Distribution of water masses and diapycnal mixing in the Cape Verde Frontal Zone

GEOPHYSICAL RESEARCH LETTERS , VOL. 35 , L07609 , doi:1O.1029 /2008GL033229, 2008 Click Here Ior Full Article Distribution of water masses and diapycnal mixing in the Cape Verde Frontal Zone A. Martínez-Marrero,J A. Rodríguez-Santana,J A. Hernández-Guerra,J E. Fraile-Nuez,2 F. López-Laatzen,2 P. Vélez-Belchí,2 and G. Parrilla2 Received 9 January 2008; revised 25 Febmary 2008; accepted 10 March 2008; published II April 2008. [1] The Cape Verde Frontal Zone separates North and South Atlantic Central Waters in the eastern North Atlantic Subtropical Gyre. CTD-02 and shipboard ADCP data from three hydrographic sections carried out in September 2003 are used to study the structure of the front. Results show the relation between spatial variations of water masses and currents, demonstrating the importance of advection in the distribution of water masses. Diapycnal diffusivities due to double diffusion and vertical shear instabilities are also estimated. Ex istence of competition between the two processes through the water column is shown. Depthaveraged diffusivities suggest that salt fingering dominates diapycnal mixing, except areas of purest South Atlantic Central Water. Here, double diffusion processes are weak and, consequently, shear of the flow is the main process. Results also show that strong mixing induced by vertical shear is associated with a large intrusion found near the front. Citation: Martínez-Marrero, A. , A. RodríguezSantana, A. Hernández-Guerra, E. Fraile-Nuez, F. LópezLaatzen, P. Vélez-Belchí, and G. Parrilla (2008) , Distribution of water masses and diapycnal mixing in the Cape Verde Frontal Zone, Ge ophys. Res. Lett. , 35, L07609, doi: 10.1029/ 2008GL033229. 1. Introduction [2] The thermoeline waters of the North Atlantic Subtropical Gyre flow southward to latitudes near Cape Blanc, where they turn west away from the coast. The southward flow is commonly identified with the Canary Current, while the westward turn is the beginning of the North Equatorial Current flowing north of the Cape Verde Frontal Zone (CVFZ). This term was introduced by Zenk et al. [1991] to characterize the eastern part of the Central Water Boundary, which is the transition zone between North Atlantic Central Water (NACW) and South Atlantic Central Water (SACW). In the past, several surveys have taken place at the CVFZ to study the distribution and variability of water masses and its relation with the upwelling off West Africa [Hughes and Bartan, 1974; Manriquez and Fraga, 1982; Bartan, 1987; Zenk et al., 1991]. The CVFZ is described as a strong meandering thermohaline front near Cape Blanc at latitudes elose to 200N. It shows sharp gradients in temperature and salinity in the upper 600 m and the presence of 'Facultad de Ciencias del Mar, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain. 2 1n stituto Español de Oceanografia, Santa Cruz de Tenerife, Spain. Copyright 2008 by the American Geophysical Union. 0094-827 6/08/2008G L033 22 9$05.00 intrusions, produced by interleaving of NACW and SACW across the front with typical vertical scales of 10 -100 m. [3] One important aspect of the CVFZ is the compensating character of the temperature and salinity fields, which cause horizontal density gradients to be relatively small across the front. This peculiarity is an important factor in reducing the vertical shear of the horizontal velocity in sorne parts of the frontal region. Kl e in and Tamczak [1994] and Kl e in and Siedler [1995] have used multiparameter analysis to analyze diapycnal mixing at the CVFZ. They found significant diapycnal flux es in the Central Water layers, with water mass composition being changed by more than 20% through diapycnal mixing. The nearly density compensated character of the front and the density ratio results, let them to suggest that double diffusion is the main cause of the observed diapycnal mixing. They conelude that double-diffusive salt flux is an important process for the salt balance in this water mass, and is balanced either by isopycnal advection or isopycnal eddy diffusion. [4] However, the role ofmixing induced by vertical shear of the current in the CVFZ has not be en quantified yet. The main purpose of our study is to estimate the relative importance of vertical shear and double diffusion processes in the diapycnal mixing in different parts ofthe frontal zone. 2. Data and Methods [5] The data presented here consist of 39 SeaBird 911 + CTD (Conductivity, Temperature, Depth) stations taken along three sections in the CVFZ between 20° and 26°N and between 18 ° and 26°W (Figure 1) . Measurements were taken during the CORICA oceanographic survey carried out aboard the RIY Thalassa in September 2003. CTD-0 2 data from the western and southern sections have been already used to investigate the spatial variability of the Canary Current, by means of an inverse box model by HernándezGuerra et al. [2005]. In this paper we concentrate on the upper 600 m of the three sections, adding the analysis of the shipboard ADCP (Acoustic Doppler Current Profiler) velocity data recorded during the cruise. Distance interva ls between stations were approximately 0.4 °, except for the African slope stations which were 0.1 ° apart and the eastern section where the stations had a spacing of 1°. Water samples were collected at each station for the CTD sensor calibration and the analysis of dissolved oxygen. The shipboard ADCP (a narrowband 75-kHz frequency) provided mean current profiles relative to the ship every 5 min in 16-meter bin lengths. Data were required to pass a test ofthe return signal (Percent Good >90%), acceptable second derivatives of horizontal and vertical velocity components with respect to depth, and reasonable error velocities as recommended by L07609 1 of 5 L07609 MARTÍNEZ-MARRERO ET AL.: DIAPYCNAL MIXING IN CVFZ L07609 .' .~ : ~ ~ . r--~--'-------, ., ~ , - , - Nov 1986 ct. I - , - Mar 1973 4 ' -, : Aug 1971 47 Western % . Figure 1. Sections and depth-averaged currents (150-550 m) measured by ADCP. CTD-0 2 stations are indicated by circles. For reference, the three historical westernmost positions of the CVFZ are shown. The March 1973 and August 1971 positions are from Manriquez and Fraga [1982], and the November 1986 position is from Zenk et al. [1991]. Firing et al. [1995]. The ADCP data shallower than 600-m depth usually satisfied these criteria. The velocities were calibrated for transducer misalignment and adjusted from ship relative currents to absolute currents using GPS position measurements. [6] The density ratio, Rr " is the parameter that dictates a system's susceptibility to double-diffusive instability [Thorpe, 2005]. Density ratios were calculated from CTD data using central differences over the same depth intervals as the ADCP data. With these Rp we were able to estimate double diffusion diapycnal diffusivities, Kd, using the Schmitt [1988] parameterization for the salt-finger type, and the Kelley [1990] parameterization for the diffusive type. With potential densities and vertical shear from CTD and ADCP measurements respectively, buoyancy frequencies and gradient Richardson numbers (Ri) were calculated. Finally, the vertical shear stress diffusivities, K", have been estimated using the Pacanowski and Philander [1981] parameterization and the Ri values. A full description of the method can be found in the work of Rodríguez-Santana et al. [1999]. The scales of the instabilities induced by vertical shear are on the order of 110 m [Pelegrí and Sangrá, 1998] implying that the depth interval used in the calculation of Ri, should be greater than these values. The use of relatively large vertical intervals produces a smoothing effect on Ri, and consequently on K,., that has been analyzed by Miller and Evans [1985]. Nevertheless, in this work, the Kd values have been calculated with the same averaging depth intervals (16 m) as for K" allowing a comparison between them to be made. 3. Results and Conclusions 3.1. Structure of the Front [7] An accepted criterion for the position of the CVFZ is the location of the 36.0 isohaline at 150 m depth [Barton, 1987; Zenk et al., 1991]. At the southern section, this criterion puts the front at station 64 (21 °W) (Figure 2). This figure suggests that the CVFZ approaches the southern section at station 66 (20.5°W). A common characteristic observed in the vertical sections oftemperature, salinity and dissolved oxygen is the fluctuation of the isolines, forming consecutive domings. The fluctuations appear to be related to the meandering of the front and the probable presence of eddies. We find upward domings around 24° W, 21 °W and 18 ° - 19 ° W, which are related to the penetration of SACW to the north. The 21 °w doming, located at station 65, forms a core of minimum salinity (~35.6) between 100-300 m depth that coincides with a core of minimum oxygen «60 ~¿mole kg -1), indicating the presence of a northward entrainment of SACW. A clear relation between domings and currents is observed near the front. The upward domings are found, approximately, in zones with northward currents, while downward domings are situated where currents flow to the south. These results suggest that water mas s distribution is correlated with mesoscale activity at the CVFZ, illustrating the importance of horizontal advection. [8] The second column of Figure 2 shows the results for the eastern section. This section al so shows mesoscale variability with alternating westward and eastward flowing currents. However, the currents reveal essentially a net westward flow along the section that is coherent with the presence of the southwest extension of the Canary Current north of the CVFZ. The front is found between stations 77 (21 °N) and 78 (22°N). The results show a meander or eddy structure that produces a SACW penetration centred at station 77. Oxygen values <60 and 1 00 ~¿mole kg -1 are observed again in the purest SACW, and at the front position, respectively. The position of the front coincides with a change of velocity direction: station 77 (21 °N) shows a northwestward current that is advecting the tongue of 2 of 5 L07609 MARTÍNEZ-MARRERO ET AL.: DIAPYCNAL MIXING IN CVFZ Southern section Eastern section 7666 771 78 79 80 81 82 ~~~~E' :~~ ~--~-~~ 200 ; o s. O~.<..:..> . • E 400 35.7 • 1:600~---------------~5.~4~~~~~~--~~ +-' c.. Q) O 200 400 600 O , 200 400 600~~~~~~~--~ -26 -24 -22 -20 -18 20 Longitude 22 24 26 Latitude L07609 Figure 2. Vertical distributions of potential temperature, salinity, velocity normal to the section measured by ADCP, and dissolved oxygen. Dots in the oxygen contours indicate the locations of the sample bottles. Station positions are shown at the topo The velocity contours are constructed with a horizontal resolution of O.l 0 , and grey contours mean eastward (northward) velocity in the eastem (southem) section. Arrows depict locations ofthe front as obtained by the position ofthe 36.0 isohaline at 150 m depth. SACW. At the westem section (not shown), the 36.0 isoline does not reach 150 meters depth, indicating that the section does not cross the CVFZ. However, the salinity, temperature and oxygen values at station 55 reveal the neamess of the front at the southem extreme of this section. The current vectors of Figure 1 evidence a mean westward flow along this section as well as in the eastem section. 3.2. Diapycnal Mixing [9] Figure 3 shows depth averaged values and standard deviations of the diffusivities obtained for the 150-600 m depth layer (approximately the Central Water layer). The results show that K,. are of the order of 10-5 ± 106 m2 S-1 and vary little along the sections, except at station 65 where we obtain the highest mean value and standard deviation (~3 x lO-5 ± 4.5 x 10-5 m2 S-I). The mean values of Kd suggest that double diffusion dominates the diapycnal mixing in the studied region, except where purer SACW is found (i.e. the eastem border ofthe southem section). Here, the diffusivity coefficients due to double diffusion are clearly small «105 m2 S-I) and, consequently, diapycnal mixing induced by the vertical shear of the flow is the Western seetíon Southern seetíon Eastern seetíon 8 55 53 51 49 47 45 43 1-55..L....J.--1-1--I-60..L....J.--1-",--:-6.J....5 ..J.-L......J...J....J70'-1lll\76 66 77, 78 79 80 81 82 6 20 22 24 -26 -24 -22 -20 -18 20 22 24 26 Latitude Longitude Latitude Figure 3. Mean values and standard deviations of the diffusivity coefficients due to shear instability (black line) and double diffusion (red line), calculated for the 150-600 meter depth layer. Arrows depict locations ofthe front as obtained by the position of the 36.0 isohaline at 150 m depth. 3 of 5 L07609 MARTÍNEZ-MARRERO ET AL.: DIAPYCNAL MIXING IN CVFZ L07609 20 / 8t.43 ••••• ~1 16 / ," ./ /" 12 '2. S ~~~ Cffi ··· ·· · · · ········ [J]i .. . I. ClTI······:,························ D ·· · ·······:·:·····:··········· 450 ...... .... ....:: 600 20 -- , /8l--53 ",' _ '2. 1 15 ,'/ F .-<' ./ '2. S 10 / 15~ L ············· ltl i. W·I .......... b ············· 300 .... ·1· . . . . . . . . 1 . . . . . . ·1· . . . . . . . . . . ............. . 450 ... . ·1···· ·· ·· .. ·1· . . . . . .. .. . ·1· .. ......... . . ..... .. .. . 600 20 -- , . • ,. •• # 21 15 .' -- .'" _ '2. S 010 / 0 ~~ [iJ d ill EJ !15~ ~ .............. 5·············· Q.I .......... E ··············· <D 17 _ '2. 1 14 '2. S 300 ... . . . . . . . . . . 1 . . . . ........... . 11 / ./ 450 ... ....... .... . . ·1· . . . . . . . ... . ·1·· ......... . ............ . 8 600 20 . '2. 1 12 '2. S ./ / 8 ~~~ lf[] :··lB" 1 CE:,············· ~ : . 450 ··1 600 17 _ '2. 1 14 15~ ~ ~~ ~ : ~ ~LiLJ ~ 11 '2. S 8 35 36 37 35 36 37 -90 -45 O 45 90 O 2 4 O 1 2 8(psu) 8(psu) Tu(deg) log(Ri) K(10-4m2 s1) -10 O 10 Velocity (cm S-1) o 125 250 02 ()lmole kg-1) Figure 4. The e-s diagrams and vertical profiles of salinity (S), current velocity normal to the section, dissolved oxygen (0 2 ), Turner angle (Tu), Richardson number (Ri), and diffusivity coefficients (K) due to shear instabilities (black line) and double diffusion (red line) in six stations. Station 43 shows NACW properties, station 72 is characteristic of SACW, and the remaining stations show mixed properties. Straight lines as proposed by Tomczak [1981] and Harvey [1982] for the e-s relationship for SACW (red line) and NACW (blue line), respectively, are shown. dominant process. The standard deviations of Kd show a tendency to increase toward the front, indicating that in the frontal area both processes compete through mixing in the water column. [10] Figure 4 shows potential temperature-salinity (e-S) diagrams and vertical distributions of different variables at six stations. Station 43 is far from the front and shows typical NACW salinity and temperature values. Station 72 shows SACW properties, and the remaining stations have intermediate values, indicating a mixture of the two water masses. An inspection of the results shows that the complex variation of currents in the vertical direction governs most of the distribution of water masses at the places situated close the front (stations 62, 65, 77). This can be se en especially at station 65, where the cores of minimum salinity and minimum oxygen (mentioned in the previous section) are in fact caused by a large intrusion of highly saline NACW that flows to the south at 300 m depth between northward-flowing SACW layers. [11] Since the dependence of the strength of double diffusive mixing on Rp is complex, the Turner angle, Tu, [Ruddick, 1983] is used in Figure 4 instead of Rp. Turner angles between -90 and - 45 ° indicate that that portion of the profile is unstable to diffusive layering, whereas Turner angles up to 45 ° indicate that the portio n of the profile is unstable to salt fingering, with the strongest activity near 90°. The Turner angles for the selected stations are shown in the third column of Figure 4. The regime of nearly constant Tu over the depth range 150-600 m in the region of undisturbed NACW (station 43) is in contrast with the situation in the frontal zone. Here, the vertical profiles of Tu show a high variability and the presence of diffusive layering regions generated by intrusions. The results for Tu al so illustrate that salt fingering in NACW is stronger than 4 of 5 L07609 MARTÍNEZ-MARRERO ET AL.: DIAPYCNAL MIXING IN CVFZ L07609 in SACW. In the SACW the values for Kd (Figure 4, sixth column) decrease strongly, especially between 200 and 400 m depth, whe re the salt-fingering decrease due to the lower vertical gradients of salinity and temperature. This result is probably a consequence of enhanced mixing in the SACW during its joumey through the equatorial current system. [1 2] Within a thermohaline front, the boundary area between two water masses increases through the occurrence of intrusions. These fe atures, which are visible as large fluctuations or inversions in the profiles, are thought to enhance double diffusive diapycnal mixing [Bartan, 1987; Kl e in and Siedler, 1995]. The calculated diffusivity coefficients shown in Figure 4 indicate that the presence of intrusions at the front increases the fluctuation of the diapycnal mixing due to double diffusion along the water column. This effect is due to the generation of layers of enhanced double diffusion: salt fingering under warm, salty intrusions and dif fusive convection abo ve; dif fusive convection under cold, fresh intrusions and salt fingering aboye. Between these two layer types, layers of di ffusive stability appear that produce the minima of K d' The results also show that large intrusions can generate important enough velocity gradients to produce gradient Richardson numbers less than one, implying strong mixing. Station 65 shows such behaviour, where maximum values of K,o of about 2 x 104 m2 S-I are obtained. Therefore, large intrusions, often observed in the past at the CVFZ, may favour diapycnal mixing induced by vertical shear in the water column to the detriment of double diffusion. The obtained results in this study suggest that estimations of diapycnal mixing in the CVFZ should inc\ude instabilities due to vertical shear, as well as double di ffusive processes. [1 3] Acknowledgments. This work has been supported by th e Spanish Ministerio de Educación y Ciencia und er the COR I CA project (REN20 0 126 49C02-0 1). References Barton, E. D. (1987), Meanders, eddies and intrusions in the thermoha1ine front off northwest Africa, Oceanol. Ac/a, 10 , 267 -283. Firing, E. , J. Ranada , and P. Ca 1d we ll (1995), Process;ng ADCP Da/a Wilh /he CODAS Sojiware Sys/em Ver s ;on 3.1, Joint In s l. for Mar. and A tmo s. Res., U ni v. of Hawaii at Manoa, Honolu1u. Harvey, J. ( 19 82), OS relationship and water masses in th e eastem North Atlantic, Deep Sea Res., 29 , 1021 -1033. Hernánd ez -Gu erra, A., E. Fraile-Nuez , F. Lóp ez -Laat zen, A. Martín ez - M arr ero, G. Parrilla, and P. Vélez-Belchí (2005), Ca nary C urr ent and North Equatorial Current from an in verse box mod el, J. Geophys. Res., 11 0, C 120 19 , doi: 1 0.1 029/2005J C0 03032. Hugh es, P. , and E. D. Barton (1974), Stratification and water ma ss analysis in the upw elling area off NW Africa in April-May 1969, Deep S ea Res., 21 , 611 -628. Ke ll ey, D. E. (1990), Fluxes through diffÍJsi ve staircases: A n ew forrnulation, J. Geophys. Res., 95 , 3365-3371. Kle in , B., and G. Siedler (1995), Isopycnal and diapycnal mixing at the Cape Verde Frontal Zone, J. Ph ys. Oceanogl: , 25 , 177 11787. Kle in , B., and M. Tomczak (1994), Identification of diap yc nal mi xing through optimum multiparameter analysis: 2. Evidence for unidirectional diapycnal mixing in th e front between North and South Atlantic Central Water, J. Geophys. Res., 99 , 25,275-25,280. Manriquez, M. , and F. Fraga (1982), The distribution of water ma sses in the upw elling region off Northwest Africa in November, Rapp. P V Re un. Cons. In/. ExplOl: Mer, 180, 39-47. M ill er, J. L., and D. L. Evans (1985), Density and velocity fine structure enhancement in oceanic eddies, J. Geophys. Res., 90,4793 - 4806. Pacanow s ki , R. c. , and S. G. H. Philander (1981), Paramete ri zation of ve rtical mixing in numerical models of tropical oceans, J. Phys. Oceanogl: , 11 , 1443 -1451. Pelegrí, J. L. , and P. Sangrá (1998), A mechanism for la ye r forrnation in stratified geophysical flows, J. Geophys. Res., 103,30,679-30,693. Rodríguez-Santana , A., J. L. Pel eg rí, P. Sangra , and A. Ma rr e roDíaz(1999), Diapycnal mixing in G ulf Stream meanders, J. Geophys. Res., 104, 25 , 89 125 ,912. Ruddick , B. R. (1983), A practical indicator of the stability of the wa ter column to double-diffusive activity, Deep Sea Res., Par/ A, 30, 11051107. Schmitt, R. W. (1988), Mixing in a thermohaline staircase, in Small-Scale Turbufence and M;x;ng;n /he Ocean, edited by J. Nihoul and B. Jamart, pp. 435 -452, Elsevier, New York. Thorpe, S. A. (2005), The Turbulent Ocean, 439 pp. C ambridg e U ni v. Pr ess, Cambridge, U.K. Tomczak, M. ( 19 81), An analysis of mi xing in th e frontal zone of South and North Atlantic Central Water off north-west Africa, Prog. Oceanogl: , 10 , 172 - 192. Zenk, w. , B. Kle in , and M. Schr6der ( 1991 ), Cape Verde Frontal Zone, Deep Sea Res., Parl A, 38 , 505-530. E. Fraile-Nuez, F. López-Laatzen, G. Parrilla, and P. Vélez-Belchí, In stituto Español de Oceanografía, E-38005 Santa Cruz de Tene rif e, Spain. A. Hernández-Guerra, A. Martínez-Marrero, and A. Rodríguez-Santana, Departamento de Física, Facultad de Ciencias del Mar , U ni versidad de Las Palmas de G. c. , Ca mpus de Tafira, E-35017 Las Palmas de Gran Canaria, Spain. ([email protected]) 5 of 5