A Stratified Diagnosis of the Indian Monsoon—Eurasian Snow Cover Relationship

Abstract

This study is aimed at reconciling inconsistencies between modeling studies of the Indian monsoon?Eurasian snow cover relationship, which support the Blanford hypothesis, and recent diagnostic studies, which have largely failed to show its existence. Recently released version 2 NOAA/NESDIS satellite-based retrievals of snow cover are used. A focus is given to diagnosing (a) spatial and temporal complexity in the Eurasian snow cover distribution, (b) the role of ENSO in modulating the snow cover?all-India rainfall (AIR) relationship, and (c) the spatial distribution of the rainfall association within India. With these goals in mind, the climatological distribution of snow cover and its interannual variability are first assessed. While snow cover is largest in northern Eurasia, its variability is most pronounced in southwestern (SW) Asia between the Black and Caspian Seas, and across the northern Indian, Himalayan, and Tibetan Plateau (NIHT) regions. In regions of large variability, and for Eurasia as a whole, AIR is found to exhibit a very weak correlation with both December?February and March?May snow cover when considered across the available data record. Patterns of snow cover anomalies associated with both ENSO and anomalous monsoon (AM) seasons are then examined in a composite analysis. Spatial complexity in snow cover anomalies during both anomalous monsoon years and ENSO events is shown to be large. Because the association between ENSO and snow cover is found to be strong across much of Eurasia, ENSO's influence on the monsoon?snow cover relationship over the climatological record is strongly suggested. Moreover, the composites suggest that many idealized simulations of the monsoon?snow cover interaction are based on forcings that are not generally reflective of variations in nature. When correlations between snow cover and monsoon intensity are calculated for regions neighboring India during anomalous monsoon seasons in which ENSO is weak (AM-neutral years), robust and statistically significant negative correlations are found, in support of the Blanford hypothesis. Specifically, snow cover in both the southwest Asian and NIHT regions are found to correlate strongly with June?September AIR during spring and winter, respectively. While snow cover over the entirety of Eurasia is also found to exhibit a modest correlation with AIR during AM-neutral years, the correlation results largely from anomalies in northern Eurasia that are distant from India and are, therefore, probably unrelated to the Blanford hypothesis. The spatial pattern of rainfall anomalies within India is also investigated. It is found that correlations between district rainfall and snow cover in the SW Asia and NIHT regions during ENSO are largely insignificant, while during AM-neutral years, significant negative correlations are found across central and northern India, in modest agreement with model simulations that show the strongest rainfall response in northern India. Together, the findings support the existence of the Blanford mechanism, while suggesting that the influence of the land surface can be overwhelmed by ENSO. A new conceptual model of the monsoon?snow cover relationship that builds on the Blanford hypothesis is, thus, proposed to better explain its lack of support from diagnostic studies that have used Eurasian mean snow cover over the entire data record.