Middle- to late-Holocene storminess in Brittany (NW France): Part II – The chronology of events and climate forcing

Abstract

This study focuses on the recurring climate conditions required for the largest storms occurring in NW France (Brittany). It is based on the analysed records of storm events along Western Brittany coast (see Part I). In this manuscript (Part II), storm recurrence is explored along with forcing mechanisms. Periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms (North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO) modes) triggering the most destructive storms. Then, palaeostorm events are discussed at the Holocene scale, from 6000 yr BP to present, to verify the forcing mechanisms. Most recorded events appear to be linked with cooling episodes, mostly in winter, a transition to or from a negative winter NAO mode, a positive AMO mode. Extreme storms occur immediately prior to the 'Medieval Warm Period' (MWP). Maximum effects are reached prior to the onset of the MWP and during the Maunder and Dalton solar minima. Low storm activity occurred during the Spörer Minimum linked to an acceleration of the Atlantic Meridional Overturning Circulation (AMOC). Main storm triggers seem to correspond to a positive AMO mode with an unstable jetstream configuration driving a negative NAO. In this study, four specific weather configurations were defined to explain each type of recorded storminess. The strongest storms correspond to low AMO and decennial-negative NAO modes (e.g. 'Little Ice Age'), or high AMO in association with dominant low NAO modes, as during the early Middle Age and present-day period. Fresh or warm oceans in association with a positive NAO mode are stormy but with very low sting storms frequency. Although in agreement with the orbital forcing and the Holocene glacial history, increasing storm frequency and intensity is most probably partly biased by continuous sea-level rise and resulting erosion.

Climate control on late Holocene high-energy sedimentation along coasts of the northeastern Atlantic Ocean

“…(Fig. 5B; HSAP, "high storm activity period" in Sabatier et al, 2012;HSP, "Holocene Storm Period" in Sorrel et al, 2012;van Vliet-Lanoë et al, 2014; SP, "storm periods" in Degeai et al, 2015;Costas et al, 2016 and references therein).…”

Holocene formation and evolution of coastal dunes ridges, Brittany (France)

“…Palaeo-storminess traces have already been discussed and events were extracted from the Holocene prism stratigraphy (Van Vliet-Lanoë et al, 2014b). The RSL curve for Brittany (Goslin, 2014;Goslin et al, 2013) was calibrated with the …”

“…This burial place was re-abandoned in the late 12th century (Giot and Monnier, 1977) with a renewed dune invasion. This period was marked by giant storms, well recorded around 900 AD in southern Brittany (Van Vliet-Lanoë et al, 2014b). A limited stabilisation was observed from 1000 to 1200 AD, which was recorded in the form of humic soils or peaty layers in the region, although it was frequently interrupted by giant storms in the form of washover fans during the 11th century (Van Vliet-Lanoë et al, 2014b).…”

“…This period was marked by giant storms, well recorded around 900 AD in southern Brittany (Van Vliet-Lanoë et al, 2014b). A limited stabilisation was observed from 1000 to 1200 AD, which was recorded in the form of humic soils or peaty layers in the region, although it was frequently interrupted by giant storms in the form of washover fans during the 11th century (Van Vliet-Lanoë et al, 2014b). Around Brittany, the ridges were often truncated and/or buried by sand drift or splay ( Carter et al, 1993;Plouguerneau, MorzadecKerfourn, 1974).…”

“…Today, the RSL keeps rising (Cazenave and Remy, 2011) and coastal retreat occurred at a rate of about 1 m/year in the bay of Audierne (1966to 1988Faye et al, 2007). Extreme storms mostly develop when the North Atlantic Oscillation (NAO) mode is negative on the decennial scale, although the NAO can be interrupted by shorter positive events, especially during a high Atlantic Multidecadal Oscillation mode (AMO; Van Vliet-Lanoë et al, 2014b). Since 2007, the storms' frequency rose, reactivating the coastal retreat, with no marked drift of sands.…”