No abstract is provided for this article.
An end-to-end comprehensive climate information system is needed to complement mitigation and adaptation as responses to the threat of human-induced climate change.
No abstract is provided for this article.
Ocean salinity records the hydrological cycle and its changes, but data scarcity and the large changes in sampling make the reconstructions of long-term salinity changes challenging. Here, we present a new observational estimate of changes in ocean salinity since 1960 from the surface to 2000 m. We overcome some of the inconsistencies present in existing salinity reconstructions by using an interpolation technique that uses information on the spatiotemporal covariability of salinity taken from model simulations. The interpolation technique is comprehensively evaluated using recent Argo-dominated observations through subsample tests. The new product strengthens previous findings that ocean surface and subsurface salinity contrasts have increased (i.e., the existing salinity pattern has amplified). We quantify this contrast by assessing the difference between the salinity in regions of high and low salinity averaged over the top 2000 m, a metric we refer to as SC2000. The increase in SC2000 is highly distinguishable from the sampling error and less affected by interannual variability and sampling error than if this metric was computed just for the surface. SC2000 increased by 1.9% ± 0.6% from 1960 to 1990 and by 3.3% ± 0.4% from 1991 to 2017 (5.2% ± 0.4% for 1960–2017), indicating an acceleration of the pattern amplification in recent decades. Combining this estimate with model simulations, we show that the change in SC2000 since 1960 emerges clearly as an anthropogenic signal from the natural variability. Based on the salinity-contrast metrics and model simulations, we find a water cycle amplification of 2.6% ± 4.4% K −1 since 1960, with the larger error than salinity metric mainly being due to model uncertainty.
Two major blocking events took place over the South Pacific Ocean east of New Zealand, 23 July-4 August and 19–27 August 1979. The configuration of the overall flow, including the split in the westerlies, the location of the blocking highs, and the wavenumber breakdown were very similar. The tendency for blocking highs to reform in the same location in close sequence is referred to as a blocking episode. The blocking events are analyzed in detail and the imprint they left on the general circulation statistics for the winter as a whole are presented. Since the blocking episode mostly prevented eddies from moving through the region, the variance of meridional velocity is small. Nevertheless, owing to the sequence of blocks the variance of geopotential height is a maximum. This signature on the general circulation is in marked contrast to that in storm track regions where the variance of geopotential height and meridional wind are both a maximum. A global-scale redistribution of mass was associated with the blocking episode.
Global warming first became evident beyond the bounds of natural variability in the 1970s, but increases in global mean surface temperatures have stalled in the 2000s. Increases in atmospheric greenhouse gases, notably carbon dioxide, create an energy imbalance at the top-of-atmosphere (TOA) even as the planet warms to adjust to this imbalance, which is estimated to be 0.5-1 W m −2 over the 2000s. Annual global fluctuations in TOA energy of up to 0.2 W m −2 occur from natural variations in clouds, aerosols, and changes in the Sun. At times of major volcanic eruptions the effects can be much larger. Yet global mean surface temperatures fluctuate much more than these can account for. An energy imbalance is manifested not just as surface atmospheric or ground warming but also as melting sea and land ice, and heating of the oceans. More than 90% of the heat goes into the oceans and, with melting land ice, causes sea level to rise. For the past decade, more than 30% of the heat has apparently penetrated below 700 m depth that is traceable to changes in surface winds mainly over the Pacific in association with a switch to a negative phase of the Pacific Decadal Oscillation (PDO) in 1999. Surface warming was much more in evidence during the 1976-1998 positive phase of the PDO, suggesting that natural decadal variability modulates the rate of change of global surface temperatures while sea-level rise is more relentless. Global warming has not stopped; it is merely manifested in different ways.
Human‐induced climate change, sometimes called “global warming,” has, unfortunately, become a “hot” topic, embroiled in controversy, misinformation, and claims and counterclaims. It should not be this way, because there are many scientific facts that provide solid information on which to base policy. There is a very strong observational, theoretical, and modeling base in physical science that underpins current understanding of what has happened to Earth's climate and why and what the prospects are for the future under certain assumptions. Moreover, these changes have impacts, which are apt to grow, on the environment and human society. To avoid or reduce these impacts and the economic and human effects of undesirable future climate change requires actions that are strongly opposed by those with vested interests in the status quo, some of whom have funded misinformation campaigns that have successfully confused the public and some politicians, leading to paralysis in political action. Without mitigation of climate change, one would suppose that at least society would plan sensibly for the changes already happening and projected, but such future adaptation plans are also largely in limbo. The implication is that we will suffer the consequences. All of these aspects are addressed in this informative and attractive book, which is written for a fairly general but technically informed audience. The book is strongly based upon the 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) and therefore has a solid scientific basis. Many figures, graphs, and maps come from the three IPCC working group reports, although the captions often do not explain the detail shown. Given that the IPCC reports totaled nearly 3000 pages, to distill the complex material down to 249 pages is no mean task, and the authors have succeeded quite well.
Annual and monthly mean values of continental freshwater discharge into the oceans are estimated at 1° resolution using several methods. The most accurate estimate is based on streamflow data from the world's largest 921 rivers, supplemented with estimates of discharge from unmonitored areas based on the ratios of runoff and drainage area between the unmonitored and monitored regions. Simulations using a river transport model (RTM) forced by a runoff field were used to derive the river mouth outflow from the farthest downstream gauge records. Separate estimates are also made using RTM simulations forced by three different runoff fields: 1) based on observed streamflow and a water balance model, and from estimates of precipitation P minus evaporation E computed as residuals from the atmospheric moisture budget using atmospheric reanalyses from 2) the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and 3) the European Centre for Medium-Range Weather Forecasts (ECMWF). Compared with previous estimates, improvements are made in extending observed discharge downstream to the river mouth, in accounting for the unmonitored streamflow, in discharging runoff at correct locations, and in providing an annual cycle of continental discharge. The use of river mouth outflow increases the global continental discharge by ∼19% compared with unadjusted streamflow from the farthest downstream stations. The river-based estimate of global continental discharge presented here is 37 288 ± 662 km3 yr−1, which is ∼7.6% of global P or 35% of terrestrial P. While this number is comparable to earlier estimates, its partitioning into individual oceans and its latitudinal distribution differ from earlier studies. The peak discharges into the Arctic, the Pacific, and global oceans occur in June, versus May for the Atlantic and August for the Indian Oceans. Snow accumulation and melt are shown to have large effects on the annual cycle of discharge into all ocean basins except for the Indian Ocean and the Mediterranean and Black Seas. The discharge and its latitudinal distribution implied by the observation-based runoff and the ECMWF reanalysis-based P–E agree well with the river-based estimates, whereas the discharge implied by the NCEP–NCAR reanalysis-based P–E has a negative bias.
Interannual variability of the atmospheric heat budget is explored via a new data set of the computed vertically integrated energy transports to examine relationships with other fields. A case study reveals very large monthly divergences of these transports regionally with El Niño‐Southern Oscillation (ENSO) and the associated changes with the Pacific‐North American teleconnection pattern, and with the North Atlantic Oscillation. In the tropical Pacific during large El Niño events the anomalous divergence of the atmospheric energy transports exceeds 50 W m −2 over broad regions for several months. Examination of the corresponding top‐of‐the‐atmosphere net radiative fluxes shows that it is primarily the surface fluxes from the ocean to the atmosphere that feed the divergent atmospheric transports. A systematic investigation of the covariability of sea surface temperatures (SSTs) and the divergence of atmospheric energy transport, using singular value decomposition analysis of the temporal covariance, reveals ENSO as dominant in the first two modes, explaining 62% and 12% of the covariance in the Pacific domain and explaining 39.5% and 15.4% globally for the first and second modes, respectively. The first mode is well represented by the time series for the SST index for Niño 3.4 region (170°W–120°W, 5°N–5°S). Regression analysis allows a more complete view of how the SSTs, outgoing longwave radiation, precipitation, diabatic heating, and atmospheric circulation respond with ENSO. The second mode indicates aspects of the systematic evolution of ENSO with time, with strong lead and lag correlations. It primarily reflects differences in the evolution of ENSO across the tropical Pacific from about the dateline to coastal South America. High SSTs associated with warm ENSO events are damped through surface heat fluxes into the atmosphere, which transports the energy into higher latitudes and throughout the tropics, contributing to loss of heat by the ocean, while the cold ENSO events correspond to a recharge phase as heat enters the ocean. Diabatic processes are clearly important within ENSO evolution.
For a number of reasons, conservation of mass in the global analyses on pressure coordinates is violated, yet this constraint is required for budget studies of all kinds. The imbalances arise from postprocessing the variables onto pressure surfaces, problems of dealing with the lower boundary and substituting an artificial atmosphere below ground, and diurnal pressure tendencies associated with the semidiurnal tide and the timing and distribution of observations. Methods are described and illustrated for May 1988 for adjusting the monthly mean global European Centre for Medium-Range Weather Forecasts analyses in three dimensions on pressure surfaces so that the mass balance is achieved, but the problems are present in analyses on constant pressure surfaces from all centers. First, a correction is needed for the global mean vertical motion. Second, it is shown that a local adjustment to the horizontal divergent velocity field is needed for regions that are below ground on constant pressure surfaces and nearby. Third, a change in the lower-boundary condition is required to remove diurnal and tidal influences, and this produces a barotropic adjustment in the horizontal velocity field as well as an adjustment in the vertical motion field that compares favorably with the semidiurnal tide in the analyses as a function of height. Solution of a three-dimensional Poisson equation is required to provide a final adjustment that minimizes the changes in the three-dimensional flow field. A vertical coordinate change is required to facilitate the solution, and the equation solves for the adjustment in the three-dimensional velocity potential using spherical harmonic expansions and finite differences in the vertical so that a matrix inversion is required for each wavenumber. Rather than any universal single-correction technique, this four-step process proves to be necessary to produce reasonable results. Even if the corrections noted here are not implemented, the diagnostic results serve as a warning to users of the analyses of potential substantial problems for certain applications. The results also indicate how operational centers could desirably alter their postprocessing procedures to ensure that the velocity field archived on constant pressure surfaces in below-ground regions satisfies the constraint of conservation of mass.
Analyses of global atmospheric observations in real time for numerical weather prediction (NWP) lack continuity over time as the operational system evolves. Reanalysis of the observations—with more complete data, improved quality control, and a constant state‐of‐the‐art assimilating model and analysis system—greatly improves the homogeneity of the record and makes it useful for examining climate variations. This whole endeavor is now referred to as “reanalysis.”
