Multispectral remote-sensing algorithms for particulate organic carbon (POC): The Gulf of Mexico

To greatly increase the spatial and temporal resolution for studying carbon dynamics in the marine environment, we have developed remote-sensing algorithms for particulate organic carbon (POC) by matching in situ POC measurements in the Gulf of Mexico with matching SeaWiFS remote-sensing reflectance. Data on total particulate matter (PM) as well as POC collected during nine cruises in spring, summer and early winter from 1997-2000 as part of the Northeastern Gulf of Mexico (NEGOM) study were used to test algorithms across a range of environments from low %POC coastal waters to high %POC open-ocean waters. Finding that the remote-sensing reflectance clearly exhibited a peak shift from blue-to-green wavelengths with increasing POC concentration, we developed a Maximum Normalized Difference Carbon Index (MNDCI) algorithm which uses the maximum band ratio of all available blue-to-green wavelengths, and provides a very robust estimate over a wide range of POC and PM concentrations (R2 = 0.99, N = 58). The algorithm can be extrapolated throughout the region of shipboard sampling for more detailed coverage and analysis

Model-based remote sensing algorithms for particulate organic carbon (POC) in the Northeastern Gulf of Mexico

Hydrographic data, including particulate organic carbon (POC) from the Northeastern Gulf of Mexico (NEGOM) study, were combined with remotely-sensed SeaWiFS data to estimate POC concentration using principal component analysis (PCA). The spectral radiance was extracted at each NEGOM station, digitized, and averaged. The mean value and spurious trends were removed from each spectrum. De-trended data included six wavelengths at 58 stations. The correlation between the weighting factors of the first six eigenvectors and POC concentration were applied using multiple linear regression. PCA algorithms based on the first three, four, and five modes accounted for 90, 95, and 98% of total variance and yielded significant correlations with POC with R2 = 0.89, 0.92, and 0.93. These full waveband approaches provided robust estimates of POC in various water types. Three different analyses (root mean square error, mean ratio and standard deviation) showed similar error estimates, and suggest that spectral variations in the modes defined by just the first four characteristic vectors are closely correlated with POC concentration, resulting in only negligible loss of spectral information from additional modes. The use of POC algorithms greatly increases the spatial and temporal resolution for interpreting POC cycling and can be extrapolated throughout and perhaps beyond the area of shipboard sampling

Global Oceans, BAMS State of the Climate in 2021, Chapter 3

Patterns of variability in ocean properties are often closely related to large-scale climate pattern indices, and 2021 is no exception. The year 2021 started and ended with La Niña conditions, charmingly dubbed a “double-dip” La Niña. Hence, stronger-than-normal easterly trade winds in the tropical south Pacific drove westward surface current anomalies in the equatorial Pacific; reduced sea surface temperature (SST) and upper ocean heat content in the eastern tropical Pacific; increased sea level, upper ocean heat content, and salinity in the western tropical Pacific; resulted in a rim of anomalously high chlorophyll-a (Chla) on the poleward and westward edges of the anomalously cold SST wedge in the eastern equatorial Pacific; and increased precipitation over the Maritime Continent. The Pacific decadal oscillation remained strongly in a negative phase in 2021, with negative SST and upper ocean heat content anomalies around the eastern and equatorial edges of the North Pacific and positive anomalies in the center associated with low Chla anomalies. The South Pacific exhibited similar patterns. Fresh anomalies in the northeastern Pacific shifted towards the west coast of North America. The Indian Ocean dipole (IOD) was weakly negative in 2021, with small positive SST anomalies in the east and nearly-average anomalies in the west. Nonetheless, upper ocean heat content was anomalously high in the west and lower in the east, with anomalously high freshwater flux and low sea surface salinities (SSS) in the east, and the opposite pattern in the west, as might be expected during a negative phase of that climate index. In the Atlantic, the only substantial cold anomaly in SST and upper ocean heat content persisted east of Greenland in 2021, where SSS was also low, all despite the weak winds and strong surface heat flux anomalies into the ocean expected during a negative phase of the North Atlantic Oscillation. These anomalies held throughout much of 2021. An Atlantic and Benguela Niño were both evident, with above-average SST anomalies in the eastern equatorial Atlantic and the west coast of southern Africa. Over much of the rest of the Atlantic, SSTs, upper ocean heat content, and sea level anomalies were above average. Anthropogenic climate change involves long-term trends, as this year’s chapter sidebars emphasize. The sidebars relate some of the latest IPCC ocean-related assessments (including carbon, the section on which is taking a hiatus from our report this year). This chapter estimates that SST increased at a rate of 0.16–0.19°C decade−1 from 2000 to 2021, 0–2000-m ocean heat content warmed by 0.57–0.73 W m−2 (applied over Earth’s surface area) from 1993 to 2021, and global mean sea level increased at a rate of 3.4 ± 0.4 mm yr−1 from 1993 to 2021. Global mean SST, which is more subject to interannual variations than ocean heat content and sea level, with values typically reduced during La Niña, was ~0.1°C lower in 2021 than in 2020. However, from 2020 to 2021, annual average ocean heat content from 0 to 2000 dbar increased at a rate of ~0.95 W m−2, and global sea level increased by ~4.9 mm. Both were the highest on record in 2021, and with year-on-year increases substantially exceeding their trend rates of recent decades

Global comparison of benthic nepheloid layers based on 52 years of nephelometer and transmissometer measurements

Global maps of maximum bottom particle concentration, benthic nepheloid layer thickness, and integrated particle mass in benthic nepheloid layers (BNL) based on 2412 global profiles collected using the Lamont Thorndike nephelometer from 1964 to 1984 are compared with maps of those same properties compiled from 6392 global profiles measured by transmissometers from 1979 to 2016. Outputs from both instruments were converted to particulate matter concentration (PM). The purposes of this paper are to compare global differences and similarities in the location and intensity of BNLs measured with these two independent instruments over slightly overlapping decadal time periods, to combine the data sets in order to expand the time scale of global in situ measurements of BNLs, and to gain insight about the factors creating/sustaining BNLs. The similarity between general locations of high and low particle concentration BNLs during the two time periods indicates that the driving forces of erosion and resuspension of bottom sediments are spatially persistent during recent decadal time spans, though in areas of strong BNLs, intensity is highly episodic. Topography and well-developed current systems play a role. These maps will help to understand deep ocean sediment dynamics, linkage with upper ocean dynamics, the potential for scavenging of adsorption-prone elements near the seafloor, and provide a comprehensive comparison of these data sets on a global scale. During both time periods, BNLs are weak or absent in most of the Pacific, Indian, and Atlantic basins away from continental margins. High surface eddy kinetic energy is associated with the Kuroshio Current east of Japan. Both data sets show weak BNLs south of the Kuroshio, but no transmissometer data have been collected beneath the Kuroshio itself. Sparse nephelometer data show moderate BNLs just north of the Kuroshio Extension, but with much lower concentrations than beneath the Gulf Stream. Strong BNLs are found in areas where eddy kinetic energy in overlying waters, mean kinetic energy near bottom, and energy dissipation within the bottom boundary layer are high. Areas of strongest BNLs include the Western North Atlantic, Argentine Basin (South Atlantic), areas around South Africa tied to the Agulhas Current region, and somewhat random locations in the Antarctic Circumpolar Current of the Southern Ocean

Determining true particulate organic carbon: bottles, pumps and methodologies

The primary means of determining particulate organic carbon (POC) concentrations in aquatic environments is by filtering from water bottles or by in situ filtration with pumps and analyzing the filters. The concentrations measured by these two methods, however, can differ by a factor of 1.2–5 in temperate waters, and by factors as large as 200 in cold, high-latitude waters. Here we report that the ratio of bottle POC to pump POC ranged between 20 and 200 in the Ross Sea during early spring and between 5 and 50 during summer. In the Antarctic Polar Front the ratio ranged between 2 and 25 in spring. A new approach to constraining POC concentrations is to use high-temperature combustion (HTC) of water samples (rather than filters), POC being the difference between measurements of total and dissolved organic carbon. POC concentrations determined by bottle filtration are in reasonable agreement with the POC concentrations obtained by HTC, independent biomass measurements, and beam-attenuation/bottle POC ratios that are similar to previous studies, thus lending credibility to the bottle POC values. Data from several studies suggest that the most likely reasons for differences between bottle and pump POC are the use of slightly larger pore-size filters with in situ pumps and higher pressure differentials across the filter during in situ pump filtration, resulting in particulate carbon being pulled through the filters. The one-to-two orders of magnitude differences in high-latitude, cold-water environments needs further investigation. If accurate measurements of POC can be obtained for calibration, beam attenuation profiles using transmissometers offer a way to quantify rapidly the distribution of POC. The JGOFS protocols for POC filtration must be modified to include a blank that accounts for DOC adsorption onto filters. The largest impact of DOC adsorption will be when POC concentrations are below ∼2 μmol/l, which includes most sub-euphotic zone waters. These findings have important ramifications for any programs involving the particulate portion of the carbon cycle

Changes in freshwater content in the North Atlantic Ocean 1955–2006

Freshwater content changes (FW) for the North Atlantic Ocean (NA) are calculated from in situ salinity profiles for the period 1955–2006 from the surface to 2,000 meters. Heat content (HC) is also calculated from in situ temperature profiles for comparison. A decrease in FW between 1955 and 2006 of ~30,000 km3 is found for the NA, despite an increase in FW of ~16,000 km3 in the subpolar North Atlantic (SNA) and Nordic Seas between the late 1960s and the early 1990s. Over the last two decades there is a pattern of decreasing FW in the upper 400 meters and increasing FW below 1,300 meters for the NA. FW and HC are strongly negatively correlated for both the SNA (r = ?0.93) and the NA (r = ?0.79). Net precipitation, from NCEP/NCAR, is found to have a strong influence on FW changes in the SNA but this relation is weaker elsewhere