Strong Stratification in the Makarov Basin, Arctic Ocean, Observed Via

Home , Halocline

22nd Australasian Fluid Mechanics Conference AFMC2020 Brisbane, Australia, 7–10 December 2020 https://doi.org/10.14264/8a0d13f

Strong stratiﬁcation in the Makarov Basin, Arctic Ocean, observed via intimate means

A. B. McCallum1 and K. Suara2

1 School of Science & Engineering University of the Sunshine Coast, Sippy Downs QLD 4558, Australia 2 School of Mechanical, Medical & Process Engineering Queensland University of Technology, Brisbane QLD 4000, Australia

Abstract collection is difficult or impossible. Daily oceanographic measurements were undertaken using Over the period 2009 to 2011 the Catlin Insurance company CTD and ADCP on an intimate over-ice expedition from the sponsored three over-ice expeditions across various segments vicinity of the North Pole towards Greenland, over a six week of the high Arctic Ocean. Ice-based traverse teams operated period, in the spring of 2011. These data were collected to bet- in ‘expedition’ mode and collected many continuous days of ter understand Arctic Ocean dynamics, particularly in this area glaciological, oceanographic and meteorological data, at high of the high Arctic where collection of high-density, undisturbed temporal and spatial resolution, across rarely visited sections of sub-ice data is extremely uncommon. We provide preliminary the Arctic Ocean. analysis of a sub-set of these data, collected in the Makarov Basin using a lightweight, portable Castaway CTD, primarily In this paper, we present a sub-set of CTD data from the 2011 validating existing observations of ocean dynamics in this data- Catlin Arctic Survey traverse of the eastern Makarov Basin, pauce region. The surface mixed layer was observed to ex- where daily CTD and ADCP sampling were conducted over a ∼ tend to a depth of ∼ 40 m and calculated buoyancy frequen- 6-week period, from the vicinity of 88o06.39N 134o08.37W to cies exceeded 0.025 s−1. This indicates very strong thermoha- 86o22.06N 94o47.05W, west of the Lomonosov Ridge (Figure line stratification, consistent with extended temporal datasets, 1). reflecting influx of fresh surface water. Intimate over-ice expeditions are relatively cheap, especially compared to alternative collection tools such as icebreakers. They provide a flexible data-collection method that can deliver high spatial and temporal resolution, in otherwise inaccessible areas of the polar oceans.

Keywords Arctic; oceanography; CTD; dynamics; stratigraphy; expeditions.

Introduction The Arctic Ocean is strongly stratified because it receives large amounts of fresh water from Eurasian runoff compared to other global oceans [9]. The system is stratified as follows: a cold relatively-fresh surface layer, a warm salty layer at depth, and the cold halocline, which is an intermediate mixed layer. Al- though not as well studied as other global oceans, efforts have Figure 1. Approximate route (red arrow) of the 2011 Catlin Arctic been made to assess Arctic ocean stratification over long tem- Survey over the period 6th April to 2nd May 2011, from 88o06.39N poral periods [2]. Such datasets can be used to simulate the o o o dynamics of different fluid layers in Arctic Ocean models [3]. 134 08.37W to 86 22.06N 94 47.05W.

Due to global warming, climate models have predicted a greater We present these data firstly, to assess the existence of stratifi- than 30% increase in the amount of fresh water entering the cation, and to examine the thickness of the upper mixed layer, Arctic Ocean by the end of this century [9]. This inflow can in this rarely sampled area of the Arctic Ocean, and secondly, alter the level of stratification in the system. Therefore, there to validate the utility of this intimate and cost-effective, over- is a need to increase the spatio-temporal coverage of observa- ice data-gathering method. With polar icebreaker costs ap- tions in the Arctic Ocean, to enable accurate model calibration. proaching $200,000 US per day (personal communication, E. o However, data from the high Arctic, north of 80 N are partic- Carmack, November 2019), the financial and research value of ularly sparse, especially outside of the summer period. This is over-ice research expeditions, that could capture data for a pe- because most data are collected from ships or ship-based par- riod of a month, for similar funding, is apparent. ties and drifter buoys. Furthermore, sea ice extent during the winter period limits the employment of such methods. A cost- Data collection and method effective, complementary approach, is the collection of data via manual deployment of instrumentation across several locations, To examine the level of stratification in the eastern Makarov using ‘intimate’ over-ice expeditions. This approach can be par- Basin during spring, in−situ depth profiles of conductivity and ticularly useful in providing high temporal and spatial data den- temperature were collected daily at ∼ 1800 GMT using a Son- sity in locations or during times when ship or buoy-based data Tek Castaway CTD. Before instrument deployment, a hole was drilled through the sea ice with a Strikemaster Mora ice auger. Figure 3 presents vertical profiles of potential temperature, The ice thickness varied from ∼ 0.3 m to > 3.0 m while the salinity and potential density, collected over the period 5 - 13th air temperature ranged from -25 oC to -5 oC. The Castaway April 2011. Daily depth-profiled CTD data were averaged; was lowered and then retrieved, using a hand-driven winch, each day contained a minimum of two profiles. Data were then mounted to a sledge. When data collection occurred in the binned into regular depth bins of 1 m; each bin had a minimum vicinity of leads through the sea ice, no hole was drilled, and of nine data points. Increasing bin size had insignificant effect the device was lowered directly into the water; a typical em- on the vertical distribution, whilst reducing the bin size resulted ployment method is shown (Figure 2). Measurement depth was in a significant number of cells having no data points. limited to 300 m because of the limited wire capacity of the hand-pulled sledges. Data were gathered during lowering and retreival of the CTD. Data were stored internally and photographs of CTD-traces were transmitted daily via satellite modem to expedition head- quarters in London, to enable real-time inspection and further post-processing. Raw data were quality controlled by remov- ing clear outliers. The boundary layer directly beneath the ice is complex and can be affected by parameters such as ice form and age, ridges and leads, buoyancy, drag coefficients and drill effects, and slush and heat produced [13]. Therefore, the upper few metres (up to 2.5 m) were omitted from the CTD datasets. Potential temperature (oC), absolute salinity (psu) and potential density (kg/m3) were derived (following the new thermodynamic equation of seawater) from in situ temperature and salinity collected by CTD each day [6]. Stratification extent within the observed Surface Mixed Layer and the halocline layer can be assessed using the Brunt-Vaisala buoyancy frequency, otherwise known as the buoyancy frequency, N. This quantifies the resistance of the fluid flow to vertical perturbation and is defined as:

g ∂ρ N2 = −( )( θ ) (1) ρo ∂z

3 where ρo = 1030 kg/m is reference density, ρθ is the potential density and g is the gravitational acceleration.

Figure 3. Vertical proﬁles of (a) Potential temperature; (b) Salinity; (c) Potential density.

Data were obtained to depths > 250 m, and the three typical layers of the upper Arctic Ocean [8] are apparent: a well- mixed, cold and fresh surface mixed layer (SML), Atlantic Wa- ter (AW) (deﬁned by T > 0oC), lying below ∼ 160 m, and the halocline, between these two layers, where salinity and density increase rapidly with depth. The relatively cold fresh surface layer insulates the surface and therefore the ice from the deeper, warmer and saltier Atlantic Water [1]. Correspondingly, Fig- Figure 2. SonTek Castaway CTD being lowered through a hand- ure 4 presents the averaged buoyancy frequency proﬁle across the transect during the period. The upper fresh surface mixed drilled hole in the sea ice utilising a sledge-mounted hand-winch. Image layer and the cold halocline are characterised with buoyancy c Martin Hartley. frequency (N2) varying from 0.5 to > 6 × 10−4 s−2.

Results Discussion Typically, expedition members travelled ∼ 10 nm per day, al- The surface mixed layer is observed to extend to a depth of ∼ though continuous variable sea ice drift meant that distance be- 40 m; this is consistent with observations by [12], [1], [18] etc. tween daily sampling sites varied. Some lateral spatial variation Throughout the data collection period it was nearly isothermal, is evident, but in this analysis, data were primarily processed with a potential temperature ∼- 1.5oC. The temperature gradi- and examined in terms of depth-variation. ent in the AW was generally limited to 0.5oC, exhibiting nearly somewhat less than the value of ∼ 0.045 s−1 reported by [16] at a depth of 40 m, in the Canada Basin in March 2010, at a latitude of ∼ 75oN, closer to shore.

Conclusions In this paper we examined a sub-set of high spatial and temporal resolution CTD data, that were collected by an intimate, cost-effective, over-ice research expedition. Analysis of these spatially and temporally constrained data with respect to depth, indicates the existence of typical Arctic Ocean stratigraphy, observed at atypical latitudes. Some spatial variation in salinity and potential density are evident, and variation in observed layer depths are consistent with literature. Spatially-averaged water column stability represents very strong stratiﬁcation, probably due to the transitioning spring regime, during which our measurements were taken. Oceanographic data paucity at high latitudes in the Arctic Ocean precludes the optimal constraining of climatological models. We propose that cost-effective expeditions such as the Catlin-funded series of over-ice expeditions, could prove valuable to address these data deﬁciencies into the future.

Figure 4. Ensemble-averaged depth profile of buoyancy frequency Acknowledgements This study draws on data obtained as part of the Catlin Arc- tic Survey whose resources were made possible by the funding constant salinity and potential salinity. support of the Catlin Group. We thank the Explorer Team of the The thickness of the halocline increased throughout the sam- Catlin Arctic Survey (Ann Daniels, Tyler Fish and Phil Coates), pling period; this may be due to the lateral injection of relatively and Geo Mission Ltd. particularly Pen Hadow, for their invalu- fresh Pacific-origin waters, that strengthen stratification at in- able support during sample collection and processing. We also termediate depths (60-220 m) [11]. This thickening is primarily thank Victoria Thorneton-Field for her provision of the CTD evident through a deepening of the AW by ∼ 20 m. This may data. All data used to produce this paper are freely available correspond with deepening as the AW approaches the Cana- from the first author with condition that acknowledgement is dian Basin, described by [14]; changes due to both mixing from given to the Catlin Arctic Survey in any reports or publications above and with shelf waters [15]. Observed AW depth (∼ 160 using such data. to ∼ 180 m) is consistent with observations for 2011 by [10], who examined change in the Arctic Ocean halocline base depth, References over the period 1981 to 2017. Small temporal lateral variations in surface salinity and poten- [1] Aagaard, K., C. L. L. and Carmack, E. (1981). On the tial density are evident, to depths of ∼ 30 m; absolute surface halocline of the arctic ocean, Deep-Sea Research, 28A, salinity values (30-31 psu) are similar to those identified within 529–545. the region by [8]. Over this short sampling transect, sea ice [2] Bourgain, P. and Gascard, J. (2011). The arctic ocean halo- thickness fluctuated between ∼ 0.5 m and ∼ 2.0 m and surface cline and its interannual variability from 1997 to 2008, air temperature varied between ∼- 15oC and ∼- 25oC. These Deep Sea Research Part I: Oceanographic Research Pa- variations, particularly sea ice thickness, may have contributed pers, 58, 745 – 756. to observed changes in surface layer salinity. The buoyancy frequency peaks at a depth of 45 m. This is [3] Carmack, E., Polyakov, I., Padman, L., Fer, I., Hunke, E., one means of estimating the depth of the surface mixed layer Hutchings, J., Jackson, J., Kelley, D., Kwok, R., Layton, [4]. This estimate is consistent with the stratification evident in C., Melling, H., Perovich, D., Persson, O., Ruddick, B., temperature observations. Magnitude of the peak buoyancy fre- Timmermans, M.-L., Toole, J., Ross, T., Vavrus, S. and quency (∼ 0.026 s−1) indicates very strong thermohaline strat- Winsor, P. (2015) Toward quantifying the increasing role ification. This value is significantly higher than the spatially of oceanic heat in sea ice loss in the new arctic, Bulletin averaged value for the Arctic Ocean (∼ 0.0015 s−1) [5], and it of the American Meteorological Society, 96, 2015, 2079– is significantly greater than maxima determined by [17], when 2105. examining variations in N across eddies in the Canadian Basin, [4] Carvalho, F., Kohut, J., Oliver, M. J. and Schofield, O. collected between the months of August and November. The (2017). Defining the ecologically relevant mixed-layer magnitude of N that we observed also exceeds that presented depth for antarctica’s coastal seas, Geophysical Research by [7], in their examination of winter sea-ice melt in the Canada Letters, 44, 338–345. Basin; they reported values of ∼ 0.01 s−1, also in April (2010); note that our data were recorded ∼ 10o further north. [5] Griffiths, S. and Peltier, W. R. (2009). Modeling of polar ocean tides at the last glacial maximum: Amplifica- However, the level of stratification that we determined is simi- tion, sensitivity, and climatological implications, Journal lar to values calculated for the Makarov Basin by [2], who ob- of Climate, 22, 2905–2924. tained a peak buoyancy frequency of ∼ 0.021 s−1, associated with large freshwater inflow into the basin. The high value that [6] IOC, SCOR and IAPSO, The international thermody- we observed probably captures increasing spring ice melt; it is namic equation of seawater–2010: calculation and use of thermodynamic properties.[includes corrections up to 31st october 2015].

[7] Jackson, J. M., Williams, W. J. and Carmack, E. C. (2012). Winter sea ice melt in the canada basin, arctic ocean, Geo- physical Research Letters, 39.

[8] Morison, J., Steele, M. and Andersen, R. (2012). Hydrog- raphy of the upper arctic ocean measured from the nuclear submarine u.s.s. pargo, Deep-Sea Research, 45, 1998, 15– 38.

[9] Nummelin, A., Ilicak, M., Li, C. and Smedsrud, L. H. (2016). Consequences of future increased arctic runoff on arctic ocean stratiﬁcation, circulation, and sea ice cover, Journal of Geophysical Research: Oceans, 121, 617–637.

[10] Polyakov, I. V., Pnyushkov, A. V. and Carmack, E. C. (2018). Stability of the arctic halocline: a new indicator of arctic climate change, Enviromental Research Letters, 13.

[11] Polyakov, I. V., Pnyushkov, A. V., Rember, R., Padman, L., Carmack, E. C. and Jackson, J. M. (2013). Winter con- vection transports atlantic water heat to the surface layer in the eastern arctic ocean, Journal of Physical Oceanog- raphy, 43, 2142–2155.

[12] Pounder, E. R. (1986). Physical oceanography near the north pole, Journal of Physical Oceanography, 91.

[13] Shirasawa, K. and Ingram, R. G. (1991). Characteristics of the turbulent oceanic boundary layer under sea ice. part 1: A review of the ice-ocean boundary layer, Journal of Marine Systems, 2, 153–160.

[14] Shu, Q., Wang, Q., Su, J., Li, X. and Qiao, F. (2019). Assessment of the atlantic water layer in the arctic ocean in cmip5 climate models, Climate Dynamics, 53, 5279– 5291.

[15] Talley, L. D., Pickard, G. L., Emery, W. J. and Swift, J. H. (2011). Descriptive physical oceanography: an introduction, 6th edn., Academic Press, London., 2011.

[16] Timmermans, M.-L. and Marshall, J. (2020). Understand- ing arctic ocean circulation: A review of ocean dynamics in a changing climate, Journal of Geophysical Research - Oceans, 125.

[17] Timmermans, M.-L., Toole, J., Proshutinsky, A., Krish- ﬁeld, R. and Plueddemann, A. (2018) Eddies in the canada basin, arctic ocean, observed from ice-tethered proﬁlers, Journal of Physical Oceanography, 38, 133145.

[18] Toole, J. M., Timmermans, M.-L., Perovich, D. K., Kr- ishfield, R. A., Proshutinsky, A. and Richter-Menge, J. A. (2010). Influences of the ocean surface mixed layer and thermohaline stratification on arctic sea ice in the central canada basin, Journal of Geophysical Research: Oceans, 115.