Stable isotopes in surface waters of the Atlantic Ocean: Indicators of ocean‐atmosphere water fluxes and oceanic mixing processes

Abstract

The surface ocean hydrological cycle is explored based on ∼300 new δ18O and δD measurements from surface waters of the Atlantic Ocean and the Mediterranean Sea over the period 2010–2016. Our approach combines these surface observations with salinity (S) and stable isotope measurements of atmospheric water vapor. The distinct regional S‐δ distributions are used to identify different surface water masses and their horizontal advection. Moreover, based on assumptions on the δ‐S characteristics of seawater sources and the isotope composition of the evaporative (δe) and meteoric water (δMW) fluxes, the δ‐S distribution is used to indicate the relative importance of evaporation (E) and meteoric water inputs (MW). Here δe is estimated from the Craig and Gordon's equation using 120 days of measurements of the ambient air above the Atlantic Ocean collected during three cruises. To provide quantitative estimates of the E:MW ratio, we use the box model from Craig and Gordon (1965). This identifies the subtropical gyre as a region where E:MW ∼2 and the tropical ocean as a region were MW:E ∼2. Finally, we show that the δ18O‐δD distribution is better represented by a linear fit than the δ‐S relationship, even in basins governed by different hydrological processes. We interpret the δ18O‐δD distribution considering the kinetic fractionation processes associated with evaporation. In the tropical region where MW exceeds E, the δ18O‐δD distribution identifies the MW inputs from their kinetic signature, whereas in regions where E exceeds MW, the δ18O‐δD distribution traces the humidity at the sea surface.

A Framework to Study Mixing Processes in the Marine Boundary Layer Using Water Vapor Isotope Measurements

“…The gray dots are data collected during convective events. The simulations from MBL‐mix are shown for different ocean conditions: (13°C, δ 18 O oc = 0.7‰, and δD oc = 4.5‰), (20°C, δ 18 O oc = 0.8‰, and δD oc = 5.5‰), and (27°C, δ 18 O oc = 1.3‰, and δD oc = 8.5‰) (average observations from the cruises; Benetti, Reverdin, et al, ; Benetti, Steen‐Larsen, et al, ). MBL‐mix simulations using different LFT d‐excess are shown with black solid (30‰) and dashed (15‰) lines.…”

PIRATA: A Sustained Observing System for Tropical Atlantic Climate Research and Forecasting

“…The most depleted isotopic values, signature of dilution processes, were recorded during periods of strong convective activity in the Gulf of Guinea. These data sets also allowed demonstrating the impact of tropical upwellings and mesoscale SSS patterns on water vapor composition, suggesting that high‐resolution isotope sampling can identify advection processes, even if salinity variations are small (Benetti, Steen‐Larsen, et al, ; Benetti, Reverdin, et al, ).…”

Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes

“…We also apply our theoretical framework to observations during the STRASSE (Sub-Tropical Atlantic Surface Salinity Experiment) cruise that took place in the northern subtropical ocean in August and September 2012 (Benetti et al, 2014). This campaign accumulates several advantages that are important for our analysis: (1) continuous δD 0 measurements in the surface water vapor (17 m) at a high temporal frequency during 1 month (Benetti et al, 2014(Benetti et al, , 2017b, (2) associated surface meteorological measurements, including SST and h 0 , (3) 22 radio soundings relatively well distributed over the campaign period and providing vertical profiles of altitude, temperature, relative humidity and pressure, (4) ocean surface water δD oce measurements (Benetti et al, 2017a), (5) a variety of conditions ranging from quiescent weather to convective conditions, (6) on many vertical profiles, a well defined temperature inversion allows to calculate the inversion altitude.…”