Bjerknes’ hypothesis on the coldness during AD 1790–1820 revisited

Abstract

The aim of this paper is to re-examine and quantify a hypothesis first put forward by J. Bjerknes concerning the anomalous coldness during the AD 1790-1820 period in western Europe. Central to Bjerknes' hypothesis is an anomalous interaction between ocean and atmosphere studied here using an oceanatmosphere coupled climate model of intermediate complexity. A reconstruction of the sea-level pressure pattern over the North Atlantic sector averaged over the period 1790-1820 is assimilated in this model, using a recently developed technique which has not been applied to paleoclimatic modelling before. This technique ensures that averaged over the simulation the reconstructed pattern is retrieved whilst leaving atmospheric and climatic variability to develop freely. In accordance with Bjerknes' hypothesis, the model results show anomalous southward advection of polar waters into the northeastern North Atlantic in the winter season, lowering the sea-surface temperatures (SSTs) there with 0.3-1.0°C. This SST anomaly is persistent into the summer season. A decrease in western European winter surface air temperatures is found which can be related almost completely to advection of cold polar air. The decrease in summer surface air temperatures is related to a combination of low SSTs and anomalous atmospheric circulation. The modelled winter and summer temperatures in Europe compare favourably with reconstructed temperatures. Enhanced baroclinicity at the Atlantic seaboard and over Baffin Island is observed along with more variability in the position of the North Atlantic storm tracks. The zone of peak winter storm frequency is drawn to the European mid-latitudes.

200 years of European temperature variability: insights from and tests of the proxy surrogate reconstruction analog method

“…One approach are data-assimilation techniques in which atmospheric states of climate models are controlled by observations. Only a few studies have focused on the adaption of this technique, developed in the field of weather forecasting, for paleoclimatic purposes using sparse proxy networks and data with an annual or lower temporal resolution (von Storch et al 2000;van der Schrier and Barkmeijer 2005;Goosse et al 2006;Crespin et al 2009;Widmann et al 2009;Goosse et al 2010). Data-assimilation approaches have great potential as they simulate multi-variate climate fields that are consistent with proxy reconstructions.…”

Daily temperature and pressure series for Salem, Massachusetts (1786–1829)

“…The two available reconstructions of sea-level pressure for the North Atlantic sector which include this period (Lamb and Johnson 1959;van der Schrier and Barkmeijer 2005) show an anomalously low pressure over the north-eastern USA. However, these reconstructions make use, amongst others, of the Holyoke record, so this latter argument has a degree of circularity in it.…”

“…However, these reconstructions make use, amongst others, of the Holyoke record, so this latter argument has a degree of circularity in it. Nevertheless, for the van der Schrier and Barkmeijer (2005) reconstruction holds that an anomalous low pressure area over the north-eastern USA was still found (albeit weaker) when they did not make use of the Holyoke record in their reconstruction.…”

“…Monthly values of the Holyoke data have previously been used by Lamb and Johnson (1959) and van der Schrier and Barkmeijer (2005) for their pressure reconstructions. Ludlum (1966) also used the data for his accounts of the Early American Winters.…”

Using data assimilation to study extratropical Northern Hemisphere climate over the last millennium

“…Moreover, DA provides estimates for variables and locations for which no empirical information exists, including large-scale atmospheric anomalies that are consistent with the local information. Using DA in palaeoclimatology has first been suggested by von Storch et al (2000). Out of the three methods discussed later in this paper one follows directly the ideas outlined in von Storch et al (2000), and a second one can be seen as a modification of it.…”