Long-term linear meridional shift of the jet structure of the Antarctic Circumpolar Current south of Africa on the basis of the satellite altimetry data: zonal distribution

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Abstract

For the period of satellite altimetry observations 1993–2018, the zonal distribution of the linear meridional shift of the jet structure and the change in the intensity of currents in the sector of the Antarctic Circumpolar Current (ACC) south of Africa (9.875°W–25.125°E) were estimated. The jet structure is understood as the alternation in the meridional direction of zones of increased and decreased values of the modulus of the absolute dynamic topography (ADT) gradient, |∇ζ|. The work was carried out using the methodology developed earlier and supplemented in this article, based on the calculations of linear regressions, which also includes an assessment of calculation errors. To estimate the zonal distributions, the sector was divided into meridional bands, for each of which a calculation was carried out. The optimum width of the band is estimated at 2.5° lon. The presence of a noticeable zonal inhomogeneity in the shift of the jet structure and changes in the intensity of the currents are shown. This inhomogeneity is obviously associated with quantitative discrepancies in the calculations of these parameters with division into bands, with their subsequent averaging over the zonal series, and without division into bands. Qualitatively, the patterns of shifts in the jet structure and changes in the intensity of flows with and without separation into bands are similar.

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About the authors

R. Yu. Tarakanov

P.P. Shirshov Institute of Oceanology RAS

Author for correspondence.
Email: rtarakanov@gmail.com
Russian Federation, Nakhimovskiy pr-t, 36, Moscow, 117997

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Supplementary files

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2. Fig. 1. Absolute dynamic topography (isolines and color fill) in the area south of Africa (9.875° west – 25.125° east) on 01.01.2003. Bold isolines -130 and 20 cm show the approximate boundaries of the AST. The abbreviation AT is the Agulhas current, in which the values of ADT also fall into the range characteristic of ADT. The darkened areas outlined by white lines correspond to areas of the ocean with depths of less than 3,000 m. The oblique lines show the main tracks of the satellites T/P, Jason-1, -2, -3. The dashed line at 39° south shows the restriction from the north for calculating the curves of dependence of h on ζ.

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3. Fig. 2. Calculation of the shift of the jet stream structure south of Africa relative to latitude and changes in the intensity of currents: (a) 26–year series of average annual distributions depending on latitude φ; (b), (d) linear shifts (°w) of the jet stream structure depending on the median latitude and the scale of the calculation, respectively, without separation and with division into meridional bands; positive shift values are a shift to the north.; the dark shading corresponds to the areas of the calculated points with a shift estimate of less than the standard calculation error, the light one corresponds to less than 95% of the probability level (according to the Student's t–criterion); (c), (e) - corresponding changes in the intensity of currents (10-3 · cm/km); 10-3 · cm/km in the studied sector approximately corresponds to 1 mm/sec. Dashed and dashed horizontal lines are the conditional boundaries of the ADT and the boundaries between the zones of increased ADT gradients within the ADT. The crosses in Fig. 2b, c, d, e denote the points corresponding to the calculation bands of SAT, UPT and UAT, as well as the ACT as a whole.; The calculation results for these points are shown in Table 1. The abbreviations in Fig. 2a: SAT – Subantarctic current, UPT – South Polar current, UAT – South Antarctic current, UT – Weddell current.

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4. Fig. 3. Estimates of the average shift of the jet structure of the ACT (° w) in calculations with the division of the source sector south of Africa into meridional bands depending on the median latitude and scale of calculation: (a) into 2 bands 17.5° wide; (b) into 4 bands 8.75° wide.; (c) – for 7 strips 5.0° wide; (d) – for 14 strips 2.5° wide; (e) – for 28 strips 1.25° wide; (f) – for 140 strips 0.25° wide. The rest is as shown in Fig. 2.

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5. Fig. 4. 26-year series of average annual distributions depending on latitude φ for individual 2.5-degree meridional bands: (a) - 7.375°– 4.375° W; (b) - 2.375°W – 0.125°W; (c) – 5.125°-7.625° VD; (d) - 12.625°-15.125° VD; (e) – 17.625°-20.125° VD; (f) – 22.625°-25.125° vd The rest is as shown in Fig. 2a.

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6. Fig. 5. Distributions of the meridional shift (° w) and changes in the intensity of the current (10-3 · cm/km) depending on longitude and median latitude, plotted according to 2.5-degree meridional bands: (a), (c), (e) – shift; (b), (d) 5a and 5b – calculation scale 3.5° w, 5b and 5g – calculation scale 7° w, 5d and 5e – calculation scale 15° w. The blue dashed lines represent the latitudes bounding the AZT.

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7. Fig. 6. Calculation of the shift of the jet structure of the ADT south of Africa relative to the given scale of ADT, ζ’ and changes in the intensity of currents: (a) – 26-year series of average annual distributions depending on ζ’; (b), (d) – linear shifts (cm) of the jet structure of currents depending on from the average values of the ADT and the calculation scale, respectively, without separation and with division into meridional bands; (c), (e) – corresponding changes in the intensity of currents (10-3 · cm/km). The crosses in Fig. 6b, c, d, and e indicate the points corresponding to the calculation bands of SAT, UPT, and UAT, as well as the ACT as a whole.; The calculation results for these points are shown in the Table. 2. The rest is as shown in Fig. 2.

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8. Fig. 7. 26-year series of average annual distributions depending on the given ADT, ζ’ for individual 2.5-degree meridional bands: (a) - 7.375°– 4.375° W; (b) – 2.375°W – 0.125°w; (c) – 5.125°-7.625° VD; (d) – 12.625°-15.125° VD; (e) – 17.625°-20.125° VD; (f) – 22.625°-25.125° vd The rest is as shown in Fig. 6a.

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9. Fig. 8. Distributions of the meridional shift (cm) and changes in the intensity of the current (10-3 · cm/km) depending on the longitude and median values of the ADT, plotted according to 2.5-degree meridional bands: (a), (c), (e) – shift; (b), (d), (e) – change in the intensity of the flow. Fig. 8a and 8b – calculation scale 36 cm, 8b and 8g – calculation scale 70 cm, 8d and 8e – calculation scale 150 cm. The blue dashed lines are isohypses that limit the ACT.

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