CANARY BASIN CIRCULATION FROM SUBSURFACE FLOATS

 

  SUBSURFACE FLOAT SET I

 

Introduction

The Azores Current and the Canary Current are part of  the eastern recirculation of the North Atlantic subtropical gyre. The Azores Current is the extension of the south-eastern branch of the Gulf Stream and it crosses the Mid-Atlantic Ridge at about 34°N (Klein and Siedler, 1989). The current flows eastward across the Canary Basin with a geostrophic transport of about 10 Sv (1 Sverdrup = 10⁶ m³ s^-1 ) in the upper 800 m. It contains the North Atlantic Central Water. The current is related to the Azores Front, which shows strong meanders and separates colder and fresher water in the north from warm and salty water in the south. The Azores Current splits into three main southward recirculation branches, which later join the westward-flowing North Equatorial Current (Siedler and Onken, 1996). These branches, which vary seasonally and interannually, are found just east of the Mid-Atlantic Ridge, in the central basin near 23°W and near the coast of West Africa, respectively (Stramma and Siedler, 1988). The easternmost branch feeds the Canary Current, an essential part of the Eastern Boundary Current System. While the large-scale flow pattern is known, the details of the processes by which the Azores Current water is incorporated in the eastern boundary flow and then transported southward and westward are, however, not well understood. Previous studies also showed evidence of westward counterflows close to the Azores Current and that some of the water reaching the continental slope near Morocco contribute to a poleward flowing upper layer slope flow reaching the Iberian peninsula (Pingree, 1997).

 

Objectives and methods

The objectives of this study are to provide a Lagrangian description of the Azores Current, to investigate  the transition between the Azores Current and the Canary Current and to study whether the Azores Current also feeds the poleward undercurrent along the African shelf. Therefore, eleven RAFOS floats from IfMK and nine RAFOS floats from AINCO (subtask 1.2.5.2.) were deployed southeast of the Azores during a cruise with NO THALASSA in July 1997 in the central Canary Basin (task 1.2.3.). The mission length of the floats was set to be either 1 year or 1.5 years. The floats were ballasted for an approximate depth level of 500 m, which corresponds to a density level of about 27.1 kg/m³. Four listening windows were installed to derive their position once a day. Before launching all floats were tested and some parameters were optimized. Information on the RAFOS floats is given in tables below.

 

To determine the position of the RAFOS floats an acoustic sound source array is used. We took advantage of the MAST II EUROFLOAT sound source array, which was moored in October 1995 and will be recovered in 1999. For the CANIGO project an additional sound source was deployed in January 1997 with FS METEOR between Madeira and the African shelf. As part of task 4.3.3. two further sound sources were moored in September 1997 with FS POSEIDON  east of the Gulf of Cadiz.

 

The floats were deployed on the second leg of the NO THALASSA cruise, which started on 14 July 1997 in Santa Cruz de La Palma and ended on 1 August 1997 in Lisbon. Along the cruise tracks CTD and ADCP, as well as XBT and thermosalinograph observations were carried out. Based on those observations the launching positions of the RAFOS floats were mainly selected to be in the area of the Azores Front and Current. Furthermore, the position of the Azores Current was derived from the stream function predictions of the quasi-geostrophic model based on altimeter data from subtask 1.2.1.3. The prediction for 22 July 1997 of the current field in 550 m shows two current branches, one flowing eastward at about 36°N and one first moving eastward at about 32.5°N and then turning southward between 21° and 23°W. This corresponds to the idea that the Azores Current splits into different branches.

 

 

References

Klein, B. and G. Siedler. 1989. On the origin of the Azores Current. J. Geophys. Res. 94: 6159-6168.

 

Pingree, R.D. 1997. The eastern subtropical gyre (North Atlantic): Flow rings recirculations structure and subduction. J. Mar. Biol. Ass.(UK) 77: 573-624.

 

Siedler, G. and R. Onken. 1996. Eastern Recirculation. In: The Warmwatersphere of the North Atlantic Ocean. W. Krauss (ed.): 339-364. Gebrueder Borntraeger, Berlin, Stuttgart.

 

Stramma, L. and G. Siedler. 1988. Seasonal changes in the North Atlantic subtropical gyre. J. Geophys. Res. 93: 8111-8118.

 

 

Table 1.  RAFOS float information Set I

 

FLOAT - ID			LAUNCHING					MISSION
IfM-No.	hex. No.	dec. No.	date	time (UTC)	j (N)	l (W)	station	start date	start time (UTC)	length (days)	stop date
418	5594E	5477	23.07.97	11:10	33° 39.99'	24° 00.01'	55	23.07.97	08:48	365	23.07.98
419	559BB	5478	24.07.97	03:58	32° 39.99'	23° 59.99'	58	23.07.97	16:15	365	23.07.98
420	446BB	4378	24.07.97	14:49	32° 00.00'	24° 00.02'	60	24.07.97	11:02	365	24.07.98
421	C53AC	12622	26.07.97	13:27	31° 59.80'	20° 00.12'	68	26.07.97	08:29	365	26.07.98
422	C53FF	12623	21.07.97	05:15	35° 59.95'	22° 29.96'	45	20.07.97	20:39	365	20.07.98
423	C5404	12624	29.07.97	13:13	35° 40.05'	22° 00.01'	78	29.07.97	10:50	365	30.07.98
424	C5475	12625	22.07.97	13:53	34° 59.94'	23° 59.91'	51	21.07.97	19:26	547	19.01.99
425	C54A2	12626	23.07.97	22:22	33° 00.03'	24° 00.00'	57	23.07.97	15:55	547	21.01.99
426	C54F1	12627	21.07.97	05:18	35° 59.95'	22° 29.96'	45	20.07.97	20:52	547	18.01.99
427	C551B	12628	27.07.97	19:24	33° 59.97'	21° 49.03'	72	27.07.97	16:18	547	25.01.99
428	C5548	12629	30.07.97	00:10	36° 15.01'	21° 30.05'	79	29.07.97	11:10	547	26.01.99

 

 

 

 

 

This table describes the CANIGO Subtask 1.2.5.2 Subsurface float set II

 

float num	hex	launched	mission length (years/days)	predicted surface date	Actual surface date	length of float data (days)	Notes
429	6866	24.7.1997	1/365	23.7.1998	23.7.1998	365	O.K.
430	6867	21.7.1997	1/365	21.7.1998	21.7.1998	365 (2)	No interim positions
431	6868	26.7.1997	1/365	25.7.1998	03.8.1997	6	Early surfacing
432	6869	29.7.1997	1/365	28.7.1998	28.7.1998	365	O.K.
434	6871	23.7.1997	1.5/547	21.1.1999	09.8.1997	15	Early surfacing
436	6873	21.7.1997	1.5/547	20.1.1999	01.7.1998	343	Early surfacing