Role of the circulation on the anthropogenic CO 2 inventory in the North-East Atlantic: A climatological analysis

Climatology-based storage rate of anthropogenic CO2 (Cant, referred to year 2000) in the North-East Atlantic (53 ± 9 kmol s−1, 0.020 ± 0.003 Pg-C yr−1) is described on annual mean terms. Cant advection (32 ± 14 kmol s−1) occurs mostly in the upper 1800 m and contributes to 60% of the Cant storage rate. The Azores and Portugal Currents act as ‘Cant streams’ importing 389 ± 90 kmol s−1, most of which recirculates southwards with the Canary Current (−214 ± 34 kmol s−1). The Azores Counter Current (−79 ± 36 kmol s−1) and the northward-flowing Mediterranean Water advective branch (−31 ± 12 kmol s−1) comprise secondary Cant export routes. By means of Cant transport decomposition, we find horizontal circulation to represent 11% of the Cant storage rate, while overturning circulation is the main driver (48% of the Cant storage rate). Within the domain of this study, overturning circulation is a key mechanism by which Cant in the upper layer (0–500 dbar) is drawdown (74 ± 14 kmol s−1) to intermediate levels (500–2000 dbar), and entrained (37 ± 7 kmol s−1) into the Mediterranean Outflow Water to form Mediterranean Water. This newly formed water mass partly exports Cant to the North Atlantic at a rate of −39 ± 9 kmol s−1 and partly contributes to the Cant storage in the North-East Atlantic (with up to 0.015 ± 0.006 Pg-C yr−1). Closing the Cant budget, 40% of the Cant storage in the North-East Atlantic is attributable to anthropogenic CO2 uptake from the atmosphere (21 ± 10 kmol s−1)

Memoria de las actividades de validación euleriana y lagrangiana

85 pages.-- Radares de alta frecuencia en el observatorio RAIA, Actividad 2: Gestión y Validación de la Información, Entregable 2.10 - Memoria de las actividades de validación euleriana y lagrangianaLa obtención de medidas precisas de las corrientes marinas constituye un desafío debido a su alta variabilidad espacio-temporal, resultado de la influencia de diversos forzamientos: las mareas, la descarga fluvial de agua dulce, el viento, etc. Las medidas de corrientes proporcionadas por instrumentos convencionales tales como correntímetros, ADCPs o boyas de deriva, aunque útiles, presentan ciertas limitaciones en términos de resolución espacial y su precisión está sujeta al estado dinámico del mar. Por ello, en las últimas décadas se han realizado notables esfuerzos para desarrollar técnicas alternativas tales como la detección remota de mapas 2D de corrientes superficiales en regiones costeras con ayuda de radares HF (High Frequency o de alta frecuencia)Este documento ha sido cofinanciado por el Fondo Europeo de Desarrollo Regional FEDER a través del Programa Interreg V-A España-Portugal (POCTEP) 2014-2020N

Cambios temporales en los transportes y la distribución de las masas de agua a lo largo de la sección 20º W de CAIBOX (Atlántico NE)

24 páginas, 3 tablas, 7 figuras[EN] The CAIBOX cruise was conducted from 25 July to 14 August 2009. Three consecutive transects (zonal, meridional, and transverse) formed a closed box to the west of the Strait of Gibraltar. This study aimed to analyze the thermohaline properties, volume transports, and water mass distributions (percentages) along the meridional section (30–41.5º N, 20º W). We identified the main geostrophic current (Azores Current) and its associated volume transport and interannual changes. Data from previous cruises (AZORES I, A16N, CLIVAR, OACES, and CHAOS) with similar tracks were employed to compare with the CAIBOX meridional section. All but one (CHAOS) were summer cruises. We estimated a mean transport for the Azores Current at 20º W of 9.3 ± 2.6 Sv. There appears to be an inverse relation between the position of this current and its associated transport, with relatively high (low) transports when the current is located roughly south (north) of 35º N. Regarding water masses, an increase of 14.4% was found for Mediterranean Water compared with the 1993, 1998, and 2003 cruises; however, Labrador Sea Water decreased its contribution and southward spreading between 1998 and 2009[ES] Entre el 25 de julio y el 14 de agosto de 2009 se llevó a cabo la campaña CAIBOX, que constó de tres transectos consecutivos (zonal, occidental y transversal) conformando una caja cerrada al oeste del estrecho de Gibraltar. El objetivo principal del presente trabajo fue el estudio de las propiedades termohalinas, los transportes (en términos de volumen) y la distribución de las masas de agua (en porcentajes) a lo largo de la sección occidental (30–41.5º N, 20º W). Se identificó la corriente geostrófica principal (Corriente de las Azores), el transporte asociado a ella y sus cambios interanuales. Adicionalmente se consideraron los datos de cinco campañas anteriores (AZORES I, A16N, CLIVAR, OACES y CHAOS) con secciones similares a las de la campaña CAIBOX. Todas excepto una (CHAOS) fueron campañas estivales. El transporte medio para la Corriente de las Azores a 20º W se estimó en 9.3 ± 2.6 Sv. Parece existir cierta relación inversa entre la posición de dicha corriente y su transporte asociado, con transportes relativamente altos (bajos) cuando la corriente se sitúa al sur (norte) de 35º N. En cuanto a las masas de agua, se observa un incremento de 14.4% del Agua Mediterránea en comparación con las campañas de 1993, 1998 y 2003; sin embargo, el Agua de Labrador ha disminuido su contribución y su propagación hacia el sur entre 1998 y 2009.An important part of the data presented in this paper was obtained through the CAIBEX project: Shelf-ocean Exchanges in the Canaries-Iberia Large Marine Ecosystem (CTM2007-66408-C02/MAR), supported by the Spanish Ministry of Education and Science. The first author, LIC, was funded by a FPU predoctoral fellowship from the National Human Resources Formation Program, within the framework of the 2008–2011 National Scientific Research, Development and Technological Innovation Plan of the Spanish Ministry of Education. The fourth author, NVR, was supported by the Isabel Barreto Program (IN840E) from the Xunta de Galicia.Peer reviewe

Surface waters of the NW Iberian margin: upwelling on the shelf versus outwelling of upwelled waters from the Rías Baixas

A set of hydrographic surveys were carried out in the Ría of Vigo (NW Spain) at 2–4 d intervals during four 2–3 week periods in 1997, covering contrasting seasons. Residual exchange fluxes with the adjacent shelf were estimated with a 2-D, non-steady-state, salinity–temperature weighted box model. Exchange fluxes consist of a steady-state term (dependent on the variability of continental runoff) and a non-steady-state term (dependent on the time changes of density gradients in the embayment). More than 95% of the short-time-scale variability of the exchange fluxes in the middle and outer ría can be explained by the non-steady-state term that, in turns, is correlated (R2>75%) with the offshore Ekman transport. Conversely, 96% of the variability of exchange fluxes in the inner ría rely on the steady-state term. The outer and middle ría are under the direct influence of coastal upwelling, which enhances the positive residual circulation pattern by an order of magnitude: from 10 2 to 10 3 m3s−1. On the contrary, downwelling provokes a reversal of the circulation in the outer ría. The position of the downwelling front along the embayment depends on the relative importance of Ekman transport (Qx, m3s−1km−1) and continental runoff (R, m3s−1). When Qx/ R>7±2 the reversal of the circulation affects the middle ría. Our results are representative for the ‘Rías Baixas’, four large coastal indentations in NW Spain. During the upwelling season (spring and summer), 60% of shelf surface waters off the ‘Rías Baixas’ consist of fresh Eastern North Atlantic Central Water (ENACW) upwelled in situ. The remaining 40% consists of upwelled ENACW that previously enters the rías and it is subsequently outwelled after thermohaline modification. During the downwelling season (autumn and winter), 40% of the warm and salty oceanic subtropic surface water, which piled on the shelf by the predominant southerly winds, enters the rias

Seasonal dynamics in the Azores–Gibraltar Strait region: A climatologically-based study

Annual and seasonal mean circulations in the Azores–Gibraltar Strait region (North-Eastern Atlantic) are described based on climatological data. An inverse box model is applied to obtain absolute water mass transports consistent with the conservation of volume, salt and heat and the equations of the thermal wind. The large-scale gyre circulation (Azores Current, Azores Counter Current, Canary Current and Portugal Current) is well-represented in climatological data. The Azores Current annual mean transport was estimated to be 6.5 ± 0.8 Sv (1 Sv = 106 m3/s) eastward, exhibiting a seasonal signal with minimum transport in the spring (5.3 ± 0.8 Sv) and maximum transport in autumn (7.3 ± 0.8 Sv). The Azores Current transport is twice that of the Azores Counter Current in spring and autumn and is four-times higher in summer and winter. The southward Portugal and Canary Currents show similar seasonal cycles with maximum transports in spring (3.5 ± 0.6 and 6.6 ± 0.4 Sv, respectively). The overturning circulation within the area has an annual mean magnitude of 2.2 ± 0.1 Sv and two seasonal extremes; the highest in summer (2.6 ± 0.1 Sv) and the lowest in winter (1.7 ± 0.1 Sv). Of the annual mean, about two thirds (1.4 Sv) of the overturning circulation results from water mass transformation west of the Strait of Gibraltar: the downwelling and recirculation of upper Central Water (0.6 Sv) in the intermediate layer, the entrainment of Central Water (0.6 Sv) into the Mediterranean Outflow and the contribution of Antarctic Intermediate Water (0.2 Sv) to the Mediterranean Outflow. The remaining 0.8 Sv relates to the overturning in the Mediterranean Sea through the two-layer exchange at the Gibraltar Strait. Accordingly, the density level dividing the upper-inflowing and lower-outflowing limbs of the overturning circulation was found to be σ1 = 31.65 kg m−3 (σ1, potential density referred to 1000 db), which is above the isopycnal that typically separates Central and Mediterranean Water (σ1 = 31.8 kg m−3). In terms of water masses, we describe quantitatively the water mass composition of the main currents. Focusing on the spread of Mediterranean Water, we found that when the northward Mediterranean Water branch weakens in spring and autumn, the westward Mediterranean Water vein strengthens, and vice versa. The maximum net transports of Mediterranean Water across the northern and western sections of the box were estimated at −1.9 ± 0.6 Sv (summer) and −0.8 ± 0.2 Sv (spring), respectively. Within the error bar (0.2 Sv), we found no significant net volume transport of Mediterranean Water across the southern section

Quasi-synoptic transport, budgets and water mass transformation in the Azores–Gibraltar Strait region during summer 2009

We describe the circulation patterns in the Azores–Gibraltar Strait region (North-Eastern Atlantic) during the 2009 CAIBOX cruise on the basis of hydrographic and direct current velocity measurements. This study offers new data for a region where importation of central waters (subpolar and subtropical modes of Eastern North Atlantic Central Water) and exports of Mediterranean Water are strongly related to large-scale dynamics in the North Atlantic Ocean (Azores Current-Mediterranean Water system). The description is backed up quantitatively by the results of a box inverse model, which was used to obtain absolute water mass transport values consistent with thermal wind equations and with conservation of volume, salt and heat. The contributions of water masses were determined in an extended Optimum Multiparameter Analysis from a quasi-synoptic point of view, providing detail in addition to volume, salt and heat transport. The surface–subsurface large-scale current system in the region consists of the Azores Current (13.1 ± 2.5 Sverdrup [Sv], 1 Sv = 106 m3 s−1), the Azores Counter-Current (5.2 ± 2.1 Sv), the Portugal Current (4.5 ± 1.4 Sv) and the Canary Current (7.1 ± 1.1 Sv). Broadly speaking, central waters are imported into the CAIBOX region at a rate of 1.6 ± 0.9 Sv, and Mediterranean Water is exported at a rate of 1.5 ± 0.4 Sv. The downwelling of central waters west of Gibraltar Strait was quantified at 1.1 Sv. Not all this volume participates in MW formation, but 0.8 Sv of entrained central waters; of which 0.5 Sv are from central waters of subpolar origin and 0.3 Sv from subtropical central waters. Of the 4.9 Sv of subtropical central waters advected by the Azores Current, about 0.7 Sv would reach the Gulf of Cadiz region either to take part in central water entrainment or to flow across the Gibraltar Strait as part of the Atlantic inflow to the Mediterranean Sea

Rapid phytoplankton response to wind forcing influences productivity in upwelling bays

Bays are often ecological hotspots within highly-productive eastern boundary upwelling systems. Though the physics of such bays are well understood, there is no consensus about the factors underlying their high productivity. Three weeks of high-temporal-resolution observations in two long, narrow bays (Rías Baixas, NW-Iberia), showed that during an upwelling pulse, deep, nutrient-rich isopycnals rose into the euphotic zone inside the rías in a few hours. The response of the isopycnals to changes in wind forcing is approximately three times faster inside the rías than the Ekman spin-up time, triggering rapid nutrient uptake and subsequent formation of a subsurface chlorophyll and production maximum. The tight coupling and rapid response of phytoplankton growth to wind forcing could explain the higher productivity of the rías, and also be at play in other upwelling bays with similar morphologies and orientations. Resolving short-term variability of physical–biological coupling is crucial to discern the future evolution of upwelling bays.Xunta de Galicia | Ref. ED481A‐2019/288Agencia Estatal de Investigación | Ref. CTM2016-75451-C2-1-

Origin and fate of a bloom of Skeletonema costatum during a winter upwelling/downwelling sequence in the Ría de Vigo (NW Spain)

Original research paperThe onset, development and decay of a winter bloom of the marine diatom Skeletonema costatum was monitored during a 10 d period in the coastal upwelling system of the Rı´a de Vigo (NW Spain). The succession of upwelling, relaxation and downwelling-favorable coastal winds with a frequency of 10 –20 d is a common feature of the NW Iberian shelf. The onset of the bloom occurred during an upwelling-favorable 1⁄2 wk period under winter thermal inversion conditions. The subsequent 1⁄2 wk coastal wind relaxation period allowed development of the bloom (gross primary production reached 8gCm–2 d–1) utilizing nutrients upwelled during the previous period. Finally, downwelling during the following 1⁄2 wk period forced the decay of the bloom through a combination of cell sinking and downward advection.Financial support came from the Spanish Ministerio de Ciencia y Tecnologı´a (MCyT) grant REN2000-0880-C02-01 and Xunta de Galicia grant PGIDT01MAR40201PN; a fellowship from the MCyT and the I3P-CSIC Program.Versión del editor0,98

Tide and wind coupling in a semienclosed bay driven by coastal upwelling

The Ría de Vigo is a semi-enclosed bay in which tidal residual currents are associated with coastal upwelling events. Both upwelling and downwelling favourable winds generate a bidirectional exchange flow with the shelf – a two-layer circulation with surface waters leaving (entering) the ria and a compensating inflow (outflow) through the bottom layer under upwelling (downwelling) conditions. This vertical circulation changes the vertical density structure inside the ria. In the ria, the tide is mainly semidiurnal (M2, S2 and K2), with some energy in the diurnal band (K1). Our velocity observations show that the vertical structure of the tidal currents in the ria do not exhibit a classic barotropic profile with a bottom boundary layer beneath uniform “free-stream” flow as the tidal bottom boundary layer is affected by stratification. This links tidal circulation to the wind-driven residual circulation, since the latter also greatly helps to control the stratification. We quantify this effect by fitting tidal ellipses to observed velocities through the water column. In addition to this indirect coupling through stratification, there is a direct interaction in which velocities in the upper and bottom layers are best correlated with winds while the mid-water velocities are best correlated with tides. These wind-tide interactions are expected to play a key role in the resuspension and transport of nutrients and phytoplankton in the Ria.CTM2012-3515