Final Report Summary - LAGAPOCE (Lagrangian approach to understand upper ocean processes)
A summary description of the project objectives
The objective of this project was to advance in the understanding of the physical processes that govern the upper-ocean circulation of the oceanic subtropical gyres, by focusing in two complementary systems, the Kuroshio Current (outgoing phase) and the Canary Current (return phase), the western and eastern components of the north Pacific and Atlantic subtropical gyres.
The particular scientific objectives were approached by an innovative methodology that combines surface lagrangian velocities and satellite data. Surface lagrangian observations provide nowadays the most accurate and feasible representation of surface ocean circulation at open and coastal ocean. This methodology, recently developed by the outgoing research group, will permit to obtain the first detailed analysis of the actual surface circulation patterns of both areas.
A description of the work performed since the beginning of the project,
The outgoing phase was carried in one of the world’s most prestigious ocean research organizations, the Scripps Institution of Oceanography (USA). Data analyses and evaluation of results was jointly carried out with scientist with wide expertise in the proposed scientific and methodological topics, and in the framework of current multidisciplinary experiments of the outgoing host group. The specific objectives of the outgoing phase, regarding the mechanisms that induce the Kuroshio to make large intrusions in the continental shelf of the East China Sea and the role of the mesoscale structures incoming from the western north tropical in the variability of the Kuroshio, have been fulfilled. The results are included in a scientific article entitled “Eddy-induced Kuroshio intrusions in the East China Sea shelf” in revision after being submitted to the Journal of Marine Research in July 2012. The third specific objective regarding the detailed description of the surface circulation of the Kuroshio in its southern part has been also fulfilled and included in the manuscript above mentioned. In that sense, an interesting feature was observed in relation with the RyuKyu current, between Taiwan and Japan. The surface drifters clearly show like this current is part of the recirculation system of the Kuroshio. A separate study about this current is being carried out. This study will contribute to the overall scientific objective of understanding the physical processes that govern the upper ocean circulation of the oceanic subtropical gyres, because recirculation plays a main role in the subtropical gyres. With the aim of finding if there was a different pattern for the eddies impinging to the Kuroshio all the eddies observed using altimetry in the area since 1992 were analyzed.
The return phase was carried in the Spanish Institute of Oceanography. Data analyses and evaluation of results was jointly carried out with the coordinator of this project and with scientist from the University of Las Palmas de Gran Canaria. The know-how of the use of lagragian data and altimeter data has been applied. The specific objectives of the outgoing phase, regarding the high resolution regional description of the of unbiased mean surface currents, eddy kinetic energies, vorticity, divergence and eddy diffusivity, to explore the components of the Canary Current systems and the its partition between its open ocean and coastal components have been partially fulfilled. The most interesting result was the subject of a scientific communication to the America Geophysical Union 2013 Fall meting entitled “The Canary Deep Poleward Undercurrent” and a note in the Argo program “Float of the month - June 2012: Riding the Canary deep Poleward Undercurrent. In this contribution a new undercurrent was described as part of the Canary Current system. Although undercurrents are common to all eastern upwelling system, the particularly of the Canary Deep Poleward Undercurrent is that it flows at 1000m, and therefore the mechanism driving it should be different from that driving the more shallower poleward flows in the other eastern upwelling system. The hypothesis establish in this scientific communication is that the pressure gradient created by the encounter of the Mediterranean outflow waters and the Antarctic intermediate waters build up the Poleward flow, in the same way that the meridional large scale pressure gradient at the surface levels drive the shallower Poleward flows in the other eastern upwelling system. The average depth where the Mediterranean outflow waters and the Antarctic intermediate waters is 1000 m. The results of this analysis were compared with a numerical model and are being drafted into a scientific paper.
Additionally a new hypothesis regarding the surface circulation in the Canary Basis has been established. In this hypothesis Rossby waves propagating from the Eastern Atlantic will be reflected in the mean circulation. In order to verify the hypothesis and since the Rossby waves are due to changes in the wind stress curl, the hypotheses was tested again the extreme changes that occurred in the central Atlantic between 1998-2004. The results indicate that up to90% of the variance can be attributed to the wind stress forcing, however the results are very sensitive to the wind product used.
Regarding the second specific objective: “To what extent the vorticity balance may be used to gain insight into the physical mechanisms that determine the Azores, Portugal and the Canary Currents has not been explored since a different mechanism”, the preliminary analysis indicated that vorticity balance was not the tool to gain insight into the physical mechanisms that determine the Azores current. Instead, data from model simulations under two different wind forcings was compared with the unbiased surface velocity field obtained to achieve the first objective. The results indicate that the Azores current is divided in two parts, east and west of the Azores archipelago, and the vertical distribution of properties indicated that two different mechanisms might contribute to this current. In on side, the beta-plume effect associate to the outflow of Mediterranean waters from the strait of Gibraltar, and in the other the modification of the flow, also through the beta-plume effect, associated to the Azores Archipelago.
The main results achieved so far and description of the expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).
The main results achieved are:
(1) The description of the different mechanisms that contribute to the intrusion of the Kuroshio into the continental shelf and the development of a proxy to predict the occurrence of these intrusions. This proxy, built up with sea level data from altimetry, permit a probabilistic estimation of the occurrence of a Kuroshio intrusion since eddies originated in the western Pacific can be observed in altimetry several weeks before they imping against the Kuroshio. Given the importance of the fishing grounds found in the area any tool that help to predict changes in the hydrodynamics conditions will have strong socio-economic impacts.
(2) The description of the Canary Deep Poleward Undercurrent, and the hypothesis regarding its driving mechanism.
The objective of this project was to advance in the understanding of the physical processes that govern the upper-ocean circulation of the oceanic subtropical gyres, by focusing in two complementary systems, the Kuroshio Current (outgoing phase) and the Canary Current (return phase), the western and eastern components of the north Pacific and Atlantic subtropical gyres.
The particular scientific objectives were approached by an innovative methodology that combines surface lagrangian velocities and satellite data. Surface lagrangian observations provide nowadays the most accurate and feasible representation of surface ocean circulation at open and coastal ocean. This methodology, recently developed by the outgoing research group, will permit to obtain the first detailed analysis of the actual surface circulation patterns of both areas.
A description of the work performed since the beginning of the project,
The outgoing phase was carried in one of the world’s most prestigious ocean research organizations, the Scripps Institution of Oceanography (USA). Data analyses and evaluation of results was jointly carried out with scientist with wide expertise in the proposed scientific and methodological topics, and in the framework of current multidisciplinary experiments of the outgoing host group. The specific objectives of the outgoing phase, regarding the mechanisms that induce the Kuroshio to make large intrusions in the continental shelf of the East China Sea and the role of the mesoscale structures incoming from the western north tropical in the variability of the Kuroshio, have been fulfilled. The results are included in a scientific article entitled “Eddy-induced Kuroshio intrusions in the East China Sea shelf” in revision after being submitted to the Journal of Marine Research in July 2012. The third specific objective regarding the detailed description of the surface circulation of the Kuroshio in its southern part has been also fulfilled and included in the manuscript above mentioned. In that sense, an interesting feature was observed in relation with the RyuKyu current, between Taiwan and Japan. The surface drifters clearly show like this current is part of the recirculation system of the Kuroshio. A separate study about this current is being carried out. This study will contribute to the overall scientific objective of understanding the physical processes that govern the upper ocean circulation of the oceanic subtropical gyres, because recirculation plays a main role in the subtropical gyres. With the aim of finding if there was a different pattern for the eddies impinging to the Kuroshio all the eddies observed using altimetry in the area since 1992 were analyzed.
The return phase was carried in the Spanish Institute of Oceanography. Data analyses and evaluation of results was jointly carried out with the coordinator of this project and with scientist from the University of Las Palmas de Gran Canaria. The know-how of the use of lagragian data and altimeter data has been applied. The specific objectives of the outgoing phase, regarding the high resolution regional description of the of unbiased mean surface currents, eddy kinetic energies, vorticity, divergence and eddy diffusivity, to explore the components of the Canary Current systems and the its partition between its open ocean and coastal components have been partially fulfilled. The most interesting result was the subject of a scientific communication to the America Geophysical Union 2013 Fall meting entitled “The Canary Deep Poleward Undercurrent” and a note in the Argo program “Float of the month - June 2012: Riding the Canary deep Poleward Undercurrent. In this contribution a new undercurrent was described as part of the Canary Current system. Although undercurrents are common to all eastern upwelling system, the particularly of the Canary Deep Poleward Undercurrent is that it flows at 1000m, and therefore the mechanism driving it should be different from that driving the more shallower poleward flows in the other eastern upwelling system. The hypothesis establish in this scientific communication is that the pressure gradient created by the encounter of the Mediterranean outflow waters and the Antarctic intermediate waters build up the Poleward flow, in the same way that the meridional large scale pressure gradient at the surface levels drive the shallower Poleward flows in the other eastern upwelling system. The average depth where the Mediterranean outflow waters and the Antarctic intermediate waters is 1000 m. The results of this analysis were compared with a numerical model and are being drafted into a scientific paper.
Additionally a new hypothesis regarding the surface circulation in the Canary Basis has been established. In this hypothesis Rossby waves propagating from the Eastern Atlantic will be reflected in the mean circulation. In order to verify the hypothesis and since the Rossby waves are due to changes in the wind stress curl, the hypotheses was tested again the extreme changes that occurred in the central Atlantic between 1998-2004. The results indicate that up to90% of the variance can be attributed to the wind stress forcing, however the results are very sensitive to the wind product used.
Regarding the second specific objective: “To what extent the vorticity balance may be used to gain insight into the physical mechanisms that determine the Azores, Portugal and the Canary Currents has not been explored since a different mechanism”, the preliminary analysis indicated that vorticity balance was not the tool to gain insight into the physical mechanisms that determine the Azores current. Instead, data from model simulations under two different wind forcings was compared with the unbiased surface velocity field obtained to achieve the first objective. The results indicate that the Azores current is divided in two parts, east and west of the Azores archipelago, and the vertical distribution of properties indicated that two different mechanisms might contribute to this current. In on side, the beta-plume effect associate to the outflow of Mediterranean waters from the strait of Gibraltar, and in the other the modification of the flow, also through the beta-plume effect, associated to the Azores Archipelago.
The main results achieved so far and description of the expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).
The main results achieved are:
(1) The description of the different mechanisms that contribute to the intrusion of the Kuroshio into the continental shelf and the development of a proxy to predict the occurrence of these intrusions. This proxy, built up with sea level data from altimetry, permit a probabilistic estimation of the occurrence of a Kuroshio intrusion since eddies originated in the western Pacific can be observed in altimetry several weeks before they imping against the Kuroshio. Given the importance of the fishing grounds found in the area any tool that help to predict changes in the hydrodynamics conditions will have strong socio-economic impacts.
(2) The description of the Canary Deep Poleward Undercurrent, and the hypothesis regarding its driving mechanism.