Bottom temperature between Iceland and Shetland 1906-1962 measured in telegraph cables

Based on the resistance measurements on two sea cables from the Faroe Islands to Iceland and Shetland respectively, average bottom temperatures along the cable tracks are reported. From 1906 until the second world war the measurements were made each month and after that four time a year. The cable temperatures for the Shetland cable are compared to quasi-simultaneous observations from research vessels close to the cable and it is found that the cable temperatures correlate well with the research vessel observations except for occasional large offsets. These offsets make the value of annually averaged cable temperatures questionable in climate studies; but the seasonal temperature variation of the cables is considered reliable for the pre-war period. The seasonal variation exhibits a striking behaviour with abnormally low seasonality in two periods around 1910. This coincides with an anomalous period with abnormally low temperatures and salinities in the region which has been compared to the 1970s “Great Salinity Anomaly” (GSA). It is argued that the anomalous seasonal variation of the cable temperatures casts doubt on the generally accepted “advective” explanation for these events as regards the 1910s anomaly which instead is suggested to be linked to dynamical changes in the flow systems with reduced Atlantic transport to the region as compared to the influence of the East Icelandic Current.