Tracking a semi-arid Eastern Mediterranean ecotone through integration of terrestrial and atmospheric earth observation data (2000–2024)

Terrestrial-atmospheric contact in the Eastern Mediterranean (Israel) was examined through: (1) MODIS Atmospheric Water Vapor Reserve (AWVR); (2) AgERA5 Vapor Pressure Deficit (VPD); (3) Landsat 8/9; and (4) Israel Meteorological Service data. The aim was to develop an Earth observation method to monitor Mediterranean shrublands in relation to climate change and subtropical high (SH) migration. The objectives juxtapose healthy shrublands, a thermodynamically live state (TLS) at disequilibrium, with plagioclimax parcels, and a thermodynamically dead state (TDS) at equilibrium. Monthly/seasonal brightness (BI), greenness (GI), and wetness (WI) indices were derived using Tasseled-Cap Transformation of the Landsat data for TDS and TLS spatial structures. An innovative Normalized Difference Climate-Line Index (NDCLI) was developed to quantify TLS non-stationarity relative to baseline TDS stationarity and monitor ecotone transition. The WI-dominant TLS component drove the NDCLI toward negative (less arid) conditions when vegetation moisture content (VMC) was high during wet periods. Results from the NDCLI were regressed against AWVR data, a proxy for SH migration, revealing an annual transfer of H2O reserves between terrestrial and atmospheric systems, as reflected in their relationship. Within the study area, from 2000 to 2024, AWVR increased (+0.43 g/cm2), including a decade-long intensification event between 2014 and 2024 in which the rate of increase nearly doubled from the previous decade. At the same time, between 2010 and 2024, there were increases in temperature (+0.75 °C), precipitation (+66.5 mm), and evapotranspiration (+33.5 mm/yr⁻¹). Based on an Aridity Index developed from these metrics and the NDCLI, aridity decreased by 13.3%, as also shown in the TCT GI, with increased Spring greening. A decrease in VPD (−0.56 hPa) between 2000 and 2024 warranted the development of a modified Clausius-Clapeyron relation to explain this confounding configuration of global warming metrics and ecotone transition relative to Mediterranean Sea evaporation and the interplay between global/synoptic circulation patterns.