No abstract is provided for this article.
Click to increase image sizeClick to decrease image size Additional informationNotes on contributorsGeorge E. BrownResearch Professor of Environmental Science University of Virginia Charlottesville President, The Science & Environmental Policy Project Fairfax, Va. Head, Climate Analysis Section National Center for Atmospheric Research Boulder, Colo. U. S. House of Representatives Washington, D.C.
Droughts occur naturally, but climate change has generally accelerated the hydrological processes to make them set in quicker and become more intense, with
Possible methods for estimating surface fluxes include (i) use of bulk fluxes and in situ observations, (ii) use of model parameterizations to interpret specified inputs and compute surface fluxes, and (iii) various indirect methods, which rely on the fact that the mass and surface heat, energy, and momentum budgets must balance and so, given computations of all the other components in the various budget equations applied to fields either within the ocean or the atmosphere, fluxes may be inferred as a residual. This paper reviews the third approach using indirect methods and outlines the advantages associated with the use of global atmospheric analyses from four-dimensional data assimilation (4DDA). The time mean increment required in producing analyses in 4DDA is identical to the systematic short-term (6 h) assimilating model forecast error and is most likely due to errors in the model physics. Therefore, the analyses include a desirable fix, which allows the sum of the “physics” to be deduced from “dynamics.” The focus is on the heat and moisture budgets to infer surface heat fluxes and freshwater fluxes, but with the recognition of the need to balance the mass budget as well. The diurnal cycle of the vertically integrated mass budget for July 1985 and January 1996 from National Centers for Environmental Prediction (formerly the National Meteorological Center) reanalyses is presented, revealing the strong semidiurnal tide and highlighting the need for at least four-times-daily data. The new results reveal that gross violations of the mass budget continue to be present, but these can be allowed for. A discussion is given of other sources of errors contributing to the heat and moisture budgets.
The energy budget of the modern-day Southern Hemisphere is poorly simulated in both state-of-the-art reanalyses and coupled global climate models. The ocean-dominated Southern Hemisphere has low surface reflectivity and therefore its albedo is particularly sensitive to cloud cover. In modern-day climates, mainly because of systematic deficiencies in cloud and albedo at mid- and high latitudes, too much solar radiation enters the ocean. Along with too little radiation absorbed at lower latitudes because of clouds that are too bright, unrealistically weak poleward transports of energy by both the ocean and atmosphere are generally simulated in the Southern Hemisphere. This implies too little baroclinic eddy development and deficient activity in storm tracks. However, projections into the future by coupled climate models indicate that the Southern Ocean features a robust and unique increase in albedo, related to clouds, in association with an intensification and poleward shift in storm tracks that is not observed at any other latitude. Such an increase in cloud may be untenable in nature, as it is likely precluded by the present-day ubiquitous cloud cover that models fail to capture. There is also a remarkably strong relationship between the projected changes in clouds and the simulated current-day cloud errors. The model equilibrium climate sensitivity is also significantly negatively correlated with the Southern Hemisphere energy errors, and only the more sensitive models are in the range of observations. As a result, questions loom large about how the Southern Hemisphere will actually change as global warming progresses, and a better simulation of the modern-day climate is an essential first step.
The implementation of a global climate project and its early results are being hailed as exciting leaps forward in climate research that have enormous potential for improving predictions of interannual variations, as well as improving the understanding and modeling, and possibly developing predictive capabilities, of decadal and longer variability and climate change. Known as the Climate Variability and Predictability (CLIVAR) project of the World Climate Research Programme (WCRP),its most recent achievements have included observations, predictions, and documentation of the evolution of the 1997–1998 El Niño and that event's progression into the 1998 La Niña. In place to accomplish this was CLIVAR's full tropical Pacific observing system and new coupled atmosphere‐upper ocean models. Even more exciting was that prediction models were able to capture aspects of the differences in teleconnection patterns between this El Niño and previous events, pointing to the importance of sea surface temperature patterns globally Exciting new possibilities also have emerged through profiling floats deployed at various depths that come to the surface every week or two to give ocean soundings of temperature and salinity, while their displacements provide estimates of ocean currents at depth.
A comparison of near-global monthly mean surface temperature anomalies to those of global Microwave Sounding Unit (MSU) 2R temperatures for 1979–95 reveals differences in global annual mean trends that are shown to be largely attributable to important physical differences in the quantities that are measured. Maps of standard deviations of the monthly mean anomalies, which can be viewed as mostly measuring the size of the climate signal, reveal pronounced differences regionally in each dataset. At the surface, the variability of temperatures is relatively small over the oceans but large over land, whereas in the MSU record the signal is much more zonally symmetric. The largest differences are found over the North Pacific and North Atlantic Oceans where the monthly standard deviations of the MSU temperatures are larger by more than a factor of 2. Locally over land, the variance of the surface record is larger than that of the MSU. In addition to differential responses to forcings from the El Niño-Southern Oscillation phenomenon and volcanic eruptions, these characteristics are indicative of differences of the response to physical processes arising from the relative importance of advection versus surface interactions and the different heat capacities of land and ocean. The result is that the regions contributing to hemispheric or global mean anomalies differ substantially between the two temperature datasets. This helps to account for the observed differences in decadal trends where the surface record shows a warming trend since 1979 of 0.18°C per decade, relative to the MSU record. While a common perception from this result is that the MSU and surface measurements of global temperature change are inconsistent, the issue should not be about which record is better, but rather that both give a different perspective on the same events.
This paper addresses the issue of residence time of atmospheric moisture and concludes that the value originally proposed by Trenberth (1998) of 8.9 days still applies.It provides a partial commentary on an earlier paper by Läderach and Sodemann (2016) which suggested that the residence time was less than half, namely 3.9 days.Two methods are used to assess the lifetime and age of moisture in the atmosphere and the basic fields used come from ERA-Interim reanalyses.
An empirical orthogonal function analysis is performed on monthly mean sea level pressure (SLP) deviations from normal (anomalies) in the Australasian area and monthly mean sea surface temperature (SST) anomalies in the Tasman Sea for 14 years of data. Only the five components of each that explain most of the variance appear to be significant. Spectral and cross‐spectral analysis of the time series of the coefficients reveals significant relations at a quasi‐biennial periodicity. An empirical model of the SLP and SST fields for this period is constructed and the amplitudes of the oscillation are found to be about 2 1/2 mb in the monthly mean SLP anomaly field and 0·4 degC in the monthly mean SST anomaly field. Physical and statistical indications are that the SST field is largely caused by the SLP field but some interdependence is present. The quasi‐biennial SLP oscillation is related to Pittock's L ‐index and appears to correspond to a change in wavenumber 3 in the Southern Hemisphere atmospheric circulation. A phase change occurred at the end of 1970, but in even years prior to 1971, and in odd years since 1970, there were increased meridional transports throughout the troposphere and lower stratosphere by the large‐scale eddies. A suggestion is made that changes in land–sea heating contrasts may be an important part of the mechanism.
No abstract is provided for this article.
<title>Abstract</title> <bold>Huge heat anomalies in the past few years are not explained by climate models</bold><sup><bold>1</bold></sup><bold>. 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</bold><sup><bold>2</bold></sup><bold>. 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 to 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 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 are not directly related to TOA radiation but are strongly evident in net surface energy fluxes. Changes in the atmospheric circulation, jet streams, and storm tracks are reflected in surface ocean Ekman transports. As well as climate change, natural variability is likely in play. Hence the atmosphere and ocean currents are systematically redistributing heat from global warming, profoundly affecting local climates.</bold>
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.
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.
A reappraisal of the messy data on upper-ocean heat content for 1993–2008 provides clear evidence for warming. But differences among various analyses and inconsistencies with other indicators merit attention. The upper ocean acts as a giant heat sink and has absorbed the majority of excess energy generated by anthropogenic greenhouse gasses. This makes ocean heat content, potentially, a key indicator of climate change. But to be useful for evaluating the global energy balance and as a constraint on climate models, the measurement uncertainties of such a key indicator need to be well understood. At present the magnitude of the oceanic heat uptake is highly uncertain, with patterns of inter-annual variability in particular differing among estimates. In a major international collaboration, Lyman et al. compare the available upper-ocean heat content anomaly curves and examine the sources of uncertainly attached to them — including the difficulties in correcting bias in expendable bathythermograph data. They find that, uncertainties notwithstanding, there is clear and robust evidence for a warming trend of 0.64 watts per square metre between 1993 and 2008.
