Upwelling systems and their productivity
Researchers: P. Marchesiello, P. Estrade, B. Dewitte, X. Capet, J. McWilliams, N. Gruber, L. Renault
Eddies and other mesoscale oceanic processes, such as fronts, can enhance biological production in the ocean, according to several open-ocean studies. The effect is thought to be particularly pronounced in low-nutrient environments, where mesoscale processes increase the net upward flux of limiting nutrients. However, eddies have been suggested to suppress production in the highly productive eastern boundary upwelling systems. The effect of eddies on tracer properties of upwelling systems was first evidenced by Marchesiello et al. (2003), Capet et al. (2008) and Marchesiello and Estrade (2009). In Gruber et al. (2011), we examined the relationship between satellite-derived estimates of net primary production, of upwelling strength, and of eddy-kinetic energy—a measure of the intensity of mesoscale activity—in the four most productive eastern boundary upwelling systems. We show that high levels of eddy activity tend to be associated with low levels of biological production, indicative of a suppressive effect. Simulations using eddy-resolving models of two of these upwelling systems support the suggestion that eddies suppress production, and show that the downward export of organic matter is also reduced. According to these simulations, the reduction in production and export results from an eddy-induced transport of nutrients from the nearshore environment to the open ocean. These conclusion are also confirmed by the work of Gutknecht et al. (2013) in the Benguela system. Eddies might have a similar effect on marine productivity in other oceanic systems that are characterized by intense eddy activity, such as the Southern Ocean.
Figure 1 (from Figure 2 of Gruber et al., 2011): Modelled impact of eddies on the distribution of primary and export production in the CalCS. Maps of model-simulated depth integrated primary production (a–c) and of organic carbon export (across 100 m) (d–f), in units of mol C m−2 yr−1 . a,d, Results from a non-eddy simulation. b,e, Results from the eddy simulation. c,f, Difference between the eddy and the non-eddy cases.
Figure 2 (from Figure 4 of Gruber et al., 2011): Conceptual diagram of the impact of mesoscale eddies on coastal circulation, nitrogen transport, and organic matter production and export. The top panel presents a non-eddy upwelling case where the circulation is only driven by Ekman transport. The bottom panel presents an eddy case where eddy advection is added to Ekman flow. Shown in blue is the total transports and circulations. The red arrows show the eddy-driven transports and (bolus) velocities. Contour lines denote potential density and green arrows the vertical export of organic matter.
Mesoscale eddies and coastal upwelling in eastern boundary systems are both resulting from the intensity of upwelling velocities and from the vertical structure of oceanic properties. Ekman transport can produce upwelling through flow divergence in the coastal area due to the coastal boundary and in the offshore area due to wind curl. In the idealized case of uniform alongshore wind and flat bottom topography, the dynamical ocean response to the offshore Ekman transport is called coastal upwelling; it results from a coastal divergence that occurs within a cross-shore length scale LCU. LCU is often confused with the Rossby radius of deformation R (Smith, 1995; Pickett and Paduan, 2003, Croquette et al., 2007). However, Estrade et al. (2008) and Marchesiello and Estrade (2010) show that the dynamic upwelling scale LCU is determined by the frictional inner shelf zone where surface and bottom Ekman layers overlap. This scale is generally much smaller than R, except in regions of large and shallow shelves (e.g., Western Sahara).
Satellite wind data (e.g. QuikSCAT product) is often used to compute coastal upwelling indices. However, there is a strong limitation in estimating Ekman pumping and coastal upwelling in the ‘blind zone’ of the satellite observation (i.e., the coastal zone where no data are available). The existence of weaker winds within a nearshore drop-off zone was first reported for the California current system (Capet el al., 2004; Dorman et al., 2006; Perlin et al., 2007). Various processes can produce wind drop-off: sharp changes of surface drag and atmospheric boundary layer at the land-sea interface (Edwards et al., 2001, Capet et al., 2004); coastal orography (Edwards et al., 2001); and SST-wind coupling (Chelton et al., 2007, Jin et al., 2009, Renault et al, 2012). These nearshore processes are difficult to assess but essential to better understand and simulate upwelling systems (Marchesiello et al., 2003; Capet et al., 2004). Nearshore wind drop-off may induce significant Ekman pumping in the coastal band while strong nearshore winds favor intense coastal upwelling. It is sometimes believed that these two effects complement each other, so that total upwelling would not depend on the profile of coastal winds (Ekman pumping from wind curl would be as efficient a process as coastal upwelling; Picket and Paduan, 2003). This proposition is contradicted by Marchesiello and Estrade, (2010), who show that the coastal upwelling scale is generally much narrower than the wind drop-off scale. Therefore, even if the total vertical transport does not depend on the coastal wind profile, peak vertical velocities do. A drop of coastal wind would increase Ekman pumping less than it would reduce coastal upwelling (in terms of vertical velocities). This would tend to reduce surface cooling as confirmed by Renault et al. (2012).
In addition, it is important to note that if both coastal upwelling and Ekman pumping can produce upwelling velocities, their action can be significantly limited by onshore geostrophic flows (or equivalently alongshore pressure gradients), as shown by Marchesiello and Estrade (2010). The limitation is of the order of –uGD/2LCU (where uG is the surface onshore geostrophic flow and D the Ekman depth) and should be added to the total upwelling velocity, i.e. to any proper upwelling index.
- Gutknecht, E., Dadou, I., Marchesiello, P., Cambon, G., Le Vu, B.,
- Gruber N., Z. Lachkar, H. Frenzel, P. Marchesiello, M. Munnich, J.C. McWilliams, T. Nagai and G.-K. Plattner, 2011: Mesoscale eddy-induced reduction in eastern boundary upwelling systems. Nature Geosciences, 4, 787–792.
- Renault L. B Dewitte, P. Marchesiello, S. Illig, V. Echevin, G. Cambon, M. Ramos and O. Astudillo, 2012: Atmospheric coastal jets off Central Chile and their oceanic impact, 2012: A modeling study of the October 2000 event. J. of Geophys. Res., 117, C02030.
- Marchesiello P., and P. Estrade, 2010: Upwelling limitation by geostrophic onshore flow. Journal of Marine Research, 68, 37-62.
- Capet X., F. Colas, P. Penven, P. Marchesiello and J.C. McWilliams, 2009: Eastern Boundary Subtropical Upwelling Systems. In: Ocean Modelling in an eddying regime, M.W. Hecht, H. Hasumi, Ed., Geophysical Monograph Series, Volume 177, 409 pp.
- Marchesiello P. and P. Estrade, 2009: Eddy activity and mixing in upwelling systems: a comparative study of Northwest Africa and California regions.International Journal of Earth Sciences, 98 (2), 299-308.
- Estrade P., P. Marchesiello, A. Colin de Verdiere, C. Roy, 2008: Cross-shelf structure of coastal upwelling : a two-dimensional expansion of Ekman's theory and a mechanism for innershelf upwelling shut down. Journal of Marine Research, 66, 589-616.
- Freon P., J. Alheit, E.D. Barton, S. Kifani, and P. Marchesiello, 2006: Modelling, forecasting and scenarios in comparable upwelling ecosystems: Californie, Canary and Humboldt. Large Marine Ecosystems, Vol 14, V. Shannon, G. Hempel, P. Malanotte-Rizzoli, C. Moloney and J. Woods (Ed.), Elsevier.
- Capet X.J. ,P. Marchesiello, and J.C. McWilliams, 2004: Upwelling response to coastal wind profiles. Geophysical Research Letters, 31 (13), L13311.
- Marchesiello P., J.C. McWilliams, and A. Shchepetkin, 2003: Equilibrium structure and dynamics of the California Current System. Journal of Physical Oceanography, 33, 753-783.