I think the abstract of the article addresses your question. Working backwards, it presents two direct causes of change in groundwater storage. The first is climatic - greater precipitation tends to increase storage and a decline in precipitation will reduce recharge and thus reduce storage. The second is the anthropogenic response to changes in climate (they focus on changes in precipitation but there may be a second effect of changes in air temperature). When there is less rain, more water will be pumped for irrigated agriculture.
These two effects are then related to the way the change in precipitation is affected by change in ocean temperature. So changing ocean temperature changes the rainfall amounts and patterns. The rainfall reduction in northwestern India increases the amount of groundwater pumping needed for agriculture and decreases groundwater storage.
Methods have been developed to separate the climate and pumping effects on waterlevels (Shapoori, V., et al. 2015).