Seasonal and interannual variability of pan-Arctic surface mixed layer properties from 1979 to 2012 from hydrographic data, and the dominance of stratification for multiyear mixed layer depth shoaling

Abstract

Using 21,406 hydrographic profiles from 1979 to 2012, we present the first observational, pan-Arctic assessment of Mixed Layer (ML) properties, including quantification of seasonal and interannual variability, and identification of multiyear ML depth shoaling.Arctic Mixed Layer Depths (MLDs) vary strongly seasonally, being deeper (∼25 to >50m) in winter than summer (∼5–30m). Eastern Arctic MLDs (regional mean ∼20m in summer, ∼70 to 100+m in winter) are deeper than western Arctic MLDs (∼8m in summer, 30m in winter). Patchiness, likely related to small-scale sea ice cover variability, is large – standard deviations ∼40% of the regional mean.By binning data into 6 regions (i.e., Chukchi Sea, Southern Beaufort Sea, Canada Basin, Makarov Basin, Eurasian Basin and Barents Sea), we quantify regional seasonal climatologies and interannual variability of ML depth, temperature and salinity. In most regions, ML changes are consistent with seasonal ice melt (∼1–3m) with a ∼1.5 times greater sea ice change required in the western Arctic than in the eastern Arctic. In the Southern Beaufort Sea and the Canada Basin, however, other freshwater sources contribute to observed seasonality.MLDs are significantly correlated with wind only during ice-free times, and even then the relationship only explains 1–20% of the MLD variance. The same wind is 2–3 times more effective at deepening the ML in the eastern Arctic than in the (more stratified) western Arctic. Changes in underlying stratification (Δρ) explain ∼60% of the MLD variance, with MLDs proportional to Δρ−0.45. Weak eastern Arctic stratification permits a wind–MLD coupling comparable to an Ekman model, while the stronger western Arctic stratifications reduce the wind’s effectiveness by a factor of 6.Remarkably, record-length (up to 30-year) trends indicate almost ubiquitous ML shoaling, order 0.5–1m/yr, both in winter and summer over all the high Arctic (Canada, Makarov and Eurasian basins) and in winter in the peripheral seas (Chukchi, Southern Beaufort and Barents seas), coincident with ML freshening and increased stratification, while wind speed trends are either not significant or decreasing. The freshwater change related to this shoaling is small – order 100km3/yr. In contrast, the Southern Beaufort Sea shows ML deepening, coincident with decreasing stratification, possibly related to river water being driven away from the coast. Changes in T–S space suggest decreased convection in the Eurasian Basin in the 2000s.Although in these results it is the absence of sea ice that allows wind-driven ML deepening, the dominance of stratification over wind in determining MLD suggests that even small changes in the Arctic freshwater budget may control MLD variability, with implications for mixing nutrients and heat up into the surface layer and photic zone.