The Impact of Small-Scale Topography on the Dynamical Balance of the Ocean

Abstract The impact of small-scale topography on the ocean’s dynamical balance is investigated by quantifying the rates at which internal wave drag extracts (angular) momentum and vorticity from the general circulation. The calculation exploits the recent advent of two near-global descriptions of topographic roughness on horizontal scales on the order of 1–10 km, which play a central role in the generation of internal lee waves by geostrophic flows impinging on topography and have been hitherto unresolved by bathymetric datasets and ocean general circulation models alike. It is found that, while internal wave drag is a minor contributor to the ocean’s dynamical balance over much of the globe, it is a significant player in the dynamics of extensive areas of the ocean, most notably the Antarctic Circumpolar Current and several regions of enhanced small-scale topographic variance in the equatorial and Southern Hemisphere oceans. There, the contribution of internal wave drag to the ocean’s (angular) momentum and vorticity balances is generally on the order of ten to a few tens of percent of the dominant source and sink terms in each dynamical budget, which are respectively associated with wind forcing and form drag by topography with horizontal scales from 500 to 1000 km. It is thus suggested that the representation of internal wave drag in general circulation models may lead to significant changes in the deep ocean circulation of those regions. A theoretical scaling is derived that captures the basic dependence of internal wave drag on topographic roughness and near-bottom flow speed for most oceanographically relevant regimes.

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Intra-annual variability of Antarctic precipitation from weather forecasts and high-resolution climate models

Annals of Glaciology ◽

10.3189/1998aog27-1-488-494 ◽

1998 ◽

Vol 27 ◽

pp. 488-494 ◽

Cited By ~ 11

Author(s):

Christophe Genthon ◽

Gerhard Krinner ◽

Michel Déqué

Keyword(s):

General Circulation ◽

General Circulation Models ◽

Atmospheric Dynamics ◽

Small Scale ◽

Weather Forecasts ◽

Annual Variability ◽

Climate Simulations ◽

Antarctic Precipitation ◽

The Antarctic ◽

The Impact

The intra-annual variability of Antarctic precipitation from the European Centre for Medium-range Weather Forecasts short-term meteorological forecasts and from climate simulations by the ARPÈGE and LMD-Zoom general circulation models is presented and discussed. The spatial resolution of forecasts and simulations is high over the Antarctic region, about 100 km, so that the impact of topography and small-scale atmospheric dynamics are better resolved than with more conventional model grids (about 300 km). All the models and forecasts show that the seasonality of precipitation is spatially very variable. Meridional coast-to-interior contrasts are marked, but equally strong variations are unexpectedly found where more homogeneity might be expected because of the homogeneity of the environment, e.g. on the high Antarctic plateau. Neither the forecasts nor the simulations confirm that precipitation is mostly maximum in winter over much of East Antarctica as suggested by scarce and potentially unreliable observations (Bromwich, 1988). Spring and fall maxima are quite frequent too, though summer maxima are rare. Daily precipitation statistics show more spatial pattern, with increasingly infrequent precipitation as distance from the coast toward the interior of the ice sheet increases Several aspects of the intra-annual variability of precipitation can be interpreted in terms of atmospheric dynamics, but at both daily and seasonal time-scales the different forecasts and climate simulations often locally and regionally disagree with each other. Discrimination between models and their ability to reproduce the dynamics of Antarctic hydrology, and progress on simulating such aspects of the Antarctic climate, is limited by the lack of reliable observation of precipitation variability.

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Twenty-first century ocean forcing of the Greenland ice sheet for modelling of sea level contribution

The Cryosphere ◽

10.5194/tc-14-985-2020 ◽

2020 ◽

Vol 14 (3) ◽

pp. 985-1008 ◽

Cited By ~ 7

Author(s):

Donald A. Slater ◽

Denis Felikson ◽

Fiamma Straneo ◽

Heiko Goelzer ◽

Christopher M. Little ◽

...

Keyword(s):

Sea Level ◽

General Circulation ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

General Circulation Models ◽

Glacier Retreat ◽

Small Scale ◽

Intergovernmental Panel ◽

Continental Scale ◽

The Impact

Abstract. Changes in ocean temperature and salinity are expected to be an important determinant of the Greenland ice sheet's future sea level contribution. Yet, simulating the impact of these changes in continental-scale ice sheet models remains challenging due to the small scale of key physics, such as fjord circulation and plume dynamics, and poor understanding of critical processes, such as calving and submarine melting. Here we present the ocean forcing strategy for Greenland ice sheet models taking part in the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), the primary community effort to provide 21st century sea level projections for the Intergovernmental Panel on Climate Change Sixth Assessment Report. Beginning from global atmosphere–ocean general circulation models, we describe two complementary approaches to provide ocean boundary conditions for Greenland ice sheet models, termed the “retreat” and “submarine melt” implementations. The retreat implementation parameterises glacier retreat as a function of projected subglacial discharge and ocean thermal forcing, is designed to be implementable by all ice sheet models and results in retreat of around 1 and 15 km by 2100 in RCP2.6 and 8.5 scenarios, respectively. The submarine melt implementation provides estimated submarine melting only, leaving the ice sheet model to solve for the resulting calving and glacier retreat and suggests submarine melt rates will change little under RCP2.6 but will approximately triple by 2100 under RCP8.5. Both implementations have necessarily made use of simplifying assumptions and poorly constrained parameterisations and, as such, further research on submarine melting, calving and fjord–shelf exchange should remain a priority. Nevertheless, the presented framework will allow an ensemble of Greenland ice sheet models to be systematically and consistently forced by the ocean for the first time and should result in a significant improvement in projections of the Greenland ice sheet's contribution to future sea level change.

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On the impact of sea ice in a global ocean circulation model

Annals of Glaciology ◽

10.3189/s0260305500013884 ◽

1997 ◽

Vol 25 ◽

pp. 111-115 ◽

Cited By ~ 3

Author(s):

Achim Stössel

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Ocean Circulation ◽

General Circulation ◽

Thermohaline Circulation ◽

Circulation Model ◽

Deep Ocean ◽

Global Ocean ◽

Convective Activity ◽

The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

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A Time History of Pre- and Post-Bomb Radiocarbon in the Barents Sea Derived from Arcto-Norwegian Cod Otoliths

Radiocarbon ◽

10.1017/s0033822200041515 ◽

2001 ◽

Vol 43 (2B) ◽

pp. 843-855 ◽

Cited By ~ 30

Author(s):

John M Kalish ◽

Reidar Nydal ◽

Kjell H Nedreaas ◽

George S Burr ◽

Gro L Eine

Keyword(s):

Time Series ◽

Ocean Circulation ◽

General Circulation ◽

Barents Sea ◽

Dissolved Inorganic Carbon ◽

Time History ◽

General Circulation Models ◽

Fine Tuning ◽

Birth Date ◽

The Barents Sea

Radiocarbon measured in seawater dissolved inorganic carbon (DIC) can be used to investigate ocean circulation, atmosphere/ocean carbon flux, and provide powerful constraints for the fine-tuning of general circulation models (GCMs). Time series of 14C in seawater are derived most frequently from annual bands of hermatypic corals. However, this proxy is unavailable in temperate and polar oceans. Fish otoliths, calcium carbonate auditory, and gravity receptors in the membranous labyrinths of teleost fishes, can act as proxies for 14C in most oceans and at most depths. Arcto-Norwegian cod otoliths are suited to this application due to the well-defined distribution of this species in the Barents Sea, the ability to determine ages of individual Arcto-Norwegian cod with a high level of accuracy, and the availability of archived otoliths collected for fisheries research over the past 60 years. Using measurements of 14C derived from Arcto-Norwegian cod otoliths, we present the first pre- and post-bomb time series (1919–1992) of 14C from polar seas and consider the significance of these data in relation to ocean circulation and atmosphere/ocean flux of 14C. The data provide evidence for a minor Suess effect of only 0.2‰ per year between 1919 and 1950. Bomb 14C was evident in the Barents Sea as early as 1957 and the highest 14C value was measured in an otolith core from a cod with a birth date of 1967. The otolith 14C data display key features common to records of 14C obtained from a Georges Bank mollusc and corals from the tropical and subtropical North Atlantic.

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Recent Sea Ice Decline Did Not Significantly Increase the Total Liquid Freshwater Content of the Arctic Ocean

Journal of Climate ◽

10.1175/jcli-d-18-0237.1 ◽

2018 ◽

Vol 32 (1) ◽

pp. 15-32 ◽

Cited By ~ 11

Author(s):

Qiang Wang ◽

Claudia Wekerle ◽

Sergey Danilov ◽

Dmitry Sidorenko ◽

Nikolay Koldunov ◽

...

Keyword(s):

Spatial Distribution ◽

Sea Ice ◽

Arctic Ocean ◽

Ocean Circulation ◽

General Circulation ◽

The Arctic ◽

Sea Ice Decline ◽

The Arctic Ocean ◽

Eurasian Basin ◽

The Impact

Abstract The freshwater stored in the Arctic Ocean is an important component of the global climate system. Currently the Arctic liquid freshwater content (FWC) has reached a record high since the beginning of the last century. In this study we use numerical simulations to investigate the impact of sea ice decline on the Arctic liquid FWC and its spatial distribution. The global unstructured-mesh ocean general circulation model Finite Element Sea Ice–Ocean Model (FESOM) with 4.5-km horizontal resolution in the Arctic region is applied. The simulations show that sea ice decline increases the FWC by freshening the ocean through sea ice meltwater and modifies upper ocean circulation at the same time. The two effects together significantly increase the freshwater stored in the Amerasian basin and reduce its amount in the Eurasian basin. The salinification of the upper Eurasian basin is mainly caused by the reduction in the proportion of Pacific Water and the increase in that of Atlantic Water (AW). Consequently, the sea ice decline did not significantly contribute to the observed rapid increase in the Arctic total liquid FWC. However, the changes in the Arctic freshwater spatial distribution indicate that the influence of sea ice decline on the ocean environment is remarkable. Sea ice decline increases the amount of Barents Sea branch AW in the upper Arctic Ocean, thus reducing its supply to the deeper Arctic layers. This study suggests that all the dynamical processes sensitive to sea ice decline should be taken into account when understanding and predicting Arctic changes.

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The southern stratospheric gravity wave hot spot: individual waves and their momentum fluxes measured by COSMIC GPS-RO

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-7797-2015 ◽

2015 ◽

Vol 15 (14) ◽

pp. 7797-7818 ◽

Cited By ~ 47

Author(s):

N. P. Hindley ◽

C. J. Wright ◽

N. D. Smith ◽

N. J. Mitchell

Keyword(s):

Southern Ocean ◽

Gravity Wave ◽

General Circulation ◽

Hot Spot ◽

Polar Vortex ◽

General Circulation Models ◽

Wave Drag ◽

Southern Andes ◽

Analysis Technique ◽

Modelling Studies

Abstract. Nearly all general circulation models significantly fail to reproduce the observed behaviour of the southern wintertime polar vortex. It has been suggested that these biases result from an underestimation of gravity wave drag on the atmosphere at latitudes near 60° S, especially around the "hot spot" of intense gravity wave fluxes above the mountainous Southern Andes and Antarctic peninsula. Here, we use Global Positioning System radio occultation (GPS-RO) data from the COSMIC satellite constellation to determine the properties of gravity waves in the hot spot and beyond. We show considerable southward propagation to latitudes near 60° S of waves apparently generated over the southern Andes. We propose that this propagation may account for much of the wave drag missing from the models. Furthermore, there is a long leeward region of increased gravity wave energy that sweeps eastwards from the mountains over the Southern Ocean. Despite its striking nature, the source of this region has historically proved difficult to determine. Our observations suggest that this region includes both waves generated locally and orographic waves advected downwind from the hot spot. We describe and use a new wavelet-based analysis technique for the quantitative identification of individual waves from COSMIC temperature profiles. This analysis reveals different geographical regimes of wave amplitude and short-timescale variability in the wave field over the Southern Ocean. Finally, we use the increased numbers of closely spaced pairs of profiles from the deployment phase of the COSMIC constellation in 2006 to make estimates of gravity wave horizontal wavelengths. We show that, given sufficient observations, GPS-RO can produce physically reasonable estimates of stratospheric gravity wave momentum flux in the hot spot that are consistent with measurements made by other techniques. We discuss our results in the context of previous satellite and modelling studies and explain how they advance our understanding of the nature and origins of waves in the southern stratosphere.

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21st century ocean forcing of the Greenland Ice Sheet for modeling of sea level contribution

10.5194/tc-2019-222 ◽

2019 ◽

Cited By ~ 1

Author(s):

Donald A. Slater ◽

Denis Felikson ◽

Fiamma Straneo ◽

Heiko Goelzer ◽

Christopher M. Little ◽

...

Keyword(s):

Sea Level ◽

21St Century ◽

General Circulation ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

General Circulation Models ◽

Glacier Retreat ◽

Small Scale ◽

Intergovernmental Panel ◽

Continental Scale

Abstract. Changes in the ocean are expected to be an important determinant of the Greenland Ice Sheet's future sea level contribution. Yet representing these changes in continental-scale ice sheet models remains challenging due to the small scale of the key physics, and limitations in processing understanding. Here we present the ocean forcing strategy for Greenland Ice Sheet models taking part in the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), the primary community effort to provide 21st century sea level projections for the Intergovernmental Panel on Climate Change 6th Assessment Report. Beginning from global atmosphere-ocean general circulation models, we describe two complementary approaches to provide ocean boundary conditions for Greenland Ice Sheet models, termed the retreat and submarine melt implementations. The retreat implementation parameterizes glacier retreat as a function of projected submarine melting, is designed to be implementable by all ice sheet models, and results in retreat of around 1 and 15 km by 2100 in RCP2.6 and 8.5 scenarios respectively. The submarine melt implementation provides estimated submarine melting only, leaving the ice sheet model to solve for the resulting calving and glacier retreat, and suggests submarine melt rates will change little under RCP2.6 but will approximately triple by 2100 under RCP8.5. Both implementations have necessarily made use of simplifying assumptions and poorly-constrained parameterisations and as such, further research on submarine melting, calving and fjord-shelf exchange should remain a priority. Nevertheless, the presented framework will allow an ensemble of Greenland Ice Sheet models to be systematically and consistently forced by the ocean for the first time, and should therefore result in a significant improvement in projections of the Greenland ice sheet's contribution to future sea level change.

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Sensitivity of simulated oxygen isotopes in ice cores and speleothems to Last Glacial Maximum surface conditions

10.5194/egusphere-egu21-2305 ◽

2021 ◽

Author(s):

André Paul ◽

Alexandre Cauquoin ◽

Stefan Mulitza ◽

Thejna Tharammal ◽

Martin Werner

Keyword(s):

Last Glacial Maximum ◽

General Circulation ◽

Ice Cores ◽

General Circulation Models ◽

Glacial Maximum ◽

Sea Surface ◽

Surface Conditions ◽

Last Glacial ◽

The Impact ◽

Mean Square Errors

In simulations of the climate during the Last Glacial Maximum (LGM), we employ two different isotope-enabled atmospheric general circulation models (NCAR iCAM3 and MPI ECHAM6-wiso) and use simulated (by coupled climate models) as well as reconstructed (from a new global climatology of the ocean surface duing the LGM, GLOMAP) surface conditions.The resulting atmospheric fields reflect the more pronounced structure and gradients in the reconstructions, for example, the precipitation is more depleted in oxygen-18 in the high latitudes and more enriched in low latitudes, especially in the tropical convective regions over the maritime continent in the equatorial Pacific and Indian Oceans and over the equatorial Atlantic Ocean. Furthermore, at the sites of ice cores and speleothems, the model-data fit improves in terms of the coefficients of determination and root-mean square errors.In additional sensitivity experiments, we also use the climatologies by Annan and Hargreaves (2013) and Tierney et al. (2020) and consider the impact of changes in reconstructed sea-ice extent and the global-mean sea-surface temperature.Our findings imply that the correct simulation or reconstruction of patterns and gradients in sea-surface conditions are crucial for a successful comparison to oxygen-isotope data from ice cores and speleothems.

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A Simplistic Approach for Assessing Hydroclimatic Vulnerability of Lakes and Reservoirs with Regulated Superficial Outflow

Hydrology ◽

10.3390/hydrology6030061 ◽

2019 ◽

Vol 6 (3) ◽

pp. 61 ◽

Cited By ~ 1

Author(s):

Kleoniki Demertzi ◽

Dimitris Papadimos ◽

Vassilis Aschonitis ◽

Dimitris Papamichail

Keyword(s):

Climate Change ◽

General Circulation ◽

Drainage Basin ◽

Management Strategies ◽

General Circulation Models ◽

Climatic Conditions ◽

Modified Model ◽

Natural Lakes ◽

The Future ◽

The Impact

This study proposes a simplistic model for assessing the hydroclimatic vulnerability of lakes/reservoirs (LRs) that preserve their steady-state conditions based on regulated superficial discharge (Qd) out of the LR drainage basin. The model is a modification of the Bracht-Flyr et al. method that was initially proposed for natural lakes in closed basins with no superficial discharge outside the basin (Qd = 0) and under water-limited environmental conditions {mean annual ratio of potential/reference evapotranspiration (ETo) versus rainfall (P) greater than 1}. In the proposed modified approach, an additional Qd function is included. The modified model is applied using as a case study the Oreastiada Lake, which is located inside the Kastoria basin in Greece. Six years of observed data of P, ETo, Qd, and lake topography were used to calibrate the modified model based on the current conditions. The calibrated model was also used to assess the future lake conditions based on the future climatic projections (mean conditions of 2061-2080) derived by 19 general circulation models (GCMs) for three cases of climate change (three cases of Representative Concentration Pathways: RCP2.6, RCP4.5 and RCP8.5). The modified method can be used as a diagnostic tool in water-limited environments for analyzing the superficial discharge changes of LRs under different climatic conditions and to support the design of new management strategies for mitigating the impact of climate change on (a) flooding conditions, (b) hydroelectric production, (c) irrigation/industrial/domestic use and (d) minimum ecological flows to downstream rivers.

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