Synoptic-scale variability of surface winds and ocean response to atmospheric forcing in the eastern austral Pacific Ocean
Authors: Pérez-Santos Iván , Seguel Romanet , Schneider Wolfgang , Linford Pamela , Donoso David , Navarro Eduardo , Amaya-Cárcamo Constanza , Pinilla Elías , Daneri Giovanni 
Published: September 2019

Abstract. In the Southern Hemisphere, macroscale atmospheric systems such as
westerly winds and the southeast Pacific subtropical anti-cyclone (SPSA)
influence the wind regime of the eastern austral Pacific Ocean. The average
and seasonal behaviors of these systems are well known, although wind
variability at different time and distance scales remains largely
unexamined. Therefore, the main goal of this study was to determine the
variabilities of surface winds on a spatiotemporal scale from 40 to 56∘ S,
using QuikSCAT, Advanced Scatterometer (ASCAT), and the fifth major global
European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) surface-wind
information complemented with in situ meteorological data. In addition,
interactions between the atmospheric systems, together with the
ocean–atmosphere response, were evaluated for the period 1999–2018. The
empirical orthogonal function detected dominance at the synoptic scale in
mode 1, representing approximately 30 % of the total variance. In this
mode, low and high atmospheric pressure systems characterized wind
variability for a 16.5 d cycle. Initially, mode 2 – which represents
approximately 22 % of the variance – was represented by winds from the
west/east (43–56∘ S), occurring mostly during spring
and summer/fall and winter at an annual timescale (1999–2008) until they
were replaced by systems cycling at 27.5 d (2008–2015). This reflects
the influence of the baroclinic annular mode in the Southern Hemisphere.
Mode 3, representing approximately 15 % of the variance, involved the
passage of small-scale low and high atmospheric pressure (LAP and HAP)
systems throughout Patagonia. Persistent Ekman suction occurred throughout
the year south of the Gulf of Penas and beyond the Pacific mouth of the
Strait of Magellan. Easterly Ekman transport (ET) piled these upwelled waters
onto the western shore of South America when winds blew southward. These
physical mechanisms were essential in bringing nutrients to the surface and
then transporting planktonic organisms from the oceanic zone to Patagonian
fjords and channels. In the zonal band between 41 and 43∘ S, the latitude of Chiloé Island, upward Ekman pumping and Ekman transport during spring and summer favored a reduced sea surface temperature and
increased chlorophyll a (Chl a) levels; this is the first time that such Ekman upwelling conditions have been reported so far south in the eastern Pacific Ocean. The influence of the northward-migrating LAP systems on the ocean–atmosphere interphase allowed us to understand, for the first time, their direct relationship with recorded nighttime air temperature maxima (locally referred to as “nighttime heatwave events”). In the context of global climate change, greater attention should be paid to these processes based on their possible impact on the rate of glacier melting and on the austral climate.