No abstract is provided for this article.
The role of the atmospheric circulation in climate change is examined. A review is given of the information available in the past record on the atmosheric circulation and its role in climate change, firstly at the surface via sea level pressure in both the northern...
No abstract is provided for this article.
The flow of energy through the climate system is described along with its perturbations arising from human-induced climate change. The role of the atmosphere, oceans, land, and ice components of the climate system are detailed along with the hydrological cycle and interactions that give rise to large variability, such as the El Niño phenomenon. The mass and specific heat of the main components play a major role in storing and moving heat and energy around. The main cause of climate change is the changes in composition of the atmosphere and the enhanced greenhouse effect from increasing carbon dioxide and other greenhouse gases, offset somewhat by pollution. This produces an Earth Energy Imbalance, and the extra heat has profound influences, including increasing the global mean surface temperature.
Monthly mean brightness temperature anomalies derived from channel 2 of the microwave sounding units (M5Us) on board NOAA satellites over the past decade are examined and compared with both weighted and pressure-level ECMWF monthly mean temperatures for the 96 months of 1982–89. Very good agreement between the MSU and channel 2 weighted ECMWF anomalies is found over most of the globe with correlation coefficients over 0.9, but the agreement falls off over the tropics, the South Atlantic, and high latitudes of the Southern Hemisphere. The ECMWF analyses agree best with the MSU data in regions of good radiosonde coverage, while lower correlations are found in regions where the analyses depend more heavily on satellite data. Systematic errors introduced into the analyses by the retrieval techniques applied to the radiance data largely explain this apparent contradiction. Additionally, changes to the analysis-forecast system at ECMWF over the decade appear as apparent changes in climate, and these discontinuities most strongly affect the tropics and are evident in regions of fewer observations. To the extent that the weighted ECMWF data data with the MSU brightness temperatures the vertical dependence of the MSU data can be examined. Correlations of the MSU data with ECMWF temperature anomalies at individual pressure levels are highest at 300 mb over the globe, a level apparently less affected by the frequent changes and improvements at ECMWF. Over regions of good data coverage, such as the Northern Hemisphere landmasses and Australia. the MSU anomalies correlate very highly with all levels of the troposphere up to 200 mb. Thus, the MSUs appear to be an extremely useful tool for measuring global tropospheric temperature fluctuations on a monthly and longer time scale. Problems in the ECMWF temperature record since 1982 am examined in detail for the tropics. In September 1982 the introduction of diabatic nonlinear normal-mode initialization resulted in significant temperature increases in the tropical middle troposphere, especially at 500 mb. In May 1985 tropical temperatures at 700 mb (950 inb) increased (decreased) after the implementation of the T106 spectral model with major accompanying changes to physical parameterizations. Tropical temperatures near the tropopause decreased substantially after the May 1986 enhancement of the vertical revolution of the model from 16 to 19 levels with 3 new stratosphere levels. Problems at 1000 mb are present throughout the 96-month study period and are directly related to the manner in which analyzed temperatures are obtained at ECMWF. Comparisons with temperatures obtained from radiosonde stations in the tropics show that the ECMWF analyses have clearly improved with time, especially after May 1985 after which the 1000–200-mb temperatures show much greater coherence. These results show the importance of realizing the inherent problems with operationally based gridded datasets, and they strongly support the need for reanalysis of all data using a state-of-the-art four-dimensional data assimilation system.
The evolution of the Southern Oscillation (SO) is examined in the time domain by computing lagged cross correlations between sea level pressures at Darwin and sea level or surface pressures at selected stations. Also, in the Northern Hemisphere, the historical and U.S. Navy sea level pressure analyses are used. All monthly time series are low-pass filtered to retain periodicities greater than 20 months in order to highlight the interannual fluctuations which are primarily associated with the SO. A detailed analysis of the post-1941 period results in plotted maps of the phase (lead or lag) and magnitude of the maximum cross correlations with Darwin, in a manner analogous to a broadband coherence and phase spectrum. The relationships within the SO are further examined, where possible, back to 1882 using time series of running decadal cross covariances. The dominant pattern reveals the two poles of the traditional standing oscillation or seesaw of the SO, with centers of opposite sign over Indonesia and the central South Pacific Ocean. But there are significant phase variations within each center and clear indications that changes over the South Pacific lead the opposite changes in the Indonesian pole by 1–2 seasons. Largest leads of three seasons begin near New Zealand but quickly spread over the subtropics of both hemispheres in the Pacific. Typically 1–3 seasons later, opposite anomalies begin over the Indian region and progress east and southeast into the western Pacific. Significant positive and negative lagged correlations occur only in the New Zealand area. For the post-1950 period, which is the basis for most recent analyses of El Niño–SO events, the SO was dominated by a three-six year quasi-periodicity which leads to ambiguity in interpreting phase relationships. The pattern of leads and lags is consistent with a progression of anomalies from southeast Australia across New Zealand and into the Pacific about two years later. The progression is not very regular, often occurring in discrete jumps. Moreover, it requires reinterpretation of the negative correlations as positive correlations that are half a period (π radians) out of phase. Eastward propagation is likely to be exaggerated by the implied cyclicity imposed by analyses in the frequency domain. Over the longer term (1882–1984) the ambiguity is lessened and the two poles are seen to be more distinct. Systematic leads are still apparent over the subtropics of the Pacific but the evidence for an eastward-propagating component extending from Australia across the southwest Pacific is not consistent throughout the record. The results show that caution must be exercised in interpreting the post-1950 period as representative of the long-term mean behavior. The importance of the tendency for changes over the South Pacific to lead the SO lies in the probable role of associated processes in setting up tropical sea surface temperature anomalies, especially during the onset stage of El Niño events. The South Pacific Convergence Zone (SPCZ) is regarded as a key feature, and the physical mechanisms likely to be important are discussed. It is noted that the SO and El Niño events do not always coincide. Tropical Pacific sea surface temperatures can be anomalously warm without a change in the SO, apparently provided that the SPCZ is not involved to any extent. However, global-scale atmospheric teleconnections are primarily associated with the SO.
Diurnal (or sub-daily) variations are large in many surface and atmospheric fields such as solar radiation, surface latent and sensible heat fluxes, surface temperature and winds, atmospheric convection, and precipitation. These diurnal variations are especially important in air-land and air-sea interactions, which are highly non-linear and thus can not be resolved using daily mean values. Current regional and global climate models still have difficulties in simulating the diurnal variations correctly. Here we analyze the diurnal variations in surface air temperature and pressure, precipitation, and cloudiness simulated by the Community Climate System Model (CCSM), a state-of-art climate system model developed by a large number of scientists from the National Center for Atmospheric Research (NCAR) and partner institutions. The CCSM simulates well some aspects of diurnal variations such as the diurnal and semidiurnal pressure tides, and the diurnal cycle of temperature over land; but it has large biases in simulating diurnal cycles in cloud amount, moist convection, and precipitation. Diurnal variations over the oceans are too weak in the CCSM mainly because the surface ocean has no diurnal cycle in the model.
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.
No abstract is provided for this article.
El Niño refers to the exceptionally warm sea temperatures in the tropical Pacific, but it is linked to major changes in the atmosphere through the phenomenon known as the Southern Oscillation (SO), so that the whole phenomenon is called El Niño–Southern Oscillation (ENSO) by scientists. The phenomenon arises because of the coupled atmosphere–ocean interactions whereby the winds create changes in sea temperatures that in turn determine where the main tropical convection and storm activity occur, and thus determine the winds. This article outlines the current understanding of ENSO and the physical connections between the tropical Pacific and the rest of the world, using the 1997–98 El Niño event as a particular illustration. The mechanisms are described along with the interannual variations in climate over time and the associated El Niño events. The impacts are described especially in terms of the associated floods and droughts around the world. An update is included to encompass the super El Niño event of 2015–16.
CC as L pruebas recabadas sugieren que el hombre influye de una forma apreciable sobre el clima global.” Con estas mesuradas palabras, el Panel Intergubernamental sobre Cambio Climatico (IPCC), financiado por la Organizacion Meteorologica Mundial y el Programa Ambiental de las Naciones Unidas, reconocia en 1995 que los seres humanos no eran en absoluto consecuentes en lo concerniente a la salud del planeta. Lo que el panel no preciso -y 10 que cientificos y politicos discuten acaloradamentees cuando, donde y cuanto se ha notado y se notara esa influencia. Hasta ahora, los cambios climaticos presuntamente relacionados con la actividad humana han sido bastante modestos. Pero las previsiones de varia indole sugieren que el cambio alcanzara una intensidad espectacular hacia mediados del siglo XXI, sobrepasando cuanto se ha visto en la naturaleza en los ultimos 10.000 anos. Aunque algunas regiones pudieran beneficiarse durante cierto tiempo, cabe esperar que, en conjunto, las alteraciones resultaran perjudiciales e incluso catastroficas. Si la ciencia pudiera determinar la cuantfa en que ciertas actividades influyen sobre el clima, se encontraria en mejor situacion a efectos de recetar remedios para las peores alteraciones. iEs posible tal cuantificacion? Creemos que si. Nos parece que puede lograrse hacia el ano 2050, pero solo a condicion de que este objetivo se convierta en prioridad internacional permanente. Pese a la incognoscibilidad inevitable de los pormenores del cambio climatico, es patente que la acci6n humana incluye en la atmosfera de formas diversas y preocupantes. La combustion de carburantes fosiles en centrales termicas y automoviles expulsa particulas y gases que alteran la composicion de la atmosfera. La contaminacion visible, debida a combustibles ricos en azufre incluye aerosoles, particulas micrometricas que crean un cielo de panza de burra. AI reflejar parte de los rayos solares hacia el espacio, estos aerosoles enfrfan la atmosfera; de un modo transitorio, pues ~610 permanecen en el aire algunos dias, hasta que la lluvia los barre y los deposita en
Various observation-based datasets are employed to robustly quantify changes in ocean heat content (OHC), anomalous ocean–atmosphere energy exchanges and atmospheric energy transports during El Niño-Southern Oscillation (ENSO). These results are used as a benchmark to evaluate the energy pathways during ENSO as simulated by coupled climate model runs from the CMIP3 and CMIP5 archives. The models are able to qualitatively reproduce observed patterns of ENSO-related energy budget variability to some degree, but key aspects are seriously biased. Area-averaged tropical Pacific OHC variability associated with ENSO is greatly underestimated by all models because of strongly biased responses of net radiation at top-of-the-atmosphere to ENSO. The latter are related to biases of mean convective activity in the models and project on surface energy fluxes in the eastern Pacific Intertropical Convergence Zone region. Moreover, models underestimate horizontal and vertical OHC redistribution in association with the generally too weak Bjerknes feedback, leading to a modeled ENSO affecting a too shallow layer of the Pacific. Vertical links between SST and OHC variability are too weak even in models driven with observed winds, indicating shortcomings of the ocean models. Furthermore, modeled teleconnections as measured by tropical Atlantic OHC variability are too weak and the tropical zonal mean ENSO signal is strongly underestimated or even completely missing in most of the considered models. Results suggest that attempts to infer insight about climate sensitivity from ENSO-related variability are likely to be hampered by biases in ENSO in CMIP simulations that do not bear a clear link to future changes.
An investigation is made into the presence of quasi-biennial oscillations (QBOs) in sea-level pressure fields over the Northern Hemisphere. Using 55 years (1925–79) of seasonally averaged sea-level pressure anomalies, a series of analyses has been performed in order to systematically isolate and describe the QBOs. A standard empirical orthogonal function (EOF) analysis of the seasonal anomalies reveals significant nonrandomness at QBO periods. The data were then band-pass filtered in order to focus on the QBO, and a second EOF analysis was performed. Finally, a new complex EOF analysis technique was applied to the data. Complex EOFs have the advantage of permitting both standing and propagating modes. The QBO variance in six standard EOFs is essentially confined to four EOFs in the filtered data set and compressed into three complex EOFs. The latter account for 56% of the filtered variance. The dominant mode of the complex EOF analysis was common to all analyses and has been found in many other studies. It is essentially a standing wave pattern corresponding to a high-latitude zonal index with departures in pressure of opposite sign in low and high latitudes. This mode includes elements of the North Atlantic Oscillation, the Pacific–North American teleconnection pattern and the Southern Oscillation, but also differs somewhat from all three. It tends to be phase locked to the annual cycle. The second complex EOF mode C2 exhibits clear propagating characteristics as it evolves in time. The third mode C3 also has some propagating characteristics and, at times, both modes strongly resemble wave trains of quasi-stationary Rossby waves. Neither C2 nor C3 is phase locked to the annual cycle. It appears likely that all three complex EOFs are normal mode responses of the atmosphere to different kinds of forcing. Statistical evidence for phase locking of all three complex EOFs to the QBO in zonal winds in the equatorial stratosphere is not convincing, and the origin of the preferred QBO periodicity remains to be determined.
