What is the characteristic of the solar signal in the Earth's climate system?

Observations from the ground and space are key to improving our knowledge of how the Sun is influencing the Earth's climate. Solar irradiance and particle flux measurements are mandatory for constraining the external solar forcing. Atmospheric observations provide a benchmark for assessing the representation of solar influence in atmospheric models. However, important experimental uncertainties in the quantification of solar forcing drivers, as well as limitations in the extraction of the solar climate signal from atmospheric and oceanic observational records remain. SOLARIS-HEPPA aims at concentrating efforts to optimize the exploitation of solar irradiance, particle, and climate observations and simulations in a coordinated manner and to provide their link to climate forecasts activities.

What is the mechanism for solar influence on climate?

Mechanisms for solar influence on climate can generally be divided into radiatively driven and particle driven components. Variations of the total solar irradiance (TSI) give raise for the „bottom-up“ mechanism, involving atmosphere-ocean coupling and introducing a positive feedback and enhancement of circulation patterns and precipitation in the tropics. The “top-down” stratospheric UV (SSI) mechanism transfers the solar signal from the stratosphere down to the surface by a chain of indirect circulation effects which are induced by variations of stratospheric ozone and related heating. Although these radiatiatively driven mechanisms are well documented by observations and model studies, large discrepancies between the observed and simulated signals, together with the large spread of model responses, have raised a controversial debate on the efficiency of these mechanisms and on the related model uncertainties. The particle driven component is further divided into Energetic Particle Precipitation (EPP) and Cosmic Ray (CR) effects. EPP affects primarily the chemical composition (e.g., NOy and O3) of the mesosphere and lower thermosphere. Vertical coupling is responsible for the transfer of this chemical signal to the stratosphere, where it adds to the “top-down” mechanism via modulation of ozone-related heating. CRs are the main source of ionization in the troposphere, but the connection between ionization and cloud production and therefore convection remains to be demonstrated. SOLARIS-HEPPA aims at combining all mechanisms in order to understand their relative importance and achieve a complete picture for solar influence on climate, in particular its importance for decadal scale and regional climate predictions.

How do the different natural and anthropogenic forcings interact?

The solar forcing does interact with a number of natural and anthropogenic forcings such as volcanic eruptions, increasing greenhouse gas emissions, or intrinsic atmospheric variations such as the El Niño Southern Oscillation (ENSO) and the Quasi-Biennual Oscillation (QBO). SOLARIS-HEPPA aims at improving our understanding of these interactions with particular emphasis on their role in solar variability in a changing climate.

How much the solar forcing can contribute to the skill of near-term predictions?

Solar forcing, in particular the periodic solar variability, could be a potential source for the annual-to-decadal predictability of the (regional) climate system, involving the stratosphere response, ozone chemistry changes, and downward coupling processes. SOLARIS-HEPPA aims to assess the influence of solar forcing on the major stratospheric events' predictability and their potential to add skill to tropospheric forecasts.