519 publications from this institution
Closure of the water and energy cycles for North America has been improved by combining several new data sets to provide an integrated view from 1979 to 2010. We use new global atmospheric reanalyses, top‐of‐atmosphere radiation, surface fluxes including evaporation E and precipitation P , streamflow and river discharge, and Gravity Recovery and Climate Experiment estimates of water storage and its tendency. The atmospheric moisture budget provides more reliable estimates and reproducible time series of E‐P than separate estimates of E and P . The excess of P over E is greatest in winter largely because of changing evapotranspiration, whereas precipitation is largest in summer. The annual mean loss of energy to space of 33 W m ‐2 is compensated for nearly equally by transports of dry static energy and latent energy onto land. The annual cycle (amplitude of ~20 W m ‐2 ) of implied downward surface flux corresponds to changes in surface and soil temperatures and seasonal snowmelt.
The United Nations Framework Convention on Climate Change (UNFCCC) is an international environmental treaty adopted on 9 May 1992 and taken up at the Earth Summit in Rio de Janeiro in June 1992. The UNFCCC objective is to “stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” It set up important annual reporting requirements, including annual meetings of the Conference of the Parties (COP) to assess and promote progress.
No abstract is provided for this article.
A global perspective is developed on a number of high impact climate extremes in 2010 through diagnostic studies of the anomalies, diabatic heating, and global energy and water cycles that demonstrate relationships among variables and across events. Natural variability, especially ENSO, and global warming from human influences together resulted in very high sea surface temperatures (SSTs) in several places that played a vital role in subsequent developments. Record high SSTs in the Northern Indian Ocean in May 2010, the Gulf of Mexico in August 2010, the Caribbean in September 2010, and north of Australia in December 2010 provided a source of unusually abundant atmospheric moisture for nearby monsoon rains and flooding in Pakistan, Colombia, and Queensland. The resulting anomalous diabatic heating in the northern Indian and tropical Atlantic Oceans altered the atmospheric circulation by forcing quasi‐stationary Rossby waves and altering monsoons. The anomalous monsoonal circulations had direct links to higher latitudes: from Southeast Asia to southern Russia, and from Colombia to Brazil. Strong convection in the tropical Atlantic in northern summer 2010 was associated with a Rossby wave train that extended into Europe creating anomalous cyclonic conditions over the Mediterranean area while normal anticyclonic conditions shifted downstream where they likely interacted with an anomalously strong monsoon circulation, helping to support the persistent atmospheric anticyclonic regime over Russia. This set the stage for the “blocking” anticyclone and associated Russian heat wave and wild fires. Attribution is limited by shortcomings in models in replicating monsoons, teleconnections and blocking.
Accurate but approximate formulae for determining the mass of the atmosphere in terms of the surface pressure p s are derived and applied to globally analyzed data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) for 1985 through 1993. The formulae take into account effects of the shape of the Earth and variations in gravity with latitude and height. Variations in total mass occur because of changes in the water vapor loading of the atmosphere. Independent computations are made of the surface pressure due to water vapor p w , which is proportional to the precipitable water, using the ECMWF analyses of specific humidity. Spurious trends in both the mass of dry air and the atmospheric moisture are found to arise from changes in the analysis system at ECMWF, confounding attempts to seek real trends associated with climate change. For the recent 4‐year period 1990 to 1993 the mean annual p s was 984.76 mbar with a maximum in July of 984.98 mbar and a minimum in December of 984.61 mbar which correspond to a total mean mass of the atmosphere of 5.1441×10 18 kg with a range of 1.93×10 15 kg throughout the year associated with changes in water vapor in the atmosphere. The global mean p w for 1985–1993 is 2.58 mbar, but values are 5 to 10% lower after mid‐1992. Using the Special Sensor Microwave Imager data to make adjustments, the best estimate of the annual global p w is 2.4 mbar, corresponding to ∼2.5 cm of precipitable water. The total atmospheric moisture as given by p w varies with an annual cycle range of 0.36 mbar, a maximum in July, and a minimum in December. Thus the mean mass of water vapor is 1.25×10 16 kg and the dry air mass is 5.132×10 18 kg, corresponding to a mean surface pressure of 982.4 mbar. Overall uncertainties are ∼0.1 mbar or 0.5×10 15 kg in total mass and about double those values for atmospheric moisture content. As well as the global means, hemispheric mean values and meridional profiles of p s and p w are presented for the mean annual cycle and as latitude‐time series to show the interannual and longer‐term variability.
The focus of this Paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the Southern Hemisphere. The analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter. Histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed. Blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more. Composite flow and anomaly fields for several cases are presented and examined in detail. The geographical distribution of the frequency of lame amplitude ⩾150 gpm anomalies at 500 mb that persist for only 1–3 days is very similar to that of the high-frequency band (2–8 day period) variances that identity the storm tracks in the Southern Hemisphere. The primary location for blocking in the Southern Hemisphere is in the New Zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerlies on a spatial scale of 60° longitude. Other maxima occur southeast of South America and over the southern Indian Ocean. Sporadically, multiple blocking events occur at more than one location and one triple blocking event is examined in detail. Zonal wave 3 is strongly evident in such cases; it also plays a dominant role in the majority of blocking events. However, on most occasions, blocking occurs in isolation as a local phenomenon, and it appears that the local wave 3 may be but part of a wave train with a great circle rather than zonal orientation. Transient eddies appear to play an important role in sustaining a blocking event by either continually reinforcing the anticyclone on its western flank or by quickly reestablishing a new anticyclone as the old one breaks down or moves away.
A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979–2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental landmasses, zonal means over land, hemispheric land means, and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high-frequency information taken from atmospheric reanalyses. Results for E − P are contrasted with those from atmospheric moisture budgets based on 40-yr ECMWF Re-Analysis (ERA-40) data. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land, especially in the Tropics and subtropics.
No abstract is provided for this article.
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.
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.
The Eliassen-Palm (E-P) flux, applied to zonal men flows, is an indicator of both the flux of eddy activity and the eddy forcing of the zonal mean flow. For time mean flows, a localized E-P flux is derived and used diagnostically to assess the impact of transient eddies on a major blocking episode that occurred over the South Pacific during the Southern Hemisphere winter of 1979. In contrast to previous studies that have focused on the mean quasi-geostrophic potential vorticity equation, the focus here is on the mean momentum equations. Eddy transports and the associated induced meridional circulation and other internal adjustments necessary to maintain the thermal wind balance, are gathered together allowing the residual circulation and the effects of the eddies to be determined. The time-mean equations of motion are thus transformed to consist of mean terms, the residual circulation and the divergence of a localized E-P flux vector. The latter is a measure of the eddy forcing of the mean flow, and the east-west component is shown to be related to the flux of wave activity. For the zonal mean case it is identical to the E-P flux. The local E-P flux is closely related to, but differs from, the E-vector of Hoskins et al. and Plumb's radiative wave activity flux, but has several advantages over both. For the blocking episode, defined as 20 July-31 August 1979, transient eddies were steered around the location of the blocking anticyclones following the two branches of the split westerly jet. However, the transient eddies in each branch differed in character, both from each other and from those in the main Southern Hemisphere storm track that extends across the southern Indian Ocean near 50°S. In the latter, the high frequency synoptic-scale baroclinic eddies are barotropically damped. The eddies have similar character to the south of the block but consist mainly of zonal wavenumbers 3 and 4 with periods shorter than a week. In contrast, the transient eddies in the subtropical branch of the jet are higher wavenumber (mostly waves 5 and 6) with periods longer than a week and, although primarily baroclinic, they are also maintained by barotropic processes. Most transient wave energy propagates eastward and wave packets can be followed around the entire hemisphere, mostly following the split westerly jet, with a period of about six days. The local E-P flux divergence is divided into barotropic and baroclinic components. The former is coherent in the vertical but strongest at 300 mb near the tropopause. The transient eddies barotropically accelerate the westerlies in the main storm track and branch south of the block, and this is partially balanced by the baroclinic component. Thus a large part of the momentum balance is between transient eddy momentum convergence and the Coriolis torque arising from the poleward heat transport induced Ferrel cell, in combination with Surface friction. Where the main westerly jet splits as part of the blocking flow configuration, both the barotropic and baroclinic local E-P flux components are acting to decelerate the westerlies and thus the transient eddies are helping to maintain the blocking episode. The main differences between the storm track and blocking regions arise in the barotropic component of the local E-P flux. It appears that the configuration of the split westerly jet acts to systematically deform the transient eddies in such a way that they feed back to help maintain the split structure.
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.
No abstract is provided for this article.
Earth's energy imbalance (EEI) drives the ongoing global warming and can best be assessed across the historical record (that is, since 1960) from ocean heat content (OHC) changes. An accurate assessment of OHC is a challenge, mainly because of insufficient and irregular data coverage. We provide updated OHC estimates with the goal of minimizing associated sampling error. We performed a subsample test, in which subsets of data during the data-rich Argo era are colocated with locations of earlier ocean observations, to quantify this error. Our results provide a new OHC estimate with an unbiased mean sampling error and with variability on decadal and multidecadal time scales (signal) that can be reliably distinguished from sampling error (noise) with signal-to-noise ratios higher than 3. The inferred integrated EEI is greater than that reported in previous assessments and is consistent with a reconstruction of the radiative imbalance at the top of atmosphere starting in 1985. We found that changes in OHC are relatively small before about 1980; since then, OHC has increased fairly steadily and, since 1990, has increasingly involved deeper layers of the ocean. In addition, OHC changes in six major oceans are reliable on decadal time scales. All ocean basins examined have experienced significant warming since 1998, with the greatest warming in the southern oceans, the tropical/subtropical Pacific Ocean, and the tropical/subtropical Atlantic Ocean. This new look at OHC and EEI changes over time provides greater confidence than previously possible, and the data sets produced are a valuable resource for further study.
The climate is changing from human activities. This has major implications for the future and for human society and ecosystems, including birds. However, it is often masked by natural variability and there is great weather-related variability. This chapter reviews observed changes in climate, with a focus on changes in surface climate including variations in major patterns (modes) of climate variability and teleconnections. Of particular importance are changes in extremes. It describes how natural and anthropogenic drivers of climate change are assessed using climate models and concludes with a brief summary of future projected changes in climate and their impacts.
No abstract is provided for this article.
[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.
