ARTICLE IN PRESS
Continental Shelf Research 27 (2007) 1875–1892 www.elsevier.com/locate/csr
Remote sensing and modelling of bio-physical distribution patterns at Porcupine and Rockall Bank, Northeast Atlantic
Christian MohnÃ, Martin White
Department of Earth and Ocean Sciences, Martin Ryan Institute, National University of Ireland Galway, Galway, Ireland
Received 28 July 2006; received in revised form 9 March 2007; accepted 28 March 2007 Available online 19 April 2007
Abstract
Monthly composites of multi-year sea surface temperature (SST) and chlorophyll-a (Chl-a) have been used in combination with ocean model simulations to study bio-physical distribution patterns at Porcupine and Rockall Bank, two large submarine banks in the Northeast Atlantic in close proximity to the European shelf edge. Seven years (January 1998–December 2004) of remotely sensed data have been collated to create monthly climatological fields and to analyse principal spatio-temporal characteristics. At both banks, a region of cooler SST is found over the summit region compared to warmer waters of the surrounding ocean, less apparent in summer when capped by the seasonal thermocline. Enhanced Chl-a levels are found over both banks with a lifetime partly exceeding the bloom period. At Rockall Bank, both SST and Chl-a signals are more pronounced and persistent showing a 30% increase in annual Chl-a levels over the summit area with an even higher ratio in spring and autumn. A combination of physical processes appears to promote the enhanced productivity over both banks through the generation of a quasi-steady dome of cold, dense water during winter convection and upwelling events. This cold dome is associated with the presence of a retentive circulation based on Taylor cap dynamics and tidal rectification processes. The larger and more persistent enhancement of Chl-a levels over Rockall Bank would appear due to its isolated nature as well as its size. In contrast, Porcupine Bank is partly attached to the Irish shelf edge and exposed to the poleward flowing shelf edge current which may strip passive particles from the central bank region. Satellite derived Chl-a spring/summer distributions over the banks have been used to initialise model simulations of passive tracer dispersion. Timescales for the observed lifetime of the remotely sensed Chl-a patches are consistent with model derived retention timescales and simple scaling for the dispersion of passive biological material over the banks. Surface particle residence times over Rockall Bank are estimated to exceed Porcupine Bank values by a factor of two. Finally, the tidal contribution to individual particle motion is found to be large in some Rockall Bank areas, but less important at Porcupine Bank. r 2007 Elsevier Ltd. All rights reserved.
Keywords: Northeast Atlantic; Porcupine Bank; Rockall Bank; Remote sensing; Numerical modelling; Particle retention
1. Introduction one of the major pathways for the propagation of warm North Atlantic waters into the Arctic Ocean. The sub-polar European continental margin and In addition to their significance for ocean climate, the Rockall Trough have always been considered as they have attracted further attention and scientific interest over the past decade with the discovery of ÃCorresponding author: Tel.: +353 91 495157. giant carbonate mound provinces and reef-forming E-mail address: [email protected] (C. Mohn). cold water corals (Roberts et al., 2006). In a linked
0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.03.006 ARTICLE IN PRESS 1876 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 series of EU funded projects and ongoing seabed 60°N mapping initiatives, these habitats were mapped and studied with a focus on environmental controls of reef formation and reef development (see Mienert HRB et al., 2004 for an overview). Benthic ecosystems are very vulnerable to environmental and human 58°N factors. The requirement of a profound and 1.0 0.2 interdisciplinary knowledge about their functioning and controls is the principal motivation of the 1.0 RT recently started EU project HERMES (Hotspot 0.5 ° 0.3 Ecosystem Research at the Margins of European 56 N 0.4 Seas) with Porcupine and Rockall Bank being one RB of the target areas (Weaver et al., 2004). Porcupine and Rockall Bank are also part of the Exclusive 2.0 Economic Zone (EEZ) of Ireland and the UK, and 54°N therefore important target areas for fisheries and 0.2 ecosystem management efforts. They are not only 3.0 0.2 0.3 spawning grounds for commercial fish species like PB 0.1 4.0 blue whiting, but also are home to a species-rich 0.4 52°N 1.0 deep-water fish community, such as Orange roughy. 0.5 Porcupine Bank is a large shelfbreak bank located 2.0 at the continental margin west of Ireland (see Fig. 1 for a map of the study area). The shallow bank PS 50°N summit is centred at 53:5 N forming an elliptical plateau inside the 200 m isobath with a summit depth of 180 m, attached to the Irish shelf break by 20°W18°W16°W14°W12°W10°W8°W a 300 m deep channel. The characteristic slope is very steep on its western side where it falls from Fig. 1. Map of the study area with major topographic features depths of 300 m at its edge to almost 4000 m into the (PB: Porcupine Bank, PS: Porcupine Seabight, RB: Rockall Bank, RT: Rockall Trough, HRB: Hatton-Rockall Basin). The northern Porcupine Abyssal Plain over a distance of bold lines indicate the location of transects used for satellite data 40 km. To the south, however, the bank is largely analysis. Depth contours are in km. isolated from the shelf edge and extends over a larger area gradually decreasing to typical water depths of 2000 m. The Rockall Bank is an isolated marine environment which clearly differs from near- topographic feature separating the Rockall Trough by oceanic far field conditions. Among these are from the Iceland and Hatton-Rockall Basin. Its closed, bottom-intensified circulation cells gener- orientation is mainly SW–NE with a large shallow ated by impinging steady currents (Taylor caps, e.g. summit plateau centred at 57:7 N. The slope Chapman and Haidvogel (1992)), the resonant characteristics are similar to Porcupine Bank with generation of large amplitude seamount-trapped an elongated southwestern tip and steeper slopes waves generated by periodic tidal flow (Chapman, along its east–west axis. 1989; Brink, 1990), enhanced turbulent vertical The role of the special flow dynamics at mixing (e.g. Kunze and Toole, 1997) and uplifting submarine banks has been recognised as an of cold, nutrient-rich water (e.g. Genin and Boeh- important factor in providing a sustainable feeding lert, 1985). The relative importance of each of these mechanism for coral growth (White et al., 2005) and processes for the local marine ecosystem depends on in supporting enhanced biological productivity and a considerably large parameter space, such as bentho-pelagic coupling, yet the full spectrum of seamount geometry, strength of the impinging flow bio-physical interactions is not fully understood. and ambient stratification conditions as well as the Isolated or partly isolated seamounts and submar- geographical latitude (for an overview see Beck- ine banks are known to support a variety of physical mann, 1999). They are considered to have an processes and phenomena which are confined to the important influence on biological material trans- local topography and may contribute to maintain a port, such as particle aggregation, trapping and ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1877 retention (e.g. Mullineaux and Mills, 1997; Goldner 7 years time series of the monthly composites of SST and Chapman, 1997; Genin, 2004), and therefore and Chl-a images (for periods and subregions see have the potential to create favourable conditions Table 1). The averaging was carried out in two for marine biology in critical parts of the seamount/ steps: first, the geometric mean (median) was used submarine bank ecosystem, e.g. pelagic, bentho- to calculate the climatological averages following pelagic and benthic boundary layer. the method outlined by Yoder et al. (2002).Ina Results from previous studies indicate that such second step, the standard deviation of all valid processes also occur at Porcupine (e.g. Mohn and pixels of the 7 years time series was calculated at Beckmann, 2002; Lynch et al., 2004) and Rockall each grid point and large outliers were eliminated by Bank (e.g. Huthnance, 1974; Dooley, 1984), but excluding all pixels with values greater than 2 systematic observations over longer periods are standard deviations. This averaging procedure was sparse. In this study, we discuss the potential of the also applied to the SST data. The resulting Porcupine and Rockall Bank as a sustainable tracer climatological data set was spatially re-binned to a reservoir. We focus on the role of the flow dynamics uniform 0:1 grid and gaps in the 7 years time series on the transport and dispersion of passive biological were closed by linear interpolation. material, such as chlorophyll. We use remotely The significance of the climatological averages sensed AVHRR (Advanced Very High Resolution was limited by the total number of available pixels Radiometer) and SeaWiFS (Sea-viewing Wide for each period. The percentage of all cloud-free Field-of-view Sensor) data to investigate spatio- pixels of SST and Chl-a observations contributing temporal characteristics of sea surface temperature to each period and subregion is listed in Table 1. (SST) and surface chlorophyll (Chl-a) patterns over Data coverage generally varied in time and space; it Porcupine and Rockall Bank. The results are was lowest during winter and also decreased combined with passive tracer simulations using a towards higher latitudes. Large variations were also three-dimensional ocean circulation model. The found between SST and Chl-a coverage. The observational data base, processing methods and number of available SST observations were rela- the physical model are introduced in Section 2. In tively consistent and ranged from a minimum of Section 3 we present results from the analysis of 68.6% (64.7%) in winter to a maximum of 90% climatological SST and Chl-a distributions over the (87.1%) in summer at Porcupine Bank (Rockall banks complemented by passive model tracer Bank). In contrast, Chl-a data coverage was higher simulations. Finally, observations and model results than 95% from March to October in both are compared and discussed in Section 4. subregions, but large data gaps occurred in winter, especially in the Rockall Bank area where no data 2. Material and methods Table 1 2.1. Remote sensing Percentage of data coverage to calculate monthly climatological averages for the period 1997–2004 at each subregion Monthly composites of remotely sensed SST and Month Porcupine Bank Rockall Bank Chl-a data were analysed for a 7 years period (20210 W, 50255 N) (20210 W, 55260 NÞ extending from December 1997 to December 2004. data coverage (%) data coverage (%) SeaWiFS Chl-a data were acquired from the NASA Goddard Earth Sciences (GES) Data and Informa- SST Chl-a SST Chl-a tion Services Center (DISC). AVHRR Pathfinder Jan (1) 67.8 60.9 69.2 5.9 version 5 SST data were obtained from the NASA Feb (2) 68.6 87.3 64.7 70.5 Physical Oceanography Distributed Active Archive Mar (3) 78.1 96.7 66.6 94.0 Center (PO.DAAC). Chl-a and SST fields are Apr (4) 83.6 99.7 77.3 99.0 processing level-3 products at a spatial resolution May (5) 87.1 99.6 87.1 99.6 Jun (6) 83.0 98.1 73.7 98.4 of 0:1 and 4 km, respectively. All data were Jul (7) 84.4 98.8 80.7 98.2 subsampled over the study area between Aug (8) 90.0 99.8 82.8 99.3 50 W260 W latitude and 20 W210 W longitude Sep (9) 86.1 99.4 84.5 99.0 to cover the Porcupine and Rockall Bank subre- Oct (10) 77.3 98.9 76.2 97.3 gions. Monthly and seasonal climatologies were Nov (11) 72.8 77.9 66.3 29.8 Dec (12) 70.9 20.3 66.9 — calculated from all available, cloud-free data for the ARTICLE IN PRESS 1878 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 were available in December throughout the ana- topography. The European continental margin lysed 7 years period. Therefore, only Chl-a compo- separates the relatively warm oceanic waters off sites for months with a data coverage of at least the continental slope from the cooler waters on the 50% in each subregion were used in this study. Irish shelf. Further offshore, the Rockall Bank divides the poleward Atlantic inflow into two major 2.2. Ocean modelling branches: Warm Eastern North Atlantic Water (ENAW), a mode water of predominantly subtro- We used the terrain-following, primitive equation pical origin enters the Rockall Trough (Holliday ocean general circulation model SPEM (Haidvogel et al., 2000), whereas the eastern Iceland Basin and et al., 1991) to simulate the influence of the Hatton-Rockall Basin region west of Rockall Bank distribution of biological material as a consequence is dominated by its colder subpolar counterpart, of physical forcing. The model domain incorporated Western North Atlantic Central Water (WNAW, the shelf and oceanic areas west of Ireland and (e.g. Schmitz and McCartney, 1993; Pollard et al., spanned an area of 1280 760 km between 1996)). The most prominent features are quasi- 45260 N latitude and 10220 W longitude. The isolated patches of cold surface water over the horizontal grid contained 320 190 grid points with central Porcupine and Rockall Bank summits which a uniform grid spacing of 4 km. The vertical grid can be found in almost every month throughout the consisted of 20 levels and was adapted to the climatological year. The spatial pattern and tem- bottom topography through a generalised s-coordi- poral evolution of the cold core over each bank nate transformation. The vertical levels were largely agree. It is most pronounced in the late stretched to increase the resolution in the bottom autumn/winter period from December to March and surface layers. The model was configured as a and covers the central bank areas inside the 300 m periodic f-plane channel centred at mid-latitude of isobath. It weakens with seasonal warming and is the model area. As a major extension of the less apparent in summer, but reconstitutes in idealised representation of the Irish shelf edge autumn. In situ observations over the Porcupine employed by Mohn and Beckmann (2002) and Bank indicate that the cold dome over the central White et al. (2005), realistic bathymetric data was summit area seems to be a persistent feature, but is taken from the satellite gravimetry based data set confined to greater depths in periods of strong ETOPO-2 (Smith and Sandwell, 1997) and mapped background stratification (Mohn et al., 2002). to the three-dimensional model domain. To reduce Monthly mean Chl-a distributions are presented possible errors resulting from the calculation of in Fig. 3. Temporal and spatial chlorophyll varia- the horizontal pressure gradient in this type of tions at shelf-ocean boundaries may well be in models (Beckmann and Haidvogel, 1993), addi- excess of 2–3 orders of magnitude at shelf-ocean tional weak smoothing was applied to the bathy- boundaries. In these regions chlorophyll is com- metry without affecting the realism of the main monly represented by a log-normal distribution topographic features. Subgridscale dissipative pro- (Campbell, 1995). We therefore log-transformed the cesses not resolved by the model were parameterised linear Chl-a concentrations prior to averaging. Chl- by constant biharmonic mixing coefficients for a patterns over the banks generally show an inverse lateral viscosity and diffusivity ðnuv ¼ nr ¼ 2:5 relationship with the seasonal cycle of the cold 108 m4 s 1Þ. An explicit linear bottom friction of dome (Fig. 3). Surface concentrations are lowest in 4 1 rb ¼ 2:5 10 ms was prescribed at the deepest winter and reach a maximum in summer. However, velocity grid point. winter levels (February–March) over the central bank areas are still slightly enhanced compared to 3. Results typical concentrations found in the Porcupine Seabight/western approaches and the southern 3.1. A seasonal climatology of the Porcupine and entrance of the Rockall Channel. But spatial Rockall Bank area variations in winter are weak and may be of the same order of magnitude as the Chl-a concentra- The monthly mean distribution of SST in the tions and therefore have to be interpreted very study area is shown in Fig. 2. It is dominated by a carefully. general poleward decrease and several transition In the Porcupine Bank area, the phytoplankton zones associated with abrupt changes of the seafloor bloom period extends from April on the Irish shelf ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1879
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Fig. 2. Monthly climatology of SST ð CÞ for the Porcupine and Rockall Bank areas. Numbers indicate months of the year. Color scaling varies for different seasons of the climatological year. ARTICLE IN PRESS 1880 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892
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Fig. 3. Monthly climatology of log-transformed Chl-a ð mg m 3Þ for the Porcupine and Rockall Bank areas. Numbers indicate months of the year (only months with a data coverage of at least 50% at both subregions are shown). Color scaling varies for different seasons of the climatological year. to July over the bank. There it reaches a maximum bank appears to be delayed by one month against from May to June with highest levels inside the Irish shelf conditions. While phytoplankton on the 400 m isobath. The bloom onset and timing over the Irish shelf seems to have propagated northwards ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1881 and to be accumulated along the shelf edge north- latitude where the most pronounced Chl-a patches east of the bank by the end of June, Chl-a levels of were observed in the seasonal climatological fields. 1mgm 3 can be observed over the central Spatio-temporal characteristics are presented as Porcupine Bank until July. Compared to Porcupine cross-shelf anomalies along the transect calculated Bank, however, locally elevated Chl-a levels over for each month. The cold dome over the bank is the central Rockall Bank area are more pronounced centred at 13:7 W, develops in late autumn and is a and appear to be a persistent feature throughout persistent feature until April. The strongest gradi- almost the whole year. Enhanced Chl-a levels in late ents appear from November to February when summer/early autumn (September–October) of surface waters over the bank are 0:4 C cooler 1mgm 3 are confined to the shallow summit than surrounding oceanic waters. Spring/summer areas inside the 200 m isobath, but extend to the sea surface warming causes a weakening of the deeper bank areas limited by the 400 m isobath from dome, being transformed into a frontal zone and April to August. The highest Chl-a concentrations separating warmer slope/shelf waters east of 13 W ðX2mgm 3Þ occur from May to August. from cooler oceanic waters to the west (Fig. 4a). Monthly Chl-a concentrations are highest in Irish 3.2. The annual climatological cycle shelf waters where the bloom period starts in April. The bloom onset over the bank is delayed by one The annual climatological cycle of SST and Chl-a month, but elevated Chl-a values exist until July is analysed in more detail along transects intersect- (Fig. 4b). The phase shift of the spring bloom onset ing the central summit areas of Porcupine and and timing between bank and shelf waters is not Rockall Bank. Figs. 4a and b show the temporal limited to the shallow summit areas of the bank, but evolution of monthly SST and Chl-a for the annual can also be found over the deeper portions further climatological cycle along a transect at 52:9 N south (White et al., 2005). During the main bloom
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Fig. 4. Annual cycle of along-transect anomalies of (a) SST ( C) and (b) Chl-a ðmg m 3Þ at Porcupine Bank (52:9 N latitude), (c) SST ( C) and (d) Chl-a ðmg m 3Þ at Rockall Bank (57:7 N latitude). Months with data coverage less than 50% were not considered. The dashed lines indicate the location of the central summit of each bank. The geographical location of the transects is shown in Fig. 1. ARTICLE IN PRESS 1882 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892
0.9 0.9 11.2 ) 12.5 ) 0.8 0.8 C) C) ° 11 ° ( 12.4 ( 0.7 0.7 (mgm (mgm PB RB PB RB 10.8 0.6 0.6 SST 12.3 SST 0.5 0.5 12.2 10.6 0.4 0.4
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Fig. 5. Annual mean profiles of (a) SST ( C) and (b) Chl-a ðmg m 3Þ at Porcupine ð52:9 N latitude) and Rockall Bank (57:7 N latitude) 2 2 and corresponding variances (c) sSST and (d) slogðchl aÞ. period, Chl-a levels over the bank are of the same In average, surface waters over Porcupine Bank are magnitude as shelf values and exceed slope (oceanic) 0:3 C cooler than surrounding oceanic waters and values by 0:5mgm 3 ð1mgm 3Þ, therefore acting 0:1 C cooler than shelf/slope waters. (Fig. 5a). The as a local source of high productivity (Fig. 4b). annual SST variability at that latitude (expressed as The same analysis was carried out for the the variance of the annual averages over the 7 years central bank summit of Rockall Bank at 57:7 N averaging period) gradually increases towards the (Figs. 4c and d). The period of isolated low SST shelf/slope region where the highest variability is values over the bank extends from October to May. observed (Fig. 5c). At Rockall Bank, enhanced In this period, temperatures over the bank are surface temperatures during the summer warming 1 C ð0:5 CÞ lower than average values in the period make only a minor contribution to the Rockall Trough (Hatton-Rockall Basin). With annual average SST distribution (Fig. 5a): Tem- increasing sea surface warming in July, SST peratures over the bank are 0:3 C and up to 0:7 C differences between the central Rockall Bank and lower than mean values in the Hatton-Rockall Rockall Trough waters drop to 0:5 C in August. In Basin and Rockall Trough, respectively. In contrast addition, Rockall Bank waters are generally warmer to Porcupine Bank, the SST variability generally than oceanic waters in the Hatton-Rockall Basin. decreases towards Rockall Bank (except a moderate Analysis of monthly climatological Chl-a distribu- increase over the summit, see Fig. 5c) indicating that tions along the summit transect confirms that Chl-a the physical conditions over the bank are relatively levels over the Rockall Bank summit are persistently invariant against seasonal variations. higher throughout the climatological year than Annual mean Chl-a levels over the Porcupine values in surrounding oceanic waters. Chlorophyll summit area are slightly elevated by 2 mg m 3 concentrations over the summit peak from April to compared to open ocean values further west, but September and exceed maximum values of sur- are lower than annual shelf/slope values by approxi- rounding oceanic levels by up to 0:6mgm 3. mately the same order of magnitude (Fig. 5b). This The annual mean SST profile is dominated by the indicates that the spring bloom is the major cold winter/spring signal over both bank summits. contributor to the elevated Chl-a levels over the ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1883 bank and any particle retention mechanism appears many parts of the European shelf edge (e.g. to be weak or limited to the bloom timing. The Chl- Huthnance et al., 2001). The total forcing velocity a variance at that latitude is shown in Fig. 5d and is prescribed as was calculated with log-transformed data to ac- X4 count for the large seasonal variations. Variability is u ¼ u þ u cosðo t þ f Þ, generally low and decreases towards the shelf with 0 i i i i¼1 some extreme values associated with different transition zones (from west to east: ocean-bank, where u0 and ui are the velocities of the steady bank-slope and slope-shelf). The annual average and tidal flow constituents, respectively (u0 ¼ 1 1 1 Chl-a concentration over the Rockall Bank summit 0:05 cm s , uM2 ¼ 3:09 cm s , uS2 ¼ 0:97 cm s , 3 1 1 is 1mgm and constitutes a pronounced local uK1 ¼ 0:45 cm s , uO1 ¼ 0:26 cm s ), oi are the maximum exceeding the highest values observed tidal frequencies and fi are the tidal phases relative ¼ ¼ over Porcupine Bank by 30%. That indicates that to May 1, 1994 (fM2 11:0 , fS2 16:8 , ¼ ¼ even in periods of light limitation a substantial fK1 42:9 , fO1 29:2 ). Since we are focusing fraction of biological material is retained over the on the summer situation only, any seasonality of the bank summit and can contribute to the observed flow and background stratification as well as wind high Chl-a levels. Chlorophyll variability is low over forcing were not included. The model spin-up time the central bank summit and high in the adjacent was 40 days to establish a quasi-stationary state of oceanic areas. The logarithmic transformation of the three-dimensional ocean circulation before the Chl-a data generally intensifies areas where passive tracers were released into the model domain. temporal variations are strong indicating that The initial tracer release areas were defined to cover enhanced Chl-a levels over Rockall Bank are a the central summit regions of both banks inside the persistent feature rather than a periodic event. 400 m isobath in agreement with the most pro- nounced patterns of remotely sensed chlorophyll 3.3. Model simulations spring distributions (see Fig. 6b). Due to the partially isolated character of Porcupine Bank, its The experimental strategy was chosen to investi- shallow eastern boundary was limited to the 12:5 W gate the role of physical forcing for the dispersion of longitude. Tracer fields were integrated over an passive tracers representing a biological variable additional period of 180 days with a time step of such as phytoplankton. The main goal of the model 240 s using the advective/diffusive algorithm of the simulations was to identify characteristic distribu- physical model. tion patterns and to compare the temporal evolu- Fig. 6a introduces the main circulation features in tion of model tracers with observed satellite the top 500 m of the water column, averaged over chlorophyll fields above the banks. The model the first 90 days after tracer release. The horizontal experiments were designed to simulate an idealised flow field at Porcupine Bank is composed of two hydrographic situation assuming late spring/early major components: a poleward flowing boundary summer conditions when the phytoplankton bloom current along the Irish continental margin and an has fully developed. To achieve this, the model was anticyclonic recirculation around the bank summit. initialised using a mean density distribution taken The mean velocity of the shelf edge current west of from in situ measurements carried out in the oceanic Porcupine Bank is 10 cm s 1 and is a realistic far field west of Porcupine Bank in the periods May representation of observed summer conditions to June 1994 and 1995 (White et al., 1998; Mohn et (Pingree et al., 1999; Huthnance et al., 2001). The al., 2002). A combination of steady and tidal forcing recirculation over the bank is controlled by a was applied to generate a realistic representation of combination of Taylor cap and tidally rectified the major components of the flow field at this flow. As a consequence, the flow is accelerated on particular time of the year. Tidal amplitudes and the western side generating an asymmetric recircu- phases for the four major semidiurnal and diurnal lation cell with a weaker return flow along the 1 barotropic tidal constituents (M2, S2, K1, O1) were eastern flank of 5cms . At Rockall Bank, an taken from the inverse global tidal model TPXO.5.1 impinging flow of comparable magnitude is missing (Egbert et al., 1994). The steady background flow and tidal rectification is the dominant process was defined to generate a shelf edge current of mean generating a symmetric residual recirculation velocity of 10 cm s 1, a realistic summer value for centred above the 500 m isobath with maximum ARTICLE IN PRESS 1884 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1040 km 1040 km
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Fig. 6. (a) Modelled flow field averaged over the upper 500 m of the water column. (b) Initial particle release areas for model tracer simulations. Only part of the model domain is shown. speeds of 20 cm s 1. Observational evidence for (Fig. 7a). Secondly, particle trapping in the bottom such a flow pattern is given by e.g. Dooley (1984) layer at summit depths (Fig. 7c) is more effective (up who reported a predominantly along-isobath flow to 80% of the tracers are retained over the summit) along the northern and eastern Rockall Bank of as a consequence of the bottom-intensified recircu- 10220 cm s 1. lation in the weaker stratified deeper layers. Further model integration shows that the decrease of tracer 3.3.1. Passive tracer distributions concentrations over Porcupine Bank is a continuous To investigate the response of passive tracer process. After 180 days, tracers are reduced to movement to the physical forcing, monthly averages summit values of 10% and 35% of their initial of modelled tracer fields in the surface and bottom values in the surface and bottom layer, respectively layers were analysed after an integration period of (Figs. 7b and d). At Rockall Bank, however, 90 and 180 days, respectively. The values represent physical conditions appear to support a consider- the remaining percentage of tracers relative to their ably stronger particle retention over the summit. initial value of 1 (0 outside the initial release area) at More than 60% of the initial concentrations are still each subregion. The resulting tracer distributions at trapped in the surface and bottom layers after the Porcupine Bank show two important results. First, first 90 days of model integration (Figs. 7a and c). most of the tracers are advected away downstream There is some indication for meso-scale surface from the release area due to the influence of the shelf variability along the outer rim of the bank whereby edge current. By this process, surface concentrations surface tracers are lost to the surroundings. But this over the bank are reduced to an average of 60% process is weak compared to the strong advective of its initial value over most of summit area after 90 character of the boundary current at Porcupine days, with the exception of a local maximum Bank. After 180 days surface and bottom layer developing further upstream of the central summit values only drop slightly to values of 50% in ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1885 920 km
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Fig. 7. 30 days averages of passive model tracer distributions in the surface layer, (a) 90 days and (b) 180 days after tracer release. Corresponding distributions in the bottom layer are shown in (c) and (d). average. The model results assume a stable physical chlorophyll patterns and indicate that particle environment without significant changes through trapping over Porcupine Bank occurs but is a wind and/or seasonality of the flow for the period temporary event, whereas the tidally dominated under consideration. Despite this idealised setting, forcing conditions over Rockall Bank are able to the modelled surface tracer distributions are in maintain a sustainable tracer reservoir over many qualitative agreement with the observed surface months. ARTICLE IN PRESS 1886 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892
3.3.2. Passive particle trajectories concentrated over the summit area over the full Lagrangian particle tracking methods have been simulation period, with only a minor fraction proven very useful in many studies at isolated leaving the initial release area (Fig. 8b). At Rockall seamounts (e.g. Goldner and Chapman, 1997) and Bank, residence times of surface and bottom floats submarine banks (e.g. Bartsch and Coombs, 2000). are strongly enhanced with particles systematically They are able to provide a more detailed picture of propagating around the central summit area the response of particle movement to the flow (Figs. 8c and d). However, surface conditions conditions, e.g. identifying up and downwelling appear more variable and part of the particles areas as well as following the lateral and vertical experience a southward translation over the course drift of individual particles through the flow field. of the simulation period, but are still located within Particle positions are determined at each model time the outer rim of the bank inside the 500 m isobath step through the updated velocity fields. Subgrids- after 90 days. cale mixing processes, such as turbulent diffusion, To get information about vertical particle dis- are not resolved explicitly, which may lead to an placements along the float tracks, we calculated the underestimation of the integrated particle disper- depth of each particle dp relative to the depth at its sion. Some model studies introduced a stochastic initial position as approach using random diffusion methods (e.g. d ðx; y; tÞ¼z ðx; y; tÞ z ðx; y; 0Þ, Visser, 1997; Proehl et al., 2005) to overcome p p p this problem, others considered the deterministic where dp is the relative vertical displacement for method to be sufficient where a fine grid resolution each particle p and zp is the in situ particle depth at is used (e.g. Vikebœ et al., 2005). In our study, position x, y and time t. dp was calculated for all we decided in favour of the simpler approach bottom floats inside the initial release areas (see Fig. considering the time-dependent velocity fields as 6b) and the resulting particle displacements along the dominant contributor to particle movement in the daily float trajectories are shown in Figs. 9a and the area. b). Positive values indicate net upward and negative Two ensembles of 262 (Porcupine Bank) and 304 values net downward drift of particles. At Porcu- (Rockall Bank) passive Lagrangian floats were pine Bank (Fig. 9a), upwelling mainly occurs over released in the surface and bottom layers of the the bank summit at shallow water depths with model domain after a spin-up time of 40 days. The additional upwelling regions along its north–south floats were evenly distributed around the central axis towards the southern rim where particles bank areas and recorded every 4 h over a period of propagate across the bathymetry. Typical scales 90 days (see Fig. 6b for the location of the release range from 50 to 150 m. Areas where floats sink areas). The resulting particle trajectories and below their initial value are located along the particle depths are summarised in Figs. 8 and 9, southeastern rim of the bank and within an respectively. From all available particles, daily extended area along the continental margin. Down- trajectories (thus omitting the tidal cycle) are shown ward particle displacements vary between 100 m in for those particles released over the central summit the northwestern Porcupine Seabight and higher areas inside the 200 m isobath. The particle trajec- values in excess of 200 m along the continental tories are color-coded according to the time after margin. At Rockall Bank (Fig. 9b), conditions are their release. Most of the surface floats initially qualitatively similar, but less intense: upward drift released over the Porcupine Bank are swept off the of bottom particles is in the range of 50–150 m and central summit area after 30 days and fall into two confined to the shallower summit areas, whereas main groups thereafter (Fig. 8a). A large fraction of downward drift primarily occurs along certain the particles is moved downstream with the shelf regions along the southern and western rim, but edge current leading to isolated regions of passive does not exceed 100 m. tracer accumulation along the continental margin, To estimate the tidal contribution to passive as seen in the Eulerian fields. A smaller group of particle transport, depths and trajectories of in- particles stays above the bank summit for an dividual bottom floats were analysed for all 4 hourly extended period, partly recirculating around the records over a period of 60 days (Fig. 10). The summit, partly carried upstream and retained over primary response to tidal forcing is an oscillating the deeper bank regions south of the summit. In particle motion, both horizontally and vertically, contrast, bottom floats are less variable and mainly superimposed on an anticyclonic residual drift ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1887 640 km 640 km
0 30 0 30 30 60 30 60 60 90 60 90
520 km 520 km 440 km 440 km
0 30 0 30 30 60 30 60 60 90 60 90
440 km 440 km
Fig. 8. Passive particle trajectories of Lagrangian float ensembles released inside the 200 m isobath in the (a) surface and (b) bottom layer over Porcupine Bank and (c) surface and (d) bottom layer over Rockall Bank, respectively. Color coding represents time after float release in days. around the banks. This effect was previously studied tidally induced vertical particle displacements over based on model simulations at idealised and realistic Rockall Bank can reach up to 150 m in the summit isolated seamounts (e.g. Goldner and Chapman, area and 100 m along the outer rim (Figs. 10c and 1997; Beckmann and Mohn, 2002). It is considered d). These values are in the same order of magnitude to have strong implications for biological particle as the residual displacements in some bank regions dynamics, but is insufficiently described by the and therefore are an important factor for the Eulerian approach. However, the overall effect of vertical scattering of passive particles in near- tidal forcing for passive particle drift seems to vary bottom layers. Horizontal tidal particle oscillations strongly between the two banks. At Porcupine Bank have typical scales of 10 km (outer rim) and up to (Figs. 10a and b), vertical tidal excursions are in the 20 km (summit), respectively. order of 20 m in the shallow summit areas and do The model results confirm that passive tracer not exceed 50 m along the drift path of a particle retention in tidally dominated systems, like Rockall released further south. Horizontal displacements are Bank, is more effective compared to systems where 4 km over the summit, but only 2 km along the steady background currents are the major source of outer shelf edge over one tidal cycle. In contrast, physical forcing. However, strong tidal activity as ARTICLE IN PRESS 1888 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892
200 100 0 440 km 640 km -100 -200
520 km 440 km
Fig. 9. Relative along-track particle depths (m) for Lagrangian floats released inside the 400 m isobath at (a) Porcupine Bank and (b) Rockall Bank.
Fig. 10. Relative along-track particle depths (m) and particle paths (plan view) for individual Lagrangian floats at Porcupine Bank (a and b) and Rockall Bank (c and d). ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1889 predicted for Rockall Bank, generates additional over Porcupine Bank are mainly determined by a levels of variability to the particle motion which combination of Taylor cap dynamics and rectifica- might be able to move particles from tidally active tion of the K1 diurnal tidal constituent. The relative to less active bank areas with a strong potential for influence of each forcing component is expected to particle redistribution within one tidal cycle. vary seasonally with the strength of the poleward flowing shelf edge current impinging on the bank. In 4. Discussion summer, this current is weak (Pingree et al., 1999; Huthnance et al., 2001) and tidal rectification 4.1. Climatology dominates the circulation over the bank (Mohn and Beckmann, 2002), whereas in winter the shelf In the present study we analysed 7 years of edge current strengthens considerably and Taylor remotely sensed data to investigate spatio-temporal cap dynamics become more influential. At Rockall distribution patterns of SST and chlorophyll in the Bank, however, a steady impinging flow of Porcupine and Rockall Bank areas. Monthly and critical strength is absent and tidal rectification is seasonal climatologies were calculated and used as the dominant force. This leads to a robust reference fields for passive tracer simulations with a recirculation along the flanks of the bank. Observa- three-dimensional ocean circulation model em- tional evidence, however, is sparse, but confirma- ployed to the study area. In summary, climatologi- tive: early current meter measurements by cal SST patterns and their temporal evolution are Huthnance (1974) showed a strong, up to 20-fold relatively consistent. An isolated dome of cold amplification of the K1 tidal constituent at Rockall water, most intense in winter and spring are a Bank compared to typical values in oceanic regions pronounced feature over both banks. Its surface off the bank, recently supported by White (pers. signature is attenuated in summer with the forma- comm.). In addition, Dooley (1984) reported a tion of the seasonal thermocline. This process persistent clockwise recirculation around the north- appears to be less intense in the annual climatolo- ern tip of the bank in the order of 10220 cm s 1 gical cycle over Rockall Bank. Systematic observa- based on current meter observations during the late tions at Porcupine Bank in 1994 (Kloppmann et al., 1970s and early 1980s. 2001) and 1995 (White et al., 1998; Mohn et al., 2002) as well as recent modelling studies by Lynch 4.2. Particle trapping et al. (2004) confirm that the cold dome penetrates to the surface layer in periods of weak stratification, Previous studies on the potential of particle but is trapped beneath the seasonal thermocline in trapping at idealised, isolated seamounts have sea surface warming periods. shown that tidally forced systems are able to retain In contrast, substantial differences can be ob- biological material more effectively than steady served in the Chl-a dispersion and retention over the forced systems where most of the particles are swept banks: while elevated Chl-a levels over Porcupine away downstream from the seamount after several Bank are restricted to the bloom period from spring advective timescales and only temporary trapping to summer, they are a pronounced and persistent occurs (Chapman and Haidvogel, 1992; Goldner feature over Rockall Bank for at least 6 months and Chapman, 1997). Using the normalised mod- from April to September. Even in winter months, elled tracer fields, we calculated residence times chlorophyll concentrations over Rockall Bank (expressed as the e-folding timescale) for all appear to be higher than in surrounding waters of particles released over the Porcupine and Rockall the Rockall Trough and the Hatton-Rockall Basin. Bank summits. Residence times are shown for the However, Chl-a data coverage in December and surface and bottom layer of the model domain and January is sparse in the region and the winter values represent spatial averages over the initial release presented are mostly derived from linear interpola- area inside the 400 m isobath (Fig. 11). The model tion of the 7-year time series at every grid point. simulations predict a surface (bottom) residence This phenomenon is confirmed by the model time of 70 (90) days for Porcupine Bank simulations and can be explained by the different (Fig. 11a) and b150 ð 150Þ days for Rockall Bank physical mechanisms at work. The cold dome is (Fig. 11b). Caution is advised in interpreting these associated with a clockwise quasi-stationary recir- values, however, as they are based on simulations culation cell. The properties of the recirculation assuming an unchanged physical environment for a ARTICLE IN PRESS 1890 C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892
1 1 surface surface bottom bottom 0.8 0.8
0.6 0.6
0.4 0.4 1/e 1/e
model particle concentration 0.2 0.2
0 50 100 150 0 50 100 150 time after tracer release [d] time after tracer release [d]
Fig. 11. Modelled surface and bottom particle residence times over (a) Porcupine and (b) Rockall Bank. Tracers were initialised after a spin-up time of 40 days and spatially averaged over each release area.
typical late spring/early summer situation, i.e. Scotland, Burrows and Thorpe (1999) have found a 3 2 1 seasonal variations of stratification and flow condi- value of Kh 4 10 m s , similar to that of 2 tions as well as wind forcing are not considered. 103 m2 s 1 found by Booth (1988) for the Rockall Strong wind events are a common phenomenon in Trough. Taking the areas for the banks (assuming the Northeast Atlantic and storm induced wind an elliptical shape encompassed by the 400 m 10 2 mixing can lead to substantial modifications of the isobath) as APB ¼ 2 10 m and ARB ¼ 5 10 2 bio-physical regime (Beckmann and Mohn, 2002; 10 m , the retention timescales are T PB ¼ 58 days Kloppmann et al., 2001). However, this process and TRB ¼ 145 days, respectively. These are likely mainly acts on a relatively short timescale which we to be upper bounds, but are comparable to the were not able to resolve with the database used for residence times predicted by the model simulations. this study. A reduction of the Porcupine Bank particle The longer retention time of passive particles over residence times is likely due to the advective nature Rockall Bank compared to Porcupine Bank may be of the slope current and its strong seasonality with a reconciled in two ways. Firstly, the larger size of major flow intensification in winter. The enhanced Rockall Bank relative to Porcupine, and secondly, and persistent chlorophyll levels over Rockall Bank the substantial impact of the slope current at may also be a consequence of stronger winter Porcupine Bank, helping to strip away particles convection in the Rockall area when nutrient-rich from the summit. A first order estimate of the water masses are made available to the deeper retention timescale based on drifter experiments by summit areas (White et al., 2005). The modelled Burrows and Thorpe (1999) and Booth (1988) can tracer fields further indicate a substantial downslope be derived from a comparison of the area of the tracer flux generated through local benthic bound- banks to the horizontal turbulent diffusivity ðKhÞ.A ary layer dynamics (see Figs. 7b and d). This process timescale for the horizontal diffusion of particles appears to be more pronounced at Rockall Bank away from the bank can be formulated by equating resulting in reduced particle residence times in the the diffusivity with the area of the bank over time. near-summit bottom layer (Fig. 11b). The resulting relationship for the retention timescale T may be written as 4.3. Implications for benthic ecology T ¼ A=K , h Submerged banks have long been recognised as where A is the area of the bank and Kh the important hot spots of bio-physical interactions, horizontal diffusion coefficient. Based on analysis of especially for fisheries oceanography (e.g. Klopp- drifter experiments in the slope current west of mann et al., 2001; Werner et al., 1993). At a smaller ARTICLE IN PRESS C. Mohn, M. White / Continental Shelf Research 27 (2007) 1875–1892 1891 spatial scale, bio-physical interactions at seamounts GOCE-CT-2005-511234, funded by the EC’s have effect in the chlorophyll and benthic dynamics Sixth Framework Programme under the priority (e.g. Dower et al., 1992; Comeau et al., 1995). ‘Sustainable Development, Global Change and A long residence time of primary production over Ecosystems’. The AVHRR Oceans Pathfinder topographic features is important for any sessile SST data were obtained through the online benthic community resident over the topography. PO.DAAC Ocean ESIP Tool (POET) at the This is because either sufficient time is needed for Physical Oceanography Distributed Active Archive the transfer of the surface productivity to higher Center (PO.DAAC), NASA Jet Propulsion Labora- trophic levels, or that organic material is required to tory, Pasadena, CA, USA. The chlorophyll data be transported directly to the benthic communities, used in this study were acquired using the GES- before it is lost to the system. At the Rockall and DISC Interactive Online Visualization ANd aNaly- Porcupine Bank margins, one important benthic sis Infrastructure (Giovanni) as part of the NASA’s ecosystem is that of the cold-water corals (Wilson, Goddard Earth Sciences (GES) Data and Informa- 1979; Roberts et al., 2003). These azooxanthellate tion Services Center (DISC). The authors wish to scleractinian corals are generally found in regions thank Aike Beckmann, Stephanie McDonagh and with high surface productivity and in regions of Jenny Ullgren for helpful comments and discussions. dynamic benthic currents. The food source for these The comments and suggestions of two anonymous corals is unclear, but it appears from stable isotope reviewers are gratefully acknowledged. analysis that a mixed zooplankton and phytoplank- ton/detrital diet is most likely (Duineveld et al., 2004). The prevalence of carbonate mounds at the References flanks of these banks (e.g. Kenyon et al., 2003) suggests that a high particle flux from the surface Bartsch, J., Coombs, S., 2000. An individual-based model of the waters over the banks to the flanks must be present. early life-history stages of mackerel (scomber scombrus)in the eastern North Atlantic embedded within a three-dimen- The persistent high chlorophyll levels found over sional physical transport model. Fisheries Oceanography 10, Rockall Bank would appear to be a significant 1–22. source of organic material for the corals. Kiriakou- Beckmann, A., 1999. Dynamical processes at isolated seamounts, lakis et al. (2004) have shown that fresh organic Habil. Thesis, Carl von Ossietzky - Universita¨ t Oldenburg, material is present at the coral-topped Darwin Fachbereich Physik. Beckmann, A., Haidvogel, D.B., 1993. Numerical simulation of mounds in the northern Rockall Trough at 1000 m flow around a tall isolated seamount. Part I: problem depth, indicating that high quality food can reach formulation and model accuracy. Journal of Physical the coral community in significant quantities. White Oceanography 23 (8), 1736–1753. et al. (2005) have suggested that these communities Beckmann, A., Mohn, C., 2002. The upper ocean circulation are a favourable place in relation to the submerged at Great Meteor Seamount. Part II: retention potential of the seamount-induced circulation. 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