Wind-induced Ekman Transport Research Articles

Interdecadal changes in SST variability drivers in the Senegalese-upwelling: the impact of ENSO

Sea surface temperature (SST) variability in the North Eastern Tropical Atlantic has its center of action in the Senegalese–Mauritanian upwelling system, where its drivers are wind-induced ocean dynamics and air–sea thermodynamic processes. Thus, a better understanding of the local wind variations, together with their predictability, contributes to a more comprehensive assessment of the SST variability in that region. In this study, we use monthly data from two ocean reanalyses, SODA and ORAS-5, and a regional forced-ocean simulation to characterize the interannual SST variability off the Senegalese Coast in the common period 1960–2008. Local indices of the mixed layer heat budget during the major upwelling season (February–March–April) exhibit pronounced interannual to decadal variability. We demonstrate that the local interannual SST variability undergoes inter-decadal fluctuation and concomitant changes in its local and remote drivers. Off-Senegal SST variability was largely controlled by wind-induced Ekman transport during the 1960s–1970s, acting under favorable thermocline and mixed layer conditions. However, from 1980s onwards, the drastically reduced Ekman impact observed on local SSTs is associated with a deeper thermocline. This shift in the effectiveness of the dynamic mechanisms coincides with a more active ENSO teleconnection with upwelling before the 1980s. An extended SODA record reveals that the multidecadal modulator of the ENSO impact on the North-eastern Tropical Atlantic resembles the negative phase of the Atlantic Multidecadal Variability. Our results bring to light the fundamental role played by the global decadal background state in the activation of the drivers and air-sea mechanisms responsible for generating the interannual off-Senegal SST variability.

Inter-annual variability of biogeography-based phytoplankton seasonality in the Arabian Sea during 1998–2017

Long-term multi-satellite-based DINEOF-reconstructed chlorophyll-a data are utilized to generate an updated biogeography of phytoplankton seasonality and to decipher the environmental controls in the Arabian Sea (AS). As in published information, the biogeographical clusters and biological boundary of phytoplankton seasonality are mostly organized and stable in four separated provinces, but also suffering significant yearly biogeographical discrepancies. Four distinct clusters are identified that correspond to the summer-peak upwelling cluster (17.5% of AS area), winter-peak entrainment cluster (17.4%), tropical flat cluster (33.7%), and transitional cluster (31.4%), appearing regularly in the AS with their boundaries tightly linked to the coast lines. Considerable inter-annual variability of the biogeography of seasonality is evident with a significant instability that occurred in the interior AS. The biogeographical shifts observed are in agreement with surface advection, upwelling-favorable wind and wind-induced entrainment anomalies. Influences of wind mixing power on the distribution of summer-peak/transitional taxa of the central AS in 2006/2009/2016 are hypothesized. The most spatial changes of the summer-peak clusters in 2015 and 2016 are strongly linked to the co-occurred extreme positive and negative IOD&ENSO events. The IOD-/ENSO-related wind-induced Ekman transport and local/remote forced upwelling Rossby waves are consolidated and combined to generate the observed unprecedented biogeographical variability in the southern tip of India in 2015 and 2016.

Structure of Submesoscale Fronts of the Mississippi River Plume

AbstractSubmesoscale currents (SMCs), in the forms of fronts, filaments, and vortices, are studied using a high-resolution (~150 m) Regional Oceanic Modeling System (ROMS) simulation in the Mississippi River plume system. Fronts and filaments are identified by large horizontal velocity and buoyancy gradients, surface convergence, and cyclonic vertical vorticity with along-coast fronts and along-plume-edge filaments notably evident. Frontogenesis and arrest/destruction are two fundamental phases in the life cycle of fronts and filaments. In the Mississippi River plume region, the horizontal advective tendency induced by confluence and convergence plays a primary role in frontogenesis. Confluent currents sharpen preexisting horizontal buoyancy gradients and initiate frontogenesis. Once the fronts and filaments are formed and the Rossby number reaches O(1), they further evolve frontogenetically mainly by convergent secondary circulations, which can be maintained by different cross-front momentum balance regimes. Confluent motions and preexisting horizontal buoyancy gradients depend on the interaction between wind-induced Ekman transport and the spreading plume water. Consequently, the direction of wind has a significant effect on the temporal variability of SMCs, with more active SMCs generated during a coastally downwelling-favorable wind and fewer SMCs during an upwelling-favorable wind. Submesoscale instabilities (~1–3 km) play a primary role in the arrest and fragmentation of most fronts and filaments. These instabilities propagate along the fronts and filaments, and their energy conversion is a mixed barotropic–baroclinic type with horizontal-shear instabilities dominating.

Lagrangian pathways in the southern Benguela upwelling system

The effect of ocean currents on fish eggs and larvae during their journey from spawning to nursery grounds in the Southern Benguela upwelling system is poorly understood. The survival and successful transport of fish eggs and larvae results from complex biological and physical processes. This study focuses on the advective processes, more specifically on how the dynamics and characteristics of the ocean currents shape the Lagrangian pathways in the Southern Benguela. A mesoscale eddy resolving interannual (1989–2010) simulation of the region, with a horizontal resolution of 7.5 km, is used to study processes impacting the connectivity between the western edge of the Agulhas Bank and the west coast upwelling region. A set of Lagrangian experiments are conducted with particles being released within the top 100 m of the water column along an across-shore transect off Cape Point (34∘ S). Transport success is given by the ratio of the number of particles that reach St Helena Bay (32∘ S) over the total number of particles released. The model results show a strong seasonal cycle in transport success which is governed by the complex three-dimensional structure of the along-shore jets, their variability, together with the wind-induced Ekman drift. Transport success is most efficient in spring when the Benguela Jet consists of one coherent intensified single-core branch that flows over the 300 m isobath, and when wind-induced Ekman transport favours the retention of particles within the jet. At this time of the year, the pathway leading to successful transport is located inshore, with 90% of particles released inshore the 300 m isobath being successfully transported to St Helena Bay in <15 days. This pathway is also characterized by low eddy kinetic energy values. During the upwelling season (December–March), transport success becomes less efficient, and less sensitive to the initial across-shore position of the particles. The inshore route no longer dominates, as the majority of particles follow offshore pathways. The Benguela Jet shifts offshore and splits into several branches due to the shoaling of the poleward undercurrent. The entrainment of particles within the offshore branch of the jet is favored by the dominating offshore wind-induced Ekman transport. Particles trapped in the offshore branch get exposed to higher mesoscale variability. Their northward progression is slower, which leads to journeys generally exceeding 20 days. This study shows that successful transport from the Agulhas Bank to the west coast upwelling region cannot be attributed to only a simple wind induced modulation of the jet. It explores how the seasonal modulation of the Benguela Jet, poleward undercurrent and offshore Ekman transport combine together with the turbulent off-shelf eddy field to set-up the characteristics of transport success.

On the Possible Mechanisms for Saltening of the Bay of Bengal

&#x0D; &#x0D; &#x0D; The Bay of Bengal (BoB) is a low saline basin owing to large influx of freshwater from precipitation and river runoff. To maintain the salt balance of the BoB, the incessant lowering of salinity is to be balanced by the inflow of saltier water into the basin. In the present work, various processes that contribute to the saltening of the BoB, viz. coastal upwelling, eddies and their interaction, lateral advection from Arabian Sea and tropical cyclones are discussed. In the near-shore regions, the coastal upwelling due to wind induced Ekman transport plays a dominant role in increasing the surface salinity. On the other hand, in the open ocean, the divergence induced by eddies and their mutual interaction contributes significantly to the salt water pumping. In the southern BoB, the advection from the Arabian Sea increases the salinity. The formation of cyclones in the BoB also leads to an increase in the surface salinity. However, the magnitude of saltening of the Bay due to these processes varies from north to south. The uplift of saltier water from subsurface levels increases the salinity in the surface layers thereby creating a salinity gradient and a salinity front.&#x0D; &#x0D; &#x0D;

Mississippi waters reaching South Florida reefs under no flood conditions: synthesis of observing and modeling system findings

In August 2014, in situ measurements revealed an intense salinity drop impacting South Florida coral reefs, between Pulley Ridge (Southwest Florida Shelf) and the Florida Keys. The low salinity waters had a surface signal of 32 (down from 35.2) and extended over a 15–20-m deep lens. Satellite observations showed that this abrupt drop in salinity was due to a southeastward export of Mississippi River waters from the Northern Gulf of Mexico (GoM), revealing strong interaction between coastal and oceanic flows. Unlike previous events of marked long-distance Mississippi water export, this episode is not associated with Mississippi flooding conditions, which makes it a unique study case. We have developed a high-resolution (~2 km) comprehensive hydrodynamic numerical model of the GoM to study the conditions that controlled the 2014 Mississippi River water export episode. It is based on the Hybrid Coordinate Ocean Model (HYCOM) and assimilates remotely sensed altimetry and sea surface temperature observations, to ensure that the simulated upper-ocean is realistic. This regional model has a detailed representation of coastal physics (especially river plume dynamics) and employs high-frequency river discharge and atmospheric forcing. The combined use of the simulation and observations reveals a unique pathway that brought Mississippi waters first eastward along the Northern GoM continental shelf, under prevailing winds and the presence of an anticyclonic Loop Current eddy, then southward along the edge of the West Florida Shelf, before reaching the deep GoM. Unlike usually observed, the offshore advection of Mississippi River waters thus took place far from the Delta area, which is another specificity of the 2014 episode. Finally, in the Florida Straits, Mississippi waters were advected from the deep ocean to the continental shelf under the influence of both deep sea (particularly a cyclonic Loop Current frontal eddy) and shelf flows (wind-induced Ekman transport). The simulation, in tandem with data, thus helped analyze processes that are likely to affect the connectivity between reefs in the southern Florida region (Florida Keys, Dry Tortugas, Pulley Ridge) and remote areas (Mississippi Delta), as well as the local connectivity between neighboring reefs.

Weakening of the Kuroshio Intrusion into the South China Sea over the Past Two Decades

Abstract Inferred from the satellite and in situ hydrographic data from the 1990s and 2000s, the Kuroshio intrusion into the South China Sea (SCS) had a weakening trend over the past two decades. Associated with the weakened Kuroshio intrusion, the Kuroshio loop and eddy activity southwest of Taiwan became weaker, whereas the water above the salinity minimum became less saline in the northern SCS. The sea surface height southwest of Taiwan increased at a slower rate compared to other regions of the SCS because of the weakened Kuroshio intrusion. Simulations using the Regional Ocean Modeling System (ROMS) Pacific model show that the strength of the Kuroshio intrusion into the SCS decreased from 1993 to 2010 with a negative trend, −0.24 sverdrups (Sv) yr−1 (1 Sv ≡ 106 m3 s−1), in the total Luzon Strait transport (LST). Although wind-induced Ekman transport through the Luzon Strait became weaker, the magnitude at 0.001 Sv yr−1 was too small to compensate for the negative trend of the LST. On the other hand, the piling up of the water induced by monsoon winds was an important mechanism for changing the pressure gradient across the Luzon Strait and eventually affecting the LST. The sea level gradient between the western Pacific and the SCS had a negative trend, −0.10 cm yr−1, corresponding to a negative trend in the geostrophic transport at −0.20 Sv yr−1. The Kuroshio transport east of Luzon Island also had a negative trend, which might also be linked to the weakening Kuroshio intrusion.

One mechanism contributing to co-variability of the Atlantic inflow branches to the Arctic

The two-branched inflow of warm and saline Atlantic Water to the Arctic is the major contributor of oceanic heat to the Arctic climate system. However, while the Atlantic Water entering the Arctic through the Fram Strait retains a large part of its heat as it flows along the Arctic continental slope, the branch flowing through the shallow Barents Sea releases a substantial amount of heat to the atmosphere. Hence, the pathway of the Atlantic Water into the Arctic to a large degree determines the short term fate of its heat. Here we show events in which the relative strengths of the two branches are affected by wind-induced Ekman-transport off the northern Barents Sea shelf. The resulting decrease in sea surface height induces a cyclonic circulation anomaly along the slope encircling the northern Barents Sea shelf area, which enhances the flow through the Barents Sea while weakening the branch flowing along the Arctic continental slope.

Variability of river plumes off Northwest Iberia in response to wind events

The Western Iberian Buoyant Plume (WIBP) is a low-salinity lens formed by river discharge and continental run-off extending along the shelf off Northwest Iberia. The variability of this structure is evaluated with a numerical model forced by real meteorological data and climatologic river discharge during late 2002, when conditions were those of a typical autumn. The direction and intensity of the wind-induced Ekman transport, but also the previous conditions and the duration of the event are found to determine plume behavior. We have identified three characteristic situations: a) confinement of the plume to the coast during downwelling — southerly-winds, b) expansion of the plume during the declining phase of the downwelling event by relaxation of the wind, and c) expansion of the plume by upwelling — northerly-winds. The short time scale of the response of the plume (1–3 h) adds timing between wind events and the phase of the tide as an additional source of variability. In all cases the Iberian Poleward Current (IPC), a saltier and warmer poleward current flowing over the slope, responds as well to wind changes. Furthermore, our simulations illustrate how topography and differences in the river discharge induce local differences in dynamics. Comparisons to available observations show a reasonable model skill. Differences between wind measurements and wind forcing applied to the model appear to be a major source of uncertainty in model results.

Variability of the western Galician upwelling system (NW Spain) during an intensively sampled annual cycle. An EOF analysis approach

The key features of the western Galician shelf hydrography and dynamics are analyzed on a solid statistical and experimental basis. The results allowed us to gather together information dispersed in previous oceanographic works of the region. Empirical orthogonal functions analysis and a canonical correlation analysis were applied to a high-resolution dataset collected from 47 surveys done on a weekly frequency from May 2001 to May 2002. The main results of these analyses are summarized bellow. Salinity, temperature and the meridional component of the residual current are correlated with the relevant local forcings (the meridional coastal wind component and the continental run-off) and with a remote forcing (the meridional temperature gradient at latitude 37°N). About 80% of the salinity and temperature total variability over the shelf, and 37% of the residual meridional current total variability are explained by two EOFs for each variable. Up to 22% of the temperature total variability and 14% of the residual meridional current total variability is devoted to the set up of cross-shore gradients of the thermohaline properties caused by the wind-induced Ekman transport. Up to 11% and 10%, respectively, is related to the variability of the meridional temperature gradient at the Western Iberian Winter Front. About 30% of the temperature total variability can be explained by the development and erosion of the seasonal thermocline and by the seasonal variability of the thermohaline properties of the central waters. This thermocline presented unexpected low salinity values due to the trapping during spring and summer of the high continental inputs from the River Miño recorded in 2001. The low salinity plumes can be traced on the Galician shelf during almost all the annual cycle; they tend to be extended throughout the entire water column under downwelling conditions and concentrate in the surface layer when upwelling favourable winds blow. Our evidences point to the meridional temperature gradient acting as an important controlling factor of the central waters thermohaline properties and in the development and decay of the Iberian Poleward Current.