Search Results

You are looking at 1 - 10 of 43 items for

  • Author or Editor: Bryan C. Weare x
  • Refine by Access: All Content x
Clear All Modify Search
Bryan C. Weare

Abstract

The spatial and temporal character of El Niño 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 Niño 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 Niño 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 equatorial and northeastern tropical Pacific by 1–4 months. On the other hand, changes in the central and eastern Pacific near 25°S tend to precede those in the equatorial region by a few months.

Full access
Bryan C. Weare

Abstract

The role of moisture variations in the initiation of Madden–Julian oscillation (MJO) variability is examined. Composite singular value decomposition (CSVD) analyses at various time lags are carried out to discern the complex space–time relationships between convection, low-level specific humidity and divergence, and surface evaporation. The utilized data are low-level moisture and winds and surface evaporation from the NCEP–NCAR reanalysis and outgoing longwave radiation (OLR) from NOAA satellite observations for the period 1981–2000. These data are filtered at each point using a 150-point Lanczos filter capturing well the 20–100-day periodicities.

The two dominant CSVDs describe an eastward-propagating quasi-wavenumber-1 system in all of the analyzed variables. The dominant low-level divergence, moisture, and latent heat flux variations lead those of OLR by approximately 10 days. The low-level convergence and positive moisture anomalies develop near and to the east of the ensuing convective perturbations; positive latent heat flux variations generally occur farther to the east. Moisture variations develop at least 15 days prior to a convective event and have stronger correlations with OLR than those of simultaneous low-level divergence. Near the centers of convection the low-level moisture increases at the same time that the 1000-hPa flow is becoming slightly more divergent. This implies that the moisture preconditioning of convective events is not driven totally by moisture convergence. This may require a modification of the frictional wave conditional instability of the second kind (CISK) hypothesis for MJO development.

Full access
Bryan C. Weare

Abstract

The relationships between the patterns of monthly sea surface temperature (SST) variations and those of precipitation in the tropical Pacific Ocean region are examined. The rainfall data utilized are derived from satellite observations of outgoing longwave radiation. A composite empirical orthogonal function (EOF) analysis of the SST and precipitation indicates a dominant mode of variation linking SST variations in the eastern equatorial region with those of MNWI about 30° westward. One-point correlation analyses show that this general relationship is present for all points in the eastern and central ocean, but that no significant SST-rainfall correlations are evident for SST points wen of about the dateline. The one-point correlation analyses also suggest that during time periods outside of El Niñ's, the rainfall response to SST changes is largely confined to the areas of climatological precipitation maxima, suggesting only a minor alteration in the large-scale circulation. On the other hand, during El Niñ there is the strong suggestion that circulation changes give rise to complete shifts in rainfall zones. The possible influence of the strong spatial autocorrelations of SST on these results are also explored using the one-point correlations. It is concluded that the observed SST-rainfall teleconnections cannot be wholly explained by the large-scale nature of the SST variations.

The inferences derived from these analyses are utilized in formulating linear regression (LR) models to specify tropical precipitation anomalies based upon a knowledge of concurrent SST perturbations. Preliminary analyses suggest that while relatively large hindcast skills are evident in various LR models, the apparent skills decrease substantially when the models are applied to new data. While these results do not prove that such models can never be very useful, they do project difficulties in greatly increasing the skill, especially as long as the available data periods are relatively short

Full access
Bryan C. Weare

Abstract

A linear shallow water model on an equatorial β-plane is driven by three different estimates of the heating rate perturbation during seven seasons of a composite El Niño episode. In two cases the heating perturbations for this model are assumed to be closely linked to the pattern of sea surface temperature anomalies. In the, third the heating is related to direct estimates of precipitation departures. The model-derived winds based upon these three estimates are compared to one another and to data available from marine weather reports. An analysis of area averaged winds, surface latent heat fluxes and moisture convergences shows general agreement with observations over the composite El Niño seasons for the results of all three forcing estimates, although the model results using the direct precipitation estimates seem generally to be the poorest. A second analysis explores the similarity of smaller-scale features of the model results to the observations. This suggests that the simple parameterized heating using sea surface temperature estimates gives the only solutions which fairly well identify smaller-scale, variations.

Full access
Bryan C. Weare

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, rather than more, negative shortwave cloud forcing. Thus the primary error in the mean shortwave cloud forcing is likely due to specification of clouds that are too reflective in the NCEP analysis model. Moderate errors in the variability of NCEP–NCAR SWCF are apparently related to errors in the analyzed seasonal variability of total cloudiness, which are exacerbated by NCEP model specification of clouds that are too bright and underestimates of the seasonal variability of the clear-sky fluxes.

Full access
Bryan C. Weare

Abstract

Zonal average temperature for each 15° latitude band of the globe for six atmospheric levels is computed for crush mouth between May 1958 and December 1974. The averages are based on reports at a total of 190 WMO stations. Monthly and annual means, interannual variances and one-month lag correlations are presented. The correlations between zonal mean temperatures at different levels within the same latitudinal zone and different zones at the same level are also illustrated. Sample power spectra are presented for middle-latitude and subtropical zonal mean temperatures.

Full access
Bryan C. Weare

Abstract

Monthly means of selected variables of the 2.5° International Satellite Cloud Climatology Project (ISCCP) C2 total cloud cover (CC), cloud-top pressure (CTP), and cloud water (CW) are statistically related to sea surface temperature (SST). The statistical tools utilized include intra- and interannual correlation, regression, and composite empirical orthogonal function (EOF) analyses.

The dominant intra- and interannual composite EOFs all show that CC, CTP, and CW departures have spatially coherent links with those of SST. The second most important intra-annual functions also show coherent relations, which are about three months out of phase with those of the dominant functions. The regression analysis suggests that this phase relation may be explained by significant correlations of the cloud variables with not only SST, but also with the time derivative of SST (dSST/dt). For instance, in the tropical Pacific increased CC is accompanied by increases in SST but decreases in dSST/dt, and increased CTP is associated with decreases in SST. However, at middle and high latitudes other relationships exist, such that larger CCs may be associated with decreased SSTs, or higher CTPs may be related to higher SSTs. These diagnosed relationships have important implications for understanding cloud and cloud radiative feedbacks in weather and climate.

Full access
Bryan C. Weare

Abstract

Zonal averages of low, middle and total cloud amount estimates derived from measurements from Nimbus-7 have been analyzed for the six-year period April 1979 through March 1985. The globally and zonally averaged values of six-year annual means and standard deviations of total cloud amount and a proxy of cloud-top height are illustrated. Separate means for day and night and land and sea are also shown. The globally averaged value of intra-annual variability of total cloud amount is greater than 7%, and that for cloud height is greater than 0.3 km. Those of interannual variability are more than one-third of these values. Important latitudinal differences in variability are illustrated. The dominant empirical orthogonal analyses of the intra-annual variations of total cloud amount and heights show strong annual cycles, indicating that in the tropics increases in total cloud amount of up to about 30% are often accompanied by increases in cloud height of up to 1.2 km. This positive link is also evident in the dominant empirical orthogonal function of interannual variations of a total cloud/cloud height complex. This function shows a large coherent variation in total cloud cover of about 10% coupled with changes in cloud height of about 1.1 km associated with the 1982–83 El Niño–Southern Oscillation event.

Full access
Bryan C. Weare

Abstract

The El Niño index suggested by Weare, Navato and Newell is extended to include the period 1974–83.This index is based upon the most important empirical orthogonal function of interannual Pacific sea surface temperature (SST) departures for the period 1949–73. The index for the entire period 1949–83 is presented in both graphical and tabular forms. It is also compared with another El Niño index based upon tropical SST departures calculated by Wright.

Full access
Bryan C. Weare

Abstract

Multilag singular value decomposition (MLSVD) analysis is developed and applied to diagnosing the impact of interannual variations of outgoing longwave radiation (OLR) on tropical stratospheric temperature changes. MLSVD is designed to analyze simultaneously variations at multiple levels and for a large number of temporal lags and leads. The two dominant MLSVDs are strongly related to El Niño–Southern Oscillation (ENSO). The associated patterns of tropical OLR are similar to the canonical ENSO SST patterns with strong negative sign regions stretching along the equator in the eastern and central Pacific. These dominant modes are strongly linked to temperature perturbations at a wide range of lags. At the lowest analyzed level (200 hPa) and zero lag positive temperatures anomalies are in the region of low OLR. In the lower stratosphere near 100 hPa, strong negative temperature perturbations replace the positive values of the lowest level. Higher in the stratosphere near 20 hPa, equatorial temperature perturbations are again positive, but with a more zonally elongated spatial pattern. Overall, the equatorial temperature anomalies propagate slowly to the east, at a speed strongly related to ocean–atmosphere coupling of less than 1 m s−1, and vertically and westward into the stratosphere by Rossby waves with a speed in the range of 30 m s−1.

Full access