One Hundred Years In The Norwegian Sea

By Paul Homewood  

 

http://www.tandfonline.com/doi/pdf/10.1080/00291950701409256

 

I mentioned this article in discussion of the sea ice years last week. It is worthwhile, though, looking in some detail at some of the earlier periods.

What will strike you is just how big the climatic changes in the Arctic have been over the last century or so.

 

 

 

The Great Chill, 1900-1920 

Their history starts with the Great Chill, which they describe:

It is instructive that around the turn of the 20th century, the northern Norwegian Sea and Barents Sea were undergoing a period of extreme cooling, with lower air and sea temperatures than have been observed since.

 

This is important to bear in mind. There is much evidence to suggest that the 19thC marked the coldest period there since the end of the ice age. Yet here we find that, after a brief recovery, temperatures were plummeting again. Any analysis of temperature trends since then needs to bear in mind just how exceptionally cold it was in the early 20thC.

They go on:

….and with very evident impacts on the local ecosystem. As the yield, weight, liver weight, and roe weight of spawning cod descended to what would prove to be a 100-year minimum (ICES 1996), Helland-Hansen & Nansen (1909) were among the first to formulate a clear link to environmental temperature, highlighting the contemporary minima in the extent of open water in the Barents Sea and in the sea temperatures at Kola, Lofoten and Stad.

Though the climatic data set for the area was only just becoming established at that time, it seems that their inference was correct. The long-sustained Russian monitoring of the Kola Section of the Barents Sea shows that the minima in this 100-year record occurred c.1900 -1903 and c.1916-1918 (Loeng 1991), while a reconstruction of annual and seasonal means of surface air temperature for the Barents Sea since 1900 (Fig. 3) confirms that these low sea temperature episodes were present as cold extrema in the air temperature record also in spring, autumn  and especially winter.

 

 

 

Is it much warmer now than then? Or was it much colder then?

 

The Warming in the North, 1920 – 1960

As our hydrographic time series is lengthened into the middle decades of the 20th century, it begins to capture evidence of one of the largest and most widespread regime shifts to affect our waters within the modern instrumental record. These were the decades of ‘the Warming in the North’, when the salinity of North Atlantic water passing through the Faroe-Shetland Channel into the Norwegian Sea reached a century-long high (Dooley et al. 1984), when salinities were so high off Cape Farewell that they were rejected as erroneous (Harvey 1962) and when a precipitous warming by more than 2C in the 5-year mean pervaded the West Greenland banks (Fig. 6), and also when the northward dislocations of biogeographical boundaries for a wide range of species, from plankton to commercially important fish, terrestrial mammals, and birds, were at their most extreme in the 20th century.

The astonishing nature of these radical events is vivid in he contemporary scientific literature.

 

Scientists cannot agree on the causes of this phenomenon.

Bengtsson et al. (2004) provide a recent review of the possible causes of Arctic warming. They conclude that its cause is not anthropogenic, pointing out that greenhouse gas forcing was then only 20% of present values and that temperatures would subsequently cool in the 1960s when greenhouse gas forcing was the same or greater. They find they are unable to judge whether a change in solar forcing was a factor, since the appropriate observational data only exist for the last two decades, but they do rule out vulcanicity, pointing out that the eruption of Katmai (Alaska) in 1912 would have had a smaller effect on climate than the eruption of Pinatubo (Philippines) in 1991, and the cooling effect of Pinatubo disappeared after less than three years.

They therefore conclude that the most likely cause of Arctic warming in the middle decades of the 20thC was internal variability in the climate system.

 

It has been suggested “an interactive mechanism involving an increased pressure gradient between Svalbard and Norway, increased warm water transport to the Barents Sea and decreased sea-ice cover.”

But Dickson and Osterhus rule this out:

However, such a localized explanation is unappealing from an oceanographic point of view for two reasons.

First, although the pressure gradient across the Barents Sea Opening does appear to show some underlying correlation with the flux of warmth into the Barents Sea throughout the 20th century , in fact this correlation holds good only for the (15-year) smoothed series. When annual values of these variables are compared , we find that a close correlation exists only after 1960 , with no correlation during the early period of Arctic warming (1920 to 1950) that Bengtsson et al. (2004) were trying to explain. Yet the type of interaction between warm water inflow and ice extent that they proposed is the type of mechanism that must surely be played out on an annual basis.

Figure 6 shows information to support the most telling counter-argument against local explanations of any sort for this early warming, namely that the warmth appears concurrently throughout the Atlantic northern gyre, from West Greenland and the Iceland Sea through the Faroes and Norwegian Sea to the Barents Sea (and, as will be shown later in this article, to the Atlantic-derived sub-layer of the Arctic Ocean).

 

In other words, not only was the rise in temperatures substantial during this period, it was a also widely spread.

 

 

Summary

What is clear from these events, as well as the Great Salinity Anomaly of 1968-82, and recent warming, is that the Arctic regularly experiences extreme climatic and environmental change. Moreover, these changes are associated with natural causes, particularly Atlantic Ocean cycles.

The authors conclude:

 

The recent reconstruction of Atlantic Water Core Temperatures across the Arctic Ocean over the whole of the past century by Polyakov et al. (2004), reproduced here as Fig. 20, is highly instructive in setting various ‘drivers’ into context. Though based, as might be expected, on a very variable data-density with time (Polyakov et al. 2004), we find to our surprise that each of the main episodes in the hydrographic history of the Norwegian Sea over the past century, as we have described them, appears to be represented in the Atlantic sublayer of the Arctic Ocean.

 

 

As in the Norwegian Sea, the series there begins with conditions of extreme cold around the beginning of the 20th century recorded by Nansen (1902) during the long polar drift of the Fine Resolution Antarctic Model (FRAM). This is plainly followed by a period of sustained warmth after the 1920s consistent with and concurrent with the ‘Warming in the North’ that pervaded the Atlantic northern gyre, until the 1960s when the passage of the Great Salinity Anomaly around the same circuit brought a sharp reimposition of cooling with an extreme south-eastward shift of the Ocean Polar Front. This was followed by the period of the amplifying NAO (1960s – 1990s) during which, as already described, successive pulses of warmth spread along the eastern boundary to produce the warmest conditions of the century in the Atlantic-derived sub-layer of the Arctic Ocean. The fundamental conclusions are:

 

1) A whole Arctic-subarctic system of change is involved here (Arctic air temperature, sea-level pressure, ice concentration, and landfast ice thickness all share the same multi-decadal periodicity, according to Polyakov et al. 2004).

2) The subarctic seas have been a continuing source of multi-decadal Arctic change over the past century.

3) The same complex of causes that drove these changes in the Nordic Seas by no means restricted to local forcing or to the NAO has contributed to change in the AWCT (Atlantic Water Core Temperature) of the Arctic.

4) Following the example cited by Polyakov et al. (2004), it can also be expected that the resulting changes in the density of the upper Arctic Ocean will dynamically alter the incoming ocean-climate signals from subarctic seas, thus internally modifying their character and impact.

5) The input of warmth from the Norwegian Sea to the Arctic Ocean still appears to be continuing.

 

The past century in the history of the Norwegian Sea has been most eventful, when episode after episode of extreme environmental change, each different in both character and forcing, have succeeded one another on a timescale of decades. Two of these episodes the Warming in the North (still unresolved), and the most recent warm pulse associated with the amplifying NAO were characterized by the poleward spread of warmth from the North Atlantic through the Norwegian Sea to the Barents Sea shelf and Arctic Ocean. In the case of two others the GSA (Great Salinity Anomaly) and the recent freshening of the subarctic seas  the ocean-climate signal has been in the opposite direction, from the High Arctic to the North Atlantic through Nordic seas.

In no case can we assume that the episode in question is the simple imposition of change from one region on another. The two main episodes of enhanced freshwater outflow from the Arctic can both be shown to have modified the Atlantic thermohaline circulation in some significant way; our records from the Arctic Ocean are now good enough to show that the recent warming, salinification and shoaling of the Atlantic-derived sublayer will have caused a sufficient change in density to moderate the inflow itself (Polyakov et al. 2004); further, in the abyss the relative shift from vertical towards horizontal exchange has brought subtle and protracted changes in the density of three deep basins (Greenland Sea, Norwegian Sea and Arctic Ocean), enough to alter the pattern of their deep interchange.

As the extreme cooling of c.1900 (not observed since) formed the backdrop to the century under discussion, we are fortunate that the pioneers of oceanography were on hand to record these extrema. At a later date, it is difficult to imagine how the dramatic yet subtle changes in the abyss of the Norwegian Sea obvious to us now might have been noticed at all if the OWS M had not contributed its half-century of precise, deep hydrography. Further, as the latest wave of warming enters the Arctic Ocean, it will be equally plain that the succession of extreme multi-decadal events we have described is likely to continue beyond the range of our past experience and are equally likely to have significant climatic impacts.

 

Although still not well understood, powerful natural forces have had, and will continue to have, an enormous effect on Arctic climate. To blame currently warm conditions on “global warming” is naive in the extreme and ignores the lessons of history.

This is, of course, the story of just one part of the Arctic, albeit a large chunk. It is a common misconception that the whole of the Arctic has one climate, that marches in tune to global warming. This could not be farther from the truth. Some parts of the Arctic respond in vastly different ways to others.

As this history of the Norwegian Sea makes clear, we have seen huge climatic changes in the past and will continue to do so in the future.