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Intertropical Convergence Zone (intertropical + convergence_zone)
Selected AbstractsLate-glacial and Holocene vegetation, climate and fire dynamics in the Serra dos Órgãos, Rio de Janeiro State, southeastern BrazilGLOBAL CHANGE BIOLOGY, Issue 6 2010HERMANN BEHLING Abstract We present a high-resolution pollen and charcoal record of a 218 cm long sediment core from the Serra dos Órgãos, a subrange of the coastal Serra do Mar, located at 2130 m altitude in campos de altitude (high elevation grass- and shrubland) vegetation near Rio de Janeiro in southeastern Brazil to reconstruct past vegetation, climate and fire dynamics. Based on seven AMS 14C ages, the record represents at least the last 10 450 14C yr bp (12 380 cal years bp), The uppermost region was naturally covered by campos de altitude throughout the recorded period. Diverse montane Atlantic rain forest (ARF) occurred close to the studied peat bog at the end of the Late-glacial period. There is evidence of small Araucaria angustifolia populations in the study area as late as the early Holocene, after which point the species apparently became locally extinct. Between 10 380 and 10 170 14C yr bp (12 310,11 810 cal yr bp), the extent of campos de altitude was markedly reduced as montane ARF shifted rapidly upward to higher elevations, reflecting a very wet and warm period (temperatures similar to or warmer than present day) at the end of the Younger Dryas (YD) chronozone. This is in opposition to the broadly documented YD cooling in the northern Hemisphere. Reduced cross-equatorial heat transport and movement of the Intertropical Convergence Zone over northeastern Brazil may explain the YD warming. Markedly extended campos de altitude vegetation indicates dry climatic conditions until about 4910 14C yr bp (5640 cal yr bp). Later, wetter conditions are indicated by reduced high elevation grassland and the extension of ARF into higher elevation. Fire frequency was high during the early Holocene but decreased markedly after about 7020 14C yr bp (7850 cal yr bp). [source] Climate dynamics of atmosphere and ocean in the equatorial zone: a synthesisINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2004Stefan Hastenrath Abstract A synopsis is offered of circulation mechanisms in the oceanic regions of the equatorial zone. Over the eastern Atlantic and Pacific, and especially in boreal summer, cross-equatorial flow from the Southern Hemisphere is strong and induces a tongue of cold surface waters, centred to the south of the equator. Upon crossing the equator in these sectors, owing to the Coriolis effect and a kinetic energy imbalance, the airstream speeds up and divergence develops, producing the Intertropical Divergence Zone (ITDZ). Once these processes result in the wind recurving from southeasterly to southwesterly, the flow slows down and becomes convergent, manifest in the Intertropical Convergence Zone, with a maximum to the south of the wind confluence. By contrast, over the western Atlantic and central Pacific and especially in boreal winter, winds in the equatorial band are predominantly from the east, upper-ocean Ekman transport is directed away from the equator, and the upwelling and cold tongue are centred on the equator. Cross-equatorial flow is insufficient to produce recurvature, the ITDZ is narrower and weaker, the divergence maximum is at the equator rather than in low northern latitudes, and the convergence maximum straddles the wind confluence. Over the Indian Ocean, the wind field is dominated by the alternation between the predominantly meridional flow of the winter and summer monsoons. Equatorial westerlies are limited to the short monsoon transition seasons. Essential for their origin is an eastward pressure gradient along the equator and weak southern trade winds, allowing recurvature somewhat south of the equator. Because the zonal pressure gradient is strongest in boreal summer and the southern trade winds are weakest in austral summer, the equatorial westerlies peak in spring and autumn. The boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal,vertical circulation cell along the Indian Ocean equator. Equatorial zonal,vertical circulation cells require well-developed zonal flow in the lower troposphere along the equator and, therefore, appear confined to the oceanic longitudes and certain seasons. Thus, they are found over the Atlantic only in boreal winter and over the Indian Ocean only in boreal autumn, whereas over the Pacific they prevail all year round. Copyright © 2004 Royal Meteorological Society [source] Interannual variability of lower-tropospheric moisture transport during the Australian monsoonINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2002Christopher R. Godfred-Spenning Abstract The interannual variability of the horizontal lower-tropospheric moisture transport associated with the Australian summer monsoon has been analysed for the 1958,99 period. The 41-season climatology of moisture flux integrated between the surface and 450 hPa showed moderate levels of westerly transport in the month before Australian monsoon onset, associated with cross-equatorial flow in the Sulawesi Sea and west of Borneo. In the month after onset the westerly moisture transport strengthened dramatically in a zonal belt stretching from the Timor Sea to the Western Equatorial Pacific, constrained between the latitudes 5 and 15 °S, and associated with a poleward shift in the Intertropical Convergence Zone and deepening of the monsoon trough. Vertical cross-sections showed this transport extending from the surface to the 500 hPa level. In the second and third months after onset the horizontal flow pattern remained similar, although flux magnitudes progressively decreased, and the influence of trade winds became more pronounced over northern Australia. Nine El Niño and six La Niña seasons were identified from the data set, and composite plots of the affected years revealed distinct, and in some cases surprising, alterations to the large-scale moisture transport in the tropical Australian,Indonesian region. During an El Niño it was shown that the month prior to onset, in which the moisture flux was weaker than average, yielded to a dramatically stronger than average flux during the following month, with a zone of westerly flux anomalies stretching across the north Australian coast and Arafura Sea. The period of enhanced moisture flux during an El Niño is relatively short-lived, with drier easterly anomalies asserting themselves during the following 2 months, suggesting a shorter than usual monsoon period in north Australia. In the La Niña composite, the initial month after onset shows a tendency to weaker horizontal moisture transport over the Northern Territory and Western Australia. The subsequent 2 months show positive anomalies in flux magnitude over these areas; the overall effect is to prolong the monsoon. Comparison of these results with past research has led us to suggest that the tendency for stronger (weaker) circulations to arise in the initial month of El Niño (La Niña) events is a result of mesoscale changes in soil moisture anomalies on land and offshore sea surface temperature (SST) anomalies, brought about by the large-scale alterations to SST and circulation patterns during the El Niño,Southern Oscillation. The soil moisture and SST anomalies initially act to enhance (suppress) the conditions necessary for deep convection in the El Niño (La Niña) cases via changes in land,sea thermal contrast and cloud cover. Copyright © 2002 Royal Meteorological Society. [source] Cross-cutting moraines reveal evidence for North Atlantic influence on glaciers in the tropical Andes,JOURNAL OF QUATERNARY SCIENCE, Issue 3 2010Jacqueline A. Smith Abstract Surface exposure dating of boulders on an exceptionally well-preserved sequence of moraines in the Peruvian Andes reveals the most detailed record of glaciation heretofore recognised in the region. The high degree of moraine preservation resulted from dramatic changes in the flow path of piedmont palaeoglaciers at the southern end of the Cordillera Blanca (10° 00, S, 77° 16, W), which, in turn, generated a series of cross-cutting moraines. Sixty 10Be surface exposure ages indicate at least four episodes of palaeoglacier stabilisation (>65, ca. 65, ca. 32 and ca. 18,15,ka) and several minor advances or stillstands on the western side of the Nevado Jeulla Rajo massif. The absence of ages close to the global Last Glacial Maximum (ca. 21,ka) suggests that if an advance culminated at that time any resulting moraines were subsequently overridden. The timing of expanded ice cover in the central Peruvian Andes correlates broadly with the timing of massive iceberg discharge (Heinrich) events in the North Atlantic Ocean, suggesting a possible causal connection between southward migration of the Intertropical Convergence Zone during Heinrich events and a resultant increase in precipitation in the tropical Andes. Copyright © 2010 John Wiley & Sons, Ltd. [source] Pollen- and diatom based environmental history since the Last Glacial Maximum from the Andean core Fúquene-7, ColombiaJOURNAL OF QUATERNARY SCIENCE, Issue 1 2003Maria Isabel Vélez Abstract The late Pleistocene,Holocene ecological and limnological history of Lake Fúquene (2580 m a.s.l.), in the Colombian Andes, is reconstructed on the basis of diatom, pollen and sediment analyses of the upper 7 m of the core Fúquene-7. Time control is provided by 11 accelerator mass spectrometry (AMS) 14C dates ranging from 19 670 ± 240 to 6040 ± 60 yr BP. In this paper we present the evolution of the lake and its surroundings. Glacial times were cold and dry, lake-levels were low and the area was surrounded by paramo and subparamo vegetation. Late-glacial conditions were warm and humid. The El Abra Stadial, a Younger Dryas equivalent, is reflected by a gap in the sedimentary record, a consequence of the cessation of deposition owing to a drop in lake-level. The early Holocene was warm and humid; at this time the lake reached its maximum extension and was surrounded by Andean forest. The onset of the drier climate prevailing today took place in the middle Holocene, a process that is reflected earlier in the diatom and sediment records than in the pollen records. In the late Holocene human activity reduced the forest and transformed the landscape. Climate patterns from the Late-glacial and throughout the Holocene, as represented in our record, are similar to other records from Colombia and northern South America (the Caribbean, Venezuela and Panama) and suggest that the changes in lake-level were the result of precipitation variations driven by latitudinal shifts of the Intertropical Convergence Zone. Copyright © 2003 John Wiley & Sons, Ltd. [source] Integrated marine and terrestrial evidence for abrupt Congo River palaeodischarge fluctuations during the last deglaciationJOURNAL OF QUATERNARY SCIENCE, Issue 8 2001Fabienne Marret Abstract We present a high-resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition (18 to 9 cal. kyr BP) based on integrated oceanic and terrestrial proxies from a Congo fan core. Pollen, grass cuticle, Pediastrum and dinoflagellate cyst fluxes, sedimentation rates and planktonic foraminiferal ,18O ratios, u37K, sea-surface temperature and alkane/alkenone ratio data highlight a series of abrupt changes in Congo River palaeodischarge. A major discharge pulse is registered at around 13.0 cal. kyr BP which we attribute to latitudinal migration of the Intertropical Convergence Zone (ITCZ) during deglaciation. The data indicate abrupt and short-lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles. The phases of enhanced Congo discharge stimulated river-induced upwelling and enhanced productivity in the adjacent ocean. Copyright © 2001 John Wiley & Sons, Ltd. [source] Interannual variability of boreal summer rainfall in the equatorial AtlanticINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2009Guojun Gu Abstract Tropical Atlantic rainfall variations during boreal summer (June,July,August (JJA)) are quantified by means of a 28-year (1979,2006) monthly precipitation dataset from the Global Precipitation Climatology Project (GPCP). Rainfall variability during boreal spring (March,April,May (MAM)) is also examined for comparison in that the most intense interannual variability is usually observed during this season. Comparable variabilities in the Atlantic maritime intertropical convergence zone (ITCZ) (15° ,35°W) strength (PITCZ) are found during both seasons. Variations in the ITCZ's latitudinal location (LatITCZ) during JJA, however, are much weaker than during MAM. PITCZ and LatITCZ are shown to be closely associated with sea surface temperature (SST) anomalies in both the tropical Atlantic and Pacific. Within the tropical Atlantic, the Atlantic Niño events (Atl3) and SST anomalies in the tropical North Atlantic (TNA) are the two major local factors modulating surface rainfall patterns and variations. Atl3 is significantly correlated with PITCZ and LatITCZ during JJA and MAM. TNA is significantly correlated to PITCZ during JJA but not to LatITCZ. In contrast, TNA is significantly correlated to LatITCZ during MAM but its correlation with PITCZ is weak. The impact of the El Niño-Southern Oscillation (ENSO) events (Nino3.4) is observed during both seasons, while the correlation between Nino3.4 and LatITCZ is slightly weak. However, with the effects of Atl3 and TNA removed, the ENSO tends to have a quite limited direct impact on the tropical Atlantic, specifically over the open ocean. High second-order partial correlation between Nino3.4 and rainfall is generally confined to the western basin and over the northeastern South America. Therefore, during JJA, the two local SST modes are of dominance for the tropical Atlantic rainfall variability. Nevertheless, the ENSO seems to still play an active role in modulating surface zonal wind anomalies in the western basin and then the Atlantic Niño mode. Copyright © 2008 Royal Meteorological Society [source] Modulation of the intraseasonal rainfall over tropical Brazil by the Madden,Julian oscillationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2006Everaldo B. De Souza Abstract Fifteen years (1987,2001) of rain gauge-based data are used to describe the intraseasonal rainfall variability over tropical Brazil and its associated dynamical structure. Wavelet analysis performed on rainfall time series showed significant peaks centered roughly in periods of 30,70 days, particularly in the eastern southeastern Amazon and northern northeast Brazil. A significant enhancement of precipitation with maximum anomalies in a northeastward oriented band over tropical Brazil is evidenced from empirical orthogonal function (EOF) analysis of 30,70-day filtered rainfall anomalies during rainy season (January to May). Lagged/lead composites revealed that, on a global scale, the Madden,Julian oscillation (MJO) is the main atmospheric-mechanism modulator of the pluviometric variations on intraseasonal timescale in the eastern Amazon and northeast Brazil. A coherent northward expansion of rainfall across tropical Brazil is evident during the passage of MJO over South America. Regionally, the establishment of a quasi-stationary deep convection band triggered by the simultaneous manifestation of south Atlantic convergence zone (SACZ) and intertropical convergence zone (ITCZ) explains the intensified rainfall over these regions. Such regional mechanisms are dynamically embedded within the eastward-propagating MJO-related large-scale convective envelope along tropical South America/the Atlantic Ocean. These features occur in association with a significant intraseasonal evolution of the lower-level wind and sea-surface temperature (SST) patterns, particularly in the Atlantic Ocean, including a coherent dynamical connection with atmospheric circulation, deep convective activity over South America and rainfall over tropical Brazil. Copyright © 2006 Royal Meteorological Society [source] Influence of the Quasi-Biennial Oscillation on the ECMWF model short-range-forecast errors in the tropical stratosphereTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 628 2007Nedjeljka, agar Abstract This paper addresses the impact of the Quasi-Biennial Oscillation (QBO) on the background-error covariances in the tropical atmosphere of the ECMWF model. The tropical short-range-forecast-error covariances are represented in terms of equatorial waves coupled to convection. By comparing the forecast-error proxy data from two different phases of the QBO, it is shown that the phase of the QBO has an effect on the distribution of tropical forecast-error variances between various equatorial waves. The influence of the QBO is limited to the stratospheric levels between 50 hPa and 5 hPa. In the easterly QBO phase, the percentage of error variance in Kelvin waves is significantly greater than in the westerly phase. In the westerly phase, westward-propagating inertio-gravity waves become more important, at the expense of Kelvin modes, eastward-propagating mixed Rossby-gravity waves and inertio-gravity modes. Comparison of datasets from two easterly phases shows that the maxima of stratospheric error variance in various equatorial modes follow the theory of the interaction of waves with descending shear zones of the horizontal wind. Single-observation experiments illustrate an impact of the phase of the QBO on stratospheric analysis increments, which is mostly seen in the balanced geopotential field. Idealized 3D-Var assimilation experiments suggest that background-error statistics from the easterly QBO period are on average more useful for the multivariate variational assimilation, as a consequence of a stronger mass-wind coupling due to increased impact of Kelvin waves in the easterly phase. By comparing the tropical forecast errors in two operational versions of the model a few years apart, it is shown here that recent model improvements, primarily in the model physics, have substantially reduced the errors in both wind and geopotential throughout the tropical atmosphere. In particular, increased wind-field errors associated with the intertropical convergence zone have been removed. Consequently, the ability of the applied background-error model to represent the error fields has improved. Copyright © 2007 Royal Meteorological Society [source] | ||||||||||