Changes In Atmospheric Forcing Research Articles

Influence of Twenty-First-Century Atmospheric and Sea Surface Temperature Forcing on West African Climate

Abstract The persistence of extended drought events throughout West Africa during the twentieth century has motivated a substantial effort to understand the mechanisms driving African climate variability as well as the possible response to elevated greenhouse gas (GHG) forcing. An ensemble of global climate model experiments is used to examine the relative roles of future direct atmospheric radiative forcing and SST forcing in shaping potential future changes in boreal summer precipitation over West Africa. The authors find that projected increases in precipitation throughout the western Sahel result primarily from direct atmospheric radiative forcing. The changes in atmospheric forcing generate a slight northward displacement and weakening of the African easterly jet (AEJ), a strengthening of westward monsoon flow onto West Africa, and an intensification of the tropical easterly jet (TEJ). Alternatively, the projected decreases in precipitation over much of the Guinea Coast region are caused by SST changes induced by the atmospheric radiative forcing. The changes in SSTs generate a weakening of the monsoon westerlies and the TEJ as well as a decrease in low-level convergence and resultant rising air throughout the midlevels of the troposphere. Experiments suggest a potential shift in the regional moisture balance of West Africa should global radiative forcing continue to increase, highlighting the importance of climate system feedbacks in shaping the response of regional-scale climate to global-scale changes in radiative forcing.

Atmospheric-induced variability of hydrological and biogeochemical signatures in the NW Alboran Sea. Consequences for the spawning and nursery habitats of European anchovy

The north-western Alboran Sea is a highly dynamic region in which the hydrological processes are mainly controlled by the entrance of the Atlantic Jet (AJ) through the Strait of Gibraltar. The biological patterns of the area are also related to this variability in which atmospheric pressure distributions and wind intensity and direction play major roles. In this work, we studied how changes in atmospheric forcing (from high atmospheric pressure over the Mediterranean to low atmospheric pressure) induced alterations in the physical and biogeochemical environment by re-activating coastal upwelling on the Spanish shore. The nursery area of European anchovy ( Engraulis encrasicolus) in the NW Alboran Sea, confirmed to be the very coastal band around Malaga Bay, did not show any drastic change in its biogeochemical characteristics, indicating that this coastal region is somewhat isolated from the rest of the basin. Our data also suggests that anchovy distribution is tightly coupled to the presence of microzooplankton rather than mesozooplankton. Finally, we use detailed physical and biological information to evaluate a hydrological–biogeochemical coupled model with a specific hydrological configuration to represent the Alboran basin. This model is able to reproduce the general circulation patterns in the region forced by the AJ movements only including two variable external forcings; atmospheric pressure over the western Mediterranean and realistic wind fields.

The response of Mediterranean thermohaline circulation to climate change: a minimal model

Abstract. Physics-based understanding of the effects of paleoclimate and paleogeography on the thermohaline circulation of the Mediterranean Sea requires an ocean model capable of long integrations and involving a minimum of assumptions about the atmospheric forcing. Here we examine the sensitivity of the deep circulation in the eastern Mediterranean basin to changes in atmospheric forcing, considered a key factor in the deposition of organic-rich sediments (sapropels). To this extent we explore the setup of an ocean general circulation model (MOMA) with realistic (present-day) bathymetry and highly idealized forcing. The model proves able to qualitatively capture some important features of the large-scale overturning circulation, in particular for the eastern basin. The response to (i) a reduction in the imposed meridional temperature gradient, or (ii) a reduction in net evaporation, proves to be non-linear and, under certain conditions, of transient nature. Consistent with previous model studies, but now based on a minimum of assumptions, we find that a reduction in net evaporation (such as due to an increase in freshwater input) may halt the deep overturning circulation. The ability to perform long model integrations allows us to add the insight that, in order to have the conditions favourable for sapropel formation persist, we must also assume that the vertical mixing of water properties was reduced. The "minimal" model here presented opens the way to experiments in which one truly follows the basin circulation into, or out of, the period of sapropel formation and where forcing conditions are continously adjusted to the precession cycle.

A classification of wave generation characteristics during large wave events on the Southern Australian margin

The Southern Australian continental margin is exposed to some of the largest waves of the global ocean, having been generated by Southern Ocean extra-tropical storms. Waverider buoy data from four buoys which span the full 3000 km length of the Southern Australian continental margin, supplemented with 10 years of numerical model output from the WAVEWATCH III wave model, have been used to describe the directional wave climate of the region. The buoy records include two records at the western end of the margin (a 10-year record from Rottnest Island, and an 8-year record from Cape Naturaliste); a 6-year record from Cape de Couedic in the central portion of the margin; and a record spanning 20 years from Cape Sorell, at the southeastern end of the margin. Cape Sorell exhibits the steepest waves, largest mean significant wave heights, and most westerly wave directions, indicating increased proximity to Southern Ocean wave generating storms. Rottnest exhibits the least steep, smallest mean significant wave heights, and most southerly wave directions, reflecting the greater distance from the Southern Ocean generating storms. All sites demonstrate highly energetic extreme events, particularly at Cape Sorell, and a predominance of swell over wind sea states. Spatial coherence between simultaneous buoy records show that wave events are easily tracked in the wave record as they propagate eastwards along the Southern Australian margin. Maximum correlation between records indicates that wave events at Cape Sorell lag those at Rottnest by approximately 80 h, and at Cape de Couedic by approximately 7 h. The speed of propagation of events between records is therefore consistent with the propagation speeds of extra-tropical cyclones along the Southern Australian margin. Synoptic typing of six-hourly mean sea level pressure (MSLP) patterns obtained from the NCEP–NCAR reanalysis has been carried out to determine the main synoptic types associated with large wave events at the Cape Sorell waverider buoy site. Significant differences occur in wave characteristics between synoptic types. At Cape Sorell, wave event magnitude shows a significant correlation to the intensity (as measured by the depth) of the forcing cyclone, with larger wave events occurring during more intense storms in the Southern Ocean. At the other buoy sites, the magnitude of wave events shows little correlation to the intensity of the storm, but is more closely related to the location of the storm, with larger events occurring when storms are closer to the Australian continent. These relationships are of importance for determining how the wave climate might change in this region (and how Australia's coastline might respond) in response to changes in atmospheric forcing which are currently being observed.

Adsorption of Water Vapor by Bare Soil in an Olive Grove in Southern Spain

Abstract Data for water vapor adsorption and evaporation are presented for a bare soil (sandy loam, clay content 15%) in a southern Spanish olive grove. Water losses and gains were measured using eight high-precision minilysimeters, placed around an olive tree, which had been irrigated until the soil reached field capacity (∼0.22 m3 m−3). They were subsequently left to dry for 10 days. A pair of lysimeters was situated at each of the main points of the compass (N, E, S, W), at a distance of 1 m (the inner set of lysimeters; ILS) and 2 m (the outer set of lysimeters; OLS), respectively, from the tree trunk. Distinct periods of moisture loss (evaporation) and moisture gain (vapor adsorption) could be distinguished for each day. Vapor adsorption often started just after noon and generally lasted until the (early) evening. Values of up to 0.7 mm of adsorbed water per day were measured. Adsorption was generally largest for the OLS (up to 100% more on a daily basis), and increased during the dry down. This was mainly the result of lower OLS surface soil moisture contents (period-average absolute difference ∼0.005 m3 m−3), as illustrated using various analyses employing a set of micrometeorological equations describing the exchange of water vapor between bare soil and the atmosphere. These analyses also showed that the amount of water vapor adsorbed by soils is very sensitive to changes in atmospheric forcing and surface variables. The use of empirical equations to estimate vapor adsorption is therefore not recommended.

Propagation of wind and buoyancy forced density anomalies in the North Pacific: Dependence on ocean model resolution

The response of the thermocline to changes in atmospheric forcing are explored in two 50-year hindcast North Pacific model runs. The model runs only differ in their resolution and horizontal viscosity values. The thermocline response is explored through a modal decomposition. The first baroclinic mode response is qualitatively similar in both model runs, with a somewhat smaller response in the lower resolution model. This mode is primarily wind driven. The second baroclinic mode shows a larger response at midlatitudes in the low-resolution model than in the higher resolution model. This is consistent with the presence of very large-scale baroclinic instability in the return flow of the subtropical gyre at low-resolution, and represents a spurious response of the ocean model to large horizontal viscosity. This spurious mode of decadal variability in the thermocline is maintained even when there is variability in atmospheric forcing. This result suggests that care must be taken in interpretation of the realism of mid-latitude modes of variability centered in western boundary current extensions of coupled ocean–atmosphere models.

Numerical studies on the dynamics of the Northwestern Black Sea shelf

The Northwestern Black Sea shelf dynamics are studied with numerical simulations based on the Princeton Ocean Model. The study focus is on buoyancy and wind driven flows and on the transport and fate of low salinity waters that are introduced through riverine sources (the Danube, Dnestr and Dnepr Rivers), under the seasonal changes in atmospheric forcing. The study is part of the DANUBS project (NUtrient management in the DAnube basin and its impact on the Black Sea). The numerical simulations show that the coastal circulation is greatly influenced by river runoff and especially that of the Danube, which is dominant with monthly averaged values ranging from 5,000 m3 to 10,000 m3. The transport of low-salinity waters associated with the Danube runoff is greatly influenced by wind stress, topographic effects and basin-scale circulation patterns, such as changes in the position of the Rim Current.

The marine copepod, Pseudocalanus elongatus, as a mediator between climate variability and fisheries in the Central Baltic Sea

Abstract Pseudocalanus elongatus is a key species in the pelagic zone of the deep basins of the Central Baltic Sea. The copepod serves as a major food organism for larval as well as for adult, pelagic planktivorous fish. Large interannual fluctuations in the standing stock of P. elongatus have been attributed to significant changes in the hydrographic environment over the last two decades. In particular, the decreasing salinity in the Baltic deep basins, a result of a change in atmospheric forcing leading to an increase in rainfall since the 1980s and of a lack of pulses of saline water intrusions from the North Sea, was found to affect reproduction and maturation of the copepod. In parallel, dramatic changes in the weight‐at‐age of herring, one of the most important commercial fishes of the Baltic Sea, have been observed since the late 1980s. Using time‐series on herring stomach contents, as well as length and weight, we provide evidence for a chain of events relating variability in climate, salinity and P. elongatus abundance to changes in diet and condition of herring in the Central Baltic Sea.

An investigation of the sensitivity of a land surface model to climate change using a reduced form model

In an illustration of a model evaluation methodology, a multivariate reduced form model is developed to evaluate the sensitivity of a land surface model to changes in atmospheric forcing. The reduced form model is constructed in terms of a set of ten integrative response metrics, including the timing of spring snow melt, sensible and latent heat fluxes in summer, and soil temperature. The responses are evaluated as a function of a selected set of six atmospheric forcing perturbations which are varied simultaneously, and hence each may be thought of as a six-dimensional response surface. The sensitivities of the land surface model are interdependent and in some cases illustrate a physically plausible feedback process. The important predictors of land surface response in a changing climate are the atmospheric temperature and downwelling longwave radiation. Scenarios characterized by warming and drying produce a large relative response compared to warm, moist scenarios. The insensitivity of the model to increases in precipitation and atmospheric humidity is expected to change in applications to coupled models, since these parameters are also strongly implicated, through the representation of clouds, in the simulation of both longwave and shortwave radiation.

Mesoscale temporal changes to foredunes at Inch Spit, south-west Ireland

The history of foredune development and loss is examined on a decadal-scale basis at Inch Spit at the head of Dingle Bay. Inch currently has a foreland area (2 km(2)) with four low foredune ridges fronting the southern section (2.5 km) of the barrier. Analysis of maps (1842 & 1898) airphotographs (1949, 1967 gr. 1973) and field surveys (1993/94) of the shoreline has supported an interpretation of two to three possible sequences of sudden erosion of the foredunes and subsequent redeposition and reworking of foredunes over periods of 30 to 50 years. The erosion and deposition of the foredunes have been analysed in terms of variation in the following; human activity, sea-level, storm activity and longterm (annual to decadal) atmospheric forcing changes, but none of these factors individually account for foredune changes. It is argued on circumstantial evidence that the role of extreme storms and associated surge is the most probable key to understanding sudden foredune disappearance. Wave refraction modelling of extreme events has helped to specify the principal control on foredune loss. The result of specific microscale triggers (days) leading to mesoscale changes (decadal) is examined.

Meltwater production in Antarctic blue-ice areas: sensitivity to changes in atmospheric forcing

In the near coastal regions of Dronning Maud Land, Antarctica, below-surface ice-melt in blue-ice areas has been observed. The low scattering coefficients of the large-grained blue-ice allow penetration of solar radiation, thus providing an energy source below the ice surface. The sub-surface meltwater is significant enough to show up on remote-sensing imagery in the form of ice-covered lakes. Adjacent snow-accumulation areas have much higher scattering coefficients and consequently limit solar radiation penetration in these regions. These snow and ice surfaces are generally below freezing, and little surface melting occurs. To assess the response of these melt features to changes in atmospheric forcings such as cloudiness, air temperature, and snow accumulation, a physically-based model of the coupled atmosphere, radiation, snow, and blue-ice system has been developed. The model consists of a heat transfer equation with a spectrally-dependent solar-radiation source term. The penetration of radiation into the snow and blue-ice depends on the surface albedo, and the snow and blue-ice grain size and density. Model simulations show that ice melt occurring in this area is sensitive to potential variations in atmospheric forcing. Under certain conditions more traditional surface melting occurs and, under other conditions, the existing melt processes can be shut down completely. In light of the sensitivity of this system to variations in atmospheric forcing, and the ability to view melt-related features using remote sensing, a tool exists to efficiently monitor variations in Antarctic coastal climate.

Meltwater production in Antarctic blue-ice areas: sensitivity to changes in atmospheric forcing

Arctic snow cover is important to life on Earth from the microscale soil microarthropod population, to reindeer at the local scale and even the global scale through its impact on the global climate. There is a consensus that global warming will be enhanced towards the Arctic. This will influence the hydrology and snow cover in these regions, which in turn will provide a feedback to climate. There is still a lack of knowledge regarding the impacts to polar hydrology and snow cover; the gaps are even larger with respect to feedback mechanisms.The objective of this study has been to improve the understanding and description of the dynamic processes of an Arctic snow cover. Here, the Arctic climate is studied from the first snow fall to the end of ablation in a series of nine publications grouped into three topics: Snow Distribution, Snowmelt and Energy Balance and Measurement Methods. The research is based on measurements and observations of climate, snow properties and snow distribution during the period 1992 to 2000 on the tundra in the vicinity of Ny-Alesund at 78°55'N, 11°56'E at Svalbard, Norway.Improvement of existing snowmelt models has been achieved by the implementation of energy balance calculations and improved description of the snow cover. Ground Penetrating Radar systems as a tool for snow surveying have been improved and used to measure and describe snow distributions over large areas. A snow drift model (SnowTran-3D) has been successfully tested for different scales and topography and was improved to better handle settling of the snow cover due to aging and melting.

Long‐term fluctuations in the statistical properties of low‐frequency variability: Dynamical origin and predictability

AbstractThe stability and predictability of the statistical properties of low‐frequency variability are investigated by studying the dynamical origin of long‐term changes arising, respectively, from the nonlinear response to variations in atmospheric forcing and from chaotic fluctuations in the energy partition between different variability patterns. Results from long integrations of a realistic quasi‐geostrophic (QG) model are analysed, and interpreted on the basis of the behaviour of simple nonlinear dynamical systems. It is shown that chaotic energy fluctuations not only modify the variance explained by different variability patterns, but also may either obscure or enhance the multimodality in the distribution of model states in periods as long as several decades. On the other hand, it is argued that significant variations in atmospheric statistics may be induced by anomalies in the atmospheric forcing originated from the lower boundary. QG simulations show that a strengthening of the planetary‐wave forcing due to the asymmetry in the divergent circulation between the western and eastern Pacific increases low‐frequency variability in the extratropical Pacific, and may turn the distribution of model states from unimodal to bimodal. Anomalies in low‐frequency variance related to changes in atmospheric forcing seem to be larger than those originated by internal dynamics, at least in certain periods and regions, suggesting a potential predictability of these statistics by coupled ocean‐atmosphere models.

Sensitivity of lake freeze-up and break-up to climate change: a physically based modeling study

To assess the response of lake Freeze-up and break-up dates to changes in atmospheric forcing, a physically based computational model of the coupled lake, lake-ice, snow and atmosphere system has been developed. Model performance is validated using meteorological and lake-ice observations from Great Slave Lake in northern Canada (1991/92) and St Mary Lake in Glacier National Park, Montana, (1992/93). Model integrations with modified atmospheric forcing indicate that air-temperature changes of ±4°C can delay or speed up the freeze-up and break-up dates by as much as 4 weeks for St Mary Lake, and 2 weeks for Great Slave Lake. For both lakes, break-up date is more sensitive to air-temperature changes than is freeze-up. Changes of ±3/10 cloud-cover fraction produce a shifting of break-tip dates by 1 week. Changes in wind speeds of ± 3 m s−1 modify the maximum ice depth of the lakes by 5–10 cm. For Great Slave Lake, lower wind speeds produced a surface temperature low enough to delay the onset of break-up by 2 weeks.

Sensitivity of lake freeze-up and break-up to climate change: a physically based modeling study

To assess the response of lake Freeze-up and break-up dates to changes in atmospheric forcing, a physically based computational model of the coupled lake, lake-ice, snow and atmosphere system has been developed. Model performance is validated using meteorological and lake-ice observations from Great Slave Lake in northern Canada (1991/92) and St Mary Lake in Glacier National Park, Montana, (1992/93). Model integrations with modified atmospheric forcing indicate that air-temperature changes of ±4°C can delay or speed up the freeze-up and break-up dates by as much as 4 weeks for St Mary Lake, and 2 weeks for Great Slave Lake. For both lakes, break-up date is more sensitive to air-temperature changes than is freeze-up. Changes of ±3/10 cloud-cover fraction produce a shifting of break-tip dates by 1 week. Changes in wind speeds of ± 3 m s−1 modify the maximum ice depth of the lakes by 5–10 cm. For Great Slave Lake, lower wind speeds produced a surface temperature low enough to delay the onset of break-up by 2 weeks.

Diagnosing Climate Change and Ocean Ventilation Using Hydrographic Data

Changes in atmospheric forcing can affect the subsurface water column of the ocean by three different mechanisms. First, warmed mixed-layer water that is subducted into the ocean interior will cause subsurface warming; second, the subducted surface water can be freshened through changes in evaporation and precipitation; and third, the properties at a given depth may be changed by the vertical displacement of isotherms and isohalines without changes of water masses. These vertical displacements of the water column can be caused either by changes in the rates of renewal of water masses or by dynamical changes (such as changes in wind stress). A method for analysing the subsurface temporal changes in hydrographic data is described in terms of these three processes: “pure warming,” “pure freshening,” and “pure heave.” Linear relations are derived for the relative strength of each process in terms of the observed changes of potential temperature and salinity in two different coordinate frames: (i) constant density surfaces, and (ii) isobaric surfaces. Inverse methods are applied to three realizations of the SCORPIO section at 43°S in the Tasman Sea. These sections were obtained in 1967, and in the austral winter and summer of 1989 and 1990, respectively. This data is used to explore the relative strengths of surface warming, surface freshening, and heave of the water column. The six-month differences for this region show small changes in Sub-Antarctic mode water (SAMW) and are not characterized by any one process, whereas below the mode waters the observed differences are well described by the heave process. In contrast, the 23-year differences show significant changes in the properties of the water that flows into the Tasman Sea: SAMW (300-700 db) is well described by pure warming of near- surface waters, while the changes observed at the depth of the salinity minimum are consistent with pure freshening. The observed changes in the interior of the ocean between adjacent seasons do not exhibit significant changes of water masses, consistent with the distance of this section from the outcrop region of the density surfaces of interest. For the 23-year differences, changed surface waters subducted into the ocean interior have sufficient time to influence the temperature-salinity correlations. The skill of our approach in discriminating between short-term changes (almost exclusively heave) and long-term changes associated with the subduction of changed surface waters is particularly encouraging. Although the observed changes could equally well be natural variability, they are qualitatively consistent with coupled numerical models of climate change in which surface waters are warmed and increased precipitation occurs south of the Sub-Antarctic Front.

Arctic ambient noise and climate change

In global warming, sea ice plays a significant role, and sea ice conditions may provide one of the most sensitive parameters to monitor. Modeling predicts that warming will be greatest at high latitude, and sea ice properties are very sensitive to warming and changes in atmospheric forcing. Therefore, sea ice might provide a sensitive marker for climate change. The mechanical properties of sea ice are related to the ice temperature, salinity, thickness, concentration, and flow size distribution. It has been established that the ambient noise field under pack ice is correlated to the mechanical properties, ice conditions, and forcing. Many studies over the last two decades have delineated some of these relationships. The hypothesis of this paper is that monitoring the ambient noise under the ice, for which there is well‐developed and reliable technology, may be an excellent way to detect a climate change signal. This paper will review the relationships between under ice ambient noise and the properties of the ice pack. Based on what is known, a monitoring system consisting of remotely operated buoys is proposed. A proposed buoy design and sampling array for the Arctic is presented. Used in conjunction with satellite remote sensing, the system could provide for continuous monitoring of the ice pack for other purposes and would be cost effective for acoustic monitoring of the climate change signal in the Arctic.

Seasonal variations of sea ice motion in the Canada Basin and their implications

Ice velocities and sea level pressure data from Arctic drifting buoys for 1979‐85 are used to examine the seasonal variation of sea ice drift in the Canada Basin. A recurring reversal of the mean anticyclonic gyre is demonstrated in late summer, in response to changes in atmospheric forcing. The identification of this reversal of ice motion is significant for numerical modeling of Arctic atmosphere‐ocean‐ice interactions.