Consistency of Estimated Global Water Cycle Variations over the Satellite Era

Abstract

Motivated by the question of whether recent interannual to decadal climate variability and a possible “climate shift” may have affected the global water balance, we examine precipitation minus evaporation (P – E) variability integrated over the global oceans and global land for the period 1979–2010 from three points of view—remotely sensed retrievals and syntheses over the oceans, reanalysis vertically integrated moisture flux convergence (VMFC) over land, and land surface models (LSMs) forced with observations-based precipitation, radiation, and near-surface meteorology. Over land, reanalysis VMFC and P − evapotranspiration (ET) from observationally forced LSMs agree on interannual variations (e.g., El Niño/La Niña events); however, reanalyses exhibit upward VMFC trends 3–4 times larger than P − ET trends of the LSMs. Experiments with other reanalyses using reduced observations show that upward VMFC trends in the full reanalyses are due largely to observing system changes interacting with assimilation model physics. The much smaller P − ET trend in the LSMs appears due to changes in frequency and amplitude of warm events after the 1997/98 El Niño, a result consistent with coolness in the eastern tropical Pacific sea surface temperature (SST) after that date. When integrated over the global oceans, E and especially P variations show consistent signals of El Niño/La Niña events. However, at scales longer than interannual there is considerable uncertainty especially in E. This results from differences among datasets in near-surface atmospheric specific humidity and wind speed used in bulk aerodynamic retrievals. The P variations, all relying substantially on passive microwave retrievals over ocean, also have uncertainties in decadal variability, but to a smaller degree.

The Observed State of the Water Cycle in the Early Twenty-First Century

“…For some time, it will be difficult to determine with certainty which changes are part of a real, long-term trend and which are related to interdecadal natural variability, but that should not discourage the effort. The analysis of Robertson et al (2014) is a step in that direction. Third, as old satellites are decommissioned and new ones are launched, it will be important to identify ensuing discontinuities in the data record (see, e.g., T11).…”

“…Sparse ground-based data and simple water budget analyses were used to estimate spatial patterns of precipitation and evapotranspiration respectively. Because long-term measurements of river discharge are also limited in availability (Alsdorf et al 2007), it is generally estimated as the difference of precipitation minus evapotranspiration in the above-mentioned studies (despite significant uncertainty therein; see Robertson et al 2014), based on assumptions of negligible long-term net water storage change, or by using a model to account for storage changes. Given current capabilities to observe terrestrial water storage changes using the NASA and German Gravity Recovery and Climate Experiment (GRACE) mission (Tapley et al 2004;Wahr et al 2004), the storage term can now be quantified with confidence in water budget analyses (Rodell et al 2004a;Syed et al 2010).…”

The Global Ocean Water Cycle in Atmospheric Reanalysis, Satellite, and Ocean Salinity

“…The version used in this study is an update of the version previously published (Yu and Weller 2007). Studies have shown that the combined E 2 P from OAFlux and GPCP produces a freshwater budget that is best consistent with the ocean salt content (Schanze et al 2010;Ren et al 2014) and the atmospheric moisture content (Robertson et al 2014) among the products chosen for evaluation. The authors in this study are aware of several other satellite-based E and P products.…”

“…However, differences between GPCP and CMAP over the ocean are noted, particularly in the tropical and high latitudes (Yin et al 2004). These differences are attributed largely to the inclusion of in situ rain gauge measurements and to the technical limitations in retrieving snowfall and coldseason P. To overcome the uncertainties associated with satellite P products, dynamically based metrics, such as the atmospheric conservation of moisture, are introduced to relate E 2 P to vertically integrated moisture convergence (Trenberth et al 2011;Robertson et al 2014). This type of approach is deemed more reliable, as it allows the fidelity of E 2 P produced by model physics to be evaluated by analyzed state variables of wind and moisture.…”

A 1DVAR retrieval applied to GMI: Algorithm description, validation, and sensitivities

“…In the last decade or more, efforts have moved from sensor-specific approaches to an emphasis on intercalibration and algorithms that can provide consistent time series of geophysical constituents from multiple satellite sensors [Berg et al, 2013;Robertson et al, 2014;Hou et al, 2014]. The approach detailed here follows previous studies that iteratively solve for geophysical parameters by forward modeling the atmosphere and finding a solution that closely matches observed radiances, balancing observations with prior knowledge of the state vector [Deblonde and English, 2003;Bettenhausen et al, 2006;Boukabara et al, 2011;Munchak et al, 2016;EK08].…”