Dynamics of the Gulf of Thailand/Andaman Sea

Suriyan Saramul Department of Marine Science, Faculty of Science, Chulalongkorn University Gulf of Thailand (GoT) Geography

• Southwest of SCS • Quite shallow, max ~> 80 m found at the central, mean ~ 45 m • 2 ridges btw. Cape Camau & Kota Bharu (25m and 50m) • 67m sill btw. 2 ridges • 4 main rivers (up north) bring freshwater to Gulf General oceanographic features of the Gulf

• A classical 2-layered, shallow water estuary • Low salinity flows out on top layer • High salinity (relatively cold water) flows in at the bottom layer & found at the central Gulf over depth of 50m • Wind prevails under monsoon condition (variable wind) General oceanographic features of the Gulf

• Northeast (winter) and Southwest (summer) monsoons play important role for the circulation features in the Gulf • Strong waves occur during northeast monsoon • Annual rainfall peaks around Aug to Nov. General oceanographic features of the Gulf

• Variable winds+tidal currents+freshwater runoff+precipitation create the upwell of low temp, high sal and low oxygen (divergence)

Buranapratheprat (2008) General oceanographic features of the Gulf

• Variable winds+tidal currents+freshwater runoff+precipitation create the downwell of high temp, low sal and high oxygen (convergence)

Buranapratheprat (2008) General oceanographic features of the Gulf

• Tides: diurnal is found in the central Gulf, while mixed mainly semi-diurnal tide is found for the rest of the Gulf • Tidal range is gradually increased from the entrance toward the head Singhruck (2002) Singhruck (2002)

Choi et al. (1997) Fang et al. (1999) Choi et al. (1997) Fang et al. (1999) Eddies in the Gulf

Short lived ~ 2-3 days (a) & (b) 2 anticyclonic eddies, upper and lower Gulf

Short lived ~ 2-3 days (a) Surface (b) 10 m (c) 30 m Singhruck (2002) (d) 40 m Circulation features: Geostrophic currents

CW & CCW eddies relate to upwelling and downwelling Sojisuporn et al. (2010) Upwelling Report

• Based on Naga Expedition (Robinson, 1963) • NE monsoon, upwelling occurs along the eastern shore of the GoT, • SW monsoon, intense upwelling occurs along the west coast of the GoT • Singhruck (2001), central Gulf along the west coast has been observed high Chl-a concentration all year Circulation features: tide-wind 3D model

Yanagi and Takao (1998) Circulation features: tide-wind 3D model

• Based on Naga Expedition: October 1959 – August 1960 • CW circulation occurs both NE & SW monsoons (nearly barotropic) Yanagi and Takao (1998) Seasonal variation of stratification features

Yanagi et al. (2001) • Naga Expedition + SEAFDEC cruises: September 1995 & April-May 1996 • Buranapratheprat et al. (2016): • Surface heat flux + tidal stirring are main control stratification followed by freshwater flux • NE monsoon: Well-mixed condition due to large tidal stirring, surface heat loss and low freshwater input • SW monsoon: Strong stratification due to high salinity intrusion and wind-driven surface outflow Andaman Sea (AS) Geography

• Andaman-Nicobar Islands (ANIs) • 3 main channels (exchange btw AS and Bay of Bengal (BoB)) • Great Channel • 10 degree Channel • Preparis Cbannel • Mean & max. depth ~ 1,100m & 4,419m • Deepest part exists at the central AS near the ANIs General oceanographic features of the AS

• Also under the influence of monsoonal winds • 4 seasons are defined • winter monsoon (December – February), • first inter-monsoon (March – May), • summer monsoon (June – August), and • second inter-monsoon (September – November) • Summer brings large rain to the area: strong stratification is established • As a consequence: shoaling/deepening of oxygen minimum zone General oceanographic features of the AS Form number (F) Tide types

Rizal et al. (2012) Dynamically important

• It favors coastal Kelvin wave, a poleward propagation of waves along the continental slope of the basin • During the inter-monsoon periods, Kelvin wave is intensified by strong remotely Wyrtki Jet causing the increasing of SSH toward east coast (it is downwelling condition) • SW, a local wind generates downwelling condition • Upwelling associates with internal wave is also observed

Wall et al. (2012) Surface circulation

• Varkey et al., (1996) and Potemra et al., (1991) showed CW circulation during NE and CCW circulation during SW monsoon • In contrast, clockwise circulation in both monsoons (Rizal et al., 2012; Wyrtki, 1961). Internal waves

• The interaction of strong barotropic tidal currents with abrupt bottom topography (Garrett and Kunze, 2007) • Normally, it is found during NE monsoon when the tidal range is large • Several channels at ANIs, cause internal tides to generate and propagate toward the AS coast (Alpers et al., 1997; Hyder et al., 2005) • From temperature profile, it is showed that internal tide forms at depth less than 100 m Jackson et al. (2012) Biological important

• The Andaman Sea is rich in its bio-diversity • However, the deep basin of Andaman Sea suffers from the very low oxygen level, due to the strong stratification and limited connection with Indian Ocean (Chatterjee et al., 2017; Dutta et al., 2007; Kamesh Raju et al., 2004) Biological important

• The large amplitude internal waves that have high oxygen concentration at the top layer and low oxygen concentration at the lower layer may break when they reach the continental shelf • Coral reefs may positively/negatively response to the change of temperature due to internal tides caused upwelling • The enhanced hypoxia condition is the consequence of high production due to the upwell of enriched nutrients generated by breaking of internal tides. Thank you very much