3.1.4. The contrast of GoM from the southwestern shelf of India
The environmental variables collected during the first in-situ sampling during 2010-2011 (Table 1) shows that the hydrography of the GoM was very similar to the waters in the SEAS along the southwest coast of India8-12. At the same time, the vertical distribution of temperature and dissolved oxygen in the GoM during the Southwest Monsoon displayed two significant features, which were strikingly different from the southwest coast of India during the same period. The water column in the GoM during the Southwest Monsoon was warm (>26°C) and without any significant vertical temperature gradients (Figure 2), unlike that on the southwest coast of India due to coastal upwelling. Also found in this data high oxygen concentration throughout the water column (>4.2 mg L-1) negating the possibility of coastal upwelling in the GoM (Figure 3). We confirmed the robustness of this feature based on a few earlier records on the environmental variables from this region23,24. The upwelling along the southwest coast of India during the Southwest Monsoon16,25,26 and its implications on the biological resources are well known2,3,27-30. During the Southwest Monsoon, westerly/north-westerly winds close to the coastline induces upwelling along the west coast of India16,26,27, which lowers the SST by 4-5°C2,27. During this process, the cold (23°C) and low oxygenated (~2 mg L-1) deep oceanic waters (from 75 - 150 m) are elevated to the surface waters along the entire west coast of India2,27-30. Both local winds and remote forcing facilitate coastal upwelling along the extensive southwest coast of India18, 26-30.
The vertical distribution of temperature and dissolved oxygen at different depth zones (10m, 25m, and 40m) in the shelf waters along the southwest coast of India up to the western proximity of the GoM (Tuticorin) clearly shows coastal upwelling signatures in terms of the surfacing of cold and low-oxygenated deep water (Figure 4). The most striking feature here in the present context is the spatial extent of the cold and low oxygenated water in the three depth zones. Along with the 10 m depth, upwelling signatures were evident up to Vizhinjum (SP2), but in 25m and 40m, it was apparent up to Kulasekharapatnam (SP4). ANOVA results confirmed the significant spatial differences in the levels of temperature and dissolved oxygen between the northern (SP1 & SP2) and southern (SP4 & SP5) sections (P<0.05; Supplementary Table 1). PCA showed that transects in the western proximity of the GoM (SP4 and SP5) were significantly warmer and more oxygenated compared to those in the north along the southwest coast of India (SP1 and SP2) (Supplementary Figure 4). All the in-situ data showed that upwelling along the Indian southwest coast does not extend towards the GoM. This feature is also consistent in the vertical sections of temperature and dissolved oxygen in ROMS, showing that the coastal upwelling prevails only up to the southern Indian tip (Cape Comorin) and does not extend into the GoM (Supplementary Figure 5). This vital feature we reaffirmed in a historical oceanographic survey data sets published, as the upwelling signatures of the southwest coast of India phase-out by reaching the western proximity of the GoM (Tuticorin) region 31 (Supplementary Figure 6 & 7). The reason for the absence of coastal upwelling in the GoM could be the alignment of its coastline differently from the Indian southwest coastline. Thus, the wind and remote forces, which cause upwelling along the adjacent southwest coast of India might not be favouring upwelling in the GoM as observed in the case of the western and eastern shores of Sri Lanka32.