Water Masses in the Black Sea as Seen by Profiling Floats. A revisit of the role of winter, slope and geothermal convectio.

<p>More than 6000 profiles from profiling floats in the Black Sea over the 2005-2020 period were used to study the ventilation of this basin from the top to the very bottom. In the upper layers and in the main pycnocline, water masses show a strong interannual variability following intermittent events of cold water formation. The density ratio decreased three times during the last 15 years, revealing the decreasing role of temperature in the vertical layering of the Black Sea halocline. The deep transition layer (DTL) between 700 and 1700 m acts as an interface between the baroclinic layer and the largest bottom convective layer (BCL) of the world oceans. On top of DTL are the warm intermediate layer (WIL) and deep cold intermediate layer (DCIL). They both showed strong trends in the last fifteen years due to warmer climate and intensification of warmer intrusions from Bosporus. A “salinity wave” was detected in 2005-2009 below ~1700 m, which evidenced for the first time the penetration of gravity flow from Bosporus down to the bottom. The layering of water masses was explained as resulting from the different distribution of sources of heat and salt, double duffusion and balances between the geothermal and salinity flows in the BCL.</p>

Influence of the Extratropical Ocean Circulation on the Intertropical Convergence Zone in an Idealized Coupled General Circulation Model

The authors present coupled model simulations in which the ocean's meridional overturning circulation (MOC) sets the zonal mean location of the intertropical convergence zone (ITCZ) in the hemisphere with deep-water production. They use a coarse-resolution single-basin sector coupled general circulation model (CGCM) with simplified atmospheric physics and two idealized land–sea distributions. In an equatorially symmetric closed-basin setting, unforced climate asymmetry develops because of the advective circulation–salinity feedback that amplifies the asymmetry of the deep-MOC cell and the upper-ocean meridional salinity transport. It confines the deep-water production and the dominant extratropical ocean heat release to a randomly selected hemisphere. The resultant ocean heat transport (OHT) toward the hemisphere with the deep-water source is partially compensated by the atmospheric heat transport (AHT) across the equator via an asymmetric Hadley circulation, setting the ITCZ in the hemisphere warmed by the ocean. When a circumpolar channel is open at subpolar latitudes, the circumpolar current disrupts the poleward transport of the upper-ocean saline water and suppresses deep-water formation poleward of the channel. The MOC adjusts by lowering the main pycnocline and shifting the deep-water production into the opposite hemisphere from the channel, and the ITCZ location follows the deep-water source again because of the Hadley circulation adjustment to cross-equatorial OHT. The climate response is sensitive to the sill depth of the channel but becomes saturated when the sill is deeper than the main pycnocline depth in subtropics. In simulations with a circumpolar channel, the ITCZ is in the Northern Hemisphere (NH) because of the Southern Hemisphere (SH) circumpolar flow that forces northward OHT.

A comparison of the variability of biological nutrients against depth and potential density

The main biogeochemical nutrient distributions, along with ambient ocean temperature and the light field, control ocean biological productivity. Observations of nutrients are much sparser than physical observations of temperature and salinity, yet it is critical to validate biogeochemical models against these sparse observations if we are to successfully model biological variability and trends. Here we use data from the Bermuda Atlantic Time-series Study and the World Ocean Database 2005 to demonstrate quantitatively that over the entire globe a significant fraction of the temporal variability of phosphate, silicate and nitrate within the oceans is correlated with water density. The temporal variability of these nutrients as a function of depth is almost always greater than as a function of potential density, with he largest reductions in variability found within the main pycnocline. The greater nutrient variability as a function of depth occurs when dynamical processes vertically displace nutrient and density fields together on shorter timescales than biological adjustments. These results show that dynamical processes can have a significant impact on the instantaneous nutrient distributions. These processes must therefore be considered when modeling biogeochemical systems, when comparing such models with observations, or when assimilating data into such models.

Climate-Driven Basin-Scale Decadal Oscillations of Oceanic Phytoplankton

Phytoplankton—the microalgae that populate the upper lit layers of the ocean—fuel the oceanic food web and affect oceanic and atmospheric carbon dioxide levels through photosynthetic carbon fixation. Here, we show that multidecadal changes in global phytoplankton abundances are related to basin-scale oscillations of the physical ocean, specifically the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. This relationship is revealed in ~20 years of satellite observations of chlorophyll and sea surface temperature. Interaction between the main pycnocline and the upper ocean seasonal mixed layer is one mechanism behind this correlation. Our findings provide a context for the interpretation of contemporary changes in global phytoplankton and should improve predictions of their future evolution with climate change.

A comparison of the variability of biological nutrients against depth and potential density

The main biogeochemical nutrient distributions, along with ambient ocean temperature and the light field, control ocean biological productivity. Observations of nutrients are much sparser than physical observations of temperature and salinity, yet it is critical to validate biogeochemical models against these sparse observations if we are to successfully model biological variability and trends. Here we use data from the Bermuda Atlantic Time-series Study and from the World Ocean Database 2005, to demonstrate quantitatively that over the entire globe a significant fraction of the temporal variability of phosphate, silicate and nitrate within the oceans is correlated with water density. The variability of these nutrients with respect to depth and neutral density is estimated and it is shown that in most regions variability against density is significantly reduced. The largest reductions in variability were found within the main pycnocline. This in principle allows nutrient distributions to be inferred from physical hydrographic measurements, a fact that can usefully be applied to modeling, assimilating, and, in the long term, for biogeochemical forecasting.