статья в журнале

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Progress in Oceanography

ISSN:00796611

2023

Mikaelyan, A.S.¹

Zatsepin, A.G.¹

Podymov, O.I.¹

Mosharov, S.A.¹

Pautova, L.A.¹

Fedorov, A.V.¹

Ocherednik, O.A.¹

10.1016/j.pocean.2023.102984

Cyclonic eddies in the ocean often increase primary production and phytoplankton biomass. The main mechanism is the rise of the nutricline in the cyclone's core, which determines the intensification of the ascending flow of deep nutrients into the photic layer. However, some cyclones suppress primary productivity, and the natural processes underlying them are little studied and are of great fundamental interest. In August 2021, a vast cyclonic zone with two mesoscale cyclones occupied the Black Sea deep-water basin. A powerful quasi-tropical atmospheric vortex amplified one of the cyclones, which increased the Ekman pumping and led to strong sea surface cooling by 4 °C. However, the inflow of thermocline water into the euphotic layer and higher nutricline gradients did not lead to elevated depth-integrated concentration of chlorophyll a (Chl) and phytoplankton biomass. Moreover, the surface Chl and the average primary production was less than in the outside waters. The proposed Stratification-Lock hypothesis explains this phenomenon. A combination of several factors led to its appearance. The drastic water cooling, followed by calm weather and vigorous summer heating, led to strong thermal stratification, dampening turbulent mixing in the near-surface layer. At the same time, the velocity and shear of current in and below the thermocline were low in the entire cyclonic zone, which prevented substantial diapycnal mixing generated at depth. As a result, a 'stratification lock' appeared in the entire water column, suppressing nutrients' upward flux. The proposed hypothesis is consistent with the effect of thermal stratification on primary productivity revealed in the entire cyclonic zone. The higher was the thermal stratification the lower was the depth-integrated total phytoplankton biomass and primary production. Also, diatoms developed in the upper layer in waters with weaker thermal stratification. The described stage lasted two weeks and was interrupted by strong wind mixing, which destroyed the 'stratification lock' and caused an increase in Chl at the sea surface. This stage of the cyclone life cycle occurs under a strictly defined combination of hydro-meteorological conditions, suggesting a relatively rare occurrence. Nevertheless, some evidence of the similar effect of a cyclone on primary productivity can be found in other studies, indicating that this natural mechanism, consisting of a complex weather-current-eddy combination, can operate in various marine areas. © 2023 Elsevier Ltd

Cyclonic eddy, Deep chlorophyll maximum, Eddy effect on phytoplankton composition, Effect of stratification on primary production, Flow of deep nutrients, Phytoplankton, Turbulence

2023-04-13 10:42:38