Abstract's details
Understanding Eastern Tropical Atlantic Ocean Dynamics in Relation to Climate Indices
CoAuthors
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: Science II: Large Scale Ocean Circulation Variability and Change
Presentation type: Type Oral
Contribution: PDF file
Abstract:
Satellite altimetric measurements have played a key role in enhancing the monitoring and comprehension of ocean dynamics within the context of the ongoing climate change scenario, revealing the non-uniformity of sea level changes across the globe.
It is now well established that local and regional effects, as well as modes of climate variability and long-term trends, have a significant influence over sea level that should not be undervalued. This study analyses the mean annual and seasonal ocean circulation along with mesoscale ocean dynamics in the Eastern Tropical Atlantic Ocean (ETAO) region (3°-30°N; 40°W-0°) over a 28-year period (1993-2020). Additionally, it investigates the correlation between these oceanic variables and the climate indices affecting the region. The choice of the ETAO as the study area is due to its status as a relatively understudied low-energy region within the eastern tropical ocean basins, in contrast to their more extensively examined western counterparts. Nevertheless, it is essential to investigate the dynamics, spatio-temporal variability, and trends in this region, particularly given the presence of significant areas of permanent upwelling, to gain a comprehensive understanding of the atmosphere-ocean interaction within this area.
The sea level anomaly (SLA), eddy kinetic energy (EKE) and the magnitude of the surface geostrophic currents show pronounced seasonality. Sea level change reveals a mean rising rate of 3.25 mm/year for the ETAO region (GIA correction not included), higher than the global average. However, this rate is not uniform along the study region. The higher values, around 5 mm/year, can be found at the north-eastern area around 28°-30°N and 40°-27°W, possibly associated with the southernmost part of the Azores Current. High values ranging from approximately 3.5 to 4 mm/year are also observed to the south of 10°N, within the domain influenced by the North Equatorial Counter Current (NECC). On the other hand, both the linear trends of mean EKE and surface geostrophic currents exhibit decreasing rates, in contrast to the positive linear trends observed globally. These trends seem to be strongly influenced by the main currents in the region, particularly to the south of 10°N where the NECC is located.
Significant correlations have been identified between SLA and several climate indices. The North Atlantic Oscillation (NAO), Tropical North Atlantic (TNA), Tropical South Atlantic (TSA), Atlantic Meridional Mode (AMM), Western Hemisphere Warm Pool (WHWP), and Southern Oscillation Index (SOI) have been considered. Composite maps of the metocean variability driving factors, including sea surface temperature (SST), sea level pressure (SLP) and wind, indicate that SST appears to be the primary driver explaining the correlation between SLA and climate indices. While wind also plays a role in explaining the link between SLA and certain indices, its primary influence is confined to specific areas within the study region. Conversely, the contribution of SLP to the correlations appears to be minimal, showing limited impact for certain climate indices and during certain seasons. In general, EKE anomalies for positive and negative indices phases across different seasons align with SST variability. Additionally, the magnitude of surface geostrophic current anomalies appears to be strongly influenced by the dominant currents in the region, particularly the NECC.
This study has contributed to enhancing the understanding of the temporal and spatial variability of SLA, EKE, and surface geostrophic currents in the ETAO. It reveals a pronounced positive trend in SLA and points out the strong interaction between the atmosphere and the ocean within this region.
It is now well established that local and regional effects, as well as modes of climate variability and long-term trends, have a significant influence over sea level that should not be undervalued. This study analyses the mean annual and seasonal ocean circulation along with mesoscale ocean dynamics in the Eastern Tropical Atlantic Ocean (ETAO) region (3°-30°N; 40°W-0°) over a 28-year period (1993-2020). Additionally, it investigates the correlation between these oceanic variables and the climate indices affecting the region. The choice of the ETAO as the study area is due to its status as a relatively understudied low-energy region within the eastern tropical ocean basins, in contrast to their more extensively examined western counterparts. Nevertheless, it is essential to investigate the dynamics, spatio-temporal variability, and trends in this region, particularly given the presence of significant areas of permanent upwelling, to gain a comprehensive understanding of the atmosphere-ocean interaction within this area.
The sea level anomaly (SLA), eddy kinetic energy (EKE) and the magnitude of the surface geostrophic currents show pronounced seasonality. Sea level change reveals a mean rising rate of 3.25 mm/year for the ETAO region (GIA correction not included), higher than the global average. However, this rate is not uniform along the study region. The higher values, around 5 mm/year, can be found at the north-eastern area around 28°-30°N and 40°-27°W, possibly associated with the southernmost part of the Azores Current. High values ranging from approximately 3.5 to 4 mm/year are also observed to the south of 10°N, within the domain influenced by the North Equatorial Counter Current (NECC). On the other hand, both the linear trends of mean EKE and surface geostrophic currents exhibit decreasing rates, in contrast to the positive linear trends observed globally. These trends seem to be strongly influenced by the main currents in the region, particularly to the south of 10°N where the NECC is located.
Significant correlations have been identified between SLA and several climate indices. The North Atlantic Oscillation (NAO), Tropical North Atlantic (TNA), Tropical South Atlantic (TSA), Atlantic Meridional Mode (AMM), Western Hemisphere Warm Pool (WHWP), and Southern Oscillation Index (SOI) have been considered. Composite maps of the metocean variability driving factors, including sea surface temperature (SST), sea level pressure (SLP) and wind, indicate that SST appears to be the primary driver explaining the correlation between SLA and climate indices. While wind also plays a role in explaining the link between SLA and certain indices, its primary influence is confined to specific areas within the study region. Conversely, the contribution of SLP to the correlations appears to be minimal, showing limited impact for certain climate indices and during certain seasons. In general, EKE anomalies for positive and negative indices phases across different seasons align with SST variability. Additionally, the magnitude of surface geostrophic current anomalies appears to be strongly influenced by the dominant currents in the region, particularly the NECC.
This study has contributed to enhancing the understanding of the temporal and spatial variability of SLA, EKE, and surface geostrophic currents in the ETAO. It reveals a pronounced positive trend in SLA and points out the strong interaction between the atmosphere and the ocean within this region.