The region of the Sunda Shelf has an average depth of
approx. 48
For investigation of marine environments with focus on the local biogeochemical processes, on the impact of marine pollution due to dense coastal human population and on changes of any kind of environmental properties, it is recommendable to have knowledge about hydrodynamical water exchange parameters of the regions of interest, which give an idea about time scales of renewal of the corresponding water bodies. Without this knowledge, it might be difficult to distinguish between different processes inducing changes of water properties (e.g., advection, internal conversions) and to interpret observational or numerical findings.
There are many different types of water exchange parameters
mentioned in the literature such as “age”, “flushing rate”,
“residence time”, “exposure time”, “transit time”, “turn
over time”, “influence time”, “half life time” and so
on. Authors like
Certain parameters like the residence time or exposure time are
actually Lagrangian properties of each water particle within the
domain of interest
For the Indonesian Seas, there are only a few estimations on the
renewal of water, they are usually done in connection with
investigation of water exchange and water mixing of deep basins
The Indonesian Seas are located in a geographical region dominated by two different and opposing wind systems: the summer monsoon lasts from approximately May or June until September with prevailing wind directions from SW to SE, depending on the geographical position, while the winter monsoon with prevailing slightly weaker winds from opposite directions (NE to NW) lasts from November to February or March. The spring and autumn transition periods in between offer highly variable winds in strength and direction while changing their main direction. This meteorological behaviour has a heavy impact on the ocean circulation. Therefore, we will also focus on the seasonal dependence of the renewal of the water masses in the regions of interest.
A numerical model system was applied to simulate the hydrodynamics
as realistic as possible. Our approach for this article is to
select certain geographical areas on the shallow Sunda Shelf and
calculate their flushing rates
A numerical model system has been applied to receive a most
realistic distribution of currents, sea surface temperature (SST)
and sea surface salinity (SSS). The first component is the global
ocean circulation model MPI-OM
Finally, freshwater inflow is required at the river mouths. These
river runoff data were generated with the MPI-HM
The requested most realistic exchange of fresh water, heat and
momentum between ocean and atmosphere limits the thickness of the
ocean surface layer. As a consequence, tidal forcing had to be
switched off (upper layer thickness is 5
The simulation period covered the years 1958 to 2012. Model results
were averaged over 24
The fourth component of the model system is a suspended particulate
matter transport model
The regions of interest for the flushing rates and residence times
are limited to depths less than 200
In order to get a clear idea about the role of the different seasons in this highly variable monsoon-dependent circulation system, the regional model's horizontal velocities were averaged to receive monthly values for each of the five decades from 1960 to 2012 (for the last decade, the years 2010 to 2012 were also added). This procedure enables us to investigate also decadal variability.
With these horizontal velocities, flushing rates
For the tracer model simulations, the same decadal monthly averaged 3-D current fields were taken. Four simulations for two years were performed using constant velocities of February, May, August and October of the decade 2000 to 2012 as representatives for the two fully developed monsoon seasons and the transition periods. The residence time was taken as the time needed for a particle to leave the corresponding region of interest for the first time. For the vertical averages displayed in the result pictures, the residence times of the layers were weighted according to the layer thicknesses.
For reasons of clear differentiation of the two applied methods in estimating the renewal rates, we will use the term flushing rate for the analytical calculation and the term residence time for the estimation of the time parameter from the tracer model simulations.
For the validation of the model results, comparisons with observed velocities at different key locations within the Indonesian Seas will be presented. Also comparison of sea surface temperature and sea surface salinity with satellite data will show the high quality of the regional model results. For the long-term results, a climatology of the Indonesian Throughflow will underline the capability of the numerical hydrodynamical model system.
For direct comparison of model results with observed data, measured
velocities from moored current meters at different depths of some
key locations were used kindly provided by the INSTANT project
Overall, a satisfying agreement can be stated to the time series as
well as to the average velocities, which show a slight
underestimation by the model at some positions. There are only
a few major discrepancies like in the Makassar 350
Most of these “disturbences” are not locally generated but
transferred from the open boundaries into the model domain. Both
open boundaries to the Pacific and the Indian Oceans are sometimes
– from outside – crossed or touched by strong large gyres or
dynamic meandering currents like the South Equatorial Current
(SEC), introducing strong horizontal barotropic pressure and
density gradients along the open boundaries.
Another presentation of the agreement of simulation results of the
regional model HAMSOM with the real condition is a comparison of
sea surface salinities (SSS) and temperatures (SST). Both are shown
in Fig.
For SST comparison, three regions A, B, C have been selected (see
map in Fig.
The comparison displayed in Fig.
For SSS comparison, SMOS (soil moisture and ocean salinity sensor)
derived data represent – with an accuracy of 0.4
A slight overestimation of SSS from the model is expected because
of the thickness of the upper layer. Precipitation has a much
larger impact on the remote sensing data than on the model
data. Since the model does not resolve any vertical profile within
the upper 5
Last but not least, the averaged transports through some sections
in the Indonesian Seas for the periods 2004 to 2006 and 1970 to
2006 are presented in Fig.
Also from these transport distributions, we can see a principle
agreement of the simulated with expected circulation in the
Indonesian Seas according to the cited literature. Remarkable
differences are the Ombai Strait and Timor Passage transports,
which correspond to the underestimated simulated currents of the
comparing time series of the same locations shown in
Fig.
For the Lifamatola Strait, the cited value of
The results of the analytically calculated flushing rates from the
simulated volume transports into the different domains are depicted
in Fig.
High flushing rates mean low water exchange or slow inflow. During the transition periods between the two monsoon phases, April/May and October, there is some stagnation of the circulation, which partly even changes its direction from one phase to the other. This leads to a deceleration of water currents and longer flushing rates. It has to be noted, if directions of transport change, stagnation is not that obvious for the throughflow domains, because there is inflow on either open side of the domain.
The standard deviation (SD) of the monthly inflowing transports (not shown), which were
calculated from daily inflowing transports, range from
approximately 9 to 43 % of the monthly mean volume transport
depending on the month and the domain, the highest occurring for
the Sunda Shelf and the South China Sea. The SD of the flushing
rates (also not shown), calculated from daily flushing rates,
ranges from 40 up to 300
There is no decadal trend visible in the monthly results, and the
decadal variability is non-specific and low. Inter-annual
variability was not investigated because of the well-known general
high variability of the current system in the Indonesian Seas
The residence times as calculated from the tracer model simulations give us a much more detailed pictures and a quite different impression of the exchange of water in the regions. The model simulated four times a period of two years, forced with constant decadal monthly velocity fields of February, May, August and October, representing the fully developed NW/NE monsoon season, transition period, the fully developed SE/SW monsoon season and the transition period from the last to the first season, respectively. May instead of April was selected because of the long flushing rates shown in the previous section.
To get an idea about the general order of residence times and the
portion of particles (representing the water masses) flushed out
after a simulation period of two years, Table
The table shows a great discrepancy to the flushing rates. For May,
August and October, most regions have not reached their half life
times after two simulation years, while the
The vertically averaged residence times for the region of the
entire Sunda Shelf are given for the different seasons in
Fig.
During the summer monsoon in August (Fig.
In May, after transition from boreal winter to summer monsoon
season (Fig.
The simulation results for October (Fig.
For the Gulf of Thailand as part of the Sunda Shelf region, the
residence time in February is simulated mostly with more than two
years. Obviously, the inflowing currents retroflect already close
to their entrance in a way that there is hardly exchange for
central and coastal parts of the Gulf. As mentioned in the previous
paragraph, during the boreal winter season, the northerly monsoon
winds pile up the water in the southern South China Sea (SCS),
which blocks the outflow from the Gulf. Additionally, the southward
flow along the Vietnamese coast is topographically hindered from
flowing into the Gulf by the Vietnamese most southeastern tip,
which directs the flow rather towards the West-Malaysia. The above
mentioned band of lower residence times south of Borneo support
this interpretation about the flow pattern. The onset of this
situation in October shows the same effect for the Gulf
area. During the SE monsoon season, the currents obviously intrude
further into the Gulf and decrease the residence time to ca. 1.75
to 2
In the Malacca Strait, we can also detect two very different
situations, which supports the seasonal flow patterns published
already by
Since we are able to follow the tracers in three dimensions, we can
also distinguish the residence times for tracers starting at
different depth levels, shown for the upper two and the fourth
model layers for the February and August simulations in
Fig.
It is interesting to see the very different residence times in the
single layer pictures with non-averaged values. In general, the
described above features are more obvious. For the surface layer,
many areas within the Sunda Shelf region show residence times of
less than 30
According to the different residence times for different layers in both simulations and the short residence times in upper layers, the local wind plays a dominant role for the direction and the strength of the water transport in the shallow Sunda Shelf region, while barotropic gradients are of minor importance.
Two different methods were applied to estimate hydrodynamic time parameters connected with renewal of water body of a certain region, here: the Sunda Shelf and its sub regions: the analytical calculation of the flushing rates and the residence times estimated from tracer model results. The two methods delivered very different results.
The analytical approach of using the formula
The application of Lagrangian tracer models to follow water particles and estimate a horizontal or 3-D distribution of the residence times from typical season-averaged flow fields is also not realistic, because there are no monthly averaged situations lasting longer than a month. However, it provides a much better idea about the time ranges for water renewal and a more detailed view on the locations with rather stagnating and rather quickly exchanged water masses. Additionally, it gives us a clearer idea about the influence of the seasons.
The connection between both methods is – for an idealized case of
uniform flow and immediate mixing of all remaining with new water
– the time reached after exchanging an amount of 63
For the Indonesian shelf seas, only the tracer model results indicated and visualized that there are big differences in water exchange between locations within the domains of interest and between seasons.
For the interdisciplinary investigation of marine environments such
as shelf seas, gulfs and bays, it is helpful to have knowledge
about basic physical parameters of those regions. One important
parameter is the rate of renewal of the water within a certain
region, which gives an idea about the importance of external
(advective) and internal processes. This is essential for basic
environmental research as well as applied investigations, e.g. for
the development of sustainable aquaculture. Two methods were
applied to estimate hydrodynamical parameters connected with the
process of water renewal within certain regions. From numerical
hydrodynamical model results, one method calculated the flushing
rates according to the formula
Focusing on the entire Sunda Shelf (average depth 49
In conclusion, the results of the two different methods to estimate the water renewal rates in certain regions recommend the application of Lagrangian tracer models rather than just a calculation with the simple formula. The formula cannot take recirculation patterns into account, which might mislead the investigators towards dramatic underestimations of the times needed under realistic conditions to exchange entire water bodies.
This project was funded by the German Ministry of Education and Research under the grant 03F0642D in the frame of the German-Indonesian cooperation SPICE III.
Some geometric data about the flushing regions: their number of horizontal grid points and 3-D grid cells in the regional model, their average depth, their area and their volume.
Percentage of particles flushed out of the regions and half life times (hlt, number of days to flush out 50
Regional hydrodynamical model domain with bathymetry. Red dots: river input points; X: locations of four moorings for comparison of simulated and observed data. Isobath of 200
Comparison of simulated (red) and measured (gray) meridional velocities in Labani Channel (Makassar) and Lombok Strait (see Fig.
Comparison of simulated (red) and measured (gray) zonal velocities in Ombai Strait and Timor Passage (see Fig.
Simulated and satellite observed sea surface salinity (SSS) and temperature (SST) at different positions. Satellite SSS is from SMOS for 2010 to 2012, satellite SST from MODIS Aqua and Terra for 2000 to 2012.
Simulated and literature referenced averaged transports through different sections for periods 2004 to 2006 and – in brackets – 1970 to 2006 in Sverdrup (
Flushing rates for different regions on the Sunda Shelf in days. Regions are indicated in upper left panel.
Residence times as simulated by the tracer model for region Sunda Shelf for 2000 to 2012 monthly mean velocity fields of February, May, August and October.
Residence times as simulated by the tracer model for three model layers of the Sunda Shelf region for 2000 to 2012 monthly mean velocity fields February and August.