The 18.6-year nodal tidal cycle and the bi-decadal precipitation oscillation over the plains to the east of subtropical Andes, South America (original) (raw)
Related papers
Principal modes of interannual and decadal variability of summer rainfall over South America
International Journal of Climatology, 2001
Using the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) product together with the Goddard Earth Observing System (GEOS) reanalysis and the National Center for Environmental Prediction (NCEP) sea-surface temperature (SST) data, we have conducted a diagnostic study of the interannual and decadal scale variability of principal modes of summer rainfall over South America for the period 1979-1995. By filtering the annual and short (B 12 months) timescale variations, results of empirical orthogonal function analysis show three leading modes of rainfall variation identified with interannual, decadal and long-term variability. Together, these modes explain more than half the total variance of the filtered data. The first mode is highly correlated with El Niñ o-Southern Oscillation (ENSO), showing a regional rainfall anomaly pattern largely consistent with previous results. This mode captures the summer season interannual variability, not only the Northeast Brazil drought but also its connection with excessive rainfall over Southern Brazil and the Ecuador coast in El Niñ o years. Another distinctive feature is the strengthening of the low-level flow along the eastern foothills of the eastern Andes, signifying an enhancement of the South American summer monsoon in response to an El Niñ o anomaly. The decadal variation displays a meridional shift of the Inter-Tropical Convergence Zone (ITCZ), which is tied to the anomalous cross-equatorial SST gradient over the Atlantic and the eastern Pacific. Associated with this mode is a large-scale mass swing between polar regions and the mid-latitudes. Over the South Atlantic and the South Pacific, the anomalous subtropical high and the associated anomalous surface wind are dynamically consistent with the distribution of local SST anomalies, suggesting the importance of atmospheric forcing at the decadal time scale. The long-term variation shows that since 1980 there has been a decrease of rainfall from the northwest coast to the southeast subtropical region and a southwards shift of the Atlantic ITCZ, leading to increased rainfall over northern and eastern Brazil. Possible links of this mode to the observed SST warming trend over the subtropical South Atlantic and to the interdecadal SST variation over the extratropical North Atlantic are discussed.
2010
Observations, atmosphere models forced by historical SSTs, and idealized simulations are used to determine the causes and mechanisms of interannual to multidecadal precipitation anomalies over southeast South America (SESA) since 1901. About 40% of SESA precipitation variability over this period can be accounted for by global SST forcing. Both the tropical Pacific and Atlantic Oceans share the driving of SESA precipitation, with the latter contributing the most on multidecadal time scales and explaining a wetting trend from the early midcentury until the end of the last century. Cold tropical Atlantic SST anomalies are shown to drive wet conditions in SESA. The dynamics that link SESA precipitation to tropical Atlantic SST anomalies are explored. Cold tropical Atlantic SST anomalies force equatorward-flowing upper-tropospheric flow to the southeast of the tropical heating anomaly, and the vorticity advection by this flow is balanced by vortex stretching and ascent, which drives the increased precipitation. The 1930s Pampas Dust Bowl drought occurred, via this mechanism, in response to warm tropical Atlantic SST anomalies. The atmospheric response to cold tropical Pacific SSTs also contributed. The tropical Atlantic SST anomalies linked to SESA precipitation are the tropical components of the Atlantic multidecadal oscillation. There is little evidence that the large trends over past decades are related to anthropogenic radiative forcing, although models project that this will cause a modest wetting of the climate of SESA. As such, and if the Atlantic multidecadal oscillation has shifted toward a warm phase, it should not be assumed that the long-term wetting trend in SESA will continue. Any reversal to a drier climate more typical of earlier decades would have clear consequences for regional agriculture and water resources. * Lamont-Doherty Earth Observatory Contribution Number 7383.
Climate Dynamics, 2014
This study uses experiments with an Atmospheric General Circulation Model (AGCM) to address the role of the oceans and the effect of land-atmosphere coupling on the predictability of summertime rainfall over northern Argentina focusing on interdecadal time scales during 1901-2006. Ensembles of experiments where the AGCM is forced with historical SST in the global, Pacific and tropical-north Atlantic domains are used. The role of land-atmosphere interaction is assessed comparing the output of simulations with active and climatological soil moisture. A Maximum Covariance Analysis between precipitation and SST reveals the impact of the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation and the equatorial-tropical south Atlantic on rainfall over northern Argentina. Model simulations further show that while the dominant influence comes from the Pacific basin, the Atlantic influence can explain a large transition from dry to wet decades over northern Argentina during the beginning of the 1970s. Analysis of anomalies before and after the transition reveals an upper level anticyclonic circulation off the Patagonian coast with barotropic structure. This circulation enhances the moisture transport and convergence in northern Argentina and, together with enhanced evaporation, increased the rainfall after 1970. The SST pattern is dominated by cold conditions in the equatorial Atlantic and warm eastern Pacific and south Atlantic. We also found that land-atmosphere interaction leads to a representation of the long term rainfall evolution over northern Argentina that is closer to the observed one. Moreover, it leads to a smaller dispersion among ensemble members, thus resulting in a larger signal-to-noise ratio.
Cross-Equatorial Flow and Seasonal Cycle of Precipitation over South America
Journal of Climate, 2002
The relationship between South American precipitation and cross-equatorial flow over the western Amazon is examined using the 15-yr (1979-93) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. A meridional wind index, the V index, is constructed to represent the variability of the crossequatorial flow, based on area-averaged (5ЊS-5ЊN, 65Њ-75ЊW) daily 925-hPa meridional winds. The V index displays large submonthly, seasonal, and interannual variabilities, and correlates well with precipitation over South America. Two circulation regimes are identified, that is, a southerly regime and a northerly regime. Linear regression shows that when the V index is southerly, precipitation is mainly located to the north of the equator. When the V index is northerly, precipitation shifts toward the Amazon basin and subtropical South America. The V index is predominately southerly in austral winter and northerly in summer. The onset (demise) of the Amazon rainy season is led by an increase in the frequency of the northerly (southerly) V index. The relation between the V index and upper-level circulation is consistent with the seasonal cycle of the South American monsoon circulation. Hence, the V index is a good indicator for precipitation change over tropical and subtropical South America.
Climate Research, 2000
The decadal variability in the structure of the annual precipitation cycle over Southern South America (SSA) is analysed with the purpose of investigating whether the lower frequency variability laid bare by annual data is also evident in the annual precipitation structure. Climatic analysis shows that the annual cycle plus the semi-annual cycle dominate the annual variability of precipitation in SSA and represent most of the physical factors responsible for the observed patterns. The percentage of variance explained by the annual cycle shows 2 local maxima, in northwestern Argentina and southern Chile, with opposite phases, summer and winter. The interdecadal analysis of the annual cycle shows 2 areas of relevant variability: one over the central east and the other over the north east. In the first area, there is a positive trend in the variance explained by the first harmonic, indicating that precipitation tends to be better represented by an annual cycle, a fact that might indicate a climatic change in so far as this variable is concerned. In view of global warming, and as a first step towards quantifying the relationship between temperature and precipitation in the region, correlation coefficients are evaluated. The correlation structure of the warmer period 1943-52 generally shows a slight correlation pattern when compared to the 2 colder periods, 1955-64 and 1966-75. In northwestern Argentina, the highest positive correlation coefficients are found at the coldest times in 1955-64, and are probably related to an increase in cloudiness. During summer months (November to February), there is an inverse relationship between precipitation and temperature over most of SSA. That is, warmer/colder summers are associated with precipitation below/above the mean. In autumn and spring months, the correlation is positive in the eastern part of Argentina and Paraguay, and represents well the mechanism of maximum precipitation in this area, which is mainly the result of cyclogenesis. Winter correlation shows a weak positive pattern over SSA and a negative correlation area to the east of the Cordillera de los Andes, which is more intense in spring.
npj climate and atmospheric science, 2018
The South America low-level jet (SALLJ) on the eastern slopes of the Andes is a unique climatological feature in the continent. The SALLJ transports large amounts of moisture and controls the spatiotemporal variability of precipitation in southeast South America. This study shows a remarkable influence of the Atlantic Multidecadal Oscillation (AMO) on decadal-to-multidecadal variability of the SALLJ. The results show a consistent pattern in which active SALLJ days during negative AMO phases are associated with negative precipitation anomalies over northern Amazon and the Atlantic Intertropical Convergence Zone (ITCZ). Increased cross-equatorial flow over northwestern South America combined with the outflow associated with the atmospheric subsidence over the negative precipitation anomalies enhances northerly winds along the eastern slopes of the Andes and Amazon. This atmospheric circulation response, which is more prevalent in the austral winter, intensifies the SALLJ. In the exit region of the SALLJ over southern Brazil, Uruguay and northern Argentina, an anomalous low-level cyclonic circulation is associated with enhanced precipitation. The influence of the AMO on the SALLJ is consistent with paleo-proxy studies showing multidecadal changes in precipitation over the La Plata River drainage basin. The analysis shows that secular trends reinforce the SALLJ and precipitation patterns. Moreover, the study highlights the importance of natural variability (i.e., decadal-multidecadal variations) occurring within long-term trends in the mean state, possibly associated with global warming, and significant changes in the SALLJ and precipitation over South America.
Theoretical and Applied Climatology, 2000
In subtropical Argentina, Paraguay and southern Brazil, precipitation is most abundant during summer but its interannual variability is large. At this time a zone of low-level convergence, upper-level divergence, and intense convection is developed to the north of this area. This feature is known as the South Atlantic convergence zone (SACZ) and seems to be related to the interannual variability of summer rainfall to its south. The aim of this work is to document this relationship. Reduced (increased) precipitation in southern Brazil, most of Uruguay and northeastern Argentina is associated with a strong (weak) SACZ and a northward (southward) displacement of it, while increased (reduced) rainfall occurs further south in subtropical Argentina. Also, warm (cold) SST in the region 20° S–40° S and west of 30° W is likely accompanied by a southward (northward) shift of the SACZ. Aside of this relation with the SACZ that affect on the precipitation field of Southeastern South America, the proximate Atlantic Ocean SST seems to force the precipitation over this region by other mechanisms as well. The result of this additional SST forcing is to enhance the signal of the SACZ in northeastern Argentina, Uruguay and southern Brazil and to oppose the SACZ effect in southern subtropical Argentina.
International Journal of Climatology, 2003
We describe the rainfall and circulation anomalies of the South American summer monsoon (SASM) during December-January-February (DJF) of 1997-98 (El Niño) and 1998-99 (La Niña). The most pronounced rainfall signals in DJF 1997-98 include (a) excessive rainfall over northern Peru and Ecuador, (b) deficient rainfall over northern and central Brazil, and (c) above-normal rainfall over southeastern subtropical South America. The rainfall anomalies in (a) and (b) are associated with the excitation of an anomalous east-west overturning cell with rising motion and low-level westerlies over the equatorial eastern Pacific, coupled to sinking motion and low-level easterlies over northern Brazil. The easterlies turn sharply southeastward on encountering the steep topography of the Andes, enhancing the summertime low-level jet (LLJ) along the eastern foothills of the Andes near 15-20°S, possibly contributing to the increased rainfall in (c). During DJF 1997-98, the sea-surface temperature-induced warming spreads and expands over the entire tropical troposphere. The eastward expansion of a warm upper tropospheric geopotential and temperature ridge from the Niño-3 region, across subtropical South America to the southeast Atlantic, enhances warming over the Altiplano Plateau, hydrostatically strengthening the Bolivia high. Similar to previous warming events, the South Pacific high is weakened, and the South Atlantic high is strengthened. During DJF 1998-99, as cold water develops over the equatorial central Pacific, the SASM anomalies in the tropics are weaker and less organized and appear to be in transition to the opposite phase to those found in DJF 97-98. In the subtropics, notable features include a weakening of the LLJ, a rainfall pattern associated with a poleward shift of the South Atlantic convergence zone, and development of the Pacific-South America teleconnection pattern. Published in 2003 by John Wiley & Sons, Ltd.
Intraseasonal Variability of Rainfall Over Northern South America and Caribbean Region
2008
Based on decadal (amounts for each ten days) precipitation data from meteorological stations situated in Northern South America and Caribbean region, a decadal precipitation index (DPI) was calculated in order to study the intraseasonal variability (ISV) of regional rainfall. The spectral analysis of DPI allows to identify signals with 20-25, 30, 40 and 50-60 days period. According to the analysis of their spatial distribution these signals are well defined over the Caribbean island and coastal sector such as in some sectors of the Andean region; the 60-days signal is presented only over Caribbean region and in some places in the Pacific sector; in the eastern lowlands of Orinoco and Amazon basin these signals are not clearly expressed. Exploring the relationship between regional ISV and Madden-Julian Oscillation correlation analysis was made. Due to the presence of signals different of 30-60 days, the correlation coefficients were very low. Considering this situation, high frequency smoothing was applied to DPI time series; after that, a relative correlation was detected between smoothed DPI and Madden-Julian Index (MJI).