Air-Sea Interactions during Cold Air Outbreaks in a coupled Mixed Layer Model

Postponed access: the file will be accessible after 2020-06-03Cold air outbreaks play a crucial role in the air-sea heat exchange in high latitudes. To explore the sensitivity to ocean coupling, the role of latent heating and the sensitivity to sea ice distributions in cold air outbreaks, we couple an atmospheric to an oceanic mixed layer model. The mixed layer model of the atmosphere is based on the equations for liquid water potential temperature and the total mixing ratio and the oceanic mixed layer model is based on the equations for temperature and salinity. A steady state is obtained through heat exchange between the atmosphere and ocean, as well as advection in the atmosphere and relaxation toward a climatological state in the ocean. The results show that the coupling with the ocean has only a marginal impact on the atmospheric boundary layer structure, but that cold air outbreaks can lead to an increase in the oceanic mixed layer depth. Latent heating acts to increase the atmopsheric boundary layer growth, which leads a reduction of sensible heat fluxes. We further investigate the effect of different sea ice distributions with and without coupling between the atmosphere and ocean, and show that the sea ice distribution does not change the effect of the cold air outbreak when integrating the fluxes from the ice edge to far downstream the fetch, but it does change the distribution of the fluxes and thereby the local response between the atmosphere and the ocean. The model shows that the oceanic mixed layer depth is deeper when there is a sharper transition from sea ice to open water.Masteroppgave i meteorologi og oseanografiGEOF399MAMN-GEO

Why has Precipitation Increased in the Last 120 Years in Norway?

We use a data set with daily precipitation observations from 55 homogeneity-tested stations in Norway from 1900 to 2019 available from MET-Norway. These observations show that precipitation in Norway has increased by 19% since 1900. Notably, over half of the overall increase occurred within the decade of 1980–1990 and is happening across all precipitation rates. To examine possible mechanisms behind the precipitation increase, we use a diagnostic model to separate the effects of changes in vertical velocity, temperature and relative humidity. We use daily vertical velocity, near-surface temperature and relative humidity from two reanalysis products, ERA-20C and 20th Century Reanalysis. The model-based precipitation correlates significantly with the observed precipitation on an annual timescale (r > 0.9), as well as captures the trend in all reanalysis products. The diagnostic model indicates that the variability in vertical velocity chiefly determines the interannual variability and long-term trend. The trend in vertical velocities contributes to more than 80% of the total modeled trend in precipitation between 1900 and 2019. However, over the last two decades (1995–2015), changes in temperature and relative humidity are the main contributors to the modeled trend in precipitation.publishedVersio

Air-Sea Interactions during Cold Air Outbreaks in a coupled Mixed Layer Model

Cold air outbreaks play a crucial role in the air-sea heat exchange in high latitudes. To explore the sensitivity to ocean coupling, the role of latent heating and the sensitivity to sea ice distributions in cold air outbreaks, we couple an atmospheric to an oceanic mixed layer model. The mixed layer model of the atmosphere is based on the equations for liquid water potential temperature and the total mixing ratio and the oceanic mixed layer model is based on the equations for temperature and salinity. A steady state is obtained through heat exchange between the atmosphere and ocean, as well as advection in the atmosphere and relaxation toward a climatological state in the ocean. The results show that the coupling with the ocean has only a marginal impact on the atmospheric boundary layer structure, but that cold air outbreaks can lead to an increase in the oceanic mixed layer depth. Latent heating acts to increase the atmopsheric boundary layer growth, which leads a reduction of sensible heat fluxes. We further investigate the effect of different sea ice distributions with and without coupling between the atmosphere and ocean, and show that the sea ice distribution does not change the effect of the cold air outbreak when integrating the fluxes from the ice edge to far downstream the fetch, but it does change the distribution of the fluxes and thereby the local response between the atmosphere and the ocean. The model shows that the oceanic mixed layer depth is deeper when there is a sharper transition from sea ice to open water

The Iceland Greenland seas project

The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway