average is 5.1 Sv, while during the La Niña months
of December 1996 to February 1997 the average is
12.5 Sv, a 2.5-fold difference. Most of the remaining variance of ITF transport once ENSO effect is
removed is explained by the annual cycle, with a
June maximum and December minimum (Gordon
et al., 1999a), and by intraseasonal events (Sprintall
et al., 2000; Susanto et al., 2000).
As 1997 was for the most part an El Niño year,
when ITF is expected to be smaller than average,
the climatic ITF within the Makassar Strait is
expected to be larger than the 1997 mean of
9.3 Sv. Assuming that the relationship of the
Makassar ITF to ENSO is defined by the 1.6-year
record (which without a longer time series is a crude
approximation), a climatic mean for the ITF may
be expected to be 13 Sv (from Fig. 4b of Gordon
et al., 1999a). An ITF transport of 10–15 Sv seems
like a fair number to use, at least until observations more accurately determine the ITF mean and
variability.
4.7.3.4 ITF transport profile
The Arlindo Makassar Strait measurements suggest a complex vertical profile of transport, with
implications for interocean thermohaline fluxes
and mass budget of the western tropical Pacific
warm pool water: the most persistent and
strongest ITF occurs within the thermocline and
not within the warm surface layer (Gordon and
Susanto, 1999; Gordon et al., 1999a). The data
indicate frequent occurrence of maximum southward speeds within the mid to lower thermocline.
The subsurface maximum occurs during times of
large transport, from April to September 1997 and
again in April 1998 to the end of the record in
June 1998.
An upward-looking ADCP at 150 m on the
Arlindo Makassar Strait moorings provided a
record of surface layer flow from 1 December
1996 to 1 March 1997. This time series shows
increasing southward speeds with increasing depth,
with surface flow varying from near zero to northward. This was not a local wind effect, as the
NSCAT-measured winds during this period were
directed towards the south. Model results suggest
similar temporal dependence. Masumoto and
Yamagata (1993) with the GFDL (Geophysical
Fluid Dynamics Laboratory) model forced by
Hellerman and Rosenstein (1983) winds show
northward surface flow within Makassar during
the winter months. Using a model driven by
ECMWF (European Centre for Medium Range
Weather Forecasting) winds, Potemra et al. (1997)
find that Ekman transport in the Indonesian Seas
is directed towards the Pacific in the winter
months, with strong Indian Ocean-bound Ekman
transport in summer. The 1/6° resolution POP
(Parallel Ocean Program) model display a surface
(upper 100 m) flow towards the north in Makassar
Strait during the winter, and towards the south in
the summer months (McClean, personal communication, February 1999).
The reality and causes of seasonal oscillations
of the surface flow must be further investigated.
Possible candidates are: strong monsoonal zonal
wind across the southern boundary of the Makassar Strait (Java and Flores Seas) relative to the
weak monsoonal winds in the northern boundary
of the Makassar Strait (Sulawesi Sea); Ekman
pumping along the southern coast of the Sunda
Islands (Potemra, 1999); or a buoyancy effect
induced by the enormous amount of very-lowsalinity Java Sea water injected into the Flores Sea
and southern boundary of the Makassar Strait
during the boreal winter.
4.7.3.5 Thermohaline fluxes
The Arlindo Makassar Strait time series reveals
that the ITF transport is linked to thermocline
depth: transport is smaller and thermocline shallower during El Niño (Bray et al., 1996, 1997;
Meyers, 1996; Ffield et al., 2000). The correlation
between variability in the average thermocline
temperature to variability in the southward
Makassar transport is r:0.67 (Ffield et al., 2000).
Using nearly 15 years of XBT data, Ffield et al.
(2000) show that the Makassar upper thermocline
temperature is highly correlated with ENSO: 0.77
for the Southern Oscillation Index; 90.80 for
NINO3 SST anomaly, and 90.82 for NINO4 SST
anomaly. The correlations increase when the ENSO
time series are lagged a month or so. Thus the
Makassar temperature field – when coupled with
the throughflow – transmits equatorial Pacific El
Niño and La Niña temperature fluctuations into
the Indian Ocean.
An estimate of the internal energy transport
(Warren, 1999) for the Makassar Strait is made by
Ffield et al. (2000) by integrating the product of
temperature, volume transport, density and specific
heat in the upper 400 db of the water column.
4.7 Interocean Exchange
309
Gordon
of December 1996 to February 1997 the average is
12.5 Sv, a 2.5-fold difference. Most of the remaining variance of ITF transport once ENSO effect is
removed is explained by the annual cycle, with a
June maximum and December minimum (Gordon
et al., 1999a), and by intraseasonal events (Sprintall
et al., 2000; Susanto et al., 2000).
As 1997 was for the most part an El Niño year,
when ITF is expected to be smaller than average,
the climatic ITF within the Makassar Strait is
expected to be larger than the 1997 mean of
9.3 Sv. Assuming that the relationship of the
Makassar ITF to ENSO is defined by the 1.6-year
record (which without a longer time series is a crude
approximation), a climatic mean for the ITF may
be expected to be 13 Sv (from Fig. 4b of Gordon
et al., 1999a). An ITF transport of 10–15 Sv seems
like a fair number to use, at least until observations more accurately determine the ITF mean and
variability.
4.7.3.4 ITF transport profile
The Arlindo Makassar Strait measurements suggest a complex vertical profile of transport, with
implications for interocean thermohaline fluxes
and mass budget of the western tropical Pacific
warm pool water: the most persistent and
strongest ITF occurs within the thermocline and
not within the warm surface layer (Gordon and
Susanto, 1999; Gordon et al., 1999a). The data
indicate frequent occurrence of maximum southward speeds within the mid to lower thermocline.
The subsurface maximum occurs during times of
large transport, from April to September 1997 and
again in April 1998 to the end of the record in
June 1998.
An upward-looking ADCP at 150 m on the
Arlindo Makassar Strait moorings provided a
record of surface layer flow from 1 December
1996 to 1 March 1997. This time series shows
increasing southward speeds with increasing depth,
with surface flow varying from near zero to northward. This was not a local wind effect, as the
NSCAT-measured winds during this period were
directed towards the south. Model results suggest
similar temporal dependence. Masumoto and
Yamagata (1993) with the GFDL (Geophysical
Fluid Dynamics Laboratory) model forced by
Hellerman and Rosenstein (1983) winds show
northward surface flow within Makassar during
the winter months. Using a model driven by
ECMWF (European Centre for Medium Range
Weather Forecasting) winds, Potemra et al. (1997)
find that Ekman transport in the Indonesian Seas
is directed towards the Pacific in the winter
months, with strong Indian Ocean-bound Ekman
transport in summer. The 1/6° resolution POP
(Parallel Ocean Program) model display a surface
(upper 100 m) flow towards the north in Makassar
Strait during the winter, and towards the south in
the summer months (McClean, personal communication, February 1999).
The reality and causes of seasonal oscillations
of the surface flow must be further investigated.
Possible candidates are: strong monsoonal zonal
wind across the southern boundary of the Makassar Strait (Java and Flores Seas) relative to the
weak monsoonal winds in the northern boundary
of the Makassar Strait (Sulawesi Sea); Ekman
pumping along the southern coast of the Sunda
Islands (Potemra, 1999); or a buoyancy effect
induced by the enormous amount of very-lowsalinity Java Sea water injected into the Flores Sea
and southern boundary of the Makassar Strait
during the boreal winter.
4.7.3.5 Thermohaline fluxes
The Arlindo Makassar Strait time series reveals
that the ITF transport is linked to thermocline
depth: transport is smaller and thermocline shallower during El Niño (Bray et al., 1996, 1997;
Meyers, 1996; Ffield et al., 2000). The correlation
between variability in the average thermocline
temperature to variability in the southward
Makassar transport is r:0.67 (Ffield et al., 2000).
Using nearly 15 years of XBT data, Ffield et al.
(2000) show that the Makassar upper thermocline
temperature is highly correlated with ENSO: 0.77
for the Southern Oscillation Index; 90.80 for
NINO3 SST anomaly, and 90.82 for NINO4 SST
anomaly. The correlations increase when the ENSO
time series are lagged a month or so. Thus the
Makassar temperature field – when coupled with
the throughflow – transmits equatorial Pacific El
Niño and La Niña temperature fluctuations into
the Indian Ocean.
An estimate of the internal energy transport
(Warren, 1999) for the Makassar Strait is made by
Ffield et al. (2000) by integrating the product of
temperature, volume transport, density and specific
heat in the upper 400 db of the water column.
4.7 Interocean Exchange
309
Gordon