Seasonal Ekman upwelling in the Southwest Sumba from INDESO Model

Southwest Sumba water is part of the Indonesian fisheries management region (WPP573). Marine fisheries resources are influenced by oceanographic phenomena such as an upwelling event. This study aims to describe characteristics of seasonal Ekman upwelling by analyzing oceanographic parameters from the validated INDESO model output (2008-2014). It shows that upwelling event in the study area occurs during the Southeast Monsoon period, which creates an Ekman drift of 0.26 Sv towards offshore. This transported water mass is then replaced by an upwelled vertical flow of sub-surface colder and nutrient-rich water at the velocity of the order of 10−4 m/s. Surface features of the upwelling event are seen from a minimum temperature (24.3 °C), sea level anomaly (0.34 m), but the maximum of chlorophyll-a (3.02 mg/m3). During this time, an uplifted isotherm of 25.5 °C is found from sub-surface to 10 m depth, but it is outcropped at the sea surface in the centre of upwelling area. Interestingly, during upwelling event, salinity stratification revealed an isohaline of 34.10 psu is much deeper at 40 m depth, and much fresher water mass from the Ombai Indonesian Throughflow water is dominant. Averaged temperature-based upwelling index between June-September is about 0.3 °C.

Can we use sea surface temperature and productivity proxy records to reconstruct Ekman upwelling?

Marine sediments have greatly improved our understanding of the climate system, but their interpretation often assumes that certain climate mechanisms operate consistently over all timescales of interest and that variability at one or a few sample sites is representative of an oceanographic province. In this study, we test these assumptions using modern observations in an idealized manner mimicking paleo-reconstruction to investigate whether sea surface temperature and productivity proxy records in the Southern California Current System can be used to reconstruct Ekman upwelling. The method uses extended empirical orthogonal function (EEOF) analysis of the covariation of alongshore wind stress, chlorophyll, and sea surface temperature as measured by satellites from 2002 to 2009. We find that EEOF1 does not reflect an Ekman upwelling pattern but instead much broader California Current processes. EEOF2 and 3 reflect upwelling patterns, but these patterns are timescale dependent and regional. Thus, the skill of using one site to reconstruct the large-scale dominant patterns is spatially dependent. Lastly, we show that using multiple sites and/or multiple variables generally improves field reconstruction. These results together suggest that caution is needed when attempting to extrapolate mechanisms that may be important on seasonal timescales (e.g., Ekman upwelling) to deeper time but also the advantage of having multiple proxy records.

Dynamics of Upwelling Annual Cycle in the Equatorial Pacific Ocean

A recent work proposed a simple theory based on the framework of Zebiak–Cane (ZC) ocean model, and successfully characterized the equatorial Atlantic upwelling annual cycle as a combination of the local wind-driven Ekman upwelling and nonlocal wind-driven wave upwelling. In the present work, utilizing the same simple framework, we examined the fidelity of the upwelling Pacific annual cycle using observations and simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We demonstrated that the theoretical upwelling annual cycles generally match the original upwelling annual cycles in the equatorial Pacific in both observations and CMIP5 simulations. Therefore, this simple formulation can be used to represent the upwelling annual cycle in the equatorial Pacific. Observationally, the equatorial Pacific upwelling annual cycle is dominated by the local wind-driven Ekman upwelling, while the remote wave upwelling is confined near the eastern boundary with little contribution. In CMIP5 simulations, though the theoretical-reconstructed upwelling well-reproduces the original upwelling, the contribution is totally different compared to the observation. The wave upwelling serves as the main contributor instead of the Ekman upwelling. We further demonstrated that such discrepancy is attributable to the bias of the central to eastern equatorial thermocline depth patterns. This amplified, westward-shift wave upwelling weakened the impacts of the Ekman upwelling, and contributes to the entire Pacific equatorial upwelling annual cycle substantially. This implies that a realistic simulation of the equatorial Pacific upwelling annual cycle in models is very sensitive to the careful simulation of the equatorial thermocline depth annual evolutions.

Can we use sea surface temperature and productivity proxy records to reconstruct Ekman Upwelling?

Marine sediments have greatly improved our understanding of the climate system, but their interpretation often assumes that certain climate mechanisms operate consistently over all timescales of interest and that variability at one or few sample sites is representative of an oceanographic province. In this study, we test these assumptions using modern observations in an idealized manner mimicking paleo-reconstruction to investigate whether sea surface temperature and productivity proxy records in the Southern California Current System can be used to reconstruct Ekman upwelling. The method uses Extended Empirical Orthogonal Function (EEOF) analysis of covariation of alongshore windstress, chlorophyll and sea surface temperature as measured by satellites from 2002 to 2009. We find that EEOF1 does not reflect an Ekman upwelling pattern, but instead much broader California Current processes. EEOF2 and 3 reflect upwelling patterns, but these patterns are timescale dependent and are regional. Thus, the skill of using one site to reconstruct the large scale dominant patterns is spatially dependent. Lastly, we show that using multiple sites and/or multiple variables generally improve field reconstruction. These results together suggest caution is needed when attempting to extrapolate mechanisms that may be important on seasonal time scales (e.g. Ekman upwelling) to deeper time, but also the advantage of having multiple proxy records.

Subantarctic and Polar Fronts of the Antarctic Circumpolar Current and Southern Ocean Heat and Freshwater Content Variability: A View from Argo

Argo profiling floats initiated a revolution in observational physical oceanography by providing numerous, high-quality, global, year-round, in situ (0–2000 dbar) temperature and salinity observations. This study uses Argo’s unprecedented sampling of the Southern Ocean during 2006–13 to describe the position of the Antarctic Circumpolar Current’s Subantarctic and Polar Fronts, comparing and contrasting two different methods for locating fronts using the same dataset. The first method locates three fronts along dynamic height contours, each corresponding to a local maximum in vertically integrated shear. The second approach locates the fronts using specific features in the potential temperature field, following Orsi et al. Results from the analysis of Argo data are compared to those from Orsi et al. and other more recent studies. Argo spatial resolution is not adequate to resolve annual and interannual movements of the fronts on a circumpolar scale since they are on the order of 1° latitude (Kim and Orsi), which is smaller than the resolution of the gridded product analyzed. Argo’s four-dimensional coverage of the Southern Ocean equatorward of ~60°S is used to quantify variations in heat and freshwater content there with respect to the time-mean front locations. These variations are described during 2006–13, considering both pressure and potential density ranges (within different water masses) and relations to wind forcing (Ekman upwelling and downwelling).

Water Mass Transformations Driven by Ekman Upwelling and Surface Warming in Subpolar Gyres

The Sverdrup relationship when applied to the Southern Ocean suggests that some isopycnals that are deep in the eastern Pacific will shoal in the Atlantic. Cold waters surfacing in the South Atlantic at midlatitudes would be warmed by the atmosphere. The potential for water mass transformations in this region is studied by applying a three-layer planetary geostrophic model to a wide ocean basin driven by the Ekman upwelling typical of the Southern Ocean surface winds. The model uses a simple physically based parameterization of the entrainment of mass into the surface layer with zonally symmetric atmospheric surface fields to find steady-state subpolar gyre solutions. The solutions are found numerically by specifying suitable boundary conditions and integrating along characteristics. With reasonable parameter settings, transformations of more than 10 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water between layers are obtained. The water mass transformations are sensitive to the strength of the wind stress curl and the width of the basin and relatively insensitive to the parameterization of the surface heat fluxes. On the western side of the basin where the cold waters are near the surface, there is a large region where there is a local balance between the Ekman pumping and the exchange of mass between layers. Simple formulas are derived for the water mass transformation rates in terms of the difference between the maximum and minimum northward Ekman transports integrated across the basin and the depths of the isopycnal layers on the eastern boundary. The relevance of the model to the Southern Ocean and the Atlantic meridional overturning circulation is briefly discussed.

Satellite Observations of Mesoscale Eddy-Induced Ekman Pumping

Three mechanisms for self-induced Ekman pumping in the interiors of mesoscale ocean eddies are investigated. The first arises from the surface stress that occurs because of differences between surface wind and ocean velocities, resulting in Ekman upwelling and downwelling in the cores of anticyclones and cyclones, respectively. The second mechanism arises from the interaction of the surface stress with the surface current vorticity gradient, resulting in dipoles of Ekman upwelling and downwelling. The third mechanism arises from eddy-induced spatial variability of sea surface temperature (SST), which generates a curl of the stress and therefore Ekman pumping in regions of crosswind SST gradients. The spatial structures and relative magnitudes of the three contributions to eddy-induced Ekman pumping are investigated by collocating satellite-based measurements of SST, geostrophic velocity, and surface winds to the interiors of eddies identified from their sea surface height signatures. On average, eddy-induced Ekman pumping velocities approach O(10) cm day−1. SST-induced Ekman pumping is usually secondary to the two current-induced mechanisms for Ekman pumping. Notable exceptions are the midlatitude extensions of western boundary currents and the Antarctic Circumpolar Current, where SST gradients are strong and all three mechanisms for eddy-induced Ekman pumping are comparable in magnitude. Because the polarity of current-induced curl of the surface stress opposes that of the eddy, the associated Ekman pumping attenuates the eddies. The decay time scale of this attenuation is proportional to the vertical scale of the eddy and inversely proportional to the wind speed. For typical values of these parameters, the decay time scale is about 1.3 yr.

Wind-induced upwelling in the Kerguelen Plateau region

In contrast to most of the Southern Ocean, the Kerguelen Plateau supports an unusually strong spring chlorophyll (Chl a) bloom, likely because the euphotic zone in the region is supplied with higher iron concentrations. This study uses satellite wind, sea surface temperature (SST), and ocean color data to explore the impact of wind-driven processes on upwelling of cold (presumably iron-rich) water to the euphotic zone. Results show that, in the Kerguelen region, cold SSTs correlate with high wind speeds, implying that wind-mixing leads to enhanced vertical mixing. Cold SSTs also correlate with negative wind-stress curl, implying that Ekman pumping can further enhance upwelling. In the moderate to high eddy kinetic energy (EKE) regions surrounding Kerguelen, we find evidence of coupling between winds and SST gradients associated with mesoscale eddies, which can locally modulate the wind-stress curl. This coupling introduces persistent wind-stress curl patterns and Ekman pumping around these long-lived eddies, which may modulate the evolution of Chl a in the downstream plume far offshore. Close to the plateau, this eddy coupling breaks down. Kerguelen has a significant wind shadow on its downwind side, which changes position depending on the prevailing wind and which generates a wind-stress curl dipole that shifts location depending on wind direction. This leads to locally enhanced Ekman pumping for a few hundred kilometers downstream from the Kerguelen Plateau; Chl a values tend to be more elevated in places where wind-stress curl induces Ekman upwelling than in locations of downwelling, although the estimated upwelling rates are too small for this relationship to derive from direct effects on upward iron supply, and thus other processes, which remain to be determined, must also be involved in the establishment of these correlations. During the October and November (2011) KErguelen Ocean and Plateau compared Study (KEOPS-2) field program, wind conditions were fairly typical for the region, with enhanced Ekman upwelling expected to the north of the Kerguelen Islands.