Interpreting Rainfall Anomalies Using Rainfall's Nonnegative Nature

Previous studies emphasized the role of transient disturbances in linking interannual variation of average rainfall (IVAR; average period>1month) to external climate forcings. Researchers conventionally use transient eddy-driven horizontal fluxes of certain variables to explain the IVAR, which is not universally applicable to disturbance-associated IVAR research because these variables' horizontal distributions can drastically affect their fluxes. Here we demonstrate that the strengthened transient disturbance activities can directly impact IVAR positively in almost all extratropical terrestrial regions in each season. This impact occurs because disturbance strengthened daily downward motions cannot generate negative rainfall to compensate for the increased rainfall generated by disturbance enhanced daily upward motions. This is much more universally applicable to disturbance-associated IVAR research because it derives transient eddies' direct and independent impacts on IVAR. Because of this impact, the horizontal heterogeneity of global warming rate can modify the long-term trend of extratropical terrestrial rainfall through disturbance activity trend. Plain Language Summary Transient disturbance activity (the day-to-day component of atmospheric circulation variance) is often used to explain monthly/seasonal-scale rainfall anomalies in many regions, which is conventionally through anomalous transient disturbances-conveyed horizontal fluxes of certain variables. Horizontal flux anomalies are drastically affected by variables' horizontal distributions, which makes the above explanation hard to be universally applied to rainfall anomaly research in various regions and seasons. In this study, we demonstrate that the enhanced transient disturbance activity can directly cause positive monthly/seasonal-scale rainfall anomalies in most extratropical terrestrial regions throughout the world, which occurs because rainfall is always nonnegative. The present revelation is much more universally applicable to research on the rainfall anomalies associated with anomalous disturbance activities than the traditional one. The new revelation will greatly improve our understanding of how external forcing (e.g., El Nino) impact terrestrial rainfall anomalies via changing transient disturbance activities.