Weakening and Poleward Shifting of the North Pacific Subtropical Fronts from 1980 to 2018

Recent evidence shows that the North Pacific subtropical gyre, the Kuroshio Extension (KE) and Oyashio Extension (OE) fronts have moved poleward in the past few decades. However, changes of the North Pacific Subtropical Fronts (STFs), anchored by the North Pacific subtropical countercurrent in the southern subtropical gyre, remain to be quantified. By synthesizing observations, reanalysis, and eddy-resolving ocean hindcasts, we show that the STFs, especially their eastern part, weakened (20%±5%) and moved poleward (1.6°±0.4°) from 1980 to 2018. Changes of the STFs are modified by mode waters to the north. We find that the central mode water (CMW) (180°-160°W) shows most significant weakening (18%±7%) and poleward shifting (2.4°±0.9°) trends, while the eastern part of the subtropical mode water (STMW) (160°E-180°) has similar but moderate changes (10% ± 8%; 0.9°±0.4°). Trends of the western part of the STMW (140°E-160°E) are not evident. The weakening and poleward shifting of mode waters and STFs are enhanced to the east and are mainly associated with changes of the northern deep mixed layers and outcrop lines—which have a growing northward shift as they elongate to the east. The eastern deep mixed layer shows the largest shallowing trend, where the subduction rate also decreases the most. The mixed layer and outcrop line changes are strongly coupled with the northward migration of the North Pacific subtropical gyre and the KE/OE jets as a result of the poleward expanded Hadley cell, indicating that the KE/OE fronts, mode waters, and STFs change as a whole system.

Diagnosing CO2 emission-induced feedbacks between the Southern Ocean carbon cycle and the climate system: A multiple Earth System Model analysis using a water mass tracking approach

Anthropogenic CO2 emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2 uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth System Model projections of 21st century climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealised ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2 (due to increasing air-sea CO2 partial pressure difference, dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2 uptake by the Southern Ocean and account for one-fifth of the global uptake of CO2 emissions. The increase in CO2 uptake is primarily dpCO2-driven, with Antarctic intermediate waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic mode waters. The process dominating the decrease in CO2 uptake is water mass-dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic mode waters, local climate impacts in Antarctic intermediate waters, and reduction in outcrop areas in circumpolar deep waters and Antarctic bottom waters. Intermodel variability in the feedbacks is predominately dpCO2–driven and should be a focus of efforts to constrain projection uncertainty.

A 2011-2018 Fukushima Perspective on North Pacific Mode Water Pathways

<p>This investigation uses the tracer information provided by the 2011 direct ocean release of radio-isotopes, (<sup>137</sup>Cs, ~30-year half-life and <sup>134</sup>Cs, ~2-year half-life) from the Fukushima Dai-ichi nuclear power plant (FDNPP) together with hydrographic profiles to better understand the origins and pathways of mode waters in the North Pacific Ocean. While using information provided by radionuclide observations taken from across the basin, the main focus is on the eastern basin and results from analyses of two data sets 2015 (GO-SHIP) and 2018 (GEOTRACES) along the 152°W meridian. The study looks at how mode waters formed in the spring of 2011 have spread and mixed, and how they have not. Our radiocesium isotope samples tell a story of a surprisingly confined pathway for these waters and suggest that circulation to the north into the subpolar gyre occurs more quickly than circulation to the south into the subtropical gyre. They indicate that in spite of crossing 6000 km in their journey across the Pacific, the densest 2011 mode waters stayed together spreading by only a few hundred kilometers in the north/south direction, remained subsurface (below ~200 m) for most of the trip, and only saw the atmosphere again as they followed shoaling density surfaces into the boundary of the Alaska Gyre. The more recent data are sparse and do not allow direct measurement of the FDNPP specific <sup>134</sup>Cs, however they do provide some information on mode water evolution in the eastern North Pacific seven years after the accident. </p>

Long-term ventilation changes of Subpolar Mode Waters in the North Atlantic Ocean and its impact on the oxygen distribution

<p>In the subpolar North Atlantic Ocean, Subpolar Mode Waters (SPMWs) are formed during late winter convection following the cyclonic circulation of the subpolar gyre. SPMWs participate in the upper flow of the Atlantic overturning circulation (AMOC) and provide much of the water that is eventually transformed into several components of the North Atlantic deep water (NADW), the cold, deep part of the AMOC. In a warming climate, an increase in upper ocean stratification is expected to lead to a reduced ventilation and a loss of oxygen. Thus, understanding how mode waters are affected by ventilation changes will help us to better understand the variability in the AMOC. In particular, we would like to address how the volume occupied by SPMWs has varied over the last decades due to ventilation changes, and what are the aspects driving the subpolar mode water formation, their interannual variations as well as the impact of the variability in the mixing and subduction and vertical dynamics on ocean deoxygenation. For this purpose, we use two observation-based 3D products from Copernicus Marine Service (CMEMS), the ARMOR3D and the OMEGA3D datasets. The first consists of 3D temperature and salinity fields, from the surface to 1500 m depth, available weekly over a regular grid at 1/4° horizontal resolution from 1993 to present. The second consists of observation-based quasi-geostrophic vertical and horizontal ocean currents with the same temporal and spatial resolution as ARMOR3D.</p>

Anthropogenic Temperature and Salinity Changes in the Southern Ocean

In this study, we compare observed Southern Ocean temperature and salinity changes with the historical simulations from 13 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), using an optimal fingerprinting framework. We show that there is an unequivocal greenhouse gas–forced warming in the Southern Ocean. This warming is strongest in the Subantarctic Mode Waters but is also detectable in denser water masses, which has not been shown in previous studies. We also find greenhouse gas–forced salinity changes, most notably a freshening of Antarctic Intermediate Waters. Our analysis also shows that non–greenhouse gas anthropogenic forcings—anthropogenic aerosols and stratospheric ozone depletion—have played an important role in mitigating the Southern Ocean’s warming. However, the detectability of these responses using optimal fingerprinting is model dependent, and this result is therefore not as robust as for the greenhouse gas response.

Long range transport of radiocaesium derived from global fallout and the Fukushima accident in the ocean interior of the Pacific Ocean since 1960s through 2017

<p>The world's oceans act as a sink for artificial radionuclides as well as for other anthropogenic pollutants released into the environment. Owing to physical and biogeochemical processes in the ocean, artificial radionuclides in the ocean are redistributed from their initial entry points which depend on the various sources. Long range transport of radiocaesium in the ocean interior were investigated and presented. Radiocaesium were derived from global fallout which occurred mainly late 1950s and early 1960s and the Fukushima accident occurred in 2011. In the ocean interior, main factor is subduction of mode water formation from surface to two mode waters, STMW and CMW. Radiocaesium then stayed long in both STMW and CMW, but relatively first recirculation and southward movement were observed in STMW for decadal time scale.</p><p>We establish database for artificial Radionuclides in the marine environment as HAM global 2018, doi: 10.34355/CRiED.U.Tsukuba.00001, and we reconstruct 137Cs activity concentration sections for 1965-1968 and 1970-1973 to understand initial conditions of 137Cs activity concentration in ocean interior just after large atmospheric fallout in early 1960s and 5 years after injection. We also carried out observations at stations between 49 deg. N and 60 deg. S along 165 deg.  E in 2002, 2012 and 2015. After that, we also observed vertical profiles in the western North Pacific Ocean.　</p><p>Basic feature of radiocaesium distribution along 165 deg. E section in 1963-1965 was dome shape distribution of which deepest places were around 30-40 deg. N and of which maximum depth were around 600- 800 meter depths. The penetration of 137Cs is found less than 800 m depth, associated with the bowl shape of isopycnals in the midlatitude region. In general, the 137Cs activity concentrations in the subsurface and intermediate water of the mid latitude region of the western North Pacific were higher than those in surface waters of the subtropical and equatorial Pacific. In 2002, we observed two 137Cs activity concentration maxima at 250 m and at 400 to 500 m depth at around 20 deg. N. The 137Cs activity concentration at the core at 400 to 500 m depth in 2002 was around 2 – 3 Bq m-3 and the start of moving in 1963-1965 was 16 Bq m-3 which indicates only one thirds of dilution occurred during about 40 years travel in the ocean interior as CMW. In 2012, we also observed two 134Cs activity concentration maxima at 150 m, 30 deg. N and at 300 m depth at 40N, while we observed a Fukushima derived at 300 m, 30 deg. N with southward movements. Basic feature of 137Cs distribution derived from atmospheric weapons test along 165 deg. E section in 2012 still keep dome shape distribution of which deepest places were around 30-35 deg. N and of which maximum depth were around 400 meters depths, while deepest places were around 20-30 deg. N in 2015. These findings strongly suggest that radiocaesium has been transporting in the ocean interior by subduction of mode waters from subarctic region to subtropical region and tropical region.</p>

Nutrient budgets in Southern Ocean mode waters controlled by nitrification

<p>Southern Ocean mode and intermediate waters supply nitrate-rich but silicate-poor waters to the lower latitudes, impeding diatom growth throughout the extra-polar ocean and weakening the ocean’s ability to absorb carbon dioxide from the atmosphere. This silicate deficiency is widely attributed to high silicate to nitrate uptake by iron-limited diatoms. Here, we show that nitrification, by rapidly regenerating nitrate in shallow waters, drives the silicate deficiency. Measurements of nitrate dual isotopes and complementary modelling independently suggest that 15-35% of the nitrate within mode waters is generated by nitrification. Our results reveal that without nitrification, the silicate deficiency would disappear, which would allow the diatomaceous niche to expand. Nitrification therefore provides a key buffering service that mitigates against change in the silicate deficit and subsequently restricts diatom dominance to the polar ocean. This insight highlights the critical importance for understanding Southern Ocean processes, such that the large-scale effects of ongoing environmental change may be realised.</p>

Effect of mesoscale eddies on subtropical mode water formation and ocean heat storage

<p><span>Mode water formation results from air-sea exchange processes in association with the dynamics and thermodynamics of ocean currents or fronts in every ocean basin. Here, a new algorithm is applied to the Argo global array to define surface mixed layer depths and to detect mode waters with homogeneous properties underneath. Specifically, we revisit the spatial and temporal evolution of South Atlantic subtropical mode water (SASTMW) using this new algorithm and find that our set of criteria is more precise than previous detections of mode water. With satellite altimetry measurements and eddy tracking algorithms (Laxenaire et al., 2018), the colocalization between mesoscale eddies and mode waters can be achieved. We then test how much the profiles indicative of mode water are matched with locations of mesoscale eddies and to what extent these eddies influence mode water variability. In addition, we investigate the relationship between the temporal integral of surface heat flux with the heat stored within the layers of the SASTMWs during the formation periods. Nearly all Argo profiles indicate that mode water formation occurs at the time and within the region where loss of latent heat flux from ocean to the atmosphere is significant. Anticyclonic eddies, specifically, play a crucial role in heat redistribution associated with mode waters advected by the subtropical gyre. </span></p>

Stability-indicating UPLC-PDA Method for Ambrisentan Tablets and Identification of a Main Degradation Product by UPLC-MS/MS

Background: Ambrisentan is a drug used to treat the pulmonary arterial hypertension symptoms, commercialized as coated tablets. Drug quality control is an essential part for the development and release of drugs for consumption; however, there are few studies related to the proposition of analytical methods and stability study for ambrisentan. Objective: The development of an UPLC assay of ambrisentan in tablets with degradation product`s elucidation was proposed. Methods: Tests with different solvents and chromatographic columns were carried out, achieving an optimal condition using mobile phase in gradient mode, Waters® BEH C18 column and detection at 260 nm. Results: Satisfactory system suitability was obtained (theoretical plates, sensitivity and resolution among peaks), with a reduced analysis time (6 minutes). The method was validated in accordance with the international guidelines and it demonstrated adequate specificity, either for the drug assay as for the identification and quantification of degradation product. It showed linearity (r= 0.999), accuracy (degradation products recovery: 98.47 - 102.44; assay recovery: 99.98 - 104.32%) and precision (RSD: 0.69), with limits of quantification and detection in suitable magnitude in order to evaluate possible drug degradation. Conclusion: UPLC method demonstrated to be fast with satisfactory robustness. The main ambrisentan degradation product formed under thermal stress conditions was elucidated by UPLC-MS/MS and its structure was suggested.