Revisiting the Moisture Budget of the Mediterranean Region in the ERA5 Reanalysis

Tootoonchi, Roshanak; Bordoni, Simona; D'Agostino, Roberta

doi:https://doi.org/10.5194/egusphere-2024-1647

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https://doi.org/10.5194/egusphere-2024-1647

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11 Jun 2024

| 11 Jun 2024

Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Revisiting the Moisture Budget of the Mediterranean Region in the ERA5 Reanalysis

Roshanak Tootoonchi, Simona Bordoni, and Roberta D'Agostino

Abstract. Moisture budget assessments from reanalyses and climate models have provided fundamental insights into the maintenance and response to perturbations of the hydrological cycle in the Mediterranean region. Here we perform similar analyses using the latest generation European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis ERA5, and we complement previous work by further decomposing the mean flow into contributions by the zonal-mean flow, which is dominated by the mean meridional circulation, and by zonally anomalous circulations and/or moisture, namely the stationary eddies. According to ERA5, in the annual mean, net evaporation (negative P – E) over the ocean and net precipitation (positive P – E) over land are primarily due to submonthly transient eddies converging moisture originating from the ocean into the surrounding land. Overall, total stationary eddies reinforce the transient tendency over the ocean but oppose it over land, with the zonal-mean flow exerting a minor drying tendency limited to the region's southernmost latitudes. The divergent total stationary-eddy moisture flux arises from a strongly divergent zonally anomalous circulation acting on the zonal-mean moisture and is opposed by a convergent pure stationary-eddy moisture flux. The relative magnitude of these terms changes over the seasonal cycle, explaining the transition from net precipitation during winter (DJF) to net evaporation during summer (JJA) over land. More specifically, as transient eddies weaken during the warm season, the strengthened divergent total stationary-eddy moisture flux becomes dominant and causes strong drying and negative net precipitation. Somewhat surprisingly, moisture flux divergence by the mean meridional circulation is found to play a minor role in the Mediterranean region across all seasons except autumn (SON).

Received: 31 May 2024 – Discussion started: 11 Jun 2024

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Roshanak Tootoonchi, Simona Bordoni, and Roberta D'Agostino

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RC1: 'Comment on egusphere-2024-1647', Anonymous Referee #1, 27 Jun 2024 reply

This papers examines the climatological moisture budget - the atmospheric branch of the hydrological cycle - in the latest ERA5 atmospheric reanalysis for the annual mean and by season.
The authors first rather usefully point out the global imbalance between P and E in ERA5 with P exceeding E and that the imbalance has reduced over time as E has increased substantially - primarily through an abrupt increase in the mid 1990s. I take this to mean that there is an artificial moisture “source” in ERA5 - though the authors say “sink” - which allows global mean P to exceed what is supplied by E. The authors also usefully show the moisture imbalance between P-E and the convergence of the vertically integrated moisture budget for the Mediterranean region. It is not small either, though it has been reducing in size over time, though not with the same time evolution as the global imbalance. Because of these matters, the authors caution on the analysis of trends in the ERA5 moisture budget, which is sage counsel.
The authors then move on their decomposition of the climatological moisture budget. Their analysis with a new version of ECMWF reanalysis largely confirms prior findings of Seager et al. (2014) with ERA-Interim. The transient eddies are found to extract moisture from the Mediterranean Sea and converge it over land everywhere. The authors compute this term as a residual form monthly data but the pattern agrees with Seager et al. who commuted this directly from daily data, which would be worth pointing out.
Unlike the prior work, the authors then decompose the mean flow moisture component into zonal mean and stationary wave components and then these into terms due to zonal asymmetries of humidity and mean circulation. This has not been shown before. To my surprise the component due to the zonal mean (Hadley Cell) overturning is very small despite this term oft being quoted as the reason for the semi-aridity of the Med region. This is rue even in winter. The work emphasizes the importance of the stationary waves and finds drying is primarily provided by the zonally asymmetric circulation (not humidity).
My main criticism of the paper is that why the contributions from the stationary eddies are what they are is not explained.
The authors should seek to explain in terms of the zonal asymmetries, why the the transport of zonally symmetric moisture by the zonally anomalous circulation is a drying in the Med while the transport of zonally anomalous moisture by the zonally symmetric circulation is a wetting and, also, why the put stationary eddy term is mostly a convergence. Currently these are presented as decomposition artifacts but instead need to be put on good physical ground.
I also wonder if the authors have thought about these decomposition into mean and zonally varying in terms of a zonally varying Hadley Cell as in Li et al. (2022, JGT-Atmospheres, 10.1029/2022JD036940). That would allow growing the zonal symmetric part with the part due to a zonally varying meridional overturning into one “Hadley Cell” term. I am not sure if this would capture all of the term due to zonal asymmetries of the mean flow shown here but it would be interesting to see.
Line 260 what term does “this term” refer to?

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Citation: https://doi.org/10.5194/egusphere-2024-1647-RC1

Roshanak Tootoonchi, Simona Bordoni, and Roberta D'Agostino

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Short summary

In this study, we explore the role of stationary circulations arising from deviations from the zonal mean in the distinct transition from net evaporation over ocean to net precipitation over land in the Mediterranean region from ERA5. Stationary eddies reinforce the wetting tendency over land and oppose the drying tendency over ocean due to transient storms. Our results have important implications for future changes in the region, previously identified as a climate change hot spot.


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