Seasonal Atmospheric Circulation Research Articles

Atmosphere-land cover feedbacks alter the response of surface temperature to CO2 forcing in the western United States

In order to test the sensitivity of regional climate to regional-scale atmosphere-land cover feedbacks, we have employed a regional climate model asynchronously coupled to an equilibrium vegetation model, focusing on the western United States as a case study. CO2-induced atmosphere-land cover feedbacks resulted in statistically significant seasonal temperature changes of up to 3.5°C, with land cover change accounting for up to 60% of the total seasonal response to elevated atmospheric CO2 levels. In many areas, such as the Great Basin, albedo acted as the primary control on changes in surface temperature. Along the central coast of California, soil moisture effects magnified the temperature response in JJA and SON, with negative surface soil moisture anomalies accompanied by negative evaporation anomalies, decreasing latent heat flux and further increasing surface temperature. Additionally, negative temperature anomalies were calculated at high elevation in California and Oregon in DJF, MAM and SON, indicating that future warming of these sensitive areas could be mitigated by changes in vegetation distribution and an associated muting of winter snow-temperature feedbacks. Precipitation anomalies were almost universally not statistically significant, and very little change in mean seasonal atmospheric circulation occurred in response to atmosphere-land cover feedbacks. Further, the mean regional temperature sensitivity to regional-scale land cover feedbacks did not exceed the large-scale sensitivity calculated elsewhere, indicating that spatial heterogeneity does not introduce non-linearities in the response of regional climate to CO2-induced atmosphere-land cover feedbacks.

Influence of topography on monthly rainfall distribution over East Africa

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 28:199-212 (2005) - doi:10.3354/cr028199 Influence of topography on monthly rainfall distribution over East Africa P. Oettli*, P. Camberlin Centre de Recherches de Climatologie, FRE 2740 CNRS, Université de Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France *Email: oettlip@u-bourgogne.fr ABSTRACT: statistical models were defined for explaining the spatial distribution of rainfall in eastern Africa (southern Kenya and NE Tanzania) on the basis of various topographical descriptors. The procedure is first presented for the month of April, and subsequently month-to-month changes in the role of topography on rainfall distribution are discussed. Predictors included quantitative estimates of slope, mean and standard deviation of elevation, and principal components that describe the topographical environments around the meteorological stations. Different window sizes were considered in order to account for processes of different scales. Linear models were defined for each mean monthly rainfall field, and cross-validated. Inclusion of interpolated residuals yield explained variances in the range of 53 to 89%. Results were compared to a simple interpolation of gauge rainfall. The models are particularly useful in areas with few rain gauges. Selected predictors, as well as their scales, change seasonally. North–south exposure contrasts are the main factor of rainfall variation, except in the northern summer. East-facing stations are wetter in the short rains season (October to December), and drier in the monsoon season. These variations coincide with seasonal atmospheric circulation changes above the region. Slopes also contribute to rainfall variability. At the scale of the study mean elevation had little effect; other predictors, especially exposure, actually describe the interaction between rainfall and topography more adequately. KEY WORDS: Rainfall · Geostatistical modelling · Orography · Kenya · Tanzania Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 28, No. 3. Online publication date: May 24, 2005 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2005 Inter-Research.

The relationship of Pacific deep tropical convection to the winter and springtime extratropical atmospheric circulation of the South Pacific in El Niño events

During some El Niño events Rossby wave trains (RWT) are observed to strongly modulate the seasonal atmospheric circulation of the South Pacific extratropics in the austral winter and spring. Here it is shown that seasonally intensified deep tropical convection is confined to the Pacific Intertropical Convergence Zone (ITCZ) close to the dateline in those events with strong RWT modulation but not in other cases. In other cases deep convection either withdraws from the ITCZ or the ITCZ strongly interacts with the South Pacific Convergence Zone (SPCZ) lying at 5–10°S. In both cases this points to a weakening of the local tropical Hadley circulation that may be crucial for Rossby wave generation. It is also found that the expected RWT response in the high latitudes can fail to occur even when SST in the central tropical Pacific are very high.

An Initial Analysis of River Discharge and Rainfall in Coastal New South Wales, Australia Using Wavelet Transforms

In many coastal catchments of south eastern New South Wales, Australia, changes in river morphology are a response to human impact superimposed on spatial and temporal patterns of variability in precipitation and discharge. Understanding, and preferably quantifying, spatial and temporal patterns of hydrologic variability are essential to understanding natural changes, and to separate these from artificial changes in river systems. Prediction and management of water resources are also dependent upon this understanding. We assess the variability in precipitation and discharge using the wavelet transform which projects the time series of data into a three dimensional surface of frequency, amplitude and time. The analysis reveals that changes across time often reflect changes in individual seasons and may be linked to changes in particular seasonal atmospheric circulation systems. Strong perturbations in the analysis of one catchment are consistent with documented, geomorphically‐effective, flooding sequences. The characteristics of the series in the transformed data reveal interesting differences at certain times and scales which may be a reflection of changes in larger scale atmospheric processes.

The influence of the 1997–99 El Niňo Southern Oscillation on extratropical baroclinic life cycles over the eastern North Pacific

AbstractThe El Niňo Southern Oscillation (ENSO) strongly influences the interannual and seasonal atmospheric circulation over the North Pacific. The present study shows that the meridional displacement of the time‐mean upper‐level jet associated with ENSO modulates the time‐mean barotropic (meridional) wind shear over the central and eastern North Pacific storm track. Earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper‐level potential vorticity (PV) waves. The present study suggests that differences in the time‐mean flow associated with the 1997–99 ENSO cycle had a similar impact on tropopause PV structure, meridional eddy fluxes of momentum and temperature, and predictability.

95/01171 Splitter a major step forward in nonexplosive excavation

Remarkable angiosperm evolution and significant vegetation changes took place during the Cenozoic on earth. Specifically, Neogene is a crucial period in forming the framework of the current vegetation and in restructing earth ecosystem after the Eocene–Oligocene “greenhouse-icehouse” climate transition. Under the background of global changes, regional vegetation in China was synchronously developed. Some local changes had made extensive influences, such as the topographic changes, especially the uplift of the Qinghai-Tibet Plateau, and the changes in seasonal atmospheric circulation and precipitation, i.e., the initiation and evolvement of Asian monsoon system, along with other geographic changes in land and sea. The expansion of xeromorphic vegetation, represented by forest steppe, steppe and desert, played an ever-increasing role, which was closely related with the development of angiospermous xerophytes. The distributions of some representative angiospermous pollen types with their parent plants of xerophilous origination in the Cenozoic are summarized in the paper. Evidence shows that the origination and evolution of the angiospermous xerophytes underwent a series of developments, which were prompted largely by environmental variations. Pollen studies from some representative sites in China show that spatial and temporal development of Cenozoic xeromorphic vegetation is largely consistent with the global changes.

95/01172 Structural characterization of the organic polymers comprising a lignite's matrix and megafossils

Remarkable angiosperm evolution and significant vegetation changes took place during the Cenozoic on earth. Specifically, Neogene is a crucial period in forming the framework of the current vegetation and in restructing earth ecosystem after the Eocene–Oligocene “greenhouse-icehouse” climate transition. Under the background of global changes, regional vegetation in China was synchronously developed. Some local changes had made extensive influences, such as the topographic changes, especially the uplift of the Qinghai-Tibet Plateau, and the changes in seasonal atmospheric circulation and precipitation, i.e., the initiation and evolvement of Asian monsoon system, along with other geographic changes in land and sea. The expansion of xeromorphic vegetation, represented by forest steppe, steppe and desert, played an ever-increasing role, which was closely related with the development of angiospermous xerophytes. The distributions of some representative angiospermous pollen types with their parent plants of xerophilous origination in the Cenozoic are summarized in the paper. Evidence shows that the origination and evolution of the angiospermous xerophytes underwent a series of developments, which were prompted largely by environmental variations. Pollen studies from some representative sites in China show that spatial and temporal development of Cenozoic xeromorphic vegetation is largely consistent with the global changes.

LATE-QUATERNARY VEGETATION RESPONSE TO CLIMATIC-GLACIAL FORCING IN NORTH PACIFIC AMERICA

Late Quaternary pollen records of biogenic deposits of the American North Pacific coast show multiple responses of vegetation to forcing by climate and glaciers. Overall trends during the Holocene have been toward development of closed forest, but intervals are evident that were both progressive and retrogressive. Vegetation apparently responded in concert with fluctuating climatic and glacial episodes. Grass-sedge tundra, widespread initially in a relatively cold, late-glacial setting, was replaced by open arborial communities in the early Holocene during milder millennia of the thermal maximum. Climate did not support the development of structurally closed forest until the cool and humid late Holocene. With reactivation of glaciers during recent millennia, vegetation has been altered by a series of stadial and interstadial events. Current interglacial climate trends appear to involve a shifting pattern in the duration and intensity of seasonal atmospheric circulation centers over the North Pacific Ocean. Climatic-glacial forcing recorded in these studies is attributed to changes in solar radiation, among a number of other factors.