Air-sea interaction and the influence of sea ice during cold-air outbreaks over the subpolar North Atlantic Ocean

Barrell, Christopher (2023) Air-sea interaction and the influence of sea ice during cold-air outbreaks over the subpolar North Atlantic Ocean. Doctoral thesis, University of East Anglia.

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Abstract

Air-sea interaction in the subpolar North Atlantic (SPNA) has a strong influence on weather in the Northern Hemisphere and controls the formation of dense waters that feed into the Atlantic Meridional Overturning Circulation (AMOC). During cold-air outbreaks (CAOs) the strongest turbulent heat fluxes occur near the ice edge, hence the distribution of sea ice is critically important for determining air-sea interaction. I evaluate coupled model simulations for the SPNA and the influence of the marginal ice zone (MIZ) in short-range weather forecasts and long-range climate projections. Coordinated atmosphere-ocean measurements made during the Iceland-Greenland Seas Project (IGP) field campaign in 2018 are used to evaluate the then semi-operational Met Office Global Coupled model 2 (GC2). It is shown to simulate cold biased sea surface temperatures up to approximately 200 km from the ice edge due to an overly wide MIZ, which contributes to forecast error downstream.

A further quantitative analysis of 10-day sea ice forecasts from the Met Office Global Coupled model 3.1 (GC3.1) was performed. Evaluation using two satellite sea ice products shows that for individual cases dynamic sea ice in the coupled model provides no clear improvement over a persistent sea ice field. However, sensitivity tests with two ocean model grid resolutions (1/4° and 1/12°) and a modified atmospheric drag parameterisation over the marginal ice zone show that the 1/12° ocean model provides advancement in the simulation of eddy driven features along the ice edge. Moreover, the largest source of error in the model experiments is the sea ice input field from the commonly used OSI-SAF 401 operational product highlighting an opportunity for improvement.

Last, the future of air-sea interaction over the SPNA is investigated using output from the HadGEM3-GC3.1 climate model, produced as part of the 6th Coupled Model Inter-Comparison Project (CMIP6). Contrary to the short-range forecasts, comparison of two model resolutions (MM: 60 km atmosphere - 1/4° ocean; HH: 25 km – 1/12°) shows that the HH model accurately simulates the historic sea ice distribution, which provides confidence in its future projections. Nonetheless, the simulations agree in predicting that under climate change the subpolar North Atlantic will see further significant reductions in wintertime air-sea turbulent fluxes later in the 21st century, which may reduce the formation of dense waters that feed into the lower limb of the AMOC.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Environmental Sciences
Depositing User:	Chris White
Date Deposited:	28 Oct 2024 14:34
Last Modified:	28 Oct 2024 14:34
URI:	https://ueaeprints.uea.ac.uk/id/eprint/97235
DOI:

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