Heat Content In The Beaufort Sea (Arctic Basin) From Ocean Currents Has Roughly Doubled In 30 Years
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Ocean heat in the Beaufort Gyre
Before the 2000s, typical BG halocline heat content per unit area was around 2 × 108 J m−2 (Fig. 2A). Since that time, there has been a sustained increase in heat content per unit area (local values reach beyond 4 × 108 J m−2 in the 2014–2017 time period), with maximal values centered over the Canada Basin coincident with the climatological BG center (Fig. 2) (1). Over the period 1987–2017, total warm halocline heat content integrated horizontally over a region encompassing the BG has nearly doubled (Fig. 3A). It is instructive to set the resulting heat content increases in context alongside sea ice. The capacity for sea ice melt of the additional heat content (the increase of ~2 × 108 J m−2 over 30 years) equates to a change of about 0.8 m in thickness, taking the latent heat of melting to be 2.67 × 105 J kg−1 and the density of sea ice to be 900 kg m−3.
Both increased temperatures and a thickening of the warm halocline layer [associated with spin-up of the BG and accumulation of freshwater; see (1)] contribute to the observed heat content increase (Fig. 3B). The contributions to changes that result from either layer thickness h change (of the 31 ≤ S ≤ 33 layer) or layer-averaged temperature change may be examined by writing the change in heat content ΔQ = Qf − Qi (subscripts i and f denote initial and final, respectively) as ΔQ = [Qi(hf/hi − 1)] + [Qf − (hf/hi)Qi], where the first term in square brackets is the contribution due to layer thickness change (from hi to hf), and the second term is the contribution due to temperature change. There is interannual variability in which contribution has a bigger influence on the overall heat content change, but neither of these presents as the dominant factor (not shown).
ACCUMULATION OF HEAT
Halocline ventilation
The source of the increased halocline heat content can be understood by first considering how the BG halocline is ventilated. The northern Chukchi Sea (NCS) region exerts major influence on the interior structure of the halocline; here, water masses with the salinity range of the warm halocline outcrop at the surface (11). In this region, which we define to be within 70°N to 75°N and 150°W to 170°W, and south of the 300-m isobath (Fig. 2E), water is pumped down from the surface (via the Ekman transport convergence as a result of the prevailing anticyclonic wind stress gradients) and transported laterally by the BG geostrophic flow into the interior gyre (9, 11). Observations suggest that the NCS is characterized by the strongest time-mean Ekman downwelling in the entire Canada Basin, with downwelling rates averaging around 20 m year−1, which corresponds to a vertical Ekman flux of around 0.05 Sv (1 Sv = 106 m3 s−1) for the region (12). This strong downwelling, associated with the region of maximum strength of the prevailing easterlies, takes place year-round with some interannual variability, but no significant trend over 2003–2014 [see Figs. 4 to 6 in (12)].
A major oceanographic feature of relevance in the NCS is a surface front in the vicinity of the Chukchi slope. The front marks the lateral transition between relatively warm (in summer/fall) and salty surface waters (and a deeper mixed layer) to the south, and cool and fresh surface waters (and a shallower mixed layer) to the north, toward the interior of the BG freshwater center. Water at the surface on the south side of the front is transferred below the mixed layer and into the interior halocline by subduction: vertical Ekman pumping plus lateral induction. It should be noted that there are likely other physical mechanisms at play in this important region that depend on the details of surface buoyancy forcing and sea ice state [for example, (13, 14)], local winds, and properties, dynamics, and stability of regional boundary currents [for example, (8, 15–17)]. The ventilation rate from this region (combination of Ekman downwelling and lateral induction) is estimated to be around 0.2 Sv (11). The cause of the warming halocline can be discerned by examining surface ocean temperatures over this region of maximum subduction, the portal for halocline ventilation.
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https://advances.sciencemag.org/content/4/8/eaat6773