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Jurnal Ilmu dan Teknologi Kelautan Tropis, Vol. 6, No. 2, Hlm. 355-371, Desember 2014

A NUMERICAL MODELING STUDY ON UPWELLING MECHANISM IN SOUTHERN STRAIT

Agus S. Atmadipoera1* and Priska Widyastuti1, 1Department of Marine Science and Technology, Agricultural University, Bogor; *E-mail: [email protected]

ABSTRACT While it has been well documented in the previous studies that upwelling events in the southern (MAK) during the Southeast Monsoon (SEM) period are associated with low surface temperature (SST) and high chlorophyll-a (Chl-a) concentrations in the seawater, the dynamic and physical processes that trigger these upwelling events are still less well understood. In the present study we proposed a mechanism of the upwelling event using a numerical model of the Regional Modeling System (ROMS). Model validations showed a high correlation of SST climatology between the model and the NOAA-AVHRR satellite data. Moreover, velocity fields of the (ITF) Makassar in Libani Channel was well reproduced by proposed model, revealing an intensification of the flow centered near 120 m depth, which is in good agreement with the observation data. The model demonstrated that during the SEM period strong southeasterly winds that blow over southern Island can increase high vertical diffusivity and heat loss through heat flux. Hence, these physical processes lead to increased vertical mixing that, in turn, generates low SST, as a proxy of upwelling event. Furthermore, the upwelling process is enhanced by the ITF Makassar jet that creates large circular eddies flow due to complex topographic within the triangle area of southern Makassar - eastern Sea - western . The eddies generate the area of convergence offshore along the ITF pathways and divergence area in the coastal waters close to southern Sulawesi Island. Model experiment with closing/opening revealed a change of intensity and area of upwelling, suggesting that the Selayar Island forms a barrier for the outflow from MAK to northern part of Flores Sea.

Keywords: Upwelling, ITF Makassar, SE monsoon winds, ROMS-AGRIF, Makassar Strait.

ABSTRAK Upwelling di bagian selatan Selat Makassar (MAK) dicirikan oleh rona khas permukaan laut dari data satelit, seperti rendahnya suhu permukaan laut (SPL) dan tingginya klorofil-a yang terjadi dalam periode monsoon tenggara (SEM). Kajian proses interaksi laut-atmosfer dan dinamika laut yang memicu terjadinya upwelling di wilayah ini masih belum banyak diteliti. Tujuan dari makalah ini untuk mengkaji mekanisme upwelling dengan menggunakan pemodelan laut ROMS-AGRIF. Hasil model menunjukkan bahwa gaya pembangkit utama upwelling adalah dorongan angin kuat monsoon tenggara yang menghasilkan transport Ekman kearah baratdaya di kawasan selatan Pulau Sulawesi. Gesekan angin permukaan ini dapat meningkatkan difusivitas vertikal dan pelepasan fluks bahang ke atmosfer. Sehingga proses tersebut meningkatkan aktivitas percampuran massa air secara vertikal, yang berimplikasi terhadap menurunnya SPL. Proses upwelling ini juga diperkuat oleh adanya resirkulasi dari jet Arus Lintas (ARLINDO) Makassar yang membentuk pusaran arus besar (eddies), yang terjadi karena konfigurasi topografi yang kompleks di sekitar wilayah studi. Pusaran arus tersebut berimplikasi terhadap terbentuknya wilayah konvergensi di laut lepas pada lintasan ARLINDO serta wilayah divergensi di perairan dekat pantai di selatan Pulau Sulawesi. Selain itu, hasil simulasi dengan eksperimen on/off Selat Selayar, menunjukkan bahwa konfigurasi Kepulauan Selayar dapat mempengaruhi intensitas dan luasan upwelling di wilayah studi.

Kata kunci: Upwelling, ARLINDO Makassar, Angin Monsun Tenggara, ROMS-AGRIF, Selat Makassar.

@Ikatan Sarjana Oseanologi Indonesia dan Departemen Ilmu dan Teknologi Kelautan, FPIK-IPB 355 A Numerical Modeling Study on Upwelling …

I. INTRODUCTION associated with a large standard deviation of SST and Chl-a concentrations (Syahdan Southern Makassar Strait (MAK), et al., 2014ab) (Figure 1). The analysis of which located in Southern Sulawesi area upwelling indication from satellite (2-8 ºS and 116-122 °E), is well-known for imagery had previously been investigated having a high abundance of marine and (Setiawan et al., 2010; Habibi et al., fisheries source in the field of fisheries. 2010), however, its dominant forcing and This area includes the Makassar Strait, mechanism are not well understood yet. A and Flores Sea. In May, MAK is better knowledge of the upwelling mecha- covered by warm sea surface temperature nisms in this area is essential. Among the (SST) of about 29-30 ºC and the concen- methods to describe the mechanism are by tration of Chlorophyll-a (Chl-a) is typica- numerical model approaches such as lly low (~0.3 mg m-3). In August, how- Regional Ocean Modeling System ever, the Chl-a concentration increases to (ROMS) and its nesting enabled version a maximum of 1.3 mg m-3 (Setiawan et (Adaptative Grid Refinement in Fortran- al., 2010). From December to March the AGRIF). These models are a free surface surface waterin this area usually has the terrain following primitive equation highest temperature and the lowest sali- hydrostatic model, configurable for realis- nity, whereas in the period from June to tic regional applications (Marta-Almeida November the temperature is low and the et al., 2010). salinity is high (Susanto et al., 2012). ROMS-AGRIF has been applied to High-salinity surface Makassar water is investigate numerous ocean phenomena, advected into Java Sea during the Sout- e.g., the meso-scale eddy-induced reduc- heast Monsoon (SEM) period (Atmadi- tion in eastern boundary upwelling sys- poera et al., 2009). tems (Gruber et al., 2011), the upwel-ling The MAK is also influenced by limitation by geostrophic onshore flow Indonesian Throughflow (ITF) jet which (Marchesiello et al., 2011), the simulation is passing the Makassar Strait as the of phytoplankton ecosystem dynamics in primary inflow path of ITF within the the California Current System (Gruber et interior Indonesian . The mean trans- al., 2006), a study of chlorophyll bloom in port volume in the Makassar Strait from the western Pacific at the end of the 1997- January 2004 through November 2006 1998 El Nino (Messie et al., 2006), and a was 11.6 ± 3.3 Sv (Sv=106 m3s-1) and high-resolution modeling of the sediment reached its maximum towards the end of erosion and the particle transport across the northwest and southeast monsoons the northwest African shelf (Karakas et (April-June), with minimum transport was al., 2006). from October to December (Gordon et al., A few research about the upwel- 2008). During the SEM, the surface ling phenomenon in this have been southeasterly winds blow steadily with the conducted. Besides, the complexity of the gradual increase of the wind speeds from topography which induces an eddy-type May to August and the winds are quasi- throughflow from Kartadikaria et al. parallel to the coastline of Southern (2012), eddy-resolving model results Sulawesi Island (Habibi et al., 2010). This proposed the hypotheses regarding the situation leads to the upwelling event, interaction to the upwelling process. This which is followed by high abundance of present study describes the dynamics and fish and other marine biota in this area physical processes of the upwelling (Setiawan et al., 2010; Habibi et al., mechanism based on the following 2010). The upwelling event is highly hypotheses: 1) coastal divergence around

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Figure 1. Standar deviation of sea surface temperature [unit in °C] (left) and surface chlophyll-a concentration [unit in mg/l] (right), showing high standard deviation of parameters in southern tip of Sulawesi Island (after Syahdan et al. 2014ab)

MAK; 2) recirculation of surface ITF ORCA05 climatology simulations (1970- affected by the SEM winds; 3) water 2003). exchange across Selayar Strait; or 4) the The model domain covers the combination of the three hypotheses is upwelling area in MAK between 2-8 ºS causing the upwelling in the MAK area. In and 116-122 ºE. The bottom topography in addition, we attempted to describe the MAK is complex, with a narrow channel impact of the interaction between topo- directing towards the southeast and a graphy and sea currents on the upwelling Dewakang sill (~860 m depth) just west of intensity, distribution of upwelling and the Selayar Island (Figure 2). Selayar Strait is annual cycle of atmospheric and oceanic located between southern tips of Makassar parameters in MAK. and Selayar Island, which may convey the water exchange between MAK and Flores II. METHODS Sea.

2.1. Data 2.2. Model Configuration Inputs of the model were obtained The model configuration was from the World Databases and Research performed by ROMS-AGRIF. ROMS Center (Penven et al., 2007). Surface resolves the primitive equations (Boussi- forcings (heat flux, air-sea parameter, nesq approximation and hydrostatic verti- freshwater flux) were obtained from cal momentum balance). ROMS is a free COADS05 (Comprehensive Ocean Atmo- surface oceanic model (short time steps sphere Data Set), bathymetry was obtai- are used to advance the surface elevation ned from ETOPO-01, high resolutions and barotropic momentum equations (9.28 km) monthly global SST was (larger time steps) are used for tempe- obtained from AVHRR (Advanced Very rature, salinity, and baroclinic momentum) High Resolution Radiometer) - Pathfinder with robust open boundaries, grid refine- Observations 1985-1997, wind stress ment, sediment, and ecosystem modules. monthly climatology was obtained from ROMS simulation needs horizontal grid QuickSCAT, sea water properties was data (grid position, grid size), topography, obtained from World Ocean Database land mask, surface forcing (wind stress, (2006) and lateral boundary conditions surface heat flux, and freshwater flux), were obtained from Drakkar INDO- initial conditions (temperature, salinity,

Jurnal Ilmu dan Teknologi Kelautan Tropis, Vol. 6, No. 2, Desember 2014 357 A Numerical Modeling Study on Upwelling …

Figure 2. Model domain in southern Makassar Strait and part of western Flores Sea and eastern Java Sea. Interval of isodepth contours is every 500 m. Libani Channel is deep narrow passage near 119E, 3S. currents, sea surface height) and lateral requires special treatment and coupling boundary conditions. between barotropic (fast) and baroclinic In the primitive equations in Carte- (slow) modes. In the vertical, the primitive sian coordinates, the momentum balance equations are discretized over variable in the x- and y-directions are: topography using stretched terrain- following coordinates. In the horizontal, 휕푢 1 휕푃 + 푢 . ∇푢 − 푓푣 = − the primitive equations are evaluated 휕푡 휌0 휕푥 휕 휕푢 using boundary-fitted, orthogonal + ∇ 퐾 . ∇ 푢 + 퐾 (1) ℎ 푀ℎ ℎ 휕푧 푀푣 휕푧 curvilinear coordinates on a staggered 휕푣 1 휕푃 + 푢 . ∇푣 − 푓푢 = − + Arakawa C-grid. The equation of state is 휕푡 휌0 휕푦 휕 휕푣 given by: ∇ℎ 퐾푀ℎ . ∇ℎ 푣 + 퐾푀푣 (2) 휕푧 휕푧 휌 = 휌 푆, 푇, 푃 (3) u and v are 2-D velocity fields; and S is salinity and T represents temperature. are advection terms; is Coriolis In the Boussinesq approximation, density parameter; is water density; is total variations are neglected in the vertical pressure; h is mixed layer depth; KMh is momentum equations except in their horizontal mixing coefficient; and KMv is contribution to the buoyancy force in the vertical mixing coefficient. The vertical momentum equation. Under the hydrostatic primitive equations for hydrostatic approximation, it is further momentum are solved using a split- assumed that the vertical pressure gradient explicit time-stepping scheme which balances the buoyancy force:

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휕푃 0 = − − 휕푔 (4) 푢 . ∇ −퐻 = 푤 (13) 휕푧 휕푢 −퐶 |푢 |푢 퐾 = 푑 (14) is total pressure; and is acceleration of 푀푣 휕푧 휌0 휕푣 −퐶 |푢 |푣 퐾 = 푑 (15) gravity. The final equation expresses the 푀푣 휕푧 휌0 continuity equation for an incompressible 휕푇 퐾 = 0 (16) fluid: 푇푣 휕푧 휕푆 휕푢 휕푣 휕푤 퐾푆푣 = 0 (17) 0 = + + (5) 휕푧 휕푥 휕푦 휕푧 Navier-Stokes equation is used in The diagnostics analysis was diagnostic analysis: computed online used tracer equation 휕푢 휕푢 휕푢 휕푢 terms: + 푢 + 푣 + 푤 − 푓푣 = 휕푡 휕푥 휕푦 휕푧 휕푇 1 휕푃 + 푢 . ∇푇 = ∇ℎ 퐾푇ℎ . ∇ℎ 푇 + − + 퐷𝑖푓푓(푢) (18) 휕푡 휌0 휕푥 휕 휕푇 퐾 (6) 휕푧 푇푣 휕푧 The model domain extends bet- 휕푆 º º º + 푢 . ∇S = ∇ 퐾 . ∇ 푆 + ween 2 S-8 S and 116 E to 122 E. The 휕푡 ℎ 푆ℎ ℎ 휕 휕푆 number of model grid is 83x72 points 퐾 (7) 휕푧 푆푣 휕푧 with a horizontal resolution of 1/12º. The model has 32 vertical levels, and the   u.T and u.S are advection terms ; vertical grid is stretched for increased boundary layer resolution. Model simula- KTh is horizontal mixing coefficient for tion has been performed for 10 years temperature; and K is vertical mixing Tv simulation. The 10th year simulation was coefficient for temperature. chosen for this study since it is considered Analysis on boundary conditions to have reached the statistical balance. are divided into two parts, with surface boundary conditions (z=η) and with 2.3. Description of Experiments bottom boundary conditions (z=-H). The A set of three experiments was equations for surface boundary conditions carried out to assess the oceanic response are: to the topography (Selayar Strait) and its 휕휂 = 푤 (8) 휕푡 sensitivity to the boundary conditions. The 휕푢 휏푥 three experiments named Scenario #1, 퐾푀푣 = (9) 휕푧 휌0 Scenario #2 and Scenario #3. Scenario 1 휕푣 휏 퐾 = 푦 (10) was the control run experiment. As a first 푀푣 휕푧 휌0 휕푇 푄 step, a year simulation was performed 퐾푇푣 = (11) 휕푧 휌0퐶푝 with the existence of Selayar Strait which 휕푆 푆 (퐸−푃) 퐾 = (12) separates Southern Sulawesi and Selayar 푆푣 휕푧 휌0 Island. where  and  are wind stress at x and y Scenario 2 was similar to Scenario x y 1, except that the Selayar Strait was directions; u,v,w are 3-D velocity fields; masked with land. This process was done -3 0 is sea water density (  = 1025 kg m by digitizing the land mask on the grid- 3 making process. In addition, the andC p = 4.1855 x 10 PSI); T is mixed slipperiness value in this scenario was +1. layer depth temperature; and K is vertical v This value shows the condition of free diffusivity coefficient (estimated by slip. Scenario 3 was also similar to ROMS KPP scheme). The equations for Scenario 2, except the slipperiness value bottom boundary conditions are:

Jurnal Ilmu dan Teknologi Kelautan Tropis, Vol. 6, No. 2, Desember 2014 359 A Numerical Modeling Study on Upwelling …

in this scenario was -1. This value showed mum velocity reaches 0.8 m/s near 150 m the condition of no slip. depth (Figure 4), which was in good agreement with previous experiments 2.4. Model Validation conducted by Susanto et al. (2012) and Annual cycle of SST was obtained Gordon et al. (2008). These studies by taking the daily SST value of the showed that in the upper 200 m velocity numerical model. It was well correlated (r clearly exhibits a thermocline intensifica- > 0.9) to the daily mean SST of NOAA tion with a maximum velocity close to AVHRR satellite imagery (Figure 3) and 120 m. Thus, the model showed its constantly showed large changes (SST suitability to adequately reproduce the minimum) from July to September. The observed flow and properties. SST time series from NOAA AVHRR was processed by taking the daily SST III. RESULTS AND DISCUSSION between 2007 and 2012 to make a daily annual cycle. Subsequently, the climatolo- 3.1. Annual Cycle of Upper Circulation gical mode was applied to the time series. The climatology of the simulated The minimum SST occurred during the circulation in MAK was analyzed at three SEM period between July and September, different depth levels (10 m, 50 m and 100 which was about a one-month delay of the m) (not shown) and seasonal periods as southeast monsoon winds. However, the shown in Figure 5. Much stronger ITF warmest (SST maximum) surface waters flow occurred during the SEM (June to occurred during the NWM from August), which generated a strong recir- December to March. culation in the southern tips of Sulawesi The meridional currents velocity Island. However, during the NWM, strong was compared between the model and surface flows came from Java Sea to previous experiments. It was found that , and a strong flow during the the daily averaged model southward maxi- SEM came from the ITF branch. During

Figure 3. The validation result of ROMS SST and NOAA AVHRR SST satellite imagery.

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a) b) c)

Figure 4. The MAK depth profiles of velocity near Libani channel (a) computed by model for the period of 1 year, (b) derived from the ADCP and current meter data for the period January 2004 through May 2009 (Susanto et al., 2012) and (c) derived from the ADCP and current meter data for the period 2004 to 2006 (Gordon et al., 2008). The vertical coordinates are given in decibar (dbar), which is approximately a meter (m). Figure 4b, 3c shows 3-month averaged velocity profile for January to March (JFM), April to June (AMJ), July to September (JAS), and October to December (OND). the SEM, the southeasterly wind speeds depth) between the Java and Flores seas are fully developed and are parallel to the indicates a possible area for eddies. The coastline. The southeasterly winds begin recirculation on the ITF pathway is to intensify in June. However, in the associated with the Southeast Monsoon second half of June until early July the that passes the southern Makassar Strait. wind speeds slightly decrease, denoting a The development of wind stress curls relaxation of the monsoon winds (Habibi during Southeast Monsoon generates an et al., 2010). anticyclonic eddy pattern in the north of The surface currents at 10 m and 50 Sumbawa Island (Southern Makassar m (not shown) are partly deflected Strait) (AMJ and JAS on Figure 5). It is in through Selayar Strait and recirculate to good agreement with Kartadikaria et al. the ITF pathway. The complexity of the (2012), that the upwelling region found to topography and coastline at the entrance the east of the Flores Eddy (FE) may of the MAK induce an Eddy-type through represent a key origin of mass transport flow rather than a straightforward flow for the convergence comprising the (Kartadikaria et al., 2012). It is also agree Ekman induced upwelling in the FE with Kartadikaria et al., (2012), which region. In figure 5 this is highlighted by found the Eddy-type throughflow based white arrows on JAS period. This process on model results and the results of drifters causes an upwelling of deep sea water to release field experiment around the target the sea surface. This water mass accu- western Flores Sea region and examined mulation shapes an anticyclonic eddy the influence of eddies on the vertical pattern that deepens the mixed layer and temperature structure through verification the thermocline in Flores Sea. using the available existing dataset. The existence of a steep deep basin (>500 m

Jurnal Ilmu dan Teknologi Kelautan Tropis, Vol. 6, No. 2, Desember 2014 361 A Numerical Modeling Study on Upwelling …

Figure 5. Seasonal variability of sea surface currents. The figure shows 3-month averaged velocity profile for January to March (JFM), April to June (AMJ), July to September (JAS), and October to December (OND).

3.2. The Interaction of Topography on June and gradually develops until August. SST and Sea Currents In September, the Southern Makassar SST modelling was conducted by Strait begins to warm up in September calculating the average of daily SST data (Setiawan et al., 2010). From this result, into the monthly SST for one year com- we could conclude that the upwelling putation. Three scenarios were applied to event in MAK was indicated by strong describe the main factors which generate surface winds during the SEM, which the upwelling system of southern potentially generate a wind-driven Makassar Strait. A control experiment upwelling. (Figure 6) showed that the upwelling The model results for Scenario 2 region of southern Makassar Strait has a (Figure 7) and Scenario 3 (not shown) lower temperature compared to other showed a much larger area of upwelling in close-by over the entire model the southern Makassar Strait. The period, with high an intensification of the presence of Selayar Island in Southern cool surface water during the Southeast Sulawesi Island played a dominant role as Monsoon. The SST reached a minimum a barrier for the (MAK) flow to the on August of less than 25 ºC. The SST Flores/Banda Seas. This condition caused startd to decrease in May and to increase the west-ward currents to generate a half- in September. A previous study showed recirculation following the ITF route. The that a low SST area clearly appears in land masses connecting Sulawesi island

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Figure 6. Evolution of SST and sea currents for Scenario 1 (control experiment). SST is computed for yearly mean in one year at depth 10m.

Figure 7. Evolution of SST and sea currents for Scenario 2. SST is computed for yearly mean in one year at depth 10m. Selayar Strait closure was digitized by defined it into land which connects the Selayar Island with Sulawesi Island.

Jurnal Ilmu dan Teknologi Kelautan Tropis, Vol. 6, No. 2, Desember 2014 363 A Numerical Modeling Study on Upwelling …

and Selayar Island generated an upwelling cement (Sterl et al., 2003). It was in the southern Makassar Strait. This was consistent with Renault et al. (2012), that caused by a narrow passage for the east- winds intensification could increase the ward currents to directly cross the heat loss by heat flux. Temperature southern Makassar Strait. This recircula- decrease by surface heat flux in surface tion developed into a convergence area for layer was distributed horizontally toward surface and subsurface currents into the offshore, which may cause an unstable ITF pathway. High transport volumes and convection condition. Strong mixing by velocity of ITF during the SEM, provoked wind caused vertical mixing in water the surface currents to form a strong eddy. column. This results in a relay process that Consequently, a coastal divergence carries the water mass to the surface occurred around the southern part of the through upwelling process. Sulawesi Island’s coast and the balance Oceanic variables in transect A-B between wind stress and Coriolis lead the (Figure 9) showed similar variations Ekman transport directed toward the between sea surface temperature, potential southwest, away from the coastline. temperature, salinity, sea surface elevation and vertical diffusivity during the 3.3. The Relations between Atmosphe- Southeast Monsoon (June-October). An ric and Oceanic Variables on increase of vertical diffusivity was Upwelling Intensity followed by low temperatures. This Atmospheric variables (Figure 8) condition was caused by strong intensity showed a distinct variation during the of winds during the Southeast Monsoon SEM. A drastic change of SST occurred in which caused a water transport of high May with an SST minimum in August. salinity water from Makassar Strait ti The SST started to increase in October Flores Sea and Banda Sea (Gordon et al., with its peak in April. A similar pattern 2003). was found in meridional wind stress with Vertical sections of oceanic variables a maximum intensity in August. Low (Figure 10) were computed for February temperatures occurred in the coastal area and August. These months represented the (highlighted by point A in Figure 8), and period of NWM and SEM seasons. Low increase at point B. This fluctuation also SST and high salinity in (MAK) indicated corroborated with wind stress (zonal and an upwelling event that occurred during meridional), and shortwave radiation the SEM. It followed the fluctuation components. pattern of zonal and meridional velocity Wind stress played a major role in and kinetic energy. During this season, SST changes during the upwelling period. zonal and meridional velocities increase Strong wind stress was correlated with and the currents flow southwestward, strong wind speed in southeasterly away from the coastal area. Intensification direction during the SEM. Heat flux of kinetic energy and velocity within the components showed that longwave sea surface contributes to strong meso- radiation and sensible heat reveal much scale activities (jets and eddies), which higher heat loss compared to the other were typical to the characteristics of components. Heat transfers between the upwelling (Lathuiliére et al., 2010). atmosphere and the sea surface are caused by conduction and convection processes 3.4. Diagnostic Analysis as well as evaporation and surface stress Diagnostic analysis was computed (tension). The increase of heat flux wa to analyze the contribution of each of the influenced by wind stress intensity enhan- surface forcing components. The vertical

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Figure 8. Hovmüller diagram of atmospheric variables over the transect A-B.

Figure 9. Hovmüller diagram of oceanic variables in transect A-B.

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Figure 10. Vertical section of oceanic variables for February (Northwest Monsoon) (left) and August (Southeast Monsoon) (right).

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advection has a higher value during the to the advection, pressure gradients and Southeast Monsoon (Figure 11), contrast vertical mixing components were much to the horizontal advection. Advection is a higher during the SEM. A high pressure process of water properties transport (heat, gradient indicated a high density gradient salinity, other properties) through sea in southern Makassar Strait. This condi- wave (Kӓmpf, 2009). The meridional tion triggerred the upwelling event in this component shows the southward flow in area. the ITF path. It is caused by a high Vertical mixing processes caused intensity of ITF’s transport in southern vertical diffusion and vertical advection. Makassar Strait. This condition results in Wind stress generated horizontal advec- a heat change and a deeper mixed layer tion and vertical mixing. These mecha- depth, raised by southeast monsoon winds nisms changed the water density. Hori- over the southern Makassar Strait. Similar zontal advection provided the most signi-

Figure 11. Components of forcing for February (Northwest Monsoon) and August (Southeast Monsoon) in Southern Makassar.

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ficant effect with a strong variability to September with strong affection of (Foltz et al., 2006). High pressure during wind stress and caused a decrease of the the SEM caused the currents flow towards SST and an increase in primary water with lower pressure, which results production in this area (Gordon et al., in horizontal and vertical gradient of 2005). density. The Coriolis effect plus the fric- Wind stress blew strongly to the tional coupling of wind and water (Ekman northwest during this season and caused transport) directed the surface currents in an increase of vertical diffusivity. More- a 90º angle to the left from the coastal area over, it resulted in a positive heat flux with a constant pressure and conducts a (heat loss). The heat floss due to heat flux coastal divergence. lead to an increase of water density and vertical mixing, which also was 3.5. Proposed Mechanism of Upwelling influenced by the combination of vertical In Figure 12 the proposed mecha- diffusivity and vertical velocity. This nism and physical processes of the upwel- condition caused upwelling in MAK. ling, as performed by the model, were Moreover, the SEM winds also summarized. The mechanism of upwelling caused a change in the advection in MAK was mainly generated by the (horizontal and vertical), the pressure Southeast Monsoon winds blowing over gradient and the vertical mixing, which in this area from June to August (Figure 12), turn, affected the water density. inducing a wind-driven upwelling associa- Both the ITF recirculation and the ted with Ekman pumping. High Ekman presence of the Selayar Strait had a role to pumping occurred in the MAK from July provoke the upwelling occurrence in

Figure 12. Proposed mechanism of upwelling in MAK.

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MAK. The presence of Selayar Strait as a The Southeast monsoon winds barrier for the current outflow to Flores could be the main factor for the upwelling Sea and the complex topography in this mechanism in the Southern Makassar area caused the sea current to return to Strait. For this reason, further research in ITF pathway and to form an Eddy, which this study is highly needed by setting up was associated with the Southeast real calendar years in computation. Monsoon. This condition set an increase Furthermore, a diagnostic analyses in the of upwelling intensity and results in a bottom boundary layer and mixed layer decrease of temperature and surface depths are needed to understand the elevation as well as an increase of salinity. contribution of each forcing component to the upwelling processes. A model experi- IV. CONCLUSIONS ment by setting different winds directions, e.g., southerly or northerly winds and The cause of upwelling in southern flattering bathymetry of MAK are also Makassar Strait, indicated by the decrease needed to investigate the role of topo- of temperature during the SEM was the graphy in the eddy formation. background for the hypotheses of the mechanism of upwelling in this area. The ACKNOWLEDGEMENTS model revealed that the mechanism of upwelling was mainly associated with The authors are grateful to the Southeast Monsoon winds, which fully ROMS developers, particularly to Dr. developed over MAK between June and Gildas Cambon and Prof. Patrick Marche- August. During this season, strong winds siello for their support, discussion, and triggered the vertical diffusivity. High comments on this work. We also wish to intensity of wind stress also enhanced the thank to the reviewers for their advice on heat loss by heat flux. This condition how to improvement early version of this resulted in a strong vertical mixing. manuscript. ITF in Makassar Strait played a role in affecting the mechanism of REFERENCES upwelling. ITF caused a recirculation by forming an eddy that caused a Atmadipoera, A., R. Molcard, G. Madec, convergence in the pathway of the ITF S. Wijfels, J. Sprintall, A. Koch- and a divergence in the coastal area. The Larrouy, I. Jaya, and A. Supangat. Selayar Strait in southern of Sulawesi 2009. Characteristics and variabi- Island was the gate for the outflow current lity of the Indonesian Throughflow from this recirculation and the current was Water at the outflow Straits. Deep partially deflected to Flores Sea. It Sea Research I, 56:1942-1954. showed that Selayar Strait had a minor England, M. and F. Huang. 2005. On the role in decreasing the intensity of interannual variability of the Indo- upwelling and the existence of Selayar nesian Throughflow and its linkage Island and complexity of topography with ENSO. J. Clim., 18:1435- triggered the ITF recirculation. From these 1444. doi:10.1175/JCLI3322.1. results we concluded that the upwelling Foltz, G.R. and McPhaden, M.J. 2006. mechanism was caused by the combina- The role of oceanic heat advection tion of the three hypotheses which were in the evolution of tropical North mainly associated with Southeast and South Atlantic SST anomalies. Monsoon winds. J. Clim., 19:6122-6138.

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