Figure 1: The eclipse track across Indonesia and the Pacific.
Figure 2: Indonesia Place Names and the Eclipse Track
Climate Discussion for the 2016 Total Eclipse
This March eclipse begins in the equatorial Indian Ocean west of Sumatera (Sumatra) and arcs northward to a sunset end in the high tropical latitudes north of Hawaii (Figure 1). The shadow passage begins at sunrise in the hot equatorial climate of Indonesia, under the influence of the Intertropical Convergence Zone and at the latter stages of the rainy monsoon season. Maximum eclipse comes in the mid-tropics, beneath the semi-permanent anticyclones (highs) that endow those latitudes with sunny skies and dry weather. In the afternoon of its passage, it moves into the trade wind circulation north of Hawaii, bringing a modest increase in cloudiness (Figure 2).
此次日全食始于苏门答腊以西的赤道印度洋，结束于夏威夷以北的高纬度热带地区（图1）。 全食带的日出部分属于印尼热赤道气候，处于雨季季风季节的后期阶段，受到热带辐合带的影响。最大食分出现在中热带，半永久性高压反气旋赋予那些地区晴朗的天空和干燥的气候。 全食带的下午部分，进入了信风流通的北方夏威夷，带来了云量的部分增长（图2）。
Except for a few tiny and difficult-to-reach islands in the Federated States of Micronesia, land-based sites (Figure 2) are restricted to Indonesia, primarily in Sumatera and on the islands of Kalimantan (Borneo), Sulawesi (Celebes), and North Maluku (Molucca). Both Sumatera and Sulawesi possess significant mountain and highland regions. The track across Kalimantan lies across the lower and flatter southern parts of the island, while the Maluku Islands have a rough but lower-level range of hills.
Indonesian climate is dictated by its geographical position, straddling the equator and affected by the annual movement north and south of the Intertropical Convergence Zone (ITCZ). The ITCZ is a belt of low pressure where the easterly trade-wind circulations from the Northern and Southern hemispheres meet, squeezing the lower levels of the atmosphere upward. The year-round equatorial heat and humidity over Indonesia and elsewhere in the tropics forms an atmosphere that is barely stable, ready to turn into convective clouds with the slightest disruption. The upward push brought on by the convergence of the ITCZ winds upsets the stability and the ITCZ is translated into a belt of frequent convective cloudiness and heavy precipitation.
The presence of the ITCZ is evident in Figure 3 as a belt of heavy cloudiness that stretches eastward across the Pacific from the Indonesian archipelago. In the islands themselves, however, the ITCZ is only part of the story, as the daily heating of the land, sea- and land-breeze circulations, and the interaction of winds with topography also promote instability and give rise to complex cloud patterns. In both the cloud and precipitation maps, the heaviest cloud and precipitation are found over Sumatera and Kalimantan, with lesser amounts over Sulawesi and the Moluku Islands. The complexity of these cloud patterns is evident in the satellite pictures displayed in Figure 7.
The ITCZ migrates north and south with the annual movement of the Sun, and so Indonesia is treated to seasonal wet and dry spells according to whether the ITCZ is overhead, or gone to higher or lower latitudes. In the western islands, the influence of the ITCZ is quite noticeable in the monthly rainfall statistics, such as those for Palembang in Figure 4. In Sumatera and Kalimantan, March is at the peak of the monsoon season. Farther east, rainfall amounts drop off and the distinct wet and dry seasons become more muted. At Ternate (Figure 4), wet and dry seasons are difficult to discern, but the important distinction is that March comes with only half the precipitation of the more westerly islands.
Figure 3: Average Daytime Cloud Cover in 10ths Along the Eclipse Track, derived from 19 years of satellite imagery. Source: NOAA Satellite Active Archive.
Figure 4: Monthly rainfall graphs for Palembang and Ternate. At Palembang, the wet and dry seasons are well defined, but at Ternate, the seasons are muted and out of phase with those in western Indonesia.
In satellite images, the most prominent moderator of cloud cover is the daily heating of the land, which builds deep convective clouds as the morning advances into afternoon. This heating, and then cooling at night, sets up a cycle of sea and land breezes, each of which creates its own pattern of cloudiness and rain. Generally speaking, the daytime sea breezes will carry cool air onto the land and make coastlines a little less cloudy than inland regions, but the difference is small, as the cloud that forms further inland then blows back over the coast at a higher level, at least on the eastern sides of the larger land masses. At night, land breezes, blowing out ward from the land, lift the atmosphere and form clouds over the water. Nevertheless, the cloud map of Figure 3, the precipitation map of Figure 5, and the “cloud along the centreline” graph in Figure 6 shows that overwater areas and small islands are distinctly less cloudy than those on the larger land masses.
Figure 5: Average March precipitation in mm/day derived from satellite observations. Source: GPCP/NOAA.
Figure 6: Average Cloud Amount Along the Eclipse Centreline. Cloud statistics are derived from 19 years of satellite imagery.
Source: NOAA Satellite Active Archive.
Observations from surface-based weather stations presented in Table 1 (below) show a modest decrease in cloudiness along the track, with mean cloud amounts at eclipse time ranging from mid-80 percent in Sumatera to mid-60 percent over the Maluku Islands. Satellite measurements of cloud cover are less distinct, with mean cloud amounts ranging from 75 percent in Sumatera to about 60 percent in the Malukus, save for a drop below 55 percent over the Molucca Sea. Ship-based observers will definitely have an advantage in this eclipse, giving them access to the lowest cloud amounts and a mobile platform to seek out better skies on eclipse day.
For those who prefer to observe from land, several small islands such as Bangka, Belitung, and Ternate offer the best prospects. From a meteorological perspective, the areas to be avoided are western Sumatera and the interiors of Kalimantan, and Sulawesi.
Beyond Indonesia, the eclipse track moves into a much sunnier climate as it crosses under the semi-permanent sub-tropical highs along the 30th parallel of latitude. Average cloudiness declines to less than 30 percent north of Truk and then increases steadily to the 60 percent mark as the track comes to its sunset ending north of Hawaii. Two small islands lie under the track, one temptingly close to the maximum point and the centre line. This is Woleai Atoll (and specifically, Falealop), which has an airport, though its use is now suspended because of the need for repairs. Ifalik, the second island, is half-way to the south limit. The islands are on the edge of the anticyclone’s influence, and cloud cover is not different from that in eastern Indonesia. (Web searches on these islands will result in endless confusion, as Falealop is usually mixed up with Falalop)
The Impact of El Niño
El Niño is a periodic large-scale meteorological event that is characterized by a pool of anomalously warm water in the Equatorial Pacific, weaker-than-normal easterly trade winds and a dramatic change in precipitation patterns across the Central Pacific. For Indonesia in particular, an El Niño brings dry weather or even a drought in the midst of the rainy season. It would seem natural to expect a change in cloud cover across the region in concert with the drier weather.Figure 7 shows that this is indeed the case, with reductions in cloud cover of 5 to 15 percent typically in March of El Niño. The map is constructed by subtracting the cloud cover measurements for El Niño years from the long-term average cloudiness. Since the long-term average includes years with an El Niño, the true difference may be slightly larger (though the dataset also includes La Niña years when cloud cover is higher).
Figure 7: Satellite images for March 9 showing the extent of cloud cover over the past 4 years. These are visible-light images acquired at 00 UTC (eclipse time) from a Japanese geostationary weather satellite.
The six images demonstrate the high frequency of cloud over land, particularly Kalimantan (Borneo) and Sulawesi. Cloud over the intervening straits is lower, sometimes significantly so. These are early-morning images, when the cloudiness over land is near its minimum daily value and that over water at its maximum. Image credit: Dundee Satellite Receiving Station, University of Dundee.
这四张图像显示了陆地上空的大量云层，特别是加里曼丹（婆罗洲）和苏拉威西。海峡上空的云量较少，有时候还特别显著。这些是清晨的卫星图片，此时云量接近每日的最小值而水汽则是最大值。图片来源: Dundee Satellite Receiving Station, University of Dundee.