12. An array of current meter moorings in the Mozambique Channel: results from a pilot study and plans for future observations

H. Ridderinkhof - Royal Netherlands Institute for Sea Research, Texel, Netherlands
W.P.M. de Ruijter - Institute for Marine and Atmospheric Research, Utrecht University, Netherlands

1. Introduction

The flow through the Mozambique Channel is thought to be one of the two routes via which the South Equatorial Current feeds the Agulhas Current in the Indian Ocean (the other being the East Madagascar Current along the south eastern tip of Madagascar) (Lutjeharms, 1976). This southward flux at the western part of the Indian Ocean balances more or less the inflow from the Pacific Ocean via the Indonesian throughflow in the eastern tropical part (Ganachaud et al., 2000). However, because of a lack of direct observations in the Mozambique Channel, there is much uncertainty about it’s magnitude and temporal variability.

It has been speculated recently that the flow in the Mozambique Channel might also be of great dynamical importance for the Agulhas Current. Satellite altimetry observations (Schouten et al., 2002) suggest that large scale eddies are often present, moving southward through the Mozambique Channel. This is confirmed by in situ and drifter observations (De Ruijter et al, 2002) and is also simulated in high resolution numerical model simulations (Biastoch et al., 1999). After leaving the Mozambique Channel these eddies may interact with the Agulhas Current resulting in the formation of a so-called Natal pulse (Lutjeharms et al., 2001) which may trigger the shedding of Agulhas Rings (Van Leeuwen et al., 2000). These Agulhas Rings form an important contribution to the leakage of Indian ocean water into the South Atlantic (De Ruijter et al, 1999). Thus this scheme suggests that eddies originating from Mozambique Channel might be a control on the exchange between the Indian and South Atlantic ocean.

Despite its possible importance for the large scale ocean circulation knowledge on the current field and hydrographic structure in the Mozambique Channel based on direct observations is very limited. Therefore the research program ACSEX (Agulhas Current Sources Experiment) (Ridderinkhof et al., 2001) was set up a few years ago in which different Dutch institutes (Utrecht University, Royal Netherlands Institute for Sea Research) collaborate with the University of Cape Town, South Africa. As part of the ACSEX program seagoing cruises with detailed hydrographic observations at a number of sections in the Mozambique Channel (Ridderinkhof et al., 2001, De Ruijter et al., 2002) have been carried out. Moreover, during the first cruise (early 2000) an array of current meter moorings was deployed across the narrowest section of the Mozambique Channel which was serviced a year later, early 2001, and these moorings were finally recovered on a cruise with the British RV Charles Darwin in late 2001.

In this paper some results from these current meter moorings are discussed. Moreover plans for future long-term observations with an array of moorings are presented.

2. Some results from an array of current meter moorings in the Mozambique Channel

In April 2000 the array of current meter moorings was deployed at the narrowest section of the Mozambique Channel (Figure 1). The design of the array reflects that the strongest currents were expected to occur near the surface and at the western side of the channel. In addition, current meters were deployed at 100 m above the bottom at the moorings ACS05 to ACS09 to observe the flow at intermediate and deep levels.

  • Surface currents

    The upper part of Figure 2 shows low-pass filtered (cut-off frequency of 0.25 day-1) current vectors for the top of all moorings over the entire period of observations. These current vectors immediately show that the current field is dominated by ‘events’ during which the current speed at all moorings increases strongly and changes direction gradually. Such an event is shown in more detail in the lower half of Figure 2 while Figure 3 shows the temporal evolution of the current field at all mooring sites over the same selected period. The temporal evolution of the current field suggests that an anti-cyclonic eddy is formed in this area. At the final stage of the event the eddy drifts southward through the western side of the channel. Mean currents in between the events are relatively small, as compared to the current strength during the events. It is interesting to note that during both regimes there is not a well-defined boundary current along the African continental slope, nor along the Madagascar continental slope. The strongest currents are found near the centre of the channel.

  • A Lagrangian observation of an anti-cyclonic eddy

    One of the moorings, ACS06, was recovered in early february 2002 after drifting for roughly one week in the Mozambique Channel while transmitting its position via satellite communication. Thus, by accident, a Lagrangian ‘drifter’ observation with a subsurface (steel) cable length of some 1900 m is available for this period. The track of this drifting mooring before recovery is shown in Figure 4. This clearly shows that the mooring followed a very regular, almost circular path confirming that the currents in the area are dominated by the presence of a large scale anti-cyclonic eddy.

  • The volume transport through the channel

    Since our observations are not dense enough to allow a detailed estimate of the total transport, we restrict ourselves to a very rough estimate based on the first year of observations. Firstly, the transport per unit height (in m2s-1) in the surface layers is estimated from the observations at the top of the moorings, in which division has been made between the transport through the western and eastern side of the channel, see Figure 5 (top). This clearly shows that the events influence the transport calculations strongly in that during the events there is a strong southward transport at the western side of the channel and an opposing transport at the eastern side.
  • In order to estimate the total volume transport additional information on the vertical structure of the current field is needed because there are no long term observations in the interior of the channel. For this, information from LADCP (Lowered ADCP, attached to the CTD frame) observations during 2 cruises was used. (Fortunately, during the first cruise there was an anti-cyclonic eddy at the mooring array, the second cruise took place in a period ‘in between events’). The result is shown in Figure 5 (bottom) which gives an estimate of the total volume transport through the complete mooring section. The dashed line shows a 20-day running mean. Despite the large variability in time, especially the 20-day running mean suggests that there is a remarkable ‘cyclic pattern’ in the net volume transport. A maximum southward transport coincides more or less with the beginning of the events and a minimum transport is found near the end of the events when an anti-cyclonic eddy has formed. In between the events the southward transport gradually increases to its maximum value, at which the formation of the anti-cyclonic eddy sets in. There are indications that this cyclic behavior is remotely controlled by anomalies that propagate across the Indian Ocean and may be induced by wind variability in the equatorial region (Schouten et al., 2002). There is no clear seasonal variability in the net transport. During this first year of observations the net transport varies remarkably regular between some 10 Sv northwards and 40 Sv southwards. The mean transport over this period is 17 Sv in a southward direction.

  • The Mozambique Undercurrent

    The near bottom current meters at the western side of the channel (Figure 6) show interesting results in that there is a more or less continuous current towards the north-east (the Mozambique Undercurrent), which is sometimes interrupted when an eddy moves through the section. At 2500 m depth the current reaches a maximum speed of 39.9 cms-1 and an average speed of 7.7 cms-1 over the entire second period (including the eddy periods). At this deepest part of the section North Atlantic Deep Water (NADW) was found during the hydrographic cruises (de Ruijter et al, 2001). The mean northward transport of NADW by the Mozambique Undercurrent across the mooring array is estimated to be some 2 Sv (van Aken et al, 2002).

    • 3. Conclusions

    Low frequency currents at the narrowest section of the Mozambique Channel are dominated both in magnitude and duration by the regular occurrence of ‘strong current events’ during which a large anti-cyclonic eddy is formed. The strongest currents are found near the centre of the channel. Thus, in this region there is no coherent boundary current along the African shelf, nor along the Madagascar shelf.

    More or less persistent northward deep undercurrents are present, both at intermediate (1500 m) and deep levels (2500 m) along the African continental slope

    The net volume transport through the channel is remarkably oscillatory with fluctuations between about 10 Sv northwards and 40 Sv southwards. The net southward transport estimated from the first year of observations, is 17 Sv.

    4. Future plans

    Recently the Netherlands Organisation for Scientific Research has funded a proposal to carry out Long-term Ocean Climate Observations (LOCO). These observations will form a Dutch contribution to international programmes like CLIVAR and GOOS. As part of this LOCO program an array of moorings will be deployed in the Mozambique Channel for a period of 5 years. The location of the moorings will be similar the moorings discussed above. However, the observations will be more detailed by using more ADCP’s, current meters and additional sensors for salinity and temperature. First deployment of this mooring array will be in November 2003.

    References

    • Biastoch, A. C.J.C.Reason, J.R.E.Lutjeharms and O.Boebel, 1999. The importance of flow in the Mozambique Channel to seasonality in the greater Agulhas Current system. Geophys. Res. Letters, 26, 3321-3324
    • Biastoch, A. and W. Krauss, 1999. The Role of Mesoscale eddies in the Source Regions of the Agulhas Current. J. Phys. Oc. 29, 2303-2317
    • De Ruijter, W.P.M., A. Biastoch, S.S. Drijfhout, J.R.E. Lutjeharms, R.P. Matano, T. Pichevin, P.J. van Leeuwen and W. Wijer, 1999. Indian-Atlantic interocean exchange: Dynamics, estimation and impact. J. Geophys. Res., 104, 20,885-20,910.
    • De Ruijter, W.P.M., H. Ridderinkhof, J.R.E. Lutjeharms, M.W. Schouten and C, Veth, 2001. Observations on the flow in the Mozambique Channel. Geophysical Res. Letters., 10.129/2001GL013714
    • Ganachaud, C. Wunsch, J. Marotzke and J. Toole, 2000. Meridional overturning and large-scale circulation of the Indian Ocean. J. Geophys. Res. 105, 26117-26134
    • Lutjeharms, J.R.E., 1976. The Agulhas Current System During the Northeast Monsoon System. J. Phys. Oc. 6, 665-670.
    • Lutjeharms, J.R.E., O. Boebel, P.C.F. van der Vaart, W.P.M. de Ruijter, T. Rossby, H.L. Bryden, 2001. Evidence that the Natal Pulse involves the Agulhas Current to its full depth. Geophys. Res. Lett. 28, 3449-3452.
    • Ridderinkhof, H., J. R. E. Lutjeharms and W. P. M. de Ruijter, 2001. A research cruise to investigate the Mozambique Current. S. Afr. J. S. 97, 461-464
    • Schouten, M.W., W.P.M. de Ruijter and P.J. van Leeuwen, 2002. Upstream control of the Agulhas Ring Shedding. Geophys. Res. Lett. In press
    • Schouten, M. W., W.P.M. de Ruijter, P.J. van Leeuwen, H.A. Dijkstra - An oceanic teleconnection between the equatorial and southern Indian Ocean - Geophysical Research Letters 10.1029/2001GL014542 (2002) IM-02-11
    • Van Aken, H.M., H. Ridderinkhof and W.P.M. de Ruijter, 2002. North Atlantic Deep Water in the south-western Indian Ocean. Submitted to Deep Sea Res.

    Figure 1 - Location of the mooring array across the narrowest section of the Mozambique Channel (top) and cross section with the type and position of the instruments (bottom).

    Figure 1.

    Figure 2 - Low-pass filtered (cut-off frequency of 0.25 day-1) current vectors as a function of time for the top of all moorings. For ACS04 and ACS05 (ADCP observations) the current vector at 300 m depth is used. The beginning and end of the strong current events (e) is indicated. In the lower panel the current vectors for mooring ACS05 to ACS09 for the period between yearday 250 and 331 are shown.

    Figure 3 - Current vectors between yearday 251 and yearday 331 with an interval of 4 days at the top of all current meter moorings, roughly at 300 m depth. The strong current event is defined from yearday 273 to yearday 331, as indicated with B and E.

    Figure 4 - Track of a drifting mooring during one week after it had surfaced. Note the almost circular path of the mooring indicating the presence of an anti-cyclonic eddy.

    Figure 5 - Volume transport per unit height in the surface layer across the western side (ACS04-ACS07) and eastern side (ACS08-ACS10) of the channel during the first period of observations (top) and volume transport (in Sv) through the mooring section as estimated from the current observations at roughly 300 m, including a 20-day running mean (bottom). For the vertical integration of the current field Lowered ADCP observations from the hydrographic cruises have been used. Positive (negative) values indicate northward (southward) transport.

    Note the strong periodicity in the volume transport which is associated with the formation of the anti-cyclonic eddies. No seasonal variability was observed.

    Figure 6 - Near bottom currents at the western side of the channel (ACS05, at intermediate depth) and in the deepest part (ACS07). Currents are predominantly in a north-east direction with a maximum value of 39.9 cms-1 at 2500 m. At this deepest part of the section North Atlantic Deep Water (NADW) was found during the hydrographic cruises.

    Modular Ocean Model