[1] The effect of the geostrophic stirring on phytoplankton variability during the northeast Atlantic spring bloom is studied by analyzing satellite derived surface chlorophyll, sea surface temperature, and sea surface height. The calculation of unstable manifolds is used as a diagnostic of the transport properties of the geostrophic velocity field (calculated from the sea surface height). We identify two mechanisms by which the geostrophic velocity field acts on chlorophyll patterns. The first mechanism is a direct effect of the horizontal transport on already formed chlorophyll. By acting as ''sticking'' transport barriers, the unstable manifolds are shown to (1) modulate the fronts of already formed phytoplankton in lobular structures, (2) create spiralling chlorophyll anomalies within eddies, and (3) produce chlorophyll filaments. The second mechanism is an indirect effect on in situ chlorophyll production mediated by nutrient upwelling. Supported by a recent study on the vertical velocities of the northeast Atlantic (Legal et al., 2006), we argue that the horizontal unstable manifolds also shape the filamentary, vertical velocity cells, and hence the patterns of in situ produced chlorophyll through submesoscale vertical nutrient injection.

Instability in homogeneous and density stratified shear flows may be interpreted in terms of the interaction of two (or more) otherwise free waves in the velocity and density profiles. These waves exist on gradients of vorticity and density, and instability results when two fundamental conditions are satisfied: (I) the phase speeds of the waves are stationary with respect to each other (“phase-locking“), and (II) the relative phase of the waves is such that a mutual growth occurs. The advantage of the wave interaction approach is that it provides a physical interpretation to shear flow instability. This paper is largely intended to purvey the basics of this physical interpretation to the reader, while both reviewing and consolidating previous work on the topic. The interpretation is shown to provide a framework for understanding many classical and nonintuitive results from the stability of stratified shear flows, such as the Rayleigh and Fjørtoft theorems, and the destabilizing effect of an otherwise stable density stratification. Finally, we describe an application of the theory to a geophysical-scale flow in the Fraser River estuary.

International audienceUsing satellite retrievals of sea surface chlorophyll and geostrophic currents we study the evolution of a distinct chlorophyll patch transported by an Agulhas ring along a ˜1,500 km track. Throughout an ˜11 months period of the total 2 years eddy lifetime, the shape of the chlorophyll patch is consistently delimited by the horizontal transport barriers associated with the eddy. Analysis of Lagrangian time series of sea surface variables in and around the eddy suggests that the evolution of the chlorophyll patch is driven by two processes (i) slow lateral mixing with ambient waters mediated by horizontal stirring in filaments, and (ii) rapid events of wind induced vertical mixing. These results support the idea that mesoscale eddies shape biological production through the combination of horizontal and vertical dynamical processes, and emphasize the important role of horizontal eddy transport in sustaining biological production over the otherwise nutrient-depleted subtropical gyres

The optimal dynamics of conservative disturbances to plane parallel shear flows is interpreted in terms of the propagation and mutual interaction of components called counterpropagating Rossby waves ͑CRWs͒. Pairs of CRWs were originally used by Bretherton to provide a mechanistic explanation for unstable normal modes in the barotropic Rayleigh model and baroclinic two-layer model. One CRW has large amplitude in regions of positive mean cross-stream potential vorticity ͑PV͒ gradient, while the second CRW has large amplitude in regions of negative PV gradient. Each CRW propagates to the left of the mean PV gradient vector, parallel to the mean flow. If the mean flow is more positive where the PV gradient is positive, the intrinsic phase speeds of the two CRWs will be similar. The CRWs interact because the PV anomalies of one CRW induce cross-stream velocity at the location of the other CRW, thus advecting the mean PV. Although a single Rossby wave is neutral, their interaction can result in phase locking and mutual growth. Here the general initial value problem for disturbances to shear flow is analyzed in terms of CRWs. For the discrete spectrum ͑which could alternatively be described using normal modes͒, the singular value decomposition of the dynamical propagator can be obtained analytically in terms of the CRW interaction coefficient and the intrinsic CRW phase speeds. Using this formalism, optimal perturbations, the disturbances which grow fastest in a given norm over a specified time interval, can readily be found. The most natural norm for CRWs is related to air parcel displacements or enstrophy. However, if an energy norm is taken, it is shown to grow due to both mutual amplification of air parcel displacements and the untilting of PV structures ͑the Orr mechanism͒ associated with decreasing phase difference between the CRWs. A generalization of the CRW description to the optimal dynamics of the complete spectrum solution is outlined. Although the dynamics then involves the interaction between an infinite number of "CRW kernels," the form of the simple interaction between any two CRW kernels is the same as in the discrete case.

Motivated by the success of potential vorticity (PV) thinking for Rossby waves and related shear flow phenomena, this work develops a buoyancy-vorticity formulation of gravity waves in stratified shear flow, for which the nonlocality enters in the same way as it does for barotropic/baroclinic shear flows. This formulation provides a time integration scheme that is analogous to the time integration of the quasigeostrophic equations with two, rather than one, prognostic equations, and a diagnostic equation for streamfunction through a vorticity inversion.The invertibility of vorticity allows the development of a gravity wave kernel view, which provides a mechanistic rationalization of many aspects of the linear dynamics of stratified shear flow. The resulting kernel formulation is similar to the Rossby-based one obtained for barotropic and baroclinic instability; however, since there are two independent variables-vorticity and buoyancy-there are also two independent kernels at each level. Though having two kernels complicates the picture, the kernels are constructed so that they do not interact with each other at a given level.

SUMMARYIt is shown that Bretherton's view of baroclinic instability as the interaction of two counter-propagating Rossby waves (CRWs) can be extended to a general zonal ow and to a general dynamical system based on material conservation of potential vorticity (PV). The two CRWs have zero tilt with both altitude and latitude and are constructed from a pair of growing and decaying normal modes. One CRW has generally large amplitude in regions of positive meridional PV gradient and propagates westwards relative to the ow in such regions. Conversely, the other CRW has large amplitude in regions of negative PV gradient and propagates eastward relative to the zonal ow there. Two methods of construction are described. In the rst, more heuristic, method a 'home-base' is chosen for each CRW and the other CRW is de ned to have zero PV there. Consideration of the PV equation at the two home-bases gives 'CRW equations' quantifying the evolution of the amplitudes and phases of both CRWs. They involve only three coef cients describing the mutual interaction of the waves and their self-propagation speeds. These coef cients relate to PV anomalies formed by meridional uid displacements and the wind induced by these anomalies at the home-bases. In the second method, the CRWs are de ned by orthogonality constraints with respect to wave activity and energy growth, avoiding the subjective choice of home-bases. Using these constraints, the same form of CRW equations are obtained from global integrals of the PV equation, but the three coef cients are global integrals that are not so readily described by 'PV-thinking' arguments. Each CRW could not continue to exist alone, but together they can describe the time development of any ow whose initial conditions can be described by the pair of growing and decaying normal modes, including the possibility of a super-modal growth rate for a short period. A phase-locking con guration (and normal-mode growth) is possible only if the PV gradient takes opposite signs and the mean zonal wind and the PV gradient are positively correlated in the two distinct regions where the wave activity of each CRW is concentrated. These are easily interpreted local versions of the integral conditions for instability given by Charney and Stern and by Fjørtoft.

A 5 year time series of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color images (SCHL) is compared with mixed layer depths (MLD) and atmospheric forcings from the Clipper model of the North Atlantic (1998–2002). This comparison is done over the region 16°–22°W, 30°–50°N, where subpolar mode waters are formed and which overlaps the region of the 2001 Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment at sea. Three production regimes are identified on the basis of the seasonal cycling of SCHL and MLD: the well‐known subpolar and subtropical regimes and a midlatitude regime. The midlatitude regime is characterized by a single broad bloom weaker than the subpolar spring bloom and stronger than the subtropical fall bloom, which starts in fall as an entrainment bloom and peaks in spring as a restratification bloom. This specific regime is found between 35°N and 40°N (±2°) in the northeast Atlantic. It corresponds to winter MLDs between Ze (the depth of the euphotic layer) and 2Ze, i.e., it lays between the region where the winter MLD is greater than Sverdrup's critical depth (subpolar regime) and the region where the mixing is never deeper than the well‐lit layer (subtropical regime). The very specific characteristics of the midlatitude regime strengthen the biological carbon pump since production is active in winter within the waters to be subducted. The midlatitude regime also may provide an explanation for the unexpectedly low f ratios sometimes observed during the bloom in the region (North Atlantic Bloom Experiment, POMME). A large interannual variability is observed for the three regimes in terms of the timing and the intensity of the blooms and of the geographical boundaries of the regimes. These variabilities appear to be mainly driven by the synoptic and the low‐frequency atmospheric variabilities. It is also shown that in addition to the northward propagation of the subpolar spring bloom from 41°N (±1.3°) to 50°N, the (fall) entrainment bloom propagates southward over the whole latitudinal range (35°–50°N).

Powerful ‘space weather’ events caused by solar activity pose serious risks to human health, safety, economic activity and national security. Spikes in deaths due to heart attacks, strokes and other diseases occurred during prolonged power outages. Currently it is hard to prepare for and mitigate the impact of space weather because it is impossible to forecast the solar eruptions that can cause these terrestrial events until they are seen on the Sun. However, as recently reported in Nature, eruptive events like coronal mass ejections and solar flares, are organized into quasi-periodic “seasons”, which include enhanced bursts of eruptions for several months, followed by quiet periods. We explored the dynamics of sunspot-producing magnetic fields and discovered for the first time that bursty and quiet seasons, manifested in surface magnetic structures, can be caused by quasi-periodic energy-exchange among magnetic fields, Rossby waves and differential rotation of the solar interior shear-layer (called tachocline). Our results for the first time provide a quantitative physical mechanism for forecasting the strength and duration of bursty seasons several months in advance, which can greatly enhance our ability to warn humans about dangerous solar bursts and prevent damage to satellites and power stations from space weather events.