Publications

The information available on different scales in the atmosphere for a number of different variables is explored using the global ECMWF analyses by examining the spatial spectra at T106 resolution. In most atmospheric spectra, a low wavenumber regime can be identified that does not follow a power law and is dominated by the stationary forced part of the flow. A higher wavenumber regime, where an approximate power law does appear to hold, can also usually be found. For the rotational part of the flow in the upper troposphere, the observed spectra follow quite closely that expected for quasi-two-dimensional geostrophic turbulence between about wavenumbers 12 and 70, with a kinetic energy spectrum falling off as n−3, where n is the total spherical harmonic wavenumber. In the lower troposphere, there is more power at high wavenumbers than would be expected from geostrophic turbulence, most likely due to the influence and close proximity of the lower boundary. Changes in the global analyses since 1979 have mainly influenced the spectra in the lower troposphere and the more recent analyses for 1988 have more power at higher wavenumbers. In the stratosphere, the spectra at high wavenumbers do not follow a power law behavior very well. The widespread practice of using a coarse grid without the appropriate truncation or smoothing first can result in unresolved scales being aliased and excessively noisy fields; an example is the 2.5° gridded fields made available by ECMWF. Appropriate procedures are described for truncating the T106 ECMWF spectral archive for scalar and vector fields. T42 resolution is an adequate representation for diagnostic calculations depicting most quantities within a few percent accuracy, although some spatial structure, which may partly be noise, is lost for the w, divergence, and moisture fields. In contrast, errors greater than 10% can occur at T21 or R15 resolution, although these truncations can be useful for displaying results.

Worsening drought, water restrictions, and wildfires have been widely featured in news reports across North America during recent years. Area burned by wildfire was at unprecedented levels in the United States in the summer of 2002, and devastating wildfires in California were in the news just a few months ago. Drought is not only gripping parts of North America, but also parts of Northern Africa and other regions worldwide, serving as a reminder of society's vulnerability to drought and its enormous economic impact. But what is the full range of past drought variability, as revealed by paleoclimate data? What role might droughts associated with abrupt climate change play? Are droughts likely to become more frequent, longer, or more extensive as we move into the future with global warming?

A comprehensive analysis has been made of the atmospheric planetary wave response to orographic and thermal forcing in midlatitudes using a simple model. Vertical heating profiles with maxima at the surface and in the mid-troposphere are considered. The model is quasi-geostrophic on a beta-plane, and has a constant zonal mean basic-state wind. With these simplifications it is possible to obtain complete analytic solutions, not only for the wave response with and without Ekman pumping, but also for the secondary effects of the waves on the zonal mean flow. The presence of diabatic heating in the waves results in significant non-zero Eliassen-Palm fluxes and violates conditions for non-acceleration of the zonal mean flow, both for propagating and trapped waves. The potential vorticity transport or, alternatively, the Eliassen-Palm flux divergence is shown to be directly related to the vertical heating profile. However, it is the interaction between orographic and thermally forced waves that is mainly responsible for change in the zonal mean flow, and the results therefore strongly depend upon the relative phase of the thermal and orographic forcing. At large heights, remote from the heating it is shown that it is possible to choose an equivalent mountain that would produce the same response in the planetary waves as the thermal forcing. The equivalent mountain height varies inversely as zonal wavenumber. In addition the mid-tropospheric heating profile produces a wave with 4-5 times the amplitude of the wave response to the surface heating profile with the same vertically integrated total heating. Consequently it is mainly in zonal waves 1 and 2 where a mid-tropospheric thermally forced wave can dominate or be comparable to the orographic waves.

[1] The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichon in March 1982 and Pinatubo in June 1991, taking into account changes from El Nino-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991–September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.

Considerable evidence has emerged of a substantial decade-long change in the north Pacific atmosphere and ocean lasting from about 1976 to 1988. Observed s

This chapter assesses the observed changes in surface and atmospheric climate, placing new observations and new analyses made during the past six years (since the Third Assessment Report TAR) in the context of the previous instrumental record. In previous IPCC reports, palaeo-observations from proxy data for the pre-instrumental past and observations from the ocean and ice domains were included within the same chapter. This helped the overall assessment of the consistency among the various variables and their synthesis into a coherent picture of change. A short synthesis and scrutiny of the consistency of all the observations is included here (see Section 3.9). In the TAR, surface temperature trends were examined from 1860 to 2000 globally, for 1901 to 2000 as maps and for three sub-periods (1910-1945, 1946-1975 and 1976-2000). The first and third sub-periods had rising temperatures, while the second sub-period had relatively stable global mean temperatures. The 1976 divide is the date of a widely acknowledged 'climate shift' and seems to mark a time when global mean temperatures began a discernible upward trend that has been at least partly attributed to increases in greenhouse gas concentrations in the atmosphere. The picture prior to 1976 has essentially not changed and is therefore not repeated in detail here. However, it is more convenient to document the sub-period after 1979, rather than 1976, owing to the availability of increased and improved satellite data since then (in particular Television InfraRed Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) data) in association with the Global Weather Experiment (GWE) of 1979. The post-1979 period allows, for the first time, a global perspective on many fields of variables, such as precipitation, that was not previously available. The availability of high-quality data has led to a focus on the post-1978 period, although physically this new regime seems to have begun in 1976/1977. Documentation of the climate has traditionally analysed global and hemispheric means, and land and ocean means, and has presented some maps of trends. However, climate varies over all spatial and temporal scales: from the diurnal cycle to El Nino to multi-decadal and millennial variations. Atmospheric waves naturally create regions of temperature and moisture of opposite-signed departures from the zonal mean, as moist warm conditions are favoured in poleward flow while cool dry conditions occur in equatorward flow. Although there is an infinite variety of weather systems, one area of recent substantial progress is recognition that a few preferred patterns (or modes) of variability determine the main seasonal and longer-term climate anomalies (Section 3.6). These patterns arise from the differential effects on the atmosphere of land and ocean, mountains, and anomalous heating, such as occurs during El Nino events. The response is generally felt in regions far removed from the anomalous forcing through atmospheric teleconnections, associated with large-scale waves in the atmosphere. This chapter therefore documents some aspects of temperature and precipitation anomalies associated with these preferred patterns, as they are vitally important for understanding regional climate anomalies and why they differ from global means. Changes in storm tracks, the jet streams, regions of preferred blocking anticyclones and changes in monsoons all occur in conjunction with these preferred patterns and other climate anomalies. Therefore the chapter not only documents changes in variables, but also changes in phenomena (such as El Nino) or patterns, in order to increase understanding of the character of change. Extremes of climate, such as droughts and wet spells, are very important because of their large impacts on society and the environment, but they are an expression of the variability. Therefore, the nature of variability at different spatial and temporal scales is vital to our understanding of extremes. The global means of temperature and precipitation are most readily linked to global mean radiative forcing and are important because they clearly indicate if unusual change is occurring. However, the local or regional response can be complex and perhaps even counter-intuitive, such as changes in planetary waves in the atmosphere induced by global warming that result in regional cooling. As an indication of the complexity associated with temporal and spatial scales measures of the magnitude of natural variability of surface temperature in which climate signals are embedded are provided. The measures used are indicators of the range: the mean range of the diurnal and annual cycles, and the estimated 5th to 95th percentiles range of anomalies. These are based on the standard deviation and assumed normal distribution, which is a reasonable approximation in many places for temperature, with the exception of continental interiors in the cold season, which have strongly negatively skewed temperature distributions owing to cold extremes. For the global mean, the variance is somewhat affected by the observed trend, which inflates this estimate of the range slightly. The comparison highlights the large diurnal cycle and daily variability. Daily variability is, however, greatly reduced by either spatial or temporal averaging that effectively averages over synoptic weather systems. Nevertheless, even continental-scale averages contain much greater variability than the global mean in association with planetary-scale waves and events such as El Nino.

The net surface energy flux is computed as a residual of the energy budget using top-of-atmosphere radiation combined with the divergence of the column-integrated atmospheric energy transports, and then used with the vertically integrated ocean heat content tendencies to compute the ocean meridional heat transports (MHTs). The mean annual cycles and 12-month running mean MHTs as a function of latitude are presented for 2000–16. Effects of the Indonesian Throughflow (ITF), associated with a net volume flow around Australia accompanied by a heat transport, are fully included. Because the ITF-related flow necessitates a return current northward in the Tasman Sea that relaxes during El Niño, the reduced ITF during El Niño may contribute to warming in the south Tasman Sea by allowing the East Australian Current to push farther south even as it gains volume from the tropical waters not flowing through the ITF. Although evident in 2015/16, when a major marine heat wave occurred, these effects can be overwhelmed by changes in the atmospheric circulation. Large interannual MHT variability in the Pacific is 4 times that of the Atlantic. Strong relationships reveal influences from the southern subtropics on ENSO for this period. At the equator, northward ocean MHT arises mainly in the Atlantic (0.75 PW), offset by the Pacific (−0.33 PW) and Indian Oceans (−0.20 PW) while the atmosphere transports energy southward (−0.35 PW). The net equatorial MHT southward (−0.18 PW) is enhanced by −0.1 PW that contributes to the greater warming of the southern (vs northern) oceans.

An analysis has been made of the interseasonal and interannual variability of mean circulation and eddy statistics for both summer and winter in the Southern Hemisphere. Total variance fields of geopotential height, the noith-wuth and east-west wind components and poleward transient eddy momentum fluxes at 500 mb are analyzed along with their contributions from two broad frequency bands covering 2-8 day and 8-64 day period fluctuations. Largest interannual variability occurs between 40-60°S in association with the main jet stream in summer or the polar jet stream in winter and the main belt of eddy activity within each season. The circulation and eddy statistics during the year of the Global Weather Experiment (GWE) are compared with the means and standard deviations over all years from 1972–80, and contrasted with individual years The GWE summer of 1978–79 is contrasted with 1976–77, and the 1979 winter is contrasted with 1980. The year of the GWE was charactrized by an exceptionally deep circumpolar trough, an increase in westerlies between 45–70°S and a decrease to the north, with a southward shift in the main westerly jet during summer 1978–79 and a considerably enhanced and southward shifted polar jet but weaker subtropical jet in winter 1979. Associated with these changes was a southward shift in storm tracks and high frequency eddy activity throughout the year. In both seasons anomalous convergence of momentum by the eddies into the jets was such that it would have helped sustain the abnormal distribution of westerlies against surface friction. Many of the anomalies in the circulation statistics during the GWE are statistically significant. most notably in winter, and their reality is supported by station data and the dynamical consistency of the relationships between the anomalous mean flow and storm tracks. In addition, the deficit of mass over the Southern Hemisphere revealed by sea level pressures in April-July 1979 is compensated by the surfeit that occurred in the North Hemisphere. Although the vastly improved observations during the GWE may have contributed to the size of the anomaly, they cannot account for the systematic change in location of the features of the flow. The circulation during the GWE appears to have been at one extreme of the large natural interannual variability that is so much a feature of the Southern Hemisphere flow. The atypical nature of the circulation should be borne in mind in analyses based solely on the GWE over the Southern Hemisphere.

An update is given of the global correlation and regression patterns of sea level pressure associated with the Southern Oscillation, based upon the reanalyses from the National Centers for Environmental Prediction–National Center for Atmospheric Research for 1958–98, a period independent of that of early work. Features over the oceans are better defined than was previously possible and most features prove to be robust, although climate changes such as the 1976 climate shift have evidently altered some important relationships, such as those with Southeast Asia. Associated surface temperature patterns are also shown over the same interval and reveal striking symmetry about the equator. For El Niño, the patterns emphasize the associated broad warming over the tropical central and eastern Pacific, as well as along the west coast of the Americas extending into high latitudes of the Pacific in both hemispheres, and cooling in the central North and South Pacific. Precipitation patterns associated with the Southern Oscillation are given based upon the post-1979 period to include satellite data over the oceans, which emphasizes that the main changes are for a global redistribution of precipitation, so that solely land-based perspectives are biased. While annual mean patterns reveal much of the geographic structure associated with the Southern Oscillation, important seasonal variations are present, especially for sea level pressure and precipitation.

No abstract is provided for this article.

No abstract is provided for this article.

Variations in the hydrological cycle and the water vapor content of the atmosphere form a vital part of the “greenhouse” feedback mechanism that can substantially enhance perturbations in the atmosphere arising from changes in forcing, such as those associated with increases in the carbon dioxide content of the atmosphere. An analysis is therefore made of the mean annual cycle, interannual variability and trends in global‐scale water vapor content of the atmosphere for December 1978 through December 1985, using global analyses from the European Centre for Medium Range Weather Forecasts. Since the variations in total atmospheric mass are almost entirely due to water vapor, two entirely independent global measures of the water vapor content can be compared. These are the total surface pressure due to water vapor from humidity analyses and the total surface pressure itself. For the mean annual cycle these show excellent agreement, and the analyses are therefore compatible with the constraint that the total mass of dry air is conserved. However, it appears that both the interannual variability and trends in water vapor are sufficiently small that they are lost in the noise level of the data. A new estimate of the total mass of the atmosphere is 5.1361×10 18 kg for the annual mean, corresponding to a mean surface pressure of 984.43 mbar. It ranges from 5.1352×10 18 kg in January to 5.1371×10 18 kg in July, owing to the annual cycle in global water vapor which has an amplitude of 1.0×10 15 kg (0.2 mbar). The total mass of dry air is estimated to be 5.123×10 18 kg (or 981.9 mbar). Also presented is the partitioning of the mass for both the total and the water vapor into the contributions from each hemisphere and as a function of latitude. Mean annual surface pressures in the northern and southern hemispheres are found to be 981.92 and 986.93 mbar, respectively. Monthly mean hemispheric fluctuations in surface pressures of ±1 mbar are not uncommon and tend to be reflected by opposite anomalies in the other hemisphere (consistent with conservation of mass), but the residual in the global monthly mean is of the order of 0.1 mbar. Since this is greater than the magnitude of the possible signal in water vapor surface pressure, the available evidence indicates that the analyzed global monthly anomalies are mostly noise.

The main problem in weather forecasting over New Zealand and the Southern Oceans is the lack of sufficient data for good analyses, especially at upper levels. Numerical weather prediction also suffers from this problem, but it can to some degree be alleviated by producing several prognoses from reasonable alternative analyses to alert the forecaster to the sensitivity of the situation to data-blank areas. This procedure allows forecasters to assess the degree of confidence that can be placed in a forecast since unresolvable problems in the initial analyses cause uncertainty in predicting the exact timing and areas that will be affected by short-wave disturbances. It also gives them greater flexibility when later information becomes available and clearly invalidates one of the forecasts. The performance of a five-layer quasi-geostrophic model developed for this purpose in New Zealand is described and illustrated with one case study of an unusual summer situation in the Australia–New Zealand region where a decaying tropical cyclone is absorbed into the frontal system of an extratropical cyclone undergoing explosive cyclogenesis.

This book has addressed the energy flows through the climate system. It has made the point that the total energy generation by humans is relatively small compared with natural flows, and the main way humans cause climate change is by interfering with the natural energy flows. Ironically, that interference comes mainly from our use of energy and the associated emissions of greenhouse gases and aerosols. In this case, “energy” encapsulates electricity, heat, transport, industrial, and agricultural activities. The latter include changes in land use. The primary source of the errant emissions is the burning of fossil fuels, generating carbon dioxide. A key issue then is how to decarbonize energy systems.

No abstract is provided for this article.

The Palmer Drought Severity Index (PDSI) was calculated globally using gridded monthly air temperature and precipitation. From 1900 to 1995, there are large multi‐year to decadal variations in the percentage areas in severe drought (PDSI < −3.0) and severe moisture surplus (PDSI > +3.0) over many land areas while secular trends are small. Since the late 1970s, however, there have been some increases in the combined percentage areas in severe drought and severe moisture surplus, resulting from increases in either the drought area (e.g., over the Sahel, eastern Asia and southern Africa) or both the drought and wet areas (e.g., over the U.S. and Europe). Although the high percentages of the dry and wet areas in the recent decades are not unprecedented during this century (except the Sahel), the recent changes are closely relate to the shift in El Niño‐Southern Oscillation (ENSO) towards more warm events since the late 1970s and coincide with record high global mean temperatures. Moreover, for any given value of ENSO indices, the PDSI anomalies tend to be larger than would be expected from previous records. These changes are qualitatively consistent with those expected from increased green‐house gases in the atmosphere.

As Earth rotates on its axis and revolves around the Sun, the basic geometry determines an excess of radiation from the Sun is received in the tropics, and there is a deficit in polar regions (Fig. 1.4). Contrasts in temperatures between the two regions would be far greater than observed (Chapter 5) were it not for the dynamic transport of energy polewards by the atmosphere and ocean. The contrast is greatest in winter, when the polar night sets in, with zero incoming radiation in the Arctic or Antarctic, and the contrast in summer is less in the northern versus southern hemisphere owing to the location of large continental land masses in mid- to high latitudes that more readily warm up than the oceans do. How and where the poleward energy transports occur greatly affect local climates.