Coordinated observational and modeling studies on the basic structure and variability of the Arctice sea ice-ocean system

第6回極域科学シンポジウム特別セッション：[S] 北極温暖化とその影響 ―GRENE 北極気候変動プロジェクトと新しい方向性―11月18日（水）　国立極地研究所 2階 大会議

Sea ice changes during the early 20th century Arctic warming in an Earth System Model

The Tenth Symposium on Polar Science/Ordinary sessions: [OM] Polar Meteorology and Glaciology, Thu. 5 Dec. / 2F Auditorium , National Institute of Polar Researc

バレンツ海における大西洋水の変質過程のモデリング研究

第6回極域科学シンポジウム分野横断セッション：[IA] 急変する北極気候システム及びその全球的な影響の総合的解明―GRENE北極気候変動研究事業研究成果報告2015―11月19日（木）　国立極地研究所 2階 大会議

Modeling basal melting of ice shelves around Antarctica and its impact on sea ice and ocean

第2回極域科学シンポジウム/第34回気水圏シンポジウム 11月17日（木） 統計数理研究所 セミナー室

High resolution modeling for long-term prediction of the Arctic sea ice

第3回極域科学シンポジウム/特別セッション「これからの北極研究」11月28日（水） 国立極地研究所 2階大会議

The Atlantic meridional overturning circulation in high resolution models

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N ‐ a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC

Pathways of basal meltwater from Antarctic ice shelves: A model study

We investigate spreading pathways of basal meltwater released from all Antarctic ice shelves using a circumpolar coupled ice shelf-sea ice-ocean model that reproduces major features of the Southern Ocean circulation, including the Antarctic Circumpolar Current (ACC). Several independent virtual tracers are used to identify detailed pathways of basal meltwaters. The spreading pathways of the meltwater tracers depend on formation sites, because the meltwaters are transported by local ambient ocean circulation. Meltwaters from ice shelves in the Weddell and Amundsen-Bellingshausen Seas in surface/subsurface layers are effectively advected to lower latitudes with the ACC. Although a large portion of the basal meltwaters is present in surface and subsurface layers, a part of the basal meltwaters penetrates into the bottom layer through active dense water formation along the Antarctic coastal margins. The signals at the seafloor extend along the topography, showing a horizontal distribution similar to the observed spreading of Antarctic Bottom Water. Meltwaters originating from ice shelves in the Weddell and Ross Seas and in the Indian sector significantly contribute to the bottom signals. A series of numerical experiments in which thermodynamic interaction between the ice shelf and ocean is neglected regionally demonstrates that the basal meltwater of each ice shelf impacts sea ice and/or ocean thermohaline circulation in the Southern Ocean

Modeling Antarctic ice shelf responses to future climate changes and impacts on the ocean

We investigate basal melting of all Antarctic ice shelves by a circumpolar ice shelf-sea ice-ocean coupled model and estimate the total basal melting of 770-944Gt/yr under present-day climate conditions. We present a comparison of the basal melting with previous observational and modeling estimates for each ice shelf. Heat sources for basal melting are largely different among the ice shelves. Sensitivities of the basal melting to surface air warming and to enhanced westerly winds over the Antarctic Circumpolar Current are investigated from a series of numerical experiments. In this model the total basal melting strongly depends on the surface air warming but is hardly affected by the change of westerly winds. The magnitude of the basal melting response to the warming varies widely from one ice shelf to another. The largest response is found at ice shelves in the Bellingshausen Sea, followed by those in the Eastern Weddell Sea and the Indian sector. These increases of basal melting are caused by increases of Circumpolar Deep Water and/or Antarctic Surface Water into ice shelf cavities. By contrast, basal melting of ice shelves in the Ross and Weddell Seas is insensitive to the surface air warming, because even in the warming experiments there is high sea ice production at the front of the ice shelves that keeps the water temperature to the surface freezing point. Weakening of the thermohaline circulation driven by Antarctic dense water formation under warming climate conditions is enhanced by basal melting of ice shelves

Diagnostic evaluation of effects of vertical mixing on meridional overturning circulation in an idealized ocean

In this study, diagnostic equations are proposed to quantitatively evaluate meridional overturning circulation (MOC) simulated in ocean general circulation models (OGCMs). Applicability of the equations is illustrated by revisiting the MOC simulated in an idealized ocean. The simulations with surface differential heating/cooling show that, for certain horizontal distribution of vertical diffusivity, the stronger vertical mixing does not intensify the MOC while it makes the deeper water less dense. This result, which is in marked contrast to the widely accepted idea that the stronger vertical mixing promotes upwelling and intensifies the MOC by making the deeper water less dense, was investigated using the diagnostic equations. It was found that geostrophy dominates the MOC, and the geostrophic flow normal to lateral boundaries induced intense upwelling/downwelling along the boundaries. These results indicate that the primary role played by the vertical mixing on the large-scale MOC is to change hydrostatic pressure fields (geostrophic flow fields), rather than to promote upwelling. The simulation with localized cooling on the other hand showed that the ageostrophic flows significantly contribute to small-scale features of the MOC, while the geostrophic flows determine large-scale structure of the MOC. The proposed equations will thus be useful to quantitatively diagnose the MOC dynamics in realistic OGCMs