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Dynamics of counter wind current along the south Sri Lanka coast during the Southwest Monsoon

Abstract

Shipboard velocity profiles collected in July 2018 are used to study coastal current in the south of Sri Lanka during the summer monsoon. The observations reveal that there is a narrow (~ 50 km wide) westward coastal current against the summer monsoon, separated the eastward southwest monsoon current (SMC) from the island of Sri Lanka. However, the climatological south Sri Lanka coastal current (SSLCC) is eastward following the direction of the SMC. The deviations between the observations and climatology of the SSLCC suggest its significant interannual variability. The dynamics of the westward SSLCC and its impact factors are thus focused on in this study. The results indicate that two main processes are responsible. First, the boreal summer intraseasonal oscillation (BSISO) plays an important role in the presence of westward SSLCC. The BSISO signal intensifies the wind strength east and south of Sri Lanka, reinforces the east India coastal current (EICC), and bends the SMC favoring occurrence of the westward SSLCC. Second, the upwelling Rossby wave signal propagates to Sri Lanka but stops at 82°E, which favors the Sri Lanka Dome developing. As the western flank of the SLD, the strengthened EICC flows southward and turns to west resulting in the westward SSLCC. Accordingly, the energy conversions by baroclinic and barotropic instability between mean flow and eddy are analyzed for both the westward and eastward SSLCC.

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Availability of data and material

ADCP data: The National Aquatic Resources Research and Development Agency (NARA), Sri Lanka.

Code availability

NCAR command language (open source).

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Acknowledgements

Thanks to the support from China-Sri Lanka Joint Centre for Education and Research (CSL-CER) and Chinese Academy of Sciences. Heartiest gratitude goes to the National Aquatic Resources Research and Development Agency (NARA), Sri Lanka, which provides me with the ADCP data. We would like to express our deepest appreciation to Mr. Ruchira Jayathilake in NARA for his hard work and for providing data. We acknowledge the Jet Propulsion Laboratory, California Institute of Technology, for providing their ECCO2 data. And our gratitude also goes to the NOAA/OAR/ESRL for their interpolated OLR data. The wind data is downloaded from the NCEP/NCAR 40-year reanalysis project. Whoever works hard to distribute free data is highly appreciated. Their download information is attached in the Appendix.

Funding

Fundings are obtained by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA20060502), the National Natural Science Foundation of China (Nos. 91958202 and 41731173), the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (No. GML2019ZD0306), the Key Research Program of the Chinese Academy of Sciences (No. ZDRW-XH-2019–2), and the Independent Research Project Program of State Key Laboratory of Tropical Oceanography (No. LTOZZ2101).

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Correspondence to Weiqiang Wang.

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The authors declare no competing interests.

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Key points

• The westward south Sri Lanka coastal current (SSLCC) against the southwest monsoon, deviated from its climatological eastward direction, is observed in both in situ measurements and reanalysis data exhibiting significant interannual variability of the SSLCC.

• The boreal summer intraseasonal oscillation (BSISO) intensifies the wind strength east and south of Sri Lanka favoring strong east India coastal current (EICC).

• The BSISO and the upwelling Rossby wave are responsible for the westward SSLCC.

Responsible Editor: Guoping Gao

Supplementary Information

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Supplementary file1 (DOCX 1561 KB)

Appendix

Appendix

Datasets

Parameter Source Period Spatial resolution
ADCP data The National Aquatic Resources Research and Development Agency (NARA), Sri Lanka 2018 4 m (150 kHz) and 16 m (75 kHz)
Wind https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html 1992–2017 2.5 × 2.5
Wind stress http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=4920 1992–2017 0.25 × 0.25
U and V current
Sea surface height
Potential temperature http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=4920
http://apdrc.soest.hawaii.edu/las_ofes/v6/dataset?catitem=71
ECCO2: 1992–2017
OFES: 1992–2012
ECCO2:0.25 × 0.25
OFES:0.1 × 0.1
Salinity
Vertical velocity
Outgoing longwave radiation https://www.esrl.noaa.gov/psd/ 1992–2012 1 × 1
Geostrophic U and V current https://coastwatch.pfeg.noaa.gov/erddap/griddap/erdTAgeo1day_LonPM180.html 1992–2012 0.25 × 0.25
Bathymetry of the Indian Ocean http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=1294 - 2 min

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Wickramage, C.H., Wang, W., Arulananthan, K. et al. Dynamics of counter wind current along the south Sri Lanka coast during the Southwest Monsoon. Ocean Dynamics 71, 1051–1067 (2021). https://doi.org/10.1007/s10236-021-01477-6

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Keywords

  • South Sri Lanka coastal current
  • Boreal summer intra-seasonal oscillation
  • Sri Lanka dome
  • Southwest monsoon current