Humberto Fuenzalida

RECENT TEMPERATURE VARIATIONS IN SOUTHERN SOUTH AMERICA

Results from a critical appraisal of surface mean air temperature in Chile and Argentina and extreme air temperature in Chile during the present century are presented. Observations were homogenized to produce a set of time series as reliable as possible. Linear trends computed for the period 1933–1992 resulted in warming rates from 1ċ3 to 2ċ0 °C 100 years−1; during the last three decades warming rates are twice as large. The generalized warming is not present around 41°S, where a cooling period from the 1950s to the 1970s prevails. Both positive and negative trends are due mostly to changes in minimum temperatures. The influence of El Niņo–Southern Oscillation on surface temperature along the Pacific South American coast from 18°S to 53°S was estimated and found to decrease southward. When its effect is extracted, warming trends become more uniform through time. In particular, the Southern Oscillation Index change around 1976 is felt in minimum temperatures at almost all stations, starting a period with higher values along the Chilean Pacific coast. Trend corrections for autocorrelation in the series introduce only small local changes.

Coastal Lows along the Subtropical West Coast of South America: Mean Structure and Evolution

The typical conditions of the eastern boundary of the subtropical anticyclone [e.g., well-defined marine boundary layer (MBL), equatorward low-level flow] that prevail along the mountainous west coast of subtropical South America are frequently disrupted by shallow, warm-core low pressure cells with alongshore and crossshore scales of 1000 and 500 km, respectively. These so-called coastal lows (CLs) occur up to five times per month in all seasons, although they are better defined from fall to spring. Marked weather changes along the coast and farther inland are associated with the transition from pressure drop to pressure rise. The mean structure and evolution of CLs is documented in this work, using a compositing analysis of 57 episodes selected from hourly pressure observations at a coastal station at 30 8S during the austral winters of 1991, 1993, and 1994, and concurrent measurements from a regional research network of nine automatic weather stations, NCEP‐NCAR reanalysis fields and high-resolution visible satellite imagery. Coastal lows tend to develop as a migratory surface anticyclone approaches southern Chile at about 408S producing a poleward-oriented pressure gradient and geostrophically balanced offshore component in the low-level wind. At subtropical latitudes the transition from negative to positive geopotential anomalies occurs around 850 hPa. Enhanced mid- and lowlevel subsidence near the coast and downslope flow over the coastal range and Andes Mountains leads to the replacement of the cool, marine air by adiabatically warmed air, lowering the surface pressure at the coast and offshore. As the midlatitude ridge moves to the east of the Andes, the alongshore pressure gradient reverts back and the easterly wind ceases to act. The recovery of the surface pressure toward mean values occurs as the cool, cloud-topped MBL returns to the subtropical coast, although the pressure rise can be attenuated by midlatitude troughing. The return of the MBL resembles a Kelvin wave propagating along the coast from northern Chile (where the MBL eventually thickened) into subtropical latitudes in about a day.

Variations in UV radiation in Chile

The first observations of UV global radiation in Chile with a portable multichannel radiometer are reported. Four UV wavelengths are included: 305, 320, 340 and 380 nm. Observation latitudes spanning from 18 degrees to 53 degrees S allow an estimation of latitudinal variations in daily maxima for both summer and winter. Measurements over Santiago deviate from a smooth latitudinal profile, probably as a consequence of urban air pollution. The main effect of this is to prevent UV solar radiation from reaching the ground, especially during winter. Altitudinal increments in UV radiation are estimated by comparing observations along the coast and valleys with others on the Andes and one isolated summit. Diurnal variations in the height increment support an increase from morning to evening. The results indicate that in rural areas the altitudinal increment is lower (4%-10% per kilometre) than that reported for Europe, reaching very low magnitudes (2% or less) in the Andean summits of desert regions.

Ozone and UV Radiation over Southern South America: Climatology and Anomalies

Abstract Ozone and UV radiation were analyzed at eight stations from tropical to sub-Antarctic regions in South America. Ground UV irradiances were measured by multichannel radiometers as part of the Inter American Institute for Global Change Radiation network. The irradiance channels used for this study were centered at 305 nm (for UV-B measurements) and 340 nm (for UV-A measurements). Results were presented as daily maximum irradiances, as monthly averaged, daily integrated irradiances and as the ratio of 305 nm to 340 nm. These findings are the first to be based on a long time series of semispectral data from the southern region of South America. As expected, the UV-B channel and total column ozone varied with latitude. The pattern of the UV-A channel was more complex because of local atmospheric conditions. Total column ozone levels of <220 Dobson Units were observed at all sites. Analysis of autocorrelations showed a larger persistence of total column ozone level than irradiance. A decreasing cross-correlation coefficient between 305 and 340 nm and an increasing cross-correlation coefficient between 305 nm and ozone were observed at higher latitudes, indicating that factors such as cloud cover tend to dominate at northern sites and that ozone levels tend to dominate at southern sites. These results highlight the value of long-term monitoring of radiation with multichannel radiometers to determine climatological data and evaluate the combination of factors affecting ground UV radiation.

The Influence of the Andes on Cutoff Lows: A Modeling Study*

Abstract A cutoff low (COL) pressure system that occurred in March 2005 (late austral summer) over the subtropical southeast Pacific is examined by means of numerical simulations using the Weather and Research Forecasting (WRF) model. The episode exhibited typical features of COLs in this region, including its formation from an elongated northwest–southeast extratropical trough and subsequent intensification off the west coast of South America. During the developing stage, the cyclonic circulation did not extend into the lower troposphere and only upper-level, nonprecipitating clouds were observed at and around the system. When the COL reached the continent it produced moderate but unseasonal rainfall along the semiarid western slope of the Andes cordillera [summit level at ∼5000 m above sea level (ASL)] at the same time that the system experienced a rapid decay. The control simulation used full physics, full topography, and a single domain (54-km grid spacing) laterally forced by atmospheric reanalysis. ...

Ultraviolet solar radiation at Valdivia, Chile (39.8°S)

Daily and annual cycles of solar ultraviolet radiation were characterised at Valdivia, Chile (39.8°S), using data collected with a multichannel radiometer between January 1995 and March 1999. First, the instruments measurements at 305 and 340 nm were analysed. In a second step, erythemal dose rates and daily doses were calculated from the measured data and their daily and seasonal variation was evaluated. In January, hourly mean irradiance at noon reaches 7.4 and 64 μW cm−2 nm−1 for 305 and 340 nm, respectively; corresponding values for July are 0.3 and 20 μW cm−2 nm−1. Erythemal daily doses vary from 300 to 6000 J m−2 between July and January. Episodes with high radiation are classified in duration and accumulated dose. A single event with very low total ozone and corresponding high UV levels is presented. Images from the Total Ozone Mapping Spectrometer (TOMS) are used to show its evolution. Variations in total ozone, measured from space, and cloud changes, inferred from observed UV-A radiation, explain 77% of the daily changes in UV-B.

Interannual variations of global UV radiation in Santiago, Chile (33.5°S)

Observations with a four-channel UV radiometer in Santiago, Chile (33.5°S, 70.6°W) from January 1992 to December 1998 are presented. Channels are centered at 305, 320, 340 and 380 nm with a 10 nm bandwidth. Measurements were made at one-minute intervals. Hourly mean values at noon for 305 and 340 nm are presented as well as instantaneous irradiances for 60° of solar zenith angle. Their aperiodic variations on a seasonal scale are discussed with respect to the quasi-biennial oscillation (QBO) and ENSO phenomena. A significant positive trend that must be caused by decreasing total ozone is found in 305 nm irradiance. On a seasonal basis, this negative trend appears strong and significant during winter.

Multichannel radiometer calibration: a new approach.

The error in irradiance measured with Sun-calibrated multichannel radiometers may be large when the solar zenith angle (SZA) increases. This could be particularly detrimental in radiometers installed at mid and high latitudes, where SZAs at noon are larger than 50 degrees during part of the year. When a multiregressive methodology, including the total ozone column and SZA, was applied in the calculation of the calibration constant, an important improvement was observed. By combining two different equations, an improvement was obtained at almost all the SZAs in the calibration. An independent test that compared the irradiance of a multichannel instrument and a spectroradiometer installed in Ushuaia, Argentina, was used to confirm the results.

Prewhitening of Climatological Time Series

Abstract The most common procedures to prewhite a climatological time series are considered and compared with those based on Fourier analysis. In particular, advantages and shortcomings of the anomaly method and the seasonal differences are noted. Some modifications for the Fourier prewhitened synthesis are introduced to preserve the smoothness of the spectra. A simple recursion filter with prewhitening ability is presented and compared with the other filter using synthetic and a real time cries. Special attention is given to the case of short time series. The anomaly method and the recursion filter appear as suitable procedures to replace a Fourier technique giving both similar outputs but differing in the details. The seasonal differences, although very simple to apply, introduce substantial modifications in the output and should be avoided. When the data series contains a few times the cycle to be eliminated the recursion filter seems to be safer than the anomaly method.

Wet and dry deposition of nitrogen compounds in the southeast Pacific coast: Montemar, central Chile

Atmospheric concentration in the marine boundary layer and direct deposition measurements of total nitrate and ammonium for an eastern South Pacific coastal site near Valparaiso, Chile, were carried out during the period January 1992 through March 1993. Concentrations in air for both nitrogen forms exhibit a seasonal pattern with low values in summer and a pronounced increase in winter, having a mean annual concentration of 32.6 nmol m−3 for total nitrate and of 44.8 nmol m−3 for total ammonium. Wet deposition is characterized by annual precipitation weighted concentrations of 6.0 and 8.1 μmol dm−3 for nitrate and ammonium, respectively. The main factor regulating wet deposition for both nitrate and ammonium is rainfall amount, so that changes in precipitation amount alone originated by climatological factors can produce changes in their relative deposition levels. Existing climatological pattern determines an overall dominance of dry over wet deposition, with totals of 12.9 and 7.34 mmol m−2 per year, the reduced forms of nitrogen being generally more abundant.