Dennis L. Hartmann

Cloud-radiative forcing and climate : Results from the Earth Radiation Budget Experiment

The study of climate and climate change is hindered by a lack of information on the effect of clouds on the radiation balance of the earth, referred to as the cloud-radiative forcing. Quantitative estimates of the global distributions of cloud-radiative forcing have been obtained from the spaceborne Earth Radiation Budget Experiment (ERBE) launched in 1984. For the April 1985 period, the global shortwave cloud forcing [-44.5 watts per square meter (W/m2)] due to the enhancement of planetary albedo, exceeded in magnitude the longwave cloud forcing (31.3 W/m2) resulting from the greenhouse effect of clouds. Thus, clouds had a net cooling effect on the earth. This cooling effect is large over the mid-and high-latitude oceans, with values reaching -100 W/m2. The monthly averaged longwave cloud forcing reached maximum values of 50 to 100 W/m2 over the convectively disturbed regions of the tropics. However, this heating effect is nearly canceled by a correspondingly large negative shortwave cloud forcing, which indicates the delicately balanced state of the tropics. The size of the observed net cloud forcing is about four times as large as the expected value of radiative forcing from a doubling of CO2. The shortwave and longwave components of cloud forcing are about ten times as large as those for a CO2 doubling. Hence, small changes in the cloud-radiative forcing fields can play a significant role as a climate feedback mechanism. For example, during past glaciations a migration toward the equator of the field of strong, negative cloud-radiative forcing, in response to a similar migration of cooler waters, could have significantly amplified oceanic cooling and continental glaciation.

The seasonal cycle of low stratiform clouds

Abstract The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans. In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6% increase in stratus fractional area coverage is associated with each 1°C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summ...

The effect of cloud type on Earth's energy balance : global analysis

Abstract The role of fractional area coverage by cloud types in the energy balance of the earth is investigated through joint use of International Satellite Cloud Climatology Project (ISCCP) C1 cloud data and Earth Radiation Budget Experiment (ERBE) broadband energy flux data for the one-year period March 1985 through February 1986. Multiple linear regression is used to relate the radiation budget data to the cloud data. Comparing cloud forcing estimates obtained from the ISCCP-ERBE regression with those derived from the ERBE scene identification shows generally good agreement except over snow, in tropical convective regions, and in regions that are either nearly cloudless or always overcast. It is suggested that a substantial fraction of the disagreement in longwave cloud forcing in tropical convective regions is associated with the fact that the ERBE scene identification does not take into account variations in upper-tropospheric water vapor. On a global average basis, low clouds make the largest contri...

Modulation of Eastern North Pacific Hurricanes by the Madden–Julian Oscillation

Abstract Hurricane and tropical storm statistics verify the modulation of eastern Pacific tropical systems by the Madden–Julian oscillation (MJO) as hypothesized by Maloney and Hartmann. Over twice as many named tropical systems (hurricanes and tropical storms) accompany equatorial 850-mb westerly anomalies than accompany equatorial easterly anomalies, and the systems that do exist are stronger. Hurricanes are over four times more numerous during westerly phases of the MJO than during easterly phases. The current study constructs a composite life cycle of the MJO during May–November 1979–95 using an index based on the 850-mb equatorial zonal wind. Equatorial Kelvin waves propagating eastward from convective regions of the western Pacific Ocean alter dynamical conditions over the eastern Pacific Ocean. Westerly (easterly) equatorial 850-mb wind anomalies are accompanied by enhanced (suppressed) convection over the eastern Pacific hurricane region. Convection locally amplifies the wind anomalies over the ea...

Wave-Maintained Annular Modes of Climate Variability*

Abstract The leading modes of month-to-month variability in the Northern and Southern Hemispheres are examined by comparing a 100-yr run of the Geophysical Fluid Dynamics Laboratory GCM with the NCEP–NCAR reanalyses of observations. The model simulation is a control experiment in which the SSTs are fixed to the climatological annual cycle without any interannual variability. The leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward. This fluctuation is strongest during the winter months. The structure and amplitude of the simulated modes are very similar to those derived from observations, indicating that these modes arise from the internal dynamics of the atmosphere. Dynamical diagnosis of both observations and model simulation indicates that variations in the zonally symmetric flow associated with the annular modes are forced by eddy fluxes in the free troposphere, while...

The Life Cycle of the Northern Hemisphere Sudden Stratospheric Warmings

Motivated by recent evidence of strong stratospheric‐tropospheric coupling during the Northern Hemisphere winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP‐NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasistationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher.

Eddy-Zonal Flow Feedback in the Southern Hemisphere

Abstract The variability of the zonal-mean zonal wind in the Southern Hemisphere is studied using EOF analysis and momentum budget diagnostics of NCEP reanalysis data (1978–97). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north–south movement of the midlatitude jet. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes accounts for the unusual persistence of EOF1 and plays an important role in the selection of the leading EOF of midlatitude variability. Further analysis also shows a propagating feedback, common to both EOF1 and EOF2, which is responsible for the poleward drift of wind anomalies with time. The observations support the following feedback mechanism. Anomalous baroclinic wave activity is generated at the latitude of anomalous temperature gradient that, by thermal wind, coincides with the latitude of the anomalous zonal jet. The net propagation of baroclinic...

Frictional Moisture Convergence in a Composite Life Cycle of the Madden–Julian Oscillation

Abstract A composite life cycle of the Madden–Julian oscillation (MJO) is constructed using an index based on the first two EOFs of the bandpass-filtered (20–80 days) 850-mb zonal wind averaged from 5°N to 5°S every 2.5° around the equator. Precipitation, 1000-mb convergence, 850-mb wind, and 200-mb wind are composited for the period 1979–95. Water vapor integrated from the surface to 300 mb is composited for the period 1988–92. Frictional moisture convergence at the equator is shown to play an important role in the life cycle of the Madden–Julian oscillation (MJO). Regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection. This convergence coincides with 850-mb easterly wind anomalies, as is consistent with Kelvin wave dynamics. Drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850-mb westerly perturbations. Possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advect...

Wave-Driven Zonal Flow Vacillation in the Southern Hemisphere

The variability of the zonal mean flow in the Southern Hemisphere during the period 1985‐94 is studied using European Centre for Medium-Range Forecasts analyses. The dominant mode of variability has approximately equivalent barotropic variations of opposite signs centered at 40 8 and 608S. This structure is dominant in all seasons and has similar variance in all seasons. The temporal variance of the amplitude of this mode is well modeled as Gaussian red noise with a correlation e-folding time of about 10 days. Zonal wind anomalies are maintained against frictional drag by variations in the zonal flow accelerations driven by transient eddies and associated mean meridional circulations. The eddy structures suggest that equatorward propagation is favored when the jet is displaced poleward and zonal propagation is favored when the jet is displaced equatorward.

Eddy–Zonal Flow Feedback in the Northern Hemisphere Winter

The variability of the zonal-mean zonal wind in the Northern Hemisphere winter (December‐March) is studied using EOF analysis and momentum budget diagnostics of NCEP‐NCAR reanalysis data (1976‐2001). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north‐south movement of the midlatitude westerlies. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes selects the leading EOF of midlatitude variability. Like the Southern Hemisphere, the baroclinic eddies reinforce the zonal wind anomalies while external Rossby waves damp the wind anomalies. In the Northern Hemisphere, the quasi-stationary eddies also reinforce the zonal wind anomalies, but the baroclinic eddies are most important for the positive eddy‐zonal flow feedback. The observations support the following feedback mechanisms. 1) Above-normal baroclinic wave activity is generated in the region of enhanced westerlies. This leads to wave propagation out of the westerlies that is associated with reinforcing eddy momentum fluxes. 2) The westerly jet is a waveguide for external Rossby waves that tend to propagate into the jet and remove momentum from it. 3) The quasi-stationary waves respond to a refractive index anomaly in the high latitudes below the tropopause. During the high (low) index this anomaly is negative (positive) leading to an acceleration (deceleration) of the zonal wind in the high latitudes.