Field observations and results of a 1-D boundary layer model for developing near-surface temperature maxima in the Western Arctic

Abstract

Gallaher, SG et al 2017 Field observations and results of a 1-D boundary layer model for developing near-surface temperature maxima in the Western Arctic. Elem Sci Anth, 5: 11, pp. 1-21, DOI: https://doi.org/10.1525/elementa.195 IntroductionRecent changes in the Arctic ice-ocean system have led to an increase in upper ocean heating. The primary source of this heating is the two-fold rise in ocean-absorbed solar radiation (Perovich et al., 2007) that results from rapidly declining summer sea ice extent (Comiso et al., 2008;Steele et al., 2010). Recent studies in the Canada Basin show that this absorbed solar heating is partitioned 0.23/0.77 between ocean heat storage and latent heat loss (basal ice melt), respectively (Toole et al., 2010;Gallaher et al., 2016). Most of the oceanic heat is accumulated in near-surface temperature maximum (NSTM) features. The NSTM is defined as an upper ocean (< 50 m) temperature maximum that: 1) is at least 0.2°C above freezing (δT); 2) has a salinity < 31; and 3) resides above a cooler water layer by at least 0.1°C (Jackson et al., 2010). Jackson et al. (2010) attribute NSTM development to the absorption of solar radiation in shallow, stratified layers beneath melting sea ice and open water during summer. Steele et al. (2011) present an additional formation process caused by cooling of the near-surface ocean under open water areas in late summer, which leaves behind a warmer subsurface layer. Although NSTM heat is gained in the summer, the release of this heat often occurs in later seasons. Observations in the Canada Basin show that the NSTM often survives into fall, and that heat from this layer can be mixed into the surface mixed layer to delay or slow freeze up (Steele et al., 2008;Jackson et al., 2010;Steele et al., 2011;Timmermans, 2015).Early studies of the NSTM during AIDJEX (Maykut and McPhee, 1995) and SHEBA (McPhee et al., 1998) found that the layer was present directly below the summer surface mixed layer, at depths between 25 and 35 m. However, the Canada Basin upper ocean is freshening (McPhee et al., 2009) Summer sea ice extent in the Western Arctic has decreased significantly in recent years resulting in increased solar input into the upper ocean. Here, a comprehensive set of in situ shipboard, on-ice, and autonomous ice-ocean measurements were made of the early stages of formation of the near-surface temperature maximum (NSTM) in the Canada Basin. These observations along with the results from a 1-D turbulent boundary layer model indicate that heat storage associated with NSTM formation is largely due to the absorption of penetrating solar radiation just below a protective summer halocline. The depth of the summer halocline was found to be the most important factor for determining the amount of solar radiation absorbed in the NSTM layer, while halocline strength controlled the amount of heat removed from the NSTM by turbulent transport. Observations using the Naval Postgraduate School Turbulence Frame show that the NSTM was able to persist despite periods of i...

Exploring the Pacific Arctic Seasonal Ice Zone With Saildrone USVs

“…The benefits of using automated, uncrewed sampling platforms to increase our ability to observe the SIZ has been demonstrated by several research programs (see Lee et al, 2017, and references therein). Recent SIZ sampling strategies include deploying instrumentation on or in the ice (e.g., Polashenski et al, 2011;Timmermans et al, 2014;Gallaher et al, 2017), or on autonomous underwater vehicles and open water platforms, such as moored-buoys and wave gliders (Wood et al, 2013). Surface drifters (e.g., Thomson, 2012;Banzon et al, 2020) capable of measuring environmental parameters such as air and water temperature and wind speed, have also been deployed in open water and partial ice cover.…”

The importance of capturing late melt season sea ice conditions for modeling the western Arctic ocean boundary layer

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“…Melting causes ponds to form on the surface of the sea ice (Figure 8a), decreasing surface albedo and initiating a strong feedback mechanism where reduced albedo increases absorption of solar radiation, driving more melt and thus further reductions in albedo. Gallaher et al (2017) use AOFB data to show that the eventual drainage of these melt ponds provides a significant source of buoyant water to maintain strong near-surface stratification. This inhibits vertical mixing and thus helps generate and preserve the near-surface temperature maximum throughout much of the open water season.…”