Bio-optical feedbacks among phytoplankton, upper ocean physics and sea-ice in a global model

<p>Sea ice affects the exchange of energy and matter between the atmosphere and the ocean from local to hemispheric scales. Salt fluxes across the ice-ocean interface that drive thermohaline mixing beneath growing sea ice are important elements of upper ocean nutrient and carbon exchange. Sea-ice melt releases freshwater into the upper ocean and results in formation of melt ponds that affect gas and energy transfer across the atmosphere-ice interface. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) provided an opportunity to follow sea-ice evolution and exchange processes over a full seasonal cycle in a rapidly changing ice cover. To this end, approximately 25 sea-ice cores were collected at 2 distinct sites, representing first-year and multi-year ice, to monitor physical, biological and geochemical processes relevant to atmosphere-ice-ocean exchange processes. Here we compare the growth and decay of first-year ice in the Central Arctic during the winter 2019-2020 to that of landfast first-year ice at Utqia&#289;vik, Alaska, from 1998 to 2016. Ice stratigraphy was similar at both sites with about 15 cm of granular ice on top of columnar ice, with a comparable growth history with a similar maximum ice thickness of 1.6-1.7 m. We aggregated the sea-ice bulk salinity and temperature profiles using a degree-day approach, and examined brine and freshwater fluxes at lower and upper interfaces of the ice, respectively. Preliminary results show lower sea-ice bulk salinity during the growth season and greater desalination at the ice surface during the melt season at the MOSAiC floe in comparison to Utqia&#289;vik.</p>

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Reduced Stability of the Atlantic Meridional Overturning Circulation due to Wind Stress Feedback during Glacial Times

Journal of Climate ◽

10.1175/2008jcli2291.1 ◽

2008 ◽

Vol 21 (23) ◽

pp. 6260-6282 ◽

Cited By ~ 14

Author(s):

Olivier Arzel ◽

Matthew H. England ◽

Willem P. Sijp

Keyword(s):

Sea Ice ◽

Wind Stress ◽

Surface Wind ◽

Meridional Overturning Circulation ◽

Nonlinear Dependence ◽

Upper Ocean ◽

Overturning Circulation ◽

Climate Conditions ◽

Meridional Overturning ◽

The Impact

Abstract A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

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Marine Bioluminescence and Upper Ocean Physics: Seasonal Changes in the Northeast Atlantic

Oceanography ◽

10.5670/oceanog.1989.29 ◽

1989 ◽

Vol 2 (1) ◽

pp. 36-36 ◽

Cited By ~ 10

Author(s):

John Marra

Keyword(s):

Seasonal Changes ◽

Upper Ocean ◽

Northeast Atlantic ◽

Ocean Physics

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Upper ocean stratification and sea ice growth rates during the summer-fall transition, as revealed by Elephant seal foraging in the Adélie Depression, East Antarctica

Ocean Science ◽

10.5194/os-7-185-2011 ◽

2011 ◽

Vol 7 (2) ◽

pp. 185-202 ◽

Cited By ~ 19

Author(s):

G. D. Williams ◽

M. Hindell ◽

M.-N. Houssais ◽

T. Tamura ◽

I. C. Field

Keyword(s):

Sea Ice ◽

Continental Shelf ◽

Mixed Layer ◽

Growth Rates ◽

Shelf Break ◽

Upper Ocean ◽

Ocean Stratification ◽

Glacier Tongue ◽

Ice Growth ◽

Continental Shelf Break

Abstract. Southern elephant seals (Mirounga leonina), fitted with Conductivity-Temperature-Depth sensors at Macquarie Island in January 2005 and 2010, collected unique oceanographic observations of the Adélie and George V Land continental shelf (140–148° E) during the summer-fall transition (late February through April). This is a key region of dense shelf water formation from enhanced sea ice growth/brine rejection in the local coastal polynyas. In 2005, two seals occupied the continental shelf break near the grounded icebergs at the northern end of the Mertz Glacier Tongue for several weeks from the end of February. One of the seals migrated west to the Dibble Ice Tongue, apparently utilising the Antarctic Slope Front current near the continental shelf break. In 2010, immediately after that year's calving of the Mertz Glacier Tongue, two seals migrated to the same region but penetrated much further southwest across the Adélie Depression and sampled the Commonwealth Bay polynya from March through April. Here we present observations of the regional oceanography during the summer-fall transition, in particular (i) the zonal distribution of modified Circumpolar Deep Water exchange across the shelf break, (ii) the upper ocean stratification across the Adélie Depression, including alongside iceberg C-28 that calved from the Mertz Glacier and (iii) the convective overturning of the deep remnant seasonal mixed layer in Commonwealth Bay from sea ice growth. Heat and freshwater budgets to 200–300 m are used to estimate the ocean heat content (400→50 MJ m−2), flux (50–200 W m−2 loss) and sea ice growth rates (maximum of 7.5–12.5 cm day−1). Mean seal-derived sea ice growth rates were within the range of satellite-derived estimates from 1992–2007 using ERA-Interim data. We speculate that the continuous foraging by the seals within Commonwealth Bay during the summer/fall transition was due to favorable feeding conditions resulting from the convective overturning of the deep seasonal mixed layer and chlorophyll maximum that is a reported feature of this location.

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Response of the Atlantic Thermohaline Circulation to Increased Atmospheric CO2 in a Coupled Model

Journal of Climate ◽

10.1175/jcli3208.1 ◽

2004 ◽

Vol 17 (21) ◽

pp. 4267-4279 ◽

Cited By ~ 39

Author(s):

Aixue Hu ◽

Gerald A. Meehl ◽

Warren M. Washington ◽

Aiguo Dai

Keyword(s):

Sea Ice ◽

Heat Transport ◽

North Atlantic ◽

Thermohaline Circulation ◽

Climate Model ◽

Coupled Model ◽

The Arctic ◽

Labrador Sea ◽

Upper Ocean ◽

Gin Seas

Abstract Changes in the thermohaline circulation (THC) due to increased CO2 are important in future climate regimes. Using a coupled climate model, the Parallel Climate Model (PCM), regional responses of the THC in the North Atlantic to increased CO2 and the underlying physical processes are studied here. The Atlantic THC shows a 20-yr cycle in the control run, qualitatively agreeing with other modeling results. Compared with the control run, the simulated maximum of the Atlantic THC weakens by about 5 Sv (1 Sv ≡ 106 m3 s−1) or 14% in an ensemble of transient experiments with a 1% CO2 increase per year at the time of CO2 doubling. The weakening of the THC is accompanied by reduced poleward heat transport in the midlatitude North Atlantic. Analyses show that oceanic deep convective activity strengthens significantly in the Greenland–Iceland–Norway (GIN) Seas owing to a saltier (denser) upper ocean, but weakens in the Labrador Sea due to a fresher (lighter) upper ocean and in the south of the Denmark Strait region (SDSR) because of surface warming. The saltiness of the GIN Seas are mainly caused by an increased salty North Atlantic inflow, and reduced sea ice volume fluxes from the Arctic into this region. The warmer SDSR is induced by a reduced heat loss to the atmosphere, and a reduced sea ice flux into this region, resulting in less heat being used to melt ice. Thus, sea ice–related salinity effects appear to be more important in the GIN Seas, but sea ice–melt-related thermal effects seem to be more important in the SDSR region. On the other hand, the fresher Labrador Sea is mainly attributed to increased precipitation. These regional changes produce the overall weakening of the THC in the Labrador Sea and SDSR, and more vigorous ocean overturning in the GIN Seas. The northward heat transport south of 60°N is reduced with increased CO2, but increased north of 60°N due to the increased flow of North Atlantic water across this latitude.

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Changes in the upper ocean mixed layer and phytoplankton productivity along the West Antarctic Peninsula

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2017.0173 ◽

2018 ◽

Vol 376 (2122) ◽

pp. 20170173 ◽

Cited By ~ 20

Author(s):

Oscar Schofield ◽

Michael Brown ◽

Josh Kohut ◽

Schuyler Nardelli ◽

Grace Saba ◽

...

Keyword(s):

Sea Ice ◽

Antarctic Peninsula ◽

Mixed Layer ◽

Carbon Fixation ◽

Upper Ocean ◽

The West ◽

Phytoplankton Productivity ◽

West Antarctic ◽

West Antarctic Peninsula

The West Antarctic Peninsula (WAP) has experienced significant change over the last 50 years. Using a 24 year spatial time series collected by the Palmer Long Term Ecological Research programme, we assessed long-term patterns in the sea ice, upper mixed layer depth (MLD) and phytoplankton productivity. The number of sea ice days steadily declined from the 1980s until a recent reversal that began in 2008. Results show regional differences between the northern and southern regions sampled during regional ship surveys conducted each austral summer. In the southern WAP, upper ocean MLD has shallowed by a factor of 2. Associated with the shallower mixed layer is enhanced phytoplankton carbon fixation. In the north, significant interannual variability resulted in the mixed layer showing no trended change over time and there was no significant increase in the phytoplankton productivity. Associated with the recent increases in sea ice there has been an increase in the photosynthetic efficiency (chlorophyll a -normalized carbon fixation) in the northern and southern regions of the WAP. We hypothesize the increase in sea ice results in increased micronutrient delivery to the continental shelf which in turn leads to enhanced photosynthetic performance. This article is part of the theme issue ‘The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change’.

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Hydrographic Preconditioning for Seasonal Sea Ice Anomalies in the Labrador Sea

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-064.1 ◽

2013 ◽

Vol 43 (5) ◽

pp. 863-883 ◽

Cited By ~ 10

Author(s):

Ian Fenty ◽

Patrick Heimbach

Keyword(s):

Sea Ice ◽

Mixed Layer ◽

Critical Role ◽

The Arctic ◽

Labrador Sea ◽

Upper Ocean ◽

Quasi Equilibrium ◽

Low Salinity ◽

Thermohaline Front ◽

Subsurface Waters

Abstract This study investigates the hydrographic processes involved in setting the maximum wintertime sea ice (SI) extent in the Labrador Sea and Baffin Bay. The analysis is based on an ocean and sea ice state estimate covering the summer-to-summer 1996/97 annual cycle. The estimate is a synthesis of in situ and satellite hydrographic and ice data with a regional coupled ⅓° ocean–sea ice model. SI advective processes are first demonstrated to be required to reproduce the observed ice extent. With advection, the marginal ice zone (MIZ) location stabilizes where ice melt balances ice mass convergence, a quasi-equilibrium condition achieved via the convergence of warm subtropical-origin subsurface waters into the mixed layer seaward of the MIZ. An analysis of ocean surface buoyancy fluxes reveals a critical role of low-salinity upper ocean (100 m) anomalies for the advancement of SI seaward of the Arctic Water–Irminger Water Thermohaline Front. Anomalous low-salinity waters slow the rate of buoyancy loss–driven mixed layer deepening, shielding an advancing SI pack from the warm subsurface waters, and are conducive to a positive surface meltwater stabilization enhancement (MESEM) feedback driven by SI meltwater release. The low-salinity upper-ocean hydrographic conditions in which the MESEM efficiently operates are termed sea ice–preconditioned waters (SIPW). The SI extent seaward of the Thermohaline Front is shown to closely correspond to the distribution of SIPW. The analysis of two additional state estimates (1992/93, 2003/04) suggests that interannual hydrographic variability provides a first-order explanation for SI maximum extent anomalies in the region.

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Employing Satellite-Derived Sea Ice Concentration to Constrain Upper-Ocean Temperature in a Global Ocean GCM

Journal of Climate ◽

10.1175/2008jcli2256.1 ◽

2008 ◽

Vol 21 (17) ◽

pp. 4498-4513 ◽

Cited By ~ 12

Author(s):

Achim Stössel

Keyword(s):

Sea Ice ◽

Deep Ocean ◽

Global Ocean ◽

Upper Ocean ◽

Ocean Temperature ◽

Freshwater Input ◽

Sea Ice Concentration ◽

Ice Concentration ◽

Ocean Properties

Abstract The quality of Southern Ocean sea ice simulations in a global ocean general circulation model (GCM) depends decisively on the simulated upper-ocean temperature. This is confirmed by assimilating satellite-derived sea ice concentration to constrain the upper-layer temperature of a sea ice–ocean GCM. The resolution of the model’s sea ice component is about 22 km and thus comparable to the pixel resolution of the satellite data. The ocean component is coarse resolution to afford long-term integrations for investigations of the deep-ocean equilibrium response. Besides improving the sea ice simulation considerably, the simulations with constrained upper-ocean temperature yield much more realistic global deep-ocean properties, in particular when combined with glacial freshwater input. Both outcomes are relatively insensitive to the passive-microwave algorithm used to retrieve the ice concentration being assimilated. The sensitivity of the long-term global deep-ocean properties and circulation to the possible freshwater input from ice shelves and to the parameterization of vertical mixing in the Southern Ocean is reevaluated under the new constraint.

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The Impact of Arctic Winter Infrared Radiation on Early Summer Sea Ice

Journal of Climate ◽

10.1175/jcli-d-14-00773.1 ◽

2015 ◽

Vol 28 (15) ◽

pp. 6281-6296 ◽

Cited By ~ 27

Author(s):

Hyo-Seok Park ◽

Sukyoung Lee ◽

Yu Kosaka ◽

Seok-Woo Son ◽

Sang-Woo Kim

Keyword(s):

Sea Ice ◽

Arctic Ocean ◽

Interannual Variability ◽

Radiative Forcing ◽

Heat Content ◽

The Arctic ◽

Upper Ocean ◽

Upper Ocean Heat Content ◽

The Arctic Ocean ◽

Arctic Summer

Abstract The Arctic summer sea ice area has been rapidly decreasing in recent decades. In addition to this trend, substantial interannual variability is present, as is highlighted by the recovery in sea ice area in 2013 following the record minimum in 2012. This interannual variability of the Arctic summer sea ice area has been attributed to the springtime weather disturbances. Here, by utilizing reanalysis- and satellite-based sea ice data, this study shows that summers with unusually small sea ice area are preceded by winters with anomalously strong downward longwave radiation over the Eurasian sector of the Arctic Ocean. This anomalous wintertime radiative forcing at the surface is up to 10–15 W m−2, which is about twice as strong than that during the spring. During the same winters, the poleward moisture and warm-air intrusions into the Eurasian sector of the Arctic Ocean are anomalously strong and the resulting moisture convergence field closely resembles positive anomalies in column-integrated water vapor and tropospheric temperature. Climate model simulations support the above-mentioned findings and further show that the anomalously strong wintertime radiative forcing can decrease sea ice thickness over wide areas of the Arctic Ocean, especially over the Eurasian sector. During the winters preceding the anomalously small summer sea ice area, the upper ocean of the model is anomalously warm over the Barents Sea, indicating that the upper-ocean heat content contributes to winter sea ice thinning. Finally, mass divergence by ice drift in the preceding winter and spring contributes to the thinning of sea ice over the East Siberian and Chukchi Seas, where radiative forcing and upper-ocean heat content anomalies are relatively weak.

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