Multidecadal warming of Antarctic waters Schmidtko, Sunke; Heywood, Karen J.; Thompson, Andrew F. ...
Science (American Association for the Advancement of Science),
12/2014, Letnik:
346, Številka:
6214
Journal Article
Recenzirano
Odprti dostop
Decadal trends in the properties of seawater adjacent to Antarctica are poorly known, and the mechanisms responsible for such changes are uncertain. Antarctic ice sheet mass loss is largely driven by ...ice shelf basal melt, which is influenced by ocean-ice interactions and has been correlated with Antarctic Continental Shelf Bottom Water (ASBW) temperature. We document the spatial distribution of long-term large-scale trends in temperature, salinity, and core depth over the Antarctic continental shelf and slope. Warming at the seabed in the Bellingshausen and Amundsen seas is linked to increased heat content and to a shoaling of the mid-depth temperature maximum over the continental slope, allowing warmer, saltier water greater access to the shelf in recent years. Regions of ASBW warming are those exhibiting increased ice shelf melt.
Patterns of deoxygenation: sensitivity to natural and anthropogenic drivers Oschlies, Andreas; Duteil, Olaf; Getzlaff, Julia ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
09/2017, Letnik:
375, Številka:
2102
Journal Article
Recenzirano
Odprti dostop
Observational estimates and numerical models both indicate a significant overall decline in marine oxygen levels over the past few decades. Spatial patterns of oxygen change, however, differ ...considerably between observed and modelled estimates. Particularly in the tropical thermocline that hosts open-ocean oxygen minimum zones, observations indicate a general oxygen decline, whereas most of the state-of-the-art models simulate increasing oxygen levels. Possible reasons for the apparent model-data discrepancies are examined. In order to attribute observed historical variations in oxygen levels, we here study mechanisms of changes in oxygen supply and consumption with sensitivity model simulations. Specifically, the role of equatorial jets, of lateral and diapycnal mixing processes, of changes in the wind-driven circulation and atmospheric nutrient supply, and of some poorly constrained biogeochemical processes are investigated. Predominantly wind-driven changes in the low-latitude oceanic ventilation are identified as a possible factor contributing to observed oxygen changes in the low-latitude thermocline during the past decades, while the potential role of biogeochemical processes remains difficult to constrain. We discuss implications for the attribution of observed oxygen changes to anthropogenic impacts and research priorities that may help to improve our mechanistic understanding of oxygen changes and the quality of projections into a changing future.
This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.
Abstract The Antarctic Slope Front (ASF) is a fundamental feature of the subpolar Southern Ocean that is still poorly observed. In this study we build a statistical climatology of the temperature and ...salinity fields of the upper 380 m of the Antarctic margin. We use a comprehensive compilation of observational datasets including the profiles gathered by instrumented marine mammals. The mapping method consists first of a decomposition in vertical modes of the combined temperature and salinity profiles. Then the resulting principal components are optimally interpolated on a regular grid and the monthly climatological profiles are reconstructed, providing a physically plausible representation of the ocean. The ASF is located with a contour method and a gradient method applied on the temperature field, two complementary approaches that provide a complete view of the ASF structure. The front extends from the Amundsen Sea to the eastern Weddell Sea and closely tracks the continental shelf break. It is associated with a sharp temperature gradient that is stronger in winter and weaker in summer. The emergence of the front in the Amundsen and Bellingshausen sectors appears to be seasonally variable (slightly more westward in winter than in summer). Investigation of the density gradients across the shelf break indicates a winter slowdown of the baroclinic component of the Antarctic Slope Current at the near surface, in contrast with the seasonal variability of the temperature gradient.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Antarctic Intermediate Water (AAIW) is a dominant Southern Hemisphere water mass that spreads from its formation regions just north of the Antarctic Circumpolar Current (ACC) to at least 20°S in all ...oceans. This study uses an isopycnal climatology constructed from Argo conductivity–temperature–depth (CTD) profile data to define the current state of the AAIW salinity minimum (its core) and thence compute anomalies of AAIW core pressure, potential temperature, salinity, and potential density since the mid-1970s from ship-based CTD profiles. The results are used to calculate maps of temporal property trends at the AAIW core, where statistically significant strong circumpolar shoaling (30–50 dbar decade−1), warming (0.05°–0.15°C decade−1), and density reductions up to −0.03 (kg m−3) decade−1 are found. These trends are strongest just north of the ACC in the southeast Pacific and Atlantic Oceans and decrease equatorward. Salinity trends are generally small, with their sign varying regionally. Bottle data are used to extend the AAIW core potential temperature anomaly analysis back to 1925 in the Atlantic and to ∼1960 elsewhere. The modern warm AAIW core conditions appear largely unprecedented in the historical record: biennially and zonally binned median AAIW core potential temperatures within each ocean basin are, with the notable exception of the subtropical South Atlantic in the 1950s–70s, 0.2–1°C colder than modern values. Zonally averaged sea surface temperature anomalies around the AAIW formation latitudes in each ocean and sectoral southern annular mode indices are used to put the AAIW core property trends and variations into context.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The supply of oxygen‐rich water to the oxygen minimum zones (OMZs) of the eastern North and South Pacific via zonal tropical currents is investigated using shipboard acoustic Doppler current profiler ...and hydrographic section data. Near the equator, the Equatorial Undercurrent (EUC), Northern and Southern Subsurface Countercurrents (SCCs), and the Northern and Southern Intermediate Countercurrents (ICCs) all carry water that is oxygen richer than adjacent westward flows, thereby providing a net oxygen supply to the eastern Pacific OMZs. The synoptic velocity‐weighted oxygen concentration difference between eastward and westward flows is typically 10–50 μmol kg−1. Subthermocline zonal oxygen fluxes reflect decreasing oxygen concentrations of the EUC, the SCCs, and the ICCs as they flow eastward. Approximately 30 year time series in well‐sampled regions of the equatorial Pacific show oxygen content decreasing as rapidly as −0.55 μmol kg−1 yr−1 in the major oxygen supply paths of the OMZs for a 200–700 m layer and similar trends for a density layer spanning roughly these depths. This finding is in gross agreement with climate models, which generally predict expanding OMZs.
The warming climate is causing a strengthening of ocean stratification. Ocean stratification, in turn, has significant impacts on physical, biogeochemical and ecological processes, such as ocean ...circulation, ventilation, air-sea interactions, nutrient fluxes, primary productivity and fisheries. How these processes are affected in detail by changing stratification still remains uncertain and are likely to vary locally. Here, we investigate the state and trend of different parameters characterizing the stratification of the global upper-ocean which can be derived from Argo profiles for the period 2006-2021. Among those parameters are mixed layer depth, magnitude and depth of the vertical stratification maximum. The summertime stratification maximum has increased in both hemispheres, respectively. During wintertime, the stratification maximum has intensified in the Northern Hemisphere, while changes in the Southern Hemisphere have been relatively small. Comparisons to mixed layer characteristics show that a strengthening stratification is mainly accompanied by a warming and freshening of the mixed layer. In agreement with previous observational studies, we find a large-scale mixed layer deepening that regionally contributes to the increasing stratification. Globally, the vertical stratification maximum strengthens by 7-8% and the mixed layer deepens by 4 m during 2006-2021. This hints to an ongoing de-coupling of the surface ocean from the ocean interior. The investigated changes can help determine the origin of existing model-observation discrepancies and improve predictions on climate change impact on upper-ocean ecology and biogeochemistry.
A surface diurnal warm layer is diagnosed from Seaglider observations and develops on half of the days in the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the ...Madden–Julian Oscillation (CINDY/DYNAMO) Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (>80 W m−2) and low wind speed (<6 m s−1) and preferentially in the inactive stage of the Madden–Julian oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4–5m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m−2that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30%–50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending toward isothermal conditions.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A strong oxygen-deficient layer is located in the upper
layers of the tropical Pacific Ocean and deeper in the North Pacific.
Processes related to climate change (upper-ocean warming, reduced
...ventilation) are expected to change ocean oxygen and nutrient inventories.
In most ocean basins, a decrease in oxygen (“deoxygenation”) and an increase
in nutrients have been observed in subsurface layers. Deoxygenation trends
are not linear and there could be multiple influences on oxygen and nutrient
trends and variability. Here oxygen and nutrient time series since 1950 in
the Pacific Ocean were investigated at 50 to 300 m depth, as this layer
provides critical pelagic habitat for biological communities. In addition to
trends related to ocean warming the oxygen and nutrient trends show a strong
influence of the Pacific Decadal Oscillation (PDO) in the tropical and the
eastern Pacific, and the North Pacific Gyre Oscillation (NPGO) in particular in
the North Pacific. In the Oyashio Region the PDO, the NPGO, the North
Pacific Index (NPI) and an 18.6-year nodal tidal cycle overlay the long-term
trend. In most eastern Pacific regions oxygen increases and nutrients
decrease in the 50 to 300 m layer during the negative PDO phase, with
opposite trends during the positive PDO phase. The PDO index encapsulates
the major mode of sea surface temperature variability in the Pacific, and
oxygen and nutrients trends throughout the basin can be described in the
context of the PDO phases. El Niño and La Niña years often influence
the oxygen and nutrient distribution during the event in the eastern
tropical Pacific but do not have a multi-year influence on the trends.
Abstract
The North Atlantic Ocean is the most intense marine sink for anthropogenic carbon dioxide (CO
2
) in the world’s oceans, showing high variability and substantial changes over recent decades. ...However, the contribution of biology to the variability and trend of this sink is poorly understood. Here we use
in situ
plankton measurements, alongside observation-based sea surface CO
2
data from 1982 to 2020, to investigate the biological influence on the CO
2
sink. Our results demonstrate that long term variability in the CO
2
sink in the North Atlantic is associated with changes in phytoplankton abundance and community structure. These data show that within the subpolar regions of the North Atlantic, phytoplankton biomass is increasing, while a decrease is observed in the subtropics, which supports model predictions of climate-driven changes in productivity. These biomass trends are synchronous with increasing temperature, changes in mixing and an increasing uptake of atmospheric CO
2
in the subpolar North Atlantic. Our results highlight that phytoplankton play a significant role in the variability as well as the trends of the CO
2
uptake from the atmosphere over recent decades.
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