A comprehensive statistical analysis of how an index of the Southern Oscillation changed from 1882 to 1995 was given by Trenberth and Hoar [1996], with a focus on the unusual nature of the 1990–1995 El Niño‐Southern Oscillation (ENSO) warm event in the context of an observed trend for more El Niño and fewer La Niña events after the late 1970s. The conclusions of that study have been challenged by two studies which deal with only the part of our results pertaining to the length of runs of anomalies of one sign in the Southern Oscillation Index. They therefore neglect the essence of Trenberth and Hoar, which focussed on the magnitude of anomalies for certain periods and showed that anomalies during both the post‐1976 and 1990‐mid‐1995 periods were highly unlikely given the previous record. With updated data through mid 1997, we have performed additional tests using a regression model with autoregressive‐moving average (ARMA) errors that simultaneously estimates the appropriate ARMA model to fit the data and assesses the statistical significance of how unusual the two periods of interest are. The mean SOI for the post‐1976 period is statistically different from the overall mean at <0.05% and so is the 1990‐mid‐1995 period. The recent evolution of ENSO, with a major new El Niño event underway in 1997, reinforces the evidence that the tendency for more El Niño and fewer La Niña events since the late 1970s is highly unusual and very unlikely to be accounted for solely by natural variability.
Evidence is mounting that global warming enhances a cyclone's damaging winds and flooding rains
No abstract is provided for this article.
No abstract is provided for this article.
The annual cycles of sea level and surface pressures and the atmospheric pressure owing to water vapor have been analyzed in detail. Global sea level pressures undergo an annual cycle of 0.5 mbar range with a maximum in the northern winter. Global surface pressures, which represent the total mass of the atmosphere, also undergo an annual cycle of 0.5 mbar range but with the maximum in the southern winter. The changes in water vapor concentrations are responsible for the latter—water vapor has a maximum in the southern winter. The constancy of the mass of dry air is used as a check on the accuracy of computations. The total mass of the atmosphere is 513.7 × 10 16 kg with a standard error of 0.02 × 10 16 kg and an annual cycle of amplitude 0.1 × 10 16 kg. The corresponding global mean surface pressure of the atmosphere is 984.68 mbar. The mean total mass of water vapor is 1.3 × 10 16 kg which corresponds to 2.53 cm of precipitable water or 12906 km 3 of water at 0°C. The distribution of pressure and mass as a function of latitude are also presented. A substantial annual exchange of mass occurs between the hemispheres amounting to 0.7 × 10 16 kg of dry air (2.7 mbar range in hemispheric averaged pressure). Maximum pressures occur in winter, and the summer loss of mass is partially compensated for by increased water vapor amounts. The maximum net meridional flow occurs across 5°N in the transition seasons. In the northern hemisphere, water vapor undergoes an annual cycle with a range of 1.5 mbar (area averaged) and a summer maximum, and the sea level pressure owing to dry air undergoes an annual cycle of range 5 mbar and a winter maximum. Of this, 54% is due to exchanges with the southern hemisphere and 46% is due to changes in the artificial atmospheric mass used to correct from the surface to sea level.
A large number of reinforced concrete buildings collapsed or were heavily damaged during the 7.4 magnitude earthquake that struck northwestern Turkey on August 17, 1999. Recorded peak ground accelerations were relatively low (0.3 g–0.4 g) compared to the magnitude of the structural damage, and the elastic acceleration response spectra from the recorded motions were comparable with the elastic design spectra specified in the current Turkish seismic code. Seismic code requirements are discussed and compared with observed details. Many structural deficiencies were highlighted by the earthquake damage, including: reinforced concrete columns with insufficient confinement and transverse reinforcement, 90-degree hooks at the end of column ties, poor detailing in beam-column joint regions, strong-beam and weak-columns, soft and weak stories, and poor quality construction. Buildings with shear wall structural elements generally performed well.
Summary form only given. Following a detailed diagnosis of the vital signs of the planet Earth by the IPCC, it has become evident that the planet is running a "fever" and the prognosis is that it is apt to get much worse. "Warming of the climate system is unequivocal" and it is "very likely" due to human activities. This is the verdict of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), known as AR4. Warming of the climate system is unequivocal as is now clear from an increasing body of evidence showing discernible physically consistent changes. Moreover these changes are now simulated in climate models for the past 100 years to a reasonable degree, adding confidence to future projections. This talk will go over the evidence for climate change, how we are able to say that it is due to humans, and what it means for the future.
A brief review and evaluation of various analyses of the Southern Hemisphere westerlies is given along with the presentation of some relent results. Several features characterize the westerlies of the Southern Hemisphere as quite different from those in the Northern Hemisphere and, in the past, thew have typically been difficult to reproduce well in general circulation modes. They are the double jet structure in winter, the stronger midlatitude tropospheric winds in summer than in winter, and the ensuing much smaller amplitude of the annual cycle which is associated with a maximum of global atmospheric angular momentum in January. New values for the hemispheric angular momentum integrals are than previously reported. Two estimates of the distribution and strength of the southern westerlies that have been widely used are considered to be seriously biased. Factors contributing to discrepancies among different results am large natural variability, missing data and biases in observing systems, and methods of analysis. Over the sparsely observed Southern Hemisphere, the latter is the main reason why biases exist in analyses based only on mean station data, and the absence of imposed dynamical constraints has led to internally inconsistent fields. Even recent estimates of the southern westerlies from global operational analyses should be used judiciously with proper consideration given to reliability and biases.
The flows of energy and water from ocean to land are examined in the context of the land energy and water budgets, for land as a whole and for continents. Most atmospheric reanalyses have large errors of up to 15 W m−2 in the top-of-atmosphere (TOA) energy imbalance, and none include volcanic eruptions. The flow of energy from ocean to land is more reliable as it relies on analyzed wind, temperature, and moisture fields. It is examined for transports of the total, latent energy (LE), and dry static energy (DSE) to land as a whole and as zonal means. The net convergence of energy onto land is balanced by the loss of energy at TOA, measured by Clouds and the Earth’s Radiant Energy System (CERES), and again there are notable discrepancies. Only the ECMWF Interim Re-Analysis (ERA-I) is stable and plausible. Strong compensation between variations in LE and DSE transports onto land means that their sum is more stable over time, and the net transport of energy onto land is largely that associated with the hydrological cycle (LE). A more detailed examination is given of the energy and water budgets for Eurasia, North and South America, Australia, and Africa, making use of Gravity Recovery and Climate Experiment (GRACE) data for water storage on land and data on river discharge into the ocean. With ERA-I, the new land estimates for both water and energy are closer to achieving balances than in previous studies. As well as the annual means, the mean annual cycles are examined in detail along with uncertainty sampling estimates, but the main test used here is that of closure.
The advantages and disadvantages of using global analyses on pressure level surfaces versus model (sigma or hybrid) level surfaces are explored. Model levels consist of a terrain-following coordinate in the lowest levels but may gradually transition to pressure with height. One major issue is that the model surface often does not correspond with the earth’s real surface. Another is that a change in horizontal resolution is not well defined because the vertical coordinate also changes in the process. However, such changes are required as comparison of analyses from one center with another or with model output requires a common vertical coordinate. Also, a reduction from high resolution with 106 waves (T106 resolution) to a moderate resolution with, say, 42 waves (T42), so as to reduce the size of datasets by a factor of about 6, is often desirable. Another issue involves the meaning and use of time averages on model surfaces and their relationship to corresponding time averages on pressure surfaces. The traditional representation on constant pressure surfaces has much to recommend it and is widely used and familiar to the community, but suffers from errors arising from interpolation to the pressure surfaces and the need to properly treat regions of those surfaces that are below ground (in regions of high topography). Spectral truncation and resolution changes are well defined on pressure surfaces. Test results are presented for cases where T106 analyses on model surfaces are (1) transformed to pressure and then truncated, (2) truncated and then transformed to pressure levels, and (3) then compared at comparable resolutions. The differences arise mainly from the ill-posed truncation on model surfaces, but also from vertical interpolations. These tests are also applied to a heat budget calculation involving nonlinear terms for one month. However, the latter comparison also brings in considerations of how best to do time averaging. Fortunately, in practical terms, differences between time-averaging approaches are quite small and probably negligible for most purposes. Other results show that truncation on model surfaces from T106 to T42 produces errors considerably larger than vertical interpolation errors. However, truncation to T63 for heat budget computations produces more acceptable, although not inconsequential, truncation errors provided that only T31 waves are retained as reasonably accurate.
On the basis of simulations of hurricane Katrina in August 2005 with the advanced Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection, empirical relationships are computed between the maximum simulated wind and the surface fluxes and precipitation and provide a reasonable fit to the data. The best track data set of global observed tropical cyclones is used to estimate the frequency that storms of a given strength occur over the globe after 1970. For 1990–2005 the total surface heat loss by the tropical ocean in hurricanes category 1 to 5 within 400 km of the center of the storms is estimated to be about 0.53 × 10 22 J a −1 (where a is year) (0.17 PW). The enthalpy loss due to hurricanes computed on the basis of precipitation is about a factor of 3.4 greater (0.58 PW), owing to the addition of the surface fluxes from outside 400 km radius and moisture convergence into the storms typically from as far from the eye as 1600 km. Globally these values correspond to 0.33 W m −2 for evaporation, or 1.13 W m −2 for precipitation. Changes over time reflect basin differences and a prominent role for El Niño, and the most active period globally was 1989 to 1997. Strong positive trends from 1970 to 2005 occur in these inferred surface fluxes and precipitation arising from increases in intensity of storms and also higher sea surface temperatures. Confidence in this result is limited by uncertainties in the best track tropical cyclone data. Nonetheless, the results highlight the importance of surface energy exchanges in global energetics of the climate system and are suggestive of the deficiencies in climate models owing to their inadequate representation of hurricanes.
The heat budget has been computed locally over the entire globe for each month of 1988 using compatible top-of-the-atmosphere radiation from the Earth Radi
Vertically integrated atmospheric energy and heat budgets are presented with a focus on the zonal mean transports and divergences of dry static energy, latent energy, their sum (the moist static energy), and the total (which includes kinetic energy), as well as their partitioning into the within-month transient and quasi-stationary components. The latter includes the long-term mean and interannual variability from 1979 to 2001 and, in the Tropics, corresponds to the large-scale overturning global monsoon and the embedded Hadley and Walker circulations. In the extratropics, it includes the quasi-stationary planetary waves, which are primarily a factor in the Northern Hemisphere winter. In addition to the mean annual cycle, results are presented for the interannual variability. In the extratropics, poleward transports of both latent and dry static energy reinforce one another. However, the results highlight strong cancellations between the transports of latent and dry static energy in the Tropics as moisture is converted into latent heat, and also between quasi-stationary and transient components in the extratropics. Hence the total energy transports and divergences are fairly seamless with latitude and the total interannual variability is substantially less than that of the components. The strong interplay between the transient and quasi-stationary waves in the atmosphere highlights the symbiotic relationship between them, as the stationary waves determine the location and intensity of the storm tracks while the transient disturbances help maintain the stationary waves. These results highlight that observationally there is a very strong constraint that the global energy budget places on atmospheric dynamics.
Two potentially important papers by Wielicki et al. (1) and Chen et al. (2) dealt with aspects of how clouds and radiation vary and change, and whether climate models simulate the changes correctly. There is ample prior evidence suggesting that models have difficulties in correctly simulating clouds, and clouds are regarded as the biggest source of uncertainty in reports by the Intergovernmental Panel on Climate Change (IPCC) (3). However, an alternative interpretation of the disagreements shown between observations and models is that the analyses of the observations may be flawed.
No abstract is provided for this article.
Past attribution studies of climate change have assumed a null hypothesis of no role of human activities. The challenge, then, is to prove that there is an anthropogenic component. I argue that because global warming is “unequivocal” and ‘very likely’ caused by human activities, the reverse should now be the case. The task, then, could be to prove there is no anthropogenic component to a particular observed change in climate, although a more useful task is to determine what it is. In Bayesian statistics, this change might be thought of as adding a ‘prior’. The benefit of doubt and uncertainties about observations and models are then switched. Moreover, the science community is much too conservative on this issue and too many authors make what are called ‘Type II errors’ whereby they erroneously accept the null hypothesis. Global warming is contributing to a changing incidence of extreme weather because the environment in which all storms form has changed from human activities. WIREs Clim Change 2011, 2:925–930. doi: 10.1002/wcc.142 This article is categorized under: Paleoclimates and Current Trends > Detection and Attribution Climate Models and Modeling > Knowledge Generation with Models
