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Lareef Zubair and C. F. Ropelewski

Abstract

Recently, it was reported that the relationship of the Indian southwest monsoon rainfall with El Niño–Southern Oscillation (ENSO) has weakened since around 1980. Here, it is reported that in contrast, the relationship between ENSO and the northeast monsoon (NEM) in south peninsular India and Sri Lanka from October to December has not weakened. The mean circulation associated with ENSO over this region during October to December does not show the weakening evident in the summer and indeed is modestly intensified so as to augment convection. The intensification of the ENSO–NEM rainfall relationship is modest and within the historical record but stands in contrast to the weakening relationship in summer. The intensification of the circulation is consistent with the warming of surface temperatures over the tropical Indian Ocean in recent decades. There is modestly intensified convection over the Indian Ocean, strengthening of the circulation associated with ENSO (Walker circulation), and enhanced rainfall during El Niño episodes in a manner consistent with an augmented ENSO–NEM relationship.

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Muthuvel Chelliah and C. F. Ropelewski

Abstract

Uncertainties in estimates of tropospheric mean temperature were investigated in the context of climate change detection through comparisons of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 40-yr reanalysis (1958–97), the National Aeronautics and Space Administration Data Assimilation Office (NASA/DAO) 14-yr reanalysis (1980–93), the European Centre for Medium-Range Weather Forecasts Reanalysis Project (ERA) 15-yr reanalysis (1979–94), and the satellite microwave sounding unit channel 2 (MSU Ch2) (1979–97) temperature data. The maximum overlap period for comparison among these datasets is the 14 full years January 1980 to December 1993. This study documents similar shifts in the relative bias between the MSU Ch2 and the ERA and the NCEP–NCAR reanalyses in the 1991–97 period suggesting changes in the satellite analysis. However, the intercomparisons were not able to rule out the changes in the reanalysis systems and/or the input data on which the reanalyses are based as prime factors for the changes in the relative bias between the MSU and ERA and NCEP–NCAR reanalyses.

These temporal changes in the relative bias among the reanalyses suggest their limitations for global change studies. Nonetheless, the analysis also shows that the pattern correlations (r) between the MSU Ch2 monthly mean fields and each of the reanalyses are very high, r > 0.96, and remain relatively high for the anomaly fields, r > 0.8, generally >0.9. This result suggests that reanalysis may be used for comparisons to numerical model–generated forecast fields (from GCM simulation runs) and in conjunction with “fingerprint” techniques to identify climate change.

In comparisons of the simple linear trends present in each dataset for the 1980–90 period, each of the reanalyses had spatial patterns similar to MSU Ch2 except that the NCEP–NCAR reanalysis showed smaller “positive” (warming) trends in comparison with the MSU while the ERA reanalysis showed larger positive trends. The NASA/DAO reanalysis showed a mixed pattern. Many regions of the globe are identified that showed consistent warming/cooling patterns among the major reanalyses and MSU, even though there were disagreements in the exact magnitude among the analyses. The spatial patterns of linear trends changed, however, with the addition of three years of data to extend the trend analysis to the 1980–93 period. This result suggests that such simple linear trend analyses are very sensitive to the temporal span in these relatively short datasets and thus are of limited usefulness in the context of climate change detection except, however, when the signal is large and shows consistency among all datasets.

The long record (1958–96) of seasonal mean 2-m temperature anomalies from NCEP–NCAR reanalysis is well correlated with gridded analyses of station-based observed surface temperature, with correlations between 0.65 and 0.85. It is argued that these correlations might suggest an upper limit to the magnitudes of the pattern correlations that might be obtained by correlating observed surface temperature analyses with those from multiyear GCM simulation runs made in the context of fingerprint climate change detection.

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C. F. Ropelewski and P. D. Jones

Abstract

No abstractt available.

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X. L. Wang and C. F. Ropelewski

Abstract

Secular changes in the spatial and temporal structures of the El Niño/Southern Oscillation (ENSO) cycle using four different versions of global sea surface temperature (SST) analysis are examined. The assessments were made for both multidecadal climate means and multidecadal measures of variability by separating the SST variations into low-frequency (periods longer than 30 years) and high-frequency components. The reliability of these estimates is also addressed.

This study substantiates a conceptual framework that views the multidecadal, low-frequency variations as a varying climate “base state” upon which ENSO-scale variability is superposed. The secular changes of the climate base state were quantified both in space and in time. The analysis suggests that multidecadal SST variability has been concentrated in the South Atlantic and Indian Ocean Basins. The Pacific is dominated by the ENSO-scale variability. The analyses reveal that variations in the climate base state and ENSO-scale variability were positively correlated; that is, ENSO-scale variability is higher (lower) when the climate mean SST is relatively warmer (colder). However, the quantification of secular changes of the ENSO-scale variability was found to be sensitive to the particular SST analysis used. Therefore, the conclusions from this study are subject to further verification by using more variables and longer records.

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J. B. Jalickee and C. F. Ropelewski

Abstract

A method for the analysis and objective classification of meteorological data is presented and illustrated. The first step in this approach is the application of an efficient method to describe the data both qualitatively and quantitatively. This approach, based on the singular decomposition theorem, extracts the important features from the data and is similar to principal component or empirical orthogonal function analysis. The extracted features are then combined in the classification step to form new features, each of which describes a subset of the data. The mathematical details are given and computational considerations are discussed.

The method is illustrated with a set of boundary-layer potential temperature profiles derived from structure sonde data taken during the GARP Atlantic Tropical Experiment (GATE). This set of profiles, taken at 3 h intervals over a 20-day period, was objectively classified into two distinct groups. Time, series of surface divergence and radar-estimated precipitation rates are used to demonstrate that one group of profiles is associated with a convectively modified boundary layer, while the other group of profiles is more typical of undisturbed situations.

Other uses and limitations of the objective classification method are discussed.

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C. F. Ropelewski and J. B. Jalickee

Abstract

Profiles of potential temperature, specific humidity and moist static energy are analyzed using the objective classification method described in Part I (Jalickee and Ropelewski, 1979). These profiles, extending from the surface to 890 mb, were derived from structure sonde data taken by the Fay and the Meteor during Phase III (30 August–18 September 1974) of GATE. The technique permits an objective separation of the data into groups and subgroups based on profile shapes. The initial classification, based on the shapes of the potential temperature profiles, yields two broad categories, i.e., those corresponding to undisturbed and disturbed boundary layers. While the potential temperature profiles in these groups are relatively homogeneous, that is, they resemble each other, the specific humidity and moist static energy profiles show large shape variations in each category.

To form more homogeneous groups, the data within each of the broad classes are further subclassified using moist static energy as a discriminator in the objective analysis. This analysis yields two, clearly defined, moist static energy profile types. The first, a subclass of the undisturbed group, comprises 25% of the soundings and contains profiles which exhibit clearly defined mixed, transition and cloud layers. The mean mixed-layer top for these profiles was close to 960 mb with a mean transition-layer thickness of near 35 mb. The second type, while a subclass of the disturbed group, contains profiles that are not associated with organized convective precipitation at the time of the sounding. In these profiles 27% of the total, moist static energy decreased rapidly with height in the lowest 20–30 mb, i.e., to ∼980 mb, and then became constant for the remainder of the profile. Profiles in this subclass may result from mass and energy transfers usually associated with organized deep convection.

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C. F. Ropelewski and M. S. Halpert

Abstract

The relationships are examined between precipitation and the high index phase of the Southern Oscillation (SO) for 19 regions of the globe which have documented low SO index-precipitation relationships (Ropelewski and Halpert 1986, 1987). The study reveals that 15 of these regions also show evidence of characteristic precipitation anomalies during the high index phase of the SO. In each of the regions, the high SO index-precipitation relationships show the opposite sign of those documented for the low index. These precipitation relationships were consistent, holding for over 70% of the high SO index years, and statistically significant. In particular, the high index phase of the SO is associated with enhanced precipitation for the monsoons of India and northern Australia as well as for the rainy seasons in northeastern South America and southeastern Africa. High SO index precipitation was found to be less than median in the central Pacific, Minicoy-Sri Lanka, eastern equatorial Africa, the Gulf of Mexico and northern Mexico region, and southeastern South America. The seasons which showed high SO index-precipitation relationship were almost identical to the masons associated with the low index in 13 of the 15 high SO index regions. Thus to a first approximation, this study suggests that the sign of the precipitation anomaly for these regions is linearly related to the phase of the SO. Since there am 25 low index and 19 high index years in the 109 yr analysis period, from 1875 to 1983, these results further suggest that, for over 40% of the years precipitation may be classified and perhaps predicted on the basis of the extreme SO phase.

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Hsin Hsing Chia and C. F. Ropelewski

Abstract

Variations in the seasonal mean (July–October) genesis positions of tropical cyclones (TCs) in the western North Pacific associated with variations in the large-scale atmospheric circulation are investigated. Analysis shows considerable interannual variability in the seasonal TC mean genesis positions (MGPs) during the 1979–99 period. The variability is shown to be related to the 200–850-hPa vertical wind shear, the west Pacific sea surface temperature (SST), the position and strength of the monsoon trough, and the position and strength of the western Pacific subtropical high (WPSH). Each of these circulation features as well as the SST is, in turn, related to the El Niño–Southern Oscillation (ENSO). However, while this study suggests that ENSO is a major factor in determining seasonal MGP, the relatively short satellite observational period also suggests that ENSO is not the sole determinant, the La Niña year of 1988 being one example. The study further suggests that the role of ENSO is complicated by the differences in the timing and evolution of individual ENSOs with respect to the peak in the mean annual cycle of the TC genesis.

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C. F. Ropelewski and P. A. Arkin

Abstract

We examine the progress in the analysis of climate variability through the lens of a 40-year series of annual Climate Diagnostics and Prediction Workshops initiated by the National Oceanic and Atmospheric Administration (NOAA) in 1976. The evolution of climate data and data access, data analysis and display, and our understanding of the physical mechanisms associated with climate variability, as well as the evolution in the character of the workshops, are documented by reference to the series of workshop proceedings. This retrospective essay chronicles the transition from the mid-1970s, when individual investigators or their organizations held much of the climate data suitable for research, to the present day, where many of the key climate datasets are freely accessible on the Internet. In parallel we also chart the evolution in data analysis and display tools from hand-drawn line graphs of single-station data to color animations of regional and global fields based on satellite data, numerical models, and sophisticated analysis tools. Discussion of these two themes is augmented by documentation of the increasing understanding of the physical climate system as climate science moved away from the “bones of bare statistics” that characterized climate analysis in the mid–twentieth century toward the “flesh of physical understanding.”

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J. E. Gaynor and C. F. Ropelewski

Abstract

A boundary-layer categorization scheme based on GATE acoustic sounder data stratifies surface data and tethered balloon data from the NOAA research ship Oceanographer. The results indicate a clear increase in the sensible heat flux across the sea surface during disturbed conditions (the gravity current and storm wake of the disturbance) but no conclusive differences of latent heat and momentum fluxes. The tethered balloon profiles show a near disappearance of the mixed layer within the gravity current and, in the storm wake, a very shallow and cool mixed layer capped by a strong stable layer relative to the undisturbed category.

We calculate the vertical motion due to buoyancy-driven entrainment for a range on entrainment parameters after exploiting the categorized tethered balloon profiles to obtain mean gradients at the top of the mixed layer. Because we and others have observed that the shallow mixed-layer depth remains nearly constant with time in the storm wake, this calculated entrainment-induced vertical motion is balanced with an hypothesized mesoscale subsidence beneath the anvil in the wake. Even though our entrainment calculation ignores the possibly important but unknown effets of vertically propagating waves, breaking waves and wind shear, the integrated divergence derived from this subsidence agrees well with the range of mesoscale divergences in the storm wake presented by Zipser (1977).

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