Search Results

You are looking at 1 - 10 of 39 items for

  • Author or Editor: Brian J. Soden x
  • All content x
Clear All Modify Search
Brian J. Soden

Abstract

Observations of the clear-sky outgoing longwave radiation and sea surface temperature are combined to examine the evolution of the tropical greenhouse effect from colder La Niña conditions in early 1985 to warmer El Niño conditions in late 1987. Although comparison of individual months can suggest a decrease in greenhouse trapping from cold to warm conditions, when the entire 4-yr record is considered a distinct increase in tropical-mean greenhouse trapping of ∼2 W m−2 is observed in conjunction with a ∼0.4 K increase in tropical-mean sea surface temperature. This observed increase compares favorably with GCM simulations of the change in the clear-sky greenhouse effect during El Niño–Southern Oscillation (ENSO). Superimposed on top of the SST-driven change in greenhouse trapping are dynamically induced changes in tropical moisture apparently associated with a redistribution of SST during ENSO. The GCM simulations also successfully reproduce this feature, providing reassurance in the ability of GCMs to predict both dynamically and thermodynamically driven changes in greenhouse trapping.

Full access
Brian J. Soden

Abstract

Satellite observations of temperature, water vapor, precipitation and longwave radiation are used to characterize the variation of the tropical hydrologic and energy budgets associated with the El Niño–Southern Oscillation (ENSO). As the tropical oceans warm during an El Niño event, the precipitation intensity, water vapor mass, and temperature of the tropical atmosphere are observed to increase, reflecting a more vigorous hydrologic cycle. The enhanced latent heat release and resultant atmospheric warming lead to an increase in the emission of longwave radiation. Atmospheric global climate models, forced with observed sea surface temperatures (SSTs), accurately reproduce the observed tropospheric temperature, water vapor, and outgoing longwave radiation changes. However, the predicted variations in tropical-mean precipitation rate and surface longwave radiation are substantially smaller than observed. The comparison suggests that either (i) the sensitivity of the tropical hydrological cycle to ENSO-driven changes in SST is substantially underpredicted in existing climate models or (ii) that current satellite observations are inadequate to accurately monitor ENSO-related changes in the tropical-mean precipitation. Either conclusion has important implications for current efforts to monitor and predict changes in the intensity of the hydrological cycle.

Full access
Jie He and Brian J. Soden

Abstract

There is a lack of consensus on the physical mechanisms that drive the anthropogenic weakening of tropical circulation. This study investigates the relative roles of direct CO2 forcing, mean SST warming, and the pattern of SST change on the weakening of the tropical circulation using an ensemble of AMIP and aquaplanet simulations. In terms of the mean weakening of the tropical circulation, the SST warming dominates over the direct CO2 forcing through its control over the tropical mean hydrological cycle and tropospheric stratification. In terms of the spatial pattern of circulation weakening, however, the three forcing agents are all important contributors, especially over the ocean. The increasing CO2 weakens convection over ocean directly by stabilizing the lower troposphere and indirectly via the land–sea warming contrast. The mean SST warming drives strong weakening over the centers and edges of convective zones. The pattern of SST warming plays a crucial role on the spatial pattern of circulation weakening over the tropical Pacific.

The anthropogenic weakening of the Walker circulation is mostly driven by the mean SST warming. Increasing CO2 strengthens the Walker circulation through its indirect effect on land–sea warming contrast. Changes in the upper-level velocity potential indicate that the pattern of SST warming does not weaken the Walker circulation despite being “El Niño–like.” A weakening caused by the mean SST warming also dominates changes in the Hadley circulation in the AMIP simulations. However, this weakening is not simulated in the Southern Hemisphere in coupled simulations.

Full access
Ákos Horváth and Brian J. Soden

Abstract

This study combines geostationary water vapor imagery with optical cloud property retrievals and microwave sea surface observations in order to investigate, in a Lagrangian framework, (i) the importance of cirrus anvil sublimation on tropical upper-tropospheric humidity and (ii) the sea surface temperature dependence of deep convective development. Although an Eulerian analysis shows a strong spatial correlation of ∼0.8 between monthly mean cirrus ice water path and upper-tropospheric humidity, the Lagrangian analysis indicates no causal link between these quantities. The maximum upper-tropospheric humidity occurs ∼5 h after peak convection, closely synchronized with the maximum cirrus ice water path, and lagging behind it by no more than 1.0 h. Considering that the characteristic e-folding decay time of cirrus ice water is determined to be ∼4 h, this short time lag does not allow for significant sublimative moistening. Furthermore, a tendency analysis reveals that cirrus decay and growth, in terms of both cloud cover and integrated ice content, is accompanied by the drying and moistening of the upper troposphere, respectively, a result opposite that expected if cirrus ice were a primary water vapor source. In addition, it is found that an ∼2°C rise in sea surface temperature results in a measurable increase in the frequency, spatial extent, and water content of deep convective cores. The larger storms over warmer oceans are also associated with slightly larger anvils than their counterparts over colder oceans; however, anvil area per unit cumulus area, that is, cirrus detrainment efficiency, decreases as SST increases.

Full access
Jie He and Brian J. Soden

Abstract

Atmospheric general circulation models (AGCMs) are often considered inadequate for studying natural climate variability because of their lack of coupling with an underlying ocean. This lack of two-way air–sea coupling results in an inconsistency in surface energetics. This study aims to determine whether the lack of two-way air–sea coupling also undermines an AGCM’s ability to simulate anthropogenic climate change. A comparison between coupled and atmospheric GCM simulations shows that anthropogenic climate change can be well reproduced by an AGCM and that errors due to the lack of two-way coupling are primarily limited to internal variability. Simulations using a stochastic linear model are shown to further support this conclusion. These results suggest a greater utility for AGCMs as computationally efficient tools for understanding and downscaling coupled model simulations of anthropogenic climate change.

Full access
Brian J. Soden and Rong Fu

Abstract

This paper combines satellite measurements of the upwelling 6.7-μm radiance from TOVS with cloud-property information from ISCCP and outgoing longwave radiative fluxes from ERBE to analyze the climatological interactions between deep convection, upper-tropospheric humidity, and atmospheric greenhouse trapping. The satellite instruments provide unmatched spatial and temporal coverage, enabling detailed examination of regional, seasonal, and interannual variations between these quantities. The present analysis demonstrates that enhanced tropical convection is associated with increased upper-tropospheric relative humidity. The positive relationship between deep convection and upper-tropospheric humidity is observed for both regional and temporal variations, and is also demonstrated to occur over a wide range of space and time scales. Analysis of ERBE outgoing longwave radiation measurements indicates that regions or periods of increased upper-tropospheric moisture are strongly correlated with an enhanced greenhouse trapping, although the effects of lower-tropospheric moisture and temperature lapse rate are also observed to be important. The combined results for the Tropics provide a picture consistent with a positive interrelationship between deep convection, upper-tropospheric humidity, and the greenhouse effect. In extratropical regions, temporal variations in upper-tropospheric humidity exhibit little relationship to variations in deep convection, suggesting the importance of other dynamical processes in determining changes in upper-tropospheric moisture for this region. Comparison of the observed relationships between convection, upper-tropospheric moisture, and greenhouse trapping with climate model simulations indicates that the Geophysical Fluid Dynamics Laboratory (GFDL) GCM is qualitatively successful in capturing the observed relationship between these quantities. This evidence supports the ability of the GFDL GCM to predict upper-tropospheric water vapor feedback, despite the model's relatively simplified treatment of moist convective processes.

Full access
Angela J. Colbert and Brian J. Soden

Abstract

This study investigates the relationship between tropical cyclone (TC) tracks and climatological variations in large-scale environmental parameters associated with the TC steering flow. By using the Atlantic Ocean hurricane database for 1950–2010, TCs that form in the main development region (MDR) are categorized into one of three track types: straight moving, recurving landfall, or recurving ocean. As expected, the straight-moving storms are associated with a westward extension and strengthening of the subtropical high, whereas the recurving ocean storms are associated with a weakening of the high. The presence of El Niño conditions in the tropical Pacific Ocean is shown to be associated with a weakening of the high, an increase in the percentage of recurving ocean TCs, and a decrease in the percentage of recurving landfall TCs. Positive phases of the Atlantic Meridional Mode are associated with an increase in the percentage of recurving ocean TCs and a decrease in the percentage of straight-moving TCs. Synthetic tracks are simulated for each storm using a beta and advection model. Sensitivity experiments using both observed and uniformly seeded genesis locations indicate that the path of straight-moving TCs is largely a reflection of their tendency to form in the southwestern portion of the MDR rather than of differences in steering flow. These experiments also suggest that the shift in TC tracks associated with El Niño/La Niña conditions is largely attributable to changes in the steering flow, whereas the track changes associated with variations in the Atlantic Meridional Mode are due to a systematic shift in genesis location.

Full access
Ryan J. Kramer and Brian J. Soden

Abstract

In response to rising CO2 concentrations, climate models predict that globally averaged precipitation will increase at a much slower rate than water vapor. However, some observational studies suggest that global-mean precipitation and water vapor have increased at similar rates. While the modeling results emphasize changes at multidecadal time scales where the anthropogenic signal dominates, the shorter observational record is more heavily influenced by internal variability. Whether the physical constraints on the hydrological cycle fundamentally differ between these time scales is investigated. The results of this study show that while global-mean precipitation is constrained by radiative cooling on both time scales, the effects of CO2 dominate on multidecadal time scales, acting to suppress the increase in radiative cooling with warming. This results in a smaller precipitation change compared to interannual time scales where the effects of CO2 forcing are small. It is also shown that intermodel spread in the response of atmospheric radiative cooling (and thus global-mean precipitation) to anthropogenically forced surface warming is dominated by clear-sky radiative processes and not clouds, while clouds dominate under internal variability. The findings indicate that the sensitivity of the global hydrological cycle to surface warming differs fundamentally between internal variability and anthropogenically forced changes and this has important implications for interpreting observations of the hydrological sensitivity.

Full access
Eui-Seok Chung and Brian J. Soden

Abstract

Contrary to a midtropospheric warming trend detected from Microwave Sounding Unit (MSU) measurements, High-Resolution Infrared Radiation Sounder (HIRS) temperature (15 μm) channels, sensitive to the thermal emission from the troposphere, produce distinct cooling trends for the period 1980–99. This apparent discrepancy in the tropospheric temperature trend is investigated through radiative transfer simulations using Geophysical Fluid Dynamics Laboratory climate model output and the profiles of the standard model atmospheres. Radiative simulations with time-invariant carbon dioxide concentration throughout the entire analysis period produce trends that are qualitatively similar to that obtained from the MSU observations, implying that the observed cooling trends of the HIRS temperature channels are attributable to increased carbon dioxide concentration over the 20-yr period. Additional simulations with the observed time-varying concentration of carbon dioxide confirm this basic result. Whereas temperature fluctuations dominate variability on monthly to interannual time scales, carbon dioxide changes dominate the decadal trends in both the observations and model simulations. Further simulations examined the sensitivity of the brightness temperature change with respect to the changes in tropospheric and stratospheric temperature. These calculations indicate that the influences of stratospheric temperature on the measured radiances are greater for the HIRS temperature channels relative to the MSU midtropospheric channel. These results highlight the contributions of time-varying carbon dioxide concentrations and stratospheric temperature to the HIRS 15-μm (temperature channel) radiance record and underscore the importance of accurately accounting for these changes when using HIRS measurements for long-term monitoring.

Full access
Eui-Seok Chung and Brian J. Soden

Abstract

Consistency of upper-tropospheric water vapor measurements from a variety of state-of-the-art instruments was assessed using collocated Geostationary Operational Environmental Satellite-8 (GOES-8) 6.7-μm brightness temperatures as a common benchmark during the Atmospheric Radiation Measurement Program (ARM) First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Water Vapor Experiment (AFWEX). To avoid uncertainties associated with the inversion of satellite-measured radiances into water vapor quantity, profiles of temperature and humidity observed from in situ, ground-based, and airborne instruments are inserted into a radiative transfer model to simulate the brightness temperature that the GOES-8 would have observed under those conditions (i.e., profile-to-radiance approach). Comparisons showed that Vaisala RS80-H radiosondes and Meteolabor Snow White chilled-mirror dewpoint hygrometers are systemically drier in the upper troposphere by ∼30%–40% relative to the GOES-8 measured upper-tropospheric humidity (UTH). By contrast, two ground-based Raman lidars (Cloud and Radiation Test Bed Raman lidar and scanning Raman lidar) and one airborne differential absorption lidar agree to within 10% of the GOES-8 measured UTH. These results indicate that upper-tropospheric water vapor can be monitored by these lidars and well-calibrated, stable geostationary satellites with an uncertainty of less than 10%, and that correction procedures are required to rectify the inherent deficiencies of humidity measurements in the upper troposphere from these radiosondes.

Full access