The Atlantic Meridional Transect: overview and synthesis of data

Progress in Oceanography 45 (2000) 257–312

Atlantic Meridional Transect (AMT) Programme

The Atlantic Meridional Transect: overview and synthesis of data J. Aiken a,*, N. Rees a, S. Hooker b, P. Holligan c, A. Bale a, D. Robins a, G. Moore a, R. Harris a, D. Pilgrim d a

CCMS Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, PL1 3DH, UK b NASA, GSFC, Greenbelt, MD, USA c Southampton Oceanography Centre, European Way, Southampton, UK d University of Plymouth, Drakes Circus, Plymouth, PL4 8AA, UK

Abstract The Atlantic Meridional Transect programme uses the twice-annual passage of the RRS James Clark Ross between the UK and the Falkland Islands, before and after the Antarctic research programme in the Austral Summer (see Aiken, J., & Bale, A. J. (2000). An introduction to the Atlantic Meridional Transect (AMT) Programme. Progress in Oceanography, this issue). This paper examines the scientific rationale for a spatially-extensive time and space series programme and reviews the relevant physical and biological oceanography of the Atlantic Ocean. The main scientific observations from the research programme are reported. These are set in the context of historical and contemporary observations pertinent to the principal objectives of the cruise, notably the satellite remotely sensed observations of ocean properties. The extent to which the programme goals have been realised by the research to date is assessed and discussed. New bio-optical signatures, which can be related to productivity parameters, have been derived. These can be used to interpret remotely sensed observations of ocean colour in terms of productivity and production processes such as the air/sea exchange of biogenic gases, which relate to the issues of climate change and the sustainability of marine ecosystems.  2000 Elsevier Science Ltd. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

* Corresponding author. E-mail address: [email protected] (J. Aiken). 0079-6611/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 0 ) 0 0 0 0 5 - 7

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2. Oceanographic review . . . . . . . . . . . . . . . . 2.1. Topography . . . . . . . . . . . . . . . . . . . . 2.2. Circulation . . . . . . . . . . . . . . . . . . . . . 2.3. Water masses . . . . . . . . . . . . . . . . . . . 2.4. Water mass analysis . . . . . . . . . . . . . . . 2.5. Near-surface water structures . . . . . . . . . . 2.6. T and S structure to 200 m — CTD sections 3.

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260 261 262 264 265 267 271

Remote sensing of physical structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

4. Coupled physical-biological structures . . . . . . . . . . . . . . . . . . . . . . . . . . 283 4.1. Undulating oceanographic recorder (UOR) sections . . . . . . . . . . . . . . . . 284 5.

Remote sensing of biological structures . . . . . . . . . . . . . . . . . . . . . . . . . 286

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Phytoplankton pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

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Primary productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

8. Ocean optics and validation of SeaWiFS . . . . . . . . . . . . . . . . . . 8.1. Comparison of Klu vs. Ked . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Comparison of Ked(ret) vs. Ked(meas.) . . . . . . . . . . . . . . . . . 8.3. Comparison of Chl(ret.) vs. Chl(meas), using a standard algorithm 9.

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301 303 303 303

Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

1. Introduction The oceans have a fundamental role in determining global climate because of their capacity to absorb and emit radiatively and chemically active trace gases (CO2, CH4, N2O, CO, S–compounds, halocarbons) and to store and to transport heat and organic matter; these processes directly and indirectly affect regional ecosystems and the Earth’s radiation budget. The oceans contain approximately 85% of the carbon circulating in the earth’s biosphere and provide the main long term control of atmospheric CO2 and the strength of the natural ‘greenhouse effect’. The ocean biota, particularly phytoplankton, contribute significantly in all these processes. Photosynthetic carbon fixation, leading to the transport of carbon to the deep oceans and sediments (the ‘biological pump’) increases the capacity of the oceans for CO2 (Moore & Bolin, 1987; Holligan, 1992). Dimethylsulfide (DMS) derived from phytoplankton is believed be the major source of cloud condensation nuclei in the marine atmosphere (Charlson, Lovelock, Andreae & Warren, 1987). The absorption and back-scattering

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of biogenic particles (Holligan & Balch, 1991) affect the oceanic albedo (surface reflectivity) and the rate of heating of surface waters (Morel & Antoine, 1994). All these processes affect the control of the temperature of the Earth’s surface. Good progress and improved understanding of these issues has been made through international programmes co-ordinated by the IGBP: e.g. Joint Global Ocean Flux Experiment (JGOFS) and the UK NERC projects Bio-geochemical Ocean Flux Study (BOFS) and Plankton Reactivity In the Marine Environment (PRIME); the 1989 North Atlantic Bloom Experiment (Watson, Robinson, Robertson, Williams & Fasham, 1991; Ducklow & Harris, 1993); Equatorial Pacific (Barber, Murray & McCarthy, 1994); Southern Ocean (Turner, Owens & Priddle, 1995); Arabian Sea (SCOR, 1990); LOICZ and WOCE. Extrapolation of studies and models to basinscales to assess global carbon budgets is timely. Progress is limited by the paucity of information on the variability in biological provinces and on the fate of atmospheric CO2 and the implications for oceanic productivity. Are there observable trends in production, ‘new production’, carbon export and recycling? (c.f. Venrick, McGowan, Cayan & Hayward, 1987; Falkowski & Wilson, 1992). Are there qualitative changes (species, taxa, calcareous versus non-calcareous) which influence the global carbon system with implications for climate change? Can we quantify the contribution of the oceanic ‘biological pump’ to the control of the natural and the anthropogenically-forced ‘greenhouse effect’? These are key issues for JGOFS (1991) and GLOBEC (1993) which need to merge oceanographic measurements with remotely sensed data to meet their objectives of characterising oceanic productivity and biologically-mediated carbon fluxes in a wide range of ecosystems. The AMT programme builds on the process-orientated research carried out by JGOFS, BOFS and PRIME with spatially-extensive measurements covering ocean basin scales. The AMT cruise track crosses major boundaries between shelf seas and the pristine zones of the oligotrophic central gyres, which provide baseline reference stations. The strong gradients at the shelf break provide a sharp contrast in the indices of pollution and biodiversity. Recent technological developments, which have provided instruments for improved data acquisition of key biological variables over ocean basin scales, have been exploited in the AMT programme. For example, autonomous pCO2 equipment (Cooper, Watson & Ling, 1998), and the Fast Repetition Rate Fluorometer (FRRF, Kolber & Falkowski 1992, 1993) for measuring photosynthetic parameters rapidly and non-destructively, have been used. When used with the Undulating Oceanographic Recorder (UOR, Aiken & Bellan, 1990), as in the IronExII experiment in the Pacific in May 1995 (Coale, Fitzwater, Gordon, Johnson & Barber, 1996; Behrenfeld, Bale, Kolber, Aiken & Falkowski, 1996) and in the AMT programme, the FRRF provides enhanced data acquisition of photosynthetic parameters which can increase our understanding of pelagic primary production. The new technologies for the measurements of zooplankton using Acoustic Doppler Current Profilers (ADCP) and the Optical Plankton Counter (OPC, Herman, 1992) have been exploited effectively. Satellite remote sensing of water colour provides synoptic, large area, spatiallyresolved observations of ocean biology (colour) on a daily basis, subject to cloud-free conditions. The AMT programme has set out to develop a synergistic combination of

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autonomous shipboard, towed measurements and satellite remotely sensed observations, as a solution to the under-sampling problem, by extrapolating measurements from research ships to basin-scales (Platt & Sathyendranath, 1988). Data from new satellite sensors, for example, OCTS (November 1996 to June 1997), SeaWiFS (September 1997–present), MODIS (launched December 1999) and MERIS (due 2001) can play an important role in defining the present state of the oceans, as a basis for evaluating the role of biology in models of the climate system and for detecting biological feedback responses to climate change. Spatially-extensive biooptical oceanography on AMT cruises has provided data to develop and validate new bio-optical models, validate and give scientific credibility to remotely sensed imagery of oceanic biology. This is an integral part of the holistic strategy, linking oceanography, remote sensing and modelling, in particular the relationships between bio-optical signatures and production processes. It has been an inherent goal of the AMT programme to examine the hypothesis of biogeochemical provinces and determine their characteristic properties. Traditionally oceanographers have partitioned the oceans on the basis of physical and biological characteristics: for the former, topography, geostrophic flows, wind driven circulation, gyres, fronts, upwelling zones and patterns of seasonal stratification; for the latter, biological productivity, as well as phytoplankton and zooplankton assemblages and community structure. Taken together, this biophysical partitioning provides the descriptors of regional ecosystems or biogeochemical provinces, each with discrete boundaries and each having distinct flora and fauna. The concept of biogeochemical provinces has been promoted by Longhurst, Sathyendranath, Platt and Caverhill (1995) as a means of evaluating basin-scale productivity from remotely sensed measurements of ocean colour, making use of province-specific physical and biological parameterisations derived from climatological values of the key variables. Considerable advance towards the validation of this concept has been achieved by the development of a method for objective analysis of physical provinces using the first and second derivatives of surface density to determine province boundaries (Hooker, Rees & Aiken, 2000).

2. Oceanographic review Since most CTD casts have been to 200 m only, physical oceanography within the AMT programme is largely superficial. It has little relevance to long term trends or shifts of ocean basin circulation and with no geostrophic reference, cannot meet the objectives of WOCE. However, the circulation and water masses of the upper 200 m impact on the biological production and help define bio-geochemical provinces and their characteristic properties. The oceanographic physical properties relevant to primary production and provinces (topography, circulation, water masses and vertical physical structure) are reviewed in this section. The classical studies of water masses are used extensively in the following reviews, notably the works of Wright and Worthington (1970) and Emery and Meincke (1986). Notable reviews of circulation are by Peterson and Stramma (1991), Tomczak and Godfrey (1994) and Reid (1994).

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2.1. Topography The standard cruise track between the UK and the Falkland Islands is shown in Fig. 1a. The Mid-Atlantic Ridge (MAR) runs the length of the North and South Atlantic, from Iceland in the north to Tristan da Cunha and Bouvet Island in the south (Fig. 1b). It divides the Atlantic into two elongated depressions, the Eastern Atlantic trough and the Western Atlantic trough. Depths exceeding 5000 m are found on either side of the ridge. The depressions on either side of the ridge are further divided into basins by transverse ridges. The AMT track spans the UK continental margin, the Porcupine and Iberian Plain, the Canary Basins and the Cape Verde plateau in the North East Atlantic, the MAR, the Brazil basin, Argentine Basin and the South American Continental Margin in the South West Atlantic. The along-track bottom topography corresponding to the AMT–2 cruise track, extracted from the US National geophysical gridded database, is shown in Fig. 2. It is evident that the cruise track is largely across ocean waters with depths of greater than 3000 m, with the notable exception of the UK continental margin (50–48°N) and South American Shelf (34–38°S and 48–50°S). Topographic features do not force changes observed in the upper 200 m, although a localised decrease in density is seen at the seamount at 20°S (Hooker et al., 2000).

Fig. 1. (a) The AMT–2 cruise track with the major surface circulation patterns adapted from Tomczak and Godfrey (1994). (b) The bathymetic chart of the Atlantic with 100 and 200 m contours showing the location of the Mid Atlantic Ridge.

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Fig. 2. The along-track bottom topography, based on the AMT–2 cruise track, extracted from the US National geophysical gridded database.

2.2. Circulation The major surface circulation patterns in the Atlantic (adapted from Tomczak & Godfrey, 1994) are shown superimposed on Fig. 1. Table 1 lists the abbreviations used for the circulation features and water masses used in this review section. Within the north–east Atlantic, the relatively warm and saline waters of the Gulf Stream (GS) enter the region in the form of two currents, the North Atlantic Current (NAC) and the Azores Current (AC). The NAC enters the region from the west and south– west of 53°N, and the AC enters from between 35°N and 36°N. The AC is a component of the northern subtropical gyre, which in the north–east Atlantic consists of the Portugal Current (PC) and Canary Current (CC) and the North Equatorial Current (NEC), with its centre located at 15°N. The southern subtropical gyre is made up of the South Equatorial Current (SEC) which extends across the equator, the Brazil Current (BC), the South Atlantic Current (SAC), and the Benguela Current (BenC). The equatorial current system has a banded structure and dominates the surface flow and hydrography of the region. The NEC is a region of broad and uniform westward flow, north of 10°N. The easterly flowing North Equatorial Counter Current (NECC), is highly seasonal, nearly disappearing in February when the trade winds in the northern hemisphere are strongest. During the Boreal Fall it is at its strongest and manifests itself between 9°N and 3°N as a band of low-density water to depths of up to 100 m. The SEC is a broad and uniform westward flow, which extends from circa 3°N to at least 15°S. The strongest equatorial current is the westward flowing Equatorial Undercurrent

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Table 1 The abbreviations used for the circulation features and water masses in the review section AC AW BenC BC CC CPC ENAW ENAWt ESW FC GS MAR NAC NAG NEC NECC PC SAC SACZ SACW SAF SAG SASW SEC STF WNAW

Azores Current Amazon Water Benguela Current Brazil Current Canary Current Circumpolar Current Eastern North Atlantic Water Eastern North Atlantic Water Tropical Equatorial Surface Water Falklands Current Gulf Stream Mid Atlantic Ridge North Atlantic Current North Atlantic Gyre North Equatorial Current North Equatorial Counter Current Portugal Current South Atlantic Current Sub Antarctic Convergence Zone South Atlantic Central Water Sub Antarctic Front South Atlantic Gyre Sub Antarctic Surface Water South Equatorial Current Subtropical Front Western North Atlantic Water

centred on the equator at a depth of 100 m. North and south of the equator at a depth of 200 m are the North Equatorial and South Equatorial Undercurrents respectively, which are swift, narrow features. The North Equatorial Counter Current is prevented from flowing north by the east–west orientation of the African coastline, except for a small portion which combines with the North Equatorial Undercurrent to drive a cyclonic gyre centred at 10°N, 22°W. The doming of the thermocline in the summer results in this gyre being known as the Guinea Dome. The associated circulation reaches to depths of 150 m and persists throughout the year, although it is weaker in winter. The westward flowing SEC is multi-banded and has a velocity maximum just north and south of the equator. The Equatorial Undercurrent causes a relative velocity minimum at the equator. The weak South Equatorial Counter Current is found between 7°S and 9°S. To the south of this, there is a distinct branch of the SEC which originates from the Benguela Current (BenC); this branch bifurcates at circa 10°S–12°S with the main portion feeding northward into the North Brazil Current and the remainder into the southerly flowing Brazil current (BC). The BC is relatively shallow (200 m) and flows southward along the S. American shelf and is the western boundary current of the South Atlantic subtropical gyre (Peterson & Stramma, 1991).

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Between 33°S and 38°S the BC separates from the S. America shelf forming the Sub-Antarctic Front (SAF) with the cold water of the Falkland Current (FC). The FC is a jet-like northward looping excursion of the Circumpolar Current (CPC). The southernmost extent of the warm Brazil current, after separation from the shelf, is between 38°S and 46°S and is linked to the formation of eddies in the region (Willson & Rees, 2000). 2.3. Water masses Western North Atlantic Water (WNAW) dominates the North Atlantic and is found in the North American Basin, the North African Basin, the southern Labrador and European Basins, and in the higher latitude portions of the northern equatorial areas (North Atlantic basins as defined by Wright & Worthington, 1970). WNAW is carried northward from the Sargasso and Caribbean Seas by the Gulf Stream and across the North Atlantic in the NAC and AC. The Mid Atlantic Ridge (MAR), centred at approximately 31°W, limits the extent of WNAW. The combination of Subpolar Water, formed in the northeastern cyclonic gyre (McCartney & Talley, 1982), and WNAW produces Eastern North Atlantic Water (ENAW) which is believed to form in the eastern North Atlantic at the surface in winter. A variant of ENAW of subtropical origin, ENAWt, is formed on the northern margins of the AC (Rios, Perez & Fraga, 1992). ENAWt probably results from the mixing of sub-tropical water and remnants of Antarctic Intermediate Water and is presumed to form southeast of the Azores (Fiuza & Halpern, 1982). South Atlantic Central Water (SACW), flows north across the equator and into the North Atlantic, mixing with ENAW. The transition between north and south Atlantic waters occurs at a front at approximately 15°N, in the western Atlantic; in the east it turns northward past 20°N, following the southern limit of the Canary Current. As the separation zone between the both waters is located some 1500 km north of the equator and the mixture of water is not returned into the current system but transported northward, there is no formation of a special equatorial water mass, as seen in the Pacific. The opposing eastward and westward flows however influence the hydrographic properties of surface SACW in the tropics (Tomczak & Godfrey, 1994). The discharge from the Amazon River creates a plume of Amazon Water (AW) which is carried eastward by the NECC into the central tropical zone. The discharge of AW is limited to a depth of approximately 100 m at about 25°C (Emery & Dewar, 1982) and salinity less than 35.0 (Muller-Karger, McClain & Richardson, 1988). AW is recorded during the Boreal Autumn AMT cruises as warm, low salinity water down to depths of 70 m, centred at 5°N. SACW is advected into the western South Atlantic region by the BC where it is characterised by temperatures higher than 10°C (up to 26°C in the surface layer) and a salinity over 35.0 and as high as 37.3 (Bianchi, Giulivi & Piola, 1993). The Subtropical Front (STF) forms the southern boundary of SACW. The BC defines a poleward extension extending southward as far as 44°S within the rather limited

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longitudes of 50–55°W, with a return to lower temperatures over the 45°W eastern limit. Sub Antarctic Surface Water (SASW) is carried northward by the FC along the continental shelf, reaching as far as 36°S at the surface. It is advected northward in the upper 500 m with a temperature lower than 10°C and a salinity lower than 34.3 (Bianchi et al., 1993). SASW is characterised by relatively high temperature and salinity at some distance below the surface, with a uniform temperature between 8– 9°C (Sverdrup, Johnson & Fleming, 1942). 2.4. Water mass analysis For the purposes of water mass identification, the vertical CTD casts represent the most comprehensive data. These data were geolocated, binned at 1 m intervals, and then analysed for water mass properties using an automated technique (Rees & Aiken, 1995). The T–S relationship of the CTD casts obtained on AMT–1 and AMT– 3, and on AMT–2 and AMT–4 are shown in Figs. 3 and 4, respectively. Fig. 5 shows the vertical distribution of water masses as a function of latitude during AMT–1 and AMT–2. The locations, and extent, of the water masses are comparable for all 4 cruises. From 20–50°N the water column is dominated by ENAW with evidence of

Fig. 3.

The T–S relationship of the CTD casts obtained on AMT–1 and AMT–3.

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Fig. 4.

The T–S relationship of the CTD casts obtained on AMT–2 and AMT–4.

ENAWt in the surface waters from 35°N, where the water is influenced by input from the AC. A decrease in salinity is shown from the surface to a depth of 100 m in the tropics and equatorial region (TER). This is caused by the high precipitation in the region. For the purposes of this discussion, it has been identified as a surface water called Equatorial Surface Water (ESW) as it has a distinct T–S relationship, different to SACW. ESW is surface SACW, which has been diluted by the excess in precipitation over evaporation, mixing in the flow of the equatorial currents and the transport of AW into the region. Low salinity at 4°N–5°N, less than 34.5 on AMT–1 and AMT–3 to a depth of 50 m, originates from AW that is transported eastwards across the Atlantic by the NECC. The region 37°S to 20°N is predominantly SACW which is a relatively stable water mass exhibiting a consistent slope in the T–S relationship. The region from 37–52°S is dominated by water advected northward from the Antarctic by the FC. The water has been categorised as SASW to a depth of 200 m. However, the effect of SACW transported into the region alters the characteristics of the water, because of the increase in temperature. This water retains the low salinity levels characteristic of Antarctic water but has a temperature profile characteristic of water from a warmer origin.

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Fig. 5. The vertical distribution of water masses as a function of latitude during AMT–1 and AMT–2.

2.5. Near-surface water structures The AMT–1 (typical of the BFAS cruises) and AMT–2 (typical of the AFBS cruises) near-surface T and S for the 50°N to 50°S transect, are shown in Fig. 6a and b, respectively. As expected, the northern hemisphere is warmest during October and the southern hemisphere is warmest during May. The inter-seasonal difference is about 2.5°C in the tropics and up to 5°C in the mid-latitudes. The large variance in the T–S measurements between 35–48°S is a feature of the extreme heterogeneity of the western boundary, in this case, as a result of the BC and FC confluence, and the plume from the River Plate. Within the ocean gyres, there is an increase in salinity from high latitudes to the subtropics, which is closely coupled with temperature, and is a manifestation of the increase in evaporation associated with higher temperatures and lower latitudes. This relationship breaks down in the tropics, from about 25°N to 15°S, because of the increase in precipitation within these latitudes. On either side of the equator, from about 15°N to 10°S, there are notable differences between AMT–1 and AMT–2 caused by seasonal forcing on the complex current systems which prevail in the area.

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Fig. 6. (a) SST from AMT–1 (solid line) and AMT–2 (dashed line), at a sampling interval of 20 km. (b): SSS from of AMT–1 (solid line) and AMT–2 (dashed line), at a sampling interval of 20 km.

Most notable is the variation in the NECC; on AMT–1 it is evident as a broad band of low salinity water below 34.5, whilst on AMT–2 the salinity reduction is less and the feature is less distinct. Figs. 7 and 8 show the near-surface T–S diagrams of AMT–1, AMT–2, AMT–3 and AMT–4, for the northern and southern hemispheres, respectively. In keeping with the circulation patterns described above, the surface data show a clear differentiation of several distinct current systems and/or physical provinces. In terms of a generalised description, the subtropical gyres have linear, nearly constant slope in T– S properties, which is consistent with a robust coupled evaporation and precipitation relationship. There are offsets between the BFAS and AFBS measurements as a result of seasonal heating and cooling in the different hemispheres. However, the consistency of these patterns is evidence that surface T–S relationships are descriptive of surface water masses and is the substantive observation, during the research on the objective analysis of physical provinces (Hooker et al., 2000). Within the TER, the banded current system of the equatorial region is evident as a series of distinct ‘steps’ in the T–S relationship. When crossed from north to south, the currents exhibit the following characteristics: the southern extent of the CC (25– 20°N) has a constant temperature and a decrease in salinity, the NEC (20–12°N) has an increase in temperature with relatively constant salinity, the NECC (9–2°N) has relatively constant temperature with a dip in salinity (circa 5°N), and the SEC (2°N to 15°S) has a small decrease in temperature coupled with an increase in salinity to a surface maximum in the southern hemisphere.

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Fig. 7. T–S relationship of surface waters for the Northern Hemisphere for AMT–1, AMT–2, AMT–3, AMT–4. Measurements in the NAG (50–25°N) are depicted by diamonds, the TER (25°N to 0°) by a star.

The South Atlantic Convergence Zone (SACZ) is notable for the heterogeneity forced by the confluence of the warm BC and the cold FC as well as the eddies created from the two current systems. The T–S relationship shows that BC water is characterised by a T of greater then 15°C and S between 35–36, while FC water is

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Fig. 8. T–S relationship of surface waters for the Southern Hemisphere for AMT–1, AMT–2, AMT–3, AMT–4. Measurements in the TER (0°–15°S) are depicted by a square, the SAG (15–35°S) by an X, and the SACZ (38–50°S) by a triangle.

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characterised by low salinity water circa 34 and a T between 5 and 10°C. The eddies created from the two current systems mixing are separated by a subset of distinct T–S relationships, notably on AMT–4 and AMT–2 and to a lesser extent on AMT–1. 2.6. T and S structure to 200 m — CTD sections One of the principle objectives of the AMT programme is the study of variations in the physical characteristics of the surface waters. By maintaining a constant CTD sampling strategy a decadal data set will be acquired which should reveal trends and variations in the basin-scale activity. The structure measured by the CTD unit to a depth of 200 m from the first four cruises (AMT–1, AMT–2, AMT–3 and AMT–4) is compared with the relevant T and S extracted from the Levitus Climatological Atlas (Levitus, 1994). The sections for each hemisphere in the relevant seasons (BFAS and AFBS) and the climatology are shown in Figs. 9–16. The Levitus climatological atlas provides a historical baseline against which the in situ data from AMT cruises can be assessed. The initial analysis has shown that the mid-latitude components of the NAG and SAG, the CC and BC respectively, are storing warmer water to a greater depth, than has been previously recorded. This can be illustrated by reference to the contoured vertical sections. In the CC between 30°N and 22°N with the Levitus climatology temperature fields show lower levels in the surface waters down to 60 m (see Figs. 9 and 11). In the climatology (October) a salinity minimum is located over the Guinea Dome at (13–8°N) Fig. 10, localised to 60 m that is more likely to be an expression of the westward flowing NECC. However, on both the AMT–1 and AMT–3 sections, this is seen as a band from 9°N to 3°N. This is supported by other measurements such as the near-surface continuous measurements and TOPEX imagery (Aiken & 18 others, 1998a). In the northern extreme of the BC, within the SAG, between 15°S and 25°S, the in situ CTD give higher temperatures in the surface layer, than the Levitus climatology. From the AMT data set, it can be seen that this region is storing warmer water to greater depth than has been recorded historically (Figs. 13 and 15).

3. Remote sensing of physical structure Many of the physical features of the AMT are shown in the satellite imagery of SST. Figs. 17 and 18 show the AVHRR weekly composites of SST compiled for AMT–2 and AMT–3. These data are zebra-contoured (Hooker, Brown & Kirwan, 1995) which helps identify the current systems and gyres of the temperate zones, north and south. The equatorial zone is poorly imaged because of the cloudy conditions which pertain and the difficulty of correcting AVHRR data for atmospheric water vapour which predominates in this region. The along track SST for AMT–2, derived from AVHRR, compares closely with the measured surface T (see Fig. 19), particularly through the temperate zones. The AVHRR sensor is lower by circa 1°C in the equatorial regions for the reasons discussed above. The AVHRR provides frequent cloud-free measurements of the Falklands Current

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Fig. 9. The along-track contoured vertical section of temperature for AMT–1 and AMT–3 CTD casts to 200 m for the Northern Hemisphere with the Levitus temperature field for the month of October. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 10. The along-track contoured vertical section of salinity for AMT–1 and AMT–3 CTD casts to 200 m for the Northern Hemisphere with the Levitus salinity field for the month of October. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 11. The along-track contoured vertical section of temperature for AMT–2 and AMT–4 CTD casts to 200 m for the Northern Hemisphere with the Levitus temperature field for the month of May. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 12. The along-track contoured vertical section of salinity for AMT–2 and AMT–4 CTD casts to 200 m for the Northern Hemisphere with the Levitus salinity field for the month of May. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 13. The along-track contoured vertical section of temperature for AMT–1 and AMT–3 CTD casts to 200 m for the Southern Hemisphere with the Levitus temperature field for the month of October. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 14. The along-track contoured vertical section of salinity for AMT–1 and AMT–3 CTD casts to 200 m for the Southern Hemisphere with the Levitus salinity field for the month of October. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 15. The along-track contoured vertical section of temperature for AMT–2 and AMT–4 CTD casts to 200 m for the Southern Hemisphere with the Levitus temperature field for the month of May. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 16. The along-track contoured vertical section of salinity for AMT–2 and AMT–4 CTD casts to 200 m for the Southern Hemisphere with the Levitus salinity field for the month of May. The locations of CTD casts are denoted by dotted line on the plots; the climatology is at 1° resolution.

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Fig. 17.

AVHRR week-by-week composite of SST for the AMT–2 cruise, April to May 1996.

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Fig. 18.

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AVHRR week-by-week composite of SST for the AMT–3 cruise, September to October 1996.

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Fig. 19.

SST for AVHRR and AMT–2 measurements 50°N to 50°S, April/May 1996.

and the S. Atlantic sub-Antarctic convergence. Fig. 20 shows data for the 26 Oct. 1995 (25–55°S, 40–70°W) during the AMT–1 cruise, with temperature fronts of 6°C in 20 km. A high resolution XBT section across the edge of a warm core ring coincident with the AVHRR data, is shown in Fig. 21. This integration of remote sensing SST data and in situ measurements provides quasi 3–D structures of complex heterogeneous zones and successive images shows how these change over short time spans. Other satellite sensors such as the TOPEX/Poseidon altimeter can show physical structure of the surface ocean, from measurements of the sea surface height (SSH). Fig. 22 shows the SSH anomaly (SSHA) from TOPEX for the 3 days up to the 4 Oct. 1997 for the whole of the Atlantic Ocean during the AMT–5 cruise. The Azores Current at circa 35°N is seen as a predominately negative SSHA, as is the North Equatorial Current at circa 10–12°N, while the North Equatorial Counter Current at circa 5°N is shown as a positive SSHA. The heterogeneous area of the sub-Antarctic convergence circa 35–50°S is shown in detail as an inset (Fig. 22a). Several ‘small’ cold water multi-poles (di-poles are the most common manifestation) with very positive SSHA, surround a large warm core water mass (very negative SSHA), detached from the warm Brazilian Current. The high resolution XBT section taken during AMT–5 through this area from 31.5–45°S (Fig. 23) shows the sharp temperature fronts which occur through these cold-core poles at 38–39°S and at 41–42°S.

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Fig. 20.

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AVHRR image of SST for the SACZ 25–55°S, 40–70°W, 26 Oct 1995 (AMT–1).

4. Coupled physical-biological structures The general features of the physical structure for the whole transect as they relate to the biological biomass and productivity are shown in Fig. 24 (T and S) and Fig. 25 (Chl and NO3) for the AMT–1 (AFBS) cruise. At the northern extreme, the water

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Fig. 21.

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High resolution XBT section (40–48°S) through the SACZ, 22–23 October 1995 (AMT–1).

column is partially or deeply mixed, from post-autumn breakdown of stratification. At the southern end, the water is partially mixed or incipiently stratified as seasonal heating warms the surface layers. In both conditions, phytoplankton biomass is concentrated at the surface as a consequence of moderately high light, adequate nutrient supplies and incipient stability of the water column. In the temperate to sub-tropical zones, stratification persists in the autumn, with a mixed layer depth of 40 to 50 m and a sub-surface phytoplankton population in the thermocline. In the permanently stratified tropical ocean both north and south of the equator, there is a deep mixed layer with very low chlorophyll concentrations (circa 0.04 to 0.05 mg m⫺3) and a deep chlorophyll maximum in the thermocline with concentrations reaching only 0.2 to 0.5 mg m⫺3. Throughout the transect, where there is a homogenous surface mixed layer, macro-nutrients (NO3) are below the levels that can be measured by colourimetry (0.2 µM). On AMT–5 and AMT–6, nano-level nitrate concentrations were measured by chemiluminescent methods, (Aiken et al., 1998a) 4.1. Undulating oceanographic recorder (UOR) sections Many detailed features of the vertical structure through the surface mixed layer and seasonal thermocline are shown in the UOR vertical sections of T, S and Chl from AMT–1; the locations of the UOR tows are shown in Fig. 26a. Fig. 26 (26– 20°N, composite of SDY 276 and 277, 1995) shows the upwelling cold, low salinity water with high surface chlorophyll concentrations (⬎1 mg m⫺3), at the fringes of the west African coastal upwelling. Fig. 27 (UOR sections 13–8°N, composite of SDY 278 and 279, 1995) shows the shallow mixed layer (20 m), the doming of the thermocline and the low salinity surface water over the Guinea Dome. With enhanced nutrient supplies from this mid-ocean upwelling, chlorophyll is concentrated in the thermocline at 40–50 m at moderate levels of 0.6 to 1.0 mg m⫺3. Fig. 28 (UOR

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Fig. 22. TOPEX image of SSH anomaly for North and South Atlantic for AMT–5 cruise September/October 1997; (a) insert is the SSH anomaly for the South Atlantic Convergence Zone (30– 55°S, 40–70°W), October 1997.

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Fig. 23.

High density XBT section 31.5–45°S through the SACZ, 11–15 October 1997.

sections 6.5°N–0°, composite of SDY 280 and 281, 1995) shows the deeper mixed layer (⬎50 m), the doubly stratified, low salinity surface layers in the NECC, and the rising depth and increasing concentration of the sub-surface chlorophyll maximum (0.6 mg m⫺3), approaching the equatorial upwelling. The characteristics of the equatorial front are shown in Fig. 29 (UOR tow A313, AMT–3, from 1.3°N– 1.8°S, SDY 279 and 280, 1996). Note the sharp drop in surface temperature (0.8°C) and the rise in salinity (0.6) across the front. The counter-flowing NECC and SEC induce an upwelling (divergence) of cold deep water, drawing phytoplankton from the sub-surface maximum in the thermocline, to the surface. South of the equator, the surface mixed layer deepens rapidly to over 100 m (see Fig. 24). This is below the limits for the UOR undulation depth range (0–80 m) at the passage speed of 22 km.h⫺1 (12 knots).

5. Remote sensing of biological structures The complex physical structures which are encountered along the AMT transect, form the physical provinces which host a wide range of ecosystems of diverse productivity. These range through mesotrophic, temperate shelf systems and mid-ocean gyres in spring and fall, to oligotrophic sub-tropical gyres, eutrophic upwellings and convergence zones of extreme variability. The AMT cruises encounter 2 seasons per transect and with 2 cruises per year, the spring and fall in both northern and southern hemispheres are sampled each year, just after the semi-annual equinoxes. The diversity of ecosystem productivity for each season is illustrated by the CZCS monthly composite images of chlorophyll concentration for April and September shown in Figs. 30 and 31, respectively. Particular features of note are: the productive spring phytoplankton blooms in the temperate N Atlantic in April and the S Atlantic in September with the less productive autumn blooms in the opposite seasons; the

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Fig. 24.

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Vertical sections of T and S for N and S Atlantic Ocean 50°N to 50°S for AMT–1.

extensive N W African upwelling in both seasons, most notably in September; the plume of highly productive water from the Amazon outflow, transported across the central Atlantic from west to east by the North Equatorial Counter Current (MullerKarger et al., 1988); the equatorial upwelling between the NEC and SEC, a weak feature on the equator; the complex zone of the south Atlantic sub-Antarctic convergence. Recent ocean colour imagery from OCTS for chlorophyll concentration, composited week-by-week during AMT–4 (April/May 1997, Fig. 32) show similar features to the CZCS climatology, though there are some significant differences in magnitude. The SeaWiFS week-by-week composite of chlorophyll concentration for AMT–5 is shown in Fig. 33. While the CZCS climatologies show surface chlorophyll concentrations in the middle of the gyres of typically 0.01 mg m⫺3, the AMT

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Fig. 25.

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Vertical sections of Chl and NO3 for N and S Atlantic Ocean 50°N to 50°S for AMT–1.

measurements were typically 0.05 (0.04 was the lowest measured on any cruise). OCTS data are suspiciously high (circa 0.2 mg m⫺3) but SeaWiFS data at circa 0.05 mg m⫺3 are very close to the measurements.

6. Phytoplankton pigments The surface layer biological properties (sea surface chlorophyll concentration) for AMT–2 and AMT–3 are shown in Fig. 34. The features of note are very similar to those seen in the satellite data discussed above. These show high concentrations of biomass in the temperate zones at both northern and southern extremes, in both seasons; the increased biomass at circa 20°N as the transect skirts the west African

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Fig. 26. UOR vertical section of T, S and Chl for tow from 24.5–19.3°N into the fringe of the West African Upwelling. (a) Chart of the location of UOR tows shown in Figs. 26–29.

upwelling; the small increase of chlorophyll concentration at the equatorial upwelling and the very low (⬍0.05 mg m⫺3) values in the sub-tropical gyres. The composition of the other phytoplankton pigments are shown in Fig. 35 and described and discussed elsewhere (Gibb, Barlow, Cummings, Rees, Trees, Holligan & Suggett, 2000). Of note are the high abundance of fucoxanthin, a biomarker of diatoms, and hexanolyoxyfucoxanthin, a biomarker for prymnesiophytes, in the temperate high productivity zones, north and south, and the moderate abundance of zeaxanthin, a biomarker of cyanobacteria and prochlorophytes, across the equatorial zones 30°N to 35°S. Divinyl chlorophyll a, an exclusive biomarker of prochlorophytes, forms circa 50% of the total chlorophyll concentration across this zone.

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Fig. 27.

UOR vertical section of T, S and Chl for tow from 13.2–8°N over the Guinea Dome.

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Fig. 28. UOR vertical section of T, S and Chl for tow from 6.5°N to 0°N over the equator on AMT1.

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Fig. 29.

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UOR vertical section of T, S and Chl for tow from 1.3°N to 1.8°S over the equator on AMT3.

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Fig. 30. CZCS monthly composite of chlorophyll concentration for the Atlantic Ocean for April.

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Fig. 31.

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CZCS monthly composite of chlorophyll concentration for the Atlantic Ocean for September.

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Fig. 32. OCTS week-by-week composite of chlorophyll concentration for the Atlantic Ocean for April, May 1997, corresponding to AMT–4.

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Fig. 33. SeaWiFS week-by-week composite of chlorophyll concentration for the Atlantic Ocean for September, October 1997 corresponding to AMT–5.

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Fig. 34.

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Sea surface chlorophyll concentration from 50°N to 50°S for AMT–2.

The measured surface layer chlorophyll concentration for AMT 5 and the alongtrack data extracted from the SeaWiFS week-by-week composite image (Fig. 33) are shown together in Fig. 36. The similarity between the data is reassuring with two reservations: firstly, the SeaWiFS data are composited week-by-week for the 5 week duration of the cruise and are not an exact ‘match-up’ (i.e. the measurements are mostly not within 2–4 h of satellite overpass); secondly, SeaWiFS was switched off from 10 October 1997 through operator error, so no contemporaneous data are available for the final 7 days of the AMT–5 cruise to 17 Oct. For these reasons the percentage of variance explained is only 57.4% and the relationship has a slope of 0.8 (SeaWiFS overestimates).

7. Primary productivity Primary productivity measurements (on deck radiocarbon incubations) and the determination of photosynthetic characteristics (P versus I measurements in the ship’s laboratory) using radiocarbon incubations, have been a major core project on AMT cruises. A comprehensive report of the measurements for cruises AMT–1 to 3 has been presented (Maran˜o´n & Holligan, 1999). The general patterns of productivity follow the distribution of biomass (chlorophyll concentration) with the highest daily rates in the mesotrophic temperate zones at the northern and southern extremes of the transect and in the fringes of the west African upwelling zone. There are, of course, significant changes in the size fractionated productivity between oligotrophic waters, with most activity in the picoplankton (0.2–2 µm), and the highly productive waters, with highest production in the largest size fractions (net plankton⬎20 µm).

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Fig. 35. Along-track surface layer pigments a) fucoxanthin (Fuc), hexanolyoxy fucoxanthin (Hex), zeaxanthin (Zea); b) carotenoid ratios for AMT–2.

A FRRF fitted in the UOR was towed throughout AMT–5 (Aiken et al., 1998a) with novel results. UOR tow number A514 in the heterogeneous sub-Antarctic convergence zone (shown in Fig. 22) crossed a cold core pole on the edge of a large warm-core ring of detached Brazilian current water; SST rose from 14.5 to 16°C in a few km as the front was crossed. Fig. 37 shows surface temperature, salinity and chlorophyll fluorescence, the FRRF measurements of the maximum fluorescence (Fm), the photochemical quantum efficiency (Fv/Fm) which is the ratio of the variable fluorescence Fv to Fm and downwelling irradiance at 490 nm (Ed490). For a full definition of FRRF symbols see Kolber and Falkowski (1993). The ratio of Fv/Fm decreased across the front from circa 0.4 to circa 0.25 which was close to, though less than, the changes seen in IronExII in going from pre-fertilised to postfertilised bloom (Behrenfeld et al., 1996), but significant in such a short distance. At the front, the low values of Fm and Fv/Fm and the high penetration of Ed490

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Fig. 36. Along-track, surface layer chlorophyll measured by HPLC and retrieved from SeaWiFS composite image for AMT–5 September/October 1997.

indicate that the water was practically free of plants, possibly induced by a convergent upwelling of deep water between two counter rotating water masses. The crosssection of photosystem 2 (σPS2) was circa 500–600×10⫺20 m2 in the productive waters and slightly lower (500–550×10⫺20 m2) across the front. The turnover time for photosynthesis (t) ranged from 1000 to 1150 µs on either side of the front. These are the first reported wide ranging, measurements of phytoplankton photosynthetic parameters obtained rapidly and non-destructively in natural conditions with a FRRF in a UOR. Throughout AMT–6 (Aiken & 17 others, 1998b) the FRRF was deployed in the UOR (24 tows, 75 hours of data), on CTD vertical casts (39 stations) and on-board, measuring pumped surface water (circa 7 m) from the non-toxic supply (continuously for 31 days). The contoured vertical sections of Fm and Fv/Fm from AMT–6, Cape Town to the UK (35°S to 50°N) are shown in Fig. 38. The high productivity zones through the Benguela and North West African upwelling are readily identified, as is the sub-surface maximum in the N Atlantic Equatorial zone. AMT–6 was notable for the high quality optical measurements, discussed in the next section. Preliminary analysis of these data and the FRRF measurements of Fv/Fm from the CTD casts on station, show a significant empirical relationship (r2=64%, all data). These analyses suggest that a functional relationship exists between the optical properties, spectral absorption and phytoplankton productivity,

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Fig. 37. Surface T, S, ChlF and contoured vertical sections of FRRF measurements from AMT–5 tow 514, Fm, Fv/Fm and Ed490 crossing a cold core pole.

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which implies that in the future, satellite imagery can be interpreted in terms of photochemical quantum efficiency or productivity.

8. Ocean optics and validation of SeaWiFS As stated in the goals of the AMT project (Aiken & Bale, 2000), the calibration and validation of satellite remotely sensed data of ocean colour is a major objective. For AMT–5 this became the priority objective of the cruise. Specific tasks to these ends were: 1. Derive downwelling irradiances (Ed, l), water leaving radiance (Lu, l) and the diffuse attenuation coefficient (Kd, l = d/dz(Ed, l)) at all SeaWiFS wavelengths and report the values daily to NASA GSFC; these parameters were derived from the daily station casts of the optical profiler (SeaOPS) and the free fall optical profiler (SeaFALLS; Aiken et al., 1998a). The optical data were quality assured by on-board calibration of the radiometers and irradiometers using the SeaWiFS Quality Monitor (SQM; Hooker & Aiken, 1998). The derived parameters were quality assured by statistical assessments, self-consistency checks on spectral shape and magnitude and empirical algorithm analyses. 2. Report measured phytoplankton pigments (by HPLC and fluorometry), notably chlorophyll a and pheopigments within 1 day to NASA GSFC. Quality assurance of these data was accomplished by inter-comparison of the measurements and by using a non-marine phytoplankton pigment (canthaxanthin) as an internal standard. 3. Compare measured and retrieved (computed) values of pigment concentration (chlorophyll a+pheopigments) from measurements of the Lw or Rrs, using a standard algorithm and the measured and retrieved values of the diffuse attenuation coefficient at 490 nm (Kd490) as a quality assurance of the data and analyses techniques. Processing procedures for SeaOPS and SeaFALLS data are described by Aiken et al. (1998a). For quality assessment, the following analyses were performed: 8.1. Comparison of Klu vs. Ked The diffuse attenuation coefficient ‘K’ is a very robust apparent optical property and values of KEd and KLu (=d/dz(Lu)) should be similar for the surface 1 to 2 optical depths. Significant error between these two coefficients derived from concurrent measurements, should flag a problem with either the measurement, or the analysis, of one or other parameter value. Individual data points can then be re-analysed or rejected if no assurance is forthcoming.

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Fig. 38.

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Contoured vertical sections of Fm and Fv/Fm from CTD casts for Amt–6 33°S to 50°N.

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8.2. Comparison of Ked(ret) vs. Ked(meas.) The comparison of the retrieved KEd (from the measured Lw, l data and a standard algorithm) and the measured KEd (from the Ed vs. depth data), provides a validation of the Lw data, given that the measured KEd data have been pre-validated by the procedure described above. In this case the SeaWiFS prelaunch algorithm for Kd490 (MT97; Mueller & Trees, 1997) and a Kd490 algorithm developed from previous AMT data (Mea97; Moore, Aiken & Rees, 1997), were used as the benchmarks. Additionally, this provided an assessment of the relative accuracy, as a fraction of the variance explained, for each of the algorithms considered. 8.3. Comparison of Chl(ret.) vs. Chl(meas), using a standard algorithm This is strictly an assessment of the accuracy of the ‘standard algorithm’ but is de facto an assessment of the quality of the optical data. In this case, the SeaWiFS pigment algorithm (AM97; Aiken, Moore, Trees, Hooker & Clark, 1995) with modified coefficients (Moore et al., 1997) and the SeaWiFS operational algorithm (O’Reilly et al., 1998) have been used as the benchmarks. With this test, if the data fit the model with a large fraction of the variance explained and with close to a 1:1 relationship, then inherently the optical data used with the retrieval algorithm, must be ‘good’, except for freak circumstances. Fig. 39a and b show the final form of the KLu vs. KEd regression for AMT–5 demonstrating a close agreement (slope=0.97), with a high percentage of the variance explained (0.996 for Case 1 and Case 2 waters and 0.96 for Case 1 waters only). Interim versions of this regression, produced as the cruise progressed, allowed ‘outliers’ to be identified, which on closer examination were modified or deleted. Nearly all data acquired passed this quality test and the only data omitted were those when the sun and cloud conditions were so variable that no good data could be retrieved. Fig. 40a shows the modelled KEd vs. KEd (meas.) for Case 1 waters only, using the SeaWiFS prelaunch algorithm (MT97) and the Mea97 algorithm. For both, the fraction of variance explained is similar (0.97), but Mea97 over-estimates the KEd by almost 20%, while MT97 underestimates the KEd by about 7%. With the whole data set, including the Case 2 water measurements (Fig. 40b), the variance explained is 0.96 for both, but both underestimate the KEd by 17% (Mea 97) and 40% (MT 97) respectively. Not too much significance should be attached to the divergence of the slopes of the regressions from unity, as the 4 points from Case 2 waters, arise from only 2 stations in the eastern English Channel, with K values of 0.35 m⫺1. These are three times greater than the next highest points and these points ‘drive’ the slope of the regression. The significance is the large fractions of variance explained in all cases which indicates that the data are of outstanding high quality and precision. The SeaWiFS Quality Monitor (SQM) which was deployed first on AMT–3 (Hooker & Aiken, 1998) showed that the stability of calibration of individual channels of the in-water radiometers remained stable to 0.3% throughout the cruise. For SeaFALLS, the freefall probe, the modelled versus measured results were of high merit (as assessed from the large fraction of the variance explained: — Mea97:

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Fig. 39. AMT–5 optics: (a) KLu versus KEd for Case 1 and case 2; (b) Case 1 data only.

slope=1.09, r2=0.97; MT97: slope=0.83, r2=0.97) emphasising the quality and precision of the optical data. These were greatly superior to anything reported previously. The conclusion is that the determination of K from satellite observations, given an appropriate algorithm, should have a standard error within ±8%. As K relates directly to the absorption coefficient ‘a’, the parameterisation of an algorithm for any production-driven process based on phytoplankton photon absorption, should have a contribution to the standard error of the same order of magnitude. Fig. 41 shows the regression analysis for the modelled (retrieved) values of chlorophyll concentration using the SeaWiFS algorithm (Aiken et al., 1995) against the

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Fig. 40. AMT–5 optics: KEd(ret) versus KEd(meas), solid dots MT97 algorithm, open circles are Mea97: (a) Case 1 and Case 2 waters; (b) Case 1 waters only.

values measured by HPLC. The regression has a slope 0.85 with 90.4% of the variance explained. Again, these results emphasise the high quality of the optical measurements (better than 1%) and the precision of the HPLC analyses for chlorophyll. New methods, using internal standards applied on AMT cruises, give the standard deviation of replicates to within ±3%. A preliminary assessment of the accuracy of SeaWiFS has been reported by NASA (McClain, Cleave, Feldman, Gregg, Hooker & Kuring, 1998). NASA report that the comparison of measured water leading radiances (Lwn) with satellite derived Lwn (from atmospheric correction of Lsat) at all wavelengths are in agreement to within 1%, on average, for the 12 ‘match-up’ data sets on AMT–5. For chlorophyll concen-

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Fig. 41.

AMT–5 optics: Chl (ret) versus Chl(meas) Case 1 waters.

tration the agreement is better than 10%. For the diffuse attenuation coefficient the agreement is better than 5%. These results are remarkable given that the programme goals were 5% for radiance and 35% for chlorophyll. AMT–6 was an equally important cruise for SeaWiFS calibration, validation and algorithm development on account of the wide diversity of phytoplankton and the range of pigment concentrations (chlorophyll a ranged from 0–8 mg m⫺3). As many as 17 different phytoplankton assemblages were encountered in the different ecosystems sampled: Benguela (6 groups), Gulf of Guinea, Equatorial upwelling, N Atlantic equatorial, NW African upwelling (2 groups), Canary–Azores region, Biscay and Western Approaches/English Channel (4 groups). The optical/bio-optical data obtained here contributed in a major way to the revision of the SeaWiFS operational algorithm for chlorophyll a (the change from the OC2 to OC2V2 algorithm), which reduced the over-estimate of high pigment values by a factor of 2. Not surprisingly the regression of Chla derived with the OC2V2 algorithm using AMT–6 optical measurements against the AMT–6 values measured by HPLC, has a slope of 1.000 and over 90.5% of the variance is explained (see Fig. 42). The AMT–5 and AMT– 6 optical and bio-optical data sets have been vital in the formulation and validation of the new bio-optical models (Moore and Aiken, personal communication) which promise to transform the accuracy and diversity of interpretations possible from satellite observations of ocean colour. The models give the IOPs ‘a(l)’ and ‘bb’ directly from RS data. From these, the pigments (Chla and the carotenoids), photosynthetic parameters, productivity and species type (combinations of ‘a(l)’ and ‘bb’) can be determined.

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Fig. 42.

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AMT–6 optics: Chl (ret) by OC2V2 algorithm verses Chl (meas) by HPLC.

9. Discussion and conclusions The 6 main objectives of the AMT programme are set out in the introductory paper by Aiken and Bale (2000). In the cruises to date there have been substantial scientific achievements on all of these research goals. 1. The suite of physical, chemical, biological, optical and atmospheric measurements have provided new insights into the structure and functioning of marine ecosystems. The results have laid down the foundation for a programme to determine decadal change of ocean productivity, biogeochemistry and ecosystem responses to climate change. Significant inter-annual variability of pigment biomass and productivity are short term observations. New observations of primary productivity in the oligotrophic gyres (Maran˜o´n & Holligan, 1999) have altered our view of the magnitude and inter-annual variability of productivity and productivity-driven processes in these and other diverse ecosystems of the Atlantic Ocean. Exploiting new technology, the FRRF has revolutionised the acquisition of photosynthetic rates and parameters at basin scales and the measurements have shown the extreme variability of these parameters and processes in natural systems. Data of such quantity and quality have been unobtainable hitherto with conventional station sampling and radiocarbon methods. Functional relationships have been derived between photosynthetic parameters and incident light (PAR) and between the spectral absorption of light and phytoplankton photosynthetic pigment ratios. These offer the possibility of novel interpretations of satellite observations of ocean colour. 2. Perhaps the most significant result to date is the validation of the concept of biogeochemical provinces as hypothesised by Longhurst et al. (1995). An objective procedure (derivative analysis) has been devised and tested to determine

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4.

5.

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physical province boundaries by an analysis of surface temperature and salinity or surface density (Hooker et al., 2000). By an extension of this approach, the method can determine province boundaries from remotely sensed observations of SST coupled with climatological salinity to form a synthetic density profile. An analysis of along-track pigment data has shown that biological (biogeochemical) provinces have geographical boundaries close to the same locations; some small differences in locations would be expected as biomass accumulates irregularly adjacent to physical boundaries (fronts). Preliminary analysis of remotely sensed bio-optical data (chlorophyll concentration and Kd from SeaWiFS) have shown that the same objective analysis can be applied to these data with comparable results for the province boundaries. The consistency of the derivative analysis for all the oceanographic variables is the most convincing evidence of the province concept, particularly for cruises of the same season though some degree of interannual variability can be detected from the analysis. The holistic strategy has been most prominent in the integration of shipboard measurements and remotely sensed data. The combination of remote sensed SST, SSH and in situ data can give 3–D structures in complex, heterogeneous areas (Willson & Rees, 2000). The in situ data have been used both for the development of analytical algorithms for pigments, attenuation coefficients, productivity and productivity-driven processes and also as a means of extrapolating shipboard measurements to provinces and full ocean basins with the aid of models. With the start of the operational phase of SeaWiFS during AMT–5, near real-time imagery was used as an aid to planing day-to-day sampling strategy during the cruises, notably AMT–6 (Aiken et al., 1998b) and AMT–7. New optical technology has been at the forefront of the goal to provide calibration and validation of satellite sensors of ocean colour (Hooker & McClain, 2000), notably the use of the SQM (Hooker & Aiken, 1998) to provide daily calibration assurance of the optical sensors used to determine the in water optical properties. The success of this goal can be judged by the results of the SeaWiFS calibration and validation exercise (McClain et al., 1998). New technology for the autonomous measurement of pCO2 has shown regions of under-saturation in the tropical Atlantic (Lefe`vre, Moore, Aiken, Watson, Cooper & Ling, 1998) and has been used to estimate pCO2 budgets over the whole Atlantic Basin (Lefe`vre & Moore, 2000). The development of coupled physical-biological models is a much slower cogitative process. Considerable progress has been made on the basis of province specific methods using new climatological data for ecosystem process parameters derived from AMT measurements. As a result of the high quality optical and biological measurements, new functional relationships have emerged. These have produced significant advances in bio-optical models and the development of new analytical algorithms linking photon absorption and production processes (Moore et al., personal communication). The optical data have been used to validate the models. The models can be used to derive algorithms for phytoplankton pigments, chlorophyll a, carotenoids, productivity and productivity-related processes such

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as biogenic gas exchange. This is a real advance for basin scale modelling of primary productivity. AMT cruises 1 to 7 (1995–1998) have seen the completion of phase 1 of the AMT programme, wherein many of the new, autonomous technologies and operational approaches have been pioneered and proven. There are obvious limitations in the programme, particularly one which has objectives related to issues of climate change. Notably, the physical oceanography is superficial, CTD casts have been limited to 200 m in most cases with no geostrophic reference and the spatial resolution of circa 400 km from typically 1 cast per day is too coarse. As a basin scale programme the AMT samples the temperate N. Atlantic poorly; there is no sampling north of 50°N. As a programme focused on climate change, a time series based on samples only twice a year has severe limitations, with no adequate resolution of the seasonal cycle in any province. Nevertheless, the fledgling four-year time series can already provide measurements of inter-annual variability, which is an essential pre-requisite for any study of decadal trends. With another 10 cruises planned over five years (1999– 2003) during phase 2, the basis of a study of climate change will be well established. During this period there must be a focus on those measurements that are sensitive to climate forcing or are known indices of anthropogenic influences on climate. Collaboration with other European national research activities is planned to improve the coverage of the seasonal cycle in the north Atlantic and create a European Atlantic Time and Space Series (EATSS) project. Core to this are the twice yearly transects of the other Antarctic research vessels, the Polarstern (Germany), the Hesperides (Spain) and the Pelagia (Netherlands) with opportunistic research cruises in the area 20–63°N, 20°W, by UK, German, French, Dutch, Belgian and Spanish vessels. If this develops, it will be true to say, that the AMT programme has laid down the foundation for a study of decadal trends in the marine ecosystems of the Atlantic Ocean with which to understand and model their responses to climate change.

Acknowledgements The AMT research programme has been supported by the NERC (UK) Community Research Programme PRIME and the NASA (USA) SeaWiFS project. We gratefully acknowledge the co-operation of the British Antarctic Survey (Directors Barry Heywood and Chris Rapley), the considerable contributions and diverse skills of Captains Jerry Burgan and Chris Elliott and the officers and crews of the RRS James Clark Ross, without whom the research work would have been impossible. This paper is No. 20 of the AMT Programme.

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