Nining Sari Ningsih

Upwelling variability along the southern coast of Bali and in Nusa Tenggara waters

Spatial and temporal variation of upwelling along the southern coast of Bali and in the Nusa Tenggara waters — Indonesia was studied by using satellite image data of sea surface temperatures and chlorophyll-a from September 1997 to December 2008. This study clearly reveals annual upwelling in the regions from June to October, associated with the southeast monsoon cycle, with the sea surface temperature (chlorophyll-a concentration) being colder (higher) than that during the northwest monsoon. In addition, this study also shows that the upwelling strength is controlled remotely by ENSO and IOD climate phenomena. During El Niño/positive IOD (La Niña/negative IOD) periods, the Bali — Nusa Tenggara upwelling strength increases (decreases).

Distributary channels in the fluvial to tidal transition zone

Coastal lowland plains under mixed fluvial-tidal influence may form complex, composite channel networks, where distributaries blend the characteristics of mouth bar channels, avulsion channels, and tidal creeks. The Kapuas coastal plain exemplifies such a coastal plain, where several narrow distributaries branch off the Kapuas River at highly asymmetric bifurcations. A comprehensive geomorphological analysis shows that trends in the channel geometry of all Kapuas distributaries are similar. They consist of a short, converging reach near the sea and a nonconverging reach upstream. The two parts are separated by a clear break in scaling of geometrical properties. Such a break in scaling was previously established in the Mahakam Delta, which suggests that this may be a general characteristic in the fluvial to tidal transition zone. In contrast to the geometrical trend similarities, a clear difference in bed material between the main and side distributaries is found. In the main distributary, a continuous trend of downstream fining is established, similar to what is often found in lowland rivers. In the side distributaries, bed material coarsens in the downstream direction. This indicates an undersupply of sediment to the side distributaries, which may contribute to their long-term stability as established from historical maps. Tides may be the main agent preventing fine sediment to settle, promoting residual transport of fine material to the coastal ocean.

Prerequisites for Accurate Monitoring of River Discharge Based on Fixed‐Location Velocity Measurements

River discharge has to be monitored reliably for effective water management. As river discharge cannot be measured directly, it is usually inferred from the water level. This practice is unreliable at places where the relation between water level and flow velocity is ambiguous. In such a case the continuous measurement of the flow velocity can improve the discharge prediction. The emergence of horizontal acoustic Doppler current profilers (HADCPs) has made it possible to continuously measure the flow velocity. However, the profiling range of HADCPs is limited, so that a single instrument can only partially cover a wide cross-section. The total discharge still has to be determined with a model. While the limitations of rating curves are well understood, there is not yet a comprehensive theory to assess the accuracy of discharge predicted from velocity measurements. Such a theory is necessary to discriminate which factors influence the measurements, and to improve instrument deployment as well as discharge prediction. This paper presents a generic method to assess the uncertainty of discharge predicted from range-limited velocity profiles. The theory shows that a major source of error is the variation of the ratio between the local and cross-section averaged velocity. This variation is large near the banks, where HADCPs are usually deployed, and can limit the advantage gained from the velocity measurement. We apply our theory at two gauging stations situated in the Kapuas River, Indonesia. We find that at one of the two stations the index-velocity does not outperform a simple rating curve.

Analisis Sinyal El Nino Southern Oscillation (ENSO) dan Hubungannya dengan Variabilitas Arus Lintas Indonesia di Selat Lifamatola

This paper describes correlation between ITF transports through the Lifamatola Strait with SOI data representing ENSO phenomenon has been carried out. The Lifamatola Strait is one of the ITF eastern routes which has sill around a depth of 1940 m. Current velocity data of INSTANT mooring deployed in 2004-2006 and SOI data were used In this study. The data analysis was done by using methods of low pass filter, spectral analysis, and cross correlation. The currents in the Lifamatola Strait flow dominantly northwestward in depths of 1000-1300 m, while in depths of 1400-2000 m the currents flow dominantly southeastward representing the ITF. Results of this study showed that there was a correlation between the velocity current (ITF transports) in the Lifamatola Strait with ENSO phenomenon, namely the strengthening/weakening southeastward transport of the ITF during El Nino/La Nina phenomenon. Cross correlation values of the current velocity and SOI data in the Lifamatola Strait are r(2-7) = 0.3718-0.7040 at depths ofl000-1500m with time lag of2-7 months and those are r(-11) = 0.5022-0.5538 with time lag of 11 months.