Subtropical High-pressure System Research Articles

Influência dos sistemas de altas e baixas pressões e da Temperatura da Superfície do Mar do Atlântico Sul no desenvolvimento de Sistemas Convectivos de Mesoescala sobre o Rio Grande do Sul

In this work the influences of the positioning of high and low pressures systems centers at South Atlantic Ocean and Sea Surface Temperature (SST) anomalies of Atlantic and Pacific Oceans are analysed in association to the occurrence of Mesoscale Convective Systems (MCS) in Rio Grande do Sul (RS) State. The selected period involves four years of February (2002-2005). It was utilized geostationary satellite imagery, values of Sea Surface Temperature (SST) anomalies of the Atlantic and Pacific Oceans and pluviometric precipitation data observed on the State. The horizontal fields of atmospheric pressure and streamslines were obtained from simulations made with the mesoscale model MM5. The results showed that for the analysed period there was one rainy February in the State and three months of drought. For the drought months, the number of observed MCS was inferior, up to six times, to the observed one in the rainy month. Still, in the dry months, the MCS presented shorter duration in relation to the ones observed in the rainy February. For all the analysed period, the MCS dissipation occurred to the east of its position of formation, in the majority of the cases. Although the differences found between all the analysed MCS, the preferential time in which they formed, 12 and 18 UTC, predominated in all period. The large scale and mesoscale circulation patterns had shown that in the rainy month, the centers of subtropical high pressure systems located on the Atlantic Ocean were positioned near the SSA coast, where the SST anomalies values were positive. The opposite were observed during the drought months.

Onset of southwesterly wind over eastern China and associated atmospheric circulation and rainfall

Using the 5-day averaged data from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, and precipitation from rain gauge stations in China for the period 1981–2000, we investigated seasonal variations and associated atmospheric circulation and precipitation of the southwesterly wind over eastern China. The southwesterly wind over eastern China begins earliest over southeastern China and strengthens gradually from spring to the early summer, as it extends northward. The strengthening of the spring southwesterly wind, the tropospheric upward motion, and the convergence of low-level water vapor over southeastern China results in the beginning of the local rainy season. The beginning of the Mei-yu (Plum rainfall) is connected with the northward march of the southwesterly wind. The southwesterly wind reaches the valley of the Yangtze River in the early summer and northern China in the middle summer. This signifies an onset of the large-scale southwesterly wind over eastern China. Accordingly, the rain belt over southeastern China moves to the valley of the Yangtze River in the early summer and to northern China in the middle summer. Moreover, the southerly wind extends southward to the South China Sea from the spring to summer, though it does not stretch from the South China Sea to southeastern China at those times. The strengthening of the southerly wind over southeastern China is associated with a weakening/strengthening of the eastward/westward subtropical tropospheric temperature gradient between southwestern China and the western North Pacific. The developments of a low-pressure system over southwestern China and the subtropical high-pressure system over the western North Pacific may contribute to the strengthening of the southwesterly wind. A northward advance of the high-pressure system favors the southwesterly wind stretching from southeastern China to northern China. The onset of the Indian summer monsoon also strengthens the summer southwesterly wind over eastern China.

Sediment geochemical records of productivity and oxygen depletion along the margin of western North America during the past 60,000 years: teleconnections with Greenland Ice and the Cariaco Basin

Many sediment records from the margins of the Californias (Alta and Baja) collected in water depths between 60 and 1200 m contain anoxic intervals (laminated sediments) that can be correlated with interstadial intervals as defined by the oxygen-isotope composition of Greenland ice (Dansgaard–Oeschger, D–O, cycles). These intervals include all or parts of Oxygen Isotope Stage 3 (OIS3; 60–24 cal ka), the Bölling/Alleröd warm interval (B/A; 15–13 cal ka), and the Holocene. This study uses organic carbon (Corg) and trace-element proxies for anoxia and productivity, namely elevated concentrations and accumulation rates of molybdenum and cadmium, in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom-water oxygen (BWO) conditions on open, highly productive continental margins. The main conclusion from these proxies is that during the last glacial interval (LGI; 24–15 cal ka) and the Younger Dryas cold interval (YD; 13–11.6 cal ka) productivity was lower and BWO levels were higher than during OIS3, the B/A, and the Holocene on all margins of the Californias. The Corg and trace-element profiles in the LGI–B/A–Holocene transition in the Cariaco Basin on the margin of northern Venezuela are remarkably similar to those in the transition on the northern California margin. Correlation between D–O cycles in Greenland ice with gray-scale measurements in varved sediments in the Cariaco Basin also is well established. Synchronous climate-driven changes as recorded in the sediments on the margins of the Californias, sediments from the Cariaco Basin, and in the GISP-2 Greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka. Millennial-scale cycles in productivity and oxygen depletion on the margins of the Californias demonstrate that the California Current System was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern North Pacific Ocean. It is likely that the Pacific and Atlantic Oceans were linked through the atmosphere. Warmer air temperatures during interstadials would have strengthened Hadley and Walker circulations, which, in turn, would have strengthened the subtropical high pressure systems in both the North Pacific and the North Atlantic, producing increased rainfall over the Cariaco Basin and increased upwelling along the margins of the Californias.

Paleoproductivity evolution off central Chile from the Last Glacial Maximum to the Early Holocene

A geochemical and paleontological reconstruction of paleoproductivity, upwelling intensity and sea surface temperature (SST) off central Chile at 35°S (GeoB3359-3) reveals marked changes from the Last Glacial Maximum (LGM) through the Early Holocene. Surface-water productivity was determined by the interaction between the atmospheric (the Southern Westerlies) and oceanographic (the Antarctic Circumpolar Current, ACC) systems from the LGM through early Termination I (TI). The northward shift of the climate zones during the LGM brought the ACC, as the main macronutrient source, closer to the GeoB3359-3, SST lowered, and surface water productivity and accumulation rates of biogenic components enhanced. With the poleward return of the Southern Westerlies and the ACC, the subtropical high-pressure system became the dominant atmospheric component southward till 35°S during the late TI and Early Holocene and caused surface water productivity to increase through enhanced upwelling.

High-resolution evidence from southern China of an early Holocene optimum and a mid-Holocene dry event during the past 18,000 years

Computer models suggest that the Holocene Optimum for East Asian summer monsoon precipitation occurred at different times in different regions of China. Previous studies indicate that this time-transgressive Holocene Optimum should have been experienced about 3000 yr ago in southern China. In this study we describe a section which allows us to test this timing directly. We have closely examined high-resolution eutrophic peat/mud sequences covering the past 18,000 cal yr at Dahu, Jiangxi, on the southern boundary of the mid subtropical zone in China. Late Pleistocene successions in the Dahu record indicate cooler and much wetter conditions relative to synchronous events in north-central China. Our results indicate that the Holocene Optimum occurred between ca. 10,000 and 6000 cal yr ago in southern China, consistent with the global pattern. Conditions were relatively dry and cold from 6000 to 4000 cal yr ago. Our data also support the conclusion that the last deglaciation to early Holocene in the south was much wetter, resulting in the formation of dense broad-leaved forests, which could have acted to moderate land temperature ∼10,000 to 6000 cal yr ago, yielding a stable early-Holocene climate. After 6000 cal yr, forest reduction led to unstable land temperatures, and possibly to a northerly shift of the subtropical high-pressure system. Whatever the mechanism, these changes resulted in decreased precipitation between 6000 and 4000 cal yr B.P. in southern China.

Quantification of topographic venting of boundary layer air to the free troposphere

Abstract. Net vertical air mass export by thermally driven flows from the atmospheric boundary layer (ABL) to the free troposphere (FT) above deep Alpine valleys was investigated. The vertical export of pollutants above mountainous terrain is presently poorly represented in global chemistry transport models (GCTMs) and needs to be quantified. Air mass budgets were calculated using aircraft observations obtained in deep Alpine valleys. The results show that on average 3 times the valley air mass is exported vertically per day under fair weather conditions. During daytime the type of valleys investigated in this study can act as an efficient "air pump" that transports pollutants upward. The slope wind system within the valley plays an important role in redistributing pollutants. Nitrogen oxide emissions in mountainous regions are efficiently injected into the FT. This could enhance their ozone (O3) production efficiency and thus influences tropospheric pollution budgets. Once lifted to the FT above the Alps pollutants are transported horizontally by the synoptic flow and are subject to European pollution export. Forward trajectory studies show that under fair weather conditions two major pathways for air masses above the Alps dominate. Air masses moving north are mixed throughout the whole tropospheric column and further transported eastward towards Asia. Air masses moving south descend within the subtropical high pressure system above the Mediterranean.

Comment on “modelling the inception of the Patagonian icesheet”

A geochemical and paleontological reconstruction of paleoproductivity, upwelling intensity and sea surface temperature (SST) off central Chile at 35°S (GeoB3359-3) reveals marked changes from the Last Glacial Maximum (LGM) through the Early Holocene. Surface-water productivity was determined by the interaction between the atmospheric (the Southern Westerlies) and oceanographic (the Antarctic Circumpolar Current, ACC) systems from the LGM through early Termination I (TI). The northward shift of the climate zones during the LGM brought the ACC, as the main macronutrient source, closer to the GeoB3359-3, SST lowered, and surface water productivity and accumulation rates of biogenic components enhanced. With the poleward return of the Southern Westerlies and the ACC, the subtropical high-pressure system became the dominant atmospheric component southward till 35°S during the late TI and Early Holocene and caused surface water productivity to increase through enhanced upwelling.

Sea‐surface temperature co‐variability in the Southern Atlantic and Indian Oceans and its connections with the atmospheric circulation in the Southern Hemisphere

AbstractThe relationship between sea‐surface temperature (SST) inter‐annual variability at the subtropical and midlatitudes of the southern Atlantic and Indian Oceans and its links with the atmospheric circulation in the Southern Hemisphere are investigated over the 1950–1999 period. Exploratory analysis using singular value decomposition and further investigations based on simple indices show that a large part of regional SST variability is common between the southwestern parts of both basins at subtropical and midlatitudes during the austral summer. Interestingly, these areas are also significantly associated with the far southwestern Pacific (Tasman Sea area). The patterns and time series of co‐variability between the southern Atlantic and Indian Oceans are shown to correspond to SST modes previously described in the literature as ‘subtropical dipoles’, independently for the Atlantic and Indian Oceans. Composite analyses show that austral summers characterized by simultaneous warm (and to a lesser extent cold) SST anomalies in the southwestern (northern) part of both southern oceans are related to atmospheric anomalies mainly involving a southward shift and a strengthening of the subtropical high‐pressure systems over both basins. These anomalies are embedded in a hemispheric signal associating two cores of positive pressure anomalies within the South Pacific anticyclone. The global picture appears to have a wave number 4 spatial structure. The associated low‐level wind and latent heat‐flux anomalies and the lags between atmospheric variables and SST anomalies are consistent with an atmospheric forcing on the ocean. Potential links of these patterns with large‐scale modes of climate variability in the Southern Hemisphere are discussed. Copyright © 2003 Royal Meteorological Society.

The instability of the East Asian summer monsoon–ENSO relations

The instability in the relation between the East Asian summer monsoon and the ENSO cycle in the long–term variation is found through this research. By instability, we mean that high inter–relation exists in some periods but low inter–relation may appear in some other periods. It is reveals that the interannual variation of the summer atmospheric circulation during the ‘high correlation’ periods (HCP) is significantly different from that during the ‘low correlation’ periods (LCP). Larger interannual variability is found during HCP for trade wind over the tropical eastern Pacific of the Southern Hemisphere, the low—level air temperature over the tropical Pacific, the subtropical high pressure systems in the two hemispheres, and so on. The correlation between summer rainfall over China and ENSO is as well different between HCP and LCP.

Mechanisms of Eastern Mediterranean Rainfall Variability

Abstract This paper presents a simple theory for the association between observed eastern Mediterranean (EM) rainfall anomalies and North Atlantic (NA) climate variability. Large-scale NA atmospheric mass rearrangements, primarily a modulation of the Icelandic low and the subtropical high pressure systems, tend to extend beyond the NA. A particularly strong such teleconnection exists between the northern NA and southern Europe and the Mediterranean Basin. Pressure anomalies over Greenland–Iceland are thus associated with reversed-polarity anomalies centered over the northern Adriatic, affecting the entire Mediterranean Basin; elevated Greenland pressure is accompanied by an anomalous cyclone over the Mediterranean, and a Mediterranean high pressure system is present when pressure over Greenland is reduced. In the EM, these anomalies result in anomalous southerlies during Greenland highs, and northerlies during Greenland lows. Eastern Mediterranean southerlies warm the EM, while northerlies cool locally. B...

Interannual Variability of Caribbean Rainfall, ENSO, and the Atlantic Ocean*

The large-scale ocean–atmosphere patterns that influence the interannual variability of Caribbean–Central American rainfall are examined. The atmospheric circulation over this region is shaped by the competition between the North Atlantic subtropical high sea level pressure system and the eastern Pacific ITCZ, which influence the convergence patterns on seasonal and interannual timescales. The authors find the leading modes of interannual sea level pressure (SLP) and SST variability associated with Caribbean rainfall, as selected by canonical correlation analysis, to be an interbasin mode, linking the eastern Pacific with the tropical Atlantic, and an Atlantic mode. North Atlantic SLP affects Caribbean rainfall directly, by changing the patterns of surface flow over the region, and indirectly, through SST anomalies. Anomalously high SLP in the region of the North Atlantic high translates into stronger trade winds, hence cooler SSTs, and less Caribbean rainfall. The interbasin mode, which manifests itself as a zonal seesaw in SLP between the tropical Atlantic and the eastern equatorial Pacific, is correlated with ENSO. When SLP is low in the eastern equatorial Pacific, it is high in the tropical Atlantic: the surface atmospheric flow over the basin is divergent, to the west, toward the eastern Pacific ITCZ, and to the east, toward the tropical North Atlantic. A weakened meridional SLP gradient in the tropical North Atlantic signifies weaker trade winds and the opportunity for SSTs to warm up, reaching peak intensity 2–4 months after the mature phase of an ENSO event. This SST anomaly is particularly evident in the Caribbean–western Atlantic basin. The tendency is for drier-than-average conditions when the divergent atmospheric flow dominates, during the rainy season preceding the mature phase of a warm ENSO event. The dry season that coincides with the mature phase of ENSO is wetter than average over the northwestern section of the basin, that is, Yucatan, the Caribbean coast of Honduras, and Cuba, and drier than average over the rest of the basin, that is, Costa Rica and northern South America. The following spring, as the atmospheric circulation transitions to normal conditions, the positive SST anomaly that has built up in the basin takes over, favoring convection. The positive precipitation anomaly spreads southeastward, from the northwest to the entire basin. At the start of a new rainy season, it is especially strong over the Greater Antilles.

The Temporal Behavior of the Atmospheric Boundary Layer in Israel

Upper-air measurements collected for three consecutive years (1987‐89) from the Israel Meteorological Service permanent sounding site, in Beit-Dagan, Israel, enabled the temporal behavior of the atmospheric boundary layer over Israel to be characterized. Data analyzed consisted of the layer depth, the thermal gradient within the layer, and occurrence frequency of radiative and elevated inversions. To adequately represent the multiyear seasonal and diurnal behavior, the 3-yr databases were merged based on the tested hypothesis that the month sample in each individual year comes from the same population. The analysis shows that the depth of the radiative ground-based inversion, its frequency, as well as its thermal profile are maximal during spring and early summer. The upper-inversion layer is well defined during the summer, its lowest base (0.5‐1 km MSL) indicating a sharp interface layer formed between the marine turbulent boundary layer at the shallow layer of the atmosphere and the subsiding downward motion caused by the subtropical high pressure system. During the other three seasons a significant temporal variation of the upper-inversion base is observed as a result of the frequent larger-scale synoptic weather systems. The diurnal variation of the mixed-layer depth is most evident during the summer because it is mainly governed by heat fluxes and the daily sea-breeze cycle that are most intensive then. Henceforth, the layer minimal depth, along the coast, usually occurs during late afternoon hours when the wind speed of the cool sea breeze reaches its minimal rate and heat fluxes dissipate rapidly, leading to a decrease of the marine turbulent boundary layer.

Simulated interannual to decadal variability in the tropical and sub‐tropical North Atlantic

The dominant pattern of tropical and subtropical North Atlantic sea surface temperature (SST) anomalies simulated in the GFDL coupled ocean‐atmosphere model is identified and compared to observations. The spatial pattern and temporal variability of that pattern resemble observational results. On interannual time scales it is shown that anomalous surface heat fluxes, consistent with variations in the intensity of the subtropical high pressure system in the atmosphere and the associated Northeasterly Trade winds, appear to be the most important process for generating this SST pattern. This relationship is also true on decadal time scales, although the relative role of oceanic heat advection is somewhat larger than on the interannual time scales.

Simulation of North Atlantic Decadal/Multidecadal Winter SST Anomalies Driven by Basin-Scale Atmospheric Circulation Anomalies

The North Atlantic winter sea surface temperature anomaly (Tsa) response to anomalous surface atmospheric circulation anomalies that vary over decadal and short-term inter-decadal periods is simulated for 1950 through 1992. Anomalous ocean variability is driven by adding monthly COADS-derived anomalous fields of vector wind stress and wind speed to the climatological annual cycle forcing. A simple model is derived relating winter Tsa to the integrated atmospheric forcing present earlier in time that is responsible for its existence. The basinscale structure of forced winter Tsa patterns depends on the structure of the atmospheric forcing along with regional differences in the dominant mixed layer processes that generate Tsa. For example, when the atmospheric subtropical high and subpolar low pressure systems are simultaneously strong, enhanced flow around, and baroclinic adjustments within, the subtropical gyre results in anomalously warm water in the Gulf Stream region off the U.S. East Coast. At the same time, however, the open ocean generally cools because the westerlies and trades are anomalously strong. By analyzing Tsa variability not driven by the atmosphere, an anomalously cold decade is identified characterized by rapid onset and termination that both occur within one year. The onset during 1968 coincides with the appearance of the great salinity anomaly, while the termination during 1977 coincides with an abrupt Northern Hemisphere climate shift that is particularly evident in the Pacific.

The Pitcairn Islands, South Pacific Ocean: plate tectonic and climatic contexts

Abstract The remote Pitcairn Group in the South Pacific Ocean comprises a volcanic island (Pitcairn Island), two low coral atolls (Oeno, Ducie) and a raised coralline island (Henderson Island). The geological history of these islands, on anomalously thin oceanic lithosphere, is related to the development of two subparallel island chains (Oeno-Henderson-Ducie; Pitcairn) associated with intra-Pacific plate `hotspot' activity; the surface manifestation of this activity has been partly determined by structural lineations in the plate inherited from past plate history. The climate of the Pitcairn Islands is determined by the position of the subtropical high pressure system and the South Pacific Convergence Zone. Variations in the strength of this atmospheric circulation system, measured by changes in the Southern Oscillation index of pressure difference, provide a partial explanation of the long-term variability of mean annual rainfall at Pitcairn Island. Knowledge of past climates in the Pitcairn Group remains speculative. Maps of the Pitcairn Islands and a report of climate at Henderson Island (2/91–1/92) are included in the paper.

Stability of Holocene Climate Regimes in the Yellowstone Region

A 12,500-yr pollen record from Loon Lake, Wyoming provides information on the climate history of the southwestern margin of Yellowstone National Park. The environmental reconstruction was used to evaluate hypotheses that address spatial variations in the Holocene climate of mountainous regions. Loon Lake lies within the summer-dry/winter-wet climate regime. An increase in xerophytic pollen taxa suggests drier-than-present conditions between ca. 9500 and 5500 14 C yr B.P. This response is consistent with the hypothesis that increased summer radiation and the expansion of the east Pacific subtropical high-pressure system in the early Holocene intensified summer drought at locations within the summer-dry/winter-wet regime. This climate history contrasts with that of nearby sites in the summer-wet/winter-dry region, which were under the influence of stronger summer monsoonal circulation in the early Holocene. The Loon Lake record implies that the location of contrasting climate regimes did not change in the Yellowstone region during the Holocene. The amplitude of the regimes, however, was determined by the intensity of circulation features and these varied with temporal changes in the seasonal distribution of solar radiation.

Spatial Variations of Holocene Climatic Change in the Yellowstone Region

The paleoecologic record of the northern Rocky Mountains shows considerable spatial variability, which has been difficult to explain as a unified response to large-scale changes in effective moisture or temperature through time. Comparison of past and present climatic patterns suggests that the spatial heterogeneity in the fossil record arises from a local response to the effects of greater summer solar radiation during the early Holocene. Solar radiation directly determines evapotranspiration and indirectly controls the strength of the summer monsoon and the eastern Pacific subtropical high-pressure system. The Yellowstone region provides an example of an area where the indirect effects of solar radiation are manifest by the juxtaposition of two contrasting climatic regimes today and where these regimes were amplified during the early Holocene. Fossil-pollen data suggest that central and southern Yellowstone (the summer-dry area) became drier in the early Holocene, while northern Yellowstone (the summer-wet area) became wetter. The boundary between regimes, however, is controlled by topography and has not changed significantly in the Holocene. The spatial variations of Holocene vegetation and climatic changes in the northern Rocky Mountains may therefore be explained as a trade-off between strengthened monsoonal circulation and a stronger subtropical high in the early Holocene and the relative importance of these precipitation regimes on a topographically complex landscape.

A cirrus cloud climatology from NOAA/HIRS

A climatology of upper trospospheric semi-transparent cirrus clouds is being compiled front multispectral infrared data collected by the HIRS on the NOAA polar orbiting satellites. This is a report on the first year of data analyzed (June 1989–May 1990). Semi-transparent clouds were found in 43% of the observations. Large seasonal changs were found in these clouds in many geographical areas. Previous studies restricted to the continental United States, indicated that semi-transparent clouds occurred in nearly the same frequency in all seasons. However, the global data from the NOAA/HIRS show that large changes occur in areas dominated by three major weather features; the ITCZ, the sub-tropical high pressure systems, and the mid-latitude storm belts. Semi-transparent clouds associated with these features, move latitudinally with the seasons. These clouds also are more frequent in the summer hemisphere than the winter hemisphere. They appear to be linked to convective cloud development and the mid-latitudinal frontal weather systems.

A procedure to estimate worst-case air quality in complex terrain

Using a straightforward synoptic climatological analysis scheme, it is shown that the potential for an area to experience air quality degradation due to local sources is highest under polar subtropical highs. With respect to polar highs, the problem is most severe when the sun angle is low and snow covers the ground, and the polar high persists for a long period of time. A simple algorithm is introduced which is designed to estimate worst-case impact in a trapping valley. The potential for the accumulation of air pollution in such valleys due to the persistence of a polar high in a region, is ignored in current regulatory air quality assessments. Trapping valleys and synoptic flow stagnation often occur in wilderness areas. Refined air quality assessments are shown to be possible using a mesoscale meteorological model and a pollution dispersion model. These tools permit quantitative assessments of pollution build-up from local sources as a result of the recirculation of the local air. This tool, along with the synoptic climatological classification scheme, also permits an evaluation of the fractional contribution of long range versus local sources in the air quality degradation in a region. Areas near the center of a polar or subtropical surface high pressure system, for instance, appear to be dominated by local sources, if they exist, whereas in the vicinity of extratropical cyclones, long-range transport is usually much more important.

Stability of rectilinear geostrophic vortices in stationary equilibrium

A hemispherical barotropic model of global-scale atmospheric motions is studied in a planar approximation using three K0 (Bessel) vortices of equal strength γ and length scale κ−1 to represent the subtropical semipermanent high pressure systems. In equilibrium these anticyclonic vortices are equally spaced on a latitude circle of radius a. One K0 vortex of strength γ0 is used to simulate the polar (cyclonic) vortex; γ0 is chosen so that the equilibrium configuration remains stationary with respect to the rotating earth. Over the range of the length parameter 0⩽κa⩽5, two distinct stable regimes are found: (1) a ’’soft’’-vortex regime, 0?κa<κcr⋅a=1.44763516... and (2) a ’’hard’’-vortex (climatological) regime, κ≳κcr. The nonlinear part of this stability investigation has been carried out mainly by numerical computation.