A consensus forecast issued by a group of South Asian countries was just released and projects near-normal monsoon rainfall for summer 2018. The South ASian Climate Outlook Forum (SASCOF) was established in 2010 and met last week, together with experts from both the World Meteorological Organization and user communities in India. They considered dynamical and statistical forecasts issued by all of the world’s major weather prediction centers, and issued this consensus statement:
“Normal rainfall is most likely during the 2018 southwest monsoon season (June – September) over most parts of South Asia. However, above normal rainfall is likely over some areas of east central India and southeastern parts of the region. Below-normal rainfall is likely over some areas of southern, northwestern and northeastern parts of South Asia.”
Visually, here is what this looks like (excerpted from their full forecast discussion):
The APEC climate center in Korea also issued new seasonal forecasts yesterday, which are generally have South Asia being a little wetter than the above consensus forecast, especially during early summer:
Globally, these are similar to the forecasts that APEC released one month ago; if we limit our focus to land regions, a major feature is that North America and southwestern Asia are dry in early summer (through July, while much of the Sahel is wet in late summer. The dryness in southwestern Asia is a big deal because Afghanistan is already in a prolongued drought, with millions of people being food insecure, according to the United Nations. Afghanistan isn’t typically thought of as a monsoon region because it receives most of its rain in winter and is dry in summer, but much of that summer dryness occurs because of the downward atmospheric motion induced over Afghanistan by the South Asian monsoon (e.g. this paper by Bollasina and Nigam).
India’s first forecast for the 2018 summer monsoon season was just released, and it is for a near-normal amount of rainfall over the country as a whole. In particular, based on their statistical model and their relatively new numerical model, they are predicting 97%±5% of the historical average. More details on the India Met Dept website (find the link in the right-hand bar of scrolling news).
They also cite a ‘low probability’ for deficient rainfall, meaning that there is a 42% chance the rainfall will be in the ‘normal’ range of 96-104% of the long-term average. It’s interesting that their probability breakdown gives a higher probability of low rainfall (44% chance) than of high rainfall (14% chance), but it’s unclear whether their statistical model has enough skill for such subtleties to be meaningful.
Columbia’s IRI also updated its global seasonal forecast on April 15, and the only notable change for monsoon regions is that Mexico is no longer forecast to be wetter than normal during the spring/early summer. Mexico didn’t have anomalously high rainfall in April thus far, and it doesn’t look like the current weather models are forecasting high rainfall there over the next week, so this actually seems like a change in the spring forecast:
Combined with their July-Sept forecast, this makes it look like India’s country-averaged, seasonal-mean rainfall might indeed be normal, but only because the country will be wet in the northeast during early summer and dry in the south during late summer:
The ENSO forecasts still have La Niña transitioning to a near-neutral ENSO index, but this again is made up of warmer water in the north Pacific and Atlantic, and relatively cold water south of the equator in both of those ocean basins. Given research showing how the tropical rainfall maximum can shift north-south into the hemisphere with the warmer ocean, this helps justify the idea that Africa may be wetter in the northern parts of the Sahel and drier in the south, and that India may also be wetter in the north and drier in the south.
The summer monsoon season will soon be starting in South Asia, northern Africa, and north America, so it’s time to check in with forecasts of seasonal rainfall. India will release its first monsoon forecast in a week or two, and a number of the seasonal forecasts discussed below will be updated in the next week, but it’s nevertheless a good time to take a first look at what might be happening.
IRI/Columbia University is predicting a wet early summer in the Philippines, Southeast Asia, and western Mexico, and a wet late summer in much of Africa (especially East Africa):
The APEC climate center in Korea has many of the same features; because their maps show oceanic rainfall, the wet anomalies over Mexico and the Philippines are more clearly seen as part of a northward shift of the East Pacific ITCZ:
This northward shift of the East Pacific ITCZ isn’t a classic La Niña pattern. Most of the models are actually forecasting a transition from La Niña to El Niño during summer, but this seems to occur with the cold SST persisting south of the equator in the East Pacific while the northeast Pacific warms, producing that northward shift in the ITCZ that may bring wetness to Mexico, the Philippines (and maybe even SE Asia?). Here are the SST forecasts from the North American Multi Model Ensemble:
The Australian BoM provides a nice overview of all the large-scale climate modes: ENSO is nearing neutral, as is the Indian Ocean Dipole. The BoM model is predicting a negative IOD during June (i.e. cooler SST in the western Indian ocean relative to the east), but there seems to be a fair amount of disagreement in models on this — the NMME forecast shown above looks to be a near neutral for JJA, while even the BoM page shows that only 2 out of 6 models forecast a clear negative IOD phase for June.
Links to the forecasts discussed above: