Bryan C. Weare

Examples of Extended Empirical Orthogonal Function Analyses

Abstract An extended empirical orthogonal function analysis technique is described which expands a data set in terms of functions which are the “best” representation of that data set for a sequence of time points. The method takes advantage of the fact that geophysical fields are often significantly correlated in both space and time. Two examples of applications of this technique are given which suggest it may be a highly useful tool for diagnosing the modes of variation of dominant sequences of events. In the first, an analysis of 300 mb relative vorticity, fairly regular advection of the major features of the spatial patterns is evident. Westward speeds of between 0.3 and 0.4 m s−1 are inferred. The second example illustrates extended functions of tropical Pacific Ocean surface temperatures. The dominant function, which is associated with El Nino, shows a high degree of persistence over a six-month sequence. The second most important function suggests opposing variations in the influences of the North a...

The Onset of Convection in the Madden–Julian Oscillation

Abstract An observational study of the onset of convection in the Madden–Julian oscillation (MJO) was performed. Composites of radiosonde data from the Comprehensive Aerological Reference Data Set were constructed for an ensemble of tropical stations located in the Indian Ocean, Maritime Continent, and western Pacific Ocean. The composites suggest that for the off-equatorial stations used in this study, the MJO period may be set by the buildup and discharge of the low-level moist static energy. This result supports the discharge–recharge hypothesis of Blade and Hartmann. MJO events appear to begin when the off-equatorial atmosphere has been destabilized through a combination of low-level moist static energy buildup and concurrent drying of the middle troposphere by subsidence in the wake of the previous cycle of MJO convection. The low-level moist static energy buildup is controlled by a corresponding increase in low-level moisture. The increase in low-level moisture is not caused by the 1000-mb convergen...

Annual Mean Surface Heat Fluxes in the Tropical Pacific Ocean

Abstract The four components of the long-term annual mean net surface heating of the tropical Pacific Ocean between 30°N and 40°S are calculated and portrayed. These flux elements were derived by using the bulk formulas and about 5 million marine weather reports for the years 1957–76. In addition to illustrating the mean solar, latent heat, infrared radiation and sensible heat fluxes, annual mean values of the atmospheric variables which contribute to those fluxes also are illustrated. A simple error analysis is carried out from which it is concluded that the 95% confidence bands for solar heating, latent heat loss and net oceanic heating are ±29, ±39 and ±49 W m−2, from the respective mean values. The validity of the results for the net heating is partially tested by comparing the horizontal heat transports required by the pattern of heating with independent estimates of those dynamical transports.

The Role of Low-Level Moisture Convergence and Ocean Latent Heat Fluxes in the Madden and Julian Oscillation: An Observational Analysis Using ISCCP Data and ECMWF Analyses

Abstract This paper examines whether or not low-level moisture convergence and surface latent heat flux act as forcing mechanisms of the Madden and Julian oscillation (MJO), as it is proposed by the theories of wave-CISK (conditional instability of the second kind) and evaporation-wind feedback. The mean brightness temperature of cloudy pixels at 11 μm, obtained from five years of International Satellite Cloud Climatology Project data, is used as a proxy for tropical convective activity. Five years of European Centre for Medium-Range Weather Forecasts analyses are used to estimate surface latent heat fluxes and moisture divergence integrated in the low levels of the troposphere. Spectral analysis of latent heat fluxes over the Indian and Pacific Oceans shows significant spectral peaks in the frequency band of the MJO. These peaks are due mainly to the oscillation in the surface wind speed rather than in the specific humidity difference. Principal component analysis and tagged correlation patterns of filte...

Uncertainties in Global Ocean Surface Heat Flux Climatologies Derived from Ship Observations

Abstract A methodology to define uncertainties associated with ocean surface heat flux calculations has been developed and applied to a global climatology that utilizes a summary of the Comprehensive Ocean–Atmosphere Data Set surface observations. Systematic and random uncertainties in the net oceanic heat flux and each of its four components at individual grid points and for zonal averages have been estimated for each calendar month and for the annual mean. The most important uncertainties of the 2° × 2° grid cell values of each of the heat fluxes are described. Annual mean net shortwave flux random uncertainties associated with errors in estimating cloud cover in the Tropics yield total uncertainties that are greater than 25 W m−2. In the northern latitudes, where the large number of observations substantially reduces the influence of these random errors, the systematic uncertainties in the utilized parameterization are largely responsible for total uncertainties in the shortwave fluxes, which usually r...

Evaluation of Total Cloudiness and Its Variability in the Atmospheric Model Intercomparison Project

Abstract Total cloudiness of 29 models participating in the Atmospheric Model Intercomparison Project is compared with the ISCCP C2 as well as the Nimbus-7 and Meteor observational estimates. The root-mean-square differences between the annual means of the model calculations and the C2 observations after global means are removed vary from about twice to nearly four times the difference between the C2 and Meteor observations. The large differences are in some cases due to the fact that although a model qualitatively has patterns of spatial variations similar to those of the observations, the magnitude of those variations is much too small. In other cases the models have produced the approximate magnitude of the spatial variability of the observations but display sizable errors in the pattern of that variability. Deficiencies with respect to the model simulations of the mean seasonal cycle are also pronounced. For instance, the differences between the zonal averages of total cloudiness for contrasting seaso...

Evaluation of the Vertical Structure of Zonally Averaged Cloudiness and Its Variability in the Atmospheric Model Intercomparison Project

Abstract Estimates of zonally averaged cloudiness at each pressure level in 24 models participating in the Atmospheric Model Intercomparison Project are compared with the ISCCP C2 as well as the Nimbus 7 (N7) and Warren et al. (hereafter WH) observations. The global means of model high cloudiness are about two to five times greater than the C2 satellite observations. The large differences are probably related to excessive high, thin cloud in most models. Nearly all of the models have the observed maximum in high cloud at the equator, but also maxima near 60°N and 60°S, which are not observed. The globally averaged annual mean low cloud in most models is generally 15%–20% less than the WH observations and 10%–15% less than the C2 observations. The meridional structure of model annual mean low cloud both as observed from below and as observed from above show excesses north of about 50°N and deficits south of about 40°S when compared with WH and C2 observations, respectively. The amplitude of the model seaso...

El Niño and Tropical Pacific Ocean Surface Temperatures

Abstract The spatial and temporal character of El Nino is explored with analyses of tropical Pacific Ocean surface temperatures for the period 1957–76. The data are derived from approximately 5×106 marine weather reports. Maps are illustrated which portray the initiation, maturation and decay of an “average” El Nino event. Empirical orthogonal functions of nonseasonal departures are displayed. The time coefficients of the dominant empirical functions are derived together with average departures for 18 regions which are usually 10° of latitude and 40–50° of longitude in size. Lag correlation and coherence-spectral analysts are carried out on all of the time series. The pattern of El Nino which is portrayed is that of a basinwide phenomenon with a time evolution lasting more than 24 months. During this evolution sea temperatures in the western Pacific tend to have departures of opposite sign to those in the cast. Variations in the eastern equatorial region are shown to precede those in the central equatoria...

Near-Global Observations of Low Clouds

Abstract This paper analyzes several near-global datasets of low cloud cover, including the the International Satellite Cloud Climatology Project (ISCCP) satellite observations, C. J. Hahn et al. surface-derived observations, and the National Centers for Environmental Prediction (NCEP) and ECMWF reanalysis products (ERA). The magnitudes of annual-mean ISCCP and C. J. Hahn observations of low cloud fraction are found to differ by up to about 0.4 for a number of locations. These differences are largely attributable to the fact that ISCCP low clouds are only those low clouds that are not obstructed by higher cloud. Those of both the NCEP and ERA low clouds, which should be comparable to the Hahn low cloud dataset, have magnitudes up to about 0.3 less than the latter. The dominant EOFs of the seasonal variation of ISCCP and Hahn observations low cloud differ substantially over much of the Northern Hemisphere, where there is a sizable number of observations in Hahn. The pattern of the dominant seasonal EOF of ...

Comparison of NCEP–NCAR Cloud Radiative Forcing Reanalyses with Observations

Abstract Longwave and shortwave cloud radiative forcing from the recently released National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses are compared to Earth Radiation Budget Experiment (ERBE) observations. The observed differences are analyzed utilizing concurrent International Satellite Cloud Climatology Project (ISCCP) estimates of cloudiness and other satellite observations. The results show that the NCEP–NCAR longwave cloud forcing agrees well with that of ERBE not only for the annual means but also for seasonal and climatic variations. Areas of disagreement are generally related to disagreements between NCEP–NCAR high cloudiness and observations. Overall, the NCEP–NCAR shortwave cloud forcing is in poorer agreement with ERBE observations. NCEP–NCAR annual means in the Tropics are often 20–30 W m−2 too negative. On the other hand the NCEP–NCAR total cloud cover in this region is 10%–20% less than the ISCCP observations, which should lead to less...