Annual June rains hit southeastern China

As 'Plum Rains' cause havoc in many parts of China, Al Jazeera's weather experts explain why they occur.

Plum Rains arrive from late May to early June and usually peak around June 10 - 15 [Gallo/Getty]

As parts of southeastern China struggle to cope with the worst flooding in more than half a century it is worth reflecting on the cause of the rains which are an annual - and often most welcome, event.
These rains are referred to as the Maiyu, or Plum Rains. 'Plum' refers to the ripening of the plum crops around this time and the rains are also welcomed by rice farmers in both China and Japan.
But a hint at possible problems caused by these rains may be gleaned from an alternative name -  mould rains - as crops can fall foul to mould and other diseases when the rains are particularly heavy or prolonged.
The Plum Rains arrive from late May to early June and usually peak around June 10 to June 15, in southeastern China.
Alas, such precision with the timing of the rains does not carry forward into predictions of the amounts of rain the region is likely to see each year.
The key to their arrival lies high in the sky, above the Himalayas. High altitude jet stream winds branch either side of the mountains during the winter and spring months.
Summer monsoon
But during the last week of May the southern portion of these winds weakens rapidly. At the same time, the rising heat of the summer causes a mass ascent of air, forming a weak area of low pressure over the Tibetan Plateau.
This combination of events allows the moist southwesterly air of the summer monsoon to begin its advance.
The intensity of the Plum Rains is often dependent on the fine balance between the advancing warm, monsoon air and slowly retreating cool, northerly air.
The struggle between these two air masses is often played out over the Yangtze and Huaihe river valleys and it is particularly intense during times of the La Nina Southern Oscillation.
Although La Nina is currently weakening, its influence may be felt for some time after its peak and this may have been the case this year.
Inevitably, the rains in the flood-affected regions will ease as they make their way slowly northeastwards, weaken dramatically as they do so, but China lives with the threat to life, livelihood and property that the Plum Rains bring every year.