Teleconnection between the Surface Wind of Western Patagonia and the SAM, ENSO, and PDO Modes of Variability

International audienceThe Southern Westerly Wind (SWW) belt is one of the most important atmospheric features of the Southern Hemisphere (SH). In Patagonia, these winds control the precipitation rates at the windward side of the southern Andes, and rainfall is very sensitive to any change (strength and/or latitudinal position) in the wind belt. The present-day behavior of the SWW, also known as westerlies, is characterized by remarkable seasonality. This wind belt also varies at interannual-to-decadal time scales, associated with the influence of atmospheric phenomena such as the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), respectively. Moreover, during the past few decades, the westerlies have shown an increase in their core strength influenced by changes in the Southern Annular Mode (SAM). However, what controls the long-term variability of the SWW at the high latitudes of the SH is still a matter of debate. This work statistically analyzes the influence of large-scale modes of variability, such as ENSO and PDO on the SAM and the frequency of the strong SWW from ERA5 reanalysis data of southwestern Patagonia (~51°S), where the current core of this belt is located. Our results confirm the relation between strong wind anomalies and the SAM. In addition, the temporal variations of strong winds are also significantly affected by the PDO, but there is no detectable influence of the ENSO on their frequency. This shows that future studies focused on reconstructing wind history from aeolian particles of lake sediments from southwestern Patagonia could also provide information about the modes of variability that influence strong wind frequency

Variability of the Southwestern Patagonia (51°S) Winds in the Recent (1980–2020) Period: Implications for Past Wind Reconstructions

The Southern Hemisphere Westerly Winds (SWW) control the amount and latitudinal distribution of rainfall in southwestern Patagonia. Recent studies have shown that SWW has intensified in the last decades, but their past behavior is not yet well understood. To understand this behavior, it is necessary to analyze climatic data from meteorological stations and reconstruct their variability through paleoclimatic evidence, such as lake cores. Nevertheless, Patagonia is an austral region characterized by its complex topography and quasi lack of a meteorological network. In this work, three reanalyses are studied (MERRA-2, ERA5, and GLDAS) and compared with the Cerro Castillo and Teniente Gallardo stations (~51°S), with the aim of simulating the winds in the past. The results indicate that ERA5 and MERRA-2 simulate well the wind variability in the study region, while GLDAS is less reliable. Therefore, the first two reanalyses could be used to extend the time series of the meteorological station and calibrate a new wind proxy based on the abundance and size of the aeolian particles, reconstructing in a direct way the intensity of the SWW in the past over southwestern Patagonia

Seasonality of erosion and eolian particle transport in the coastal Atacama Desert, Chile (23°S) Estacionalidad de la erosión y el transporte eólico de partículas en el desierto costero de Atacama, Chile (23°S)

The coast of the Atacama Desert is characterized by the occurrence of strong south- and southwesterly winds. The arid climate, flat geomorphology and the characteristics of the superficial sediments on Pampa Mejillones, located in northern Chile (ca. 23°S), favor the study of eolian erosion and transport processes in this coastal desert. In situ measurements of the horizontal particle flux (G) and winds in the area, together with sedimentological analyses of the uppermost part of the pampa sediments, allowed determination of the significant parameters in the eolian erosion and particle transport process and its seasonality. A local surface roughness parameter (z0) of about 10-3 m was calculated, which is a typical value of desert plains. Seasonally, values of G up to 2,100 g cm-1 month-1 were observed, associated with friction velocities (u*) between 0,4 and 0,5 m s-1 during the spring of 2000. Similar results were obtained for the years 1999 and 2001. The sedimentology (granulometry

Assessing the Origin and Variability of Eolian Lithic Material For High-Resolution Paleoceanographic Reconstructions Off Northern Chile (23 S)

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Estacionalidad de la erosión y el transporte eólico de partículas en el desierto costero de Atacama, Chile (23°S) Seasonality of erosion and eolian particle transport in the coastal Atacama Desert, Chile (23°S)

La costa del Desierto de Atacama se caracteriza por la ocurrencia de vientos fuertes del sur y suroeste. El clima árido, la geomorfología plana y las características granulométricas de los depósitos superficiales de la Pampa Mejillones, ubicada en la costa del norte de Chile (23°S), favorecen el estudio de los procesos de erosión y transporte eólico de partículas en este desierto costero. Mediciones in situ del flujo horizontal de partículas (G) y de vientos, junto con análisis sedimentológicos de la capa superficial de esta pampa, permitieron determinar los parámetros relevantes en el proceso de erosión y transporte eólico de partículas, así como estimar su estacionalidad. Se determinó un coeficiente de rugosidad superficial (z0) promedio del orden de 10-3 m, típico de planicies desérticas. Estacionalmente se observaron valores de G de hasta 2.100 g cm-1 mes-1 asociados a velocidades de fricción (u*) entre 0,4 y 0,5 m s-1 durante la primavera del año 2000. Resultados similares se obtuvieron para los años 1999 y 2001. La sedimentología (granulometría y textura superficial) y mineralogía de las partículas del suelo y del material capturado en las trampas de polvo, permitieron determinar que la fracción transportada está caracterizada principalmente por cuarzo y feldespatos, y modas granulométricas entre 75 y 1.000 um, similares a las observadas en los sedimentos cercanos y al sur (viento arriba) del área de captura. Los resultados obtenidos y la correlación preliminar entre los vientos medidos en distintas estaciones meteorológicas en la península de Mejillones, permiten establecer una relación directa entre el flujo horizontal de partículas hacia la bahía homónima y los vientos regionales del sur-suroeste, especialmente durante los meses de primavera y verano, en concomitancia con la intensificación de los vientos favorables a la surgencia costera en el área oceánica adyacente.The coast of the Atacama Desert is characterized by the occurrence of strong south- and southwesterly winds. The arid climate, fíat geomorphology and the characteristics of the superficial sediments on Pampa Mejillones, located in northern Chile (ca. 23°S), favor the study of eolian erosion and transport processes in this coastal desert. In situ measurements of the horizontal particle flux (G) and winds in the área, together with sedimentological analyses of the uppermost part of the pampa sediments, allowed determination of the significant parameters in the eolian erosión and particle transport process and its seasonality. A local surface roughness parameter (z0) of about 10-3 m was calculated, which is a typical value of desert plains. Seasonally, values of G up to 2,100 g cm-1 month-1 were observed, associated with friction velocities (u*) between 0,4 and 0,5 m s-1 during the spring of 2000. Similar results were obtained for the years 1999 and 2001. The sedimentology (granulometry and surface texture) and mineralogy of the surface and captured material in the dust traps show that the transported fraction is characterized mainly by quartz and feldspars, with grain-size modes between 75 and 1,000 um, similar to those observed in nearby soils and south (upwind) of the capture area. The results obtained and preliminary correlation of measured winds at the different meteorological stations on the Mejillones Península, indicate a direct relationship between the horizontal particle flux towards Mejillones Bay and the regional south-southwest winds, especially during spring and summer, simultaneous with the intensification of winds favoring coastal upwelling in the area

Using image-based size analysis for determining the size distribution and flux of eolian particles sampled in coastal northern Chile (23 degrees S)

The aim of this study was to determine the size distribution and flux of the mineral fraction collected in marine sediment traps in Mejillones Bay (23 degrees S) using image-based size analysis. In this bay the material settling in the water column is a mixture of biogenic particles and lithic grains. The sediments analyzed correspond to four samples collected in the austral spring of 2008. We used image-based size analysis because it presents several advantages, which makes it an ideal method to study fine marine sediments. This analysis also yielded important information about the aspect and the morphology of the particles. In order to analyze the lithic fraction only, different pretreatments were applied and size analyses were carried out with and without image information. Besides the main lithic components (quartz, feldspar, amphiboles, and micas), some nonlithic particles corresponding mainly to organic-matter remains were observed in the images, showing that the pretreatment was not fully effective. Biogenic silica and unexpected dark particles of probable anthropogenic origin were also detected. To avoid the influence of nonlithic grains in the analysis, the images were filtered based on the aspect of the particles using various discrimination tools. Before and after this removal, the size distributions of the samples were estimated and used to calculate the total flux. As compared to the flux of lithic particles, this total flux displays a systematic overestimation of up to 12%. As a consequence, using image-based size analysis allows not only enhancing the determination of the size distribution and flux but also controlling and improving the pretreatments protocols

Recent Changes in the Low-Level Jet along the Subtropical West Coast of South America

Surface winds along the subtropical west coast of South America are characterized by the quasi-weekly occurrences of low-level jet events. These short lived but intense wind events impact the coastal ocean environment. Hence, identifying long-term trends in the coastal low-level jet (CLLJ) is essential for understanding changes in marine ecosystems. Here we use ERA5 reanalysis (1979–2019) and an objective algorithm to track anticyclones to investigate recent changes in CLLJ events off central Chile (25–43 °S). Results present evidence that the number of days with intense wind (≥10 ms−1), and the number and duration of CLLJ events have significantly changed off central Chile in recent decades. There is an increase in the number of CLLJ events in the whole study area during winter (June-July-August; JJA), while during summer (December–January–February; DJF) a decrease is observed at lower latitudes (29–34 °S), and an increase is found at the southern boundary of the Humboldt system. We suggest that changes in the central pressures and frequency of extratropical, migratory anticyclones that reach the coast of South America, which force CLLJs, have played an important role in the recent CLLJ changes observed in this region

A simple model accounting for the uptake, transport, and deposition of wind-eroded mineral particles in the hyperarid coastal Atacama Desert of northern Chile

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