Dry Anomalies Research Articles

Intraseasonal variations of water vapor and cirrus clouds in the tropical upper troposphere

Space‐time variations of tropical upper tropospheric water vapor and cirrus clouds associated with the intraseasonal oscillation (ISO) are investigated using data from the Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS). Composite moisture and meteorological fields based on five ISO events selected in two boreal winters (1991–1993) are analyzed using 20–80 day band‐pass‐filtered data. At 215 and 146 hPa, wet anomalies with frequent appearance of cirrus clouds exist over the convective system and move eastward from the Indian Ocean to the central Pacific, suggesting a direct effect of convective activity up to this level. At 100 hPa, however, the moisture field seems to be indirectly affected by convective activity through the dynamical response to the convective heating. Dry anomalies are observed over the Indian Ocean around the developing stage and over the eastern Pacific around the mature‐to‐decaying stage of the ISO. Cirrus clouds are frequently found over the cold region located to the east of the convective system. These structures around the tropopause level are closely related to the eastward moving Kelvin and Rossby wave responses to the convective heating with the equatorial cold anomaly and with the subtropical anticyclonic gyres. Between the two gyres the easterly wind blowing through the equatorial cold region may cause dehydration through cirrus formation when the convective system develops over the Indian Ocean and the western Pacific. As the northern gyre intensifies, tropical dry air is transported to the subtropical Pacific and eventually to the equatorial eastern Pacific. It is suggested that the temperature and flow variations due to the coupled Kelvin‐Rossby wave structure play an important role in dehydrating air in the tropical and subtropical tropopause region.

State of the Climate in 2003

Abstract The earth's climate was influenced by a moderate El Nino in the tropical Pacific Ocean at the beginning of 2003. This ENSO warm event developed during October–November of 2002, and eventually dissipated during March–April 2003, giving way to near-neutral ENSO conditions for the remainder of the year. Despite the cessation of El Nino during the boreal spring, the ENSO warm event affected regional precipitation anomalies over a broad area of the Pacific basin, including wet anomalies along the west coast of South America, and dry anomalies in eastern Australia, the southwest Pacific, and Hawaii. The global mean surface temperature in 2003 was within the highest three annual values observed during the period of regular instrumental records (beginning in approximately 1880), but below the 1998 record-high value. Global surface temperatures in 2003 were 0.46°C (0.83°F) above the 1961–90 mean, according to one U.K. record, which ranked as third highest in this archive. In the U.S. temperature archive, ...

How do La Ni�a events disturb the summer monsoon system in Brazil?

The rainy season in most of Brazil is associated with the summer monsoon regime in South America. The quality of this season is important because it rains little during the rest of the year over most of the country. In this study, the influence of La Nina events on the summer monsoon circulation, rainfall and temperature is analyzed with seasonal and monthly resolution, using data from a dense network of stations, giving a comprehensive view of the impact of these events. The expected precipitation percentiles during the monsoon season of La Nina events are calculated, as well as anomalies of surface temperature and thermodynamic parameters. This information is analyzed jointly with anomaly composites of several circulation parameters. The analysis shows that some anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. There are abrupt changes of anomalies within the summer monsoon season, suggesting the prevalence of regional processes over remote influences during part of the season. In spring there are positive precipitation anomalies in north and central-east Brazil and negative ones in south Brazil. These precipitation anomalies are favored by the perturbation in the Walker and Hadley circulation over the eastern Pacific and South America, and by perturbations in the rotational circulation over southern South America. Northerly moisture inflow from the Atlantic into northern South America is emphasized and diverted towards the mouth of the Amazon by the low-level cyclonic anomaly north of the equator. In December and January, probably triggered by anomalous surface cooling during the spring, there is an anomalous low-level divergence and an anticyclonic anomaly over southeast Brazil. This anomalous circulation directs moisture flux towards south Brazil, causing moisture convergence in part of this region and part of central-west Brazil. The thermodynamic structure in central-east Brazil does not favor precipitation over this region, and the wet anomalies in north Brazil are displaced northward. The dry anomalies in south Brazil almost disappear and even turn positive. In February, after the strongly below normal precipitation of January, the surface temperature anomalies turn positive over southeast Brazil. The low-level anticyclonic anomaly is much weaker than in January. There are positive rainfall anomalies in north Brazil and in the South Atlantic Convergence Zone, and negative ones return to south Brazil.

Significant Events of Interhemispheric Atmospheric Mass Exchange: Composite Structure and Evolution

Abstract The various modes of atmospheric mass redistribution characterize the principal variations of the general circulation of the atmosphere. Interhemispheric exchanges of atmospheric mass occur with considerable regularity on subseasonal time scales. Observational evidence from previous studies indicates that anomalous and persistent regional atmospheric mass distributions (e.g., atmospheric blocking) may often be related to interhemispheric atmospheric mass exchange. Using the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis surface pressure, significant events when the Northern Hemisphere (NH) loses dry atmospheric mass on subseasonal time scales during the boreal winter from 1968 to 1997 are identified. A total of 25 events is found, with a preferred time scale of 9 days from the time of maximum to minimum NH dry atmospheric mass. The linear correlation coefficient between the dry atmospheric mass anomalies for the NH and Southern Hemi...

Asymmetry and non-linearity in upper tropospheric humidity variability

[1] Changes in upper tropospheric humidity (UTH) are important in understanding the climatic response to increased anthropogenic forcing. This paper explores the asymmetry and non-linearity in the response of subtropical UTH to tropical intraseasonal forcing. During northern winter, subtropical UTH anomalies, varying on intraseasonal timescales, propagate eastwards from northern Africa to the central Pacific. There is a discontinuity in the propagation of moist anomalies over the Tibetan Plateau, whilst dry anomalies show reduced propagation in the lee. This is consistent with the expected evolution of cyclones and anticyclones incident on high topography. Correlations of five-day mean UTH show no significant linear correlation between moist anomalies over Indonesia and subtropical anomalies to the north. This appears to be explained by the limitation of dry anomalies in the region of low ambient vorticity south of the East Asian Jet Stream.

A feedback mechanism between soil‐moisture distribution and storm tracks

AbstractHow the distribution of soil moisture impacts the location and persistence of the storm track and the resulting distribution of precipitation is investigated. Particular attention is given to North America where an extensive series of numerical experiments using a regional climate model are performed to investigate these issues. The findings suggest that soil‐moisture distribution has a pronounced impact on the large‐scale dynamics via a meridional displacement of the storm track. The displacement tends to enhance the local soil moisture–rainfall feedback over the region of the anomaly. The initial feedback is induced by the local effects of soil moisture on the boundary‐layer energy budget and then propagated to the large‐scale through modifications to the geopotential heights. Because of soil moisture's impact on the large‐scale dynamics, a local soil‐moisture anomaly can induce both flood‐ and drought‐like conditions in surrounding regions. Experiments with a dry anomaly applied either over the upper Midwest or Great Plains display a northward shift in the storm track. This shift tends to result in drought‐like conditions in the region of the anomaly and both flood‐ and drought‐like conditions in surrounding regions. Anomalous wetting in the Southwest impacts the distribution of precipitation not only locally, but also over most of the United States; drought‐like conditions are simulated to the north and east of the anomaly region while flood‐like conditions are simulated locally and eastward. Overall, the impacts of soil‐moisture distribution on the large‐scale dynamics and the location and intensity of the storm track play a significant role in determining summer rainfall distribution. Soil‐moisture anomalies over relatively small regions can induce floods and droughts not only locally, but also over remote regions. Copyright © 2003 Royal Meteorological Society.

The El Niño Impact on the Summer Monsoon in Brazil: Regional Processes versus Remote Influences

The El Nino impact on Brazil's summer monsoon has not been adequately assessed through seasonal analysis because it shows significant subseasonal variations. In this study, the El Nino influence on the summer monsoon circulation, rainfall, and temperature is analyzed with monthly resolution, using data from a dense network of stations. The expected precipitation percentiles during the monsoon season of El Nino (EN) events are calculated, as well as anomalies of surface temperature and thermodynamic parameters. This information is analyzed jointly with anomaly composites of several circulation parameters. The analysis shows that some precipitation and circulation anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. There are abrupt changes of anomalies within the summer monsoon season, suggesting the prevalence of regional processes over remote influences during part of the season. The probable role of remote influences and regional processes is assessed. The anomalous heat sources associated with El Nino perturb the Walker and Hadley circulations over South America and generate Rossby wave trains that produce important effects in the subtropics and extratropics. In the early summer monsoon season, remotely produced atmospheric perturbations prevail over Brazil. Anticyclonic low-level anomalies predominate over central-east Brazil, in the Tropics and subtropics, due to the subsidence over the Amazon and to Rossby waves in the subtropics. Easterly moisture inflow from the Atlantic is favored, but diverted toward northern South America (SA) and south Brazil. There are negative precipitation anomalies in north and central-east Brazil and positive ones in south Brazil. These precipitation anomalies are favored by the perturbation in the Walker and Hadley circulation over the east Pacific and South America, and by a Rossby wave train over southern SA that originates in the eastern Pacific. In January, with the enhancement of the continental subtropical heat low by anomalous surface heating during the spring, there is anomalous low-level convergence and cyclonic circulation over southeast Brazil, while at the upper levels anomalies of divergence and anticyclonic circulation prevail. This anomalous circulation directs moisture flux toward central-east Brazil, causing moisture convergence in this region. A favorable thermodynamic structure enhances precipitation over central-east Brazil, the dry anomalies in north Brazil are displaced northward, and the anomalies in south Brazil almost disappear. In February, after the above-normal precipitation of January, the surface temperature anomalies turn negative and the precipitation diminishes in central-east Brazil. There are negative rainfall anomalies in north Brazil and in the South Atlantic convergence zone (SACZ) and positive ones in south Brazil. Influence function analysis shows that while the anomalies of circulation over southeast Brazil in the spring of El Nino years are mostly due to remote influences from the tropical east Pacific, those in January are probably due to local influence. During this month the monsoonlike circulation is enhanced. Simultaneous and lagged correlation analysis of SST and rainfall in central-east Brazil shows that SST anomalies in the Atlantic Ocean off the southeastern coast of Brazil fluctuate on the same timescale as the circulation and precipitation anomalies.

A Southward Migration of Centennial-Scale Variations of Drought/Flood in Eastern China and the Western United States

Abstract Several studies of the established warm season climate records for eastern China (1470–1997) showed alternating dry and wet periods at centennial scales. The spatial patterns show that when a dry condition or drought was observed in southern China, a wet or flood situation was found in the northern part of eastern China and vice versa. These patterns suggest a meridional variation of the centennial-scale wet/dry anomalies. This study analyzed the same data and showed that the dry and wet anomalies initially appeared in the northern part of eastern China and then migrated southward to affect the low latitudes. An extension of this analysis to the United States revealed a similar southward migration of dry/wet anomalies that first developed in the high latitudes in the western part of the country. The average speed of the migrations in both areas is about 3.0° of latitude per 10 years. The results suggest that mechanisms in mid- and high latitudes may play critical roles in the development of droug...

Climate Variability in Southern South America Associated with El Niño and La Niña Events

A comprehensive view is given of the precipitation and circulation anomalies associated with the various stages of El Nino (EN) and La Nina (LN) events all over southern South America (SSA). This view comprises the delineation of coherent regions with respect to precipitation anomalies, the identification of the seasons of maximum anomalies, the indication of their magnitude, and the assessment of their consistency during those events. In addition, the spatial and temporal variability of these anomalies is detailed by calculating the expected precipitation percentiles and the consistency of wet and dry anomalies for each station and each three-month running season during EN and LN events. Composites of circulation anomalies and an assessment of their consistency are also presented and their connection with the precipitation anomalies is discussed. Southern Brazil presents the strongest average signal in EN events. The general behavior toward opposite signals in the precipitation and circulation anomalies over SSA during almost the same periods of the EN and LN events indicates a large degree of linearity in the response to these events. The timing of the anomalies changes throughout SSA, leading to the identification of eight different coherent regions in the EN case and six in the LN case. This regionalization is mostly caused by different processes leading to precipitation anomalies in SSA during those events. All these regions show a significant response in some part of each event. The magnitude and consistency of this response show a large spatial variability and some areas present very strong and consistent anomalies sometimes not disclosed when large coherent regions are analyzed. In spite of the differences in timing, some features of the precipitation anomalies are rather uniform throughout the region during EN and LN events. In EN episode, there is a tendency to lower than median precipitation in the year before the event, which continues until March of the year of the event. In a vast region, east of the Andes, the strongest positive precipitation anomalies occur in spring of this year, when the circulation anomalies concur to enhance rainfall over several regions. During the summer of the mature stage the positive precipitation anomalies almost disappear and then reappear in some regions in late summer-early autumn and in winter of the year following the starting year of the event. This description holds partially for the LN event, but with opposite signs, although there is a larger spatial variability in the LN-related anomalies in the following year and some shifts in timing. As for precipitation, the symmetry of the geopotential height anomaly fields with opposite signs between LN and EN cases is also remarkable, especially during the year (0).

Exploring the correlation between Southern Africa NDVI and Pacific sea surface temperatures: Results for the 1998 maize growing season

Several studies have identified statistically significant correlations between Pacific sea surface temperature anomalies and NDVI anomalies in Southern Africa. The potential predictive value of the relationship was explored for the 1998 maize growing season. Cross-validation techniques suggested a more useful relationship for regions of wet anomaly than for regions of dry anomaly. Observed 1998 NDVI anomaly patterns were consistent with this result. Wet anomalies were observed as expected, but wide areas of expected dry anomalies exhibited average or above-average greeness.

Precipitation Anomalies in Southern Brazil Associated with El Niño and La Niña Events

The impact of El Niño and La Niña events (warm and cold phases of the Southern Oscillation) on rainfall over southern Brazil is investigated through the use of a large dataset of monthly precipitation from 250 stations. This region is partly dominated by rough orography and presents different climatic regimes of rainfall. As previous global studies on Southern Oscillation–precipitation relationships used data from only two stations in southern Brazil, this region was not included in the area of consistent Southern Oscillation–related precipitation in southeastern South America. The present analysis is based on the method by Ropelewski and Halpert, the sensitivity of which is assessed for this region. The spatial structure of the rainfall anomalies associated with warm (cold) events is analyzed and subregions with coherent anomalies are determined. Their distribution indicates the influence of relief, latitude, and proximity to the ocean. These areas are subjected to further analysis to determine the seasons of largest anomalies and assess their consistency during warm (cold) events. The whole of southern Brazil was found to have strong and consistent precipitation anomalies associated with those events. Their magnitude is even larger than in Argentina and Uruguay. All of the subregions have consistent wet anomalies during the austral spring of the warm event year, with a pronounced peak in November. The southeastern part also shows a consistent tendency to higher than average rainfall during the austral winter of the following year. There is also a consistent tendency to dryness in the year before a warm event. During the spring of cold event years strong consistent dry anomalies prevail over the whole region, also with maximum magnitude in November. They are even stronger and more consistent than the wet anomalies in warm event years. Consistent anomalies do not occur over large areas in the years before and after cold events. The wet anomalies during the austral spring of the warm event year weaken and even reverse during the following January. The same tendency, though not so clear, is observable in the dry anomalies of cold events. The seasons of largest anomalies disclosed by this study differ from those found by previous global studies for other regions in southeastern South America. This study expands the area of consistent warm (cold) event-related precipitation defined by previous studies in southeastern South America by including a region of larger anomalies, and provides a spatial and temporal refinement to the warm (cold) event–precipitation relationship.

Asymmetric Recovery from Wet versus Dry Soil Moisture Anomalies

Abstract The energy and moisture state in the soil and new-surface atmosphere evolve due to fluxes that are themselves a function of these states. Because the soil moisture and temperature are negatively correlated (dry warm or cool moist), physical mechanisms that tend to restore each state individually (soil moisture control of evaporation and temperature dependence of saturation specific humidity, for example) act as anomaly enhancing (positive) feedback mechanisms for the other state. Model and observational studies in the literature indicate that dry hydrologic anomalies tend to persist longer than moist ones, suggesting that thee feedback mechanisms are stronger when the soil layer is dry. A conceptual land-atmosphere model, consisting of heat and moisture balance for a soil layer and a turbulently mixed atmospheric boundary layer, is subjected to stochastic forcing. The covariability structure evolves through the state-dependent turbulent and radiative fluxes in the land-atmosphere system and is no...

Investigating impacts of anomalous land‐surface conditions on Australian climate with an advanced land‐surface model coupled with the BMRC GCM

AbstractA new and complex land‐surface model, the bare essentials of surface transfer (BEST), has been incorporated into the Australian Bureau of Meteorology Research Centre (BMRC) atmospheric general circulation model (GCM). A series of experiments with anomalous land‐surface conditions using the BEST‐GCM were conducted.Three sets of conditions were used for the winter, two of which were related to the soil moisture conditions. The soil over the entire Australian continent was saturated (hereafter WET) and dried (hereafter DRY) on the first time step of 4 July, and then allowed to evolve normally. In another experiment, values of all the 15 BEST parameters over the continent were set to those at a point in the Great Desert and kept unaltered throughout the integration period (hereafter DESERT). Two additional WET and DRY experiments were carried out for the summer, from 6 January for 60 days.The results of these experiments differ significantly from those conducted with bucket‐type land‐surface parameterization schemes in that: WET or DRY anomalies evolve differently in specific regions. The geographical distributions of responses are more disorganized, possibly because of the inclusion of vegetation, which may enhance the regional features and the non‐linear treatment of soil moisture content and surface wetness. Magnitude of responses are larger because of the greater number of feedback mechanisms. For example, when the feedback of soil moisture and surface albedo is allowed, the increase of soil moisture leads to an increase of evaporation. The increase of soil moisture results in a decrease in albedo which in turn leads to an increase of absorbed solar radiation available for evaporation. Time‐scales of adjustment are different for DRY and WET anomalies. WET anomalies persist much longer, have stronger responses and are more organized than DRY anomalies. The summer WET or DRY anomalies have larger responses than the corresponding winter anomalies. These results are physically interpreted and their implication in seasonal precipitation forecasting is discussed.

Sudden Stratospheric Warming and Anomalous U.S. Weather

Severe distortion of tropospheric circulation is associated with major sudden stratospheric warming (SSW) events. This distortion consisting primarily of weakening of smaller-scale synoptic mats and development of strong blocking activity, is reviewed in the context of associated weather anomalies over the United States. Temperature, precipitation and cyclone anomalies of 17 January and February months experiencing SSW are contrasted with months of no stratospheric warming. The SSW is marked by intense cold, dry anomalies centered in the Ohio Valley, with below average temperatures covering the entire United States. In January, slight wet anomalies occur in the lee of the Rockies, and in February the desert southwest becomes quite wet. Cyclone activity decreases by 25% throughout the United States during SSW, most strongly along the West Coast and in association with the disappearance of the Colorado low. Storm track anomalies are documented over the eastern Pacific, the Gulf and Atlantic seaboard, and northeastern Canada. Slight variations of the anomaly patterns in February over the western United States are linked to the temporal evolution of SSW.

ANOMALOUS WET AND DRY SPELLS IN THE CENTRAL UNITED STATES, 1931–75

The Palmer Index (PI) was employed to compare anomalous dry spells with anomalous wet spells for 76 climatic divisions in the central United States. Comparisons were made in terms of the frequency, severity and persistence of wet and dry climatic episodes for the period 1931–75. The results indicated more frequent dry anomalies in the southwest (New Mexico and west Texas) portion of the study area and more frequent wet anomalies in the northeast (Illinois and Iowa). Not surprisingly, these are the most arid and most humid portions respectively. The results of the analysis for persistence were less clear. Typically dry anomalies lasted longer than wet anomalies, especially as the minimum length was increased. This was especially apparent when comparing the very longest dry spell with the very longest wet spell during the 45-year study period. Thus there are more but shorter wet spells and less frequent, but typically longer lasting dry spells. This final relationship posits the question of feedback process...

Simulation of the atmospheric response to soil moisture anomalies over Europe

Positive and negative anomalies in the initial soil moisture over central Europe were introduced in turn into integrations of a global model of the July atmosphere. The results show that by modifying the partitioning of the turbulent fluxes such anomalies can have major effects on the modelled rainfall, humidity and temperature during the following 50 days over the anomaly area and that the anomalies can propagate into adjacent land areas. Similar results were obtained with a lower-resolution version of the model. The effects on rainfall were already important on day 3 of the integration over the anomaly area and on day 6 over parts of Scandinavia outside the anomaly area. The experiment was repeated with a version of the model in which the prevailing flow over Europe was less weak with increased moist westerly flow from the Atlantic. In this case the dry anomaly persisted for only about 20 days before it became too weak to affect the evaporation significantly.

Paleo-environmental perspectives on the Sahel drought of 1968–73

A review of stream, dune and lake phenomena in several well-studied sectors of the Sahel indicates an informative record of repeated environmental change. Blowing sand encroached far into the now semi-arid savanna between approximately 20,000 and 12,500 years ago, which lakes had substatially greater volumes and rivers higher discharges between 9000 and 5000 years ago. But rapid change and high-amplitude variability have always been the rule. During the last 2500 years conditions fluctuated about the modern median, with strong short and medium-term oscillations. The Sahel drought after 1968 falls within the norm of at least 6 earlier, dry anomalies verfied since 1400 AD. Whether or not the coincidence of cumulative overgrazing and population pressure with severe drought has triggered a geomorphologic response that is unique in the record, can only be decided after much more systematic work has been carried out.

Simulation of the atmospheric response to soil moisture anomalies over Europe

AbstractPositive and negative anomalies in the initial soil moisture over central Europe were introduced in turn into integrations of a global model of the July atmosphere. The results show that by modifying the partitioning of the turbulent fluxes such anomalies can have major effects on the modelled rainfall, humidity and temperature during the following 50 days over the anomaly area and that the anomalies can propagate into adjacent land areas. Similar results were obtained with a lower‐resolution version of the model.The effects on rainfall were already important on day 3 of the integration over the anomaly area and on day 6 over parts of Scandinavia outside the anomaly area.The experiment was repeated with a version of the model in which the prevailing flow over Europe was less weak with increased moist westerly flow from the Atlantic. In this case the dry anomaly persisted for only about 20 days before it became too weak to affect the evaporation significantly.

Large-Scale Aspects of Deep Convective Activity over the GATE Area

Using digital IR data obtained by SMS I satellite, the large-scale behavior of convective activity was investigated over the tropical Atlantic within the GATE area. Spectral analysis has revealed the existence of two remarkable periodicities which show good association with the large-scale atmospheric field, namely, a 4-5 day mode and a diurnal mode. The area of the enhanced convection associated with the 4-5 day mode moves westward from Africa through the tropical Atlantic. Its mean phase speed turned out to be about 7° day-1 and the mean wavelength about 30° in longitude. The vertical structure of the corresponding disturbance, was investigated by the time-composite technique applied to upper air data over the GATE A/B area. The result shows that the enhancement of convection is associated with deep upward motion throughout the troposphere and southerly (northerly) winds in the lower (upper) troposphere. It also shows that the enhancement is accompanied by moistening in the cloud layer, while drying appears in the subcloud layer. As for the diurnal mode, composited variation shows the maximum convective activity in the late afternoon over the GATE A/B area. Horizontal distributions of the composited anomalies revealed that the late afternoon maxima have a zonal distribution roughly along 10°N rather than the confined one in the vicinity of West Africa. It also revealed the coexistence of the region which shows the early morning maximum. The latter takes place to the south of the former one over the middle Atlantic. Diurnal components of the large-scale kinematic and thermal fields were also examined over the A/B area. During the daytime, the vertical motion shows upward anomalies and the temperature shows distinct warm anomalies in the upper troposphere. As for the moisture variation, the atmosphere tends to show dry anomalies near local noon and moist anomalies near midnight.