Drifter deployment program was carried during 2013 to 2015 out as the framework of Air-Sea Interaction in the Indian Ocean (ASIRI) project off the east coast of Sri Lanka. The program consists of different objectives such as coastal boundary currents and water mass exchange between Bay of Bengal (BoB) and the Arabian Sea (AS) (Wijesekera et al., 2015; Lee et al., 2016). The perspectives of ASIRI were to improve the present knowledge of coupled air sea interactions in the Indian Ocean region (Lucas et al., 2014).
Indian Ocean is the least explored and the most complex ocean among the world’s three major oceans. Indian Ocean is divided into two subdivisions namely the Arabian Sea (AS) and Bay of Bengal (BoB) which located in the same latitude and comprise with unique characteristics. Both basins are influenced by Indian and Asian monsoons. The surface circulation characterizes with the seasonally reversing currents (Fig. 1) as a response to the seasonal winds (Schott and McCreary, 2001).
Figure 1. A schematic representation of identified current branches during the Southwest Monsoon, including some choke point transport numbers (Sv=106 m3/s). Current branches indicated are the South Equatorial Current (SEC), South Equatorial Countercurrent (SECC), West Indian Coast Current (WICC), Laccadive High and Low (LH and LL), East Indian Coast Current (EICC), Southwest and Northeast Monsoon Current (SMC and NMC), Sri Lanka Dome (SD). a. Summer monsoon (January/Februay); b. winter monsoon (January/Februay) (Schott and McCreary, 2001).
The BoB receives substantial freshwater flux from both precipitation and river discharges during southwest monsoon where precipitation well exceeds evaporation (Sengupta et al., 2006; Seo et al., 2009). Seasonal freshwater influx (precipitation and river runoff) and evaporation force to changes the salinity levels. The observed salinity levels suggest that AS is consisted of high (low) salinity water. Thus, higher evaporation over AS and higher freshwater influx to the BoB creates a hydrological imbalance between the AS and BoB (Kumar et al., 2004).
The seasonally reversing circulation plays a key role in controlling the imbalance between AS and the BoB which maintains long-term average salinities (Jensen, 2001). The observations based on earlier studies (Shetye et al., 1991; Schott et al., 1994; Vinayachandran and Yamagata, 1998; Gopalakrishna et al., 2005) and model simulations (Jensen, 2001; Zhang and Du, 2012; de Vos et al., 2014) pointed out that eastward flowing. Southwest Monsoon Current (SMC) brings high saline AS waters into the BoB during summer monsoon which increases the salinity in the BoB. Also, it was noticed that the westward propagation of Northeast Monsoon Current (NMC) carries low saline BoB waters into the AS during winter monsoon. Thus, the circulation within the BoB is affected by the seasonal variability of general current patterns in the Indian Ocean. East Indian Coastal Currents (EICC), which runs along the western boundary of the BoB reverses its direction twice a year (Vinayachandran et al., 2005; de Vos et al., 2014). EICC flows towards northeast from February to September with a strong peak in March to April and southwestward from October to January with the strongest flow in November (Schott and McCreary, 2001).Within this seasonal cycle, EICC plays a crucial role in the freshwater balance of the northern Indian Ocean during northeast monsoon since it is an efficient conduit for the export of low salinity water from the BoB to the Arabian Sea (Hacker et al., 1998; Han and McCreary, 2001; Jensen, 2001).
McPhaden et al. (2009) pointed out that ocean-atmospheric interaction in the region is highly dynamic, involving seasonal current reversals associated with monsoon wind forcing and significant exchange of heat across the air-sea interface. Lin et al. (2013) suggested that the current pattern reversals in the Indian Ocean are specially related to the heat and moisture transport effect on climate changes within the region. There are two possible pathways available for the transport of low salinity water from the BoB to the southeastern AS (Vinayachandran et al., 2005). One low salinity pathway is located along the coastline around Sri Lanka and the southern tip of India during October–November. Other one is running through the Palk Strait during northeast monsoon from December–March which is fed by the EICC (Shankar et al., 2002). The low salinity water mass is observed in the southeastern AS during November–December (Boyer et al., 2005). The numerical models show that the EICC flows around India and Sri Lanka in to the Arabian Sea (Vinayachandran et al., 1996; Han and McCreary, 2001) suggest that the EICC is a definite conduit (Jensen, 2001).
However, it is not yet firmly confirmed that the exchange of water mass between AS and BoB is either around the southern tip of Sri Lanka or passing through the Palk Strait due to lack of systematic in-situ measurements.
The monsoon activation is a natural phenomenon and timely variability have been investigated in the literature. The wind data from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) clearly shows the change in monsoon onsets between 2013 and 2014 (Table 1).
Source SWM activation NEM activation Dickey et al. (1998) June–September November–February Shankar et al. (2002) May–September November–February Tomczak and Godfrey, (2003) June–October December–April Shenoi (2010) June–September November–February Rao et al. (2012) June–September November–January Glejin et al. (2013) June–September October–January
Table 1. Variation of monsoon onsets in northern Indian Ocean (AS and BoB)
After analyzing the time series data from RAMA moorings during 2013–2014, it is noted that the shifting of monsoon onset over the region (Fig. 2). The observations indicated that northeast monsoon seems to be activated in the mid December while southwest monsoon in mid-May in 2013. The seasonal shifts observed in 2014 indicated the northeast monsoon activated in mid-November and southwest monsoon in early June. Thus, it is important to know that how the variability of monsoon onset period and strength of monsoon is affected on the wind driven circulation in the region. However, the drifter data utilized in this investigation does not cover the entire northern Indian Ocean both spatially and temporally, and it makes difficulties to address the changes in the wind driven circulation in the region.
Figure 2. Variation of wind speed at 15°N, 90°E (a), 12°N, 90°E (b) during 2013 and 15°N, 90° E (c), 12°N, 90°E (d) during 2014. Red lines indicate the activation of SWM and green lines indicate the activation of NEM (data source: RAMA moorings, NOAA/PMEL).
The current study is an attempt to explain the monsoonal impact on circulation pathways in the Indian Ocean utilizing drifter data with other in-situ (RAMA) and satellite observations during 2013–2015. The paper is organized as introduction, methodology, results, discussion and conclusion as in different sections.
Monsoonal impact on circulation pathways in the Indian Ocean
- Received Date: 2018-07-30
- Available Online: 2020-04-21
- Publish Date: 2020-03-01
Abstract: Monsoon driven water mass exchange between the Bay of Bengal (BoB) and Arabian Sea (AS) is the common experience. However, it is not yet firmly confirmed that the exchange pathway is either passing through southern tip of Sri Lanka or Palk Strait. Local circulation patterns impact the pathways followed by the East Indian Coastal Currents (EICC) that drive exchange, thereby modulating mixing and water mass transformation in the Bay of Bengal around Sri Lanka. In this study, observations from surface drifters were incorporated with the satellite derived data to understand the monsoonal impact on circulation patterns in the Indian Ocean. This was the first multi-national scientific effort which was conducted in the BoB and AS during 2013 to 2015 to understand the monsoonal impact on circulation patterns in the complex region. The results indicated that seasonally reversing monsoonal currents of southern Sri Lanka, traced by the wintertime freshwater export pathways of the EICC. The deflection of monsoon currents running along the east coast of Sri Lanka by forming cyclonic and anti-cyclonic eddies, which influence the mixing and stirring associated with these flows. Results further indicate the low salinity cold water flows from the BoB to AS along the western boundary of the BoB during northeast monsoon. In the same way, reverses the phenomena during southwest monsoon, transporting high salinity warm water from AS to the BoB. This maintain the bay status which occurred due to freshwater influx from large rivers and high saline water from AS. However, no evidences were observed for the exchange through Palk Strait during the study. Also, there are some mis-matches in in-situ and remotely sensed measurements which imply the necessity of systematic observation system for the complex region as an alternative approach.
|Citation:||Sinhalage Udaya Priyantha Jinadasa, Gayan Pathirana, Pradeep Nalaka Ranasinghe, Luca Centurioni, Verena Hormann. Monsoonal impact on circulation pathways in the Indian Ocean[J]. Acta Oceanologica Sinica, 2020, 39(3): 103-112. doi: 10.1007/s13131-020-1557-5|