When the Land Surface Shifts Gears

Abstract

Northern Europe is typically characterized by wet conditions, where the total evaporation and transpiration (together, "evapotranspiration") largely depend on atmospheric energy supply. Dirmeyer et al. (2021) report that in the hot and dry summer of 2018 evapotranspiration became water-limited such that decreased evaporative cooling amplified the heatwave temperatures. This way, drought magnitude increased beyond a critical point with consequent disruptions in ecosystem services.The land surface provides essential ecosystem services for society. Soils and vegetation take up atmospheric CO 2 (Heimann & Reichstein, 2008), their moisture can attenuate wildfires (Forkel et al., 2019, O.S., Hou, et al., 2020, and their evaporative cooling can mitigate hot temperatures (Seneviratne et al., 2012). These services depend on the meteorological conditions; high radiation and low rainfall can induce water stress in ecosystems and consequently limit their services (Figure 1). In semiarid regions experiencing dry seasons such as Central North America and Southern Europe, the lack of water supply limits evapotranspiration, a condition known as water limitation (Mueller & Seneviratne, 2012). This is usually not the case for northern Europe, where normally wet and cloudy weather creates conditions where evaporation rates are limited by energy supply. ORTH

Extreme Compound and Seesaw Hydrometeorological Events in New Zealand: An Initial Assessment

“…With relatively minor precipitation changes across growing seasons, the emergence of soil moisture declines, together with the strong correlation within SM-T relationships, further evidences the importance of SM-T coupling for New Zealand. The strong SM-T coupling during the growing season indicates a phase change in land states for these typically wet regions during dry seasons, revealing potential "hot spot" areas of land-atmosphere coupling like that witnessed during the 2018 summer drought and heatwave across the wet, energy-limited regions of Northern Europe and the United Kingdom (Dirmeyer et al, 2021;Orth, 2021).…”

“…9) sits in contrast to the increase in hot days found by Harrington (2021), while the high number of compounding months sits somewhat more in agreement spatially to hot day occurrence. The discrepancy suggests that soil moisture plays a less important role in compound event occurrence across the upper north and northeast of the North Island which results in a more stable land state during dry phases (Orth, 2021), particularly when viewed collectively with the weak to positive covariation in SM-T throughout these typically wet or transitional regions (Fig. 6).…”

Observed Landscape Responsiveness to Climate Forcing

“…It also does not consider water availability's non‐linear influence on land surface fluxes. Specifically, evaporation's relationship with soil moisture is non‐linear and widely described in terms of two distinct regimes (Eagleson, 1978; Orth, 2021; Seneviratne et al., 2010; Vargas Zeppetello et al., 2019). The drier state is typically referred to as the water‐limited evaporative regime, defined by the energy flux sensitivity to soil moisture variations (Figure 1).…”

Mapping terrestrial evaporation regimes : a data-driven analysis of land-atmosphere interactions under climate change

“…In energy-limited conditions, it is the near-surface weather and related energy availability that can limit ET. Therefore, the apparent evaporative regime denotes whether the land surface is driving the near-surface weather or vice-versa (Orth, 2021). Subsequently, the transition between these regimes matters, because it entails a change of causal direction (Santanello et al, 2018).…”

“…In regions that are water-limited (energy-limited), plants usually benefit from water (energy) surpluses and suffer from respective deficits. Many studies confirm that, depending on the evaporative regime, vegetation productivity follows the temporal evolution of influential variables such as soil moisture or temperature which summarize the water or energy dynamics (Beer et al, 2010;Li et al, 2021b;Madani et al, 2017;Orth, 2021;Piao et al, 2020;Seddon et al, 2016).…”