Modulation of the sea‐surface temperature in the Southeast Pacific by the atmospheric low‐level coastal jet

Abstract

[1] The atmospheric low-level coastal jet (LLCJ) in the Southeast Pacific (SEP) region is characterized as either a strong-forcing jet (colder and drier air) or weak-forcing jet (warm and moist) based on the location of the Southeast Pacific high-pressure system (SEPH). The sea-surface temperature (SST) changes corresponding to a particularly strong-forcing jet (29-30 October) and weak-forcing jet (22-23 November) are investigated in this study using the two-way air-ocean coupled model COAMPSV R and satellite observation data. Results indicate that the coupled simulation reduces the overall absolute bias 50% for the surface wind speed, 70% for the cloud liquid water path, and 15% for SST as compared to the uncoupled simulation. The coupled simulation reduces excessive SST cooling, especially during the strong-forcing jet period along the coastal area where offshore transport of upwelled cold water is too strong from the uncoupled simulation. The coupled simulation also reduces the excessive warming from the uncoupled simulation by providing better cloud coverage. The prominent mechanisms in cooling SST along the coast are the same for both the strong-forcing and weak-forcing jets, namely vigorous upwelling and horizontal advection. However, the mechanisms along the jet path differ from along the coast, with air-sea heat exchange the most important process, resulting in cooling SST during the strong-forcing jet period but warming SST during the weak-forcing jet period. The advances and differences of the present study as compared with previous studies are discussed in detail in the paper.

Sensitivity of the Near‐Shore Oceanic Circulation Off Central Chile to Coastal Wind Profiles Characteristics

“…Ekman transport (horizontal advection) and Ekman suction (vertical upward advection) are processes that tend to cool the SST near the coast, which is balanced by the surface heat flux forcing and mixing for the long‐term mean and at seasonal time scales. The rate of SST change due to Ekman upwelling writes as follows (notation similar to Hong et al, ): where T is the temperature, t is time, z is the vertical coordinate, and W e is the Ekman vertical velocity (units m/s). The vertical gradient of temperature in equation is estimated from the difference of the temperature at the surface and at the base of the mixed layer.…”

“…The mean and seasonal cycle of the SST change rate due to Ekman suction (Figure a) exhibit naturally higher amplitude in the drop‐off experiments than in CR (mean change rate: −0.0038, −0.0201, −0.0246, and −0.0354 (°C/day) for CR, DO36, DO12, and DO4, respectively). Its magnitude is in phase with the seasonal warming during spring and summer, which increases the vertical gradient of temperature, providing the positive precondition for the surface cooling (Hong et al, ). On the other hand, the rate of SST change due to horizontal advection of temperature (Figure b) is weaker in the drop‐off experiments compared to CR (mean change rate: −0.18, −0.1173, −0.1175, and −0.1053 (°C/day) for CR, DO36, DO12, and DO4, respectively) owing to the reduced coastal divergence in the sensitivity experiments compared to CR.…”

“…Ekman transport (horizontal advection) and Ekman suction (vertical upward advection) are processes that tend to cool the SST near the coast, which is balanced by the surface heat flux forcing and mixing for the long-term mean and at seasonal time scales. The rate of SST change due to Ekman upwelling writes as follows (notation similar to Hong et al, 2013):…”

“…In order to assess the effect of the changes in Ekman processes associated to the different wind profiles on SST along the coast, we follow the approach of Hong et al () to provide off‐line estimates (see section for details of the calculation) of their relative contributions. The seasonal cycle is considered in the analysis owing to the marked seasonality of the atmospheric circulation off Central Chile, in particular associated to the meridional migration of the low‐level jet (Garreaud & Muñoz, ; Rahn & Garreaud, ; Renault et al, ).…”