519 publications from this institution
Major efforts are underway to improve climate models both for the advancement of science and for the benefit of society. But early results could cause problems for the public understanding of climate change.
One of the best-known and most well-respected researchers in the area of global climate change, Dr. Kevin E. Trenberth, has concluded that there can no longer be any ‘reasonable disagreement’ that human activities, and especially fossil-fuel related energy production, are causing global climate change. In a recent meeting with Refocus US Correspondent ???, Dr. Trenberth articulated his scientific judgment that the risk of not addressing climate change, through efforts such as increased emphasis on renewable energies, could be catastrophic for the planet.
No abstract is provided for this article.
An analysis and evaluation has been performed of global datasets on column-integrated water vapor (precipitable water). For years before 1996, the Ross and
A survey is made of the published estimates of the components of the poleward flux of energy by the atmosphere in the Southern Hemisphere in order to determine the total atmospheric transport. Together with recent measurements by satellite of the Earth's radiation budget this allows a new estimate of the required poleward energy transport by the oceans in the Southern Hemisphere for mean annual conditions. Results show that the ocean and atmosphere each contribute similar amounts for 0–30°S and that the ocean probably also transports about one third of the total at 60°S. The latter is in contrast to similar latitudes in the Northern Hemisphere where the ocean transport is negligible, but consistent with the different distribution of land and sea in the two hemispheres.
The tendency for more frequent El Niño events and fewer La Niña events since the late 1970's has been linked to decadal changes in climate throughout the Pacific basin. Aspects of the most recent warming in the tropical Pacific from 1990 to 1995, which are connected to but not synonymous with El Niño, are unprecedented in the climate record of the past 113 years. There is a distinction between El Niño (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Niño region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Pacific, which are better linked to the SO, remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal‐timescale variation. One test fits an autoregressive‐moving average (ARMA) model to a measure of the SO given by the first hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990–1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases.
Huge heat anomalies in the atmosphere and ocean in recent years are not yet explained. Strong characteristic patterns in temperatures for upper layers of the ocean occurred from 2000 to 2023 in the presence of global warming from increasing atmospheric greenhouse gases. Here we show that the deep tropics are warming, although sharply modulated by El Niño-Southern Oscillation events, with strong heating in the extratropics near 40°N and 40-45°S, but little heating near 20°N and 25-30°S. The heating is most clearly manifested in zonal mean ocean heat content and is evident in sea surface temperatures. Strongest heating is in the Southern Hemisphere, where aerosol effects are small. Estimates are made of the contributions to heating of top-of-atmosphere (TOA) radiation, atmospheric energy transports, surface fluxes of energy, and redistribution of energy by surface winds and ocean currents. The patterns of change are not directly related to TOA radiation but are evident in net surface energy fluxes and inferred ocean heat transports, underscoring their coupled origin. Changes in the atmospheric circulation through a poleward shift in ocean jet streams and storm tracks are reflected in surface wind-driven ocean Ekman transports. As well as human-induced climate change, internal natural variability is likely in play. Hence the atmosphere and ocean currents are systematically redistributing heat from global warming, profoundly affecting local climates.
Nearly eight years of daily Southern Hemisphere analyses at 500 mb have been used to define the spatial dependence of the variance fields of geopotential height and the two geostrophic wind components, the corresponding covariance fields, and the transient kinetic energy. The fields are further examined in the frequency domain by using Lorenz' (1979) “poor man's spectral analysis” technique. In view of the small variation in eddy statistics as a function of the time of the year in the SH, this study removes the first four harmonies of the annual cycle and then considers all data together, so that contributions from all time scales from 2 to 4096 days (∼11 years) can be resolved. The main results are based on analyses from May 1972–January 1978 but are verified with analyses from the relatively data-rich FGGE period. Results for the zonal mean statistics are compared with those from previous studies. The zonal means of the geopotential height and westerly wind component have spectra which roughly follow that of red noise with an autocorrelation of about 0.5, whereas the northward wind component spectra closely resembles red noise with autocorretation of 0.2, resulting in considerable anisotropy in the wind fields. The northward component of transient kinetic energy is larger than the eastward component at high frequencies in middle latitudes but the reverse is true for periods of greater than two months. The westerly momentum flux by the transient eddies has a broad spectral peak at 8–32 days and is dominated by contributions from fluctuations of less than about two weeks period. The geographical dependence of the eddy statistics is mapped for four broad frequency bands covering periods of roughly less than one week, one week to two months, two months to two years, and greater than two years, thereby separating out contributions from transient baroclinic eddies, episodes of blocking, and intermonthly and interannual variability. The spatial patterns of the statistics are interpreted in the light of synoptic behavior of systems and storm tracks as defined by synoptic studies and satellite observations in the Southern Hemisphere. For periods less than a week, variances are largest in the southern Indian Ocean and relationships between the storm tracks and eddy statistics are similar to those found in the Northern Hemisphere by Blackmon, Lau, Wallace and others. However, there also are differences associated with the differences in the mean flow in each hemisphere and these are discussed in the context of baroclinic theory. At periods longer than a week geopotential height variances are largest near southern New Zealand and, to a lesser extent, southeast of South America and appear to be related to the incidence of blocking in the Southern Hemisphere. The corresponding transient kinetic energy has a maximum further north in association with cutoff cold-centered lows. In general, the high-frequency transient eddies play a much larger role in the circulation of the Southern Hemisphere than is true for the winter circulation of the Northern Hemisphere, and the eddy statistics are more zonally symmetric.
Because of a lack of observations, historical simulations of land surface conditions using land surface models are needed for studying variability and changes in the continental water cycle and for providing initial conditions for seasonal climate predictions. Atmospheric forcing datasets are also needed for land surface model development. The quality of atmospheric forcing data greatly affects the ability of land surface models to realistically simulate land surface conditions. Here a carefully constructed global forcing dataset for 1948–2004 with 3-hourly and T62 (∼1.875°) resolution is described, and historical simulations using the latest version of the Community Land Model version 3.0 (CLM3) are evaluated using available observations of streamflow, continental freshwater discharge, surface runoff, and soil moisture. The forcing dataset was derived by combining observation-based analyses of monthly precipitation and surface air temperature with intramonthly variations from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis, which is shown to have spurious trends and biases in surface temperature and precipitation. Surface downward solar radiation from the reanalysis was first adjusted for variations and trends using monthly station records of cloud cover anomaly and then for mean biases using satellite observations during recent decades. Surface specific humidity from the reanalysis was adjusted using the adjusted surface air temperature and reanalysis relative humidity. Surface wind speed and air pressure were interpolated directly from the 6-hourly reanalysis data. Sensitivity experiments show that the precipitation adjustment (to the reanalysis data) leads to the largest improvement, while the temperature and radiation adjustments have only small effects. When forced by this dataset, the CLM3 reproduces many aspects of the long-term mean, annual cycle, interannual and decadal variations, and trends of streamflow for many large rivers (e.g., the Orinoco, Changjiang, Mississippi, etc.), although substantial biases exist. The simulated long-term-mean freshwater discharge into the global and individual oceans is comparable to 921 river-based observational estimates. Observed soil moisture variations over Illinois and parts of Eurasia are generally simulated well, with the dominant influence coming from precipitation. The results suggest that the CLM3 simulations are useful for climate change analysis. It is also shown that unrealistically low intensity and high frequency of precipitation, as in most model-simulated precipitation or observed time-averaged fields, result in too much evaporation and too little runoff, which leads to lower than observed river flows. This problem can be reduced by adjusting the precipitation rates using observed-precipitation frequency maps.
Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10–20°N and 18°W–20°E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi‐year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology 24 : 547–554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded. Copyright © 2004 Royal Meteorological Society
A review is given of the meaning of the term “El Niño” and how it has changed in time, so there is no universal single definition. This needs to be recognized for scientific uses, and precision can only be achieved if the particular definition is identified in each use to reduce the possibility of misunderstanding. For quantitative purposes, possible definitions are explored that match the El Niños identified historically after 1950, and it is suggested that an El Niño can be said to occur if 5-month running means of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°–170°W) exceed 0.4°C for 6 months or more. With this definition, El Niños occur 31% of the time and La Niñas (with an equivalent definition) occur 23% of the time. The histogram of Niño 3.4 SST anomalies reveals a bimodal character. An advantage of such a definition is that it allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Niños begin in the northern spring or perhaps summer and peak from November to January in sea surface temperatures.
No abstract is provided for this article.
There is a substantial global redistribution of atmospheric mass associated with the mean annual cycle which results in large exchanges of mass between hemispheres. Interannual fluctuations in the distribution of mass both within each hemisphere and for the “hemispheric” mean over 17.5°–90° latitude are examined. The long‐period (1899–1980) historical sea level pressures over the northern hemisphere are analyzed and found to exhibit three modes of interannual zonal mean mass redistribution that are common to all seasons. The first is one where mass is mostly redistributed within the hemisphere, while the other two modes imply that compensation of mass occurs south of 17.5°N. A similar analysis over the southern hemisphere for 1972–1983 reveals remarkably similar results. Large and significant fluctuations in the hemispheric mean sea level pressure are found. These include record anomalies during the northern spring and summer of 1979, the year of the Global Weather Experiment. Opposite anomalies in the southern hemisphere, through the constraint of conservation of mass, imply the reality of a substantial global‐scale redistribution of atmospheric mass.
An evaluation is carried out of the moisture fields, the precipitation P and evaporation E, and the moisture transport and divergence in the atmosph
To explore the vertical coherence of the vertical temperature structure in the atmosphere, an analysis is performed of the full three-dimensional spatial structure of the temperature field monthly mean anomalies from the 40-yr ECMWF Re-Analysis (ERA-40) for a core region of the Tropics from 30°N to 30°S, with results projected globally. The focus is on the first three empirical orthogonal functions (EOFs), two of which have primary relationships to El Niño–Southern Oscillation (ENSO) and feature rather different vertical structures. The second (EOF-2) also has a weak ENSO signature but a very complex vertical structure and reflects mainly nonlinear trends, some real but also some in large part spurious and associated with problems in assimilating satellite data. The dominant pattern (EOF-1) in its positive sign features highly coherent zonal mean warming throughout the tropical troposphere from 30°N to 30°S that increases in magnitude with height to 300 hPa, drops to zero about 100 hPa at the tropopause, and has reverse sign to 30 hPa with peak negative values at 70 hPa. Spatially at low levels it shows warmth throughout most of the Tropics although with weak or slightly opposite sign in the western tropical Pacific and a strong reversed sign in the Pacific subtropics. Coherent wave structures below 700 hPa at higher latitudes cancel out in the zonal mean. However, the structure becomes more zonal above about 700 hPa and features off-equatorial maxima straddling the equator in the eastern Pacific in the upper troposphere with opposite sign at 100 hPa, as a signature of a forced Rossby wave. The corresponding sea level pressure pattern is similar to but more focused in equatorial regions than the Southern Oscillation pattern. The time series highlights the 1997/98 El Niño along with those in 1982/83 and 1986/87, and the 1988/89 La Niña, and correlates strongly with global mean surface temperatures. Missing, however, is the prolonged sequence of three successive El Niño events in the early 1990s, which are highlighted in EOF-3 as part of a mainly lower-frequency decadal variation that features modest zonal mean warming below 700 hPa, cooling from 700 to 300 hPa, and warming above 300 hPa, peaking at 100 hPa and extending from 40°N to 50°S. Spatially at the surface this pattern is dominated by Southern Oscillation wave-1 structures throughout the Tropics and especially the subtropics. The regional temperature structures are coherent throughout the troposphere, with strongest values in the Pacific and extending well into the extratropics, with a sign reversal at and above 100 hPa. Strong Rossby wave signatures are featured in the troposphere with a distinctive quadrupole pattern that reverses at 100 hPa. The vertical coherence of all patterns suggests that they should be apparent in broad-layer satellite temperature records but that stratospheric anomalies are not independent. The quite different three-dimensional structure of these different patterns highlights the need to consider the full structure outside of the Pacific and at all vertical levels in accounting for impacts of ENSO, and how they relate to the global mean.
