Model results of flow instabilities in the tropical Pacific Ocean

A two‐and‐a‐half‐layer model of the tropical Pacific Ocean is used to investigate the energy source for the intraseasonal dynamic‐height variability observed near 6°N. A simulation of equatorial circulation is produced by forcing the model with mean‐monthly wind‐stress climatology. Two westward‐propagating waves appear in the upper layer in the central and eastern portion of the model basin. These two waves are distinguished by period and meridional structure. An off‐equatorial wave with period of 30 days and wavelength of 1100 km has a meridional sea‐level maximum near 6°N similar to that of the 30–50 day intraseasonal wave observed in the ocean. The meridional velocity signal also is asymmetric with respect to the equator, with maximum near 4°N. The second wave with period of 15 days has a strong meridional velocity signal centered on the equator. The sea‐level and zonal velocity signals associated with this equatorial wave have maxima near 1.5°N and 1.5°S. The eddy‐energy budget reveals strong conversions from the mean‐flow to eddy field through baroclinic and upper‐layer barotropic conversion terms. Conversion terms north of the equator exhibit a bimodal structure: one maximum between the equator and 3°N is dominated by upper‐layer barotropic conversion spatially coincident with the cyclonic shear along the equatorward edge of the South Equatorial Current (SEC), and a second smaller maximum between 3°N and 5°N is a combination of upper‐layer barotropic conversion along the poleward edge of the SEC (anticyclonic shear) and baroclinic conversion near the core of the SEC. The two peaks in the conversion terms, combined with similar structure in the flux‐divergence terms in the model eddy‐energy budget, provide evidence that two wave processes are generated at the different source regions: one near the equator and a second between 2°N and 5°N

Absolute geostrophic velocity within the Subantarctic Front in the Pacific Ocean

Velocity measurements from a shipboard acoustic Doppler current profiler (ADCP) are used as a reference for geostrophic current calculations on six sections across the Subantarctic Front (SAF) in the Pacific Ocean. The resulting cross‐track velocity estimates near the bottom range from 4 to 10 cm s−1 to the east in the eastward jet at the SAF; in adjacent regions of westward surface flow, the near‐bottom velocity is usually to the west. On one section where simultaneous lowered ADCP velocity profiles are available, they confirm the results from the shipboard ADCP. Annual mean velocity sections from the Parallel Ocean Program numerical model also show near‐bottom velocities exceeding 5 cm s−1, with the same tendency for the zonal velocity component near the bottom to match the direction of the surface jets. Transport across the entire Antarctic Circumpolar Current (ACC) cannot be estimated accurately from ADCP‐referenced geostrophic sections because even a very small cross‐track bias integrates to a large error. A preliminary look at the 1992 model transport stream function shows that the effect of bottom‐referencing varies from section to section; it can cause 40‐Sv recirculations to be missed, and can cause net transport to be underestimated or overestimated by O (30 Sv)

Within- and trans-generational plasticity: Seed germination responses to light quantity and quality

Plants respond not only to the environment in which they find themselves, but also to that of their parents. The combination of within- and trans-generational phenotypic plasticity regulates plant development. Plants use light as source of energy and also as a cue of competitive conditions, since the quality of light (ratio of red to farred light, R:FR) indicates the presence of neighbouring plants. Light regulates many aspects of plant development, including seed germination. To understand how seeds integrate environmental cues experienced at different times, we quantified germination responses to changes in light quantity (irradiance) and quality (R:FR) experienced during seed maturation and seed imbibition in Arabidopsis thaliana genotypes that differ in their innate dormancy levels and after treatments that break or reinduce dormancy. In two of the genotypes tested, reduced irradiance as well as reduced R:FR during seed maturation induced higher germination; thus, the responses to light quantity and R:FR reinforced each other. In contrast, in a third genotype, reduced irradiance during seed maturation induced progeny germination, but response to reduced R:FR was in the opposite direction, leading to a very weak or no overall effect of a simulated canopy experienced by the mother plant. During seed imbibition, reduced irradiance and reduced R:FR caused lower germination in all genotypes. Therefore, responses to light experienced at different times (maturation vs. imbibition) can have opposite effects. In summary, seeds responded both to light resources (irradiance) and to cues of competition (R:FR), and trans-generational plasticity to light frequently opposed and was stronger than within-generation plasticity.Fil: Vayda, Katherine. University of Duke; Estados UnidosFil: Donohue, Kathleen. University of Duke; Estados UnidosFil: Auge, Gabriela Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; Argentina. University of Duke; Estados Unido

The Polar Front in Drake Passage: A composite‐mean stream‐coordinate view

The Polar Front (PF) is studied using 4 years of data collected by a line of current‐ and pressure‐recording inverted echo sounders in Drake Passage complemented with satellite altimetry. The location of the PF is bimodal in latitude. A northern and southern PF exist at separate times, separated geographically by a seafloor ridge—the Shackleton Fracture Zone—and hydrographically by 17 cm of geopotential height. Expressed in stream coordinates, vertical structures of buoyancy are determined with a gravest empirical mode analysis. Baroclinic velocity referenced to zero at 3500 dbar, width, and full transport (about 70 Sv) of the jets are statistically indistinguishable; the two jets alternate carrying the baroclinic transport rather than coexisting. Influences of local bathymetry and deep cyclogenesis manifest as differences in deep reference velocity structures. Downstream reference velocities of the PF‐N and PF‐S reach maximum speeds of 0.09 and 0.06 m s−1, respectively. Buoyancy fields are indicative of upwelling and poleward residual circulation at the PF. Based on potential vorticity and mixing lengths, the northern and southern PF both act as a barrier to cross‐frontal exchange while remaining susceptible to baroclinic instability

CPIES Data Collected Near Hydrostation S Southeast of Bermuda from June 2016 to June 2017

This report focuses on data collected from four current meter equipped pressure inverted echo sounders (CPIES), two with respectively two and one Popeye Data Shuttles (PDS) on them, and two dual-pressure CPIES each with a Paroscientific stable oceanographic sensor (SOS) and a 46K sensor that has a long track record of previous deployments with low-drift, deployed from June 2016 to June 2017 near Hydrostation S, 25 km southeast of Bermuda (Figure 1). The CPIES were moored at similar depths, ranging from approximately 3400 to 3600 m, at sites numbered clockwise around Hydrostation S as P1, P2, P3, and P4

Variability in the central equatorial Pacific, 1985–1989

We describe variability in the equatorial Pacific Ocean near 160°W during the 5‐year period 1985–1989, encompassing “normal”, El Niño, and La Niña conditions. This description is based on conductivity‐temperature‐depth and acoustic Doppler current profiler data acquired during five cruises between 21°N and 4°S and on dynamic‐height time series from an array based mainly on the Line Islands. At Jarvis Island, near the equator, the time series of dynamic height and near‐surface temperature go back to 1981 and show the 1986–1987 El Niño anomalies starting later in the year and having longer duration than those of the 1982–1983 El Niño. Dynamic‐height anomaly was less strong for the 1986–1987 event, but the near‐surface temperature anomaly was of similar magnitude for the two El Niños. The Jarvis near‐surface temperature drop from 1986–1987 El Niño maximum to 1988–1989 La Niña minimum was 8°C. Empirical orthogonal function analysis of the time series shows that interannual and interseasonal variability in dynamic height was dominated by a mode with meridional form similar to a first‐vertical‐mode Kelvin wave, while intraseasonal variability had a primary mode with a single peak at 6°N and a secondary mode with peak at 6°N and trough at 2°N. While the equatorial thermocline deepened to the east and shoaled to the west during the 1986–1987 El Niño, at 160°W it did not change depth during either this El Niño or the subsequent La Niña. Nevertheless, just before El Niño and just after La Niña, the thermocline was observed to be about 50 m deeper than at other times. The South Equatorial Current and North Equatorial Countercurrent had markedly reduced (increased) transports during this El Niño (La Niña). However, the Northern Tsuchiya Jet strengthened during El Niño and weakened during La Niña

What Can Hydrography Between the New England Slope, Bermuda and Africa Tell us About the Strength of the AMOC Over the Last 90 years?

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper-ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream\u27s northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope-Bermuda upper-ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020

Divergent Eddy Heat Fluxes in the Kuroshio Extension at 144°–148°E. Part I: Mean Structure

The Kuroshio Extension System Study (KESS) provided 16 months of observations to quantify eddy heat flux (EHF) from a mesoscale-resolving array of current- and pressure-equipped inverted echo sounders (CPIES). The mapped EHF estimates agreed well with point in situ measurements from subsurface current meter moorings. Geostrophic currents determined with the CPIES separate the vertical structure into an equivalent-barotropic internal mode and a nearly depth-independent external mode measured in the deep ocean. As a useful by-product of this decomposition, the divergent EHF (DEHF) arises entirely from the correlation between the external mode and the upper-ocean thermal front. EHFs associated with the internal mode are completely rotational. DEHFs were mostly downgradient and strongest just upstream of a mean trough at ~147°E. The downgradient DEHFs resulted in a mean-to-eddy potential energy conversion rate that peaked midthermocline with a magnitude of 10 × 10−3 cm2 s−3 and a depth-averaged value of 3 × 10−3 cm2 s−3. DEHFs were vertically coherent, with subsurface maxima exceeding 400 kW m−2 near 400-m depth. The subsurface maximum DEHFs occurred near the depth where the quasigeostrophic potential vorticity lateral gradient changes sign from one layer to the next below it. The steering level is deeper than this depth of maximum DEHFs. A downgradient parameterization could be fitted to the DEHF vertical structure with a constant eddy diffusivity κ that had values of 800–1400 m2 s−1 along the mean path. The resulting divergent meridional eddy heat transport across the KESS array was 0.05 PW near 35.25°N, which may account for ~⅓ of the total Pacific meridional heat transport at this latitude