Friday, November 4, 2011

Back to the big picture for a minute

We have just had a fantastic first month of DYNAMO. The project was designed to study how MJO events start in the Indian Ocean. With only a two-month intensive observing period, though, it was not guaranteed that we would see a really good case of MJO initiation. Sometimes the MJO is inactive, and with a typical MJO cycle lasting 1-2 months, there was a chance there would be an event but we wouldn’t catch it at its start. Instead, nature has cooperated beautifully. When the experiment started in late September, we were in a suppressed phase, with clear weather over much of the Indian ocean. Then, during the first weeks of October, we had a gradual transition to moister, more active conditions, transitioning to a full-on MJO in mid-late month. Now, the MJO is moving eastward into the “maritime continent” (the region around Indonesia and the Malay peninsula) and the Indian ocean is drying out again.

Just to illustrate the big picture, here is a series of satellite images spaced roughly five days apart, starting on October 1 and continuing to now. These are infrared images, meaning that it doesn’t matter if it’s night or day. (All but the last of these images are from 12 GMT, which is 5pm in the Maldives.) White represents thick, high clouds (which generally go with rain), black is clear skies and dry conditions, and the lighter grays generally indicate partially humid or cloudy conditions. (This is the “water vapor channel”, meaning that the image becomes lighter either due to clouds or to high atmospheric humidity, or both.)

October 1: There is convection in the eastern and Indian Ocean, in the Bay of Bengal and near Sumatra; and in the southern Indian ocean, south of the equator. (Note Gan island, marked in the image, is right about on the equator.) This convection may be normal seasonal monsoon activity, or partly associated with the MJO event that passed through a little earlier, before the experiment had really begun. The westernmost Indian ocean, just off Africa, and the Arabian sea as well, are dead quiet.
October 5: very similar to the previous image. Nothing much has changed.
October 10: somewhat similar again, except the Indian Ocean weather has dried out even more. Almost the entire western portion is completely tranquil, except for a thin "intertropical convergence zone" - thin line of convective clouds - in the southern hemisphere. This is the suppressed phase of the MJO.
October 15: Small signs of a little more action brewing. The southern ITCZ is a little stronger, and a little convection starting to be seen in the Arabian sea just west of southern India. Between this image and the next one, the MJO reached a record amplitude for "phase 1", where it's over the Atlantic and Africa. So we're not quite seeing it in the Indian Ocean yet, but at this point all the models and forecasters were very confidently - more confidently than usual - predicting that it would be coming soon.
October 20: The MJO cometh. The whole basin is looking more active esp. in the western Indian ocean north of the equator. If you look closely you can see two prongs of active convection in the west, one around 10N (parallel to the tip of India) and another around 5S (parallel with Diego Garcia, marked as "FJDG"), with a dry slot in between. This feature has persisted for weeks and is due to westerly (west-east) winds bringing in dry air from Africa and the subtropics. We'll write about that more later.
October 25: Now we're talking. Looks something like the previous image, but more white in the picture, indicating more convective clouds and rainfall throughout the basin.
October 30: The MJO has matured in the Indian ocean - we call this "phase 3". The western region is starting to dry out again as the convection moves east. In the northern Arabian Sea you can see some swirling in the clouds indicating formation of a tropical depression. Tropical cyclones often form in the latter periods of MJO active phases, just like now. This system has hung around for a long time and as I write this was recently named Tropical Cyclone Keila, just as it fizzles (not before bringing some rain to Oman and Yemen, it looked like from the satellite images).
November 4, 2130 GMT: The active phase is just about past us and the Indian ocean weather quiets down. The easternmost region is again the most active. However, the remnants of the MJO are visible in the twin blobs of white about 10 degrees either side of the equator just west of the DYNAMO array. The one in the middle of the Arabian Sea may well turn into another tropical cyclone, according to some of the models.

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