Western boundary currents are the locus of intense nutrient transport, or nutrient streams. The largest fraction of this transport takes place in the upper-thermocline layers, between the surface layers (where speed reaches a maximum) and the nutrient bearing strata of the subtropical gyres (where nutrient concentration is maximum). The core of the nutrient stream of the North Atlantic subtropical gyre is located slightly offshore the Gulf Stream, its density coordinate centered on the 26.5-27.30rband, approximately constant along the axis of the stream. During !ate spring 2nd wmmer the n~frient stresim reaches fhe surfare rearnnal mixed layer a t the outcropping of this isopycnal band. We argue that this must be a principal factor sustaining the seasonal high productivity of the subpolar North Atlantic Ocean. Additionally, we investigate the possibility of intermittent shearinduced diapycnal mixing in the upper-thermocline layers of the Gulf Stream, i n d~c e d by frmtegenesis t u b g p!ae during scme phase ~f ?he memderc. Uere we illustrate that diapycnal mixing has a maximum a t the location of the nutrient stream, being associated to observed nutrient anomalies. We suggest that diapycnal mixing associated to the passage of steep meanders brings nutrients from the nutrient stream to the shallow photic layers, and sustains intermittent (day-toweek) patchy (10-100 km) productivity over the stream itself.

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We use a simple two-state two-box ocean to simulate the CO₂ signal during the last four glacial-interglacial transitions in the earth system. The model is inspired by the similarity in spatial organization and temporal transition patterns between the earth and other complex systems, such as mammals. The comparison identifies the earth's metabolic rate with net autotrophic primary production in the upper ocean, sustained through new inorganic carbon and nutrients advected from the deep ocean and organic matter remineralized within the upper ocean. We view the glacial-interglacial transition as a switch of the upper ocean from a basal to an enhanced metabolic state, with energy supply initially relying on the remineralization of the local organic sources and the eventual steady state resulting from the increased advective supply of inorganic deep sources. During the interglacial-glacial transition the opposite occurs, with an initial excess of advective supply and primary production that allows the replenishment of the upper-ocean organic storages. We set the relative change in energy supply from the CO₂ signal and use genetic algorithms to explore the sensitivity of the model output to both the basal recirculation rate and the intensity-timing of the maximum recirculation rate. The model is capable of reproducing quite well the long-term oscillations, as shown by correlations with observations typically about 0.8. The dominant time scale for each cycle ranges between about 40 and 45 kyr, close to the 41 kyr average obliquity astronomical period, and the deep-ocean recirculation rate increases between one and two orders of magnitude from glacial to interglacial periods

The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.