Search for: All records

Abstract There is great interest in improving our understanding of the respective roles of the ocean and atmosphere in variability and change in weather and climate. Due to the sparsity of sustained observing sites in the open ocean, information about the air–sea exchanges of heat, freshwater, and momentum is often drawn from models. In this paper observations from three long-term surface moorings deployed in the trade wind regions of the Pacific and Atlantic Oceans are used to compare observed means and low-passed air–sea fluxes from the moorings with coincident records from three atmospheric reanalyses (ERA5, NCEP-2, and MERRA-2) and from CMIP6 coupled models. To set the stage for the comparison, the methodologies of maintaining the long-term surface moorings, known as ocean reference stations (ORS), and assessing the accuracies of their air–sea fluxes are described first. Biases in the reanalyses’ means and low-passed wind stresses and net air–sea heat fluxes are significantly larger than the observational uncertainties and in some case show variability in time. These reanalyses and most CMIP6 models fail to provide as much heat into the ocean as observed. In the discussion and conclusions section, long-term observing sites in the open ocean are seen as essential, independent benchmarks not only to document the coupling between the atmosphere and ocean but also to promote collaborative efforts to assess and improve the ability of models to simulate air–sea fluxes. 

Ocean surface radiation measurement best practices have been developed as a first step to support the interoperability of radiation measurements across multiple ocean platforms and between land and ocean networks. This document describes the consensus by a working group of radiation measurement experts from land, ocean, and aircraft communities. The scope was limited to broadband shortwave (solar) and longwave (terrestrial infrared) surface irradiance measurements for quantification of the surface radiation budget. Best practices for spectral measurements for biological purposes like photosynthetically active radiation and ocean color are only mentioned briefly to motivate future interactions between the physical surface flux and biological radiation measurement communities. Topics discussed in these best practices include instrument selection, handling of sensors and installation, data quality monitoring, data processing, and calibration. It is recognized that platform and resource limitations may prohibit incorporating all best practices into all measurements and that spatial coverage is also an important motivator for expanding current networks. Thus, one of the key recommendations is to perform interoperability experiments that can help quantify the uncertainty of different practices and lay the groundwork for a multi-tiered global network with a mix of high-accuracy reference stations and lower-cost platforms and practices that can fill in spatial gaps. 

Abstract A budget approach is used to disentangle drivers of the seasonal mixed layer carbon cycle at Station ALOHA (A Long‐term Oligotrophic Habitat Assessment) in the North Pacific Subtropical Gyre (NPSG). The budget utilizes data from the WHOTS (Woods Hole—Hawaii Ocean Time‐series Site) mooring, and the ship‐based Hawai'i Ocean Time‐series (HOT) in the NPSG, a region of significant oceanic carbon uptake. Parsing the carbon variations into process components allows an assessment of both the proportional contributions of mixed layer carbon drivers and the seasonal interplay of drawdown and supply from different processes. Annual net community production reported here is at the lower end of previously published data, while net community calcification estimates are 4‐ to 7‐fold higher than available sediment trap data, the only other estimate of calcium carbonate export at this location. Although the observed seasonal cycle in dissolved inorganic carbon in the NPSG has a relatively small amplitude, larger fluxes offset each other over an average year. Major supply comes from physical transport, especially lateral eddy transport throughout the year and entrainment in the winter, offset by biological carbon uptake in the spring. Gas exchange plays a smaller role, supplying carbon to the surface ocean between Dec‐May and outgassing in Jul‐Oct. Evaporation‐precipitation (E‐P) is variable with precipitation prevailing in the first half and evaporation in the second half of the year. The observed total alkalinity signal is largely governed by E‐P with a somewhat stronger net calcification signal in the wintertime. 

Abstract Estimates of turbulence kinetic energy (TKE) dissipation rate (ε) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates ofεwith concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates ofεat a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scaleand the newer Langmuir stability length scale, suggesting that ocean‐side friction velocityimplicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes driftandthat is likely facilitated by the steady Southeast trade winds regime. In certain regimes,, whereis the von Kármán constant andis instrument depth, and surface buoyancy flux capture our estimates ofwell, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based onand, scalesεwell at times but is overall less consistent than. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes. 

Abstract Upper-ocean turbulence is central to the exchanges of heat, momentum, and gasses across the air/sea interface, and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed-layer depths and sea surface temperature. In part, progress has been limited due to the difficulty of measuring turbulence from fixed moorings which can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring Turbulent Kinetic Energy (TKE) dissipation rates, ϵ , from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-ocean Regional Study (SPURS) to collect two year-long data sets. We find the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate two-week missions for (10 −8 ) ≤ ϵ ≤ (10 −5 ) m 2 s −3 . Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1-10 km. We also find that dissipation estimates from two different moorings at 12.5 m, and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory. 

Abstract. Ship-based time series, some now approaching over 3 decades long, are critical climate records that have dramatically improved our ability to characterize natural and anthropogenic drivers of ocean carbon dioxide (CO2) uptake and biogeochemical processes. Advancements in autonomous marine carbon sensors and technologies over the last 2 decades have led to the expansion of observations at fixed time series sites, thereby improving the capability of characterizing sub-seasonal variability in the ocean. Here, we present a data product of 40 individual autonomous moored surface ocean pCO2 (partial pressure of CO2) time series established between 2004 and 2013, 17 also include autonomous pH measurements. These time series characterize a wide range of surface ocean carbonate conditions in different oceanic (17 sites), coastal (13 sites), and coral reef (10 sites) regimes. A time of trend emergence (ToE) methodology applied to the time series that exhibit well-constrained daily to interannual variability and an estimate of decadal variability indicates that the length of sustained observations necessary to detect statistically significant anthropogenic trends varies by marine environment. The ToE estimates for seawater pCO2 and pH range from 8 to 15 years at the open ocean sites, 16 to 41 years at the coastal sites, and 9 to 22 years at the coral reef sites. Only two open ocean pCO2 time series, Woods Hole Oceanographic Institution Hawaii Ocean Time-series Station (WHOTS) in the subtropical North Pacific and Stratus in the South Pacific gyre, have been deployed longer than the estimated trend detection time and, for these, deseasoned monthly means show estimated anthropogenic trends of 1.9±0.3 and 1.6±0.3 µatm yr−1, respectively. In the future, it is possible that updates to this product will allow for the estimation of anthropogenic trends at more sites; however, the product currently provides a valuable tool in an accessible format for evaluating climatology and natural variability of surface ocean carbonate chemistry in a variety of regions. Data are available at https://doi.org/10.7289/V5DB8043 and https://www.nodc.noaa.gov/ocads/oceans/Moorings/ndp097.html (Sutton et al., 2018).