Chester F. Ropelewski

The NCEP/NCAR 40-Year Reanalysis Project

The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by differe...

Quantifying Southern Oscillation-precipitation relationships

Abstract A series of earlier studies has identified regions of the world in which precipitation appears to have a consistent relationship with the Southern Oscillation (SO). In this paper, the authors attempt to quantify this relationship based on shifts in the statistical distribution of precipitation amounts with emphasis on shifts in the median, which are associated with the warm (low SO index) and cold (high SO index) phases of the SO. This paper is partially an attempt to provide long-range forecasters with some guidance in making seasonal and multiseasonal predictions. Observed SO-related shifts in the median precipitation amounts, expressed as percentiles with respect to “climatological” conditions, can he used as a simple indication of the “typical” SO response for a given region. In general, the authors find that for many of the large areas identified in previous studies, median precipitation amounts shift on the order of 20 percentile points, that is, from the median to either the 30th percentil...

Surface Temperature Patterns Associated with the Southern Oscillation

Abstract The “typical” global and large-scale regional temperature patterns associated with the low (warm) and high (cold) phases of the Southern Oscillation (SO) are investigated. A total of 12 separate regions were found to have consistent temperature patterns associated with low phase of the SO, while 11 areas were found to have temperature patterns associated with the high phase. Of these areas, 9 have expected temperature patterns during both phases of the SO. In the tropics, temperature anomalies are of the same sign as the SO-related sea surface temperature (SST) anomaly in all land regions except for one area in the west Pacific. Three extratropical responses to the low phase of the SO are found over North America and one is found in Japan. High SO-temperature patterns were found in the extratropies for Japan, western Europe, and northwestern North America. The identified temperature responses are more consistent in tropical regions than in the extratropies. The SO can influence the estimation of ...

THE BIENNIAL COMPONENT OF ENSO VARIABILITY

Abstract Using surface marine wind and sea surface temperature data from the period 1950–1987, together with sea surface temperature and sea level pressure data from several stations in the Pacific, we have identified two dominant time scales of El Nino-Southern Oscillation (ENSO) variability. One is a biennial mode, with periods near 24 months, the other a lower frequency concentration of variance in periods of four to five years. A particularly well defined standing biennial component of ENSO variability, tightly phase-locked with the annual cycle, appears in the surface wind field of the equatorial eastern Indian Ocean. This is part of a larger scale biennial circulation over the low latitude eastern Indian Ocean-western Pacific sector. This biennial circulation is a fundamental element of ENSO variability. It exhibits a strong tendency for phase locking with the annual cycle and introduces a degree of regularity into the ENSO cycle. However, it varies in amplitude from cycle to cycle and sometimes changes phase. “Warm” and “cold” episodes are not unrelated events, but reflect opposite phases of the biennial cycle, modified by the low-frequency mode. Variations in ENSO swings can be formally described in terms of relative phasing and amplitude of the biennial and low frequency components. While the low-frequency mode typically exhibits about the same amplitude as the biennial fluctuations, it does not seem to exhibit the features associated with “ENSO episodes”, which are captured by the biennial mode. The problem of understanding biennial variability seems to be intimately linked with an understanding of the mean annual cycle. The observational results point to the annual cycle as a fundamental pacemaker of the ENSO cycle. They strongly suggest that coupled mode theory must be developed in the context of the annual cycle. They also indicate that the ENSO cycle cannot be fully described and understood without consideration of the entire Indian Ocean-Pacific Ocean sector and thus raise questions regarding the limitations of modeling the tropical Pacific as an isolated coupled ocean/atmospheric system.

Detection and Attribution of Recent Climate Change: A Status Report

This paper addresses the question of where we now stand with respect to detection and attribution of an anthropo- genic climate signal. Our ability to estimate natural climate variability, against which claims of anthropogenic signal detection must be made, is reviewed. The current situation suggests control runs of global climate models may give the best estimates of natural variability on a global basis, estimates that appear to be accurate to within a factor of 2 or 3 at multidecadal timescales used in detection work. Present uncertainties in both observations and model-simulated anthropogenic signals in near-surface air tem- perature are estimated. The uncertainty in model simulated signals is, in places, as large as the signal to be detected. Two different, but complementary, approaches to detection and attribution are discussed in the context of these uncertainties. Applying one of the detection strategies, it is found that the change in near-surface, June through August air tem- perature field over the last 50 years is generally different at a significance level of 5% from that expected from model- based estimates of natural variability. Greenhouse gases alone cannot explain the observed change. Two of four climate models forced by greenhouse gases and direct sulfate aerosols produce results consistent with the current climate change observations, while the consistency of the other two depends on which models anthropogenic fingerprints are used. A recent integration with additional anthropogenic forcings (the indirect effects of sulfate aerosols and tropospheric ozone) and more complete tropospheric chemistry produced results whose signal amplitude and pattern were consis- tent with current observations, provided the models fingerprint is used and detection carried out over only the last 30 years of annually averaged data. This single integration currently cannot be corroborated and provides no opportunity to estimate the uncertainties inherent in the results, uncertainties that are thought to be large and poorly known. These results illustrate the current large uncertainty in the magnitude and spatial pattern of the direct and indirect sulfate forcing and climate response. They also show detection statements depend on model-specific fingerprints, time pe- riod, and seasonal character of the signal, dependencies that have not been well explored. Most, but not all, results suggest that recent changes in global climate inferred from surface air temperature are likely not due solely to natural causes. At present it is not possible to make a very confident statement about the relative con- tributions of specific natural and anthropogenic forcings to observed climate change. One of the main reasons is that fully realistic simulations of climate change due to the combined effects of all anthropogenic and natural forcings mecha- nisms have yet to be computed. A list of recommendations for reducing some of the uncertainties that currently hamper detection and attribution studies is presented.

Observed tropospheric biennial variability and its relationship to the southern oscillation

Abstract Tropospheric biennial variability in several components of the Southern Oscillation (SO) is defined and described through analysis of observational data from the Comprehensive Ocean-Atmosphere Data Set (COADS), as well as through investigation of several SO index time series. The analysis suggests that the temporal behavior of the SO can be described in terms of three components: 1) a pervasive biennial pulse, which appears to be strong in both the Indian Ocean and the west Pacific surface zonal winds as well as in several SO indices, 2) the annual cycle, which tends to set the phase of biennial variability for the major SO excursions, and 3) a low-frequency, or residual, variability, which may be associated with temporal scales between large SO episodes. This study also supports recent papers in suggesting that complete models of the SO must include the Indian Ocean basin.

Optimal averaging of seasonal sea surface temperatures and associated confidence intervals (1860-1989)

Abstract Optimal averaging (OA) is used to compute the area-average seasonal sea surface temperature (SST) for a variety of areas from 1860 to 1989. The OA gives statistically improved averages and the objective assignment of confidence intervals to these averages. The ability to assign confidence intervals is the main advantage of this method. Confidence intervals reflect how densely and uniformly an area is sampled during the averaging season. For the global average, the early part of the record (1860–1890) and the times of the two world wars have largest uncertainties. Analysis of OA-based uncertainty estimates shows that before 1930 sampling in the Southern Hemisphere was as good as it was in the Northern Hemisphere. From about 1930 to 1950, uncertainties decreased in both hemispheres, but the magnitude of the Northern Hemisphere uncertainties reduced more and remained smaller. After the early 1950s uncertainties were relatively constant in both hemispheres, indicating that sampling was relatively con...

Reanalyses-Based Tropospheric Temperature Estimates: Uncertainties in the Context of Global Climate Change Detection

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 whic...

Quantifying Southern Oscillation-Precipitation Relationships from an Atmospheric GCM

Abstract This paper is an extension of a study by C. Ropelewski and M. Halpert, which examines observed precipitation relationships with the Southern Oscillation. Here, the authors repeat their analysis using atmospheric general circulation model precipitation from the average of a 13-run ensemble. The GCM is the atmospheric component of the coupled model used for seasonal prediction at the National Centers for Environmental Prediction, except that in this study, the observed sea surface temperatures were specified for the ensemble runs. Results are compared and contrasted with the observed Southern Oscillation–related precipitation behavior. These comparisons show that the multiple ensemble simulations compare favorably to the observations for most areas in the Tropics and subtropics. However, outside of the deep Tropics, the model simulations show large shifts or biases in the location of the Southern Oscillation–related anomalies. In particular, anomalies shown by the observations to occur in the south...

The Global Climate for June to August 1990: Drought Returns to Sub-Saharan West Africa and Warm Southern Oscillation Episode Conditions Develop in the Central Pacific

Although the general monsoon circulation evolved relatively normally over most of the globe, dry conditions returned to sub-Saharan West Africa. The Northern Hemisphere summer surface temperature continued to be above normal over most land areas, but in general the anomolies were less extreme. The equatorial sea surface temperatures continued to move toward warm episode Southern Oscillation conditions in the central Pacific, but without an accompanying warming in the traditional eastern Pacific El Nino areas.