is present in each of the model experiments (control, CO2 only, and CO2 + sulfates), but the magnitude decreases significantly in the CO2-only experiment. This decrease is associated with changes in the base-state climate that include a reduction in the magnitude (roughly 5%–20% or more) of wind stress and ocean currents in the upper 100 m in most ocean basins and a weakening of meridional overturning (about 50%) in the Atlantic. These weakened circulation features contribute to decreasing the amplitude of global decadal surface temperature variability as seen in a previous sea-ice sensitivity study with this model. Thus the superposition of low-frequency variability patterns in the radiative forcing increases climate variability for periods comparable to those of the forcing (greater than about 20 yr). However, there are decreases in the amplitude of future decadal (10–20-yr period) variability in these experiments due to changes of the base-state climate as a consequence of increases in that forcing.

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W. M. Washington, B. T. O'Lear, J . Takamine, and D. Robertson

The recent development of hemispheric and global general circulation models has produced a need for an efficient method of contour analysis. We will discuss a cathode ray tube (CRT) method of processing contoured data on standard map projections. A detailed explanation of the methods of making black-and-white and color movies is also included.

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David W. Pierce, Tim P. Barnett, Krishna M. AchutaRao, Peter J. Gleckler, Jonathan M. Gregory, and Warren M. Washington

Abstract

Observations show the oceans have warmed over the past 40 yr, with appreciable regional variation and more warming at the surface than at depth. Comparing the observations with results from two coupled ocean–atmosphere climate models [the Parallel Climate Model version 1 (PCM) and the Hadley Centre Coupled Climate Model version 3 (HadCM3)] that include anthropogenic forcing shows remarkable agreement between the observed and model-estimated warming. In this comparison the models were sampled at the same locations as gridded yearly observed data. In the top 100 m of the water column the warming is well separated from natural variability, including both variability arising from internal instabilities of the coupled ocean–atmosphere climate system and that arising from volcanism and solar fluctuations. Between 125 and 200 m the agreement is not significant, but then increases again below this level, and remains significant down to 600 m. Analysis of PCM’s heat budget indicates that the warming is driven by an increase in net surface heat flux that reaches 0.7 W m−2 by the 1990s; the downward longwave flux increases by 3.7 W m−2, which is not fully compensated by an increase in the upward longwave flux of 2.2 W m−2. Latent and net solar heat fluxes each decrease by about 0.6 W m−2. The changes in the individual longwave components are distinguishable from the preindustrial mean by the 1920s, but due to cancellation of components, changes in the net surface heat flux do not become well separated from zero until the 1960s. Changes in advection can also play an important role in local ocean warming due to anthropogenic forcing, depending on the location. The observed sampling of ocean temperature is highly variable in space and time, but sufficient to detect the anthropogenic warming signal in all basins, at least in the surface layers, by the 1980s.

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N. L. Miller, A. W. King, M. A. Miller, E. P. Springer, M. L. Wesely, K. E. Bashford, M. E. Conrad, K. Costigan, P. N. Foster, H. K. Gibbs, J. Jin, J. Klazura, B. M. Lesht, M. V. Machavaram, F. Pan, J. Song, D. Troyan, and R. A. Washington-Allen

A Department of Energy (DOE) multilaboratory Water Cycle Pilot Study (WCPS) investigated components of the local water budget at the Walnut River watershed in Kansas to study the relative importance of various processes and to determine the feasibility of observational water budget closure. An extensive database of local meteorological time series and land surface characteristics was compiled. Numerical simulations of water budget components were generated and, to the extent possible, validated for three nested domains within the Southern Great Plains—the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Cloud Atmospheric Radiation Testbed (CART), the Walnut River watershed (WRW), and the Whitewater watershed (WW), in Kansas.

A 2-month intensive observation period (IOP) was conducted to gather extensive observations relevant to specific details of the water budget, including finescale precipitation, streamflow, and soil moisture measurements that were not made routinely by other programs. Event and seasonal water isotope (d18O, dD) sampling in rainwater, streams, soils, lakes, and wells provided a means of tracing sources and sinks within and external to the WW, WRW, and the ARM CART domains. The WCPS measured changes in the leaf area index for several vegetation types, deep groundwater variations at two wells, and meteorological variables at a number of sites in the WRW. Additional activities of the WCPS include code development toward a regional climate model that includes water isotope processes, soil moisture transect measurements, and water-level measurements in groundwater wells.

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