Cod and Climate Change - ICES

ICES mar. Sei. Symp., 198: 311-322. 1994

Climatic changes and environmental conditions in the Northwest Atlantic, 1970-1993 E. Colbourne, S. Narayanan, and S. Prinsenberg

Colbourne, E ., Narayanan, S., and Prinsenberg, S. 1994. Climatic changes and environmental conditions in the Northwest Atlantic, 1970-1993. - ICES mar. Sei. Symp., 198: 311-322. Oceanographic, meteorological, and ice data were examined to establish the average conditions that prevailed on the Newfoundland and L abrador continental shelves during recent decades, and to describe the variability on seasonal and interannual scales. The results indicate that the below-normal air tem peratures established in 1989 persisted over much of eastern C anada, resulting in increased ice growth and reduced ice melt during the early 1990s. Since 1990, ice conditions have been more severe and have lasted longer, with 1991 being the worst in 30 years. The net effect of these conditions was a delayed and reduced heat input into the ocean, which resulted in a thinner and fresher mixed layer and a lower heat content in the water column. Conditions during similar anomalous periods of the early 1970s and mid-1980s were also examined. In general, the large negative tem perature and salinity (fresher than normal) anomalies of the early 1990s were found to be similar to events of the early 1970s and to a lesser extent of the mid-1980s and were associated with colder than normal winter air tem perature and heavier than normal ice conditions in the coastal region of the Northwest Atlantic. E. Colbourne and S. Narayanan: Department o f Fisheries and Oceans, Northwest Atlantic Fisheries Center, PO B ox 5667, St. John ’s, NF, Canada A 1C 5X2. S. Prinsenberg: Department o f Fisheries and Oceans, Bedford Institute o f Oceanography, PO Box 1006 Dartmouth, NS, Canada B 2 Y 4A2.

Introduction In contrast to forecasted global warming trends world wide, the Northwest Atlantic has experienced three anomalously cold periods since the early 1970s, particu larly in Atlantic Canada’s Newfoundland and Labrador areas. The latest cold period, which began in early 1989 (Findlay and Deptuch-Staphf, 1991) continued into the summer of 1993. These lower temperatures are associ ated with more persistent and stronger winds from the northwest. Anomalous meteorological conditions have given rise to increased ice growth and ice transport from northern latitudes, increased extent of ice cover in more southern areas and delayed ice melt during the spring and summer months (Narayanan et al., 1994). For example, 1991 has been identified as the worst ice year on record in terms of both ice-cover extent and duration. As a result, more of the atmospheric heat flux is required

to melt the ice, leaving less to heat the oceanic surface layer. The net effect of these anomalous atmospheric and ice conditions was to create corresponding periods of colder than normal surface layers over large areas, which gradually deepened over the continental shelf through mixing and convection during winter storms and buoyancy flux due to salt rejection from the growing ice cover. In addition, the enhanced ice production during winter and increased ice melt during spring also resulted in a larger than normal amplitude of the annual salinity cycle over most of the shelf. Coincident with the recent climatic anomalies in the Northwest Atlantic, the stock size and distribution of several fish species, both pelagic and bottom dwelling, have experienced considerable variability. Since the beginning of the most recent cold period very few aggregations of the once-abundant Atlantic cod (Gadus

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E. Colbourne, S. Narayanan, and S. Prinsenberg

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morhua) were found in N A FO Division 2J off the Labrador Shelf (Fig. 1), and in Divisions 3K and 3L the stock size has diminished to such a low level that a moratorium on fishing for northern cod was imposed in July 1992. Pelagic species such as capelin and salmon have also experienced large declines in mean fish sizes, delayed spawning, and overall reductions in stock sizes and distribution. To assess what environmental factors might have contributed to the variability in the fisheries during the anomalous periods, an evaluation of the climatic con ditions, in particular the major differences among the three recent cold periods, is needed. The objective of this paper is to examine how the environmental con ditions during recent years compare with the average over the last several decades and with those that existed during other anomalous periods of the early 1970s, mid1980s, and the early 1990s.

Meteorological and ice conditions Over the N orth Atlantic, the winter atmospheric circu lation is dominated by a low pressure centred over Iceland (the Icelandic Low) generating northerly to westerly wind patterns in the Northwest Atlantic. In the course of a year, the intensity and position of this system vary, effecting an annual cycle with strong cyclonic wind stress forcing during winter and spring (Thompson and H azen, 1983). Following the establishment of the cyclonic circu lation over the Northwest Atlantic every year during the fall and winter months, both the air and sea-surface temperatures decrease, the stratification in the water column starts to break down, and ice production begins. The strength, duration, and persistence of wind forcing from the northwest during the winter months deter mines to a large degree the extent and duration of ice

Climatic changes and environmental conditions in the Northwest Atlantic

ICES m ar. Sei. Symp., 198 (1994)

cover and hence the volume of ice that eventually melts on the Newfoundland and Labrador shelves.

A tm o s p h e ric circu latio n A convenient index representing the strength of the winter circulation has been termed the North Atlantic Oscillation (NAO) index and is defined as the difference in the winter sea level air pressure between the Azores and Iceland (Rogers, 1984). The periods from 1970 to 1975 and from 1988 to 1992 show strong positive NAO index anomalies (Fig. 2) indicating stronger than normal cyclonic circulation over the Northwest Atlantic during the winter months, bringing more cold Arctic air to the coastal areas of Labrador and Newfoundland. During the mid-1980s, however, the N A O index anomaly am plitude was somewhat lower compared with the other two periods and persisted for a shorter duration. The geostrophic winds from one location on the Labrador Shelf and another from the G rand Bank,

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computed by the Atmospheric Environment Service of Canada, were analysed to examine the interannual vari ability in the frequency of occurrence of northwesterly winds. A time series of occurrence of northwesterly winds for the Labrador and the G rand Banks areas was determined by averaging the percentage occurrence of winds of all speeds between ± 22.5° from northwest over the three winter months (December to February). A t both sites there is an increase in the frequency of northwesterly winds during the early to mid-1970s and from 1988 onwards, with values reaching 40% in 1974 compared to values of less than 15% in the late 1970s, consistent with the strong positive anomalies in the NAO index (Fig. 2). However, during the mid-1980s, though there appears to be a slight increase in the percentage occurrence of the northwest wind, the in crease is less pronounced and of shorter duration. In addition, the upward trend towards more frequent northwest winds is more apparent at the northerly site off Labrador compared to the Grand Bank.

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A ir te m p e r a tu r e s A ir tem perature anomaly data from two meteorological stations, Cartwright on the Labrador coast and St John’s, Newfoundland (Fig. 1), are presented in Figure 3. Daily air tem perature data from these two locations were averaged to compute a weekly tem perature time series, which is then used to compute the annua! cycle based on a 25-year period from which the anomalies are calculated. A three-m onth moving average filter was then applied to the anomalies to highlight the seasonal and interannual scales by suppressing higher frequency components. The superimposed 25-month running mean shows the decadal variations in the anomalies. Interannual variability in the air temperatures is clearly evident at both sites (Fig. 3), even when the weekly data were three-m onth averaged. The amplitude of the vari ability is more pronounced at Cartwright, and at more northern latitudes (not shown here). For example, at G odthåb, Greenland, and Iqualuit, N.W .T. Canada, air temperatures during 1983-1984 were 2.5 to 3.5°C below normal (Drinkwater el al., 1992), whereas at Cartwright the temperature was about 1.0°C below normal.

The air temperature anomalies at both sites clearly show three cold periods during the last 20 years. The 1970s started with positive anomalies but changed to extremely cold conditions in 1972 and lasted until 1977. A warming trend was then established at Cartwright until the middle of 1981, whereas at St John’s the negative anomalies ended around 1975, after which a warming trend commenced. From 1982 to 1986, Cartw right temperatures were below normal, but not as severe as those in the 1972-1977 period. A t St Jo h n ’s, the air tem perature anomalies were more or less positive after 1975 until the fall of 1984, when the trend reversed. A fter a brief period of near normal temperatures during 1987 at Cartwright and 1988 at St John's, extreme cold conditions were once again re-established over the entire region. The intense negative air tem perature anomalies established in the late 1980s continued into mid-1993, making this the longest cold period experi enced this half of the century. These colder than normal air temperatures are associated with positive NAO index anomalies and more frequent winter northwest erly winds, especially for the 1970 and 1990 cold period.

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For the mid-1980s period, however, it appears that the colder and windier weather pattern did not reach as far so u th as St J o h n ’s.

S ea ice Ikeda et al. (1988), using a simple two-layer model of the Labrador Shelf and observed meteorological forcing, were able to reproduce the interannual variability of the annual advance and retreat of the L abrador ice pack for the years 1964 to 1974. These simulations show that although ice production is a function of the air tem pera tures at northern latitudes, the year-to-year variability in ice cover over the southern Labrador and Newfound land shelves is largely driven by wind forcing with local temperatures affecting the melt rate. The ice extent anomalies calculated from a 25-year average of weekly ice cover data for the winter and spring months for the Labrador and Newfoundland shelves and the Grand Bank (Fig. 4) clearly show increased ice cover and longer durations during the

three cold periods mentioned above, namely 1972-1974, 1983-1985, and 1990 onwards. In terms of duration 1972, 1974, 1985, and 1991 were among the worst ice years in the 30-year period, with 1991 being particularly unfavourable in the inshore areas due to strong onshore flow during the spring which kept the ice locked into the bays. These heavy ice periods coincided with the large meteorological anomalies discussed above, positive N A O index anomalies, more frequent northwesterly winds, and cold air tem perature anomalies during the winter months. The net result of increased ice cover and extended ice duration is to reduce the amount of solar heat flux into the ocean surface layers, both delaying and reducing the annual warming of the water column on the shelf and along the coast. In addition, the eventual melting of increased volumes of ice generates lower than normal salinity anomalies near the surface which propagate downward through the water column owing to vertical diffusion and vertical mixing when the surface mixed layer deepens in fall and winter.

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peated here. However, to provide a background for the discussion on the anomalies and to indicate the temporal The oceanographic conditions along the continental structure of, and the scatter from the mean, tem perature shelf of the Northwest Atlantic are also experiencing a and salinity values at depths of 0,2 0 ,5 0 , and 75 m for the cooling trend that began in the late 1980s and continued 3LNO area are shown in Figure 5. As expected, the into the spring and summer of 1993 (Colbourne, 1993a). largest seasonal cycle in the water tem perature occurs in These colder and fresher than normal conditions are a the upper layers, where the strongest coupling to the direct result of the strong atmospheric and sea-ice annual solar flux exists. The amplitude of the seasonal anomalies discussed above. The present cold period is cycle for both tem perature and salinity decreases with the third such event that has occurred since the early depth and is not statistically significant at 75 m depth for 1970s. The cold period of the early 1970s was the subject this location. Maximum tem perature at the surface of a special meeting of ICN AF (ICNAF, 1975), and the occurred during the summer months with the phase of more recent cold periods have received extensive re the maximum increasing with depth at an approximate views (Petrie et al., 1992; Narayanan et al., 1992; rate of 1.4 days m ~ ' in the upper w ater column. Drinkwater, 1993; Colbourne, 1993b). In this section Petrie et al. (1991) have shown that although the we attempt further to investigate the oceanographic tem perature phase is nearly uniform horizontally, the anomalies corresponding to these periods, over wide salinity phase has significant latitudinal and cross-shelf temporal and spatial scales in N A FO Divisions 2J to variability. During ice production, near-surface salini 3NO. W ater properties, especially those at larger ties increase because of salt rejection, so that the annual depths, are partly a result of processes that occurred salinity cycle will have a maximum during this period. earlier in the year far up north from the actual locations Similarly, low salinities occur in areas where large vol on the Newfoundland Shelf. The seasonal variability at umes of ice melt during the spring and summer months. one location is thus a reflection of processes that Thus the heavy ice production, retention, and melt occurred over a larger area and are not just reflected by affect both the high and low of the salinity cycle, which local atmospheric and ice-cover conditions. propagates to lower latitudes through advection and results in a phase lag with latitude. Furtherm ore, for any given latitude the salinity phase has more scatter with T e m p o ra l a n o m a lie s in te m p e r a tu r e a n d salinity depth and with longitude compared to the tem perature because of the cross-shelf variability in the along-shore To investigate the spatial variability in the tem perature current (producing different advection rates) and be and salinity anomaly fields over time, the historical data cause of shelf-edge oscillations. set for the region was grouped into three areas in N AFO The time series of tem perature anomalies at Station Divisions 2J, 3K, and 3LNO, indicated by the shaded 27 at various standard depths (Fig. 6a) are characterized blocks in Figure 1, together with Station 27 data. The by three major cold periods: in the early 1970s, early to average water depths in these areas ranged from 70 to 80 mid-1980s, and the early 1990s. The time series exhibit m in 3LNO, 250 to 300 in 3K, and 150 to 200 in 2J. lower frequency variations in the anomalies in deeper Station 27 was established as a hydrographic monitoring water compared to the shallower depths consistent with station in 1946 and is located about 8 km off St John’s, the slower response of the deeper water to atmospheric Newfoundland; it has a water depth of 176 m. These forcing. For all three periods, the negative temperature areas were selected based on the local bathymetry and anomalies at the bottom were established first and on the available data. appear to last the longest. The cold period beginning in The data for each area for all years were then sorted 1972 continued until late 1973 in the upper layers and by Julian day to determine the annual cycle. Following until 1975 near the bottom at 175 m depth, with anomal the general methods of A kenhead (1987), the mean ies reaching peak values of —2.0°C by early 1973 over seasonal cycles in the tem perature and salinity fields at the upper water column. From late 1975 to late 1983 the selected water depths were determined by fitting a leasttem perature anomalies showed a high degree of vari squares regression to the mean and a sum of four sines ability with fluctuations of ± 2.0°C in the upper water and cosines pairs representing four harmonics. Time column and a stronger tendency towards positive series of tem perature and salinity anomalies for each anomalies near the bottom. area were then formed, taking each observation and During the second period, by early 1984 in the top subtracting the least squares fitted value for the same 50 m of the water column and early 1983 in the deeper day of the year. The time series of anomalies were then water, intense negative tem perature anomalies had low-pass filtered to suppress variations shorter than one returned with peak amplitudes reaching —3.0°C at 30 m year. depth by late 1984. This cold period lasted until midAn in-depth discussion of the annual harmonic was 1986 over most depths. By late 1986 near the bottom, by carried out by Petrie et al. (1991), and hence not re

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to the 1990s at all depths, and compared to the 1980s in the upper layers. Like the tem perature anomalies the largest negative anomaly amplitude is in the upper mixed layer, where the influence of ice melt is the largest. Time series of temperature and salinity anomalies for the shaded areas in Figure 1 at standard depths, refer enced to a 1950-1992 m ean, are shown in Figure 7. Unlike the time series at Station 27, these time series are based on a smaller data set distributed over a wider geographical area; hence the anomalies are subject to larger spatial and temporal biasing. In Division 2J the time series show large temperature anomalies in the early 1970s, early to mid-1980s, and to a

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lesser extent in the early 1990s below 20 m depth. The salinity was lower than normal almost throughout the 1970s near the surface (the G reat Salinity Anomaly, Dickson et al., 1988) and during the mid-1980s and again around 1989 at mid-depths and 1990 near the bottom (175 m). In Division 3K the time series is again charac terized by three colder and fresher than normal periods: the early 1970s, mid-1980s, and early 1990s, throughout the water column. In Divisions 3LNO the cold periods are again clearly evident, the most pronounced occur ring in the early to mid-1970s when tem peratures fell to 1.5°C below normal at 50 m depth. The cold period of the mid-1980s in Divisions 3LNO was of a shorter duration than in 2J and 3K, particularly in the upper

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Figure 7. Time series of tem perature and salinity anomalies for the shaded areas in Figure 1 at standard depths for N A FO divisions 2J, 3K, and 3LNO, referenced to a 1950-1992 mean.

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layers. In addition, the peak of the events occurred up to a year later in Divisions 3LNO compared to 2J, particu larly in the mid-1970s and 1980s. By the end of 1992 though, the temperatures remained below normal over all areas throughout the water column, the salinity anomaly had disappeared over most areas except at the surface and in the area of the G rand Bank bordering Divisions 3LNO. In general, these anomalies are very similar to conditions at Station 27, particularly in deep water, and are all associated with severe meteorological and ice conditions experienced over the same time intervals. S p atial te m p e r a tu r e a n o m a lie s A dominant feature of the vertical tem perature struc ture off the east coast of Newfoundland on the continen tal shelf is the cold intermediate layer (CIL) (Petrie et al. , 1988). The layer is formed in winter due at first to fall cooling and later by convective overturning resulting from salt rejection from the growing ice cover. In sum mer, spring ice melt and seasonal heating increases the stratification in the upper layers to a point where heat transfer to the lower layers is inhibited. The result is a cold layer of water confined to the continental shelf, with temperatures ranging from 0.0 to —1.8°C sandwiched between the warm seasonal upper layer and warmer slope water near the bottom. In the winter months the surface layer effectively disappears as it cools down to near freezing tem pera tures and becomes part of the CIL owing to winter cooling and strong surface mixing. By late April and early May the warming of the surface layer commences and the cross-sectional area of colder than 0.0°C water decreases from winter maximum to a fall minimum due to summer heating. The mean offshore extent of the Cape Bonavista (Fig. 1) CIL is about 220 km with a standard deviation of 44 km, centred at approximately 100 m depth with a mean thickness about 200 m and a standard deviation of 35 m (Fig. 8a). The average cross-sectional area of water less than 0.0°C is 26.4 km2, with a standard deviation of 8.7 km2. In 1972 the offshore boundary of the CIL was not defined by the survey extending more than 335 km offshore. In 1984 the CIL reached about 325 km off Cape Bonavista with a cross-sectional area of about 50 km2, more than 80% above normal. The thickness of the CIL during cold years is also above normal, with 0.0°C water reaching close to the bottom at mid-shelf. Figure 8b shows the intensity or minimum core tem peratures of the CIL for the Bonavista transect for the summer from 1948 to 1992. The time series are charac terized by four major cold periods with minimum occur ring in the early 1950s, early to mid-1970s, mid-1980s, and early 1990s. Minimum temperatures at the core of

IC ES m ar. Sei. Symp., 198 (1994)

the CIL range from —1.84°C in these cold periods (1984 and 1991) to —1.4°C in a warm year (1986). The average core tem perature from 1948 to 1992 is —1.64°C centred at about 70 km offshore at approximately 80 to 100 m depth. The time series of the CIL cross-sectional area anom aly (Fig. 8c) shows a gradual warming trend starting in 1950 and lasting until the mid-1960s. Since the early 1970s the time series is characterized by three major cold periods with peaks in 1972, 1984, and in 1990/91. The area anomaly during these cold periods range from 60% to 88% above average, with 1984 showing the greatest areal extent of sub-zero water. The warmest years were the mid-1960s, mid-1970s, and the late 1980s, when the cross-sectional areas were from 30% to 50% below average.

Summary Since the early 1970s the oceanography in the Northwest Atlantic has been dominated by three anomalous periods; early 1970s, mid-1980s, and the early 1990s. During these periods, widespread negative water tem perature anomalies and lower than normal salinities occurred for several years over the Northwest Atlantic. In general, it was shown that the oceanographic anom al ies during the three periods were very similar, especially in deep water, but variations, both spatial and temporal, in the anomalies are significant. The meteorological and ice-cover data also show similar long period trends as the oceanographic conditions. The results also indicated differences between the mid-1980s and the other two anomalous periods. Both the N A O index and the northwesterly winds were more pronounced in the 1970s and 1990s, whereas the N A O index anomaly was positive during the 1980s, but the frequency of the northwest winds was not much higher than other average years. Furtherm ore, the air tem pera ture in St John’s and water tem perature over the southern G rand Bank were near normal during the mid1980s, whereas both were well below normal during the other two periods. Thus the conditions during the mid1980s were less severe (in spite of the large July CIL area off Cape Bonavista) over the southern portion of 2J3KL compared to the other two anomalous periods. The results also indicated that both the meteorologi cal and oceanographic anomalies have m oderated only briefly (1986-1987) since the early 1980s. For example, the long-term trends in the Cartwright air temperatures (Fig. 3) and water temperatures at all depths at Station 27 (Fig. 6) have all been below normal since the mid1980s. Recent data from the spring and summer of 1993 indicate a continuation of below-normal air tempera-


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