Showing papers on "Forcing (mathematics) published in 2006"

Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of Houston using a fully coupled meteorology-chemistry-aerosol model

Abstract: [1] A new fully coupled meteorology-chemistry-aerosol model is used to simulate the urban- to regional-scale variations in trace gases, particulates, and aerosol direct radiative forcing in the vicinity of Houston over a 5 day summer period. Model performance is evaluated using a wide range of meteorological, chemistry, and particulate measurements obtained during the 2000 Texas Air Quality Study. The predicted trace gas and particulate distributions were qualitatively similar to the surface and aircraft measurements with considerable spatial variations resulting from urban, power plant, and industrial sources of primary pollutants. Sulfate, organic carbon, and other inorganics were the largest constituents of the predicted particulates. The predicted shortwave radiation was 30 to 40 W m−2 closer to the observations when the aerosol optical properties were incorporated into the shortwave radiation scheme; however, the predicted hourly aerosol radiative forcing was still underestimated by 10 to 50 W m−2. The predicted aerosol radiative forcing was larger over Houston and the industrial ship channel than over the rural areas, consistent with surface measurements. The differences between the observed and simulated aerosol radiative forcing resulted from transport errors, relative humidity errors in the upper convective boundary layer that affect aerosol water content, secondary organic aerosols that were not yet included in the model, and uncertainties in the primary particulate emission rates. The current model was run in a predictive mode and demonstrates the challenges of accurately simulating all of the meteorological, chemical, and aerosol parameters over urban to regional scales that can affect aerosol radiative forcing.

Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations

Abstract: Nine different global models with detailed aerosol modules have independently produced instantaneous direct radiative forcing due to anthropogenic aerosols. The an- thropogenic impact is derived from the difference of two model simulations with prescribed aerosol emissions, one for present-day and one for pre-industrial conditions. The differ- ence in the solar energy budget at the top of the atmosphere (ToA) yields a new harmonized estimate for the aerosol di- rect radiative forcing (RF) under all-sky conditions. On a global annual basis RF is 0.22Wm 2 , ranging from +0.04 to 0.41Wm 2 , with a standard deviation of ±0.16Wm 2 . Anthropogenic nitrate and dust are not included in this esti- mate. No model shows a significant positive all-sky RF. The corresponding clear-sky RF is 0.68Wm 2 . The cloud-sky RF was derived based on all-sky and clear-sky RF and mod- elled cloud cover. It was significantly different from zero and ranged between 0.16 and +0.34Wm 2 . A sensitivity anal- ysis shows that the total aerosol RF is influenced by consid- erable diversity in simulated residence times, mass extinction coefficients and most importantly forcing efficiencies (forc- ing per unit optical depth). The clear-sky forcing efficiency (forcing per unit optical depth) has diversity comparable to that for the all-sky/ clear-sky forcing ratio. While the di- versity in clear-sky forcing efficiency is impacted by factors such as aerosol absorption, size, and surface albedo, we can show that the all-sky/clear-sky forcing ratio is important be- cause all-sky forcing estimates require proper representation of cloud fields and the correct relative altitude placement be- tween absorbing aerosol and clouds. The analysis of the sul- phate RF shows that long sulphate residence times are com- pensated by low mass extinction coefficients and vice versa. This is explained by more sulphate particle humidity growth and thus higher extinction in those models where short-lived sulphate is present at lower altitude and vice versa. Solar atmospheric forcing within the atmospheric column is esti- mated at +0.82±0.17Wm 2 . The local annual average max- ima of atmospheric forcing exceed +5Wm 2 confirming the regional character of aerosol impacts on climate. The annual average surface forcing is 1.02±0.23Wm 2 . With the cur- rent uncertainties in the modelling of the radiative forcing due to the direct aerosol effect we show here that an estimate from one model is not sufficient but a combination of several model estimates is necessary to provide a mean and to ex- plore the uncertainty.

Simulation of Global Land Surface Conditions from 1948 to 2004. Part I: Forcing Data and Evaluations

Abstract: Because of a lack of observations, historical simulations of land surface conditions using land surface models are needed for studying variability and changes in the continental water cycle and for providing initial conditions for seasonal climate predictions. Atmospheric forcing datasets are also needed for land surface model development. The quality of atmospheric forcing data greatly affects the ability of land surface models to realistically simulate land surface conditions. Here a carefully constructed global forcing dataset for 1948–2004 with 3-hourly and T62 (∼1.875°) resolution is described, and historical simulations using the latest version of the Community Land Model version 3.0 (CLM3) are evaluated using available observations of streamflow, continental freshwater discharge, surface runoff, and soil moisture. The forcing dataset was derived by combining observation-based analyses of monthly precipitation and surface air temperature with intramonthly variations from the National Center...

Assessment of Twentieth-Century Regional Surface Temperature Trends using the GFDL CM2 Coupled Models

Abstract: Historical climate simulations of the period 1861–2000 using two new Geophysical Fluid Dynamics Laboratory (GFDL) global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. All-forcing runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multicentury control runs with no external forcing. Observed warming trends on the global scale and in many regions are simulated more realistically in the all-forcing and anthropogenic-only forcing runs than in experiments using natural-only forcing or no external forcing. In the all-forcing and anthropogenic-only forcing runs, the model shows some tendency for too much twentieth-c...

Diurnal Coupling in the Tropical Oceans of CCSM3

Abstract: New features that may affect the behavior of the upper ocean in the Community Climate System Model version 3 (CCSM3) are described. In particular, the addition of an idealized diurnal cycle of solar forcing where the daily mean solar radiation received in each daily coupling interval is distributed over 12 daylight hours is evaluated. The motivation for this simple diurnal cycle is to improve the behavior of the upper ocean, relative to the constant forcing over each day of previous CCSM versions. Both 1- and 3-h coupling intervals are also considered as possible alternatives that explicitly resolve the diurnal cycle of solar forcing. The most prominent and robust effects of all these diurnal cycles are found in the tropical oceans, especially in the Pacific. Here, the mean equatorial sea surface temperature (SST) is warmed by as much as 1°C, in better agreement with observations, and the mean boundary layer depth is reduced. Simple rectification of the diurnal cycle explains about half of the sh...

Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions

Abstract: Previous research has identified links between changes in sea surface temperature (SST) and hurricane intensity. We use climate models to study the possible causes of SST changes in Atlantic and Pacific tropical cyclogenesis regions. The observed SST increases in these regions range from 0.32°C to 0.67°C over the 20th century. The 22 climate models examined here suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system. We employ model simulations of natural internal variability to make probabilistic estimates of the contribution of external forcing to observed SST changes. For the period 1906–2005, we find an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model “20th-century” simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. In experiments in which forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th-century SST increases in both tropical cyclogenesis regions.

Estimated PDFs of climate system properties including natural and anthropogenic forcings

Abstract: [1] We present revised probability density functions (PDF) for climate system properties (climate sensitivity, rate of deep-ocean heat uptake, and the net aerosol forcing strength) that include the effect on 20th century temperature changes of natural as well as anthropogenic forcings. The additional natural forcings, primarily the cooling by volcanic eruptions, affect the PDF by requiring a higher climate sensitivity and a lower rate of deep-ocean heat uptake to reproduce the observed temperature changes. The estimated 90% range of climate sensitivity is 2.1 to 8.9 K. The net aerosol forcing strength for the 1980s shifted toward positive values to compensate for the volcanic forcing with 90% bounds of −0.74 to −0.14 W/m2. The rate of deep-ocean heat uptake is reduced with the effective diffusivity, Kv, ranging from 0.05 to 4.1 cm2/s. This upper bound implies that many AOGCMs mix heat into the deep ocean (below the mixed layer) too efficiently.

Climate Forcings and Climate Sensitivities Diagnosed from Coupled Climate Model Integrations

Abstract: A simple technique is proposed for calculating global mean climate forcing from transient integrations of coupled atmosphere–ocean general circulation models (AOGCMs). This “climate forcing” differs from the conventionally defined radiative forcing as it includes semidirect effects that account for certain short time scale responses in the troposphere. First, a climate feedback term is calculated from reported values of 2 × CO2 radiative forcing and surface temperature time series from 70-yr simulations by 20 AOGCMs. In these simulations carbon dioxide is increased by 1% yr−1. The derived climate feedback agrees well with values that are diagnosed from equilibrium climate change experiments of slab-ocean versions of the same models. These climate feedback terms are associated with the fast, quasi-linear response of lapse rate, clouds, water vapor, and albedo to global surface temperature changes. The importance of the feedbacks is gauged by their impact on the radiative fluxes at the top of the a...

SAM and regional rainfall in IPCC AR4 models: Can anthropogenic forcing account for southwest Western Australian winter rainfall reduction?

Abstract: [1] Winter rainfall over southwest Western Australia (SWWA) has decreased by 20% since the late 1960s. Why has the reduction occurred in the Southern Hemisphere (SH) winter months but not in summer? To what extent is this reduction attributable to anthropogenic forcing and congruent with the Southern Annular Mode (SAM)? Using reanalysis data and the Intergovernmental Panel on Climate Change 4th Assessment Report (IPCC AR4) 20th century model experiments, we show that a SAM-SWWA relationship exists in winter and not in other seasons. An ensemble result from 71 experiments reveals that anthropogenic forcing contributes to about 50% of the observed rainfall decline. Approximately 70% of the observed trend is congruent with the SAM trend, whereas for the models it is 46%. Our result suggests that other forcing factors must be invoked to fully account for the observed rainfall reduction.

Attribution of the Late-Twentieth-Century Rainfall Decline in Southwest Australia

Abstract: There was a dramatic decrease in rainfall in the southwest of Australia (SWA) in the mid-1960s. A statistical method, based on the idea of analogous synoptic situations, is used to help clarify the cause of the drying. The method is designed to circumvent error in the rainfall simulated directly by a climate model, and to exploit the ability of the model to simulate large-scale fields reasonably well. The method uses relationships between patterns of various atmospheric fields with station records of rainfall to improve the simulation of the local rainfall spatial variability. The original technique was developed in a previous study. It is modified here for application to two four-member ensembles of simulations of the climate from 1870 to 1999 performed with the Parallel Climate Model (PCM). The first ensemble, called “natural,” is forced with natural variations in both volcanic activity and solar forcing. The second ensemble, called “full forcing,” also includes three types of human-induced for...

Aerosol optical depth, physical properties and radiative forcing over the Arabian Sea

Abstract: The Arabian Sea region (4° N–20° N to 50° E–78° E) has a unique weather pattern on account of the Indian monsoon and the associated winds that reverse direction seasonally. The aerosol data, collected using ship-borne and island platforms (for 8 years from 1995 to 2002) along with MODIS (onboard TERRA satellite) data (from 2000 to 2003) have been used to evolve a comprehensive characterisation of the spatial and temporal variation in the physical, chemical, and radiative properties of aerosols over the Arabian Sea. The aerosol optical depth (AOD) was found to increase with latitude between the equator and 12° N. Over the northern Arabian Sea (regions lying north of 12° N), AODs do not show significant latitudinal variations; the average aerosol optical depth for this region was 0.29±0.12 during winter monsoon season (WMS; November to March) and 0.47±0.14 during summer monsoon season (SMS; April/May to September). The corresponding Angstrom exponents were 0.7±0.12 and 0.3±0.08, respectively. The low values of the exponent during SMS indicate the dominance of large aerosols (mainly dust particles >1 µm). The latitudinal gradient in AOD in the southern Arabian Sea is larger during SMS compared to WMS. The size distribution of aerosols shows two well-defined modes, one in the accumulation size regime and the other in the coarse size regime. During WMS, a third mode (nucleation) also appears in the sub micron range below ∼0.1 µm. The single scattering albedo does not show significant seasonal variations (remains within ∼0.93 to 0.98 through out the year). During WMS (SMS), top of the atmosphere diurnally averaged aerosol forcing remains around −6.1 (−14.3)W m−2 over the northern Arabian Sea up to around 12° N and decreases southwards till it attains a value of −3.8 (−3.4)W m−2 at the equator. The surface forcing remains around −16.2(−15.2)W m−2 over the northern Arabian Sea up to 12° N and decreases southwards to a value of −5.5 (−3.5)W m−2 at the equator. Over the north Arabian Sea, instantaneous forcing (flux change) at the surface can be as high as −50 W m−2. The instantaneous forcing decreases with latitude in the southern Arabian Sea at a rate of ∼3 W m−2deg−1.

In defense of Milankovitch

Abstract: [1] The Milankovitch hypothesis is widely held to be one of the cornerstones of climate science. Surprisingly, the hypothesis remains not clearly defined despite an extensive body of research on the link between global ice volume and insolation changes arising from variations in the Earth's orbit. In this paper, a specific hypothesis is formulated. Basic physical arguments are used to show that, rather than focusing on the absolute global ice volume, it is much more informative to consider the time rate of change of global ice volume. This simple and dynamically-logical change in perspective is used to show that the available records support a direct, zero-lag, antiphased relationship between the rate of change of global ice volume and summertime insolation in the northern high latitudes. Furthermore, variations in atmospheric CO2 appear to lag the rate of change of global ice volume. This implies only a secondary role for CO2 – variations in which produce a weaker radiative forcing than the orbitally-induced changes in summertime insolation – in driving changes in global ice volume.

The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing

Abstract: Air traffic condensation trails, or contrails, are believed to have a net atmospheric warming effect, although one that is currently small compared to that induced by other sources of human emissions. However, the comparably large growth rate of air traffic requires an improved understanding of the resulting impact of aircraft radiative forcing on climate. Contrails have an effect on the Earth's energy balance similar to that of high thin ice clouds. Their trapping of outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) is partly compensated by their reflection of incoming solar radiation (negative radiative forcing). On average, the longwave effect dominates and the net contrail radiative forcing is believed to be positive. Over daily and annual timescales, varying levels of air traffic, meteorological conditions, and solar insolation influence the net forcing effect of contrails. Here we determine the factors most important for contrail climate forcing using a sophisticated radiative transfer model for a site in southeast England, located in the entrance to the North Atlantic flight corridor. We find that night-time flights during winter (December to February) are responsible for most of the contrail radiative forcing. Night flights account for only 25 per cent of daily air traffic, but contribute 60 to 80 per cent of the contrail forcing. Further, winter flights account for only 22 per cent of annual air traffic, but contribute half of the annual mean forcing. These results suggest that flight rescheduling could help to minimize the climate impact of aviation.

Simulation of the 1976/77 Climate Transition over the North Pacific: Sensitivity to Tropical Forcing

Abstract: This study examines the contribution of tropical sea surface temperature (SST) forcing to the 1976/77 climate transition of the winter atmospheric circulation over the North Pacific using a combined observational and modeling approach. The National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 3 (CAM3) simulates approximately 75% of the observed 4-hPa deepening of the wintertime Aleutian low from 1950–76 to 1977–2000 when forced with the observed evolution of tropical SSTs in a 10-member ensemble average. This response is driven by precipitation increases over the western half of the equatorial Pacific Ocean. In contrast, the NCAR Community Climate Model version 3 (CCM3), the predecessor to CAM3, simulates no significant change in the strength of the Aleutian low when forced with the same tropical SSTs in a 12-member ensemble average. The lack of response in CCM3 is traced to an erroneously large precipitation increase over the tropical Indian Ocean whose dynamical im...

Elicitation of Expert Judgments of Aerosol Forcing

Abstract: A group of twenty-four leading atmospheric and climate scientists provided subjective probability distributions that represent their current judgment about the value of planetary average direct and indirect radiative forcing from anthropogenic aerosols at the top of the atmosphere. Separate estimates were obtained for the direct aerosol effect, the semi-direct aerosol effect, cloud brightness (first aerosol indirect effect), and cloud lifetime/distribution (second aerosol indirect effect). Estimates were also obtained for total planetary average forcing at the top of the atmosphere and for surface forcing. Consensus was strongest among the experts in their assessments of the direct aerosol effect and the cloud brightness indirect effect. Forcing from the semi-direct effect was thought to be small (absolute values of all but one of the experts' best estimates were ≤0.5 W/m2). There was not agreement about the sign of the best estimate of the semi-direct effect, and the uncertainty ranges some experts gave for this effect did not overlap those given by others. All best estimates of total aerosol forcing were negative, with values ranging between −0.25 W/m2 and −2.1 W/m2. The range of uncertainty that a number of experts associated with their estimates, especially those for total aerosol forcing and for surface forcing, was often much larger than that suggested in 2001 by the IPCC Working Group 1 summary figure (IPCC, 2001).

Uncertainties in the GSWP-2 precipitation forcing and their impacts on regional and global hydrological simulations

Abstract: The Global Soil Wetness Project (GSWP) is an international initiative aimed at producing global data sets of soil wetness and energy and water fluxes by driving land surface models with state-of-the-art 1° by 1° atmospheric forcing and land surface parameters. It also provides a unique opportunity to develop and test land surface parameterizations at the global scale, using multi-year off-line simulations that are not affected by the systematic errors found in atmospheric models. Nevertheless, the accuracy and reliability of the 10−year GSWP-2 atmospheric forcing remain questionable. A first comparison using the high-resolution Rhone-AGGregation (Rhone-AGG) database reveals that the baseline GSWP-2 precipitation forcing is drastically overestimated over the Rhone river basin. Hydrological simulations driven with each dataset and using the ISBA land surface model and the MODCOU river routing model are also compared. The simulated river discharges are validated against a dense network of river gauges and are generally less realistic when using the GSWP-2 instead of the Rhone-AGG precipitation forcing. Secondly, the GSWP-2 precipitation forcing is compared with three alternative data sets (GPCP-2, CRU-2, CMAP) at the global scale. Moreover, the results of a global sensitivity study to the precipitation forcing conducted with six land surface models are shown. The TRIP river routing model is used to convert daily runoff from all models into river discharges, which are compared at 80 gauging stations distributed over the globe. In agreement with the regional evaluation, the results reveal that the baseline GSWP-2 precipitation forcing is generally overestimated over the mid and high latitudes, which implies systematic errors in the simulated discharges. This study reveals that the empirical wind corrections applied to the GSWP-2 precipitation forcing are exaggerated, whereas the GPCP satellite adjustments seem to be useful for simulating realistic annual mean river discharges over the East Siberian river basins.

Evaluation of the Second Global Soil Wetness Project soil moisture simulations: 2. Sensitivity to external meteorological forcing

Abstract: [1] The quality of meteorological forcing data has a strong impact on the simulation of the land surface component of the hydrological cycle. In this paper, the sensitivity of soil moisture simulations to combinations of different external meteorological forcing and vegetation parameters supplied by the International Satellite Land-Surface Climatology Project (ISLSCP) Initiative II (II2) as part of the Second Global Soil Wetness Project (GSWP-2) is evaluated by using the SSiB land surface model. The simulated plant-available soil moisture in the top 1 m of soil is compared against in situ observations over grasslands and agricultural regions in the former Soviet Union, United States (Illinois), China, and Mongolia from the Global Soil Moisture Data Bank. It is found that the skill of the simulations is very sensitive to the source of meteorological forcing and that hybridization of reanalysis products with observational data substantially improves the soil moisture simulations compared to reanalysis data alone. Sensitivity is highest among products of precipitation, radiation, and vegetation class. The range of changes in the skill of soil moisture simulations with the same land surface model in 13 different sensitivity studies is as large as that resulting from 11 different land surface models driven with the same meteorological forcing from the baseline integrations of GSWP-2. Assuming differences among versions of meteorological forcing fields are indicative of uncertainties in our knowledge of these drivers of the land surface climate, the impact of that uncertainty on land surface hydrology is as large as that from the variations among land surface models.

Gravity wave influence on the global structure of the diurnal tide in the mesosphere and lower thermosphere

Abstract: [1] Observations of the diurnal tide from instruments aboard the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) explorer and from the Upper Atmosphere Research Satellite (UARS) show that the vertical wavelength of the tide is significantly shorter than what is predicted by tidal theory. The observed vertical structure of the tide can be reproduced in a mechanistic model by including gravity wave interaction. The model tide amplitude and phase are sensitive to the amplitude and phase of the diurnal component of momentum forcing that arises from gravity wave breaking. The phase of the momentum forcing relative to the tide determines whether the tide amplitude is increased or diminished by gravity wave forcing, while the amplitude of the momentum forcing determines how rapidly the tide phase will change with height. The momentum forcing profile is shaped by the structure of the gravity wave source spectrum. By comparing both the model tide amplitude and phase profiles to observations, we can provide constraints on both the gravity wave source spectrum that should be used in a gravity wave parameterization scheme and on the eddy diffusion that acts on the tide. We examine differences between the effects that two gravity wave schemes have on the tide. The role that gravity waves may play in producing tide variability is discussed in light of the results presented here.

Twentieth century simulation of the southern hemisphere climate in coupled models. Part 1: large scale circulation variability

Abstract: The ability of five, global coupled climate models to simulate important atmospheric circulation characteristics in the Southern Hemisphere for the period 1960–1999 is assessed. The circulation features examined are the Southern Hemisphere annular mode (SAM), the semi-annual oscillation (SAO) and the quasi-stationary zonal wave 3 (ZW3). The models assessed are the National Center for Atmospheric Research Community Climate System Model Version 3 (CCSM3), the Commonwealth Scientific and Industrial Research Organisation Mark 3, the Geophysical Fluid Dynamics Laboratory Model, the Goddard Institute for Space Studies Model ER (GISS-ER) and the UK Meteorological Office Hadley Center Coupled Model Version 3. The simulations were compared to the NCAR–NCEP reanalyses. The models simulate a SAO which differs spatially from the observed over the Pacific and Indian oceans. The amplitudes are too high over the southern ocean and too low over the midlatitudes. These differences are attributed to a circumpolar trough which is too deep and extends too far north, and to the inability of the models to simulate the middle to high latitude temperature gradient. The SAM is well-represented spatially by most models but there are important differences which may influence the flow over the Pacific and in the region extending from the Ross to Weddell Seas. The observed trend towards positive polarity in the SAM is apparent in the ensemble averages of the GISS-ER and CCSM3 simulations, suggesting that the trend is due to external forcing by changes in the concentration of ozone and greenhouse gases. ZW3 is well-represented by the models but the observed trend towards positive phases of ZW3 is not apparent in the simulations suggesting that the observed trend may be due to natural variability, not external forcing.

Evolution of chemical, biological, and physical water properties in the northern California Current in 2005: Remote or local wind forcing?

Abstract: [1] The spring onset of persistent upwelling-favorable winds was later than usual in the northern California Current system in 2005, resulting in delayed provision of inorganic nutrients to the upper waters of the coastal ocean. This study uses water column measurements to illustrate the evolution of temperature, salinity, nitrate and chlorophyll a prior to and after the onset of persistent local upwelling-favorable winds, including recovery to “typical” conditions. Warm, nutrient- and chlorophyll-depleted surface conditions similar to those in an El Nino were observed from Vancouver Island to central Oregon, and extended to depths greater than 500 m. Return to typical conditions was more rapid than suggested by time-integrated local wind stress but consistent in timing with “remote” forcing of water properties in this region by upwelling-favorable winds off northern California. Alongshore advection also likely contributed to the observed recovery, but was much less effective than upwelling.

Abrupt transition from natural to anthropogenic aerosol radiative forcing: Observations at the ABC‐Maldives Climate Observatory

Abstract: [1] Using aerosol-radiation observations over the north Indian Ocean, we show how the monsoon transition from southwest to northeast flow gives rise to a similar transition in the direct aerosol radiative forcing from natural to anthropogenic forcing. These observations were taken at the newly built aerosol-radiation-climate observatory at the island of Hanimaadhoo (6.776°N, 73.183°E) in the Republic of Maldives. This observatory is established as a part of Project Atmospheric Brown Clouds (ABC) and is referred to as the ABC-Maldives Climate Observatory at Hanimaadhoo (ABC_MCOH). The transition from the southwest monsoon during October to the northeast monsoon flow during early November occurs abruptly over a period of few weeks over ABC-MCOH and reveals a dramatic contrast between the natural marine aerosols transported from the south Indian Ocean by the southwest monsoon and that of the polluted aerosols transported from the south and Southeast Asian region by the northeast monsoon. We document the change in the microphysical properties and the irradiance at the surface, to identify the human signature on aerosol radiative forcing. We first establish the precision of surface radiometric observations by comparing simultaneous observations using calibrated Kipp & Zonen and Eppley pyrheliometers and pyranometers for direct, diffuse and global solar radiation. We show that the direct, diffuse and global radiation can be measured within a precision of about 3 to 5 Wm−2. Furthermore, when we include the observed aerosol optical properties as input into the Monte Carlo Aerosol Cloud Radiation (MACR) model (developed by us using Indian Ocean Experiment data), the simulated fluxes agree with the observed direct, diffuse and global fluxes within the measurement accuracy. A steady southwest monsoon flow of about 5 to 7 ms−1 persists until middle of October which switches to an abrupt change in direction to northeast flow of similar speeds bringing in polluted air from south Asia. However, it is not until end of November that a steady northeasterly flow is well established. The abrupt transition is accompanied by a large increase in aerosol optical depth from about 0.1 in October to as high as 0.4 during January, the SSA decreases from 1 to about 0.9, and the Angstrom coefficient increases from about 0.5 (suggesting large particles > 1 micron) to about 1.2 in January (submicron particles) and an increase in aerosol extinction below 3 km altitude. These changes are consistent with the transport of continental pollution from south and Southeast Asia (about 1000 to several 1000 km away from ABC_MCOH) to the north Indian Ocean during the northeast monsoon. The direct aerosol forcing, determined solely from radiometric observations without resorting to models, changes from −5 Wm−2 during October to −22 Wm−2 during January. About 50% of this forcing occurs in the photosynthetically active part of the solar spectrum (0.4 to 0.7 micron). MACR shows that the decrease in SSA from 1 to 0.9 changes the aerosol forcing efficiency by a factor of about 2 from about −40 Wm−2 (per AOD) in October to −80 Wm−2 (per AOD) in January. Thus the arrival of the brown clouds from south and Southeast Asia has a large seasonal dimming effect over remote parts of the north Indian Ocean. The observational results presented here should be used for validating climate models that attempt to simulate the anthropogenic effects of aerosol forcing on climate. The observational and model results presented in this study shows how near continuous surface based observations can be used to differentiate the human impact on aerosol forcing which is a major challenge for models.

Sea Surface Temperature Variability along the Path of the Antarctic Circumpolar Current

Abstract: The spatial and temporal distributions of sea surface temperature (SST) anomalies in the Antarctic Circumpolar Current (ACC) are investigated, using monthly data from the NCEP–NCAR reanalysis for the period 1980–2004. Patterns of atmospheric forcing are identified in observations of sea level pressure and air–sea heat fluxes. It is found that a significant fraction of SST variability in the ACC can be understood as a linear response to surface forcing by the Southern Annular Mode (SAM) and remote forcing by ENSO. The physical mechanisms rely on the interplay between atmospheric variability and mean advection by the ACC. SAM and ENSO drive a low-level anomalous circulation pattern localized over the South Pacific Ocean, inducing surface heat fluxes and Ekman heat advection anomalies. A simple model of SST propagating in the ACC, forced with heat fluxes estimated from the reanalysis, suggests that surface heat fluxes and Ekman heat advection are equally important in driving the observed SST variability. Further diagnostics indicate that SST anomalies, generated mainly upstream of Drake Passage, are subsequently advected by the ACC and damped after a couple of years. It is suggested that SST variability along the path of the ACC is largely a passive response of the oceanic mixed layer to atmospheric forcing.

Evidence of Decadal Climate Prediction Skill Resulting from Changes in Anthropogenic Forcing

Abstract: It is argued that simulations of the twentieth century performed with coupled global climate models with specified historical changes in external radiative forcing can be interpreted as climate hindcasts. A simple Bayesian method for postprocessing such simulations is described, which produces probabilistic hindcasts of interdecadal temperature changes on large spatial scales. Hindcasts produced for the last two decades of the twentieth century are shown to be skillful. The suggestion that skillful decadal forecasts can be produced on large regional scales by exploiting the response to anthropogenic forcing provides additional evidence that anthropogenic change in the composition of the atmosphere has influenced the climate. In the absence of large negative volcanic forcing on the climate system (which cannot presently be forecast), it is predicted that the global mean temperature for the decade 2000–09 will lie above the 1970–99 normal with a probability of 0.94. The global mean temperature anomaly for this decade relative to 1970–99 is predicted to be 0.35°C with a 5%–95% confidence range of 0.21°–0.48°C.

The Annular Response to Tropical Pacific SST Forcing

Abstract: The leading pattern of Northern Hemisphere winter height variability exhibits an annular structure, one related to tropical west Pacific heating. To explore whether this pattern can be excited by tropical Pacific SST variations, an atmospheric general circulation model coupled to a slab mixed layer ocean is employed. Ensemble experiments with an idealized SST anomaly centered at different longitudes on the equator are conducted. The results reveal two different response patterns—a hemispheric pattern projecting on the annular mode and a meridionally arched pattern confined to the Pacific–North American sector, induced by the SST anomaly in the west and the east Pacific, respectively. Extratropical air–sea coupling enhances the annular component of response to the tropical west Pacific SST anomalies. A diagnosis based on linear dynamical models suggests that the two responses are primarily maintained by transient eddy forcing. In both cases, the model transient eddy forcing response has a maximum ...

Advective transport of CO2 in permeable media induced by atmospheric pressure fluctuations: 2. Observational evidence under snowpacks

Abstract: [1] Meadow and forest CO2 amounts sampled beneath an approximately meter deep (steady state) snowpack at a subalpine site in southern Rocky Mountains of Wyoming are observed to vary by nearly 200 ppm over periods ranging from 4 to 15 days. This work employs the model of periodic, pressure-induced, advective transport in permeable media developed in part 1 of this study to investigate these CO2 fluctuations. With the aid of this physically based model, inferences are made about the nature of the physical properties of both the forcing mechanism and the snowpack that contribute to these periodic variations in undersnow CO2. Results are consistent with the hypothesis that the undersnow CO2 is being driven by advective flows induced by pressure fields created when the wind interacts with the local aerodynamic roughness elements (nearby mountain peaks, forest edges, snowdrifts). Nonharmonic spectral and cospectral techniques indicate that the wind modulates the low-frequency temporal dynamics of the undersnow CO2, whereas comparisons of the modeled and observed time lag between the surface forcing and the response of the undersnow CO2 suggest that site topography determines the horizontal structure of the wind (surface pressure) forcing. It is also suggested here that the snowpack is at most a weakly dispersive medium. Finally, because the model includes a CO2 source term in the soil underlying the snowpack, other findings suggest that both the wintertime CO2 fluxes emanating from the snowpack and the soil respiration rates may vary significantly between a meadow soil and a forest soil at this site.

Physical and optical properties of aerosols over an urban location in western India: Implications for shortwave radiative forcing

Abstract: [1] We discuss results on implications of seasonal and interannual variabilities in aerosol parameters measured over Ahmedabad, an urban location in western India, for the regional-scale shortwave aerosol direct radiative forcing. Results on physical and optical properties of aerosols are discussed in a companion paper. A discrete ordinate radiative transfer model has been used to carry out the radiative transfer computations. Two different approaches are followed to generate spectral values of aerosol parameters required as input for the radiative transfer computations, and the estimated values are found comparable for both methods. Magnitudes of surface forcing are found to be highest during postmonsoon (-63 ± 10 W/m 2 ), followed by dry (-54 ± 6 W/m 2 ) and lower values during premonsoon (-41.4 ± 5 W/m 2 ) and monsoon (-41 ± 11 W/m 2 ) seasons. In case of TOA, radiative forcing are found to be negative during dry (-26 ± 3 W/m 2 ) and postmonsoon (-22), while positive values are obtained during monsoon (14) and premonsoon (8). Large differences between TOA and surface forcing during monsoon and premonsoon indicate large absorption of radiant energy (∼50 W/m 2 ) within the atmosphere during these seasons. Different properties of aerosols and differences in their vertical distribution give rise to different heating rates within the atmosphere for different seasons. Heating rates at the surface are found to be highest during postmonsoon (5.6°K/day) but decreases sharply with increase in height. Atmosphere is heated strongly at higher levels between 1 and 2 km during monsoon. Results from several sensitivity studies have emphasized the importance of solar zenith angle and other related factors in modulating the values of aerosol radiative forcing.