Progress toward understanding the causes of and physical mechanisms involved in the 1988 North American drought is reported. An earlier study demonstrated that major sea surface temperature (SST) anomalies in the tropical Pacific Ocean, in association with the 1988 La Niña, may have disrupted atmospheric heating patterns by changing the location and intensity of the intertropical convergence zone and that such heating anomalies could have initiated the circulation anomalies across North America responsible for the drought. A key issue of when the drought circulation anomalies developed and their relation to changes in tropical Pacific SSTs is examined. Although unusually dry soil moisture and heat waves persisted into August, the anomalous atmospheric conditions that brought on the drought occurred in April, May, and June of 1988. The evolution of the Pacific SSTs and tropical convection, as revealed by outgoing longwave radiation, is shown to be consistent with the development of the conditions favorable for initiating the drought circulation pattern in April through June of 1988. On the equator at 110°W, SST anomalies exceeded −2.75°C in only April, May, and June and were largest (−4.1°C) in May 1988. The issues of how the 1988 La Niña differed from those in the past and the importance of the whole SST field in determining the anomalous diabatic heating are also discussed. Diagnostic calculations of atmospheric diabatic heating confirm that atmospheric heating anomalies existed in the tropical Pacific in association with the major SST anomalies during this time. The link between the anomalous heating and the tropical SSTs supports the view that influences external to the atmosphere were important and that the drought was not generated solely by mechanisms internal to the atmosphere. The distribution of diagnosed heating anomalies over North America, together with a planetary wave model response to idealized forcing, is described to clarify the possible role of soil moisture anomalies in perpetuating the drought. It is argued that feedback-caused soil moisture anomalies may have been secondary sources for the drought circulation but could not have been the primary instigator. For the most part, other diagnosed heating anomalies during the drought are found to have little influence on the North American region. Criteria to help judge the ability of general circulation models to simulate the drought are discussed.
Issues relevant to achieving an accuracy of better than 10 W m−2 on 250-km scales for monthly means in the atmospheric energy balance are explored from the standpoint of the formulation and computational procedures using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR, hereafter referred to as NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses. The focus is on the vertically integrated energy components, their monthly tendencies, transports, and divergences, using the most accurate computations in model and pressure coordinates. Approximate equations have often been used previously; although relatively small compared with the moist static energy, kinetic energy transports should be taken into account, as divergences can exceed several tens of watts per square meter. Changes in energy storage terms over a month are not negligible, as they are typically over 25 W m−2 in storm track regions. Transports of energy are meaningful only if the mass budget is closed. Typical magnitudes of the divergence of sensible heat and potential energy are very large (several hundred watts per square meter), but partly cancel when combined as dry static energy, reflecting the role of isentropic flow. The latent energy and sensible heat contributions are strongly positively correlated because of the dominance of low-level flow, and the latent energy divergence also cancels a large component of the dry static energy divergence, leaving a modest residual. This arises from the dominance of moist adiabatic processes in the Tropics and subtropics as the net divergent transports depend on temperature departures in the vertical from the saturated adiabatic lapse rate and their covariability with wind. Careful numerical treatments are required or else small errors in the large terms that should cancel can be amplified. Common assumptions that diagnostics can be computed on model terrain-following coordinates, which therefore vary from day to day as the surface pressure changes, lead to errors in energy budgets of the order of 5 W m−2 owing to the covariability of energy terms with surface pressure. How well model coordinate results can be replicated in pressure coordinates has been explored along with the role of vertical resolution using a postprocessor developed at NCAR. The standard 17-level reanalysis pressure level archive does not adequately resolve the atmosphere, and we propose a new set of 30 pressure levels that has 25-mb vertical resolution below 700 mb and 50-mb vertical resolution in the rest of the troposphere. The diagnostics reveal major problems in the NCEP reanalyses in the stratosphere that are inherent in the model formulation, making them unsuitable for quantitative use for energetics in anything other than model coordinates. In addition, small flaws are found in the ECMWF postprocessing onto pressure levels. These stem from the way the vector fields are truncated, which is a necessary step to avoid aliasing before putting the values out on a 2.5° grid. Moreover, it is desirable to compute the gridpoint values exactly rather than interpolating them from the Gaussian grid, as currently done by ECMWF. The diagnostic results computed with 30 levels replicate the full model level vertically integrated energy divergences to within about 2 W m−2 over the ocean, while errors exceed 10 W m−2 in small spots over Greenland, Antarctica, and the Himalayan–Tibetan Plateau complex.
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No abstract is provided for this article.
No abstract is provided for this article.
Seven years of daily global surface pressure (Ps,) analyses derived from European Centre for Medium Range Forecasts (ECMWF) data are examined to describe more fully interhemispheric mass exchanges and intraseasonal variability. Extreme events in hemispheric mean Ps are determined, and composited grid point differences show that hemispheric anomalies are mainly determined by pressures in the North Pacific, western North Atlantic, northern Asia and the Southern Hemisphere (SH) circumpolar trough. Seasonal differences in the composites indicate that the regional anomalies occur farther poleward in the winter hemisphere, and the tropical anomalies tend to have the same sign as that of the summer hemispheric mean anomaly. Long-lasting, localized, extreme Ps anomalies are identified in 18 significant events of hemispheric mass imbalance, and are found to be highly favored when the hemispheric mean departs significantly from normal. The result implies that regionally persistent anomalies are related to global-scale mass redistributions, rather than being totally the result of more localized redistributions. The global atmospheric angular momentum exhibits significant changes during interhemispheric mass imbalances that exceed one standard deviation (about 0.4 mb). There is a strong tendency for the hemisphere in which a deficit of mass occurs to experience, on average, a 5% increase in hemispheric angular momentum. Zonal complex empirical orthogonal functions are used to describe the Ps cos ϕ anomalies, filtered for 30–75 day fluctuations. Dominant modes are found in which each hemisphere, independently, produced intrahemispheric exchanges between polar and temperate latitudes. An interhemispheric mode indicates exchanges of mass between the midlatitudes of the Northern Hemisphere and the entire tropics plus the SH subtropics. The interhemispheric mode displays a southward propagation of anomalies from the tropical belt into the SH.
<strong class="journal-contentHeaderColor">Abstract.</strong> Global responses of the hydrological cycle to climate change have been widely studied but uncertainties of temperature responses to lower-tropospheric water vapor still remain. Here, we investigate the trends in global total precipitable water (TPW) and surface temperature from 1958 to 2021 using improved ERA5 and JRA-55 reanalysis datasets and further validate these trends by using radiosonde, Atmospheric Infrared Sounder (AIRS), and Microwave Satellite (SSMI(S)) observations. Our results indicate a global increase in total precipitable water (TPW) of 0.66 % per decade according to ERA5 data and 0.88 % per decade in JRA-55 data. These variations in TPW reflect the interactions of global warming feedback mechanisms across different spatial scales. Our results also revealed a significant near-surface temperature (T<sub>2m</sub>) warming trend at the rate of 0.14 K dec<sup>-1</sup> and a strong water vapor response to temperature at a rate of 4–6 % K<sup>-1</sup> globally, with land areas warming approximately twice as fast as the oceans. The relationship between TPW and T<sub>2m</sub> or surface skin temperature (T<sub>s</sub>) showed a variation around 6–8 % K<sup>-1</sup> in the 15–60° N latitude band, aligning with theoretical estimates from the Clausius–Clapeyron equation.
Utilizing radiation data from Earth Radiation Budget Experiment (ERBE), circulation statistics from NCEP reanalysis, and assimilated ocean data for the tropical Pacific basin, we show that the surface ocean warming during the 1986–87 El Niño is not only accompanied by significant increases in the cloud reflection of the solar radiation, but also by marked increases in the poleward energy transport in both the atmosphere and ocean. Measured over the equatorial region, the feedback from the ocean dynamics is twice as large as from the atmospheric dynamics which in turn is twice as large as the feedback from the cloud albedo. The three feedbacks constitute a strong regulatory effect upon the equatorial SST. The results reveal a prominent role of El Niño in the heat removal from the equatorial Pacific.
Climate change is a general term for long-term changes in climate of either sign. The term “global warming” was popularized by Wally Broecker in 1975 when he published a paper in Science magazine titled “Climatic change: are we on the brink of a pronounced global warming?”; although the term had been used as early as 1957 about Roger Revelle’s research. It has most commonly been interpreted to be synonymous with rises in global mean surface temperature (GMST) and associated with increasing carbon dioxide in the atmosphere, as put forward by Broecker. However, the term is ambiguous because warming can also refer to “heating,” which is more appropriate, and just one consequence of heating is an increase in temperature (another, for example, is “melting”). In fact, this would be a better way to use the term. Another ambiguity is whether it refers to all global temperature rise, for whatever reason, or whether it refers to only anthropogenic temperature increases. In any event, many people did not like it and climate change skeptics, in particular, preferred the term “climate change” to embrace both natural and anthropogenic sources, as well as the possibility that decreases could occur.
A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.5° grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r = 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r = 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Niño– Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The global very dry areas, defined as PDSI &lt; −3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI &gt; +3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ∼20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.
The climate is changing. The main reason is because of human-induced changes in atmospheric composition which produce warming from increased greenhouse gases. This is referred to as a forcing of the climate system. There are many other forcings, both natural and anthropogenic. The issue then is to determine the consequences in terms of the change in climate and its impacts. There is a direct response to just about any forcing, and in some cases that is the answer we seek. But in many or most cases, it is not so simple. Rather, the initial change provokes other responses, especially in the atmospheric and ocean circulation, that in turn cause other changes to occur. If the response amplifies the original change, then it is referred to as a positive feedback. Whereas if the response offsets and reduces the outcome, then it is a negative feedback. The size of some effects is quantified in Section 13.5.
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No abstract is provided for this article.
