Braer storm satellite image
Data in table extracted from Lamb (1991) and Hickey (2001) on major storms in the northern North Sea with known or suspected major impacts on Shetland
The two lowest barometric readings recorded in the Atlantic Ocean have occurred in the eighties and nineties of the 20th century. The first was the Atlantic Cyclone of 15 December 1986, which deepened to 916 hPa, and the other was the storm of 10-11 January 1993 which is much better documented. According to Gulev et al. (2001), Northern Hemisphere winter cyclones have intensified and produced deeper central mean sea level pressures in recent decades. However, these winter cyclones also appear to have shorter life cycles and dissipate more quickly.
Note: Wave data to compare the storms of 1/1/1992 and January 1993 is sparse. Comparisons of photos from 1992 and 1999 at Villians of Hamnavoe indicates that wave run up in 1993 was below the level attained in the previous year, when many large blocks were moved.
Dawson et al. (2002), for example, scoured daily meteorological records of the Royal Meteorological Society held in the archives of the Society's Scottish Office in Edinburgh for Stornoway (Outer Hebrides), Lerwick (Shetland Islands), Wick (Caithness) and Fair Isle (west of the Shetland Islands), recovering all data pertaining to gale-force winds over the period 1876-1996, which enabled them to reconstruct a history of storminess for that period for northern and northwestern Scotland.
Analysis of the data showed that although North Atlantic storminess and associated North Atlantic wave heights have indeed increased over the past two decades (see Woolf, D. and Coll, J., 2006), "storminess in the North Atlantic region was considerably more severe during parts of the nineteenth century than in recent decades." In addition, whereas the modern increase in storminess appears to be associated with a recent spate of substantial positive values of the North Atlantic Oscillation (NAO) index, they say "this was not the case during the period of exceptional storminess at the close of the nineteenth century." During that earlier period, the conditions that determine modern storminess were apparently overpowered by something even more potent, i.e., cold temperatures. The cold temperatures, in the view of the authors, led to an expansion of sea ice in the Greenland Sea, which expanded and intensified the Greenland anticyclone, which then led to the North Atlantic cyclone track being displaced farther south. A similar hypothesis has been expressed by Clarke et al. (2002), who postulate that a southward spread of sea ice and polar water results in an increased thermal gradient between 50°N and 65°N that intensifies storm activity in the North Atlantic.
The results of the Dawson et al. analysis indicate that increased storminess and wave heights observed in the North Atlantic Ocean over the past two decades do not appear to be the result of global warming. Rather, they are associated with the most recent periodic increase in the NAO index. Furthermore, a longer historical perspective reveals that North Atlantic storminess was even more severe than it is now in the latter part of the nineteenth century, when it was significantly colder than it is now. In fact, the storminess of that much colder period was so great that it was actually decoupled from the NAO index. Hence, the long view of history suggests that the global warming of the past century or so has actually led to an overall decrease in North Atlantic storminess.