Showing papers in "Journal of Climate in 1990"

Simulation of Regional Climate Using a Limited Area Model Nested in a General Circulation Model

Abstract: A Limited Area Model (LAM) is nested in a General Circulation Model (GCM) to simulate the January climate over the western United States. In the nesting procedure, the GCM output is used to provide the initial and lateral atmospheric boundary conditions necessary to drive the LAM. In this approach, the GCM is used to simulate realistic large-scale atmospheric behavior over an area of interest and the LAM to describe the effect of local, sub-GCM grid scale forcings (such as those induced by the complex western United States topography) on regional patterns of climatic variables. Two versions of the National Center for Atmospheric Research (NCAR) Community Climate Model [the seasonal CCM1 at 4.5° × 7.5° (R15) and 2.89° × 2.89° (T42) latitude-longitude resolution] are used to drive a version of the Pennsylvania State University/NCAR mesoscale model (MM4 at 60 km resolution), which includes sophisticated soil hydrology calculations. The CCM1 large-scale January climatology over the region is analyzed...

Patterns of Low-Frequency Monthly Sea Level Pressure Variability (1899–1986) and Associated Wave Cyclone Frequencies

Abstract: Spatial patterns of low-frequency sea level pressure (SLP) variability are identified by performing rotated principal component analysis (RPCA) on a long-term (1899–1986) Northern Hemisphere gridded dataset. The analysis is limited to the region 160°E eastward to 40°E due to missing data early in the century. The objective is to identify a comprehensive set of highly recurrent SLP teleconnection patterns; to examine some aspects of their seasonality; and to identify the associated mean winter pressure fields and cyclone frequencies occurring at times of opposite eigenvector polarity. The results are further described in the context of the Southern Oscillation and known midtropospheric teleconnection patterns. Four low-frequency variability patterns are identified over the Atlantic-European sector, including (i) the North Atlantic 0scillation (NAO), and spatial patterns with SLP variability centers over (ii) the eastern Atlantic (EATL), (iii) southern Europe and the northern Mediterranean basin (S...

Large-scale atmospheric circulation features of warm and cold episodes in the tropical Pacific

Abstract: Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Nino events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented. SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Nino events are characterized by strong positive SST anomalies along the coasts of Ecuador a...

A Method of Relating General Circulation Model Simulated Climate to the Observed Local Climate. Part I: Seasonal Statistics

Abstract: Important surface observations such as the daily maximum and minimum temperature, daily precipitation, and cloud ceilings often have localized characteristics that are difficult to reproduce with the current resolution and the physical parameterizations in state-of-the-art General Circulation climate Models (GCMs). Many of the difficulties can be partially attributed to mismatches in scale, local topography. regional geography and boundary conditions between models and surface-based observations. Here, we present a method, called climatological projection by model statistics (CPMS), to relate GCM grid-point flee-atmosphere statistics, the predictors, to these important local surface observations. The method can be viewed as a generalization of the model output statistics (MOS) and perfect prog (PP) procedures used in numerical weather prediction (NWP) models. It consists of the application of three statistical methods: 1) principle component analysis (FICA), 2) canonical correlation, and 3) infla...

Empirical Data on Contemporary Global Climate Changes (Temperature and Precipitation)

Abstract: New data are presented on the changes of mean global surface air temperature and annual precipitation over extratropical continents of the Northern Hemisphere. Global warming occurred during the last century with a mean trend of 0.5°C/100 years. It is shown that for the same period the annual precipitation over the land in the 35°–70°N zone increased by 6%. The observed variations of precipitation coincide with the results of general circulation modeling of doubled CO2 equilibrium climate change by sign but contradict by scale.

Spatial Patterns of Atmosphere-Ocean Interaction in the Northern Winter.

Abstract: The variability of monthly mean sea-surface temperature (SST) anomalies over the extratropical North Atlantic and Pacific and its relation to atmospheric circulation anomalies over the Northern Hemisphere during wintertime is investigated, by applying eigenvector analysis to a 39-year dataset and correlating the time series of the resulting expansion coefficients with the hemispheric 500 mb height and sea-level pressure fields. In agreement with previous studies, the simultaneous correlation between the tone series of the expansion coefficient of the first empirical orthogonal function (EOF 1) of North Pacific sea surface temperature (SST) and the hemispheric 500 mb height field resembles the Pacific/North American pattern, and the corresponding pattern for Atlantic SST resembles the North Atlantic Oscillation. In the vicinity of the conters of action of thew patterns, the SST fluctuations associated with these modes explain on the order of half the variance of 500 mb height and slightly less of ...

On Trends in Historical Marine Wind Data

Abstract: Long-period variations which include a trend toward strengthening winds over the last three decades have on the one hand been suggested to be real climatic changes, and on the other artifacts of the evolution of measuring techniques. An examination is presently conducted of individual ship reports from three regions with high data densities, in order to resolve this dispute. Even with corrections for instrumental effects, the pre-1950 winds appear weaker than post-1950 winds; the most probable explanation is the absence of universal sea state and Beaufort force standards prior to 1946.

On the relationship between water vapor over the oceans and sea surface temperature

Abstract: Monthly mean precipitable water data obtained from passive microwave radiometry were correlated with the National Meteorological Center (NMC) blended sea surface temperature data. It is shown that the monthly mean water vapor content of the atmosphere above the oceans can generally be prescribed from the sea surface temperature with a standard deviation of 0.36 g/sq cm. The form of the relationship between precipitable water and sea surface temperature in the range T(sub s) greater than 18 C also resembles that predicted from simple arguments based on the Clausius-Clapeyron relationship. The annual cycle of the globally integrated mass of Scanning Multichannel Microwave Radiometer (SMMR) water vapor is shown to differ from analyses of other water vapor data in both phase and amplitude and these differences point to a significant influence of the continents on water vapor. Regional scale analyses of water vapor demonstrate that monthly averaged water vapor data, when contrasted with the bulk sea surface temperature relationship developed in this study, reflect various known characteristics of the time mean large-scale circulation over the oceans. A water vapor parameter is introduced to highlight the effects of large-scale motion on atmospheric water vapor. Based on the magnitude of this parameter, it is shown that the effects of large-scale flow on precipitable water vapor are regionally dependent, but for the most part, the influence of circulation is generally less than about + or - 20 percent of the seasonal mean.

Behavior of Deep Convective Clouds in the Tropical Pacific Deduced from ISCCP Radiances

Abstract: The characteristic features, the diurnal cycle, and the spatial distribution of deep convection over the equatorial Pacific and the relationship of deep convection to SST and surface-wind convergence were examined using a combined visible-IR (VS-IR) threshold method and an IR-only threshold method for diagnosing deep convection clouds (DCCs). Results suggest that deep convection is latitudinally confined to a much smaller spatial scale than that suggested by maps of outgoing long-wave radiation. The results suggested that there are two types of relationships between deep convection, SST, and surface-wind convergence: the west Pacific type and the east Pacific type. The latter relationship is observed in the east Pacific only when SST is not abnormally warm.

Mechanisms of Interannual Variability of the Southwest United States Summer Rainfall Maximum

Abstract: The mid-summer rainfall singularity of the Southwest United States (principally Arizona) exhibits marked variations on interannual and decadal time scales. Examination of the synoptic mechanisms involved in these variations is undertaken here. In particular, associations between the rainfall, the dominant latitude of the summertime mid-tropospheric subtropical ridge (STR) over the southwest United States, and the sea surface temperatures (SSTs) of the equatorial and North Pacific region are documented. The analysis utilizes a composite approach for sets of extreme years chosen on the basis of the rainfall and circulation anomalies. It is found that northward (southward) displaced seasonal STR is associated with wetter (drier) summers in Arizona. Further, these extremes have tended to follow winters characterized by positive (negative) phases of the Pacific-North America (PNA) teleconnection pattern. The latter association arises, at least in part from the “memory” imparted to the atmosphere by th...

Maximum Likelihood Estimation for the Gamma Distribution Using Data Containing Zeros

Abstract: A method for fitting parameters of the gamma distribution to data containing some zero values using maximum likelihood methods is presented. The procedure is based on a conceptual model of the data having resulted from a censoring process so that the number, but not the numerical values of observations failing below a detection limit are known. For the case of precipitation data, this detection limit is related to the threshold value for reporting occurrence or nonoccurrence. The procedure is shown to provide parameter estimates that are more efficient (i.e., precise) than those obtained using the method of moments.

Intermonthly Variability of Cyclone Tracks in the Mediterranean

Abstract: Objective calculated monthly cyclonic tracks in the Mediterranean are presented. The intermonthly variations within each season are shown to be quite significant and an particularly notable for December-January, April-May and June-July. We suggest that composite seasonal pictures are not faithful representations of the cyclonic routes over a complex terrain region like the Mediterranean.

Dependence of Rainfall Variability on Mean Rainfall, Latitude, and the Southern Oscillation

Abstract: The relationship between the relative variability of annual rainfall, the long-term mean annual rainfall, the latitude, and the correlation between annual rainfall and the Southern Oscillation Index is examined, using data from 974 stations. A nonlinear relationship between these variables accounts for 94% of the variance in annual rainfall variability. Relative variability typically increases as mean annual rainfall decreases, as latitude decreases, and as the effect of the Southern Oscillation increases. There is an interaction between latitude and the Southern Oscillation so that the effect of the Southern Oscillation on variability weakens as latitude increases.

Global, seasonal cloud variations from satellite radiance measurements. II - Cloud properties and radiative effects

Abstract: Cloud and surface radiative properties and their effects on the earth and surface radiation budgets are obtained based on global daily visible and IR radiance measurements. The magnitude of cloud property variations and their effects on radiation increase strongly with decreasing space/time scales. Cloud properties are systematically different between land and ocean, with ocean having larger cloud cover with somewhat larger optical thicknesses and lower cloud top altitudes. Although cloud variations appear to be the primary cause of regional radiation budget variability at 5-30 daytime scales, the effects of their seasonal variations at larger spatial scales are less important than the changes associated with changes in solar declination and atmospheric/surface temperatures. The largest seasonal variations in radiation occur in the 30-60 deg latitude band in each hemisphere. Although clouds have a net cooling effect on the global, annual mean radiation balance at both the top of the atmosphere and the surface, their net effect on regional, seasonal balances is much more varied.

A General Circulation Model Study of the Atmospheric Response to Extratropical SST Anomalies Observed in 1950–79

Abstract: A 30-year experiment with an atmospheric general simulation model has been performed. The lower boundary condition at all oceanic grid points between 38°S and 60°N has been prescribed to follow the observed month-to-month variation of the sea surface temperature (SST) field during the 1950–79 period. Much of the model diagnosis presented here pertains to the midlatitude atmospheric response to recurrent SST patterns in the North Pacific and North Atlantic in winter. The principal modes of variability of the seasonally averaged 515 mb height and SST fields have been identified using rotated principal component (RPC) analysis. The extrema of the first atmospheric mode reside over the North Atlantic and Eurasia, whereas the second mode is associated with height anomalies in the North Pacific/North American sector. Cross-correlation analysis reveals that these two atmospheric modes am linked to leading patterns of the SST field in the North Atlantic and North Pacific, respectively. It is also demonst...

Global atmospheric temperature monitoring with satellite microwave measurements - Method and results 1979-84

Abstract: This paper describes a method for determining global atmospheric-temperature anomalies by means of satellite microwave radiometry. It is shown that microwave measurements of molecular oxygen thermal emission by the Microwave Sounding Units (MSUs) flying aboard the NOAA-6 and NOAA-7 can be used to monitor tropospheric temperature anomalies on global basis to a high level of precision. Comparisons between monthly MSU-derived hemispheric temperature anomalies with those computed from surface thermometer data show a very good agreement over the United States, although not for the hemispheres, especially the Southern Hemisphere. In this latter case, the poor agreement is ascribed to weaker thermal coupling between the ocean and the deep troposphere than that over the U.S. Annual anomalies for the hemispheres exhibit better correlations than do monthly anomalies.

Are There Any Satisfactory Geologic Analogs for a Future Greenhouse Warming

Abstract: There have been numerous attempts to propose past warm time periods as “analogs” for a future greenhouse warming. In this paper it is argued that, although paleoclimate studies may provide important insights into process operating in the climate system, there may be no warm time period that is a satisfactory past analog for future climate. The future greenhouse warming may represent a unique climate realization in earth history. This conclusion is based on the following considerations: 1 ) comparisons with Holocene (9000 BP) or Eemian climates (120 000 BP) may be inappropriate because much of the variations in these climates can be explained in terms of seasonal rather than mean-annual forcing it has yet to be demonstrated that increased warmth for these intervals involved mew annual temperature increases that were globally synchronous, 2) comparisons with older and warmer climates (ex., Pliocene, Eocene, or Cretaceous) can be misleading because these warm periods had reduced polar ice cover, whe...

Development of a Two-Dimensional Zonally Averaged Statistical-Dynamical Model. Part III: The Parameterization of the Eddy Fluxes of Heat and Moisture.

Abstract: A number of perpetual January simulations are carried out with a two-dimensional zonally averaged model employing various parameterizations of the eddy fluxes of heat (potential temperature) and moisture. The parameterizations are evaluated by comparing these results with the eddy fluxes calculated in a parallel simulation using a three-dimensional general circulation model with zonally symmetric forcing. The three-dimensional model's performance in turn is evaluated by comparing its results using realistic (nonsymmetric) boundary conditions with observations. Branscome's parameterization of the meridional eddy flux of heat and Leovy's parameterization of the meridional eddy flux of moisture simulate the seasonal and latitudinal variations of these fluxes reasonably well, while somewhat underestimating their magnitudes. New parameterizations of the vertical eddy fluxes are developed that take into account the enhancement of the eddy mixing slope in a growing baroclinic wave due to condensation, and also the effect of eddy fluctuations in relative humidity. The new parameterizations, when tested in the two-dimensional model, simulate the seasonal, latitudinal, and vertical variations of the vertical eddy fluxes quite well, when compared with the three-dimensional model, and only underestimate the magnitude of the fluxes by 10 to 20 percent.

Parameterizations for the Absorption of Solar Radiation by O2 and CO2 with Application to Climate Studies

Abstract: Simple and accurate parameterizations have been developed for computing the absorption of solar radiation due to O2 and CO2. The parameterizations are based on the findings that temperature has a minimal effect on the absorption and that the one-parameter scaling can be applied to take into account the effect of pressure variation along a path. Overlapping of the absorption due to CO2 and water vapor is treated accurately in the parameterizations. Simulations with a zonally averaged multilayer energy balance model show that the absorption of solar radiation due to O2 and CO2 has a small, albeit nonnegligible, effect on climate. The global surface solar radiation is reduced by 2.2 W/sq m, and the warming of the surface temperature due to a doubled CO2 concentration is reduced by 10 percent in the Northern Hemisphere.

Towards Monitoring Droughts from Space

Abstract: The utility of the midafternoon satellite derived surface temperatures for detecting drought events is examined using the NOAA-9 AVHRR data over the U.S. Great Plains during 1986–88. The interannual differences in monthly mean clear-sky temperature and in monthly mean normalized difference vegetation index are compared to the corresponding differences in the Palmer Drought Index. Results indicate that the thermal data from polar orbiters may be very useful in detecting the interannual changes in surface moisture when the change in vegetation index fails to produce a significant signal.

Arctic Contribution to Upper-Ocean Variability in the North Atlantic

Abstract: The potential climatic leverage of salinity and temperature anomalies in the high-latitude North Atlantic is large. Substantial variations of sea ice have accompanied North Atlantic salinity and temperature anomalies. Atmospheric pressure data are used here to show that the local forcing of high-latitude North Atlantic Ocean fluctuations is augmented by antecedent atmospheric circulation anomalies over the central Arctic. These circulation anomalies are consistent with enhanced wind-forcing of thicker older ice into the Transpolar Drift Stream and an enhanced export of sea ice (fresh water) from the Arctic into the Greenland Sea prior to major episodes of ice severity in the Greenland and Iceland seas.

Diagnostics and Prediction of Anomalous River Discharge in Northern South America

Abstract: The annual cycles of rainfall and river discharge in northern South America are dominated by the seasonal latitude migration of the intertropical convergence zone. The catchments in the west (Madden. Magdalena and Orinoco) have their high stands between August and December, while those of the Guyanas (Cuyuni, Essequibo, Suriname, Maroni and Oyapock) peak between May and July. Anomalously abundant discharge is in almost all catchments (except Orinoco) associated with the high SO phase (defined by anomalously high/low pressure at Tahiti/Darwin), weakened Caribbean tradewinds. and accelerated cross-equatorial southerly flow over the eastern Pacific. In a series of experiments a sharp distinction was kept between a “dependent” dataset (1940–70, or the part available in the river series) used as training period and an “independent” portion of the record (1971–87) reserved for prediction. Stepwise multiple regression models for bimonthly “seasons” used as input river discharge as regressand, and as reg...

Variation in United States Cloudiness and Sunshine Duration between 1950 and the Drought Year of 1988

Abstract: The variations in United States cloudiness (percent of sky covered by clouds, as estimated subjectively by observers at 100 National Weather Service stations) and sunshine duration (percent of possible sunshine, as estimated objectively by sunshine recorders at these same 100 stations) are examined for years 1950–88. During this period, the correlation between annual values of cloudiness and sunshine duration within the contiguous United States was −0.86, significant at the 1% level. The years of maximum cloudiness and minimum sunshine duration were 1972 and 1982, when strong El Ninos began. The year of maximum sunshine duration was 1988, but the years of minimum cloudiness were 1952–56 (mini dust bowl); the discrepancy a result of the greater long-term increase in cloudiness than decrease in sunshine duration. In the spring of 1988 them were anomalous values of cloudiness (below average) and sunshine duration (above average) in north central, south central and southeast regions of the United Sta...

Detection of Climatic Change in the Western North American Arctic Using a Synoptic Climatological Approach

Abstract: Studies which utilize a long-term temperature record in determining the possibility of a global warming have led to conflicting results We suggest that a time-series evaluation of mean annual temperatures is not sufficiently robust to determine the existence of a long-term warming We propose the utilization of an air mass-based synoptic climatological approach, as it is possible that local changes within particular air masses have been obscured by the gross scale of temperature time-series evaluations used in previous studies of this type An automated synoptic index was constructed for the winter months in four western North American Arctic locations to determine if the frequency of occurrence of the coldest and mildest air masses has changed and if the physical character of these air masses has shown signs of modification over the past 40 years It appears that the frequencies of the majority of the coldest air masses have tended to decrease, while those of the warmest air masses have increas

Prediction of Northeast Brazil Rainfall Anomalies

Abstract: Expanding on earlier research, empirically based methods are developed for predicting March-September rainfall in northern Northeast Brazil. From a network of 27 raingage stations in Brazil's Nordeste, new rainfall index series are constructed for March-September (MS) and October-January (OJ). Data input to stepwise multiple regression models further include January values of the Tahiti minus Darwin pressure index, an index of sea surface temperature (SST) in the equatorial Pacific (PWT), and indices of the fields of meridional (v) and zonal (u) wind components and of SST in the tropical Atlantic between 30°N and 30°S (AFV, ARJ, and AFT, respectively). Empirical orthogonal function (EOF) analysis of the v, u, and SST fields was used in constructing the latter three indices. Throughout the study, a sharp distinction is kept between a “dependent” dataset (1921–42 and 1948–57) used as a training period and an “independent” portion of the record (1958–87) reserved for prediction experiments. Preseaso...

Antarctic Temperatures over the Present Century—A Study of the Early Expedition Record

Abstract: Air temperature records for 26 expeditions to Antarctica that have overwintered, for periods of at least 9 months, between 1898 and 1958 have been assembled. Using a map of 1957–75 average annual temperature over Antarctica, the results for the expedition sites were made compatible with modern data. The expedition records are unfortunately confined mainly to two regions, the Antarctic Peninsula and the Ross Sea sectors. It is difficult, therefore, to generalize the results to the entire continental area. The expeditions are also somewhat irregularly spaced in time with relatively few occurring in the 1917–34 period. The best guess that can be made is that Antarctic air temperatures now appear to be warmer, by at least 1°C, than those prevailing during the first decade of the twentieth century. The result is broadly consistent with temperature changes that have been reported for both land and marine regions over the rest of the Southern Hemisphere. This result is, however, in contrast with the cou...

Tropospheric Influence on Solar Ultraviolet Radiation: The Role of Clouds.

Abstract: Measurements obtained from several Robertson-Berger (RB) meters over the course of one year define the role of cloud cover in moderating biologically effective ultraviolet radiation at the Earth's surface. In an annual mean sense, clouds reduce the erythemal irradiance to levels from 62% to 78% of the values that would exist if skies over the measurement sites remained clear and free of pollutants. The RB meter results combined with a simple model of radiative transfer allow one to estimate the response of erythemal irradiance to variations in fractional cloud cover and cloud optical thickness. If local fractional cloud cover during June and July varied by ± 10% of its monthly mean value, erythemal irradiance at the different sites would undergo charm ranging from 1.2% to 6.4% with the opposite sign. Changes in cloud optical thickness of ±10% generally have a smaller impact on surface irradiance than do changes in fractional cloud cover. Variations in erythemal irradiance predicted in these scena...

The Impact of Sea Surface Temperature Anomalies on the Rainfall over Northeast Brazil

Abstract: The response of the tropical atmosphere to the sea surface temperature (SST) anomalies in the Northern Hemisphere spring of 1984 is investigated. The methodology for investigation consists of comparing simulations with and without the global distribution of SST anomalies in the boundary conditions of the UCLA General Circulation Model (GCM). At low levels, the response includes weaker southeast trade winds over the Atlantic, increased precipitation off the northeast coast of Brazil, and reduced precipitation west of this region. The increased precipitation is due to enhanced convergence of moisture advected by the southeast trade winds, although the trades themselves are weaker. The results for the western equatorial Atlantic am in apparent agreement with the observed anomalous southern migration of the ITCZ in years with warm SST anomalies in the southern tropical Atlantic. There are strong anomalous trade winds over the Pacific extending east of the date line and weak wind anomalies over the ma...

Precipitation Fluctuation over Semiarid Region in Northern China and the Relationship with El Niño/Southern Oscillation

Abstract: In recent years the semiarid region of northern China, which has total annual precipitation between 200 and 500 mm, has shown signs of severe desertification. Intensive theoretical and observational studies are currently underway to examine the climate changes and other contributing factors. In this study, we used the 1951–86 monthly precipitation measurements in this region to study their fluctuations and relationship with the El Nino/Southern Oscillation. Three main features are identified: 1) a 2–3 year quasi-periodic fluctuation, 2) a tendency for rainfall deficiency for the whole region during ENSO years, and 3) a significant correlation between the precipitation fluctuation in the southern part of this region and Southern Oscillation index, with the former lagging the latter by 2–5 months. These features are also evident from analysis of the proxy data during the last hundred years. Discussions on the possible link between the precipitation fluctuation, the summer monsoon, the western Pacif...

The Effect of Serial Correlation on Statistical Inferences Made with Resampling Procedures

Abstract: Resampling procedures include hypothesis testing methods based on Permutation Procedures and interval estimation methods based on bootstrap procedures. The former are widely used in the analysis of climate experiments conducted with general circulation models (GCMs) and in the comparison of the simulated and observed climates. The latter are used less frequently than their flexibility and utility warrants. Both resampling techniques are powerful tools, which provide elegant means of overcoming fundamental statistical difficulties encountered in the analysis of observed and simulated climate data. Unfortunately, inference based on both resampling schemes are as sensitive to the effects of serial correlation as classical statistical methods. These tools must therefore be used with the same amount of caution as other statistical methods when it is suspected that the data might be serially correlated.