Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Alvaro Díaz x
  • Refine by Access: All Content x
Clear All Modify Search
Alvaro DÍaz
and
Patricio Aceituno

Abstract

Regional and large-scale circulation anomalies associated with periods of enhanced and reduced convective cloudiness over Uruguay are studied for austral spring and summer, when rainfall associated with deep convection is more frequent in this region. The analysis was performed at a submonthly timescale, considering that the essential nature of the mechanisms producing rainfall is not well captured by anomalies calculated on a monthly or seasonal basis in regions where precipitation is highly episodic.

Periods of enhanced and reduced convective cloudiness over Uruguay are characterized by a marked dipolar structure in the outgoing longwave radiation anomaly field along eastern South America from 10° to 40°S, with the centers of the dipole located over the South Atlantic convergence zone (SACZ) and over a broad region including Uruguay, southern Brazil, and northeastern Argentina. This dipole, which corresponds to one of the key factors of climate dynamics in South America during spring and summer, seems to be part of a much larger wavelike quasi-barotropic structure that includes alternating centers of negative and positive geopotential height and temperature anomalies in the southern portion of the continent, and farther upstream in the southern Pacific. At the regional scale, periods of enhanced convection and rainfall over Uruguay are associated with the following features: a warm-core anticyclonic circulation anomaly in the middle and upper troposphere, centered on 34°S, 45°W, approximately; an intensified Chaco low in northwestern Argentina that favors a reinforced northwesterly flow of warm and moist air from the Amazon basin; and an anomalously strong subtropical jet along eastern South America. Periods with reduced convective cloudiness over Uruguay are characterized by circulation anomalies that are broadly opposite to those described before, although some significant asymmetries in their intensity are documented. No major differences were detected in the circulation anomaly patterns between spring and summer, although some changes in the wavelike structure associated to the dipole were found.

Considering the extent of circulation anomalies described here for the austral summer semester, it seems plausible that they also characterize rainfall anomalies over a broader region in southeastern South America.

Full access
Aldo Montecinos
,
Alvaro Díaz
, and
Patricio Aceituno

Abstract

The seasonality of the simultaneous relationship between tropical Pacific SST and rainfall, as well as rainfall predictability one season in advance in subtropical South America (25°–40°S), is studied using different multivariate techniques. This study shows that ENSO-related rainfall anomalies in subtropical South America are restricted mostly to regions on the eastern and western sides of the continent and mainly during the second half of the year. The relationship is almost exclusively of the warm–wet/cold–dry type, but a more widespread impact is found when anomalously warm conditions prevail in the equatorial Pacific. A spatially coherent region with a significant warm–wet/cold–dry signal is detected in southeastern South America during austral spring (October–November), including southern Brazil, southern Paraguay, Uruguay, and eastern Argentina. This signal moves inland toward the west from spring to early summer. During late winter (July–August), a similar SST–rainfall relationship is found in subtropical Chile and southern Brazil. In Chile, a southward propagation of the signal is observed from winter to spring.

Most significant ENSO-related rainfall anomalies seem to occur after the maximum in the precipitation annual cycle. The combined analysis of seasonal diagnostics and predictability of rainfall show that the seasonal rainfall predictability in subtropical South America based on tropical Pacific SST to a greater extent is restricted to a specific time of the year and regions that broadly coincide with those where the simultaneous SST–rainfall relationship is significant. This fact suggests that persistence of tropical Pacific SST anomaly is the major source of seasonal rainfall predictability in this region, when SST is used as a predictor.

Full access
Gabriel Pisciottano
,
Alvaro Díaz
,
Gabriel Cazess
, and
Carlos R. Mechoso

Abstract

The relationships between rainfall over Uruguay (in southeastern South America) and the El Niño-Southern Oscillation phenomenon are investigated. Long time series of data from a dense network of rainfall stations are analyzed using an empirical method based on that proposed by Ropelewski and Halpert. The spatial patterns of the relationships and their temporal variability for the entire region and four subregions are studied in detail.

It is found that years with El Niño events tend to have higher than average rainfall, especially from November to the next January. Further, years with high values of the Southern Oscillation index (501) tend to have lower than average rainfall, especially from October through December. These findings are in general agreement with previous studies. It is also found that the period from March through July tends to have higher than average rainfall after El Niño years and lower than average rainfall after high-SOI years. For the southern part of Uruguay, the wet anomalies during El Niño events are relatively weak, but the dry anomalies during high-SOI events are significant for the two periods identified. The dry anomalies disappear, and even revere, during January and February after high-SOI years. This feature does not have a symmetric counterpart during January and February after El Niño years.

This study, therefore, provides both a verification and an extension of other studies that have emphasized southeastern South America but have used data from only a very few stations in the region.

Full access
Alvaro F. Diaz
,
Caarem D. Studzinski
, and
Carlos R. Mechoso

Abstract

This study focuses on precipitation in Uruguay and the Brazilian state of Rio Grande do Sul, which extend along the Atlantic coast of southern South America. The present paper has two principal goals: 1) to describe the annual cycle of precipitation and 2) to investigate the relationships between its anomalies and those in sea surface temperature (SST) in the Pacific and Atlantic oceans. The dataset is provided by 40 rainfall stations almost evenly distributed in space and covers the period 1917–80. The tools used in support of this research include principal component and canonical correlation analyses.

It is found that total precipitation tends to be evenly distributed during the year. The largest spatial variability in the monthly deviations from the annual mean appears as a west–east (inland–coastal) dipole with the largest positive values in the west during early fall and midspring, and in the east along the Atlantic coast during winter. The second mode of rainfall variability appears as a north–south dipole with the largest positive values in the south during late summer and late fall, and in the north during early spring and early summer. The third mode appears primarily as a north–south dipole along the western boundary with the largest positive values in the southwest during fall and in the northwest during early spring. These modes explain 60%, 19%, and 8% of the total variance. Five subregions are identified according to similarities between the characteristics of the annual cycles in their rainfall stations.

It is shown that there are significant relationships between anomalies in rainfall and in SST in the Pacific and Atlantic oceans. Some of these relationships confirm the results of previous studies, such as the links between the El Niño–Southern Oscillation phenomenon in the equatorial Pacific Ocean and rainfall anomalies in Uruguay during late spring–early summer and late fall–early winter. Other relationships have not been reported before, such as the links between SST anomalies in the southwestern Atlantic Ocean and rainfall anomalies in the entire region during October–December and April–July. It is also found that when SST anomalies are considered in both oceans simultaneously, their links with rainfall anomalies are in some cases enhanced and in others weakened.

Full access
Alvaro Avila-Diaz
,
David H. Bromwich
,
Aaron B. Wilson
,
Flavio Justino
, and
Sheng-Hung Wang

ABSTRACT

Atmospheric reanalyses are a valuable climate-related resource where in situ data are sparse. However, few studies have investigated the skill of reanalyses to represent extreme climate indices over the North American Arctic, where changes have been rapid and indigenous responses to change are critical. This study investigates temperature and precipitation extremes as defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) over a 17-yr period (2000–16) for regional and global reanalyses, namely the Arctic System Reanalysis, version 2 (ASRv2); North American Regional Reanalysis (NARR); European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis; Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2); and Global Meteorological Forcing Dataset for Land Surface Modeling (GMFD). Results indicate that the best performances are demonstrated by ASRv2 and ERA5. Relative to observations, reanalyses show the weakest performance over far northern basins (e.g., the Arctic and Hudson basins) where observing networks are less dense. Observations and reanalyses show consistent warming with decreased frequency and intensity of cold extremes. Cold days, cold nights, frost days, and ice days have decreased dramatically over the last two decades. Warming can be linked to a simultaneous increase in daily precipitation intensity over several basins in the domain. Moreover, the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) distinctly influence extreme climate indices. Thus, these findings detail the complexity of how the climate of the Arctic is changing, not just in an average sense, but in extreme events that have significant impacts on people and places.

Full access