Sensitivity of atmospheric moisture transport into the Arctic to sea surface temperature changes over the North Atlantic region

Abstract

Water vapor is one of the most important components of the atmosphere, which provides the energy for the formation of weather systems and climate systems. In addition, it is an important trace gas and greenhouse gas due to its participation in the processes of cloud formation, radiation, and energy exchange in the ocean–atmospheric system. In the Arctic, it affects the surface energy balance, the amount of fresh water in the Arctic Ocean, and ice and snow cover. The northerly transport caused by the temperature difference in the upper and lower latitudes plays an important role in the moisture budget of the Arctic region because the amount of evaporation is usually small over areas covered with snow and ice. Another factor in the transport of atmospheric moisture is sea surface temperatures, which control the heat content of the oceans and also have a significant impact on regulating the climate. The Atlantic Meridional Overturning Circulation, or AMOC, is a large system of ocean currents, and it is important in ocean circulation systems by transporting hot and salty water from the tropics to the North Atlantic. It is basically defined as the northward flow of warm water near the surface and the southward flow of deep cold water. It is a major system for climate variability, as well as for the redistribution of heat. In previous studies, it is stated that sea surface temperatures in the North Atlantic region change a lot in a short time, and also strong sea surface temperature variations in mid-latitude regions affect ocean-atmosphere interactions. Many studies have been conducted to determine extreme moisture transport events and their effects on Arctic sea-ice fraction, surface air temperature, and surface energy flow, and which weather systems are related. As a result, it is shown that Arctic surface air temperature and sea ice are highly affected by excessive moisture transport. Moreover, it has been shown that there is an increase in meridional moisture transport by examining the regional and seasonal trends of horizontal moisture transport in the Arctic. It has also been noted that atmospheric circulation and the North Atlantic storm track may be severely affected in the future due to the changes in ocean circulation and sea surface temperatures. The purpose of the study is to identify the effects of sea surface temperature changes in the North Atlantic region on moisture transport into the Arctic region. The study consists of two parts. In the first part, climatological features and anomalies in the North Atlantic region were studied based on the analysis of ERA5 data for the years 1979-2019 to address the modeling experiments. In the ERA5 analysis, monthly means of sea surface temperature, mean sea level pressure, meridional moisture flux, and surface evaporation covering the years 1979-2019 were used. As a result of the ERA5 data analysis, it was found that there has been a significant increase in the trend of moisture transport into the Arctic over the past 10 years compared to the long-term climatological averages. A yearly comparison of moisture transport over the past 10 years has been made. In addition, the anomalies of sea surface temperature, meridional moisture flux, and surface evaporation were analysed. The analysis revealed three notable anomalies in 2016: strong negative SST anomalies over the North Atlantic region from August to October, a strong meridional moisture flux anomaly over the northeastern North Atlantic towards the Arctic in September and October, and negative surface evaporation, ie. condensation, over the Arctic region where strong meridional moisture flux occurred. Also, in ERA5, the area where negative SST anomalies over the North Atlantic tend to correlate with meridional moisture flux over Greenland and Scandinavia in September and October. In line with the ERA5 analysis, it was decided to examine the period covering August-October 2016 with the model, and in the second part of the study, simulations with different sea surface temperatures over the North Atlantic region were carried out using the OpenIFS model. These three simulations include observed sea surface temperatures provided by the ECMWF, climatological sea surface temperatures from ERA5 climatology, and sea surface temperatures with doubling (increased) anomalies relative to ERA5 climatology. In addition, each simulation includes one control member and 10 perturbed ensembles. The stochastic physics method was used for ensemble forecast. In summary, in the study, it is discussed how the moisture transport into the Arctic region is affected when the anomalies of sea surface temperatures in the North Atlantic region are removed and increased. As a result of the modeling, three different simulations with a 3-month length covering the period of August-October 2016 were obtained. In the model results, both control forecasts and ensemble members were compared based on monthly means of mean sea level pressure, 500 hPa geopotential height and jet, total column water vapor, surface evaporation, and meridional moisture flux. In the OpenIFS sensitivity experiments, the parameters most sensitive to changes in sea surface temperature were mean sea level pressure and geopotential height, thus influencing surface and upper level wind flows. The strongest effects, such as dipole and tripole patterns, were seen for mean sea level pressure from the northeastern North Atlantic to the Arctic and over Scandinavia. Also, as changing sea surface temperatures affect the temperature distribution over the North Atlantic, the mid-level jet was also affected by the lower temperature changes. The amount of total column water vapor was also affected by sea surface temperature changes, with notable differences between the central North Atlantic and the western North Atlantic, particularly in the Nordic Seas. Surface evaporation was affected by mean sea level pressure, wind, and the amount of total column water vapor in the sensitivity experiments. Relatively dry air showed higher evaporation with higher winds. However, evaporation was limited by the presence of strong positive meridional moisture flux as well as the amount of total column water vapor, so it was difficult to mention that the variation of sea surface temperatures contributes effectively to the transport of moisture into the Arctic. The most important factors on the transport of meridional moisture flux into the Arctic in OpenIFS sensitivity experiments were the occurrence of stronger cyclonic activity in Iceland with the expanding high over Scandinavia. The region in which meridional moisture flux varied most as a result of changes in sea surface temperatures was the northeastern North Atlantic to the Arctic as well as northern Europe. However, strong influences on the transport of meridional moisture flux into the Arctic by the impact of sea surface temperature variation were not observed in OpenIFS sensitivity experiments. At the same time, according to the results of the comparison of the modified sea surface temperatures simulations with the observed sea surface temperature simulation, the use of climatological sea surface temperatures had a fewer effect, while increasing the sea surface temperature anomaly values had partially more impact. That is, increasing sea surface temperature anomalies resulted in greater variation among meteorological variables and also resulted in the transport of partly greater meridional moisture flux to higher latitudes. In this study, ensemble forecasts were also performed for each simulation and the stochastic physics approach was used. As the ensemble standard deviation (ensemble spread or variability) increased with the forecast time, it took quite large values compared to the ensemble mean values, thus making it difficult to explain the effects of changing sea surface temperatures on the meteorological variables. Furthermore, one of the challenges encountered in the study is that the 3-month length forecast period is a difficult time scale for prediction. Another limitation is that sea surface temperature anomalies are not constant and vary over time. For this reason, a constant value impact such as climatology minus 5K (climatology-5K) in the region may lead to more effective and various results. Additionally, the stochastic physics approach was used to perform ensemble predictions in the study. Because ensemble spread also increased with forecast time, it took large values compared to ensemble mean values, making it difficult to explain the effects of changing sea surface temperatures. Also, due to technical limitations, there are only 10 perturbed ensemble members for each simulation, which might be insufficient to make a difference in the ensemble forecasts. Experimenting with different ensemble forecast approaches or increasing the number of perturbed ensemble members can result in different and varied outputs. In conclusion, the study discusses how moisture transport conditions to the Arctic are affected in relation to sea surface temperature changes over the North Atlantic sector. The findings may contribute to studies of moisture transport into the Arctic and its relationship to changes in sea surface temperatures in the North Atlantic region.