Abstract's details
Evaluating Spatio-Temporal Variability in Sea Level and Eddies in the Azores Region
CoAuthors
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: Science III: Mesoscale and sub-mesoscale oceanography
Presentation type: Type Poster
Contribution: PDF file
Abstract:
The understanding of oceanic current variability and trends holds utmost significance, not only in the context of climate change but also in addressing critical research areas such as marine litter transport, deep-sea mining activities, climate tipping points, among others. To achieve this objective, satellite altimetry emerges as a pivotal data source, providing measurements that significantly advance the understanding of ocean dynamic processes, phenomena, and their evolving patterns.
Focused on the Azores region due to the existence of hydrothermal vents, seamounts and highly sensitive ecosystems, this study intends to represent and understand the spatial and temporal variability of oceanographic variables derived from satellite altimetry (sea level anomaly - SLA, eddy kinetic energy - EKE, geostrophic velocity anomalies and the eddy field), over a 28-year period, from 1993 to 2020. The selected data were the Data Unification and Altimeter Combination System (DUACS) L4 vDT2021 gridded products, extracted from the Copernicus Marine Service (CMEMS), which allow the study of mesoscale signals in the context of ocean circulation at global and regional scales due to the inclusion of all the satellites available to compute SLA grids with respect to a twenty-year mean reference period from 1993 to 2012. Before conducting the analysis, SLA grids underwent correction to align with the study period. Subsequently, variables dependent on SLA, such as geostrophic velocity magnitude and anomaly, were recalculated, and eddy tracking was performed within the region.
From the SLA data, it was possible to identify the Azores Current (AzC) around the 34°N axis, with a striking asymmetric division between higher water levels in the south and lower water levels in the north. The time series of the SLA for the region reveals temporal variability linked to seasonal cycles, with a maximum peak during fall and a minimum peak during spring. The time series also shows the existence of SLA interannual variability, with positive trends up to 7 mm/year, considering the Glacial Isostatic Adjustment (GIA) correction.
EKE values for the AzC reveal a quasi-zonal band of maximum energy value decreasing from west to east. The shape of the band suggests that the current is not propagating perfectly zonally but tends to move north-eastwards as it spreads eastwards. The temporal variability of EKE shows periods of high activity linked to the seasonal cycle and to the zonal geostrophic currents.
Geostrophic velocity data reveal an unstable current, generating eastwards propagating meanders and eddies around its main axis. The mean geostrophic velocity of the AzC was determined to be 5 cm/s, yet this variable also exhibits interannual variability.
The analysis of the eddy field reveals that the highest count of eddies occurs during the winter season and is also closely associated with EKE. On average, the distribution of cyclonic and anticyclonic structures remains consistent over time, with comparable translation speeds of approximately 3.5 km/day for both types. Cyclones exhibit stronger amplitudes, while anticyclones have longer lifetimes, although most observed eddies were ephemeral, lasting less than 4 weeks. A distinct north-south distribution of eddies is evident along the axis of the AzC, with anticyclones predominantly found in the northern region and cyclones in the southern region. Analysis of their trajectories reveals that these structures propagate westward from their formation zone, akin to the behavior of Rossby waves, while their radii expand during propagation.
This study has contributed to depict the space-time mesoscale oceanic variability in the Azores region from multiple perspectives, revealing a highly energetic area characterized by a substantial number of mesoscale structures. To comprehend the annual and interannual variability of the altimetry-derived variables, correlation studies with meteorological parameters and climatic indices will be conducted.
Focused on the Azores region due to the existence of hydrothermal vents, seamounts and highly sensitive ecosystems, this study intends to represent and understand the spatial and temporal variability of oceanographic variables derived from satellite altimetry (sea level anomaly - SLA, eddy kinetic energy - EKE, geostrophic velocity anomalies and the eddy field), over a 28-year period, from 1993 to 2020. The selected data were the Data Unification and Altimeter Combination System (DUACS) L4 vDT2021 gridded products, extracted from the Copernicus Marine Service (CMEMS), which allow the study of mesoscale signals in the context of ocean circulation at global and regional scales due to the inclusion of all the satellites available to compute SLA grids with respect to a twenty-year mean reference period from 1993 to 2012. Before conducting the analysis, SLA grids underwent correction to align with the study period. Subsequently, variables dependent on SLA, such as geostrophic velocity magnitude and anomaly, were recalculated, and eddy tracking was performed within the region.
From the SLA data, it was possible to identify the Azores Current (AzC) around the 34°N axis, with a striking asymmetric division between higher water levels in the south and lower water levels in the north. The time series of the SLA for the region reveals temporal variability linked to seasonal cycles, with a maximum peak during fall and a minimum peak during spring. The time series also shows the existence of SLA interannual variability, with positive trends up to 7 mm/year, considering the Glacial Isostatic Adjustment (GIA) correction.
EKE values for the AzC reveal a quasi-zonal band of maximum energy value decreasing from west to east. The shape of the band suggests that the current is not propagating perfectly zonally but tends to move north-eastwards as it spreads eastwards. The temporal variability of EKE shows periods of high activity linked to the seasonal cycle and to the zonal geostrophic currents.
Geostrophic velocity data reveal an unstable current, generating eastwards propagating meanders and eddies around its main axis. The mean geostrophic velocity of the AzC was determined to be 5 cm/s, yet this variable also exhibits interannual variability.
The analysis of the eddy field reveals that the highest count of eddies occurs during the winter season and is also closely associated with EKE. On average, the distribution of cyclonic and anticyclonic structures remains consistent over time, with comparable translation speeds of approximately 3.5 km/day for both types. Cyclones exhibit stronger amplitudes, while anticyclones have longer lifetimes, although most observed eddies were ephemeral, lasting less than 4 weeks. A distinct north-south distribution of eddies is evident along the axis of the AzC, with anticyclones predominantly found in the northern region and cyclones in the southern region. Analysis of their trajectories reveals that these structures propagate westward from their formation zone, akin to the behavior of Rossby waves, while their radii expand during propagation.
This study has contributed to depict the space-time mesoscale oceanic variability in the Azores region from multiple perspectives, revealing a highly energetic area characterized by a substantial number of mesoscale structures. To comprehend the annual and interannual variability of the altimetry-derived variables, correlation studies with meteorological parameters and climatic indices will be conducted.