Upper ocean variability between Iceland and Newfoundland, 1993–1998 (original) (raw)

On the mechanism of interannual variability of the Irminger Water in the Labrador Sea

Journal of Geophysical Research, 2011

1] The mechanism of variability of the North Atlantic subpolar gyre (SPG) and its relation to the North Atlantic Oscillation (NAO) is investigated using an ocean general circulation model. In this study we conducted three model experiments. The first two were forced with idealized positive (NAO + ) and negative (NAO − ) NAO-like forcing, including variations at decadal time scales. The third experiment was forced with the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The decadal variability of the volume transport, the sea surface temperature in the North Atlantic Current (SSTA1), and the Irminger Water temperature (IWT) in the NAO − experiment have 2-3 times smaller magnitude than in the NAO + experiment. The decadal variations in the strength of circulation in the NAO + experiment covaries negatively with SSTA1 and IWT anomalies. A similar covariability of these parameters is not found in the NAO − simulations. The results from the model experiment forced with the NCEP/NCAR reanalysis from 1958 to 2005 show a shift in the subpolar ocean response to the atmospheric variability in the early 1980s. The amplitude of quasi-decadal variability of the IWT and SSTA1 and their correlation are high after 1980 (r = 0.79) and weaker in the period between 1958 and 1980 (r = 0.1). The IWT after 1980 is well correlated (r = 0.67) to the subpolar gyre transport index (SPGI, defined as the minimum value of the annual mean anomaly of the model barotropic stream function). This correlation is weaker and negative (r = −0.09) in the period from 1958 to 1980. We explain this shift in the covariability of the SSTA1, IWT, and SPGI with the antisymmetric response of the SPG to atmospheric variations at decadal time scale under positive and negative NAO index. The NAO sign changed in the early 1980s from predominantly negative to positive phase. In the 1980s and 1990s the model SPGI variability follows closely the decadal variations of the NAO index with a delay of about 3 years. Similar covariability between the SPGI and the NAO and related negative covariance of SSTA1 and IWT with the SPGI are not observed in the model simulations of the period from 1958 to 1980.

Role of Greenland Freshwater Anomaly in the Recent Freshening of the Subpolar North Atlantic

Journal Of Geophysical Research: Oceans, 2019

The cumulative Greenland freshwater flux anomaly has exceeded 5,000 km 3 since the 1990s. The volume of this surplus freshwater is expected to cause substantial freshening in the North Atlantic. Analysis of hydrographic observations in the subpolar seas reveals freshening signals in the 2010s. The sources of this freshening are yet to be determined. In this study, the relationship between the surplus Greenland freshwater flux and this freshening is tested by analyzing the propagation of the Greenland freshwater anomaly and its impact on salinity in the subpolar North Atlantic based on observational data and numerical experiments with and without the Greenland runoff. A passive tracer is continuously released during the simulations at freshwater sources along the coast of Greenland to track the Greenland freshwater anomaly. Tracer budget analysis shows that 44% of the volume of the Greenland freshwater anomaly is retained in the subpolar North Atlantic by the end of the simulation. This volume is sufficient to cause strong freshening in the subpolar seas if it stays in the upper 50-100 m. However, in the model the anomaly is mixed down to several hundred meters of the water column resulting in smaller magnitudes of freshening compared to the observations. Therefore, the simulations suggest that the accelerated Greenland melting would not be sufficient to cause the observed freshening in the subpolar seas and other sources of freshwater have contributed to the freshening. Impacts on salinity in the subpolar seas of the freshwater transport through Fram Strait and precipitation are discussed. Plain Language Summary Accelerated Greenland ice sheet loss has contributed about 5,000 km 3 of freshwater into the subpolar North Atlantic since 1993, which is half of the freshwater volume propagating across the North Atlantic with the Great Salinity Anomaly in the 1970s. The volume of the Greenland freshwater anomaly is expected to cause substantial freshening in the North Atlantic and impact the Arctic and subarctic climate. Analysis of hydrographic observations identifies freshening signals in the subpolar seas in the 2010s possibly related to the increased Greenland freshwater flux. In order to verify this relationship, numerical experiments with passive tracers released at freshwater sources along the coast of Greenland are employed to track propagation, mixing, and accumulation of the Greenland freshwater flux anomaly. The model experiments demonstrate that a substantial volume of the Greenland freshwater anomaly is retained in the subpolar North Atlantic but is mostly mixed in the upper 500 m of the water column resulting in smaller magnitudes of the freshening signal compared to the observations. Thus, the simulations suggest that the accelerated Greenland melting would not be sufficient to cause the observed freshening in the subpolar seas and other sources of freshwater have contributed to the freshening.

The interannual variability of potential temperature in the central Labrador Sea

Journal of Geophysical Research, 2012

1] The interannual variability of potential temperature in the central Labrador Sea is studied with a suite of numerical simulations with an eddy-resolving regional ocean model and compared with available observations. The model successfully reproduces the observed variations in potential temperature at depths comprised between 150 and 2000 m over the period 1980-2009, capturing also the warming trend of the last decade and the deep water formation event in 2008. The suite of experiments allows for quantifying the contribution from the physical forcings responsible for the interannual variability of potential temperature in the region. The local atmospheric forcing drives the interannual signal by driving convection, while the incoming current system along the east coast of Greenland is responsible for about half of the warming trend ($0.3-0.4 C) during the last decade through restratification process. The lateral transport of Irminger water in the convective region into the central Labrador Sea is further analyzed integrating a passive tracer. It is found that the overall amount of Irminger water transported in the convective region of the Labrador Sea is directly correlated with the amount of vertical convective mixing. In the last decade, following the decrease in convective activity, the model reveals a substantial decrease in concentration of Irminger Current water below 500 m in the Labrador Sea interior: by 2010 the overall amount is less than half than in the previous 20 years. Citation: Luo, H., A. Bracco, I. Yashayaev, and E. Di Lorenzo (2012), The interannual variability of potential temperature in the

The role of the Atlantic Water in multidecadal ocean variability in the Nordic and Barents Seas

Progress in Oceanography, 2014

The focus of this work is on the temporal and spatial variability of the Atlantic Water (AW). We analyze the existing historic hydrographic data from the World Ocean Database to document the long-term variability of the AW throughflow across the Norwegian Sea to the western Barents Sea. Interannual-to-multidecadal variability of water temperature, salinity and density are analyzed along six composite sections crossing the AW flow and coastal currents at six selected locations. The stations are lined up from southwest to northeast -from the northern North Sea (69°N) throughout the Norwegian Sea to the Kola Section in the Barents Sea (33°30 0 E). The changing volume and characteristics of the AW throughflow dominate the hydrographic variability on decadal and longer time scales in the studied area. We examine the role of fluctuations of the volume of inflow versus the variable local factors, such as the air-sea interaction and mixing with the fresh coastal and cold Arctic waters, in controlling the long-term regional variability. It is shown that the volume of the AW, passing through the area and affecting the position of the outer edge of the warm and saline core, correlates well with temperature and salinity averaged over the central portions of the studied sections. The coastal flow (mostly associated with the Norwegian Coastal Current flowing over the continental shelf) is largely controlled by seasonal local heat and freshwater impacts. Temperature records at all six lines show a warming trend superimposed on a series of relatively warm and cold periods, which in most cases follow, with a delay of four to five years, the periods of relatively low and high North Atlantic Oscillation (NAO), and the periods of relatively high and low Atlantic Multidecadal Oscillation (AMO), respectively. In general, there is a relatively high correlation between the year-to-year changes of the NAO and AMO indices, which is to some extent reflected in the (delayed) AW temperature fluctuations. It takes about two years for freshening and salinification events and a much shorter time (of about a year or less) for cooling and warming episodes to propagate or spread across the region. This significant difference in the propagation rates of salinity and temperature anomalies is explained by the leading role of horizontal advection in the propagation of salinity anomalies, whereas temperature is also controlled by the competing air-sea interaction along the AW throughflow. Therefore, although a water parcel moves within the flow as a whole, the temperature, salinity and density anomalies split and propagate separately, with the temperature and density signals leading relative to the salinity signal. A new hydrographic index, coastal-to-offshore density step, is introduced to capture variability in the strength of the AW volume transport. This index shows the same cycles of variability as observed in temperature, NAO and AMO but without an obvious trend.