Exploring trends in wet-season precipitation and drought indices in wet, humid and dry regions

Abstract

This study examines wet season droughts using eight products from the Frequent Rainfall Observations on GridS database. The study begins by evaluating wet season precipitation totals and wet day counts at seasonal and decadal time scales. While we find a high level of agreement among the products at a seasonal time scale, evaluations of 10 year variability indicate substantial non-stationary inter-product differences that make the assessment of low-frequency changes difficult, especially in data-sparse regions. Some products, however, appear more reliable than others on decadal time scales. Global time series of dry, middle, and wet region standardized precipitation index time series indicate little coherent change. There is substantial coherence in year-to-year variations in these time series for the better-performing products, likely indicative of skill for monitoring variations at large spatial scales. During the wet season, the data do not appear to indicate widespread global changes in precipitation, reference evapotranspiration (RefET) or Standardized Precipitation Evapotranspiration Index (SPEI) values. These data also do not indicate a global shift towards increasing aridity. Focusing on SPEI values for dry regions during droughts, however, we find modest increases in RefET and decreases in SPEI when wet season precipitation is below normal. Dry region SPEI values during droughts have decreased by −0.2 since the 1990s. The cause of these RefET increases is unclear, and more detailed analysis will be needed to confirm these results. For wet regions, however, the majority of products appear to indicate increases in wet season precipitation, although many products perform poorly in these regions due to limited observation networks, and estimated increases vary substantially. Synopsis: Our analysis indicates a lack of increasing aridity at global scales, issues associated with non-stationary systematic errors, and concerns associated with increases in reference evapotranspiration in global dry regions during droughts.

Intercomparison of annual precipitation indices and extremes over global land areas from in situ, space-based and reanalysis products

“…For example, in R95p and R99p, MERRA2 has an increasing trend throughout the timeseries while CFSR appears to show a trend starting in about 2000. These trends are likely data artefacts as they are not present in any of the other products and could be responsible for the non-stationary systematic errors identified in Funk et al(2019) in this Focus Collection. These issues will be investigated further as part of the wider remit of the IPWG/GDAP/WCRP extreme precipitation project.…”

Robust Future Changes in Meteorological Drought in CMIP6 Projections Despite Uncertainty in Precipitation

“…It is widely reported that droughts and aridity will worsen under increasing greenhouse gas concentrations (Dai, 2013; Dai et al, 2018; Mirzabaev et al, 2019; Park et al, 2018; Sherwood & Fu, 2014), but this is not supported by recent observations of precipitation (Funk, Harrison, et al, 2019; Orlowsky & Seneviratne, 2013) and other hydrological quantities, including runoff, actual evapotranspiration, and pan evaporation (Roderick & Farquhar, 2002; Scheff, 2018; Ukkola & Prentice, 2013). The previous suggestions of more severe droughts largely arise from uncoupled modeling studies (Sheffield et al, 2012) that do not capture the various climate interactions and generally quantify droughts using potential evapotranspiration in addition to precipitation (Dai, 2013).…”

“…Many devastating droughts have occurred in recent decades, such as those in California (Griffin & Anchukaitis, 2014), the Horn of Africa (Funk, Shukla, et al, 2019), Europe (Ciais et al, 2005), and Australia (van Dijk et al, 2013), risking regional food and water security. Between 1998 and 2017, droughts are estimated to have impacted 1.5 billion people and accounted for a third of all natural disaster impacts (Funk, Harrison, et al, 2019; United Nations, n.d.). Climate change may be increasing the severity and frequency of droughts (Dai, 2013; Trenberth et al, 2014), posing challenges for water management, agriculture, and natural ecosystems.…”

Robust future changes in meteorological drought in CMIP6 projections despite uncertainty in precipitation

“…Many devastating droughts have occurred in recent decades, such as those in California (Griffin & Anchukaitis, 2014), the Horn of Africa (Funk, Shukla, et al, 2019), Europe (Ciais et al, 2005), and Australia ( van Dijk et al, 2013), risking regional food and water security. Between 1998 and 2017, droughts are estimated to have impacted 1.5 billion people and accounted for a third of all natural disaster impacts (Funk, Harrison, et al, 2019;United Nations, n.d.). Climate change may be increasing the severity and frequency of droughts (Dai, 2013;Trenberth et al, 2014), posing challenges for water management, agriculture, and natural ecosystems.…”