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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.

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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

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Bryan C. Weare

Abstract

Diabatic heating rates calculated in the UCLA general circulation model are analyzed. The heating rates are for the December–January–February season for the mean of four climatological runs and the mean of four runs with observed 1982/83 sea surface temperatures as boundary conditions. Vertically integrated total diabatic heating for the climatological calculations agrees well with observations. Unfortunately, the available observations of zonally averaged vertical structure differ from each other so substantially that comparisons with the model are inconclusive. Nevertheless, the vertical structure of the model seems quite realistic and in general agreement with analyses of tropical cloud clusters or middle latitude cyclones.

The model diabatic heating is stratified in a number of ways. Zonal average height-latitude cross sections of the heating due to cumulus and large-scale precipitation and radiation show complex profiles for the rates associated with precipitation and much less structure with those associated with radiation except near the ground. Sample vertical profiles for regions in the tropics and new 45°N suggest that the zonal averages are generally representative of the individual heating profiles except near the surface.

Anomalous heating rates associated with the model 1982/83 season, in which a a strong El Niño was observed, show a moderate alteration in diabatic heating over the breadth of the tropical Pacific. In the eastern equatorial Pacific, increases are evident for the heating rates associated with cumulus and large-scale precipitation and radiation. Reductions in cumulus and radiative heating over other parts of the tropics are also apparent.

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Bryan C. Weare

Abstract

The role of moisture variations in the initiation of Madden–Julian oscillation (MJO) variability is reexamined through composite singular value decomposition (CSVD) analyses using the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr Re-Analyses (ERA-40) data. The CSVD analyses at various time lags are carried out to discern the complex space–time relationships between convection identified using outgoing longwave radiation and 1000-hPa divergence, 850-hPa specific humidity, and surface evaporation. The most striking difference from the earlier analyses using NCEP–NCAR reanalysis data is that the observed relations between 20–100-day filtered OLR and · V 1000 anomalies are weaker and less significant in the current analyses. On the other hand, both analyses show increasing low-level moisture near and to the east of the developing convection. Thus, both results imply that moisture preconditioning of convective events is not totally driven by boundary layer moisture convergence.

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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.

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Bryan C. Weare

Abstract

A shallow water equations model of the tropical atmosphere is developed and tested. The model includes a simple parameterization of convective rainfall so that the model heating is a function of fixed sea surface temperatures and model dependent circulation. To achieve quasi-steady states the model is iterated a number of steps.

For equatorial sea temperature perturbations imposed on an idealized Pacific Ocean, the model shows two distinct modes of response. In one mode local evaporation is slightly more important than moisture convergence. This results in a relatively weak, equatorially trapped perturbation circulation. In the other mode moisture convergence is dominant and the associated circulation is stronger and less equatorially trapped. The necessary condition for the second “enhanced” mode is that the positive sea temperature perturbations overlap sufficiently with the climatologically warm waters of the western ocean.

Subtropical sea temperature perturbations generally result in a steady state heating with a relatively broad meridional scale and moderate magnitude. The final states are relatively insensitive to the position of the sea temperature perturbations relative to the mean sea temperature field.

While these results must be viewed as quite preliminary due to the simple nature of the dynamical model, they do suggest that actual atmospheric heating perturbations may be the result of complex interactions between oceanic and atmospheric conditions by way of both the surface evaporation and moisture convergence processes.

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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.

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Bryan C. Weare

Abstract

A new method for combining satellite and surface-based cloud observations into a self-consistent three-dimensional field is presented. This method derives the probabilities of the cloud states, which are most consistent with all of the observations and assumptions concerning the nature and relative uncertainties of the observations. It is applied to a three-layer atmosphere using monthly satellite- and surface-based cloud observations. The reconstructions of the observed fields usually lead to modifications of the surface-observed low cloud amount of less than 0.008 fractional cloud cover. Over the ocean the satellite-view low cloud amounts are usually decreased by between 0.06 and 0.12 for most of the middle latitudes and southeastern tropical Pacific. Over land the adjustments in the satellite low cloud amounts are generally smaller. The method leads to increases in satellite high cover of between 0.03 and 0.09 over most regions, and increases in middle cloud cover of up to around 0.03 over the subtropical oceans. Comparisons between derived total cloud cover and that calculated with the commonly used random and mixed overlap assumptions suggest that the mixed assumption generally better fits the results. On the whole there is overall fairly good agreement between the percent low cloud relative to total cloud cover in the reconstructed observations and a global climate model, but the model has a far larger percentage of high clouds nearly everywhere, especially in the tropical convective regions and over the Indian subcontinent.

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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.

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Bryan C. Weare

Abstract

The relationships between the net radiation at the surface and the top of the atmosphere in the UCLA general circulation model are investigated. These suggest that it may be possible to formulate statistical models from limited observations relating satellite derived net radiation at the top of the atmosphere to net radiation at the earth's surface. However, the results suggest it may not be possible to develop comparable statistical models to infer net infrared radiation at the surface or the vertically integrated radiative heating of an atmospheric column.

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