Time-series analysis of chlorophyll-a, sea surface temperature, and sea surface height anomalies during 2003–2014 with special reference to El Niño, La Niña, and Indian Ocean Dipole (IOD) Years

ABSTRACT

The present study was carried out to find the variability of chlorophyll-a (chl-a) concentration, sea surface temperature (SST), and sea surface height anomalies (SSHa) during 2003–2014, covering the Bay of Bengal (BoB) and Arabian Sea (AS) waters. These parameters were linked with El Niño, La Niña, and Indian Ocean Dipole (IOD) years. The observed results during 2003–2014 were evaluated and it was found that the monthly mean value for 12-year data ranged as follows: chl-a (0.11–0.46 mg m⁻³), SST (27–31 °C), and SSHa (−0.2 to 20 cm). The annual mean range of chl-a for 12-year data was 0.1–0.23 mg m⁻³, the SST range was 27–28 °C, and the SSHa range was 2.14–13.91 cm. It has been observed that with the SST range of 27–28 °C and the SSHa range of 7–9 cm, the chl-a concentration enhanced to 0.20–0.23 mg m⁻³. With a higher SST range of 28–29 °C and with a positive SSHa range of 11–14 cm, the chl-a concentration appeared to be low (0.17–0.18 mgm⁻³). During normal years, SSHa was positive with the >5 to <10 cm range during the months of April–June, which coincided with an increase in SST, >2 to <4 °C. During the normal years, SSHa (>−0.2 to <−10 cm) was observed to be negative during October–December, with a decrease in SST (<3 °C) observed. The high monthly mean chl-a concentration (>0.3 to <0.5 mg m⁻³) was noticed during December–February in the BoB and AS. Compared to the BoB chl-a range (<0.4 mg m⁻³), a high chl-a concentration was observed in AS (>0.4 mg m⁻³). However, during the phenomenon years, the study area had experienced low chl-a (<0.2 mg m⁻³), high SST (>5 °C), and more positive SSHa (>10 to <20 cm) during January–March and October–December in AS and BoB. The present study infers that a positive IOD leads to low chl-a concentration (<2 mg m⁻³) and low primary productivity in AS. El Niño caused the down-welling process, it results in a low chl-a concentration (<1 mg m⁻³) in BoB and AS. La Niña caused the upwelling process, and it results in a high chl-a concentration (>2.0 mg m⁻³) in BoB and AS. In the recent past years (2003–2014), the intensity and frequency of El Niño, La Niña, and IOD have been increasing, evidenced with few studies, and have impacts on the Indian Ocean climate. Therefore, the influences of the relative changes of these phenomena on the BoB and AS need to be understood for productivity assessment and ocean state monitoring.

Acknowledgements

The authors are thankful to Dr. Mini Raman, Head, Marine Ecosystem Division; Group Director, BPSG, Dr. Prakash Chauhan; Deputy Director, EPSA, Dr. Raj Kumar and Director, Space Applications Centre, Ahmedabad, for necessary guidance and facilities for carrying out the work. The authors acknowledge NASA/MODIS server for Aqua data sets and Physical Oceanography Distributed Active Archive Centre (PODAAC) website for providing Jason-2 satellite altimeter data sets.

Disclosure statement

No potential conflict of interest was reported by the authors.