Soy. J. Phys. Oceanogr., Vol.3, No.6, pp.475--480(1992)

@ vsP 1992.

The major objectives and results of investigations carried out during cruise 41 of R/V Akademik Vernadsky* N. P. BULGAKOV, V. V. KNYSH and A. A. SHCHIPTSOV Abstract -- The objectivesof cruise 41 of R/V Akademik Vernad~ky and the data provided on the RAZREZY project are reported. Investigations on cruise 41 of R/V Akademik Vernadsky (20 January-ll May 1990) were carried out in the framework of the national scientific projects RAZREZY (part of WOCE and TOGA), KOSMOS, and MIKROSTRUKTURA. The hydrographical data derived in the course of the research were applied to some problems from the VOLNA project. Alongside this, cruise investigations dealt with such programmes as EKOSYSTEMA, PRIBOROSTROENIE, and GASOI. The major goal of the expedition was to compile and process experimentally derived data in the economic zones of Brazil, Surinam, Guyana, and Barbados together with the adjoining oceanic areas with the purpose of sharpening up our understanding of the mechanisms and qualitative characteristics of heat transport in that region of the tropical Atlantic Ocean during the winter]spring period. Oceanographic investigations were carded out in two test areas (Fig. 1): the Brazil test area (test area 1) and the Surinam-Barbados one (test area 2). Current velocity measurements were taken in the areas of major long currents of the studied ocean region (see Fig. 1 and Table 1). Concurrently, oceanographic investigations on the coordinated programme were carried out by R/V Vladimir Parshin in the adjacent area from 25 February to 8 March 1990. The two research vessels exchanged the data retrieved outside the economic zones. At each oceanographic station, CTD sounding was performed by the MGI-4101 probe down to a depth of 1100m along with water sampling by a rosette of bathometers for subsequent chemical analysis. With the ship underway, CTD measurements were performed in the upper 6m layer with MGI-4205 probe mounted in the vessel's shaft. At moored stations and from on board the ship, the currents were observed by MGI-1301 (DISK-2) self-recorders. To improve the accuracy of the MGI-4102 oxygen channel, pioneering hydrochemical experiments were carried out using/n situ oxygen measurements with the original data treated by personal computer. During the whole period of the investigations, meteorological and actinometrical observations were carried out, along with the receiving of facsimile charts of the bade topography. As distinct from the previous expeditions in the region, pilot complex investigations in the economic zones of Brazil, Surinam, and Barbados were implemented. This has *Translated by V. Puchldn. LIDK 551A6(X61).

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Figure 1. Arrangementof test areas and sections,stations, anchored moorings,and suspendedstations. permitted the hydrophysical fields in the near-equatorial region and oceanic part of the Lesser Antilles Islands to be described in finer detail [1, 2]. W e will consider briefly the data acquired on the R A Z R E Z Y project. The thermohaline structure of waters in the western tropical Atlantic during February-April 1990 evolved due to the coupling of the Guiana current, equatorial countercurrent, equatorial undercurrent (Lomonosoff current), north equatorial current, Antilles current, and the water flow in the Caribbean Sea. THE GUIANA CURRENT The Guiana current surface temperature was in excess of 27"C. From the open sea, this current transported relatively saline (>37.2psu (practical salinity units)) tropical water from the Southern hemisphere. North of the equator, it entrained the Amazon water transported along the shelf/continental slope in the north-western direction. The transformed riverain waters near 55* W reached almost 15" N. A well-pronounced salinity maximum of the southern subtropical waters at a depth of 70-90 m was observed up to 1-2" N. As Table 1 shows, the Guiana current is distinctly outlined in the 25-500m layer (section 2). Three velocity maxima can be observed in its vertical structure. The basic core with maximum velocity values achieving 75-78eros -1 is located in the 25-100m layer. Another core (33eros -z) was registered at a depth of 200m. A core having a velocity of about 46eros -z was observed at the 300m level. The Guiana current's velocity decreasedwith depth and was 15 cms -z at 500 m.

477

Cruise 41 of R / V Akademik Vernadsky

Table 1.

In situ currents observations during cruise 41 of R/V Akademik Vemadsky

Type of observation

Area of observation

Tune and discreteness of observation

No. of stations

Buoy stations with involvement of MGI-1301 probes at the Brazil test aria

Section 1 along the course 220* from 05"40I to 04*101N every 30--40 miles

18 February-10 March; 5 rain

5 buoy stations

Section 2 along the course 220* from 000531N to 00"281S every 25-30 miles

22 February--4 March; 5min

4 buoy stations

Moorings featuring MGI-1301 probes at the Surinam-Barbados test area

Surinam economic zone along the 55*3(YW meridional from 09"13' to 07"48'N every 40 miles

20-29 March; 5 min

3 buoy stations

Barbados economic zone along 59"00'W from 12"00' to 13'00' N

24 March--4 April; 5 min

2 buoy stations

Moorings using MGI-1301 and MGI-1308 probes

Brazil and Surinam shelf areas

11 and 29 March, 29-30 March, 31 March; 5 min

5 moored stations; duration 25 h; 23 levels

Suspended drift stations u s i n g MGI-L301 and MGI-1308 probes

Brazil shelf area, the equatorial countereurrent zone

8-9 April; 5 rain

2 suspended stations; duration 37 h; 15 levels

Determination of surface currents by ship drift

Brazil and Surinam-Barbados test areas

8 February-15 April

237 determinations

The structure of the main upper core of the Guiana current changed considerably with time. Initially (22-23 February), it was documented at a depth of 100 m, its velocity being 67 cm s -a. The maximum velocity (up to 97 crn s -1) was observed on 26 February. At the end of the observational period (1-2 March), the core rose up to 25 m, its velocity being 88 cm s -1. The rate of flow at section 2 in the 25-500 m layer was approximately 13.3 Sv. With the volume transport in the top 0-25 m layer considered and the current velocity being about 1 m s -~ (by the ship drift data), the maximum rate of flow of the Guiana current was 20 Sv. At the kinematic section 1 (northern section of the Brazil test area), the Guiana current velocity at depths of 100-200 m reached nearly 50 cm s -1. A relatively high velocity was documented at a depth of 5 0 0 m (in excess of 3 0 c m s - 1 ) . Over the continental slope and in the shelf area, the Guiana current velocity in the surface layer attained 200 eros -~. As the instrument observations indicate, the G u i a n a current's rate of flow across section 1 in the 25-500 m layer amounted to about 6 Sv. With the volume transport in the 0-25 m layer, the coastal configuration, and the seabed relief taken into

478

N. P. Bulgakov, E V. Knysh and .4. ,4. Shchiptsov

account, volume transport by the Guiana current in the area was 10-15 Sv, and heat transport at section 1 was (10-14)• 1014watts. The data collected at the Surinam section allow the deduction that the Guiana current near 55* 30' W was meridionally oriented in the upper 0-100m layer. The current's rate of flow there was 5-10Sv. At depth, currents with south and east velocity components were observed. At the surface, the Guiana current had a relatively high oxygen concentration and low biogenie matter content. An oxygen maximum occurred in the subsurface layer. Analysis of the thermohaline characteristics and charts of the dynamic topography has permitted a vortical formation to be singled out near 9--10" N and 53-55 ~W. The maximum zonal current velocity there was about 60 cms -1, and the rate of flow across its radius between 10" and 11"45'N in the 0-500m layer was 13Sv. With the CTD data compiled by R/V Vladimir Parshin analysed, one can claim that the eddy at the time of its observation represented an isolated vortical feature rather than the western periphery of the Guiana current reversing eastward. During the cruise, the Guiana current was not observed to veer toward the equatorial countercurrent either by the distribution of the thermohaline and hydrochemical characteristics, or by the data retrieved from diagnostic calculations. The data from current observations at the two sections in the Brazil test area were expanded into series using a system of complex empirical orthogonal functions. The initial eigenvectors (depth function) and complex amplitudes (time functions) were analysed. The period of variability of the first expansion coefficient was 25-30 days, and of the last three coefficients a few days. The first mode may be obviously identified with the long Rossby or Kelvin waves, the latter having a period of 30 days in the region under study. Analysis of the hydrographical parameters observed by the MGI-4205 device with the ship underway (discreteness of measurements was 10 rain) and at the hydrographic stations enabled the spatial distribution of freshened waters and the location of fronts to be specified. In the coastal section of the Brazil test area between 44* and 48* W, the Amazon water reached 2* 30"-3* N. The transformed riverain waters having a salinity below 30psu and a high silicon concentration (ranging from 20#mol 1-1 near the northwestern estuary to 200 #tool 1-1 in the central part of the estuarine area) were entrained by the Guiana current along the shelf in the north-west direction. Analysis of the silicon distribution in surface waters demonstrated that over the time of the experiments at section 1 the transformed Amazon waters were not observed outside the shelf area. This was also confirmed by the distribution of the light attenuation index at the sea surface. On the other hand, such waters were observed beyond the shelf area borders in winter time during cruise 30 of R/V A k a d e m i k Vemadsky. This fact allows speculation as to the appreciable interannual variability of their propagation. It was found from the distribution of silicon, salinity, and light attenuation index that a large lens of freshened water having a salinity of about 30 psu and a relatively large Si concentration (< 13 #mol 1-1) was accommodated by the 0-50 m layer near Barbados (between 12 and 15" N at 55* W). This supports the inference that during the winter and spring of 1990 the freshened waters were transported northward. THE EQUATORIAL COUNTERCURRENT Combined analysis of the charts of dynamic topography, maps of currents determined through diagnostic calculations by the non-linear model, the thermohaline characteris-

Cruise 41 of R/V AkademikVernadsky

479

tics distribution, and/n s/tu observations showed that during February-March 1990 the equatorial countercurrent was observed as a surface flow in the area bounded by 7 and 9* N and 42 and 50* W. East of 42* W, the equatorial countercurrent did not exhibit wellmarked boundaries and core, owing to its active interaction with the north equatorial current. At the ocean surface, the equatorial countercurrent had a signature in the form of a frontal zone between cold (< 23.5"C), saline (> 36.2 psu) north tropical Atlantic water and warmer, less-saline water from the equatorial Atlantic. The subsurface salinity maximum in the equatorial countercurrent area did not exceed 36.6 psu; it was formed by the north subtropical waters and stretched in the form of a tongue in the eastern-south-eastern direction. At the vertical sections in the equatorial countercurrent area, a core was observed whose salinity was 02-0.3 psu. The main source of water for the equatorial countercurrent was the waters of the north subtropical gyre. It was determined that the largest geostrophic velocity in the equatorial countercurrent occurred at the sea surface. This increased from 40-50eros -1 in the eastern section of test area 1 up to 70080cms -1 in the western section. The countercurrent's rate of flow was basically concentrated in the 0-200 m layer and equalled 15 Sv. Diagnostic calculations by the numerical non-linear model indicated that the core of maximum zonal velocity (60cms -:) of the countercurrent was at a depth of 25-30m. At 100m, the countercurrent's vector velocity modulus equalled 50 era s-1. Heat transport in the 0--200 m layer across the 47* 30' W section, where the equatorial countercurrent was most pronounced, was 16x 10:4watts. By its position and peculiarities of the kinematic structure, the equatorial countercurrent observed in February-March 1990 was similar to its state in February-March 1985. However, ship drift-based calculation of the surface currents showed that the 1990 flow was mainly oriented eastward. Manifestation of the equatorial countercurrent on the sea surface during the winter and spring of 1990 was obviously an anomalous phenomenon. Analysis Of the temporal dependences of the tangential wind stress divergence, which was calculated for two pairs of 5* x5* squares situated symmetrically with respect to the equator, their centres being at 32* 30" and 28* 30' W, respectively, allowed the conclusion that from 8 February to 19 March 1990 the axis of the intratropical convergence zone (1TCZ) in the West Atlantic was quite close to the equator and was not seasonally shifting to the north. This deduction is supported by the kinematic chart for bade maxima movement, based on facsimile charts, as well as the charts of cloudiness compiled from remotely sensed data. From the middle of March, the ITCZ in the eastern tropical Atlantic vacillated relative to the equator shifting from one hemisphere to tlae other. The observed cycle was 8 days long. THE EQUATORIAL UNDERCURRENT The equatorial undercurrent was observed in the equatorial area (002 ~ N). Its velocity at a depth of 100m attained 50eros -1. Isotherm expanding in the upper thermodine (50-200 m) and the presence of a high-salinity (> 36.6 psu) core were characteristic for the current. As analysis of the undersurface maximum salinity distribution,/n s/tu observations, and adaptative calculations by the non-linear equatorial model has shown, the equatorial undercurrent was fed by waters from the south and north of the anticydonie gyre located south of the equatorial countercurrent. According to the diagnostic calculations, the

N. P. Bulgakov," I~. Y. Knysh and A. A. Shchiptsov

480

undercurrent had maximum velocity (about 100cms -1) at the site of its genesis (the near-equatorial zone at 41-43" W). The equatorial undercurrent was characterized by the presence of a core of maximum salinity and by the high oxygen concentration in its core (100-125m) amounting to 3.96-4.85 ml 1-1. THE NORTH EQUATORIAL CURRENT In the north-western section of test area 2, the north equatorial current was observed to veer to the south (north of 12~N). This reversal caused cold saline waters to flow southward. The maximum oxygen concentration (4.8 ml 1-1) occurred in the upper mixed layer. REFERENCES 1. Report on Cruise 41 of R / V Akademik Vemadsky, 20 .ranuwy--ll May 1990, Vol. 1, Part 2. Sevastopol:

IVlHI (1990): 2. Report on Cruise 41 of R / V Akademik Vemadsky , 20 Yanuaty--ll May 1990, Vol. 1, Part 3. Sevastopol:

MaI 099O).