When Does the Rain Arrive? Declaring the Start of a Season Billions Depend On

The seasonal reversal of winds that brings the rain is driven by the differing heat capacity of land and sea. Through spring the land mass heats far faster than the surrounding ocean, producing low pressure over the continent and drawing in moist air from the sea, which rises over the warm land and over the mountains and releases its water. In winter the pattern reverses, with cool high pressure over the land pushing dry air outwards. The rain-bearing phase advances across the region over several weeks rather than arriving everywhere at once, moving up from the south and progressing inland, and the progression is mapped and published as a series of dates. The mountain barrier to the north is essential to the system, since it blocks the exchange of air that would otherwise moderate the temperature contrast.

Declaring an onset requires criteria, since rain alone is not enough:

• •Sustained rainfall over a defined area and number of consecutive days, not a single downpour
• •A specified depth of westerly winds through the lower atmosphere
• •Outgoing radiation at the top of the atmosphere below a threshold, indicating widespread deep cloud
• •Measurement at a designated set of stations rather than anywhere
• •The criteria were formalised in 2006 after earlier declarations proved unreliable
• •Bogus onsets, where early rain is followed by a dry spell, are what the criteria exist to exclude

The dependence is direct and enormous. A large share of the region's annual rainfall arrives in this season, agriculture on rain-fed land is timed around it, and sowing too early wastes seed while sowing too late shortens the growing season, so the decision farmers make rests on when they expect the rain. Reservoir management, hydroelectric generation and urban water supply all depend on the total. Failure or delay has historically produced famine and continues to produce economic contraction visible in national accounts, while excess produces flooding that is equally damaging. Government policy on food reserves, on imports and on rural credit is adjusted according to the forecast. The seasonal outlook issued before the season is therefore among the most closely watched forecasts anywhere, and its accuracy has improved substantially and remains limited.

Comparable seasonal wind reversals occur in several regions and comparing them clarifies what drives them. West Africa has a system bringing rain northwards over the Sahel through the northern summer, with failures there producing severe drought in the 1970s and 1980s and with the mechanisms substantially different in detail. East Asia has a rain band that moves north through the summer and produces prolonged frontal rain in China, Korea and Japan, with its own local names and its own forecasting problems. Northern Australia has a wet season driven by the same seasonal migration of the tropical rain belt. Parts of the Americas show weaker versions. What the cases share is a seasonal reversal driven by the contrast between land and ocean heating, and what differs is the terrain, the ocean conditions and how sharply the season begins and ends.

Predicting the total and the timing months ahead is one of the harder problems in the field. The system responds to conditions across the whole tropical ocean, including the periodic warming and cooling of the Pacific and a comparable pattern in the Indian Ocean, and those relationships are real but not strong enough to determine the outcome, and they have weakened and strengthened over decades in ways not fully explained. Within the season the rain comes in active and break phases lasting a week or two, and predicting those matters as much to a farmer as the seasonal total. Snow cover to the north, soil moisture and aerosol loading all appear to influence the system. Model resolution is a limiting factor, since the rainfall is produced by processes smaller than the grid of a seasonal model, and the region's complex terrain compounds it.

The seasonal wind reversal is driven by land heating faster than ocean, and the rain-bearing phase advances across the region over weeks rather than arriving at once. Declaring the start needs formal criteria covering sustained rain, wind depth and cloud, adopted in 2006 to exclude false starts. The seasonal outlook is among the most watched forecasts anywhere and remains difficult.