Igor Yashayaev - Academia.edu (original) (raw)

Papers by Igor Yashayaev

Oceanography, Dec 1, 2021

Journal Of Geophysical Research: Oceans, Aug 1, 2019

AGU Fall Meeting Abstracts, Dec 1, 2012

AGUFM, Dec 1, 2006

ABSTRACT

AGUFM, Dec 1, 2010

ABSTRACT The mechanism of variability of North Atlantic sub-polar gyre (SPG) and its relation to ... more ABSTRACT The mechanism of variability of North Atlantic sub-polar gyre (SPG) and its relation to the North Atlantic Oscillation (NAO)is investigated in three model experiments. The first two are forced with idealized positive NAO+ and negative NAO- forcing and the third one with the NCEP/NCAR reanalysis. The results herein suggest that the decadal variability of volume transport, SST in the North Atlantic Current (SSTA1) and the Irminger Water temperature(IWT) in the NAO- run have two to three times smaller magnitude than in the NAO+ case. The decadal variations of the strength of circulations the NAO+ run is co-varies negatively with SSTA1 and IWT anomalies. Similar co-variability of these parameters was not found in the NAO- run. The model simulations forced with the NCEP/NCAR reanalysis from 1958 to 2005 show a presence of shift in the sub-polar ocean response to the atmospheric variability at decadal time scale in early 1980s. The amplitude of quasi-decadal variability of the IWT and SSTA1 and their correlation were stronger after 1980 (r=0.79) than in the period between 1958 and 1980 (r=0.1). The IWT after 1980 was well correlated (r=0.67) to the the sub-polar gyre transport index (SPGI, defined as the minimum value of the annual mean anomaly of the model barotropic stream function). This correlation was weaker and negative (r=-0.09) in the period from 1958 to 1981. We explain the shift in the co-variability of the SSTA1, IWT and SPGI with the antisymmetric response of the SPG to atmospheric variations at decadal time scale under positive and negative NAO index and the change of the NAO sign in the early 1980s from predominantly negative phase to positive.

Progress in Oceanography, Aug 1, 2022

Marine Environmental Research, Apr 1, 2019

Zooplankton form a trophic link between primary producers and higher trophic levels, and exert si... more Zooplankton form a trophic link between primary producers and higher trophic levels, and exert significant influence on the vertical transport of carbon through the water column ('biological carbon pump'). Using a MultiNet we sampled and studied mesozooplankton communities (i.e. > 0.2 mm) from six locations around Bermuda targeting four depth zones: ∼0-200 m, ∼200-400 m, ∼400-600 m (deep-scattering layer), and ∼600-800 m. Copepoda, our focal taxonomic group, consistently dominated samples (∼80% relative abundance). We report declines in zooplankton and copepod abundance with depth, concurrent with decreases in food availability. Taxonomic richness was lowest at depth and below the deep-scattering layer. In contrast, copepod diversity peaked at these depths, suggesting lower competitive displacement in these more food-limited waters. Finally, omnivory and carnivory, were the dominant trophic traits, each one affecting the biological carbon pump in a different way. This highlights the importance of incorporating data on zooplankton food web structure in future modelling of global ocean carbon cycling.

Irminger Water (IW) is a prominent water mass in the subpolar North Atlantic (SPNA). It is warm a... more Irminger Water (IW) is a prominent water mass in the subpolar North Atlantic (SPNA). It is warm and saline and originates from the North Atlantic Current and the Irminger Current. The water mass delivers anomalously large amounts of heat and salt to the Labrador Sea. Like any other water mass, IW is subject to temporal and spatial variability, which needs to be adequately identified and tracked.To separate IW from ambient waters, previous studies identified IW at different times using static thresholds of salinity, temperature, and density (i.e., constant over time within the individual studies). However, given the tremendous variability in the region, such static definitions often do not detect IW sufficiently since these definitions do not account for shifts in the large-scale hydrographic state of the SPNA. To address this issue, this study aims to identify non-static thresholds (i.e., incorporating temporal variability) to analyze IW variability. We refer to the method of identifying IW based on non-static thresholds as the phenomenological approach. To do so, we utilize the observation-based data set ARMOR3D between 1993 and 2022. This new approach allows us to compare estimates of IW properties and volume transports to respective estimates obtained from the static approach.In the case of the static approach being applied to the AR7W section in the eastern part of the Labrador Sea as a test region, the water column was anomalously saline in years of high IW volume transport. Hence, the static approach identified more IW and thus overestimated its volume transport. In contrast, the water column was anomalously fresh in years when the static approach reveals a low IW volume transport. Hence, applying the static approach, less IW is identified, and thus its volume transport is underestimated. In contrast, the phenomenological approach reveals less pronounced decadal variability of the IW volume transport.Applying a static IW definition will likely create stronger gradients between IW and ambient water masses when both are fresher. In turn, these gradients may impose or modulate unrealistic changes in the IW volume transport simply because the actual boundary of IW does not coincide with a certain isohaline or isotherm. Any correlated change or shift in IW properties and, for example, Labrador Sea Water will relocate the IW boundary causing the transport to change. The phenomenological approach introduced in our study resolves this issue.

Journal Of Geophysical Research: Oceans, Nov 28, 2022

The Subpolar North Atlantic plays a critical role in the formation of the deep water masses which... more The Subpolar North Atlantic plays a critical role in the formation of the deep water masses which drive Atlantic Meridional Overturning Circulation (AMOC). Labrador Sea Water (LSW) is formed in the Labrador Sea and exported predominantly via the Deep Western Boundary Current (DWBC). The DWBC is an essential component of the AMOC advecting deep waters southward, flowing at depth along the continental slope of the western Atlantic. By combining sustained hydrographic observations from the Labrador Sea to 26.5°N, we investigate the signal propagation and advective timescales of LSW via the DWBC from its source region to the Tropical Atlantic through various approaches using robust neutral density classifications. Two individually defined LSW classes are observed to advect on timescales that support a new plausible hydrographically observed advective pathway. We find each LSW class to advect on independent timescales, and validate a hypothesized alternative‐interior advection pathway branching from the DWBC by observing the arrival of LSW outside of the DWBC in the Bermuda basin on timescales similar to arriving at 26.5°N, 10–15 yr after leaving the source region. Advective timescales estimated herein indicate that this interior pathway is likely the main advective pathway; it remains uncertain whether a direct pathway plays a significant advective role. Using LSW convective signals as advective tracers along the DWBC permits the estimation of advective timescales from the subpolar to tropical latitudes, illuminating deep water advection pathways across the North Atlantic and the lower‐limb of AMOC as a whole.

Nature Communications, Feb 2, 2022

The original version of this Article contained an error in the caption of Figure 2, in which the ... more The original version of this Article contained an error in the caption of Figure 2, in which the total Ekman transport was incorrectly listed as −0.7 ± 0.01 Sv. The correct value is −1.5 ± 0.02 Sv. The corrected article also has an updated data product link in Reference 56. These have been corrected in both the PDF and HTML versions of the Article.

Communications earth & environment, Mar 27, 2024

Labrador Sea winter convection forms a cold, fresh and dense water mass, Labrador Sea Water, that... more Labrador Sea winter convection forms a cold, fresh and dense water mass, Labrador Sea Water, that sinks to the intermediate and deep layers and spreads across the ocean. Convective mixing undergoes multi-year cycles of intensification (deepening) and relaxation (shoaling), which have been also shown to modulate long-term changes in the atmospheric gas uptake by the sea. Here I analyze Argo float and ship-based observations to document the 2012-2023 convective cycle. I find that the highest winter cooling for the 1994-2023 period was in 2015, while the deepest convection for the 1996-2023 period was in 2018. Convective mixing continued to deepen after 2015 because the 2012-2015 winter mixing events preconditioned the water column to be susceptible to deep convection in three more years. The progressively intensified 2012-2018 winter convections generated the largest and densest class of Labrador Sea Water since 1995. Convection weakened afterwards, rapidly shoaling by 800 m per year in the winters of 2021 and 2023. Distinct processes were responsible for these two convective shutdowns. In 2021, a collapse and an eastward shift of the stratospheric polar vortex, and a weakening and a southwestward shift of the Icelandic Low resulted in extremely low surface cooling and convection depth. In 2023, by contrast, convective shutdown was caused by extensive upper layer freshening originated from extreme Arctic sea-ice melt due to Arctic Amplification of Global Warming. The Labrador Sea is the coldest (Fig. 1) and freshest deep subpolar North Atlantic (SPNA) basin 1 where intense vertical mixing driven by high winter surface heat losses (WSHL) produces Labrador Sea Water (LSW)-a major North Atlantic intermediate-depth water mass 2-8. Cold fresh dense oxygenrich LSW spreads across the ocean renewing and ventilating its intermediate and deeper layers 9-15. The LSW formation process, deep convection, defines interannual and longer-term trends in these layers 15-20 , spins the North Atlantic Subpolar Gyre 21 and affects the exchanges between the Subpolar and Subtropical Gyres 22,23. LSW enters 19,24 and, arguably, controls 25,26 the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The contribution of the Labrador Sea to the AMOC was recently challenged by the Overturning of the Subpolar North Atlantic Program (OSNAP), reporting it to be much lower 27 than, e.g., the volumetricallydefined LSW export 7. It should be noted that there are arguable reasons for an underestimation of the Labrador Sea contribution based on the OSNAP observations. Only a few of these reasons are given below. The interior LSW recirculation and exit flows, present on the seaward sides of the moorings, are not accounted for by the OSNAP array. LSW spreading from the Labrador Sea (Fig. 1d, e) to the Irminger Sea and Iceland Basin 10-12 is entrained by the boundary currents and dense overflows in vicinities of the continental slope and rise, and Mid-Atlantic Ridge 19 , thus contributing to the AMOC outside the LSW source (the entrainment of LSW by the Iceland-Scotland Overflow Water will be revisited in the "Results" section). Located outside the deep convection zone in the central Labrador Sea, the OSNAP array is exposed to convection in the eastern part, reducing the Labrador Sea contribution there. Lacking vertical resolution and accuracy, the mooring-based density measurements underestimate the LSW thickness changes entering the transport calculations. Identifying and understanding the limitations, omissions, and inconsistencies in the OSNAP observations will allow us to address and resolve the controversies in results and improve the program design.

AGU Fall Meeting Abstracts, Dec 1, 2010

Since 1995, the annual occupation of AR7W in the Labrador Sea has usually included LADCP data in ... more Since 1995, the annual occupation of AR7W in the Labrador Sea has usually included LADCP data in addition to hydrographic measurements and tracers. We have previously presented results discussing the section-wide circulation for particular years, comparison with geostrophic velocities, and heat flux as determined from individual as well as composite sections. In this work, we present boundary current transports for

American Geophysical Union eBooks, Feb 1, 2016

AGUFM, Dec 1, 2006

ABSTRACT

EAEJA, Apr 1, 2003

ABSTRACT

Ocean Sciences Meeting 2020, Feb 17, 2020

Oceanography, Dec 1, 2021

Journal Of Geophysical Research: Oceans, Aug 1, 2019

AGU Fall Meeting Abstracts, Dec 1, 2012

AGUFM, Dec 1, 2006

ABSTRACT

AGUFM, Dec 1, 2010

ABSTRACT The mechanism of variability of North Atlantic sub-polar gyre (SPG) and its relation to ... more ABSTRACT The mechanism of variability of North Atlantic sub-polar gyre (SPG) and its relation to the North Atlantic Oscillation (NAO)is investigated in three model experiments. The first two are forced with idealized positive NAO+ and negative NAO- forcing and the third one with the NCEP/NCAR reanalysis. The results herein suggest that the decadal variability of volume transport, SST in the North Atlantic Current (SSTA1) and the Irminger Water temperature(IWT) in the NAO- run have two to three times smaller magnitude than in the NAO+ case. The decadal variations of the strength of circulations the NAO+ run is co-varies negatively with SSTA1 and IWT anomalies. Similar co-variability of these parameters was not found in the NAO- run. The model simulations forced with the NCEP/NCAR reanalysis from 1958 to 2005 show a presence of shift in the sub-polar ocean response to the atmospheric variability at decadal time scale in early 1980s. The amplitude of quasi-decadal variability of the IWT and SSTA1 and their correlation were stronger after 1980 (r=0.79) than in the period between 1958 and 1980 (r=0.1). The IWT after 1980 was well correlated (r=0.67) to the the sub-polar gyre transport index (SPGI, defined as the minimum value of the annual mean anomaly of the model barotropic stream function). This correlation was weaker and negative (r=-0.09) in the period from 1958 to 1981. We explain the shift in the co-variability of the SSTA1, IWT and SPGI with the antisymmetric response of the SPG to atmospheric variations at decadal time scale under positive and negative NAO index and the change of the NAO sign in the early 1980s from predominantly negative phase to positive.

Progress in Oceanography, Aug 1, 2022

Marine Environmental Research, Apr 1, 2019

Zooplankton form a trophic link between primary producers and higher trophic levels, and exert si... more Zooplankton form a trophic link between primary producers and higher trophic levels, and exert significant influence on the vertical transport of carbon through the water column ('biological carbon pump'). Using a MultiNet we sampled and studied mesozooplankton communities (i.e. > 0.2 mm) from six locations around Bermuda targeting four depth zones: ∼0-200 m, ∼200-400 m, ∼400-600 m (deep-scattering layer), and ∼600-800 m. Copepoda, our focal taxonomic group, consistently dominated samples (∼80% relative abundance). We report declines in zooplankton and copepod abundance with depth, concurrent with decreases in food availability. Taxonomic richness was lowest at depth and below the deep-scattering layer. In contrast, copepod diversity peaked at these depths, suggesting lower competitive displacement in these more food-limited waters. Finally, omnivory and carnivory, were the dominant trophic traits, each one affecting the biological carbon pump in a different way. This highlights the importance of incorporating data on zooplankton food web structure in future modelling of global ocean carbon cycling.

Irminger Water (IW) is a prominent water mass in the subpolar North Atlantic (SPNA). It is warm a... more Irminger Water (IW) is a prominent water mass in the subpolar North Atlantic (SPNA). It is warm and saline and originates from the North Atlantic Current and the Irminger Current. The water mass delivers anomalously large amounts of heat and salt to the Labrador Sea. Like any other water mass, IW is subject to temporal and spatial variability, which needs to be adequately identified and tracked.To separate IW from ambient waters, previous studies identified IW at different times using static thresholds of salinity, temperature, and density (i.e., constant over time within the individual studies). However, given the tremendous variability in the region, such static definitions often do not detect IW sufficiently since these definitions do not account for shifts in the large-scale hydrographic state of the SPNA. To address this issue, this study aims to identify non-static thresholds (i.e., incorporating temporal variability) to analyze IW variability. We refer to the method of identifying IW based on non-static thresholds as the phenomenological approach. To do so, we utilize the observation-based data set ARMOR3D between 1993 and 2022. This new approach allows us to compare estimates of IW properties and volume transports to respective estimates obtained from the static approach.In the case of the static approach being applied to the AR7W section in the eastern part of the Labrador Sea as a test region, the water column was anomalously saline in years of high IW volume transport. Hence, the static approach identified more IW and thus overestimated its volume transport. In contrast, the water column was anomalously fresh in years when the static approach reveals a low IW volume transport. Hence, applying the static approach, less IW is identified, and thus its volume transport is underestimated. In contrast, the phenomenological approach reveals less pronounced decadal variability of the IW volume transport.Applying a static IW definition will likely create stronger gradients between IW and ambient water masses when both are fresher. In turn, these gradients may impose or modulate unrealistic changes in the IW volume transport simply because the actual boundary of IW does not coincide with a certain isohaline or isotherm. Any correlated change or shift in IW properties and, for example, Labrador Sea Water will relocate the IW boundary causing the transport to change. The phenomenological approach introduced in our study resolves this issue.

Journal Of Geophysical Research: Oceans, Nov 28, 2022

The Subpolar North Atlantic plays a critical role in the formation of the deep water masses which... more The Subpolar North Atlantic plays a critical role in the formation of the deep water masses which drive Atlantic Meridional Overturning Circulation (AMOC). Labrador Sea Water (LSW) is formed in the Labrador Sea and exported predominantly via the Deep Western Boundary Current (DWBC). The DWBC is an essential component of the AMOC advecting deep waters southward, flowing at depth along the continental slope of the western Atlantic. By combining sustained hydrographic observations from the Labrador Sea to 26.5°N, we investigate the signal propagation and advective timescales of LSW via the DWBC from its source region to the Tropical Atlantic through various approaches using robust neutral density classifications. Two individually defined LSW classes are observed to advect on timescales that support a new plausible hydrographically observed advective pathway. We find each LSW class to advect on independent timescales, and validate a hypothesized alternative‐interior advection pathway branching from the DWBC by observing the arrival of LSW outside of the DWBC in the Bermuda basin on timescales similar to arriving at 26.5°N, 10–15 yr after leaving the source region. Advective timescales estimated herein indicate that this interior pathway is likely the main advective pathway; it remains uncertain whether a direct pathway plays a significant advective role. Using LSW convective signals as advective tracers along the DWBC permits the estimation of advective timescales from the subpolar to tropical latitudes, illuminating deep water advection pathways across the North Atlantic and the lower‐limb of AMOC as a whole.

Nature Communications, Feb 2, 2022

The original version of this Article contained an error in the caption of Figure 2, in which the ... more The original version of this Article contained an error in the caption of Figure 2, in which the total Ekman transport was incorrectly listed as −0.7 ± 0.01 Sv. The correct value is −1.5 ± 0.02 Sv. The corrected article also has an updated data product link in Reference 56. These have been corrected in both the PDF and HTML versions of the Article.

Communications earth & environment, Mar 27, 2024

Labrador Sea winter convection forms a cold, fresh and dense water mass, Labrador Sea Water, that... more Labrador Sea winter convection forms a cold, fresh and dense water mass, Labrador Sea Water, that sinks to the intermediate and deep layers and spreads across the ocean. Convective mixing undergoes multi-year cycles of intensification (deepening) and relaxation (shoaling), which have been also shown to modulate long-term changes in the atmospheric gas uptake by the sea. Here I analyze Argo float and ship-based observations to document the 2012-2023 convective cycle. I find that the highest winter cooling for the 1994-2023 period was in 2015, while the deepest convection for the 1996-2023 period was in 2018. Convective mixing continued to deepen after 2015 because the 2012-2015 winter mixing events preconditioned the water column to be susceptible to deep convection in three more years. The progressively intensified 2012-2018 winter convections generated the largest and densest class of Labrador Sea Water since 1995. Convection weakened afterwards, rapidly shoaling by 800 m per year in the winters of 2021 and 2023. Distinct processes were responsible for these two convective shutdowns. In 2021, a collapse and an eastward shift of the stratospheric polar vortex, and a weakening and a southwestward shift of the Icelandic Low resulted in extremely low surface cooling and convection depth. In 2023, by contrast, convective shutdown was caused by extensive upper layer freshening originated from extreme Arctic sea-ice melt due to Arctic Amplification of Global Warming. The Labrador Sea is the coldest (Fig. 1) and freshest deep subpolar North Atlantic (SPNA) basin 1 where intense vertical mixing driven by high winter surface heat losses (WSHL) produces Labrador Sea Water (LSW)-a major North Atlantic intermediate-depth water mass 2-8. Cold fresh dense oxygenrich LSW spreads across the ocean renewing and ventilating its intermediate and deeper layers 9-15. The LSW formation process, deep convection, defines interannual and longer-term trends in these layers 15-20 , spins the North Atlantic Subpolar Gyre 21 and affects the exchanges between the Subpolar and Subtropical Gyres 22,23. LSW enters 19,24 and, arguably, controls 25,26 the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The contribution of the Labrador Sea to the AMOC was recently challenged by the Overturning of the Subpolar North Atlantic Program (OSNAP), reporting it to be much lower 27 than, e.g., the volumetricallydefined LSW export 7. It should be noted that there are arguable reasons for an underestimation of the Labrador Sea contribution based on the OSNAP observations. Only a few of these reasons are given below. The interior LSW recirculation and exit flows, present on the seaward sides of the moorings, are not accounted for by the OSNAP array. LSW spreading from the Labrador Sea (Fig. 1d, e) to the Irminger Sea and Iceland Basin 10-12 is entrained by the boundary currents and dense overflows in vicinities of the continental slope and rise, and Mid-Atlantic Ridge 19 , thus contributing to the AMOC outside the LSW source (the entrainment of LSW by the Iceland-Scotland Overflow Water will be revisited in the "Results" section). Located outside the deep convection zone in the central Labrador Sea, the OSNAP array is exposed to convection in the eastern part, reducing the Labrador Sea contribution there. Lacking vertical resolution and accuracy, the mooring-based density measurements underestimate the LSW thickness changes entering the transport calculations. Identifying and understanding the limitations, omissions, and inconsistencies in the OSNAP observations will allow us to address and resolve the controversies in results and improve the program design.

AGU Fall Meeting Abstracts, Dec 1, 2010

Since 1995, the annual occupation of AR7W in the Labrador Sea has usually included LADCP data in ... more Since 1995, the annual occupation of AR7W in the Labrador Sea has usually included LADCP data in addition to hydrographic measurements and tracers. We have previously presented results discussing the section-wide circulation for particular years, comparison with geostrophic velocities, and heat flux as determined from individual as well as composite sections. In this work, we present boundary current transports for

American Geophysical Union eBooks, Feb 1, 2016

AGUFM, Dec 1, 2006

ABSTRACT

EAEJA, Apr 1, 2003

ABSTRACT

Ocean Sciences Meeting 2020, Feb 17, 2020