Prevailing wind along the west coast of South America is equatorward, driven by the southeast Pacific anticyclone. The wind induces strong coastal upwelling that supports one of the most important fisheries in the world. This region lacks a dense network of in situ observations, so the high resolution (0.313°) NCEP Climate Forecast System Reanalysis is used here to present a synoptic climatology of the coastal wind along the Chile/Peru coast. Covariability between the alongshore pressure gradient and alongshore wind, which was previously identified for synoptic time scales near central Chile, is generalized for the whole coast and over annual time scales. Particular attention is paid to three prominent upwelling regions: Pisco (14.8°S), Punta Lengua de Vaca (30.0°S), and Punta Lavapie (36.4°S). Previous work has identified local maxima at these points but these are embedded in a broader low‐level jet that exhibits a marked seasonal cycle of strong wind days due to the migration of the anticyclone and is associated with a shift of both the mean wind and a more frequent recurrence of strong wind events. Alongshore wind near Pisco is normally distributed year‐round with a seasonal shift in the mean. Larger variability in the mean and distribution is found at Lavapie, associated with the seasonal change in storm tracks. The synoptic evolution that drives high‐wind events at each location is characterized. A midlevel trough and surface cyclone precede wind maxima at each location and are followed by strong midlevel ridging and a strengthened surface anticyclone. © 2013 Royal Meteorological Society

Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave. Along 20° S the MBL is generally deeper offshore (1600 m at 85° W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74–76° W. Weather Research and Forecasting (WRF) simulations underestimate MBL height the most near the coast but improve offshore. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ~74° W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18–25° S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion results in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth. Interference is clearly seen in the soundings at Iquique which are driven by a strong upsidence wave contrary to the radiation-driven cycle, leading to a diurnal cycle opposite of the other sites. Because WRF simulations have a lower MBL height, the speed of the simulated gravity wave is slower than observations and accounts for most of the discrepancy between observed and simulated phase speeds.

In the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. In the prognostic local MBL height equation the height change, the horizontal MBL height advection, and the large scale vertical velocity at MBL top are calculated explicitly from the simulation. The entrainment velocity is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes.

The climate of the ocean’s eastern boundaries is strongly influenced by subtropical anticyclones, which drive a surface wind stress that promotes coastal upwelling of nutrient-rich subsurface water that supports high primary productivity and an abundance of food resources. Understanding the projected response of upwelling-favourable winds to climate change has broad implications for coastal biogeochemistry, ecology, and fisheries. Here we use a reanalysis, an ensemble of global climate simulations, and an objective algorithm to track anticyclones to investigate the projected changes in upwelling-favourable wind events at the California, Canary, Humboldt, and Benguela coastal upwelling systems. Except for the north Pacific, we find consistent poleward shifts of mean and upper percentile daily winds over the ocean basins. We propose that extratropical, synoptic-scale migratory anticyclones that force intense coastal upwelling events—which become more frequent at higher latitudes and less frequent at lower latitudes in the future—play an important role in the projected changes in upwelling-favourable wind events in these coastal upwelling systems. These changes complement large-scale processes such as the poleward shift of the subtropical ridge (STR) and stationary subtropical highs. Hence, both extratropical and tropical processes need to be considered to fully explain projected changes at the coastal upwelling systems under anthropogenic climate change.

The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) was a major field experiment conducted in spring of 2008 off southern Peru and northern Chile, aimed at better understanding the coupled climate systems of the southeast Pacific. Because of logistical constrains, the coastal area around 30° S was not sampled during VOCALS-REx. This area not only marks the poleward edge of the subtropical stratocumulus cloud regime (thus acting as a source of transient disturbances) but is also one of the most active upwelling centers and source of surface ocean kinetic energy along the Chilean coast. To fill such an observational gap, a small, brief, but highly focused field experiment was conducted in late spring 2009 in the near-shore region around 30° S. The Chilean Upwelling Experiment (CUpEx) was endorsed by VOCALS as a regional component. CUpEx included long-term monitoring, an intensive two-week field campaign and off-shore research flights. Our goal was to obtain an atmospheric/oceanic dataset with enough temporal and spatial coverage to be able to document (a) the mean diurnal cycles of the lower-troposphere and upper-ocean in a region of complex topography and coastline geometry, and (b) the ocean-atmosphere response to the rapid changes in coastal winds from strong, upwelling-favorable equatorward flow (southerly winds) to downwelling-favorable poleward flow (northerly winds). In this paper we describe the measurement platforms and sampling strategy, and provide an observational overview, highlighting some key mean-state and transient features.