Upwelling response to atmospheric coastal jets off central Chile: A modeling study of the October 2000 event

Abstract

[1] The spatial and temporal variability of nearshore winds in eastern boundary current systems affect the oceanic heat balance that drives sea surface temperature changes. In this study, regional atmospheric and oceanic simulations are used to document such processes during an atmospheric coastal jet event off central Chile. The event is well reproduced by the atmospheric model and is associated with the migration of an anomalous anticyclone in the southeastern Pacific region during October 2000. A robust feature of the simulation is a sharp coastal wind dropoff, which is insensitive to model resolution. As expected, the simulated oceanic response is a significant sea surface cooling. A surface heat budget analysis shows that vertical mixing is a major contributor to the cooling tendency both in the jet core area and in the nearshore zone where the magnitude of this term is comparable to the magnitude of vertical advection. Sensitivity experiments show that the oceanic response in the coastal area is sensitive to wind dropoff representation. This is because total upwelling, i.e., the sum of coastal upwelling and Ekman pumping, depends on the scale of wind dropoff. Because the latter is much larger than the upwelling scale, coastal wind dropoff has only a weak positive effect on vertical velocities driven by Ekman pumping but has a strong negative effect on coastal upwelling. Interestingly though, the weakening of coastal winds in the dropoff zone has a larger effect on vertical mixing than on vertical advection, with both effects contributing to a reduction of cooling.

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼ 30° S) based on a high-resolution atmospheric regional model (WRF)

“…Here a total of 42 η levels were used in the vertical with increasing resolution toward the surface, 20 of them in the lowest 1.5 km with ∼ 30 m in the vertical for the surface level. Such telescopic resolution is a common choice in precedent studies to properly simulate the marine boundary layer depth over the ocean Rahn and Garreaud 2013;Toniazzo et al, 2013;Renault et al, 2012;Rutllant et al, 2013).…”

“…Although previous studies have documented the importance of the wind stress curl near the coast of central Chile (Renault et al, 2012;Aguirre et al, 2012), the impact of the abrupt transition of the wind near the coast (i.e., dropoff) and its seasonal variability on upwelling are still poorly understood. Here, prior to addressing this issue from an oceanic perspective, our objective is to document the wind stress curl (drop-off) and its seasonal variability off centralnorthern Chile (∼ 30 • S) using a high-resolution (∼ 4 km) atmospheric model.…”

“…Within central-northern Chile the area around 30 • S is characterized by the most intense upwelling favorable winds (Shaffer et al, 1999;Rutllant and Montecino, 2002). Additionally, local high-frequency forcing in the region is associated with atmospheric coastal jets with periods less than 25 days, which are related to synoptic dynamics of the mid-latitude pressure perturbations, in this case high pressures that migrate toward the east Rahn and Garreaud, 2013) and play a major role in coastal upwelling (Renault et al, 2009(Renault et al, , 2012Aguirre et al, 2012). All these features make the region a natural laboratory to explore the forcing mechanisms and describe the physical processes that modulate coastal upwelling.…”

“…Traditionally a simple relationship based on wind stress along the coast has been used as an index of the coastal upwelling intensity (Bakun, 1973); this approximation does not consider other more complex physical processes, such as the wind curl (Pickett and Paduan, 2003;Capet et al, 2004;Jacox and Edwards, 2012) and the geostrophic flow toward the coast, which is in balance with the alongshore pressure gradient and could potentially limit upwelling Marchesiello and Estrade, 2010). In the case of the wind curl, several modeling studies from different upwelling systems suggest that wind stress decreases within a narrow coastal band of 10-80 km called wind "drop-off" (Capet et al, 2004;Bane et al, 2005;Perlin et al, 2007;Renault et al, 2012Renault et al, , 2015 that is highly sensitive to the resolution of the model. Thus, regional ocean modeling studies show that the upwelling response is sensitive to the transition in the structure of the wind near the coast (Capet et al, 2004;Jacox and Edwards, 2012), where the structure and physical forcing of the transitional coastal wind profile is not well understood (Jin et al, 2009).…”

“…In a recent modeling study Renault et al (2012) analyzed the main physical processes that explain changes in sea surface temperature in an upwelling event during the occurrence of an atmospheric coastal jet along the centralnorthern Chile region. The results showed a clear drop-off of the coastal wind that was not observed in the QuikSCAT data due to the "blind zone" in the satellite measurements (∼ 25 km offshore).…”