Global transpiration data from sap flow measurements : the SAPFLUXNET database

Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land-atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The "sapfluxnetr" R package - designed to access, visualize, and process SAPFLUXNET data - is available from CRAN.Peer reviewe

Equatorial Upwelling in the Western Pacific Warm Pool

Vertical velocity on the equator in the western Pacific warm pool is investigated using data from the Coupled Ocean‐Atmosphere Response Experiment enhanced monitoring array (EMA) centered at 0°, 156°E. The data consist of hourly subsurface horizontal velocity time series from August 1991 until April 1994. Vertical velocity is calculated using horizontal velocity components and the application of the continuity equation. During the first year, from March 1992 until February 1993, data are available from five moorings of the EMA and thus provide nine different combinations of moorings from which to calculate vertical velocity. Four moorings were available during the remaining time period. Random errors are found to be \u3c 10−5 m s−1, while systematic errors (finite difference error, systematic instrument error, and error due to surface extrapolation) may be larger. It is suggested that errors, including finite difference errors, are not larger than the vertical velocity estimate. The estimates of vertical velocity are valid on spatial scales the size of the array (∼400 km) and timescales longer than a few days. They reveal a seasonal cycle manifested during a moderate El Niño. Results indicate upwelling, on average from 70 m down to 250 m over the 2 year time period, being slightly stronger in 1992 coincident with the stronger El Niño year. The divergence of horizontal velocity components, resulting in positive vertical velocity, is due to geostrophic divergence on the equator produced from a westward directed zonal pressure gradient force. Meridional divergence and zonal wind stress are uncorrelated, suggesting that Ekman convergence due to local westerly winds is only of partial influence. Consequently, downwelling is not found near the surface, where contributions from local winds and geostrophic divergence are in opposition. This estimate of vertical velocity indicates that water is upwelled in the warm pool from much deeper than but with comparable magnitude to the central and eastern Pacific

A Cloud-Free, Satellite-Derived, Sea Surface Temperature Analysis for the West Florida Shelf

Clouds are problematic in using Advanced Very High Resolution Radiometer (AVHRR) imagery for describing sea surface temperature (SST). The Tropical Rainfall Measuring Mission Microwave Imager (TMI) observes SST through clouds, providing daily, 1/4° maps under all weather conditions excepting rain. A TMI limitation, however, is coarse resolution. Optimal interpolation (OI) is used to generate a cloud-free, 5-km, daily SST analysis for the West Florida Shelf (WFS) by merging the high-resolution (cloud-covered) AVHRR with the coarse-resolution (cloud-free) TMI SST products. Comparisons with in-situ data show good agreements. Given large spatial gradients by coastal ocean processes, this regional analysis has advantage over the global, weekly, 1° Reynolds SST. A 5-year (1998-2002) OI SST analysis is diagnosed using Empirical Orthogonal Functions. The first two modes represent annual cycles, one by surface heat flux and another by shelf circulation dynamics

Satellite-Derived Surface Current Divergence in Relation to Tropical Atlantic SST and Wind

The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steady-state model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north–south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(±6) × 106 m3 s−1. Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations

Numerical Modelling of Tidally Generated Internal Wave Radiation From the Andaman Sea Into the Bay of Bengal

Semi-diurnal internal waves generated by tides in a high resolution numerical model that includes the Andaman Sea archipelago are found to propagate into the central Bay of Bengal and reach the coasts of India and Sri Lanka. The waves are also present in subsurface temperature records from RAMA moorings, and their propagation speed across the Bay of Bengal agrees well with satellite remote sensing from MODIS imagery in spite of the hydrostatic nature of the model. The internal waves are simulated by a fully coupled ocean-atmosphere prediction system, exchanging surface fluxes between the air and sea at high frequency and at high resolution. For the ocean, a hydrostatic model including diurnal and semi-diurnal tides provides a 2 km resolution representation of the entire Bay of Bengal. In the ocean model simulations, the semi-diurnal internal waves interact with the mesoscale circulation and surface waves and modify the flow and the stratification. By comparing coupled ocean-wave model runs with tides and without tides, but forced by identical surface fluxes from the atmosphere, it is demonstrated that the inclusion of diurnal and semi-diurnal tides act to cool the core of the thermocline while increasing the temperature above and below it along the pathway of the internal waves, a result that likely is due to vertical mixing by the waves

Quantifying Wavelengths Constrained by Simulted SWOT Observations in a SubMesoscale Resolving Ocean Analysis/Forecasting System

Using a suite of Observing System Simulation Experiments (OSSEs), the utility of simulated Surface Water Ocean Topography (SWOT) observations is estimated in a high-resolution (1 km) ocean analysis/forecasting system. Sampling a Nature Run provides observations for the OSSEs and the realism of the Nature Run is established by comparison to climatological data and an independent ocean analysis/forecast system. Each OSSE experiment assimilated different sets of simulated observations including traditional nadir altimeters, satellite sea surface temperature (SST), in situ profile data, and SWOT. OSSE evaluation metrics include area-averaged errors and wavenumber spectra with the latter providing much finer differentiation between experiments. 100 m temperature, sea surface height (SSH), and mixed layer depth (MLD) errors across the observed wavenumber spectra were reduced by up to 20% for OSSEs assimilating the simulated SWOT observations. The minimum constrained wavelength was found to be 130 km when both nadir altimetry and SWOT observations were used. The experiment using only nadir altimetry produced a value of 161 km. This 31 km gain in skill of predictable scales suggests that ocean forecasts can expect substantial gains in capability when utilizing the forthcoming SWOT data. Experimentation with the analysis decorrelation length scale suggests that emerging multi-scale assimilation methodologies will provide additional advancements in predictive skill

Temperature versus salinity gradients below the ocean mixed layer

We characterize the global ocean seasonal variability of the temperature versus salinity gradients in the transition layer just below the mixed layer using observations of conductivity temperature and depth and profiling float data from the National Ocean Data Center's World Ocean Data set. The balance of these gradients determines the temperature versus salinity control at the mixed layer depth (MLD). We define the MLD as the shallowest of the isothermal, isohaline, and isopycnal layer depths (ITLD, IHLD, and IPLD), each with a shared dependence on a 0.2 degrees C temperature offset. Data are gridded monthly using a variational technique that minimizes the squared analysis slope and data misfit. Surface layers of vertically uniform temperature, salinity, and density have substantially different characteristics. By examining differences between IPLD, ITLD, and IHLD, we determine the annual evolution of temperature or salinity or both temperature and salinity vertical gradients responsible for the observed MLD. We find ITLD determines MLD for 63% and IHLD for 14% of the global ocean. The remaining 23% of the ocean has both ITLD and IHLD nearly identical. It is found that temperature tends to control MLD where surface heat fluxes are large and precipitation is small. Conversely, salinity controls MLD where precipitation is large and surface heat fluxes are small. In the tropical ocean, salinity controls MLD where surface heat fluxes can be moderate but precipitation is very large and dominant