Local Forcing Mechanisms Research Articles

Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers

Abstract. Frontal ablation has caused 32 %–66 % of Greenland Ice Sheet mass loss since 1972, and despite its importance in driving terminus change, ocean thermal forcing remains crudely incorporated into large-scale ice sheet models. In Greenland, local fjord-scale processes modify the magnitude of thermal forcing at the ice–ocean boundary but are too small scale to be resolved in current global climate models. For example, simulations used in the Ice Sheet Intercomparison Project for CMIP6 (ISMIP6) to predict future ice sheet change rely on the extrapolation of regional ocean water properties into fjords to drive terminus ablation. However, the accuracy of this approach has not previously been tested due to the scarcity of observations in Greenland fjords, as well as the inability of fjord-scale models to realistically incorporate icebergs. By employing the recently developed IceBerg package within the Massachusetts Institute of Technology general circulation model (MITgcm), we here evaluate the ability of ocean thermal forcing parameterizations to predict thermal forcing at tidewater glacier termini. This is accomplished through sensitivity experiments using a set of idealized Greenland fjords, each forced with equivalent ocean boundary conditions but with varying tidal amplitudes, subglacial discharge, iceberg coverage, and bathymetry. Our results indicate that the bathymetric obstruction of external water is the primary control on near-glacier thermal forcing, followed by iceberg submarine melting. Despite identical ocean boundary conditions, we find that the simulated fjord processes can modify grounding line thermal forcing by as much as 3 °C, the magnitude of which is largely controlled by the relative depth of bathymetric sills to the Polar Water–Atlantic Water thermocline. However, using a common adjustment for fjord bathymetry we can still predict grounding line thermal forcing within 0.2 °C in our simulations. Finally, we introduce new parameterizations that additionally account for iceberg-driven cooling that can accurately predict interior fjord thermal forcing profiles both in iceberg-laden simulations and in observations from Kangiata Sullua (Ilulissat Icefjord).

Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers

Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers

Contrasting surface warming of a marginal basin due to large-scale climatic patterns and local forcing

The Mediterranean Sea is one of the first regions where sea surface temperature (SST) increase was linked to greenhouse effects and global warming. Due to its sensitivity to climate variability and its high impact on local and remote climate conditions, much effort has been made to assess the SST variability in the Mediterranean as a whole. However, the Mediterranean is composed of several basins, each of which plays a different role in its conveyor belt’s function. This study focuses on the basin of the Tyrrhenian Sea which represents one of the crucial areas for deep mixing of the Mediterranean main water masses. Thirty-seven years (1982–2018) of satellite-derived data were used to investigate the SST variability in relation to large-scale and local forcing mechanisms. A significant warming trend of 0.034 ± 0.004 °C/year was found, which led to an average warming of 1.288 ± 0.129 °C over the considered period. The observed warming presents time-dependent spatial patterns as well as changes in the seasonal cycle. Our results highlight that the Tyrrhenian’s individual long-term surface variability has different characteristics than the Mediterranean as a whole and provide insight into the relative influence of large-scale teleconnection patterns and local air-sea interaction on this variability.

Late Pleistocene-Holocene vegetation and climate change in the Middle Kalahari, Lake Ngami, Botswana

Pollen, spores, and microscopic charcoal from a sediment core from Lake Ngami, in the Middle Kalahari, reflect paleovegetation and paleoclimatic conditions over the last 16,600 cal years BP. The location of Lake Ngami allows for the receipt of moisture sourced from the Indian and/or Atlantic oceans, which may have influenced local rainfall or long distance water transport via the Okavango system. We interpret results of statistical analyses of the pollen data as showing a complex, dynamic system wherein variability in tropical convective systems and local forcing mechanisms influence hydrological changes. Our reconstructions show three primary phases in the regional precipitation regime: 1) an early period of high but fluctuating summer rainfall under relatively cool conditions from ∼16,600–12,500 cal BP, with reduced tree to herb and shrub ratio; 2) an episode of significantly reduced rainfall centered around c. 11,400 cal BP, characterized by an increase in xeric Asteraceae pollen, but persistent aquatic elements, suggesting less rainfall but cool conditions and lower evaporation that maintained water in the basin; and 3) a longer phase of high, but fluctuating rainfall from ∼9000 cal BP to present with more woody savanna vegetation (Vachellia (Acacia) and Combretaceae). We propose a model to relate these changes to increased Indian Ocean-sourced moisture in the late Pleistocene due to a southerly position of the African rain belt, a northerly contraction of tropical systems that immediately followed the Younger Dryas, and a subsequent dominance of local insolation forcing, modulated by changes in the SE Atlantic basin.

Late Holocene climate and environmental changes in Kamchatka inferred from the subfossil chironomid record

This study presents a reconstruction of the Late Holocene climate in Kamchatka based on chironomid remains from a 332 cm long composite sediment core recovered from Dvuyurtochnoe Lake (Two-Yurts Lake, TYL) in central Kamchatka. The oldest recovered sediments date to about 4500 cal years BP. Chironomid head capsules from TYL reflect a rich and diverse fauna. An unknown morphotype of Tanytarsini, Tanytarsus type klein, was found in the lake sediments. Our analysis reveals four chironomid assemblage zones reflecting four different climatic periods in the Late Holocene. Between 4500 and 4000 cal years BP, the chironomid composition indicates a high lake level, well-oxygenated lake water conditions and close to modern temperatures (∼13 °C). From 4000 to 1000 cal years BP, two consecutive warm intervals were recorded, with the highest reconstructed temperature reaching 16.8 °C between 3700 and 2800 cal years BP. Cooling trend, started around 1100 cal years BP led to low temperatures during the last stage of the Holocene. Comparison with other regional studies has shown that termination of cooling at the beginning of late Holocene is relatively synchronous in central Kamchatka, South Kurile, Bering and Japanese Islands and take place around 3700 cal years BP. From ca 3700 cal years BP to the last millennium, a newly strengthened climate continentality accompanied by general warming trend with minor cool excursions led to apparent spatial heterogeneity of climatic patterns in the region. Some timing differences in climatic changes reconstructed from chironomid record of TYL sediments and late Holocene events reconstructed from other sites and other proxies might be linked to differences in local forcing mechanisms or caused by the different degree of dating precision, the different temporal resolution, and the different sensitive responses of climate proxies to the climate variations. Further high-resolution stratigraphic studies in this region are needed to understand the spatially complex pattern of climate change in Holocene in Kamchatka and the surrounding region.

Formation and spreading of Antarctic deep and bottom waters inferred from a chlorofluorocarbon (CFC) simulation

The formation of deep and bottom waters along Antarctica's perimeter is determined by ocean interaction with the atmosphere, sea ice, ice shelves, and bottom topography. It initiates a chain of processes that contribute to the ventilation of the global abyss. To identify the formation sites and investigate the combined effects of the local forcing mechanisms on water mass transformation and spreading in the Southern Ocean, chlorofluorocarbon (CFC) simulations with the regional ocean circulation model (BRIOS‐1) were performed. The model uses terrain‐following vertical coordinates to better represent both near‐bottom and mixed layer processes, and includes an explicit formulation of the ice shelf–ocean interaction. In agreement with observations, the results show the main deep and bottom water formations sites to be located in the Ross Sea, Prydz Bay, and southwestern Weddell Sea. The Ross Sea ventilates the South East Pacific and Australian Antarctic Basins. Both Ross Sea and Prydz Bay ventilate via the Antarctic Coastal Current the Weddell‐Enderby Basin. The latter signal is overprinted by sources in the Weddell Sea which ventilate the south Scotia Sea and also the Weddell‐Enderby Basin. Despite the general agreement between observed and simulated quantities like bottom layer CFC distribution and inventories along the Greenwich Meridian, the model tends to underestimate the ventilation of the abyssal ocean like other models with coarse resolution.

Decadal temperature changes in the Tasman Sea

Ocean temperature changes between 1991 and 2005 in the eastern Tasman Sea were analysed. This area was chosen because of a combination of data availability, low mesoscale variability and because of its importance in determining the climate of the downwind New Zealand landmass. A large warming extending to the full depth of the water column (c. 800 m) was found to have occurred between 1996 and 2002. This warming was seen in measurements by expendable bathythermographs and also in satellite sea surface temperature and sea surface height products, and has a clear impact on New Zealand's terrestrial temperature. The nature of the warming is discussed, together with likely forcing mechanisms. No local forcing mechanisms are consistent with the observed warming, leading to the conclusion that the signal seen in the Tasman Sea is part of a larger South Pacific‐wide phenomenon.

Time scales of circulation and mixing processes of San Francisco Bay waters

Conceptual models for tidal period and low-frequency variations in sea level, currents, and mixing processes in the northern and southern reaches of San Francisco Bay describe the contrasting characteristics and dissimilar processes and rates in these embayments: The northern reach is a partially mixed estuary whereas the southern reach (South Bay) is a tidally oscillating lagoon with density-driven exchanges with the northern reach. The mixed semidiurnal tides are mixtures of progressive and standing waves. The relatively simple oscillations in South Bay are nearly standing waves, with energy propagating down the channels and dispersing into the broad shoal areas. The tides of the northern reach have the general properties of a progressive wave but are altered at the constriction of the embayments and gradually change in an upstream direction to a mixture of progressive and standing waves. The spring and neap variations of the tides are pronounced and cause fortnightly varying tidal currents that affect mixing and salinity stratification in the water column. Wind stress on the water surface, freshwater inflow, and tidal currents interacting with the complex bay configuration are the major local forcing mechanisms creating low-frequency variations in sea level and currents. These local forcing mechanisms drive the residual flows which, with tidal diffusion, control the water-replacement rates in the estuary. In the northern reach, the longitudinal density gradient drives an estuarine circulation in the channels, and the spatial variation in tidal amplitude creates a tidally-driven residual circulation. In contrast, South Bay exhibits a balance between wind-driven circulation and tidally-driven residual circulation for most of the year. During winter, however, there can be sufficient density variations to drive multilayer (2 to 3) flows in the channel of South Bay. Mixing models (that include both diffusive and dispersive processes) are based on time scales associated with salt variations at the boundaries and those associated with the local forcing mechanisms, while the spatial scales of variations are dependent upon the configuration of the embayments. In the northern reach, where the estuarine circulation is strong, the salt flux is carried by the mean advection of the mean salt field. Where large salinity gradients are present, the tidal correlation part of the salt flux is of the same order as the advective part. Our knowledge of mixing and exchange rates in South Bay is poor. As this embayment is nearly isohaline, the salt flux is dominated entirely by the mean advection of the mean salt field. During and after peaks in river discharge, water mixing becomes more dynamic, with a strong density-driven current creating a net exchange of both water mass and salt. These exchanges are stronger during neap tides. Residence times of the water masses vary seasonally and differ between reaches. In the northern reach, residence times are on the order of days for high winter river discharge and of months for summer periods. The residence times for South Bay are fairly long (on the order of several months) during summer, and typically shorter (less than a month) during winter when density-driven exchanges occur.