Calvin W. Mordy

Changes in Ocean Heat, Carbon Content, and Ventilation: A Review of the First Decade of GO-SHIP Global Repeat Hydrography.

Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earths climate system, is taking up most of Earths excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcing and ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the oceans overturning circulation.

Hydrography, nutrients, and carbon pools in the Pacific sector of the Southern Ocean: Implications for carbon flux

We investigated the hydrography, nutrients, and dissolved and particulate carbon pools in the western Pacific sector of the Antarctic Circumpolar Current (ACC) during austral summer 1996 to assess the regions role in the carbon cycle. Low fCO2 values along two transects indicated that much of the study area was a sink for atmospheric CO2. The fCO2 values were lowest near the Polar Front (PF) and the Subtropical Front (STF), concomitant with maxima of chlorophyll a and particulate and dissolved organic carbon. The largest biomass accumulations did not occur at fronts, which had high surface geostrophic velocities (20–51 cm s−1), but in relatively low velocity regions near fronts or in an eddy. Thus vertical motion and horizontal advection associated with fronts may have replenished nutrients in surface waters but also dispersed phytoplankton. Although surface waters north of the PF have been characterized as a “high nutrient-low chlorophyll” region, low silicic acid (Si) concentrations (2–4 μM) may limit production of large diatoms and therefore the potential carbon flux. Low concentrations (4–10 μM Si) at depths of winter mixing constrain the level of Si replenishment to surface waters. It has been suggested that an increase in aeolian iron north of the PF may increase primary productivity and carbon export. Our results, however, indicate that while diatom growth and carbon export may be enhanced, the extent ultimately would be limited by the vertical supply of Si. South of the PF, the primary mechanism by which carbon is exported to deep water appears to be through diatom flux. We suggest that north of the PF, particulate and dissolved carbon may be exported primarily to intermediate depths through subduction and diapycnal mixing associated with Subantarctic Mode Water and Antarctic Intermediate Water formation. These physical-biological interactions and Si dynamics should be included in future biogeochemical models to provide a more accurate prediction of carbon flux.

Vertical modeling of the nitrogen cycle in the eastern tropical South Pacific oxygen deficient zone using high-resolution concentration and isotope measurements

Marine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss have been debated, and traditional methods for measuring these rates are labor-intensive and may miss hot spots in spatially and temporally variable environments. Here we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high-resolution concentration and natural abundance stable isotope measurements of nitrate (NO3−) and nitrite (NO2−). These measurements were used to estimate process rates using a two-step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO3− reduction and NO2− oxidation. Nevertheless, we addressed two questions surrounding the fates of NO2− in the ODZ: (1) Is NO2− being primarily reduced to N2 or oxidized to NO3− in the ODZ? and (2) what are the contributions of anammox and denitrification to NO2− removal? Depth-integrated rates from the model suggest that 72–88% of the NO2− produced in the ODZ was oxidized back to NO3−, while 12–28% of NO2− was reduced to N2. Furthermore, our model suggested that 36–74% of NO2− loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long-term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.

Return of warm conditions in the southeastern Bering Sea: Physics to fluorescence

From 2007 to 2013, the southeastern Bering Sea was dominated by extensive sea ice and below-average ocean temperatures. In 2014 there was a shift to reduced sea ice on the southern shelf and above-average ocean temperatures. These conditions continued in 2015 and 2016. During these three years, the spring bloom at mooring site M4 (57.9°N, 168.9°W) occurred primarily in May, which is typical of years without sea ice. At mooring site M2 (56.9°N, 164.1°W) the spring bloom occurred earlier especially in 2016. Higher chlorophyll fluorescence was observed at M4 than at M2. In addition, these three warm years continued the pattern near St. Matthew Island of high concentrations (>1 μM) of nitrite occurring during summer in warm years. Historically, the dominant parameters controlling sea-ice extent are winds and air temperature, with the persistence of frigid, northerly winds in winter and spring resulting in extensive ice. After mid-March 2014 and 2016 there were no cold northerly or northeasterly winds. Cold northerly winds persisted into mid-April in 2015, but did not result in extensive sea ice south of 58°N. The apparent mechanism that helped limit ice on the southeastern shelf was the strong advection of warm water from the Gulf of Alaska through Unimak Pass. This pattern has been uncommon, occurring in only one other year (2003) in a 37-year record of estimated transport through Unimak Pass. During years with no sea ice on the southern shelf (e.g. 2001–2005, 2014–2016), the depth-averaged temperature there was correlated to the previous summers ocean temperature.

Innovative technology development for Arctic Exploration

The US Arctic and sub-Arctic regions are rapidly changing, creating potentially large impacts to marine ecosystems and ecosystem services. However, much of the current observing technology is ill suited to fully quantify these dynamic changes. The harsh, remote environment, expansive area, and extremely fine scale features present clear barriers to the efficient collection of effective environmental intelligence. In order to meet these challenges, NOAAs Pacific Marine Environmental Laboratory, with support from Ocean and Atmospheric Research Division, has created the Innovative Technology for Arctic Exploration (ITAE) program to facilitate the development of new autonomous platforms and high-resolution sensing technologies that may be able to address this critical gap in mission capabilities. During the programs primary field testing year, ITAE successfully completed two large-scale research missions in the Bering and Chukchi Seas involving multiple new Arctic-capable platforms, including the Saildrone unmanned autonomous surface vehicle (Saildrone, Inc.), the Profiling Crawler (PRAWLER; NOAA-PMEL), a moored instrument drastically improving vertical resolution of data collection; and the Expendable Ice Tracking (EXIT) Floats, which allow for under-ice data collection (NOAA-PMEL). Through these platforms, ITAE also tested a variety of novel sensing technologies, such as the recently developed microfluidic nitrate sensor, the Lab-on-a-Chip (National Oceanography Centre, University of Southampton). Together, these developments helped to assess important and previously inaccessible aspects of the sea ice melt season. However, important technical challenges remain, including autonomous ecosystem assessment tools that could effectively monitor and aid management of the regions multi-billion dollar annual commercial and subsistence fishing industries.

The use of Saildrones to examine spring conditions in the Bering sea

New technologies can help scientists measure and understand Arctic warming, sea ice loss and ecosystem change. NOAA has worked with Saildrone, Inc., to develop an unmanned surface vehicle (USV)-Saildrone-to make ocean surface measurements autonomously, even in challenging high-latitude conditions. USVs augment traditional research ship cruises, mitigate ship risk in high seas and shallow water, and make lower cost measurements. Under remote control, USV sampling strategy can be adapted to meet changing needs. Two Saildrones conducted 97-day missions in the Bering Sea in spring-summer 2015, reliably measuring atmospheric and oceanic parameters. Measurements were validated against shipboard values. Following that, the Saildrone sampling strategies were modified, first to measure the effects of sea-ice melt on surface cooling and freshening, and then to study the Yukon River plume.

Results of the First Arctic Heat Open Science Experiment

AbstractSeasonally ice-covered marginal seas are among the most difficult regions in the Arctic to study. Physical constraints imposed by the variable presence of sea ice in all stages of growth and melt make the upper water column and air–sea ice interface especially challenging to observe. At the same time, the flow of solar energy through Alaska’s marginal seas is one of the most important regulators of their weather and climate, sea ice cover, and ecosystems. The deficiency of observing systems in these areas hampers forecast services in the region and is a major contributor to large uncertainties in modeling and related climate projections. The Arctic Heat Open Science Experiment strives to fill this observation gap with an array of innovative autonomous floats and other near-real-time weather and ocean sensing systems. These capabilities allow continuous monitoring of the seasonally evolving state of the Chukchi Sea, including its heat content. Data collected by this project are distributed in near–...

Nutrient and phytoplankton dynamics on the inner shelf of the eastern Bering Sea

The nitrogen cycle on the inner shelf of the southeastern Bering Sea is complicated due to limited nutrient replenishment across this broad shelf, and substantial nitrogen loss through sedimentary processes. While diffusion at the inner front may periodically support new production, the shelf is generally hypothesized to be a regenerative system. This study uses a combination of hydrographic surveys, and measurements of nitrogen assimilation and benthic fluxes to examine nitrogen cycling on the inner shelf, and connectivity between the middle and inner shelves of the southern and central Bering Sea. Results establish the inner shelf as primarily a regenerative system even in spring, although new production can occur at the inner front. Results also identify key processes that influence nutrient supply to the inner shelf, and reveal coupling between the middle shelf nutrient pool and production on the inner shelf. This article is protected by copyright. All rights reserved.