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Regional Weather

WIND CONDITIONS

The following is a MSL (Mean sea level) Pressure map for the Indonesian regions, issued by the Australian Government:

 

Most Recent Regional Weather Map

 

For many people, this does not mean a great deal. All you really need to know is that each line represents an equal level of pressure. If the lines are close together you can expect stronger winds. A higher pressure gradient. It is perhaps more easily visualised with this graphic:

 

Windfinder

 

The Indonesian archipelago is made up of over 17,000 Islands, stretched over 2000nm with many of its island peaks rising above 3 km, these peaks form part of the famous ring of fire; and have a large effect on local wind conditions experienced on each Island. Therefore local knowledge for land and sea breezes is critical, however most Islands in Indonesia are within 4 degrees of the equator. so they will all experience South East Asian Monsoon.

 

The following link shows the last years wind conditions as recorded from Selaparang/Lombok. Central to the archipelago and also our trips aboard Bulan Purnama.

 

 

Wet Season, Nov-April: The monsoon is experienced as West, NorthWest, bringing rain in the Southern Hemisphere summer.

Dry Season, May-Oct: The monsoon is experienced as SouthEast in these islands, keeping things dry in the Souther Hemisphere winter.

 

In general, the winds follow the monsoon expectations. With one notorious exception in February, where it will blow erratically from the North. Although regional winds can sometimes become challenging, since we are close to the equator simple science and coriolis effect dictates that we will experience less volatile low pressure storm activity in this region. Most of the time it's "smooth sailing". Current flow activity on the other hand is a totally different story!

CURRENT & TIDE CONDITIONS

Firstly you can watch this CSIRO video to get an overall understanding of the current flow in this region. When studying the current activity of Indonesia it immediately becomes clear that the Indonesia archipelego acts like a giant sieve for water flowing from the Pacific Ocean in the North to the Indian Ocean in the South, creating large throughflow currents in the straights between the islands. This through flow can in turn produce a standing wave when faced with high underwater ridges, running perpendicular to the flow, at the bottom of the straight, this is the culprit for the mixture of diurnal and semi diurnal tides experienced in many of these islands… .which makes for a  complex tidal  system.

The direction of the transport is dependent on seasonal and annual climate, although the total net annual transport is mostly southward from the Pacific Ocean into the Indian Ocean.

In the available data, an annual net northerly directed transport through the southern boundaries was only seen in 1998, and this can be explained as a post El Nifio effect. However, it also directly illustrate the relation between the global climate and the Indonesian Throughflow. An excellent study on the Indonesian Throughfolw can be seen here:

Indonesian ThroughFlow

An important feature of the Indonesian Throughflow is that because the water in the western equatorial Pacific Ocean has a higher temperature and lower salinity than the water in the Indian Ocean, the Throughflow transports large amounts of relatively warm and fresh water to the Indian Ocean. When the Indonesian Throughflow through Lombok Strait and the Timor passage enters the Indian Ocean it is advected towards Africa within Indian South equatorial current. Here it eventually exits the Indian Ocean with the Agulhas Current around South Africa into the Atlantic Ocean. So the Indonesian Throughflow transports a significant amount of Pacific Ocean heat into the southwest Indian Ocean, which is 10,000 km away from the Lombok Strait.

The Lombok Strait is an excellent example of how current flows not only effect tidal condition in the region, but also effect the sea state, critical to a comfortable passage on any vessel. Lombok Strait is strait located at 155.37'E-116.02'E and 8.20'S-8.50'S between the islands of Lombok and Bali in the Australasian Mediterranean Sea. This is one of many possible and the second largest of the passages for the Indonesian throughflow from the Pacific to the Indian Ocean, and connects the western Flores Sea to the Indian Ocean. Bulan Purnama will cross this straight on any trip between Bali and Komodo. The strait spans a length of about 60 km with a north-south orientation. It is about 40 km wide and 1400 m deep at its northern opening, but only 18 km wide and 250 m deep at the sill in the southern opening.

Despite its small size, the Lombok Strait is considered critical for the Indonesian Throughflow. This is because of a 600 m deep trough along the edge of the Sunda Shelf extending from south of Makassar Strait to north of Lombok Strait that effectively funnels a maximum salinity water mass from Makassar Strait directly to Lombok Strait. Its position at the edge of the Sunda Shelf also allows it to transport both Java Sea and Flores Sea surface waters to the Indian Ocean.

In the northern and central strait, there is a deep channel with water depth up to 1400m. At the southern outlet, Nusa Penida Island divides the strait into two channels. The western channel, Bandung Strait, is shallower. Its water depth is less than 100 m. The eastern channel is deeper and has an east-west running sill along its bottom. The channel serves as a major passage for water exchange from the Indian Ocean into the strait. The latest measurement carried out during the 2005 Indonesian throughflow cruise showed that the sill-crest depth is about 250 m. The northern part of the LombokStrait has mixed tides that have a predominantly diurnal cycle (Chong et al., 2000). However, the tide at the sill region is predominantly semi-diurnal; tidal velocity there can exceed 3.5 m/s. Nonlinear interactions between the semi-diurnal and diurnal tidal components induce a strong tide with a period close to 14 days (Ffi eld and Gordon, 1996; Susanto et al., 2000). Because of the presence of stratified water, rough topography, and strong tidal currents, the Lombok Strait is characterized by intensive internal-wave generation. An excellent study on this internal wave generation within the Lombok Straight can be read here:

Internal Waves Observed in th Lombok Strait

 

For general further reference, please look up the Lombok straight Experiment: S. P. Murray and D. Arief. Throughflow into the Indian Ocean through the Lombok Strait.

Nature, 333:-447, 1988. Local Tides

It was Isaac Newton's universal theory of gravitation that first enabled an explanation of why there were two tides a day, not one, and, via calculation of the forces, offered hope of detailed understanding. Although it may seem that tides could be predicted via a sufficiently detailed knowledge of the astronomical forcing terms, the actual tide at a given location is determined by the response of the oceans to the astronomical forces accumulated over a period of many days. To calculate this response requires a detailed knowledge of the shape of all the ocean basin; their bathymetry and coastline shape. Instead of a direct calculation, the procedure for analysing tides is pragmatic: At each place of interest, the tide heights are measured for at least a lunar cycle. The tide heights are compared to the known frequencies of the astronomical tide-raising forces. The behaviour of the tide heights is expected to follow the behaviour of the tide force, with the amplitude and delays of those responses remaining constant. Because astronomical frequencies and phases can be calculated with certainty, the tide height at other times can be predicted once the response to the astronomical states has been found. The below Tide predictions are made available by Indo surf. These predictions are done pragmatically and are relevant for this indicated area of Indonesia only.

 

Bali tides

 

Note that an outgoing tide, or ebbing tide will flow north in the Lombok strait, as in most of these indonesian stairs. The NET Flow however, if you have read any of the information above, you will have realise will only ever be a combination of Wind Season, Current, Standing Waves and the Moon Based local tides . It is the NET flow that mariners are interested in, unfortunately no table can be given for this. Only a combination of experience and knowledge of the forces at play will put you in good stead to safely and consistently navigate these waters.