Climate of the last millennium at the southern pole of the North Atlantic Oscillation: an inner-shelf sediment record of flooding and upwelling

Abstract

Continental and marine conditions during the last millennium off Porto, Portugal (the southern pole of the North Atlantic Oscillation, NAO), are reconstructed from a sediment archive through a high-resolution multiproxy study and instrumental evidence. Results show multidecadal variability and sea surface temperatures (SSTs) that correlate well with previously published landand sea-based Northern Hemisphere temperature records, and appear to be responding to long-term solar insolation variability. Precipitation was negatively correlated with the NAO, whereas strong flooding events occured at times of marked climate cooling (AD 1100−1150 and 1400−1470) and transitions in solar activity. AD 1850 marks a major shift in the phytoplankton community associated with a decoupling of δ 18O records of 3 planktonic foraminifera species. These changes are interpreted as a response to a reduction in the summer and/or annual upwelling and more frequent fall−winter upwelling-like events. This shift's coincidence with a decrease in SST and the increase in coherence between our data and the Atlantic Multidecadal Oscillation (AMO) confirms the connection of the upwelling variability to the North Atlantic Ocean's surface and thermohaline circulation on a decadal scale. The disappearance of this agreement between the AMO and our records beyond AD 1850 and its coincidence with the beginning of the recent rise in atmospheric CO 2 supports the hypothesis of a strong anthropogenic effect on the last ~150 yr of the climate record. Furthermore, it raises an important question of the use of instrumental records as the sole calibration data set for climate reconstructions, as these may not provide the best analogue for climate beyond AD 1730.

Historical Climate off the Atlantic Iberian Peninsula

“…The transition to the Industrial Era, in the northern Porto site, although distinct at 1850 CE, starts at 1730−1740 CE (Abrantes et al, 2011) and is accompanied by the return of SSTs to the values observed pre 1300 CE. A pattern that coincides with the atmospheric temperature rise detected in NE Spain, 25…”

“…The transition into the Industrial Era is also marked by a shift in the phytoplankton community and water column stratification over the northern Porto site, which was interpreted by Abrantes et al, (2011), as a response to a reduction in the summer and/or annual upwelling and more frequent fall−winter upwelling-like events, on the basis of published evidence ( (Abrantes et al, 2011;Alvarez et 35 Clim. Past Discuss., doi:10.5194/cp-2017-39, 2017 Manuscript under review for journal Clim.…”

“…The new sites lack such data, but given that Fe and lipid compounds synthesized by higher plants, such as C23-C33 n-alkanes ([n-alc]), show a coherent pattern of variability at both the 25 Tagus and Porto sites (Abrantes et al, 2005b(Abrantes et al, , 2011, a Pearson correlation of 0.47 at p>0.01 and n=250, and that [n-alc] is available for all sites, we will use this proxy to qualify the intensity of River runoff.…”

“…A local calibration for the Porto site, via SST data from a 5° grid provided by the Climatic Research Unit shows the alkenone SST as a Spring / Fall record Abrantes (Abrantes et al, 2011). In order to confirm that indication for other sites, the same exercise was performed, but now using a smaller grid satellite estimated mean annual and seasonal temperatures for the Porto, Lisbon and 5…”

“…(Hernández et al, 2015;JambrinaEnríquez et al, 2016;Morellón et al, 2009;Moreno et al, 2008;Valero-Garcés et al, 2006); 5 speleothems (e.g. (Martín-Chivelet et al, 2011) and marine sediments (e.g (Abrantes et al, 2005;Abrantes et al, 2011;Lebreiro et al, 2006;Pena et al, 2010), have provided individual records and compiled evaluations of climate evolution over Iberia during the historical period (e.g. (Moreno et al, 2011;Sánchez-López et al, 2016).…”