Abstract. The temporal evolution of a surface chlorophyll a bloom sampled in the western tropical South Pacific during the 2015 Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise is examined. This region is usually characterized by largely oligotrophic conditions, i.e. low concentrations of inorganic nutrients at the surface and deep chlorophyll a maxima. Therefore, the presence of a surface bloom represents a significant perturbation from the mean ecological state. Combining in situ and remote sensing datasets, we characterize both the bloom's biogeochemical properties and the physical circulation responsible for structuring it. Biogeochemical observations of the bloom document the bloom itself, a subsequent decrease of surface chlorophyll a, significantly reduced surface phosphate concentrations relative to subtropical gyre water farther east, and a physical decoupling of chlorophyll a from a deep nitracline. All these characteristics are consistent with nitrogen fixation occurring within the bloom. The physical data suggest surface mesoscale circulation is the primary mechanism driving the bloom's advection, whereas balanced motions expected at submesoscales provide little contribution to observed flow. Together, the data provide a narrative where subtropical gyre water can produce significant chlorophyll a concentrations at the surface that is stirred, deformed, and transported great distances by the mesoscale circulation. In this case, for the time period considered, the transport is in an easterly direction, contrary to both the large-scale and mean mesoscale flow. As a result, future studies concerning surface production in the region need to take into account the role complex mesoscale structures play in redistributing subtropical gyre water.

Abstract. The patterns of the large-scale, meso- and submesoscale surface circulation on biogeochemical and biological distributions are examined in the western tropical South Pacific (WTSP) in the context of the OUTPACE cruise (February–April 2015). Multi-disciplinary original in situ observations were achieved along a zonal transect through the WTSP and their analysis was coupled with satellite data. The use of Lagrangian diagnostics allows for the identification of water mass pathways, mesoscale structures, and submesoscale features such as fronts. In particular, we confirmed the existence of a global wind-driven southward circulation of surface waters in the entire WTSP, using a new high-resolution altimetry-derived product, validated by in situ drifters, that includes cyclogeostrophy and Ekman components with geostrophy. The mesoscale activity is shown to be responsible for counter-intuitive water mass trajectories in two subregions: (i) the Coral Sea, with surface exchanges between the North Vanuatu Jet and the North Caledonian Jet, and (ii) around 170∘ W, with an eastward pathway, whereas a westward general direction dominates. Fronts and small-scale features, detected with finite-size Lyapunov exponents (FSLEs), are correlated with 25 % of surface tracer gradients, which reveals the significance of such structures in the generation of submesoscale surface gradients. Additionally, two high-frequency sampling transects of biogeochemical parameters and microorganism abundances demonstrate the influence of fronts in controlling the spatial distribution of bacteria and phytoplankton, and as a consequence the microbial community structure. All circulation scales play an important role that has to be taken into account not only when analysing the data from OUTPACE but also, more generally, for understanding the global distribution of biogeochemical components.

The origins of the upper branch of the Atlantic meridional overturning circulation (AMOC) are traced with backward-in-time Lagrangian trajectories, quantifying the partition of volume transport between different routes of entry from the Indo-Pacific into the Atlantic. Particles are advected by the velocity field from a recent release of "Estimating the Circulation and Climate of the Ocean" (ECCOv4). This global time-variable velocity field is a dynamically consistent interpolation of over 1 billion oceanographic observations collected between 1992 and 2015. Of the 13.6 Sverdrups (1 Sv = 10 6 m 3 /s) flowing northward across 6 • S, 15% enters the Atlantic from Drake Passage, 35% enters from the straits between Asia and Australia (Indonesian Throughflow), and 49% comes from the region south of Australia (Tasman Leakage). Because of blending in the Agulhas region, water mass properties in the South Atlantic are not a good indicator of origin. Plain Language Summary Particle trajectories in the upper limb of the Atlantic meridional overturning circulation (AMOC) are traced from the equatorial Atlantic to different sections of origin in the Southern Ocean. The velocity moving the particles is an estimate combining over 1 billion observations with a global ocean model that conserves mass, momentum, temperature, salinity, and sea-ice over a regular grid. Ninety-seven percent of the particles enters the Atlantic from the tip of South Africa, as a relatively warm and salty water mass, while the remaining 3% enters from the tip of South America cold and fresh. The thermodynamical properties of waters of different origin are blended at the tip of South Africa: Water mass analysis cannot serve as a substitute of particle trajectories to track water routes. The MOC includes northward flow of intermediate and upper waters from the Southern Ocean into the Atlantic, which are eventually transformed into North Atlantic Deep Water (NADW) in the Labrador and Nordic Seas. NADW then flows southward at depth upwelling in the Southern Ocean to close the mid-depth cell (red contours in Figure 1). An equivalent mid-depth cell is absent in the Indo-Pacific sector (Cessi, 2019). Water that has upwelled from the lower, southward branch of the mid-depth cell in the Indo-Pacific sector (south of 30 • S) can return to the North Atlantic through two pathways: the warm route, that is, westward and northward around the tip of South Africa (Gordon, 1986), or the cold route, that is, eastward and northward around Drake Passage (Rintoul, 1991). The quantitative contributions of these two routes differ among estimates, but this partition is important for the transport of heat and freshwater into the Atlantic. Water that enters the South Atlantic through the warm route is warm and salty, while that entering through the cold route is fresh and cold. Many model simulations have shown that if the cold route prevails, then the MOC is robust to freshwater perturbation in the high latitudes of the North Atlantic and Arctic. Vice versa, if the exchange is mo...