On the longitudinal shifts of the Agulhas retroflection point

Weiwei Zhang Xiaoyi Yang Wei Zhuang Xiaohai Yan

Weiwei Zhang, Xiaoyi Yang, Wei Zhuang, Xiaohai Yan. On the longitudinal shifts of the Agulhas retroflection point[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-023-2295-x
Citation: Weiwei Zhang, Xiaoyi Yang, Wei Zhuang, Xiaohai Yan. On the longitudinal shifts of the Agulhas retroflection point[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-023-2295-x

doi: 10.1007/s13131-023-2295-x

On the longitudinal shifts of the Agulhas retroflection point

Funds: The National Key R&D Program of China under contract No. 2019YFA0606702; the National Natural Science Foundation of China under contract Nos 42176222, 91858202, 41630963, and 41776003; the National Science Foundation under contract No. NSF-IIS-2123264; the fund suported by the National Aeronautics and Space Administration under contract No. NASA-80NSSC20M0220.
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  • Figure  1.  The climatology of mean kinetic energy (MKE) (a) and eddy kinetic energy (EKE) (b) in the Agulhas system during 1993 to 2018, which is computed using the geostrophic velocity from the gridded L4 satellite altimetry product, the difference between MKE and EKE (c), and the time series of EKE and MKE in the black rectangular box (35°−42°S, 10°−18°E) in c, which denotes the region where MKE is much less than EKE and is roughly the origin of Agulhas leakage as well (d). The thick red and blue lines in d are the 13-month running mean of the MKE and EKE.

    Figure  2.  The climatology of absolute dynamic topography (ADT) (colored shading), and the corresponding geostrophic velocity (black vectors) during the period of January 1993–December 2018. The thick black line is the mean location of the Agulhas Current jet path derived for each available longitudinal grid (with (1/4)° increment). The grey lines are all the identified Agulhas Current jet path. The dashed black line at 26°E approximately divides the Agulhas Current jet path into upstream and downstream sections. It is noted that the variation of the paths gets much higher downstream.

    Figure  3.  Normazlied monthly time series for various physical factors. a. AC strength integrated along the upstream AC jet path (black line) and its 13-month moving average (blue line). b. Same as a, but integrated along the downstream AC jet path. c. Spatial standard deviation (std) along the downstream jet path, the black and the blue lines represent monthly and 13-month running mean, respectively. d. Normalized longitudinal displacement of the retroflection point, which is defined as the westernmost longitude of the AC jet path. The black , blue, and red lines denote the monthly, the 13-month running mean, and the 5-year low-pass filtered displacements, respectively. The grey dashed lines indicate ±1.5 standard deviations from the mean displacements, which is the thresholds for composite analyses. The normalization of the time series is achieved following the formula as $ i=\dfrac{x-\mu }{\sigma } $, where $ \mu $ is the climatological mean and $ \sigma $ is the standard deviation of $ x $.

    Figure  4.  The ETOPO1 bottom topography in colored shadings overlapped with the 21 westernmost AC jet paths (a) and the 24 easternmost AC jet paths (b), with ±1.5 standard deviation of the retroflection longitudes as selecting criteria, respectively. The black crosses denote the final retroflection points for each path according to our algorithm. ETOPO1 is a 1 arc-minute global relief model of Earth’s surface that integrates land topography and ocean bathymetry.

    Figure  5.  Absolute dynamic topography (ADT) on the north side are associated with the retroflection position. a and b. The composites of the ADT for the 24 most eastward and the 21 most westward retroflection months. In a and b the black vectors denote the corresponding surface velocity; the black rectangular box is approximately where a cyclonic feature coincides with the westward elongation of the AC jet path in the westward composite. c. The blue curve is the 15-d running average of the spatial mean ADT within the black box in a, 45 d before the most eastward retroflection events until 105 d afterwards; the red curve is the same but referenced to the most westward retroflection events; the grey shading in c indicates the composited differences between the westward and the eastward retroflection events are significant at the 95% confidence level of t-test.

    Figure  6.  Wind stress curl anomalies at the north flank lead to the retroflection position shift. a. The wind stress curl anomalies corresponding to the composite of the 24 most eastward retroflection events. b. Same as a but composite of the 21 most westward retroflection events. In a and b, the black rectangular box is approximately where a cyclonic feature coincides (same as in Fig. 5b), and the grey box is chosen to represent an area of maximal negative wind stress curl anomaly. c. 15-d moving average of the spatial mean wind stress curl in the black rectangular box to the north, from 15 d before the event till 15 d afterwards. d. Same as c but for the mean wind stress curl in the grey box to the south. The grey shading indicates the composited differences between the westward and the eastward retroflection events are significant at the 95% confidence level of t-test.

    Figure  7.  The mean absolute dynamic topography (ADT) derived from altimetry data in 2017 (a) and 2002 (b). Notice the cyclone to the north of the retroflection point in 2002.

    Figure  8.  Configurations of the wind forcing for numerical experiments. The anomaly of the ERA5 wind stress curl and 10 m wind stress vector in 2017 (a) and 2002 (b), referenced to the 1979 to 2019 climatology. The difference between a and b is shown in c. d. Non-dimensional weighting function, which will be multiplied to the wind stress difference (2002–2017) and then added to the Case1 forcing to produce the perturbed run (Case 2). The cyan box is approximately where a cyclonic feature coincides (same as the black rectangular box in Figs 6a and b), and the dark and light green contours represent the mean sea surface height to indicate the retroflection position for the year of 2017 and 2002, respectively.

    Figure  9.  Modeled longitudinal position of the retroflection. Mean sea surface height (SSH) in the retroflection area from control run forced by 2017 mean wind stress (a) and the ensemble of the three perturbed runs forced by 2017 wind stress plus a local 2002 anomaly around the retroflection area (b). The black dashed curve represents the 0.5 m SSH isoline for the control run, and the cyan curve represents the 0.5 m SSH isoline for the ensembled perturbed runs. c. The probability distribution of the retroflection longitudes for the first three model years, the black curve and circles represent the control run, while the magenta, the green, and the red represent the three perturbed runs. d. Same as c, but the result from the latter two model years.

    Figure  10.  Time evolution of Agulhas ring shedding process for the westward retroflection events. The composite of 15-d mean ADT leading to extreme westward retroflection events. a. Mean ADT over the period 30 d to 15 d before the extreme eastward retroflection events. b and c. Same as a, but from 22 d to 7 d, and 15 d to 0 d before the extreme westward retroflection events respectively. d. Mean ADT from 8 d before the event to 7 d after the event. e and f. Same as d, but for the mean ADT from the day of the event to 15 d afterwards and from 16 d to 23 d after the event, respectively. Red solid contours include areas of negative ADT value. Black dashed lines indicate the 0.7 m ADT value, which has been usually used to estimate the retroflection position.

    Figure  11.  Time evolution of Agulhas ring shedding process for the eastward retroflection events. The composite of 15-d mean (ADT) leading to extreme eastward retroflection events . a. Mean ADT over the period 30 d to 15 d before the extreme eastward retroflection events. b and c. Same as a, but from 22 d to 7 d, and 15 d to 0 d before the extreme westward retroflection events respectively. d. Mean ADT from 8 d before the event to 7 d after the event. e and f. Same as d, but for the mean ADT from the day of the event to 15 d afterwards and from 16 d to 23 d after the event, respectively. Red solid contours include areas of negative ADT value. Black dashed lines indicate the 0.7 m ADT value, which has been usually used to estimate the retroflection position.

    Figure  12.  Westward retroflection leads to warming in the South Atlantic (SA) and cooling in the Indian Ocean (IO), corresponding to the enhanced Agulhas leakage. Composited monthly SSTa differences between the westward and the eastward retroflection events from OISST dataset (a) and EAR5 dataset (b). c. Same as b, but with SSTa lag retroflection events for one month. The grey contours include the areas where the differences are significant at 95% confidence level of t-test. The black solid and dashed rectangular boxes denote the SA region and the IO region, respectively.

    Figure  13.  Significant differences between the westward and eastward retroflection events in ALSST. a. Lead-lag compositions of ERA5 ALSST for the westward retroflection events (red crosses) and for the eastward retroflection events (blue circles). The ALSST is computed from the difference between the SA (solid box in Fig. 12) mean SST anomalies and the IO (dashed box in Fig. 12) mean SST anomalies. b. The ALSST differences between the westward and the eastward retroflection differences are statistically significant at 95% confidence level of t-test, whereas the values overlapped by the blue dashed bars are insignificant.

    Figure  15.  Time evolution of Agulhas ring shedding processes for the extreme early (eastward) retroflection during November 2000 to February 2001 recorded by Dencausse et al. (2010a). a. November 2000; b. December 2000; c. January 2001; d. February 2001.

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  • 收稿日期:  2023-11-20
  • 录用日期:  2024-01-24
  • 网络出版日期:  2024-03-15

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