1Department of Geophysics, University of Chile, Santiago, Chile
2Centro de Estudios Avanzados en Zonas Aridas (CEAZA), La Serena, Chile
Abstract. Surface coastal observations from two automatic weather stations at Paposo (~25° S) and radiosonde observations at Paposo and Iquique (~20° S), were carried out during VOCALS-REx. Within the coastal marine boundary layer (MBL), characteristic sea-land breezes are superimposed on the prevailing southerlies, resulting in light northeasterly winds from mid-night to early morning and strong near-surface southwesterlies in the afternoon. The prevailing northerly wind above the MBL and below the Andes top is modulated by the onshore–offshore (zonal) flow components induced by the diurnal cycle of net radiation along the western slope of the Andes. This diurnal cycle of the zonal regional circulation is consistent with an enhanced afternoon coastal subsidence manifested in a lower inversion base and a slight warming at its top. A numerical simulation of this zonal atmospheric circulation in a regional domain captures the afternoon zonal wind divergence and resulting subsidence along a narrow (~10 km) coastal strip.
Day-to-day variability during VOCALS-REx shows subsynoptic oscillations in the MBL depth, aside from two major disruptions in connection with a deep trough and a cutoff low, as described elsewhere. These oscillations are phase-locked to those in sea-level pressure and afternoon alongshore southerlies, as found in connection with coastal lows farther south.
From a simple scale analysis, one can tentatively conclude that the mean offshore transport of sulfur dioxide from inner, elevated sources could be associated with the afternoon seaward flow with a delay of the order of at least one-day. Within the MBL, biogenic dimethylsulfide (DMS) could be more easily degassed in the afternoon due to the strengthening of the SW winds, while other coastal sources could contribute preferentially at dawn, coinciding with the maximum coastal low-cloud cover.