Hydrographic and transport variability on the Halifax section

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C11, 8003, doi: /2001jc001267, 2003 Hydrographic and transport variability on the Halifax section John W. Loder, Charles G. Hannah, Brian D. Petrie, and Elizabeth A. Gonzalez Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada Received 13 December 2001; revised 7 November 2002; accepted 13 December 2002; published 17 September [1] Archived data and geostrophic computations are used to examine variability in hydrographic properties and along-shelf transport on the Scotian Shelf, with focus on the Halifax section and a decadal-scale hydrographic anomaly during the late 1950s and early/ mid 1960s. The long-term annual cycle shows strong seasonal variations in baroclinic transport on the inner shelf and at the shelf edge, and an associated steric change in adjusted sea level (ASL) at Halifax. Regional wind-forcing and barotropic currents make smaller contributions to the annual cycle in ASL. In contrast, regional wind-forcing contributes about 40% of the ASL variability for periods of 6 30 days. Hydrographic sections indicate that the 1950s/1960s anomaly arose from episodic extensions of Labrador Slope Water along the shelf edge followed by on-shelf intrusions, primarily during the fall-spring periods of and It ended with excursions of Warm Slope Water into the region in An analysis of monthly temperature anomalies indicates that slope temperatures can account for 50% of the variance of deep shelf temperatures 3 6 months later. Correlations between the hydrographic and baroclinic transport variations indicate increased southwestward flow associated with lower temperatures and salinities, but no evidence was found for large local current events at the onset and termination times of the 1950s/1960s anomaly. Positive correlations between wind-adjusted ASL and steric anomalies point to the potential for sea level being helpful in monitoring baroclinic transport variability. INDEX TERMS: 4223 Oceanography: General: Descriptive and regional oceanography; 4219 Oceanography: General: Continental shelf processes; 4215 Oceanography: General: Climate and interannual variability (3309); KEYWORDS: Scotian Shelf, Scotian Slope, Halifax section, hydrographic variability, transport variability Citation: Loder, J. W., C. G. Hannah, B. D. Petrie, and E. A. Gonzalez, Hydrographic and transport variability on the Halifax section, J. Geophys. Res., 108(C11), 8003, doi: /2001jc001267, Introduction [2] Recent findings that the seasonal circulation in the Scotian Shelf region is primarily baroclinic [Han et al., 1997; Hannah et al., 2001; Sheng et al., 2001], together with evidence of substantial decadal-scale and interannual changes in hydrographic properties of advective origin [Petrie and Drinkwater, 1993; Drinkwater et al., 1999; Smith et al., 2001], point to the potential for significant decadal-scale and interannual circulation changes. However, a major difficulty in the identification of such changes is the absence of long time series of current measurements. In particular, there are few current observations from the late 1950s and 1960s when there was a pronounced hydrographic anomaly extending from the Gulf of St. Lawrence to the Middle Atlantic Bight [Petrie and Drinkwater, 1993] (hereinafter referred to as PD93). On the other hand, diagnostic model computations from multiyear composite hydrographic fields indicate a substantial increase in the equatorward transport of subpolar water on the Scotian Shelf and Slope during this period [Greatbatch et al., 1991; Loder et al., 2001]. Copyright 2003 by the American Geophysical Union /03/2001JC [3] While the hydrographic database for the Scotian Shelf has provided a realistic representation of the long-term annual variation in 3-D temperature and salinity [e.g., Petrie et al., 1996; Loder et al., 1997], as well as indices of interannual variability at some sites (e.g., PD93), it is generally not adequate for providing 3-D seasonal distributions in individual years. The most frequent sampling of Scotian Shelf hydrographic structure comes from repeated occupations of the Halifax section, a set of seven stations extending across the shelf and slope from Halifax to the 2700-m isobath (Figure 1). The section was occupied approximately seasonally from 1950 to the mid 1970s which includes the period of the anomalous conditions in the late 1950s and early 1960s. The sampling was then discontinued because of concerns about aliasing of highfrequency variability [Mann and Needler, 1967]. Though the recent identification of decadal-scale hydrographic changes indicates that the concerns were overestimated for hydrographic properties, questions about aliasing effects remain for estimates of high-frequency hydrographic variability and baroclinic circulation (through geostrophic computations) from sparse data. [4] Our goal is to describe and understand the temporal and cross-shelf variability of the 1950s/1960s hydrographic and circulation changes on the Scotian Shelf, drawing on GLO 4-1

2 GLO 4-2 LODER ET AL.: HALIFAX SECTION VARIABILITY Figure 1. Map of the Scotian Shelf showing major topographic features and the location of the cross-shelf sections used in the study. Station locations and cross-shelf zones on the Halifax section are indicated. The inner portions of the Lahave and SWNS sections are shoreward of the perpendicular bar on the section lines. the full temporal resolution of Halifax section, coastal sea level and wind stress observations. Attention is focused on the onset and termination of the anomalous conditions, and the transport variability in the region s two major flows: the Nova Scotian Current on the inner shelf and the shelf-edge current. We also examine the relationships among sea level, wind stress, steric height, and transport estimates, and evaluate the extent of the coupling between shelf and slope hydrographic anomalies. [5] We start with a description of the data sets and analysis methods in section 2. We then discuss the longterm (climatological) annual cycle in the hydrography and other variables (section 3), as background for the interannual variability evaluations. The interannual variability results are presented in section 4, starting with the variability indices, then focusing on particular events and the shelfslope coupling, and finally examining the inter-relations among the variability indices. The results are summarized and discussed in section Methods 2.1. Observational Data and Fields [6] The primary information source is the historical ocean temperature and salinity database [e.g., Petrie et al., 1996] which includes data from dedicated occupations of the Halifax section [Taylor, 1961, 1966; de la Ronde, 1972; Drinkwater and Taylor, 1982; Dobson, 1975, 1977, 1978]. Measurements were taken with reversing thermometers and bottle samples prior to 1967, after which a conductivitytemperature-depth profiler was generally used. During most occupations of the Halifax section (typically completed in a day), profiles were obtained at seven stations (Figure 1). Sampling was usually over the whole water column for the shelf stations (stations 1 5), but often only over the upper m for the slope stations. [7] Using 4-D optimal linear interpolation [Bretherton et al., 1976], cross-shelf fields of temperature, salinity, and density were estimated for the long-term annual cycle (over ) on three cross-shelf sections (Figure 1): (1) the Halifax section extending offshore to the 2700-m isobath, and to a depth of 500 or 1000 m; (2) the Lahave section (6 stations) on the western Scotian Shelf extending offshore to the 400-m isobath; and (3) the SW Nova Scotia section (six stations) extending from Cape Sable across Browns Bank to the middle of Northeast Channel (maximum depth of 245 m). The spatial grid had 5-km horizontal spacing and standard vertical positions of increasing separation with depth (and extending below the seafloor to allow near-bottom interpolation). The time grid point was taken as the mid-day of each section occupation, or the first day of each month for the annual cycle. The horizontal correlation scales in the assumed covariance function [Loder et al., 1997] were specified as 60 and 30 km in the along- and cross-shelf directions. Specified vertical correlation scales increased from 10 m at the surface to 75 m at 500 m, and the temporal correlation scale was 45 days. [8] On the basis of data availability, two sets of fields were estimated for individual occupations of the Halifax section (with nearby data included) during : (1) a first set extending offshore to station 6 near the 1000-m isobath and to a depth of 300 m, which gave more realizations; and (2) a second set extending offshore to station 7 and to a depth of 500 m, which provided greater spatial coverage over the slope. The spatial correlation scales were the same as in the long-term estimates and the temporal correlation scale was 20 days. Sensitivity studies indicated that the details of the hydrographic and velocity structure were often dependent on the correlation scales, but the net transport estimates had limited sensitivity to these scales. Two types of data screening were used. First, a visual inspection of the data distribution was used to eliminate occupations that had substantial areas without data. Second, hydrographic indices were included in subsequent analyses only when their average relative error (ratio of standard deviation of estimated value to the standard deviation of the field) was under 0.45 (typically, the relative error was 0.2). [9] Time series of wind stress from Sable Island and adjusted sea level (ASL) from Halifax (Figure 1) were used to interpret the transport variability inferred from the hydrographic sections. The wind stresses were computed from hourly wind observations using a quadratic stress law and the drag coefficients from Smith [1988], and then averaged to daily values. Hourly sea level data from Halifax were adjusted for the inverse barometer effect using local atmospheric pressure data, and then averaged to daily values. The mean and trend during (the study focal period) were removed from each time series, providing time series of daily de-meaned, de-trended values. The removal of the trend over this period was especially important for sea level because of a secular change of 7.1 cm, about half of which can be accounted for by regional subsidence [Douglas, 1991]. The annual cycle was computed by averaging these values over all years, and smoothing with a 30-day box-car filter. Daily anomalies of both wind stress and ASL were then obtained by subtracting the long-term cycle from the daily de-meaned, de-trended values. Box-car averages of the anomalies were used to obtain bi-daily, weekly, and monthly anomalies for use in the interannual variability investigations.

3 LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4-3 [10] Finally, in order to examine shelf-slope hydrographic coupling with maximum temporal resolution, monthly subsurface temperature anomalies were constructed for the inner shelf ( m; area 14 of Petrie et al. [1996]), Emerald Basin ( m; area 12), and the Slope Water ( m; area 33). There was at least 1 observation in 466 (472, 472) months for the inner shelf (Emerald Basin, Slope Water) during the 615 months from January 1950 to March The means and annual cycles were estimated and removed, and gaps in the time series were filled by linear interpolation Baroclinic Velocities and Transports [11] Estimates of the along-shelf baroclinic flow (associated with horizontal density gradients) were obtained from geostrophic computations on the cross-shelf density fields. For each density field, a baroclinic velocity field was computed to balance the sum of the local depth-varying pressure gradient (due to density gradients) and the depthinvariant pressure gradient (due to a sea surface slope) that gives zero total pressure gradient (and hence no motion) at the seafloor or a specified reference level. The associated sea surface elevation obtained by integrating the surface slope along the section is referred to as the steric elevation (relative to the offshore starting point of the integration and the reference level). [12] Baroclinic transports were computed through vertical, followed by horizontal, summations of the baroclinic velocities over specified grid intervals (and so may not represent the peak transport in a particular direction). Cumulative transport refers to the net value of depthintegrated transport between specified horizontal positions. Barotropic transport is taken to be the component estimated when the near-bottom velocity is used for the entire water column. [13] In the examination of the (long-term) annual cycles, the reference level was 500 or 1000 m for the Halifax section, and the seafloor for the other sections. In the individual-occupation evaluations for the Halifax section, it was 300 m (first set) or 500 m (second set). Estimates of long-term hydrography, velocity, transport and steric elevation were also obtained for the date of each occupation of the Halifax section using the same reference level, and the resulting fields were subtracted from the individual-occupation fields to obtain anomaly fields. In addition, long-term coastal steric heights relative to 250 m in Emerald Basin were computed using composite monthly-mean density data from station 1 for 0 75 m, station 2 for m, and station 3 for m Statistical Analyses and Circulation Model [14] Variability indices for the Halifax section were computed from the hydrographic and velocity fields (or their anomalies) by spatial averaging over specified areas. The section was divided into four cross-shelf and two vertical zones. The cross-shelf zones were designated Inner (coast to station 3), Mid (station 3 5), Outer (station 5 6), and Offshore (station 6 7) (Figure 1), with the Inner zone expected to include most of the Nova Scotian Current and the Outer zone most of the shelf-edge current. Spatiallyaveraged hydrographic indices were computed for the Upper (100 m and above) and Lower (125 m and below) portions of the estimated fields in these areas. Indices of depth-integrated transport and cross-shelf steric elevation differences were computed for each of the four cross-shelf zones (and combinations thereof). An index was retained in the analyses when the zone-averaged relative errors for all contributing zones (e.g., local zone for local hydrography, or both vertical zones for transport) were less than High-resolution temperature and salinity time series from moored meters on the inner shelf [Anderson and Smith, 1989] and at the shelf break [Smith and Petrie, 1982] were used to quantify the overall temperature and density variability. Temperature standard deviations ranged from 0.5 C in the Inner zone to 1.0 C in the Outer zone, which imply uncertainties of C in the temperature indices. Representative uncertainties in the transport indices were estimated to be 0.1 Sv in the Inner zone, 0.35 Sv in the Outer zone, and 0.5 Sv in the Offshore zone. [15] The hydrographic, steric elevation and transport indices, together with the observed Halifax ASL anomalies and estimates of wind-forced anomalies from either a regional circulation model or a wind stress regression relation, were used in standard statistical evaluations (variances, correlations, regressions, coherence, etc.) of the relationships among variables. Model predictions of regional wind-forced contributions to transport and sea level were obtained with the Lynch et al. [1992] linear barotropic finite element model (as used by Greenberg et al. [1997]), on a grid extending from the eastern Scotian Shelf to Cape Cod. Spatially-variable friction coefficients were taken from nonlinear model solutions for tidal and seasonal-mean density and wind-forcing [Hannah et al., 2001], with the effect of an additional background current magnitude of 10 cm/s included. The wind stress anomalies from Sable Island were used to force the model solutions, with zero elevation specified on the upstream boundary. Thus the model-predicted wind influences are only associated with stress over the Scotian Shelf, and do not include upstream influences which can account for 30 40% of the coastal sea level variance at Halifax for periods of 2 20 days [e.g., Schwing, 1989]. As an alternative, a linear regression relation between Halifax ASL and Sable Island wind stress was also used to estimate the wind-forced contribution to Halifax ASL anomalies. For the interpretation of the annual cycle of Halifax ASL, the circulation model was used with the long-term monthly-mean stresses from Sable Island to estimate the regional wind-forced contribution. 3. Annual Cycle 3.1. Hydrography and Baroclinic Velocity [16] The long-term annual cycle of hydrography and baroclinic velocity on the Halifax section is illustrated by the cross-shelf fields in Figure 2. Aspects of this climatology have been described by several authors [e.g., Smith et al., 1978; Drinkwater et al., 1979; Umoh and Thompson, 1994; Petrie et al., 1996; Loder et al., 1997], so only a brief summary will be given here. [17] The basic vertical structure of the shelf hydrography is an upper layer of relatively-fresh water originating from the Gulf of St. Lawrence and the Newfoundland Shelf/Slope, overlying more saline water of offshore (slope) origin in Emerald Basin (Figure 2). In winter

4 GLO 4-4 LODER ET AL.: HALIFAX SECTION VARIABILITY Figure 2. Annual cycle of temperature, salinity and along-shelf baroclinic velocity ( positive southwestward, green/yellow) on the Halifax section, extending offshore to station 7 with a reference level of 500 m. Contour intervals are 2 C, 0.5 and 10 cm/s, respectively. Distributions are shown for 1 February (day 32, winter), 1 May (day 121, spring), 1 August (day 213, summer), and 1 November (day 305, fall). Standard station locations are indicated in the upper left panel. and spring, temperature and salinity show similar structure, with values generally increasing with depth and distance offshore, and the largest horizontal gradients occurring on the inner shelf and at the shelf edge. From spring through fall, a warm shallow near-surface layer evolves associated with solar heating, resulting in a remnant cold intermediate layer that extends offshore to the shelf edge with minimum temperatures on the inner shelf. The seasonal variations of the upper and intermediate layers are determined largely by air-sea interactions and along-shelf advection [e.g., Umoh and Thompson, 1994], while the properties of the deep basin waters are determined primarily by exchanges through the Scotian Gulf [e.g., Smith et al., 1978]. Over the upper continental slope, there is a wedge of the relatively fresh upper-layer shelf water, overlying a layer of Warm Slope Water (temperature of 9 12 C, salinities above 35) centered at m, and Labrador Slope Water (4 9 C, ) at depth [Gatien, 1976]. [18] The dominant features of the baroclinic velocity fields on the Halifax section are surface-intensified southwestward flows on the inner shelf and near the shelf edge over the upper slope (Figure 2). The inner-shelf flow, centered near the 150-m isobath and extending from station 1 3, is strongest in winter (peak near 30 cm/s) and weakest in summer (peak of cm/s). This is the Nova Scotian Current that was described by Drinkwater et al. [1979] who found substantially lower geostrophic transports due to a shallower (100-m) reference level. The upper-slope flow is strongest (20 30 cm/s) in winter and spring, and weakest (10 20 cm/s) in summer and fall, with its central position fluctuating between the 200- and 500-m isobaths. This flow was not detected in the Drinkwater et al. [1979] analysis, but is consistent with the downstream remnant of the shelf-edge Labrador Current [Han et al., 1997; Loder et al., 1998] and the cyclonic Slope Water gyre over the Scotian Slope [Csanady, 1979]. The velocity fields also show other weaker and generally smaller-scale features whose reliability is uncertain. An exception is the predominantly northeastward (but weak) flow over the northern half of Emerald Bank (station 3 5) which is consistent with the shelf-edge flow

5 LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4-5 Figure 3. Annual cycle in cumulative baroclinic transport shoreward of selected isobaths on (a) the Halifax section, (b) the Lahave section, and (c) the SW Nova Scotia section. See Figure 1 for locations. Solid lines indicate transports on the Inner shelf, and dashed and dash-dotted lines total transports between the coast and positions on the upper slope. The transports (positive southwestward) are referenced to a level of no motion at the seafloor in all cases. The measured transports (solid dots) for the Inner Halifax section from Anderson and Smith [1989] are included in Figure 3a, and for part of the SWNS section from Smith [1983] in Figure 3c, with their statistical uncertainty indicated by the vertical lines. 1 Sv = 10 6 m 3 s 1. making an onshore meander that moves counterclockwise around Emerald Basin [e.g., Thompson and Griffin, 1998; Hannah et al., 2001]. [19] The annual cycles of baroclinic transport on the Halifax section and the two downstream sections on the western Scotian Shelf are summarized in Figure 3. In each case, the variation in cumulative transport is shown for the inner shelf, and for the whole shelf extending beyond the shelf edge. For the inner shelf, the (southwestward) transports are much larger on the Halifax section where they vary from about 1.1 Sv in winter to 0.5 Sv in summer, than on the western Scotian Shelf where they are in the 0.1 to 0.3 Sv range. This alongshelf decrease is consistent with that described by Hannah et al. [1996, 2001] from numerical model diagnoses of 3-D climatological density fields, and with their suggestion that the western Scotian Shelf is a dynamical barrier to the equatorward penetration of subpolar water. The winter transport estimates for the inner Halifax section are within the uncertainty estimates of measured transports (Figure 3) computed from the CASP current meter array (late November 1985 to early April 1986 [Anderson and Smith, 1989]). Similarly, the present estimates for the inner SW Nova Scotia section compare favorably with measured values from the Cape Sable array (April 1979 to March 1980 [Smith, 1983]). The larger magnitudes of the velocities and transports on the inner Halifax section compared to those estimated by Drinkwater et al. [1979] point to the importance of the baroclinic pressure gradient field below 100 m (note the sloping near-bottom isohalines in Figure 2 [see also Sheng et al., 2001]. [20] The cumulative transports for the whole shelf are larger than those for the inner shelf, reflecting the southwestward shelf-edge flow. On the Halifax section, there is a net increase of Sv out to the 340-m isobath with largest magnitude in winter, and a further increase of Sv (referenced to the seafloor) out to the 1000-m isobath in winter and spring (Figure 3). The result is a large seasonal variation in total southwestward (baroclinic) transport over the shelf (Inner+Mid+Outer), with winter and spring peaks of 2 and 2.2 Sv, and an early summer minimum of 0.4 Sv. The analyses also indicate that, in the Offshore zone (stations 6 7; Figure 2), the flows are generally weaker and tend to be more northeastward. However, the reliability of the slope transport estimates is limited by the strong variability and undersampling of the hydrographic structure, as well as uncertainty in the reference level velocity. [21] On the Lahave section, the transport out to the 400-m isobath also increases (compared to the inner shelf) by Sv with a peak increase in winter, resulting in a seasonally varying shelf-wide transport of Sv southwestward. The shelf-wide transport (to the middle of Northeast Channel) on the SW Nova Scotia section (across the entrance to the eastern Gulf of Maine) also varies seasonally, with winter and summer maxima of 0.8 and 0.6 Sv, respectively, primarily associated with the shelf-edge/channel flow. These maxima are qualitatively consistent with moored measurements in Northeast Channel [Ramp et al., 1986] but larger because the measurements did not include the upper 75 m. The present transport estimates also differ from Smith s [1983] estimates, because the latter did not capture most of the shelfedge flow [Hannah et al., 2001, Figure 11]. The magnitudes and spatial patterns of the present estimates are similar (as expected) to those in recent numerical model diagnoses of the seasonal-mean circulation [Han et al., 1997; Hannah et al., 2001], indicating the importance of

6 GLO 4-6 LODER ET AL.: HALIFAX SECTION VARIABILITY the shelf-edge flow and variability off Halifax to downstream regions [Loder et al., 1998] Steric Elevation and Observed Sea Level [22] The southwestward flows described above (Figures 2 and 3) arise from generally lighter (less saline) water and hence higher (computed) surface elevations as one proceeds onshore across the shelf. The cross-shelf steric elevation differences (Figure 4a) associated with the baroclinic transport variations on the Halifax section (Figure 3) show seasonal variations with peak coastal set-ups in winter and an annual range of cm depending on the offshore reference position. By zone, the largest seasonal change is the 11-cm range across the Inner shelf (Nova Scotian Current), but there is also a 5-cm range across the Outer shelf (shelf-edge current). [23] The annual variation in steric elevation difference across the Inner shelf is similar to the long-term annual cycle of observed ASL at Halifax (Figure 4b), providing further support for the predominant role of baroclinic flow in the seasonal circulation (the steric elevation differences with other zones included show less similarity to the ASL). The annual ASL cycle has a broad maximum from October to January, a broad minimum from April to August, and an annual range of about 10 cm. The broad maximum is consistent with the October peak in ocean heat content on the inner shelf and in Emerald Basin (areas 12 and 14 of Petrie et al. [1996]; also see Umoh and Thompson [1994]), and with the December January salinity minimum associated with the passage of the annual freshwater pulse from the Gulf of St. Lawrence [Sutcliffe et al., 1976]. [24] While there is approximate agreement (rms misfit of 2.5 cm) between the annual cycles of Halifax ASL and steric elevation difference across the Inner shelf, there is even better correspondence (rms misfit of 1.9 cm) between the annual cycles of ASL and steric height calculated from the monthly-mean density profiles at station 1 3 (Figure 4b). The model estimate of the regional wind-forced contribution to the annual Halifax sea level cycle is only 1 2 cm, and its inclusion (shown as the wind-adjusted steric height in Figure 4b) does not improve the correspondence (rms misfit of 2.3 cm) with the ASL (probably because the wind influence is at the noise level in the ASL and steric estimates). This small effect of wind is consistent with Thompson s [1986] finding of a secondary influence of regional (compared to ocean-scale) wind-forcing on seasonal sea level variability. [25] The difference between the annual cycles of observed Halifax ASL and wind-adjusted steric height has a notable seasonal pattern (Figure 4c). During winter (November March), the ASL exceeds the adjusted steric height by 3 cm on average, while during April October, the ASL is generally lower than the adjusted steric height by an average of 1 cm. This seasonal change may partly reflect a seasonal variation in the barotropic (associated with a nonzero velocity at the seafloor) component of shelf or slope flow, involving more southwestward flow in winter. Assuming that the associated elevation is confined to the Inner shelf and linearly distributed between the coast and station 3, each centimeter of coastal sea level change corresponds to a current change of m s 1 and a transport change of 0.11 Sv. Current measurements within 10 m of the seafloor on the shelf part of the Halifax section are only available for the 100-m isobath in April October, and indicate a northeastward mean flow of m s 1 [Gregory and Bussard, 1996]. The associated coastal setdown of 1 cm is consistent with the ASL-steric difference in Figure 4c, suggesting a small northeastward barotropic transport in spring-fall. During winter, the deepest available measurements from the Inner shelf (at heights of m above bottom at four CASP sites [Anderson and Smith, 1989]) indicate southwestward mean currents of m s 1, and if taken to represent barotropic flow, imply a coastal set-up of about 3 cm that also fits reasonably well with the ASL-steric difference (Figure 4c). However, the present geostrophic estimates (Figure 2) as well as diagnostic model studies [e.g., Han et al., 1997] indicate significant baroclinic shear at the CASP measurement depths, so that the above values probably overestimate the barotropic flow. Thus the seasonal change in ASL-steric difference (Figure 4c) probably reflects a combination of a seasonal barotropic transport change of up to (but probably less than) 0.4 Sv in the Nova Scotian Current, the annual cycles of baroclinic (Figure 4a) and barotropic [Greenberg and Petrie, 1988] current variations on the outer shelf, and larger-scale influences. 4. Variability About the Halifax Section Annual Cycle 4.1. Hydrographic and Baroclinic Transport Variability Indices [26] The hydrographic variability about the annual cycle on the Halifax section is illustrated by the zone-averaged anomalies of temperature in Figure 5. The period had the best temporal and spatial resolution, with occupations typically seasonally with the slope included. The dominant anomalous feature identified by PD93 of a shelf-wide reduction in temperature by about 5 C during the late 1950s and early-to-mid 1960s is apparent. The salinity (not shown here) had a similar variation with a range of 1. The variations have similar patterns in the Inner, Mid, and Outer zones, with increased amplitudes in the latter (consistent with the findings of PD93). In view of the reduced sampling since 1970 and the large anomaly in the late 1950s and 1960s, we focus on the period. [27] The detailed timing of the 1950s/1960s hydrographic anomaly, and the geostrophic transport variations (500-m analysis) are shown in Figure 6. The temperature anomaly for the Inner-Lower zone was about 1 C below normal in the mid 1950s, decreased an additional 2 C during the winter of , slowly recovered in the early 1960s, decreased 3 4 C during the winter of followed by a slow recovery, and returned to near-normal values in Both the and decreases were preceded (3 6 months) by periods of below-normal temperatures at the shelf edge (Figure 6c, Outer-Upper), and the 1968 warming was associated with above-normal shelf-edge temperatures. [28] In spite of the large changes in hydrographic properties, the baroclinic transport anomalies on the Inner shelf were typically under 0.5 Sv (compared to the longterm annual variation of Sv), indicating considerable persistence of the Nova Scotian Current. The absolute

7 LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4-7 Figure 4. Annual cycles in (a) cross-shelf steric elevation difference across various zones of the Halifax section; (b) smoothed Halifax ASL (thick line) for with standard errors in the monthly means indicated by vertical lines, steric heights (solid dots with standard errors indicated by vertical lines) estimated from monthly mean density profiles at stations 1 3, steric heights adjusted for regional wind contributions (triangles), and steric elevation difference across the Inner shelf (squares; repeated from Figure 4a); and (c) the difference between monthly mean ASL and the wind-adjusted steric height, with combined standard errors (solid dots and vertical lines). The broken lines in Figure 4c are the coastal elevations for barotropic currents estimated from measured currents: for winter, from the CASP data [Anderson and Smith, 1989], and for April October, from Gregory and Bussard [1996].

8 GLO 4-8 LODER ET AL.: HALIFAX SECTION VARIABILITY Figure 5. Indices of temperature variability (relative to the annual cycle based on ) for the various Upper (100 m; solid dots) and Lower (125 m; open dots) crossshelf zones from occupations of the Halifax section. The indices for the Inner, Mid, and Outer zones are from the 300-m analysis, and those for the Offshore zone from the 500-m analysis. transport estimates for the Outer shelf were generally (but not always) southwestward, with magnitudes again typically under 0.5 Sv, while the absolute estimates for the Offshore zone were approximately evenly distributed between northeastward and southwestward flow, with magnitudes typically of order 1 Sv and ranging up to 2 Sv. Considering the coarse spatial sampling and estimated uncertainties (earlier), many of the Outer and Offshore transport estimates are not statistically significant, so they must be interpreted with caution. For example, it was found that the transport anomaly estimates for the Outer and Offshore zones were generally not reliable, apparently because of the combined uncertainties in the individual-section and long-term estimates. [29] On the basis of the PD93 finding that cold temperatures on the Scotian Shelf were associated with increased transport on the Newfoundland Shelf, and the Loder et al. [2001] finding of increased southwestward transport on the Scotian Slope during the cold period, large positive (negative) transports might be expected to be associated with the negative (positive) temperature anomalies. These are not readily apparent in Figure 6. Correlation analyses (Table 1) between the Inner/Mid temperature anomalies and the transport estimates do show significant negative correlations, but they are not large. They indicate that the present Figure 6. Indices of temperature (from Figure 5) and along-shelf baroclinic transport variability for various zones on the Halifax section from The temperature and Inner-shelf transport indices are anomalies relative to the annual cycle, while absolute transports (positive southwestward) from the 500-m analysis are used for the Outer and Offshore zones. Table 1. Correlation Coefficients Between Temperature Anomalies and Baroclinic Transport Indices (Figure 6) for the Halifax Section During a Temperature Anomalies Inner-Lower Mid-Lower Inner transport anomaly 0.29 Outer transport Offshore transport a All the correlations are significant with p = 0.99.

9 LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4-9 estimates of Outer and Offshore transport variability can account for about 20% of the temperature variance on the shelf below 125 m. It is noteworthy that the correlation between shelf temperatures and Inner transport anomalies is even lower, indicating that variability in the Nova Scotian Current is not a predominant factor in the low-frequency shelf hydrographic variability Onset of the 1950s/1960s Anomalous Hydrographic Conditions [30] We next describe the hydrographic transition periods in more detail by examining individual hydrographic sections. In October 1958 (Figure 7), shelf and upper-slope temperatures were near or just below normal, while those offshore were 4 C below normal indicating an increased presence of Labrador Slope Water. By February 1959, a large change in upper-slope properties had occurred, with temperatures at m reduced to about 2 C, more than 6 C below normal. Transports on the Outer shelf were near-normal (southwestward) during these occupations (Figure 6d), while those Offshore were weak (<0.5 Sv; Figure 6e). There was a strong seasonal cooling of Upper shelf waters during this period, and a slight cooling of the deep waters in Emerald Basin but the latter remained only slightly cooler than normal. [31] By May 1959 (Figure 7), upper-slope temperatures had warmed to only about 2 C below normal, while deep Emerald Basin water had cooled to 5.4 C [Taylor, 1961], about 3 C below normal. Similar shelf and upper-slope distributions (below the seasonal surface layer) persisted through the July and October 1959 occupations. The above sequence indicates that a major part of the onset of the hydrographic anomaly occurred between October 1958 and May 1959, and involved an upper-slope change followed by a shelf-basin change. The spring-fall 1959 occupations (Figure 7) show a cooler-than-normal intermediate layer extending from the coast to the slope, indicating the broad extent of the anomaly. [32] A somewhat similar sequence of events occurred during the winter of In September 1963 (Figure 8), shelf temperatures at depth had warmed to near normal while shelf-edge temperatures had cooled since spring 1963 to about 4 C below normal (Figure 6c). A similar subsurface pattern existed in December 1963 (not shown). By March 1964, the slope cooling had intensified (anomalies exceeding 6 C) and broadened, and shelf temperatures at depth had fallen to about 4 C below normal. The shelf anomaly persisted through July 1964, September 1964 (not shown) and subsequent years with some amelioration. The slope anomaly was considerably reduced by July 1964, but re-intensified in fall 1964 and remained until (Figure 5). [33] The and sequences indicate that the onset and re-intensification of the Scotian Shelf hydrographic anomaly both occurred through a combination of a more widespread fall-winter distribution of Labrador Water over the slope, followed by intrusions onto the shelf. However, examination of other winters indicates that anomalously cold shelf-edge water can also occur without major effects on inner-shelf conditions (Figure 5c). For example, cool shelf-edge anomalies exceeding 2 C were observed in the winters of 1954 and 1956, with limited influences on Emerald Basin bottom waters. Thus it appears that the onset of anomalous shelf hydrographic properties requires both anomalous water at the shelf edge and an on-shelf intrusion mechanism. [34] The present analyses have not found direct evidence of increased southwestward transport over the Scotian Slope during or Nevertheless there is mounting collective evidence that increased Labrador Current transport around the Grand Bank (PD93) was a major factor in the origin of the anomalous conditions. Evaluations of decadal-scale changes have indicated relativelycold and -fresh upper-ocean anomalies centered on the Scotian Slope but extending to the Newfoundland Slope during the late 1950s [Levitus, 1989; Kushnir, 1994], and increased equatorward transports in the Slope Water [Greatbatch et al., 1991]. Studies of year-to-year variability have indicated that, in 1959, there was a major cold anomaly in the Slope Water [Volkmann, 1962; Worthington, 1964], as well as a large westward flow of Labrador Current Water south of the Grand Bank [Pickart et al., 1999]. While many questions remain regarding the temporal variability and mechanisms involved in the large-scale circulation fluctuations, it seems likely that they were an important factor to the anomalous hydrographic conditions on the Scotian Shelf/Slope in the 1950s/1960s End of the 1950s/1960s Anomalous Hydrographic Conditions [35] The below-normal subsurface temperatures persisted on the inner shelf and upper slope of the Halifax section into April 1967 (Figure 9); the temperature anomaly decreased over the slope and at depth in Emerald Basin from November 1966 to January 1967, and subsequently intensified from January to April There was then a 14-month gap until the section s next occupation in June 1968, at which time the deep-shelf and upper-slope temperatures had warmed (by more than 3 C) to values about 1 C below normal, and Warm Slope Water had returned to the shelf edge. The warming continued through October 1968 when shelf temperatures returned to near normal and upper-slope temperatures were more than 4 C above normal associated with a further northward excursion of Warm Slope Water (Figure 9). The Outer transport estimates from the April 1967 and June and October 1968 sections were small (Figure 6d), while the corresponding Offshore estimates (Figure 6e) were northeastward with magnitude near 1.5 Sv in June and October [36] Other hydrographic data provide indications of the evolution between April 1967 and June For the upper slope, they indicate the presence of Warm Slope Water (10.5 C) from May to August 1967, followed by a return of Labrador Slope Water ( C) in November 1967, and then a return of warmer waters (8.7 C) as early as late January For Emerald Basin they indicate some warming at depth by August 1967 ( C), a slight relapse in January 1968 ( C), and then warmer waters (7 8.2 C) dominating by May [37] Collectively, these observations indicate that the end of the anomalous shelf hydrographic conditions was associated with increasing prevalence of Warm Slope Water near the shelf edge starting in May 1967 and intensifying during early The sparse geostrophic estimates suggest north-

10 GLO 4-10 LODER ET AL.: HALIFAX SECTION VARIABILITY Figure 7. Temperature distributions from individual occupations of the Halifax section during (left panels), and anomalies relative to the annual cycle on the same calendar day (right panels). The contour interval is 2 C.

11 LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4-11 Figure 8. Temperature distributions from individual occupations of the Halifax section during (left panels), and anomalies relative to the annual cycle on the same calendar day (right panels). The contour interval is 2 C. eastward flow over the middle slope (Offshore zone) in 1968, consistent with the change in hydrographic properties. The hydrographic data indicate that the warming at depth in Emerald Basin progressed gradually between summer 1967 and fall By December 1968 the deep salinity in Emerald Basin had increased to above-normal values over 35, which Petrie and Smith [1977] suggested may have been associated with a measured onshore deep current of 0.02 m/s and eastward wind stress of Pa during October December. Above- or near-normal shelf temperatures then persisted through the 1970s, with variable but generally above-normal slope temperatures. [38] It is unclear whether the penetration of hydrographic anomalies from the shelf edge into Emerald Basin is predominantly related to the long-term seasonal-mean circulation or other variability. Using moored measurements at the shelf edge, Smith and Petrie [1982] found that the on-shelf fluxes were dominated by the seasonal-mean contribution. Recent diagnostic modeling studies [Hannah et al., 2001; Shore et al., 2000] have identified a persistent on-shelf meander of the shelf-edge flow around/over Emerald Bank. On the other hand, wind-driven shelfedge upwelling [Petrie, 1983] followed by topographic channeling [Greenberg et al., 1997] is a candidate mechanism for episodic on-shelf penetration. This is supported by the occurrence of upwelling-favorable wind stress anomalies during the winters of , and , although these were not the only winters with such anomalies. Another possible factor in the end of anomalous conditions in Emerald Basin is the circulation associated with the 10-cm negative anomaly in Halifax- Yarmouth sea level difference during late spring 1967 (relatively high sea level at Yarmouth [Sandstrom, 1980, Figure 8], when there was a large negative baroclinic transport anomaly in the Inner zone and northeastward transport in the Offshore zone (April 1967; Figure 6) Shelf-Slope Coupling [39] In this subsection, we use the monthly subsurface temperature anomalies from the inner shelf, Emerald Basin and the Slope Water during to quantitatively evaluate the inter-relation between shelf and slope hydro-

12 GLO 4-12 LODER ET AL.: HALIFAX SECTION VARIABILITY Figure 9. Temperature distributions from individual occupations of the Halifax section during (left panels), and anomalies relative to the annual cycle on the same calendar day (right panels). The contour interval is 2 C.

13 LODER ET AL.: HALIFAX SECTION VARIABILITY GLO 4-13 Table 2. Root-Mean Square Temperatures ( C) From the m Monthly Anomalies for the Inner Shelf, Emerald Basin, and Slope Water During a Time Series Inner Basin Slope Unfiltered Low-pass High-pass a The low-pass filtered time series (interannual variability) were created using a 13-month running-average filter. The high-pass filtered time series (seasonal variability) are the difference between the unfiltered and low-pass filtered time series. graphic variability. The mean temperatures (removed) were 3.9, 7.4, and 9.1 C for the inner shelf, Emerald Basin and Slope Water, respectively, while the annual cycles had ranges of 1.8, 0.6, and 1.2 C. [40] The Slope Water has the largest overall, interannual and seasonal variance (Table 2). The inner shelf and Emerald Basin have similar interannual variance but the inner shelf has greater seasonal variance, likely the result of its shallower depth and location in a coastal upwelling zone [Petrie et al., 1987]. [41] Figures 10a 10c show that the temperature anomalies are closely related at the lower frequencies and this is supported by correlation analyses (Table 3). For the lowpass filtered time series (interannual band), the Slope Water temperature fluctuations can account for 83% of the variance in the Basin and 64% of the variance in the inner shelf. The time lags of 3 5 months between the Slope and Basin anomalies, and 5 months between Slope and inner shelf (unfiltered) anomalies confirm that changes over the slope precede changes in the Basin, and imply a reasonable advective speed of 1 cm/s. On the other hand, the analyses indicate that the peak low-frequency correlation between the Slope and inner shelf anomalies occurs for lag near 0; however, the cross-correlation peak between these series is quite broad and at 5 months lag, the Slope series can account for 59% of the low-frequency variance on the inner shelf (which is not significantly less than for zero lag). [42] To further evaluate the predictability of basin and inner-shelf temperature anomalies by the Slope Water anomalies, we constructed a filter that predicts the temperature at time t = t* from the Slope temperature at times t t* (a causal filter). Two filter families were tried: [43] 1. Filter with a delay of m months and with filter weights that decay exponentially with timescale t months. The filter weight is zero for m 1 months previous to the current time t*, a maximum m months before t* and decays exponentially beyond that. [44] 2. A simple average of the previous n months. Both filters had unit amplitude and were not tuned for gain. [45] For the Slope-Basin connection, filters with delays m =0 3 months and decay timescales of t =3 18 months yielded predictions that captured 50 55% of the variance of the Basin temperature anomalies, compared with 37% for the best linear regression. An example is shown in Figure 10d. The simple average gave results similar to those from the decay filter with t = n and m = 0 months, but captured a few per cent less variance. If either filter was used to predict the Slope anomalies from the Basin anomalies the results were much worse, which supports our conclusion that the Slope Water is influencing the Basin temperatures and not vice versa. For the connection between the Slope and inner-shelf anomalies, filters with lags of 0 3 months and decay timescales of 1 24 months yielded predicted time series that capture 25 32% of the variance of the inner-shelf time series (Figure 10e), compared with 21% for the best linear regression. This is about half of the fraction of variance predicted for the Basin time series from the Slope anomalies Comparison With Observed and Wind-Forced Sea Level Indices [46] The continuous time series of Halifax ASL and Sable Island wind stress allow further interpretation of the temperature and transport indices: [47] 1. The annual cycle of baroclinic transport is clearly apparent in coastal sea level (Figure 4) so other variations in baroclinic transport may be detectable as well. [48] 2. Wind-forced transport variability can be expected to modify the density field [e.g., Petrie et al., 1987]. Stormband wind variations can cause high-frequency variability in baroclinic flow that could be aliased into apparent lowerfrequency variability, while low-frequency wind variability could be a forcing of the low-frequency transport variations of interest. On the other hand, Thompson and Sheng s [1997] finding of a baroclinic current mode on the inner shelf (near station 2 on the Halifax section) with no correlation with wind or sea level suggests that windforcing is a secondary influence on baroclinic transport variability. [49] 3. Coastal sea level may provide indications of important barotropic flows associated with shelf waves or large-scale ocean circulation fluctuations that need to be included in transport variability indices. [50] We start with a comparison for the period of the anomalies of observed ASL and modelpredicted wind-forced sea level at Halifax. A significant contribution from regional wind-forcing to observed ASL is clearly apparent in the monthly anomalies (Figure 11a), consistent with previous findings [Sandstrom, 1980; Schwing, 1989]. Coherence analysis (not shown) using bi-daily anomalies indicates broad coherence ( , mean = 0.64 ± 0.12) at periods of 6 30 days, with gains between 1 and 2 (mean = 1.4 ± 0.3) and phase lags of hours (mean = 30 ± 8 hours) for ASL relative to wind stress, with both gain and phase generally increasing with period. Thus over these periods, about 40% of the variance in ASL can be explained by regional winds. This is consistent with the results of Schwing [1989], who found that about 90% of the subtidal sea level variance at periods of 1 40 days during the winter of could be related to wind and upstream forcing, with about half of this due to regional wind-forcing. [51] The relation of the cross-shelf (coastal relative to 500 m at station 6) steric anomalies to the ASL anomalies during is illustrated in Figure 11b where the ASL anomalies are shown as both monthly means (continuous curves) and weekly means for the weeks centered on the section occupations. The steric anomalies have comparable magnitudes to the ASL anomalies, with similar fluctuations apparent at times and a significant positive correlation (r 2 = 0.13; p = 0.98, linear slope = 0.32) between