The Makassar Strait throughflow and its jet

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jc007809, 2012 The Makassar Strait throughflow and its jet B. Mayer 1 and P. E. Damm 1 Received 12 December 2011; revised 25 April 2012; accepted 9 June 2012; published 26 July [1] A nested numerical model system has been set up to realistically simulate almost 40 years of the Indonesian throughflow (ITF). A global circulation model delivered the boundary values for sea surface height, temperature and salinity distribution to a fine resolution regional model of the ITF. The results of the regional model are in good agreement with measured data regarding velocity distribution, stratification as well as transported water masses, even though the division of the ITF volume transport into its western and eastern branches differs slightly from estimates from other model results or measurements. The results show a current system highly variable in space and time. Here, the analysis of model results focuses on the western branch of the ITF, the Makassar Strait throughflow, which is estimated to account for up to 50 to 80% of the entire ITF. The climatology and the mean vertical structure are presented. The model results show that the Makassar Strait throughflow occurs as a distinct current, which we termed Makassar Current. It is regionally developed as a subsurface jet, and it behaves like a western boundary current as it is attached to the western boundary (Sunda Shelf edge) along its entire path from the Sulawesi Sea through the Makassar Strait to its direct exit through the Lombok Strait. It appears from model experiments that the local wind stress counteracts the Makassar Strait throughflow. Citation: Mayer, B., and P. E. Damm (2012), The Makassar Strait throughflow and its jet, J. Geophys. Res., 117,, doi: /2011jc Introduction [2] Exchange of water and heat from the Pacific Ocean into the Indian Ocean occurs through the Indonesian throughflow, which is the only natural connection between two tropical oceans on earth. The ITF is divided into western and eastern branches. The western pathway carries waters from the northern tropical Pacific Ocean via the Mindanao Current through the Sulawesi Sea, the Makassar Strait into the Flores Sea and then through the Lombok Strait or through the Banda Sea and Ombai Strait into the Indian Ocean. According to our own model results and to other authors (for a summary, see Du and Qu [2010]), a contribution may also come from the Kuroshio Current entering the South China Sea through the Luzon Strait, then joining the aforementioned part of the ITF ahead of the Makassar Strait via the Sulu Sea in the Sulawesi Sea, or in boreal winter season after it via the Karimata Strait and Java Sea in the Flores Sea. This would also be water from the northern tropical Pacific Ocean, since both the Mindanao and the Kuroshio Currents are fed by the North Equatorial Current (NEC). The eastern branch consists of two sub branches, both transporting water from the southern 1 Institute of Oceanography, Center for Marine and Atmospheric Sciences, University of Hamburg, Germany. Corresponding author: B. Mayer, Institute of Oceanography, Center for Marine and Atmospheric Sciences, University of Hamburg, Bundesstr. 53, DE Hamburg, Germany. (bernhard.mayer@zmaw.de) American Geophysical Union. All Rights Reserved /12/2011JC tropical Pacific Ocean through the Maluku or the Halmahera Seas and the Banda Sea into the Indian Ocean. [3] A recent description of the present general understanding of the ITF is given by Tillinger [2011]. Due to the high spatial and temporal variability of the ITF, it is very difficult to investigate the structure and dependencies of the ITF. With this, it is also difficult to make an educated guess as to the relative importance of the Makassar Strait throughflow. Estimations of its contribution to the ITF, as performed from measurements and numerical model experiments, for example, by Gordon et al. [2008], Metzger et al. [2010], and Tillinger and Gordon [2010], range from about 50 to 80% of the total ITF volume transport, indicating that it is a very important fraction of the ITF. [4] A very interesting feature of the Makassar Strait throughflow is its vertical structure: a strong jet-like current occurs most of the time in the upper water regions from surface or subsurface down to depths of 300 to 500 m. Within the narrow passage of the Labani Channel and south of it, the core of this current is located at 110 to 150 m depth, carrying water with speeds exceeding 1 m s 1. The thermocline maximum in the Labani Channel has been measured [Gordon et al., 1999, 2003], estimated from SODA reanalysis data [Du and Qu, 2010] and also simulated [Mayer et al., 2010; Metzger et al., 2010]. This nearly year-round existing feature, with intensified values during boreal summer monsoon season, can be explained by the wind-induced circulation of surface layer waters south of and within the Makassar Strait [Gordon et al., 2003]. 1of14

2 [5] Within the German-Indonesian cooperation project Long-term Indonesian Throughflow Model Simulation (LITHMOS), the ITF was simulated for the period 1970 to The overall goal of the joint project was to numerically model the ITF as realistically as possible. The simulations were made with a nested model system consisting of an OGCM and a fine resolution regional model that used most realistic topographic and forcing data available. An overview of some interesting findings from the model results is given in Mayer et al. [2010]. [6] This article focuses on the presentation and analysis of simulation results with respect to the Makassar Strait throughflow, even though the model results reveal a lot more interesting results. We will follow it to its direct connection with the ITF outflow through the Lombok Strait. We will not consider the outflow regions of Ombai Strait and Timor Passage, which balance most part of the Makassar Strait throughflow. Section 2 explains the applied numerical model system and verifies the simulation results. The climatology of the Makassar Strait throughflow is described in section 3. Sections 4 and 5 give some aspects on the current doing the main part of the transport, and the subsurface jet measured and simulated in the Labani Channel. Section 6 relates the strong Makassar current to western boundary currents. Section 7 presents interesting results of two model experiments, while the last section contains the conclusion. 2. Applied Model System and Verification of Model Results [7] A nested model system has been set up and applied for a period of 37 years (1970 to 2006) to simulate as realistically as possible the ITF and the Indonesian Seas circulation: The global ocean circulation model MPI-OM [Marsland et al., 2003; Jungclaus et al., 2006], which was used for the IPCC model simulations for the estimation of the development of the world s climate, simulated the global circulation on a tri-polar grid with a horizontal resolution of 24 ( 44 km in lower latitudes). The vertical grid resolution is based on a z-coordinate system with 40 layers of increasing thickness from 6 m for the most upper layers to a few hundred meter for the lowest layers. Meteorological forcing was taken from the NCEP/NCAR reanalysis data [Kalnay et al., 1996]. For the region itself, the three-dimensional ocean model HAMSOM (Hamburg Shelf Ocean Model) [Backhaus, 1985; Pohlmann, 1996, 2006] was applied with horizontal resolution of 5 (approx. 9 km), also working on a z-coordinate grid with 31 layers of increasing thickness (6 m for the most upper layers to a few thousand m for the lowest layer). The simulations were driven with the same NCEP forcing as the global model. The topography for the regional model was composed of different worldwide data sets by GEBCO Team [2003] and ETOPO5 [NOAA, 1988], which were partly corrected using satellite images and navigational charts. This was especially done for some of the narrow passages between islands. The left panel of Figure 1 presents the bathymetry for the domain of the regional model. [8] Initial temperature and salinity distributions were interpolated from monthly and seasonal data of the World Ocean Atlas [Levitus, 1982], version 2 with 0.25 resolution [Boyer et al., 2005]. At the open boundaries of the regional model, simulation results (daily averages) from the global model were used for the sea surface heights as well as the vertical structure of temperature and salinity. This barotropic and baroclinic structure (pressure and density gradients) influenced the velocity components adjacent to the open boundaries according to the equations of motion. The velocity components along the open boundaries were then calculated using the Sommerfeld radiation condition following Orlanski [1976] in order to decrease numerical disturbances due to reflection of waves from the interior of the model domain. With this, only pressure and density distributions, but no transport (no velocity), were prescribed through or along the open boundaries. [9] Tidal forcing was not included, because the focus of this study was on long-term phenomena. Kartadikaria et al. [2011] point out from comparing ocean model results with observations that the tides are important due to the change of vertical mixing and hence the vertical structure of the water column. In our simulations, larger horizontal and vertical exchange coefficients were used to account for the effect of tidal mixing. The exchange coefficients are generally seen as depending on the shear of the velocity. In HAMSOM, the horizontal turbulent mixing coefficient is calculated according to Smagorinsky [1963], also depending on the shear (and the convergence/divergence). In ocean regions with naturally high velocity shear (turbulence), like shallow areas, coastal areas or areas close to large topographic gradients, the tidal influence on the mixing coefficient is stronger than in low velocity shear (laminar) regions of the ocean. By using an experimentally found tidal factor when calculating the exchange coefficients, this most important behavior is roughly included, even though non-linear effects are not. [10] The results of the regional model HAMSOM are in good agreement with measurements. For comparison, measured data have kindly been made available by the INSTANT project (International Nusantara Stratification and Transport project) [Sprintall et al., 2004]. An overview of the most important findings from the INSTANT project is given by Gordon et al. [2010]. [11] Figure 2 shows time series of measured and simulated daily averaged velocities at depth levels 50 m, 150 m, 350 m and 750 m for the Makassar Strait for the year The locations of the time series cover the red part of the line in Figure 1 (right) across the Labani Channel. For comparison at other locations, the reader is referred to Mayer et al. [2010]. When the measured data are filtered using a 14 day low-pass filter to eliminate the semi-diurnal and diurnal as well as the spring-neap tide oscillations. Therefore, we do the comparison without further filtering. [12] The model essentially reproduces magnitude and variability of the measured velocity. In deeper regions, the model seems to slightly underestimate the current speed. This could be an effect of comparing point measurements with integrated model data, as the measured data are from moored instruments at a particular depth, while model velocities are valid for a certain width of the model cell (approx. 9 km) and for a finite depth range, i.e., layer thickness, which increases with depth, e.g., at 750 m depth, the corresponding layer thickness is 150 m. It can be seen from Figure 2 that the highest velocities at the section of the Makassar Strait occur within the upper depth range and close to the western boundary of the channel. The velocity in the most eastern 2of14

3 Figure 1. (left) Model domain of the regional model HAMSOM for the Indonesian throughflow and (right) an enlargement of the Makassar Strait region. Bathymetry and scale depth are in meters. The orange-red line in Figure 1 (right) depicts the location of the section across the Labani Channel. The red part of the line covers the locations of INSTANT observations used here for comparison of measured and simulated data. water column (j = 77, black line) is the slowest at all depths and at all times. The most western one (j = 73, red line) is as high as the central ones at 50 m and 150 m depth, meaning that the strongest part of the flow in the Labani Channel seems to be attached to its western boundary. This holds even for northward directed flow for this depth range. At 350 m depth, the speed at the outer sides are quite similar and both lower than the central velocities, while at 750 m depth, the current s center still remains at the western side of the channel, as the most eastern velocity (black line) is the slowest (the most western ones do not exist anymore because of insufficient depth). 3. Climatology and Context of the Makassar Strait Throughflow [13] The monthly averages for the Makassar Strait flow through a section in the Labani Channel in sverdrups (1 Sv = 10 6 m 3 s 1 ) is shown in Figure 3 for different depth ranges for the period of For the location, the reader is referred to Figure 1 (right), where the orange-red line across the Labani Channel depicts this section. This period exhibits a slightly increased volume transport for the Makassar Strait throughflow compared to the entire longterm simulation by approx. 20%. An enhanced total ITF volume transport for the three-year period in comparison to earlier estimations of about 25 30% has been simulated within this project and also estimated from measurements by the INSTANT group [Sprintall et al., 2009]. The vertical structure, however, is about the same in our model results for both the short and the long periods. Here, the climatology of the short three-year period is presented, because later in this article it will be compared with results from two additional model experiments. [14] A clear seasonal signal is seen in the monthly mean throughflow at almost all depth ranges (vertical bold blue bars in Figure 3). During boreal summer months, June to August, when the SE monsoon prevails, the throughflow is strongest at all depth ranges, while during the boreal winter season (NW monsoon season), especially in December and January, the throughflow is weakest. In this season, the upper 50 m and the water region of m show a northward net transport. Both can be explained by the direct windforcing, which counteracts the throughflow during the NW monsoon phase. As explained by other authors [Gordon et al., 2003; Tozuka et al., 2007; Du and Qu, 2010], the NW monsoon flushes buoyant water masses from the South China Sea around Kalimantan through the Karimata Strait and the Java Sea into the Flores and Banda Seas, where it piles up and introduces a barotropic pressure gradient along the Makassar Strait from south to north. This leads to a reduction of the subsurface southward jet and a northward transport above and below this jet. During transition to the SE monsoon season, this pressure gradient is phased out, allowing for a fully developed ITF. 3of14

4 Figure 2. Comparison of simulated and measured velocities for the year 2004 at four depths of the INSTANT moorings in the Makassar Strait. The lines j =73 to j =77 are the time series in the model water columns covering the deep part of the Makassar Strait from west to east (red part of line across Makassar Strait in Figure 1). The line measured is the observed time series at the western INSTANT mooring. Measured data are along-channel velocities, model data are northward component of velocities, both positive to north. [15] In the second last section of this article, we will see that the direct wind stress seems to oppose the pressure gradient induced northward flow as well as the southward oriented surface flow after relaxation of the boreal winterly pressure gradient. [16] An important characteristic of the ITF and the Makassar Strait throughflow is the immense variability of the current system. This is evident from Figure 3, where the vertical thin red lines depict the standard deviations from the mean monthly values (vertical bold blue lines). Very often, they are even larger than the mean values themselves demonstrating that any picture of the current system is only a snapshot and can be very different just a few weeks later. [17] The blue horizontal lines show the overall average of the monthly mean transports for the different depth ranges (listed in Table 1). The main part of the average Makassar Strait throughflow, about 65%, happens within the depth range of m, obviously made by a subsurface strong current, which is located here. The contributions of the upper 50 m and the range of m is about 15% and 19%, respectively, while the currents below 700 m carry less than 2% of the water on the average because of the Makassar sill (Dewakang Sill, see Figure 1, right) at the southern end of the strait, which is about 680 m deep. [18] The embedding of the Makassar Strait throughflow into a greater current system can be seen from Figure 4a, which presents the simulated averaged volume transport for the period within the upper 420 m of the water column. The Makassar Strait throughflow appears to be part of a transport band connecting the north equatorial Pacific Ocean directly with the Indian Ocean: a minor part of the Mindanao Current (MC) flows into the Sulawesi Sea toward the coast of Borneo, where it is joined by another current coming from South China Sea through the leaky Mindoro Strait and the Sulu Sea [Qu et al., 2009]. Both proceed partly further southward into the Makassar Strait and further to the Indian Ocean, while another part of this joint flow returns eastward either back into the Pacific Ocean or to join a large gyre in the Sulawesi Sea. Similar patterns have been presented by other authors [Schmitz, 1996; Godfrey, 1996; Qiu et al., 1999; Fallon and Guilderson, 2008] suggesting that the MC and the Makassar Strait throughflow are more or less directly connected to each other. Since the distribution of the transport shown in Figure 4a is a three year average and the variability is very high (see Figure 3 and also Mayer et al. [2010]), this is, however, only another indication adding to 4of14

5 Figure 3. Monthly mean of the simulated volume transport through the Labani Channel section of Makassar Strait as depicted in Figure 1 in Sv for the entire depth and for different depth ranges. Blue bold bars are monthly mean values for the time period , red thin lines are the standard deviation from the mean. The blue horizontal lines show the overall average volume transports. Negative values mean southward transport. the speculation that the Makassar Strait throughflow is an extension of the MC. 4. The Distinct Makassar Current [19] The Makassar Strait throughflow occurs mainly with a distinct strong current, which enters the Makassar Strait as a surface current extending to depths of 200 or 300 m, depending on the season. Figure 4 shows the simulated horizontal distribution of the volume transport (in Sv) in October 2004 for three different depth ranges: Figure 4b depicts the wind-influenced upper 52 m of the water column, Figure 4c with depth range of m, where the subsurface jet is located, and Figure 4d displays the depth region m with the sub-jet part. Due to the aforementioned high variability, this is a snapshot but represents well an average situation. The current can be clearly seen in all three panels. [20] In the following, this distinct current will be termed the Makassar Current. Even though the major portion of the Makassar Strait throughflow is balanced by the exits through Ombai Strait and Timor Passage, our definition of the Makassar Current concerns only the flow connecting directly to the Lombok Strait exit. Water masses leaving the flow along this direct connection do not belong to the current anymore, which is a western boundary current as will be shown later. We will therefore follow this current along its direct path to the Lombok Strait. [21] To get an idea about the vertical position and extension of the current, Figure 5 shows vertical Hovmöller diagrams of the along-current velocity component at six different locations along the Makassar Current (locations are depicted in Figure 4) from north of the entrance to south of the exit of the Makassar Strait. Looking at the two most northern ones (a and b), the current exhibits its strongest markedness mid of every year at the surface. The third position c seldom shows a weak core below the surface, which is clearer at the fourth position d. In the Labani Channel (position e), there has obviously been developed a strong subsurface current with a core at depths of 100 to 150 m. South of this, at crossing from Makassar Strait into Flores Sea (position f), the current is still located at those depths. Table 1. Average Volume Transport in Sv Through the Makassar Strait for the Total Depth and Different Depth Ranges for the Period a Depth Ranges (m) Simulation Total Below 700 Reference Barotropic No wind a One sverdrup is 10 6 m 3 s 1. The core of the subsurface jet is located within m. Reference simulation means the model run, of which the results were analyzed and presented in this article. 5of14

6 Figure 4. (a) Horizontal distribution of simulated volume transport as average for the upper 420 m and (b d) as a daily average on 1 October 2004 for the Makassar Strait and the region south of it to the Lesser Sunda Islands for different depth ranges. Shading colors show magnitude in Sv according to the color bar, arrows show direction of the transport. Depth contour lines are shown for 100 m, 300 m, 500 m and 1000 m depth. Figure 4b displays the transport within the upper 52 m, Figure 4c for the range of m (thickness of range: 370 m), and Figure 4d for the depth range of m (thickness: 450 m). The encircled crosses represent positions of water columns for the vertical Hovmöller diagrams showing the development of along-current velocity component in Figure 5. 6of14

7 Figure 5. (a f) Time series of velocity component in the main current direction (along-current velocity component) for the upper 800 m. Positions are depicted in Figure 4. Positive values (reddish shading) are into the given direction, negative (blueish shading) out of given direction. Direction angles are clockwise from north. 7of14

8 Close to the Lombok Strait (not shown here), the Makassar Current, which became a subsurface jet, is back to surface. [22] To summarize, the Makassar Strait throughflow is part of a strong distinct current system, which enters the Makassar Strait at its northern narrow entry as a surface current, deepens north of the Labani Channel to become a subsurface jet in the Labani Channel with a core at m depth, remains a subsurface current at that depth until the current has crossed the Flores Sea and come back to the surface as it gets close to the Lombok Strait. During this journey, it loses a great portion of its volume, as described at the end of this section. [23] Gordon et al. [2010] explain a general upwelling within the Indonesian seas that results from the divergence between inflow and outflow as estimated from the INSTANT program observations. The upward movement of the simulated Makassar Current is probably a manifestation of that, especially since it extends to more than 400 m depth in the northern Flores Sea (see Figure 5f), whereas the Lombok Strait is not more than 260 m deep. [24] Measurement campaigns do usually take place in the Labani Channel, which is an important place for the Makassar Strait throughflow and the ITF transport. The observed vertical structure might be expected to exist in the entire Makassar Strait. The model results suggest, however, that the vertical structure varies and that the jet is the deepened distinct Makassar Current. [25] The reason for the deepening of the strong current is the low-density fresh Java Sea water transported during the NW monsoon season from the west into the Flores Sea and into the Makassar Strait, as described in the previous section about the climatology. During this season (boreal winter), model results show buoyant surface water entering through the western open boundary across the Flores Sea and spreading out into the Makassar Strait and as far as the Banda Sea. The spreading occurs due to surface transport northward into the Makassar Strait forcing the strong southward throughflow to submerge underneath it. This is in good agreement with the description and explanation published by Gordon et al. [2003] and Du and Qu [2010]. Also, Tozuka et al. [2007] show the importance of the role of South China Seawater flowing through the Karimata Strait in deepening the Makassar Strait throughflow and generating the subsurface maximum during the NW monsoon season, with an ocean general circulation model. [26] At two locations south of the Makassar Strait, the current occasionally bifurcates into a branch further to the south and one toward the east, see Figure 4. The first location of bifurcation is the group of shallow coral islands around Pulau Kalukalukuang, which is located about 135 km west of the city of Makassar (see Figure 1). Here, the narrow outlet between this island group and the Sunda Shelf edge sometimes does not offer sufficient space for the entire jet to pass, especially when it is wider than this outlet and also extending to greater depths. So, a portion passes east of these islands across the Dewakang Sill and around the southwestern tip of Sulawesi Island to proceed via the northern Flores Sea into the western Banda Sea, where it partly joins the eastward flow along the northern coasts of the Lesser Sunda Islands. The second bifurcation happens just in front of the Lombok Strait: while the Makassar Current leaves through this narrow outlet, another portion of the volume produces the above mentioned eastward flow through the southern Flores Sea along the northern side of the Lesser Sunda Islands. It is not clear yet, whether the saturation of the Lombok Strait throughflow is due to the narrowness of the Lombok Strait (hydraulic reasons) or due to a pressure gradient to the Indian Ocean [Wijffels et al., 2008]. In any case, at this export gate, the Makassar Current is reduced to a lesser portion of the Makassar Strait throughflow. [27] The southern subsurface part of the Makassar Current has been detected from XBT measurements above the Sunda shelf edge in the middle of the Flores Sea, south of the first bifurcation position and a bit south of our position f in Figure 4. It has been referred to as the Makassar Jet by Wijffels et al., [2008]. The authors realized that this is the same current, which has been observed as the subsurface jet in the Labani Channel. 5. The Makassar Strait Jet [28] The well-known jet within the Labani Channel of the Makassar Strait is a strong current, of which the core with speeds exceeding 1 m s 1 is located at depths of 100 to 150 m. It is very variable in vertical and horizontal extension, strength and time. The fully developed subsurface current extends from just below the water surface down to about 400 m, occasionally to 500 m, but it might also have a thickness of about 100 m, reaching from 100 m depth to 200 m depth. This is reproduced by the model, and has been estimated from measurements, for example by the INSTANT project [Gordon et al., 2008] as well as from SODA outputs [Du and Qu, 2010], and from other model simulations [Metzger et al., 2010]. [29] The simulated vertical structure of the flow at the Labani Channel section (upper 800 m, location see the orange-red line across Makassar Strait in Figure 1) is presented in Figure 6. Figures 6 (left) and 6 (middle) show the vertical distribution of the daily average northward velocity component at the section across the Makassar Strait for two different times. The panel for 19 February 2004 displays a very well developed southward jet with weak countercurrents above and below it. This situation is obtained close to the end of the winter monsoon season, when the NW winds piled up buoyant water masses south of the Makassar Strait introducing the pressure gradient. and a sub-jet northward flow due to the barotropic pressure gradient. Figure 6 (middle) shows the situation for 24 July The subsurface jet itself is not as strong as in February, but the southward flow extends to surface and to greater depths within the section. [30] The seasonal variation of the strength and the depth of the jet can be seen from Figure 6 (right) presenting the simulated vertical profiles of the along-channel velocities as quarterly averages for the period and as overall average for that period. Clearly, it can be seen that the jet is strongest in the second quarter of a year and weakest in the last quarter of a year. It is also obvious that the kernel of the jet is deepest, at about 110 to 120 m depth, during the first quarter of a year, which is toward the end of the NW monsoon season. During July to September (fully developed SE monsoon), the peak velocity occurs shallowest above 100 m depth. This supports the description for the deepening of the Makassar Current in the previous section. For comparison 8of14

9 Figure 6. (left and middle) Vertical section (upper 800 m) from west to east across the Makassar Strait. Northward velocities (positive, reddish shading colors) are into the page, depths are in m, velocities in m s 1. Location is depicted in Figure 1 (right), by the orange-red line in the Makassar Strait, where the red part covers the deep eastern region being the Labani Channel. (right) The vertical profile of the along-channel velocities as quarterly and overall average of simulated along-channel velocities for the upper 800 m. with INSTANT observations, the reader may be referred to Gordon et al. [2008]. [31] The Makassar Current with its subsurface part is not horizontally centered within the Makassar Strait. Even in the narrow Labani Channel, it is shifted to the western boundary of the strait. This can be clearly seen from both panels in Figure 6. Velocity measurements with moorings across the Labani Channel support the western intensification of the current [Susanto and Gordon, 2005] as well as other model simulations [see, e.g., Rosenfield et al., 2010]. [32] The importance of the subsurface jet in the Makassar Strait becomes clear, if we look at its relative contribution to the entire ITF volume transport. According to our regional model simulations, which principally reconstruct the total volume transport of the ITF, the entire Makassar Strait throughflow accounts for about half of the entire ITF volume transport, which means that the part being the subsurface jet contributes about 30% to the ITF, which is on average a bit more than 4 Sv for the three year period. [33] Estimates from measurements result in an even greater importance of the jet concerning the total ITF volume transport. According to Gordon et al. [1999, 2008] and Sprintall et al. [2009], the contribution of the whole Makassar Strait throughflow to the entire ITF is about 70%. If we combine this with our simulation of the vertical structure within the Makassar Strait throughflow, the fraction of the subsurface jet covers about 50% of the entire ITF or approx. 7.5 Sv for the period mentioned above. [34] A dependency of the strong subsurface current on the Mindanao Current (MC) has been detected in the model results. A cross-correlation of the long-term time series ( ) of the simulated volume transports of both currents (see Figure 7) shows a maximum correlation coefficient of 0.76 at an eight day lag with an annual recurrence (variations in the MC transport can be seen eight days later in the transport of the subsurface jet). Note that only daily averaged model results have been analyzed. A similar but weaker correlation with the MC volume transport can be found for the entire Makassar Strait throughflow as well. [35] The correlation supports the idea already described in section 3 that the Makassar Strait throughflow is mainly fed by the north equatorial Pacific as estimated from observed TS diagrams [Gordon, 1995] and from chemical tracer concentrations [Gordon and Fine, 1996]. It also agrees with the Figure 7. Cross-correlation of the volume transport of the Mindanao Current and the transport through the depth range m of the Makassar Strait (Labani Channel) for the entire simulation period ( ). The maximum correlation coefficient is 0.76 with a time lag of 8 days meaning that a signal in the Mindanao Current transport arrives 8 days later at the Makassar Strait section. 9of14

10 Figure 8. Temporal development of the volume transport within the upper 420 m of the Makassar Strait. Shading shows the magnitude in Sv according to the color bar, arrows give direction of flow. Daily averages for 30 Jan, 04 Feb, 09 Feb and 19 Feb 2004 are presented. opinion that the Makassar Strait throughflow is essentially an extension of the tropical northwestern Pacific current system [e.g., Gordon et al., 1999; Wajsowicz et al., 2003] from which we can conclude that it is also correlated to the ENSO. This and a correlation to the Indian Ocean Dipole is, however, subject to a future publication. 6. The Makassar Current: A Western Boundary Current [36] The Makassar Current behaves like a strong western boundary current. As mentioned in the previous section, the core of the subsurface jet in the Makassar Strait is not horizontally centered but attached to the western walls of the strait. This is also valid for the surface parts of the Makassar Current. From Figure 4c, which shows the volume transport of the depth range containing the jet, it is easy to see that the jet is part of an intense current band along the entire western boundary. Its origin is usually not located within the Makassar Strait. It exists already before entering the Makassar Strait, and it flows southward attached to the Sunda Shelf edge extending from north of the Makassar Strait all the way to the Lombok Strait, connecting the Sulawesi Sea and often even the Sulu Sea with the Indian Ocean. According to our model results, it looks like this subsurface jet is an extension of the Mindanao Current, which is occasionally joined by another western boundary current coming via the Sulu Sea. [37] Some authors [e.g., Wajsowicz et al., 2003; Du and Qu, 2010; Metzger et al., 2010] have detected the western intensification in their analyses of observations, SODA outputs or from other model applications and have mentioned previously a western boundary jet both within and to the south of the Makassar Strait. Wajsowicz et al. [2003] states that the flow through the Makassar Strait is essentially that of the Indo-Pacific basin western boundary current. Wajsowicz [1996] applied the mathematical model of the island rule [Godfrey, 1989] and explained the western intensification within the Indonesian throughflow not directly but by analogy with an electrical circuit, where the frictional straits, set up in series and in parallel, were compared with electrical resistors. Even though the author s closed circuit does not correspond to a closed circuit in the ocean circulation, the equations derived from the island rule for that circuit show nicely the fact that the transport tends to flow through the most western strait, until friction limits further throughflow through that strait. Then, the next most western strait is used and so on. We found similar behavior in our model results, and this has not been shown before except by the aforementioned analogy. [38] Model simulations (also in the barotropic mode) produced setup situations after a period of relatively low throughflow, where a strong current through the Makassar Strait developed within two or three days. For these cases of a rapid development of the distinct strong current, it first connected the entrance and the exit of the Makassar Strait directly. It was not attached to the western boundary of the Makassar Strait, but located rather close to its eastern wall, the coast of Sulawesi. However, it slowly moved to the western wall of the strait within two to three weeks. Figure 8 presents the daily averages of volume transport in the Makassar Strait within the upper 420 m for four different 10 of 14

11 days. It shows the following development from left to right: firstly, the throughflow has just increased and connects the entrance and exit of the strait more or less straight and directly. Secondly, it has been further enlarged and widened and a distinct flow is visible, which is centered in the strait. Thirdly, the flow is now a marked current, which has already moved a little westward. Finally it is a current attached to the western boundary of the strait. [39] This behavior of the meridional current, which is attached to the western boundary along its entire passage, cannot be explained using the arguments for the western intensification in large ocean basins, where wind vorticity is introduced into the ocean circulation leading to the strong boundary currents along the western limitations of these ocean basins (see Stommel and Munk theories). The westward shift of the Makassar Current is due to the meridional gradient of the Coriolis force, as model experiments suggest and as indirectly shown by the analogy in Wajsowicz [1996]. This is subject to further investigation. 7. Model Experiments: Barotropic Mode and No Local Wind [40] Two model experiments show the influence of the baroclinic mode and of the local wind on the Makassar Strait throughflow. The monthly mean volume transports of these experiments for the period is shown in Figure 9 for the total flow through the Labani Channel and for different depth ranges to make it comparable with Figure 3. Additionally, Table 1 lists the average volume transports for the reference simulation, of which the results were presented in the previous sections, and these two model experiments for the period [41] For the barotropic case, where the separating pycnocline is absent, the throughflow extends to the entire depth of the Makassar Strait. During summer season with high southward volume transport, the peak in August shows a transport more than twice as much as for baroclinic mode. During boreal winter season, the throughflow is decreased in normal baroclinic mode, while it is opposite directed in barotropic mode. The pressure gradient between Flores Sea in the south and Sulawesi Sea in the north, which is set up by the NW monsoon with water masses coming from the South China Sea via Karimata Strait and Java Sea, is effective for the entire water column. It reverses the entire flow through the Makassar Strait. In January, the northward average volume transport reaches 16 Sv and is even higher than the regular average southward summer throughflow, of which the maximum amounts to almost 12 Sv in July. [42] The reversing of the throughflow during the winter monsoon leads to a slightly lower average throughflow of approx. 5 Sv; in the baroclinic mode, it amounts to approx. 6.2 Sv for the same period. [43] From analyses of measurements in the Makassar Strait, Waworuntu et al. [2001] concluded that a model of at least three layers is required to appropriately describe the Makassar Strait throughflow, which is consistent with our findings, where the bulk of this throughflow occurs in and is limited to the thermocline. [44] The case study without local wind-forcing does not change much of the throughflow. Nevertheless, it is interesting to see from Figure 9 that the Makassar Strait throughflow is slightly enhanced in this case, where local wind stress is switched off. The water mass transport within the upper 700 m is a little bit increased with higher southward flow in boreal summer and higher northward flow during winter season. [45] The explanation for this is the direct wind-forcing, which is contradirectional to the pressure gradient forcing also set up by the wind: during boreal winter, the prevailing NW winds push buoyant upper ocean water masses from the South China Sea through the Kalimata Strait into the Flores and the Banda Seas, where they pile up due to the narrow outlets, leading to an increase of sea surface height. This produces a pressure gradient forcing water masses to flow northward from the Flores Sea into the Makassar Strait, while the NW wind stress acts in the opposite direction slowing down the surface flow. During boreal summer, similar contradirectional forcing is applied but this time into the other directions: while the SE monsoon winds enable a relaxation of the pressure gradients from the winter monsoon leading to the regular southward flow in the Makassar Strait, the southeasterly monsoon winds push the surface waters in northward direction again resulting in a reduced transport. [46] From this, we conclude that the Ekman theory does not apply in this region. Otherwise, the profiles would show different near-surface values with opposite trends (the red profile would tend to lower near-surface southward velocities, the blue profile to higher southward velocities). The SE monsoon wind stress seems to be more effective compared to the NW monsoon stress, because on the overall average the local wind stress plays a decelerating role for the Makassar Strait throughflow and for the entire ITF [Mayer et al., 2010]. [47] The theoretical estimation of the directly wind induced transport in the Makassar Strait would fill a paper on its own as it is quite complicated due to north-south varying average wind stresses (directions) and the fact that the Makassar Strait extends from 1 N to about 4 S crossing the equator and leading to different Ekman transports with different directions and a wind channel behavior in between. 8. Conclusions [48] The regional model HAMSOM has been successfully applied to the region of the ITF. The flow through the Makassar Strait makes up an important part of the volume transport of the ITF, about 50% according to our model results. [49] It has not been shown before that the Makassar Strait throughflow occurs as a distinct strong current most of the time. Probably due to the b-effect, it behaves like a western boundary current, which we termed Makassar Current. Even after a rapid development, when it directly and straight connects the entrance and the exit of the Makassar Strait located rather next to the eastern boundary of the strait, it moves within two or three weeks westward to the western boundary. [50] The Makassar Current enters the Makassar Strait as a surface current, deepens underneath a surface layer to become a strong and narrow subsurface jet before passing the Labani Channel, exits the strait, crosses the Flores Sea still as a subsurface current with a wider core and returns back to the surface before leaving through the Lombok Strait. The subsurface jet develops within the range of about 50 to 300 m 11 of 14

12 Figure 9. Monthly mean volume transports for (top) simulation in barotropic mode and (bottom) without local wind-forcing through the section of Makassar Strait as depicted in Figure 1 in Sv for the entire depth and for different depth ranges. Blue bars are monthly mean values for the time range , red lines are standard deviation from mean. The blue horizontal lines show the overall average transports. Negative values mean southward transport. 12 of 14

13 depth with a core at about 100 to 150 m depth. According to our simulation results, the jet contributes about 65 to 70% to the Makassar Strait throughflow and about one third to the entire volume transport of the ITF. [51] Along its path from the Makassar Strait to the Indian Ocean, the Makassar Current bifurcates at two locations, releasing most of its volume toward the eastern branch of the ITF. The first bifurcation occurs at the topographic ridge around the coral island group of Kalukalukuang, forcing a portion of the current to flow southeastward across the Dewakang Sill along the southwestern tip of Sulawesi and then eastward into the northern Flores Sea. The second bifurcation occurs just north of the narrow Lombok Strait causing an eastward flow into the southern Flores Sea. As a result, the Makassar Current transports only a minor portion of the Makassar Strait throughflow through the Lombok Strait into the Indian Ocean. [52] The transport climatology and the standard deviations from the mean values show in the same manner as for the ITF itself a high temporal variability. Even though the volume transport of the Mindanao Current is difficult to estimate due to its varying location, the model results show a correlation between the transports of the Makassar subsurface jet in the Labani Channel and the Mindanao Current with a peak value at a time lag of about 8 days, pointing out that signals occurring in the Mindanao Current close to the northeastern boundary of the model domain take about 8 days to show up at the Labani Channel in the Makassar Strait. [53] Local wind stress plays a contradictory double role in this region during the boreal winter season. Model results and experiments have shown that the Ekman theory does not apply in the vicinity of the Makassar Strait, leading to an ocean surface flow in the same direction as the wind. Because of this, during boreal winter, the direct effect of the NW monsoon slightly accelerates the southward surface flow (or slows down a possibly northward directed flow), which, in turn, is contra-directional to the effect of the barotropic pressure gradient introduced by the same wind. The direct SE monsoon wind stress acts in a northern direction, slowing down the southward flow. This is valid for the Makassar Strait and the entire ITF. [54] Acknowledgments. We would like to thank Thomas Pohlmann, Kieran O Driscoll, and Simon Schoof for their valuable discussions and their great help. This project was funded by the German Research Foundation under the number Da773/3-1. References Backhaus, J. O. (1985), A three-dimensional model for the simulation of shelf sea dynamics, Dtsch. Hydrogr. Z., 38, Boyer, T., S. Levitus, H. Garcia, R. Locarnini, C. Stephens, and J. 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