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CLIMATE CHANGE IS WARMING THE DEEP OCEAN

Posted on: October 14, 2020

Climate change and the ocean
Ocean Heat Content And The Importance Of The Deep Ocean
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THIS POST IS A CRITICAL REVIEW OF A PHYS.ORG ARTICLE ON AN OCEAN WARMING EFFECT OF ANTHROPOGENIC GLOBAL WARMING.

LINK TO THE ARTICLE: https://phys.org/news/2020-10-deep-sea-slowly.html

PART-1: WHAT THE PHYS.ORG ARTICLE SAYS

The deep sea is slowly warming: by American Geophysical Union

New research reveals temperatures in the deep sea fluctuate more than scientists previously thought and a warming trend is now detectable at the bottom of the ocean. In a new study in AGU’s journal Geophysical Research Letters, researchers analyzed a decade of hourly temperature recordings from moorings anchored at four depths in the Atlantic Ocean’s Argentine Basin off the coast of Uruguay. The depths represent a range around the average ocean depth of 3,682 meters, with the shallowest at 1,360 meters and the deepest at 4,757 meters. They found all sites exhibited a warming trend of 0.02 to 0.04 degrees Celsius per decade between 2009 and 2019, a significant warming trend in the deep sea where temperature fluctuations are typically measured in thousandths of a degree.

According to the study authors, this increase is consistent with warming trends in the shallow ocean associated with anthropogenic climate change, but more research is needed to understand what is driving rising temperatures in the deep ocean.

In years past, everybody used to assume the deep ocean was quiescent. There was no motion. There were no changes but each time we go look we find that the ocean is more complex than we thought. The challenge of measuring the deep.

Researchers today are monitoring the top 2,000 meters of the ocean more closely than ever before, in large part due to an international program called the Global Ocean Observing System. Devices called Argo floats that sink and rise in the upper ocean, bobbing along in ocean currents, provide a rich trove of continuous data on temperature and salinity.

The deep sea, however, is difficult to access and expensive to study. Scientists typically take its temperature using ships that lower an instrument to the seafloor just once every ten years. This means scientists’ understanding of the day-to-day changes in the bottom half of the ocean lag far behind their knowledge of the surface.

NOAA is carrying out a rare long-term study at the bottom of the ocean, and the four devices they had moored at the bottom of the Argentine Basin were collecting information on ocean currents and a temperature sensor was built into the instrument’s pressure sensor used to study currents and it had collected temperature data for the entirety of their study period.

“So we went back and we calibrated all of our hourly data from these instruments and put together what is essentially a continuous 10-year-long hourly record of temperature one meter off the seafloor,” Meinen said.

Dynamic depths

The researchers found at the two shallower depths of 1,360 and 3,535 meters (4,460 feet and 11,600 feet), temperatures fluctuated roughly monthly by up to a degree Celsius. At depths below 4,500 meters (14,760 feet), temperature fluctuations were more minute, but changes followed an annual pattern, indicating seasons still have a measurable impact far below the ocean surface. The average temperature at all four locations went up over the course of the decade, but the increase of about 0.02 degrees Celsius per decade was only statistically significant at depths of over 4,500 meters.

According to the authors, these results demonstrate that scientists need to take the temperature of the deep ocean at least once a year to account for these fluctuations and pick up on meaningful long-term trends. In the meantime, others around the world who have used the same moorings to study deep sea ocean currents could analyze their own data and compare the temperature trends of other ocean basins.

“There are a number of studies around the globe where this kind of data has been collected, but it’s never been looked at,” Meinen said. “I’m hoping that this is going to lead to a reanalysis of a number of these historical datasets to try and see what we can say about deep ocean temperature variability.”

A better understanding of temperature in the deep sea could have implications that reach beyond the ocean. Because the world’s oceans absorb so much of the world’s heat, learning about the ocean’s temperature trends can help researchers better understand temperature fluctuations in the atmosphere as well.

“We’re trying to build a better Global Ocean Observing System so that in the future, we’re able to do better weather predictions,” Meinen said. “At the moment we can’t give really accurate seasonal forecasts, but hopefully as we get better predictive capabilities, we’ll be able to say to farmers in the Midwest that it’s going to be a wet spring and you may want to plant your crops accordingly.”

OTHER REPORTS OF THIS RESEARCH

Deep diving robots find warming accelerating in South Pacific Ocean waters
More information: Christopher S. Meinen et al, Observed Ocean Bottom Temperature Variability at Four Sites in the Northwestern Argentine Basin: Evidence of Decadal Deep/Abyssal Warming Amidst Hourly to Interannual Variability During 2009–2019, Geophysical Research Letters (2020). DOI: 10.1029/2020GL089093. Journal information: Geophysical Research Letters
Provided by American Geophysical Union. Facebook. Twitter.

PART-2: CRITICAL COMMENTARY

It is generally recognized that the the Antarctic Ocean is subject to significant geothermal heat that plays a role in the ice melt events and also explains the relative warmth of the Deep Circumpolar Current in that region. The South Atlantic and specifically the Argentine Basin is located immediately north of this geologically active area and it is known to be geologically active such that the observed abyssal warming of 0.02 to 0.04C over a decade can be explained as a geological event. A relevant bibliography is provided below that generally supports this evaluation.

The additional consideration is that a brief decadal warming event in the geographically limited region of the Argentine Basin of the Southwest Atlantic must be understood as an internal earth system climate variability that does not have an interpretation in terms of anthropogenic global warming and climate change that relates to long term trends in global mean temperature and not to temperature events particularly so when they are geographically constrained.

Under these conditions, it is not possible to understand the localized decadal warming event in a geologically active region in terms of anthropogenic global warming. LINK TO RELATED POST ON INTERNAL CLIMATE VARIABILITY: https://tambonthongchai.com/2020/07/16/the-internal-variability-issue/

RELEVANT BIBLIOGRAPHY: GEOLOGICAL FEATURES OF THE ARGENTINE BASIN.

  1. Hofmann, M., and M. A. Morales Maqueda. “Geothermal heat flux and its influence on the oceanic abyssal circulation and radiocarbon distribution.” Geophysical Research Letters 36.3 (2009). Geothermal heating of abyssal waters is rarely regarded as a significant driver of the large‐scale oceanic circulation. Numerical experiments with the Ocean General Circulation Model POTSMOM‐1.0 suggest, however, that the impact of geothermal heat flux on deep ocean circulation is not negligible. Geothermal heating contributes to an overall warming of bottom waters by about 0.4°C, decreasing the stability of the water column and enhancing the formation rates of North Atlantic Deep Water and Antarctic Bottom Water by 1.5 Sv (10%) and 3 Sv (33%), respectively. Increased influx of Antarctic Bottom Water leads to a radiocarbon enrichment of Pacific Ocean waters, increasing Δ14C values in the deep North Pacific from −269‰ when geothermal heating is ignored in the model, to −242‰ when geothermal heating is included. A stronger and deeper Atlantic meridional overturning cell causes warming of the North Atlantic deep western boundary current by up to 1.5°C. FULL TEXT; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL036078
  2. Purkey, Sarah G., and Gregory C. Johnson. “Warming of global abyssal and deep Southern Ocean waters between the 1990s and 2000s: Contributions to global heat and sea level rise budgets.” Journal of Climate 23.23 (2010): 6336-6351.: Abyssal global and deep Southern Ocean temperature trends are quantified between the 1990s and 2000s to assess the role of recent warming of these regions in global heat and sea level budgets. The authors 1) compute warming rates with uncertainties along 28 full-depth, high-quality hydrographic sections that have been occupied two or more times between 1980 and 2010; 2) divide the global ocean into 32 basins, defined by the topography and climatological ocean bottom temperatures; and then 3) estimate temperature trends in the 24 sampled basins. The three southernmost basins show a strong statistically significant abyssal warming trend, with that warming signal weakening to the north in the central Pacific, western Atlantic, and eastern Indian Oceans. Eastern Atlantic and western Indian Ocean basins show statistically insignificant abyssal cooling trends. Excepting the Arctic Ocean and Nordic seas, the rate of abyssal (below 4000 m) global ocean heat content change in the 1990s and 2000s is equivalent to a heat flux of 0.027 (±0.009) W m−2 applied over the entire surface of the earth. Deep (1000–4000 m) warming south of the Subantarctic Front of the Antarctic Circumpolar Current adds 0.068 (±0.062) W m−2. The abyssal warming produces a 0.053 (±0.017) mm yr−1 increase in global average sea level and the deep warming south of the Subantarctic Front adds another 0.093 (±0.081) mm yr−1. Thus, warming in these regions, ventilated primarily by Antarctic Bottom Water, accounts for a statistically significant fraction of the present global energy and sea level budgets. LINK TO FULL TEXT; https://journals.ametsoc.org/jcli/article/23/23/6336/32723
  3. Roden, Gunnar I. “Thermohaline fronts and baroclinic flow in the Argentine Basin during the austral spring of 1984.” Journal of Geophysical Research: Oceans 91.C4 (1986): 5075-5093. Thermohaline fronts, structure, and baroclinic flow in the central Argentine basin are investigated on the basis of a 1984 field experiment. The Brazil Current, after initial overshoot, meanders northeastward toward subtropical latitudes with speeds of the order of 0.3 m s−1. The meanders have a wavelength of about 400 km and an amplitude of 200 km. Brazil Current signatures, as expressed by dynamic height, are recognizable to depths of several kilometers. The Brazil and Antarctic Circumpolar currents do not meet in the central Argentine basin to form common eastward flow, as was expressed in classical descriptions, but instead diverge sharply near 42°W. This is seen also in the trajectories of satellite‐tracked drifters. The region between the currents is marked by cyclonic and anticyclonic eddies. Strong thermohaline fronts accompany the boundaries of these currents. The Brazil Current and sub-Antarctic fronts are well separated in the central basin. Brazil Current density fronts are deep and extend from the surface to 3000 m, while the associated temperature and salinity fronts are intermittent over this depth interval. Temperature fronts virtually vanish at the interface between the Antarctic Intermediate Water and the North Atlantic Deep Water. Salinity fronts reverse their polarity beneath the core of the former. At depths between 3000 and 4000 m, abyssal temperature and salinity fronts are observed which are largely density compensating. At the subantarctic and cold core eddy fronts, horizontal temperature and salinity gradients in the upper mixed layer compensate each other in such a way that no density front is found. Deep subpycnocline mixed layers occur in the poleward lobes of the Brazil Current during austral spring, suggestive of previous winter convection.
  4. Kouketsu, Shinya, et al. “Deep ocean heat content changes estimated from observation and reanalysis product and their influence on sea level change.” Journal of Geophysical Research: Oceans 116.C3 (2011). We calculated basin‐scale and global ocean decadal temperature change rates from the 1990s to the 2000s for waters below 3000 m. Large temperature increases were detected around Antarctica, and a relatively large temperature increase was detected along the northward path of Circumpolar Deep Water in the Pacific. The global heat content (HC) change estimated from the temperature change rates below 3000 m was 0.8 × 1022 J decade−1; a value that cannot be neglected for precise estimation of the global heat balance. We reproduced the observed temperature changes in the deep ocean using a data assimilation system and examined virtual observations in the reproduced data field to evaluate the uncertainty of the HC changes estimated from the actual temporally and spatially sparse observations. From the analysis of the virtual observations, it is shown that the global HC increase below 3000 m during recent decades can be detected using the available observation system of periodic revisits to the same sampling sections, although the uncertainty is large. LINK TO FULL TEXT; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JC006464
  5. Piecuch, Christopher G., et al. “Sensitivity of contemporary sea level trends in a global ocean state estimate to effects of geothermal fluxes.” Ocean Modelling 96 (2015): 214-220. Geothermal fluxes constitute a sizable fraction of the present-day Earth net radiative imbalance and corresponding ocean heat uptake. Model simulations of contemporary sea level that impose a geothermal flux boundary condition are becoming increasingly common. To quantify the impact of geothermal fluxes on model estimates of contemporary (1993–2010) sea level changes, two ocean circulation model experiments are compared. The two simulations are based on a global ocean state estimate, produced by the Estimating the Circulation and Climate of the Ocean (ECCO) consortium, and differ only with regard to whether geothermal forcing is applied as a boundary condition. Geothermal forcing raises the global-mean sea level trend by 0.11 mm yr−1 in the perturbation experiment by suppressing a cooling trend present in the baseline solution below 2000 m. The imposed forcing also affects regional sea level trends. The Southern Ocean is particularly sensitive. In this region, anomalous heat redistribution due to geothermal fluxes results in steric height trends of up to ± 1 mm yr−1 in the perturbation experiment relative to the baseline simulation. Analysis of a passive tracer experiment suggests that the geothermal input itself is transported by horizontal diffusion, resulting in more thermal expansion over deeper ocean basins. Thermal expansion in the perturbation simulation gives rise to bottom pressure increase over shallower regions and decrease over deeper areas relative to the baseline run, consistent with mass redistribution expected for deep ocean warming. These results elucidate the influence of geothermal fluxes on sea level rise and global heat budgets in model simulations of contemporary ocean circulation and climate.
  6. Purkey, Sarah G., and Gregory C. Johnson. “Antarctic Bottom Water warming and freshening: Contributions to sea level rise, ocean freshwater budgets, and global heat gain.” Journal of Climate 26.16 (2013): 6105-6122. Freshening and warming of Antarctic Bottom Water (AABW) between the 1980s and 2000s are quantified, assessing the relative contributions of water-mass changes and isotherm heave. The analysis uses highly accurate, full-depth, ship-based, conductivity–temperature–depth measurements taken along repeated oceanographic sections around the Southern Ocean. Fresher varieties of AABW are present within the South Pacific and south Indian Oceans in the 2000s compared to the 1990s, with the strongest freshening in the newest waters adjacent to the Antarctic continental slope and rise indicating a recent shift in the salinity of AABW produced in this region. Bottom waters in the Weddell Sea exhibit significantly less water-mass freshening than those in the other two southern basins. However, a decrease in the volume of the coldest, deepest waters is observed throughout the entire Southern Ocean. This isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential temperature maximum. The water-mass freshening of AABW in the Indian and Pacific Ocean sectors is equivalent to a freshwater flux of 73 ± 26 Gt yr−1, roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Isotherm heave integrated below 2000 m and south of 30°S equates to a net heat uptake of 34 ± 14 TW of excess energy entering the deep ocean from deep volume loss of AABW and 0.37 ± 0.15 mm yr−1 of sea level rise from associated thermal expansion. LINK to Full Text: https://journals.ametsoc.org/jcli/article/26/16/6105/33911

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