Figure 1: Mean monsoon-season (June-September) precipitation (mm) for South Asia, 1901-2016. Land grid cells where precipitation was under 100 mm are left unshaded and are not included in the study domain.
Densely populated South Asia, a leading agricultural area, derives most of its water supply from intense summer rainfall (Figure 1). The rainy season is closely linked to the South Asia Summer Monsoon (SASM), in which moisture flows to the Indian subcontinent from the Arabian Sea and the Bay of Bengal, driven by the land-sea temperature contrast. Interannual variation in the amount and spatial distribution of summer precipitation in the SASM monsoon zone is considerable, with severe consequences for agricultural productivity and human well-being. Prediction of monsoon precipitation has therefore been pursued for over a century, using several predictors such as snow cover and air pressure patterns.
With average precipitation of 35 inches per four-month season over an area encompassing most of the Indian subcontinent, the South Asia summer monsoon is intense, only partly understood, and notoriously difficult to predict. A frequent visitor to the region, stretching from Nepal to Sri Lanka, Krakauer has devised a methodology that allows forecasts potentially up to a year in advance. Currently, most predictions are made about two months in advance of the South Asia monsoon season that runs from June to September, but it is not known how far ahead skillful forecasts might be possible.
"People usually use one or two predictors for forecasts," said the Grove School of Engineering associate professor who is also affiliated with the CCNY-based NOAA-CREST. Many of these predictors are one or another pattern of sea surface temperatures. His question was how do you find which patterns are important for forecasting the monsoon—the amount of rain and where it will be?" Unlike other forecasters who use only the sea surface temperature readings from neighboring waters, Krakauer looked at the predictive potential of all the common patterns in the sea surface temperature map. He developed prediction methods using global sea surface temperature and monsoon precipitation data from between 1901 to 1996, and tested the performance of his prediction methods on data from 1997-2017. The results show that two methods seem to do a good job of forecasting the monsoon. Sea surface temperatures at the beginning of the monsoon as well as four years back were analyzed.
His finding was that, generally, the closer to the beginning of the monsoon season, the more accurate forecasts that are based on sea surface temperature can be. But predictions with some accuracy can be made as far as a year in advance. Getting a better sense of how much water will be available is particularly important given that the rainfall is getting more intense in South Asia while the total amount remains constant, meaning that more rain is falling in a shorter period. This could be problematic for farmers in the region.
By considering linear and nonlinear forecast methods using SST modes as predictors, prediction skill for spatially distributed monsoon-period precipitation over South Asia was found to decay with a timescale of 5-12 months, but with residual skill at several-year lead times due to long-term climate trends. While these methods are not definitive and could likely be further improved, the present findings suggest that South Asia monsoon period precipitation can be predicted with longer lead time than the subseasonal to seasonal leads usually attempted now.
Nir Y Krakauer. Year-ahead predictability of South Asian Summer Monsoon precipitation. Environmental Research Letters, 2019; DOI: 10.1088/1748-9326/ab006a
To learn more about other climate-related stories occurring cross Asia, be sure to click here!
© 2018 Oceanographer Daneeja Mawren
This MODIS satellite image on August of 2005 captures a dust storm crossing Iraq and portions of the Middle East.
When one thinks of the desert, maybe an image such as the one above comes to mind. The MODIS satellite shows a mainly barren landscape with a dust storm scraping across portions of the Middle East.
A well-known concept is that deserts are some of the driest places on the planet and cannot support much plant life. Many deserts have a wet season where just about all the yearly precipitation falls (an amount typically minuscule in comparison to other regions) and a dry season where little if any precipitation falls. A previous article outlining the active fall weather across the Middle East is located here.
The images above are from Columbia University and showcase three-month precipitation anomalies from fall into winter across the Middle East. Notice the persistent above to well above normal precipitation across the region.
Putting all of this into perspective, the desert portions of the Middle East are bone dry during the summertime months while precipitation is much more common in periods during the fall and winter. Persistent anomalous rainfall has contributed to a transformation across portions of western Iran, as well as much of Iraq and Syria. Precipitation anomaly maps are posted above showing in some cases a three-month period where precipitation was 50-100+ mm (about 2 to 4 inches or more) above normal. So, what was the result of this excess precipitation in a desert like landscape?
MODIS satellite imagery from February 2018 (above) and 2019 (below). Notice the amount of greenery that is present across northern and central Syria, northern and central Iraq, western Iran, and even portions of southwest Iraq into northern Saudi Arabia.
Examine the MODIS satellite images above both from February (one in 2018 the other in 2019). The difference is rather astounding and significant. Vegetation exists all throughout Syria and Iraq this year, the same locations that were barren desert landscapes in 2018. This begs the question, with that amount of vegetation, will the wet season hang on a little longer with all the extra moisture from evapotranspiration in the plants? Will this not be a factor once the synoptic pattern switches over to the dry season and all the plants die off? Or possibly a combination of the two in some fashion.
Nobody knows for sure but stay tuned to GWCC to find out over the coming months and click here to learn more about Western Asia.
©2019 Meteorologist Joe DeLizio