High‐Resolution Nudged Isotope Modeling With ECHAM6‐Wiso: Impacts of Updated Model Physics and ERA5 Reanalysis Data

Abstract

Abstract We present here results of new isotope‐enabled simulations with an enhanced ECHAM6‐wiso model version nudged to the ERA5 reanalyses, at two different spatial resolutions, for the period 1979–2018. The isotopic content of snow on sea ice is considered, yielding surface water vapor with lower isotope ratios over sea ice covered areas, and the kinetic fractionation factors for oceanic evaporation are assumed as independent of wind speed. Also, the supersaturation equation was slightly re‐tuned for a better agreement with the Antarctic isotope observations. In addition to the spatial resolution, the impacts of the improved ECHAM6 model physics and the chosen updated ERA5 reanalyses data set on our simulation results are investigated. For this purpose, detailed comparisons to simulation results obtained from the predecessor ECHAM5‐wiso model nudged to ERA5 reanalyses and from ECHAM6‐wiso nudged to ERA‐Interim are performed. Compared to the ERA‐Interim reanalyses, the nudging to ERA5 does not result in substantial changes of modeled surface isotope values on a global scale but water transport over the tropics clearly changed, with increased precipitation amounts over the Amazonian area and related changes of isotopic contents in water vapor in the high troposphere. An ECHAM6‐wiso simulation at high spatial resolution (0.9° horizontal resolution and 95 vertical levels) generally further improves the modeling of annual mean isotope values. The modeling of temporal isotopic variations at seasonal and (sub‐)daily time scales is also slightly improved for the high spatial resolution configuration. The latter will be very useful for an improved interpretation of various water isotope records.

Keywords

• GCM
• water isotopes
• ERA5
• nudging
• water cycle