sign in sign up
<<
Home , Azores Current, Portugal Current

Modern Environmental Science and Engineering (ISSN 2333-2581) August 2019, Volume 5, No. 8, pp. 754-759 Doi: 10.15341/mese(2333-2581)/08.05.2019/011 Academic Star Publishing Company, 2019 www.academicstar.us

Portuguese Maritime Space: A Study about Currents and Water Masses, with a Climatology and Software Approach

Nádia Nogueira Marques, and Carlos Lopes da Costa NRP João Roby, Escola Naval and Centro de Investigação Naval (CINAV), Portuguese Navy

Abstract: There are many phenomena that characterize the water masses in the Portuguese maritime space: regional phenomena such as the Mediterranean outflow water, or the and Counter Current, and more extensive phenomena such as the subtropical gyre. The thermal and salinity analysis, and consequent sound velocity characteristics and density of these waters allows us to understand better their behavior in the geographic space. In this study, we tried to get a visual perspective of the different types of water masses and their circulation, from developed software, and with the more updated data available. The Mediterranean Outflow Water, the Azores Current and Countercurrent are clearly shown. However, it’s not clear or consistent the Slope Current and the Portugal and Canary currents.

Key words: Mediterranean outflow water, Azores current, database

45°N and the longitudes 040°W-005°W is shown in 1. Introduction  Fig. 1, including the general bathymetry From an oceanographic point of view, the representation. geographical area of Portuguese maritime space can be 2. Water Masses Distribution and characterized by its current’s dynamics, diverse water Characterization masses and consequent hydrological phenomena such as mesoscale eddies. The currents of general 2.1 Mediterranean Outflow Water circulation in this zone are associated to the northeast In the subtropical North Atlantic, the characteristics part of the subtropical gyre, encompassing currents that stand out the most are the properties, such as the North Atlantic Drift Current, the Portugal well-marked in high degree of salinity and temperature, and the Canary currents. Also associated to these being its cause coming from the Mediterranean “saline currents are the changes caused by the outflow and tongue” [1]. The surface water that comes from the inflow of water out and into the Mediterranean Sea, , and that progressively enters the which consequently influence all the behaviour of Mediterranean Sea, begins to undergo successive peripheral water masses. Based on the created software transformations in its composition. According to and climatology knowledge, it is tried to find these Kinder and Bryden (1987), apud Bozec (2011) [2], in structures. The study area between the latitudes 30°N- the Mediterranean Sea, there is an high evaporation rate which leads to the production of a water with about Corresponding author: Carlos Lopes da Costa, Master of 2.1 more practical salinity units (psu), and Science in Physical , Navy Captain; research area/interest: . E-mail: consequently a saline and denser water. It is in the [email protected].

Portuguese Maritime Space: A Study about Currents and Water Masses, 755 with a Climatology and Software Approach

Strait of Gibraltar that a flow of Mediterranean Water and along the continental slope, probably transporting (MOW) is seen, and this one is denser in comparison it as far as the Porcupine Bank (50°N), west of Ireland with the surrounding Atlantic Ocean water, reason for [2]. descending to about 1000 m depth, along the 2.2 Azores Current and Countercurrent continental slope, on the eastern part of the Cadiz’s Gulf (Fig. 2). The Azores Current (AC) is a branch of the Gulf In trying to understand the behavior of MOW along Stream and flows southeast to the Atlantic Middle the Portuguese coast, were found saline lenses [1]. Ocean Ridge at about 34°N-37°N, Azores’ southwest These coherent and energetic vortices, better known as [3]. According to Pailet and Mercier (1997) apud Kida meddies, contain large amounts of water from (2007) [4], as the AC approaches the eastern ocean Gibraltar’s outflow, distinguished by their warm and boundary, it bifurcates into a branch to the south that salty content, up to 1 psu and 4.0°C higher than their ends up joining the , and another branch surrounding environment. that flows to the Cadiz’s Gulf, being investigated its The MOW, after reaching its neutral fluctuation, extension from the Newfoundland Rise. near 008°W, ends up being intensified by the Slope Transportation to the east of the (Corrente da Vertente in portuguese), or between 32°N and 35°N by AC is observed at about Mediterranean undercurrent. This current, near St. 10-12 Sv, most of which is concentrated in the upper Vicent Cape, Southwest Portugal, continues northwards 1000 meters layer of the ocean, at speeds exceeding 10 cm/s [3]. On the opposite direction, to the west, and on the north side of the AC, between 37° and 38° N, Onken (1993), apud Kida (2007) [4], was the first to suggest the existence of another zonal current, Azores Counter Current (ACC), with a transport of 2 to 8 Sv and well pronounced in the layer of 0-800 meters. Stramma and Muller (1989), apud Kida (2007) [4] observed a maximum velocity of 7 cm/s for the ACC. Some other studies indicate that there is a dependence Fig. 1 Study area representation between latitudes between this and the Mediterranean Sea, also related to 30°N-45°N and longitudes 040°W-005°W, including general bathymetry. the concept of beta-plume dynamics introduced by Stommel (1982), apud Volkov (2010) [3]. According to L. da Costa (2016) [5], it is believed that the beta-plume mechanism, associated with the AC and ACC, begins to be topographic beta-plume along the slope in the northeast of Cadiz's Gulf that, with the horizontal turbulent friction, for example associated with meddies, extends to offshore and west of St. Vicent Cape, with the bathymetry losing control of the plume, passing to be controlled by the planetary

Fig. 2 Inflow and outflow process representation of vorticity gradient. Mediterranean Water1.

1 http://www.euroargo-edu.org/floatdata.php?float=6900701.

756 Portuguese Maritime Space: A Study about Currents and Water Masses, with a Climatology and Software Approach

2.3 Portugal Coastal Dynamics study is comprised between latitudes 30°N and 45°N, and longitudes 040°W and 005°W (Fig. 1), and the data The currents system, in Portugal coastal maritime was extracted from the World Ocean Database 2018 area, is poorly defined due to interactions between the (WOD18) of the National Oceanic and Atmospheric coast and the large-scale currents, the ocean floor Administration (NOAA), since it is globally topography and the water masses. Taking this into considered one of the largest climate data files. account, we choose to use the guidelines used by Perez et al. (2001) and Martins et al. (2002), apud Aguiar 4. Graphic Interpretation (2011) [6], considering that this system extends from A general panorama analysis within the the 36°N to 46°N of latitude, and from the Iberian investigation area is shown in Fig. 3, having been coasts to the 024°W of longitude. This currents’ system calculated the average, maximum and minimum of the is mainly supported by the subtropical gyre zone, and it four variables under study, as the depth increases. stands between the and Counter Examining Fig. 3a, the maximum values of the Current, this also known as “Corrente da Vertente”, thermal signal between 700 and 1500 meters vary being its northern limit the North Atlantic Current, and between 13.0°C and 5.0°C, with evidence of a its southern limit the AC. maximum relative at about 1400 meters. As for Fig. 3b, In this area, the salinity and temperature in the upper the maximum salinity value varies between 550 and 100 meters, vary respectively between 35.8-36.0 psu 1500 meters with values ranging from 36.2-35.3 g/kg, and between 14.0°C-19.0°C, depending on the but with a relative maximum at depth of about 1200 (cooler) or (warmer) meters with 36.6 g/kg, evidencing the MOW presence. conditions. The drawn conclusion from these values’ interpretation 3. Data and Computational Programming is that MOW has a presence from 600 to 1500 meters, and its core is between 1100 m and 1300 meters depth. The temperature and salinity data acquired allowed In Fig. 3c, a practically linear continuity in the us to analyze and interpret the results from different density vertical profile is evident, with the less dense perspectives. The salinity (g/kg) has an influential role waters superimposed on the denser ones. In Fig. 3d, as in the density of water masses: the density increasing consequence of the changes in temperature and salinity with increasing salinity and pressure and decreasing seen above, it also suffers variations in its behavior by with increasing temperature. Density (kg/m3) is an emphasizing two sound channels (centered on the extremely important variable because it is related to the relative minimums of sound velocity): the upper one force that causes the internal circulation in the ocean, between 500 and 1200 meters with a relative minimum by influencing the buoyancy forces and vertical of 1509 m/s at 650 meters, and the lowest between motions, and the horizontal pressure gradients forces 1200 and 3100 meters with a relative minimum of 1502 and horizontal motions. The temperature (°C) in the m/s at 2000 meters. An immersion analysis, at fixed ocean upper layers, up to 500-1000 meters deep, in the levels, was also performed, thus allowing a horizontal low and medium latitudes is the parameter that most assessment of the water masses along the longitude and influences the density. As the acoustic energy latitude. propagation velocity (m/s), practically doesn’t change In Figs. 4a and 4b, high N/S gradients are visible, with the salinity, temperature and pressure changes are varying between 15.5°C-22.0°C at the surface and most influent for changes in sound velocity. 13.0°C-19.0°C at 100 meters, being notorious the For this study, we choose to use the Matlab software presence of lower temperatures in the north and along for graphic interpretation. The chosen area for this

Portuguese Maritime Space: A Study about Currents and Water Masses, 757 with a Climatology and Software Approach

the Portuguese coast, which can be justified by the coastal upwelling — the rise of deeper, colder waters. Even at these depths it’s possible to verify low E/W gradients to the north, which increase greatly to the south and southeast. At the Fig. 4c, we can see that both gradients (N/S and E/W) decrease, since that at this depth there is no more influence of the meteorological or thermodynamic agents at the surface. The temperature (a) at this immersion level varies between 11.0°C to the north and northeast and 13.5°C to the southwest of the area. In Fig. 4d, there is a temperature rise for the 11.0°C near the coast that extends to the north beyond 45°N,

(b)

(a) (b)

(c)

(c) (d)

(d) Fig. 3 Graphical results of the Matlab program, (e) (f) Fig. 4 Graphical results of the Matlab program, included representing the average (blue), maximum (orange) and the research area. Temperature distribution (°C) at depths: minimum (grey) values of vertical profiles of temperature a), a) surface, b) 100 m, c) 500 m, d) 1000 m, e) 1500 m and f) salinity b), density c) and speed of sound d) variables 2000 m. included in the research area.

758 Portuguese Maritime Space: A Study about Currents and Water Masses, with a Climatology and Software Approach and to the west with remarkable presence up to 025°W, directions predominate in this area. In Fig. 5a, between with 8.0°C. As for Fig. 4e, there is already a fading of latitudes 32°N-35°N with eastward direction and with the thermal signal, with 8°C at the coast, which values reaching up to 0.1 m/s in longitudes 32°N-35°N, additionally fade to 020°W with 6.0°C. This its signal being vanished with the approach to the temperature behaviour is MOW indicative. The coastal zone, what appears to be AC. As for Fig. 5b, we thermal characteristic evolution, from 1000 meters to can see a decrease of the AC signal to values of 0.08 1500 meters, helps the conclusion that MOW core is m/s between 028°-035°W, and here we can already closer to 1000 meters and not so much to 1500 meter, notice the presence of an AC contrary current, north of giving strength to the assertion that this nucleus this one and westbound, the ACC. remains between 1100 meters and 1300 meters. In Fig. 5c, the AC present signal is small, however, With the previous graph’s analysis, the stratification the ACC contour between 35°N-37°N latitudes is components of the , along with the depth, already well delineated. are generally perceptible. For a complete notion of the The vertical structure of the AC (33°N-36°N) is very water masses behaviour, geostrophic currents were clear and ACC (36°N-37°N), though less intense is quite computed, relative to 1500 dbar, and analyzed in the evident at subsurface. Both are well documented in the N/S and E/W components. vertical cross section of the E/W component of the As for the N/S component, both at the surface and geostrophic flow, at longitude 011°W, shown in Fig. 6. along the depth, it is difficult to see if there is any coherence and consistency in its direction. However, with respect to the E/W component, it is already clearly discernible which structures of the currents and

(a) (b)

Fig. 6 Vertical cross section of the E/W component of the , relative to 1500 dbar, at longitude 011.5°W, where are depicted the CA and CCA.

4. Conclusion

Throughout this study, it was verified that the main water masses present in this study area, is MOW, at

(c) (d) intermediate levels, giving its thermal and salt unique Fig. 5 Graphical results of the Matlab program. Analysis characteristics. Beyond this, the AC and the ACC are of the geostrophic current, relative do 1500 dbar, in the E/W evident and relevant in , a persistent component, along the research area, in the different and important climatological feature. The southward immersion levels: a) surface, b) 100 m, c) 500 m and d) 1000 m depth. flowing Portugal Current followed by the Canary

Portuguese Maritime Space: A Study about Currents and Water Masses, 759 with a Climatology and Software Approach

Current, and the northward flowing Slope Current [2] Bozec Alexandra et al., On the variability of the Mediterranean outflow water in the North Atlantic form (“Corrente da Vertente”) are not clear in the 1948 to 2006, Journal of Geophysical Research 116 (2011) geostrophic calculation plots from the data. 1-18. This methodology supported by climatological data [3] Volkov Denis L., On the reasons for the formation and is good to represent the average flow, but is not variability of the Azores Current, Journal of Physical Oceanography 40 (2010) 2197-2220. adequate to resolve the important and energetic [4] Kida Shinichiro, Price James F. and Yang Jiayan, The mesoscale time and space varying features. upper-oceanic response to overflows: A mechanism for the Azores Current, Journal of Physical Oceanography 38 Acknowledgement (2007) 880-894. [5] Lopes Da Costa C., Mecanismo da pluma β e a sua Thanks to the Portuguese Naval Academy (Escola aplicação o sistema da Corrente e Contracorrente dos Naval) and the Naval Research Centre (CINAV) for Açores, in: Actas das 4as Jornadas de Engenharia providing and supporting research activities. Hidrográfica, 21 a 23 de Junho de 2016, Instituto Hidrográfico, Almada , Escola Naval, 2016, pp. 133-136. References [6] Aguiar A. C. and Barbora et al., Zonal struture of the mean flow and eddies in the Azores Current System, Journal of [1] Bower Amy S., Ambar Isabel and Serra Nuno, Structure Geophysical Research 116 (2011) 53-67. of the Mediterranean Undercurrent and Mediterranean

Water spreading around the southwestern Iberian Peninsula, Journal of Geophysical Research 107 (2002) 3-19.