From the sea, knowledge

South Asia’s agriculture, water management, and indeed much of the sub-continents’ way of life, is dependent on the mercy and whims of the South Asian Summer Monsoon rainfall and circulation. At the same time highly-varying and dependable (while it is certain to manifest each year, the amount and timing of the crucial rainfall can be shrouded in uncertainty), the monsoon is the subject of much continuous research that ever strives for better sub-seasonal, seasonal, and interannual forecasts, which can help South Asia’s people to better manage, and prepare, for its rains. In particular for agriculture, finding robust, reliable, predictors for monsoon variability is a crucial determinant for regional food production and security, as producers rely on advance weather information to decide what and when to plant, and how to allocate their management resources. If the natural variability, the monsoon’s ups and downs, baked into the system weren’t enough, the whole region stands be further impacted by rising temperatures and altered circulation patterns that accompany global climate change.

In this vein, a new study by Roxy et al. (2015) in Nature Communications has found a strikingly strong relationship between rising Indian Ocean sea surface temperatures (SSTs), and declining rainfall around the Indian sub-continent. The logic goes that as the Indian Ocean SSTs rise, particularly in the western portion of the basin, the major centers of convection (heated, moisture filled air that may rise high into the atmosphere, eventually contributing to rainfall) start to shift around the region, along with the transport of moisture by the canonical monsoon winds, for which the system is named. Roxy et al. suggest that as these SSTs increase, as they have been, the winds have concurrently weakened, causing less transport of moisture over the continent, leading to declines in rainfall over the land surface.

 Sea surface temperatures (absolute, not anomalies) and land vegetation over the Indian Ocean are seen below in a visualization created with data from 1994 to 2005 from the Pathfinder satellite dataset. Impacts of rising Indian Ocean SSTs have also been linked to changes in rainfall over East Africa. Find more here:     Credit:  NASA

Sea surface temperatures (absolute, not anomalies) and land vegetation over the Indian Ocean are seen below in a visualization created with data from 1994 to 2005 from the Pathfinder satellite dataset. Impacts of rising Indian Ocean SSTs have also been linked to changes in rainfall over East Africa. Find more here:

Credit: NASA

In fact, the idea that changes in Indian Ocean SSTs can modulate the monsoon rainfall and circulation has been floating around for quite some time now. In 1999, Saji et al. and Webster et al. suggested that there was a source of internal variability (meaning, perhaps not forced by climate change, but occurring naturally) in the Indian Ocean, referred to as the Indian Ocean Dipole (IOD) Mode, or Equatorial Oscillation, among other names. In the simplest description, this “mode” seems to flip between alternating states that leaves the western Indian Ocean warmer (in the positive phase) or cooler (in the negative phase) than the eastern Indian Ocean, thereby also changing the overlying atmospheric convection and circulation. Since then, much research has been done to understand: how independent and reliable this IOD actually is; how the Indian Ocean varies with the better known El Nino Southern Oscillation (ENSO) cycle of altered SSTs and circulation in the Pacific; and how the Indian Ocean will “adjust” to rising global temperatures, both at the surface and in its deeper circulation (Gadgil et al., 2004; Ashok et al., 2004; Ihara et al. 2007; Ihara et al., 2008; Ummenhofer et al., 2011; and many more).

Monsoon rainfall forecasts seasonally issued for South Asian agriculture have historically relied heavily on ENSO indices, which, again historically, have explained approximately 30% of the interannual variability and are normally associated with below-normal rainfall totals. However, this relationship did not hold during the 1997/1998 major ENSO event, and the above studies suggest that other Indian ocean/atmosphere dynamics and interactions may be important for future study and forecasts. The uniqueness of the Roxy et al., study is that it demonstrates a surprisingly strong relationship between declining monsoon rainfall totals over portions of South Asia and rising (western) Indian Ocean SSTs. More work is called for to better articulate the mechanisms by which this may be occurring, further elucidating the series of interactions between the sea (and land) surface, the weakened monsoon circulation, and the declines in rainfall. As a relevant aside, the modification of the South Asian land surface via agricultural production may also be contributing to local, if not regional, shifts in circulation and rainfall patterns, and should also be considered (Douglas et al. 2006, 2009; Lee et al., 2008; Niyogi et al., 2010; Guimberteau et al., 2011; Shukla et al., 2014). Given these findings, and that historical ENSO-monsoon relationships are changing in their reliability (Krishna Kumar et al., 1999; and refs above), farmers and other agriculturalists therefore need additional metrics and predictors by which to plan their food production systems that fully consider the impact of changing Indian Ocean SSTs and interactions.

A better understanding of these Indian Ocean interactions would lead to the development of new, and additional, indices that account for interannual, and perhaps seasonal and sub-seasonal, monsoon rainfall variability. Krishna Kumar et al., 2004 demonstrates how influential Indian Ocean SSTs are in retrospectively understanding changes in interannual crop yields, particularly in the central and western portions of India. The body of work investigating the interactions between the Indian Ocean, the Pacific Ocean, and the monsoon system has since made great strides and continues to develop, and more studies should be undertaken to understand how these interactions, such as the findings of Roxy et al., control agricultural production.  Furthermore, we should move to incorporate these emerging insights into better, more sophisticated, forecasts and assessments - at the stakeholder level - for current and future South Asian agricultural production.



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