Another phase of eruptions from volcano Aoba (Vanuatu)

By HansPeter Roesli (Switzerland)

The activity between 5 and 14 April can be seen in the Himawari-8 animation using Volcanic Ash RGBs from AHI at 10-minute intervals (Figure 1). The animation shows two major eruptions with a pause (shown in fast-forward) between 6 April 06:00 UTC and 9 April 16:20 UTC. Coincidentally, during the lull the area was frequently shrouded in convective cloud.

On 15 April the Wellington VAAC office reported that the eruption had ceased. The different phases of the eruptions are commented on in more detail below.

SO2 plume travelling to the Coral Sea

A map of SO₂ concentration extracted from IASI on Metop, tweeted by Simon Proud (@simon_sat ), showed a patch of high values east of Australia. The map was from 10 April and the tweet hinted that the SO₂ was being ejected from the volcano Aoba.

In fact, the first part of the animation in Figure 1 shows an eruption on 5 April after 13:50 UTC, with an SO₂ plume (green colour) that morphed into a ring while travelling west (see Figure 2).

According to the Ash&SO₂ RGB sequence (Figure 3) the SO₂ passed north of New Caledonia and arrived over the Coral Sea 24 hours later, very close to where IASI detected the large SO₂ patch.

The peculiar ring-shape of the SO₂ plume may be explained by the closest radio-soundings at Nouméa in New Caledonia (540 km south-west of Aoba) on 5 April 12:00 UTC and 6 April 00:00 UTC. They show a temperature inversion at/above 700hPa and moderate easterly winds below.

Most probably, the plume has evolved under this temperature inversion in a rather laminar flow that favoured its regular horizontal expansion.

SO2 and ash going east

After the lull, from 9 to 14 April, the skies cleared up somewhat and the renewed eruptions reached greater heights than before, most of them ending up in the westerly regime above the boundary layer.

The left-hand panel of Figure 4 shows a plume where the ejection of mainly ash (pink to grey colour) alternated with one of mainly SO₂ (green colour). In contrast, one day later (right-hand panel) an almost pure SO₂ plume encountered a turbulent regime and meandered considerably.

Going west again

After 11 April the eruptions weakened and the effluents were again driven westwards by the winds in the boundary layer. Sentinel-3A’s OLCI instrument scanned the area in daylight on 12 April.

The True Colour RGB from 12 April 22:20 UTC (Figure 5, left panel) shows a northwest pointing plume. Strangely enough, on the Himawari-8 Volcanic Ash RGB from the same time (Figure 5, right panel) the streak is hardly recognisable — very weak SO₂ colouring but no ash colours.