José A. Rutllant

Circulation and Low-Frequency Variability near the Chilean Coast: Remotely Forced Fluctuations during the 1991–92 El Niño

Results are reported from the first long, recording current meter observations over the slope off Chile. These observations, at 308S during the 1991-92 El Nino event, are analyzed together with observations of currents at a local deep sea site; local wind and sea level; sea level from the Peru and Chile coasts; and wind, temperature, and currents from the equatorial Pacific. Mean poleward flow of 12 cm s21 was observed within the Peru-Chile Undercurrent over the slope. Mean flow in the depth range of Antarctic Intermediate Water was not distinguishable from zero in the presence of strong, low-frequency (LF) variability, which dominated slope currents at all depths. The strongest LF fluctuations had periods of about 50 days, but periods of 10 and about 5 days were also observed. Significant, local wind forcing of slope currents was only found in the period band 6-10 days and may be related to coastal-trapped waves in the atmosphere. Our analysis shows that free, coastal-trapped waves in the ocean, arriving from the north, dominated the LF variability over the shelf and slope off northern and central Chile during the 1991-92 El Nino event. Strong 50-day period fluctuations there started their journey about two months earlier—and 15 000 km farther up the coastal-equatorial waveguide—near the date line in the equatorial Pacific as equatorial Kelvin waves forced by westerly wind events of similar period. Upon reaching the South American coast, these waves forced coastal- trapped waves, which propagated along the Peru coast into the study region. Likewise, a scenario of equatorial- trapped waves forcing coastal-trapped waves may explain 10-day as well as 6-day and 4.5-day period coastal- trapped waves off Chile stemming from mixed Rossby-gravity and inertia-gravity waves trapped at the equator. Since the large, 50-day period, coastal-trapped waves may strongly modify coastal upwelling source water, such remotely forced waves may have a significant influence on the pelagic ecosystem off Chile, at least during El Nino events.

Meteorological air pollution potential for Santiago, Chile: towards an objective episode forecasting

The geography and climate of the Santiago basin are, in general, unfavorable for the diffusion of air pollutants. Consequently, extreme events occur frequently during the high pollution season extending from April to August. The meteorological conditions concurrent with those extreme events are mainly associated with the leading edges of coastal lows that bring down the base of the semipermanent temperature inversion reducing the dirunal growth of the surface mixed layer. In order to produce an objective 12 to 24-hour episode forecast, a two-way multivariate discriminant analysis has been used in the definition of a meteorological air-pollution potential index (MAPPI), separating high and low meteorological air-pollution potential days. The same procedure has been applied in the selection of the most efficient predictors for the MAPPI objective forecast, based on 12 and 24 UTC radiosonde data at Quintero, about 100 km to the NW of Santiago. Results indicate about 70% correctly forecasted days, with satisfactory skill-scores relative to persistency. The strong persistency characterizing the most efficient predictors in the 12-hour objective forecast scheme, makes the prediction of the first and last days of any particular air-pollution potential episode particularly difficult. To overcome this problem, a new set of predictors based on continuous measurements near the level of the top of the temperature inversion layer (900 hPa during air-pollution episodes) is being tested. Preliminary results indicate that the time-integrated zonal wind component at that level is a reliable precursor for both the onset and the end of air-pollution potential episodes.

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.

Coastal Lows along the Subtropical West Coast of South America: Numerical Simulation of a Typical Case

Abstract Subsynoptic, warm core low pressure areas are frequently observed along the west coast of subtropical South America during austral winter. These so-called coastal lows (CLs) tend to develop as an upper-air, midlatitude ridge is approaching the subtropical Andes and, therefore, while pressure is increasing aloft and farther to the south. These CLs have a profound impact in the coastal weather associated with a rapid transition from clear skies and stronger than average equatorward low-level flow to overcast conditions and relaxed equatorward (or even poleward) flow. Weather conditions inland mostly reflect the associated changes in the strength and height of the base of the subsidence inversion. In this work, a mesoscale simulation of a typical CL episode is performed using a numerical weather prediction model [the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)] Comparison with observations reveals that the model simulation properly captures the large-scale pattern as we...

Southeast Pacific Stratocumulus: High-Frequency Variability and Mesoscale Structures over San Felix Island

Abstract Stratocumulus cloud cover patterns and their relationship to drizzle were characterized at San Felix Island (SFI; 26.5°S, 80°W) in the southeast Pacific Ocean. Small closed, large closed, and open cells were identified in about 65% of the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images during 2003. The MODIS imagery was combined with ceilometer and surface meteorological measurements, human observations of cloud types and drizzle, and large-scale meteorological analyses for January through June. The authors identified two drizzle regimes: a synoptically quiescent summer (January–March) regime characterized by a strong anticyclone, large closed cells, and frequent drizzle, and an autumn (April–June) regime characterized by a weaker anticyclone, small closed cells and open cells, and precipitation that was mainly associated with synoptic activity. The large closed cells had higher mean cloud bases and tops than the small closed cells and accounted for 45% of the cumulus-under...

Assessing the Influence of the MJO on Strong Precipitation Events in Subtropical, Semi-Arid North-Central Chile (30°S)

AbstractAnnual precipitation in subtropical, semiarid north-central Chile (30°S) during rainy years comprises a few (3–5) strong events in the fall and winter, which are presumably modulated by the Madden–Julian oscillation (MJO). Precipitation from 1979–2009 was recorded daily at three stations along the Elqui Valley. The relationship between the MJO and precipitation is investigated from two perspectives: 1) examining a MJO index (MJOI) based on the actual precipitation events and 2) examining the likelihood of precipitation based on a favorable MJOI. About 80% of the strong precipitation events at the coast in La Serena are related to an active MJO near the central equatorial Pacific. These events are often typified by broad, slow moving synoptic systems in phase with the MJO propagation. Blocking in the far southeast Pacific is associated with precipitation 75% of the time, while deep troughs make up the rest.A relationship between a MJOI and strong rainfall suggests that, though it could be used as a...

The Low-Level Atmospheric Circulation near Tongoy Bay–Point Lengua de Vaca (Chilean Coast, 30°S)

AbstractStrong southerly, terrain parallel winds often occur along the coast of north-central Chile (25°–35°S) embedded in the marine atmospheric boundary layer and the lower part of the capping temperature inversion. Their offshore structure and variability have received considerable attention because of the effect on open-ocean processes and connection with the southeast Pacific cloud layer. Mesoscale low-level circulations linked to the coastal topography (e.g., coastal jets and sea breeze) are less studied in Chile, but are particularly relevant as they alter the upper-ocean circulation and the cloud pattern in the nearshore strip.Surface, radiosonde, and airborne meteorological observations near point Lengua de Vaca (LdV)–Tongoy Bay (TB) at 30°S are used alongside numerical modeling to understand the local circulation near a prominent upwelling center. Most observations were gathered during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Chilean Upwelling Exp...

Boundary-layer dynamics of the extremely arid northern part of Chile

A boundary-layer field experiment in the extremely arid northern part of Chile was carried out in July, 1970. Captive radiosonde data, pibal soundings and surface observations in a three-point zonal cross-section at 23 deg lat. S are analyzed. Evidence of a thermal response in the boundary-layer circulation and daytime subsidence enhancement is presented. Surface windspeeds seem to increase in the general upslope direction while wind components along the contour lines are apparent in the central strip, in accordance with earlier results for Pampa de la Joya, Peru.

The Coastal Boundary Layer at the Eastern Margin of the Southeast Pacific (23.4°S, 70.4°W): Cloudiness-Conditioned Climatology

Abstract A basic climatological description of 29 years of surface and upper-air observations at a coastal site (23.4°S, 70.4°W) in northern Chile is presented. The site is considered to be generally representative of the eastern coastal margin of the southeast Pacific stratocumulus region, which plays an important role in the global radiative balance. The analysis focuses on two of the main elements affecting coastal weather in this region: low-level cloudiness and the state of the subsidence temperature inversion. The objectives of the paper are 1) to present the basic climatological features of these elements and 2) to document the differences in the structure of this coastal boundary layer (BL) associated with the presence or absence of low-level clouds. Low-level clouds (defined here as ceilings less than 1500 m AGL) occur at the site mostly in the night, especially during austral winter and spring. Elevated subsidence inversions show a very large prevalence in the 1200 UTC [0800 local time (LT)] rad...

Estacionalidad de la erosión y el transporte eólico de partículas en el desierto costero de Atacama, Chile (23°S)

Resumen es: La costa del Desierto de Atacama se caracteriza por la ocurrencia de vientos fuertes del sur y suroeste. El clima arido, la geomorfologia plana y las car...