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1
Long-term eddy modulation inhibited the meridional asymmetry of
halocline in the Beaufort Gyre
Jinling Lu1,2, Ling Du1,2, Shuhao Tao1,2
1 Frontier Science Center for Deep Ocean Multispheres and Earth System (FDOMES) and Physical Oceanography Laboratory,
Ocean University of China, Qingdao, 266100, China
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2 College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao, 266100, China
Correspondence to: Ling Du (duling@ouc.edu.cn)
Abstract. Under the background of wind forcing change along with Arctic sea ice retreat, the mesoscale processes undergoing
distinct variation in Beaufort Gyre (BG) region are more and more significant to oceanic transport and energic cascade, and
then these changes put oceanic stratification into a new state. Here the varying eddies and eddy kinetic energy (EKE) in the
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central Canada Basin (CB) and Chukchi–Beaufort continental slope are obtained based on mooring observations (2003–2018),
altimetry measurements (1993–2019) and reanalysis data (1980–2021). In this paper, the variability of halocline in BG
representing adjustment of stratification in the upper layer is shown so as to analyze how it occurs under significantly changing
mesoscale processes. We find that the halocline depth has deepened by ~40 m while that in the north has deepened by ~70 m
in the in the last nearly two decades by multiple data sets. The halocline depth lifting to the north initially was shifted to a final
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nearly symmetric structure. Eddy strength and Eddy induced low salinity water transportations have been continuously
increasing toward the central basin at the mean time the halocline depth and strength among the southern and northern parts
in the basin have reached a nearly identical and stable regime. It is clearly clarified that the long-term dynamical eddy
modulation through eddy fluxes facilitating the freshwater redistribution inhibited the meridional asymmetry of halocline of
the BG. Further research into high-resolution observations and data simulations can helps us to better understand the eddy
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modulation processes and its influence on large-scale circulation.
1 Introduction
Global temperatures have continued to rise since 1970s. The Arctic Ocean as the focal point of climate change research is the
region with the most dramatic global surface temperature warming (Huang et al., 2018), with the warming range as high as
1.2 °C /10a, more than twice the global average warming range, which is called “Arctic amplification” phenomenon (Serreze
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and Barry, 2011). These variations not only affect the upper ocean circulation, but also expose the Arctic atmosphere–ice–sea
system to rapid changes (Timmermans and Marshall, 2020). In this context, with summer sea ice declining in the Arctic
(Stroeve et al., 2007, 2014; Niederdrenk and Notz, 2018) shown by satellite derived data, the existence of more freshwater in
the upper layer makes local stratification alter and results in the redistribution of water masses. Meanwhile, the emergence of
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broader areas of open water in the Canada Basin (CB) leading to more active ocean–atmosphere interaction and more
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susceptible to atmospheric forcing have attracted more and more attention to the mesoscale processes.
The Beaufort Gyre (BG) located in the CB, a large-scale wind-driven anticyclonic circulation feature, storing a substantial
amount of freshwater in the CB (Proshutinsky et al., 2009, 2019), is accompanied by prevalent mesoscale eddies (Doddridge
et al., 2019; Manucharyan and Spall, 2016; Zhao and Timmermans 2015; Zhao et al. 2016). The halocline in the CB, a thick
layer with a double peak of stratification, is considered to be an insulating “density barrier” between the surface mixed layer
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and Atlantic water layer underneath (Bourgain and Gascard, 2011). The asymmetric stratification of the BG and halocline
vertical structure are payed attention in the recent researches (Kenigson et al., 2021; Zhang et al., 2023). The gyre is highly
asymmetric associated with surface forcing and topography with isohalines steeper in the south and east compared with those
in the north and west (Zhang et al., 2023). The increase of isopycnal slope with depth can be attributable to the eddy-induced
streamfunction (Kenigson et al., 2021). Besides, the freshwater content (FWC) accumulated by Ekman convergence has
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increased between 2003 and 2008 and remained relatively constant between 2008 and 2012 (Timmermans and Toole, 2023).
Likewise, observations indicated that Pacific Winter Water (PWW) layer has generally deepened during 2004–2018 while the
layer thickness has increased (Kenigson et al., 2021), which was identified an isopycnals deepening by 70 m during 2004–
2011 (Zhong et al., 2018), suggesting a spin-up of the gyre. Previous works about eddies in the CB or the Arctic Ocean were
mostly based on satellite (e.g., Kozlov et al., 2019; Kubryakov et al., 2021, Raj et al., 2016), in situ hydrographic data (e.g.,
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Fer et al., 2018; Timmermans et al., 2008; Zhao et al., 2014, 2016; Zhao and Timmermans, 2015), high eddy-resolving
simulations (e.g., Reagan et al., 2020; Wang et al., 2020) and etc. Eddy activity, a common feature in the halocline of the BG,
is also focused by many past studies. Moreover, the kinetic energy in the halocline of BG was mainly dominated by mesoscale
eddy activities (Zhao et al., 2016, 2018). Eddies are distributed at different depths in the Arctic Ocean and mainly concentrated
at subsurface (Zhao et al., 2014) even they may extend to thousands of meters in depth. The depth of maximum value is
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generally found about 70–110 m in the halocline (Wang et al., 2020). Based on 127 eddies observed at drifting sea ice stations,
Manley and Hunkins (1985) found that the eddy kinetic energy (EKE) accounted for about one-third of the total kinetic energy
(TKE) of the upper 200 m in the CB. From the perspective of the horizontal pattern, EKE derived by satellite is also higher
along main boundary currents and continental shelves in the Arctic Ocean (Timmermans and Marshall 2020). Zhao et al. (2016)
kept Ice Tethered Profiler (ITP) measurements for temperature, salinity and current between 2005 and 2015 to survey the
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changes of eddy field in the CB. They found that eddies were mostly distributed in the western and southern parts of the CB.
As was showed that the number of eddies in the lower halocline doubled from 2005–2012 to 2013–2014 (Zhao at al., 2016)
with the past increasing of FWC, the gyre areas and strength (Regan et al., 2019; Timmermans and Toole, 2023; Zhang et al.,
2016). The response of TKE and EKE to the spin-up of the gyre during 2003–2007 in particular showed that EKE at subsurface
has generally strengthened (Regan et al., 2020). It was also demonstrated by a recent research that with wind energy input
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increasing into BG due to significant loss of sea ice after 2007 eddy activities would also be more active (Armitage et al.,
2020).
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Mesoscale eddies can transmit momentum, heat, water masses and chemical compositions, not only contributing to
atmospheric circulation, mass distribution and marine biology, but also playing an important role in global ocean heat balance
(Chelton et al., 2007). Eddies are not only exhibiting unprecedented changes but also playing a crucial role in the Ekman-
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driven BG stability in the context of sea ice loss (Manucharyan et al., 2016). They can balance atmosphere–ocean and ice–sea
stress input, gradually weaken the slope of isopycnals and geostrophic currents and counteract the accumulation of FWC driven
by Ekman pumping through dissipating available potential energy (APE). The eddy activity as a key physical process affects
the release and accumulation of freshwater and ultimately influences the formation of halocline (Manucharyan and Spall,
2016). Except that, the Ekman pumping and sea ice are also major factors affecting the dynamics of halocline. This balance
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between halocline and eddies is thought to occur on different time scales in realistic models (Doddridge et al., 2019;
Manucharyan et al., 2017) and suggests a link between small-scale features and changes to the large-scale circulation.
However, with sea ice condition changing due to global warming, long-term variability of eddies in the central basin and basin
boundary regions is still unsolved. Furthermore, according to the standpoint about possible gyre’s stabilization in recent years
(Proshutinsky et al., 2019; Zhang et al., 2016) the eddy modulation in the halocline of the BG on a long timescale is still
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unknown. Due to the influence of the measurement conditions, limited satellite observation, now continuous observation data
of eddies in space and time is relatively scarce. Data coverage in space and time is yet to be improved (Zhao et al., 2016). The
results of numerical simulation lack effective data to support, so researches on oceanic mesoscale eddies remain uncertain to
some extent. Here we used multiple data sets containing moored, in situ and satellite altimetry observations, in comparison
with reanalysis data, to quantify the strength of mesoscale processes by SLA and horizonal currents. The stationary eddies and
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EKE as well as the transformation of halocline across the basin are both pointed out to assess the low frequency variability of
halocline in BG under significantly changing mesoscale eddies. Section 2 presents the details of data and methodology. Section
3 demonstrates the halocline variability especially on its meridional asymmetry in the BG region. And eddy distribution and
interannual changes are discussed in Section 4. Section 5 explains the correlation of EKE and geostrophic currents as well
significant eddy modulation in the halocline. Section 6 is summary and discussion in this paper.
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2 Data and methods
2.1 Observations and ocean reanalysis data
In this paper we used multiple data sets including hydrographic observations, satellite altimetry and reanalysis data sets. The
hydrographic data are in situ measurements from Conductivity Temperature Depth (CTD) and mooring data from McLane
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Moored Profilers (MMPs) at four moorings that are all deployed under the Beaufort Gyre Exploration Project (BGEP,
http://www.whoi.edu/beaufortgyre/data). The reanalysis data sets used here mainly consists of World Ocean Atlas 2018
(WOA18) and Simple Ocean Data Assimilation (SODA, version 3.4.2).
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Annual hydrographic survey through ship based CTD has been conducted in the BG region each year between August and
October. CTD data between 2004 and 2021 are used to mainly investigate spatio-temporal variability of oceanic stratification
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across the CB. The positions of CTD instruments deployed is shown in Fig. 1a. Plus, to supplement long-term trends and
changing characteristics of halocline and to capture mesoscale eddies at representative stations in the CB, mooring data
deployed at four corners around the basin (Fig. 1b) between mid 2003 and mid 2018 above 500 m are also analyzed. Each
mooring system included a MMP that returned profiles of horizontal velocity, temperature, salinity, pressure and etc. A pair
of upgoing/downgoing profiles (separated by 6 hours) was returned every other day, data were processed to a vertical resolution
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of 2 dbar. The shallowest moored measurement varies from about 50–90 m (depending on the mooring and sampling period)
to avoid collisions with ice keels and the deepest measurements are to 2000 m.
Figure 1. (a) The positions of in situ sites of CTD measurement from BGEP in a certain months during 2004–2021. The purple bar
indicates an artificially selected meridional transect with a width of 36km mostly along 150°W but partially bent at the southwestern
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continental slope in Beaufort Sea. (b) A map of the Canada Basin and the bathymetric contours upper than 4000 m isobath. Coloured
diamonds denote the locations of four BGEP moorings. The two chosen regions are shown by green (AL, Alaskan coast) and black
(BSS, Beaufort Sea slope) boxes respectively. (c) The distribution of mean kinetic energy at 50 m. Vectors denote the directions of
mean currents. Gray lines denote the 300 m, 1000 m and 3000 m bathymetry. (d) The distribution of horizontal gradient of potential
density (shading and vector) at 50 m. Vectors point in the directions of increasing potential density. The results of (c) and (d) are
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calculated from the 2005–2017 WOA climatology.
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The SODA reanalysis data set is developed by the University of Maryland based on the Global Simple Ocean Data Assimilation
System, which adopted in this paper is the 5 days averaged from 1980 to 2021, with a horizontal resolution of 1/2°×1/2° and
vertically divided into 50 layers with unequal spacing. We obtained the gridded altimetry data (product identifier:
SEALEVEL_GLO_PHY_L4_REP_OBSERVATIONS_088_047) over the years in the 1993–2019 period from the
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Copernicus Marine Environmental Monitoring Service (CMEMS). This product consists of daily gridded maps of dynamic
topography in ice-free regions that have been derived as a sum of mapped sea level anomalies (SLA) calculated from combined
measurements by different satellites and mean dynamic topography (MDT).
2.2 Methods
For estimating EKE to assess the strength of eddy activities, we used ocean current data from and SODA and altimetry.
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Geostrophic velocities are calculated from sea level hight. The horizontal velocity is decomposed into annual mean velocity
and abnormal value (Penduff et al., 2004; Rieck et al., 2015, 2018; Regan et al., 2020):
(1)
Note that the EKE in this paper is estimated by a low-frequency ‘‘eddy’’ which defined as a departure from a long-term
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temporal mean, with a period (it depends on temporal resolution of the data) of greater than 5 days or 1 day (Lucke et al.,
2017). In addition, the vertical velocity shear can be related to the large-scale density field by the thermal wind relation
(Meneghello et al., 2021)
(2)
where U is the horizontal current field, N is Brunt-Väisälä buoyancy frequency which represents oceanic stratification,
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is the isopycnal slope, ρ is potential density of sea water, ρo is the average density of seawater, g is the
gravity acceleration, and z is depth. Developed by (3), the horizontal velocity field is calculated by the integration with depth
from bottom to surface. The maps of horizontal velocity field (Fig. 1c) and density gradient (Fig. 1d) at 50 m in the CB are
shown, the main circulation feature is clearly discerned and southwestern basin near continental slopes is the key region for
varying currents tending towards high EKE and instability.
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For investigating the variation of halocline to understand shifting of oceanic stratification, we consider the depth of potential
density surface =27.4 (25) kg·m−3 to approximately represent the base (top) of the halocline (Timmermans et al., 2020).
Based on the upper and lower boundary of halocline, APE is defined as the amount of potential energy in a stratified fluid
available for mixing and conversion into kinetic energy (Huang 1998; Munk and Wunsch 1998) is following Eq. (3) (Polyakov
et al., 2018; Bertosio et al., 2022, partial modification):
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(3)
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where z1 and z2 represent the depth of halocline upper and lower boundary, and ρref is potential density at the base of the
halocline.
Furthermore, for discerning the critical role of mesoscale eddies in balancing the halocline, we consider the eddy advection
velocity in the (y, z) plane can be defined from an eddy streamfunction as
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(4)
and is represented as (Manucharyan et al., 2016; Manucharyan and Spall, 2016; Manucharyan and Isachsen, 2019)
(5)
where
is the average meridional eddy salt flux and
is the average vertical salt gradient (Marshall and Radko, 2003).
Here bars and primes correspond to annul mean and perturbation variables. Due to buoyancy mainly controlled by salinity in
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Arctic, represents the cumulative effects of eddy thickness fluxes that arise from correlations between eddy velocities and
eddy induced isopycnal displacements. Overall, when vertical salt gradient is generally negative in the CB a positive value of
indicates a southward (northward) transportation of high(low) salinity water and vice versa.
If eddy genesis is related to baroclinic instability, baroclinic growth rate ω is correlated with EKE. The baroclinic growth rate
ω can be estimated here by (Simth, 2007)
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(6)
where
is the Richardson number. We call the inverse of this quantity ω–1=T the “Eady timescale”.
The Eady timescale should be short where there is anomalously high EKE or weak stratification.
3 BG halocline variability
This section is aimed to investigate the spatio-temporal variability of halocline in the BG region, particularly its varying
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asymmetry inside is the main focus of this article. The halocline’s depth, thickness and strength and vertical structure are
detailedly analyzed below, all of which indicate its meridional asymmetry at the mean time.
3.1 Temporal variation of the halocline
Under the spin-up of the BG, isopycnals of the PWW layer in the cold halocline have deepened (Kenigson et al., 2021). We
have chosen the special isopycnal surface to characterize the top and base of the halocline. Figure 2a and b show the
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discontinuous variation of the halocline upper/lower boundary and thickness at four moorings from MMP. As a whole, the
rangeability of the halocline top is much smaller than that of the halocline base, and the depth of the halocline upper/lower
boundary at single mooring shows basically consistent trends, so we mainly focus on the variation of halocline base depth. But
there are different characteristics of variation during 2003–2018 despite of void measurements in time. Finite results at mooring
C show that an increasing trend of depth and thickness of the halocline before 2008. Besides, other moorings provided results
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over a longer term, which captured a deepening of the halocline base and an increasing of thickness over the years from 2003
to 2018. The thickness of halocline at mooring B located in the northwestern part of CB increased steadily by about 70 m, at
the same time the depth of halocline base deepened by up to 70 m over the years 2003–2018. The thickness of halocline in the
southern part of the basin (moorings A and D) both increased by about 30 m company with the halocline base deepening
approximately 40 m. It’s worth noting that the depth of halocline has a stagnant phase even opposite development over the
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years between 2003–2007 and 2015–2018. Particularly, linear trends and mean values of the halocline depth and thickness in
three periods (2003–2007; 2008–2014; 2015–2018) are computed (Table 1). A negative trend of halocline depth is clearly
during 2008–2014 in the southern sites of the basin (moorings A and D), but the former and latter periods both mostly exhibit
positive trends in halocline depth and thickness. The variation at the only northern site (mooring B) covering three periods
shows entirely different features, the halocline thickness reveals a negative trend in the third period (after 2015) that eventually
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remains a steady level while the depth of that still keeps on deepening. In final, the halocline thickness and depth at every site
tends to be homogeneously distributed and the differences are obviously shrunken than before.
Figure 2. Time series of (a) depth of isopycnals 25 kg/m3 (upper coloured lines) and 27.4kg/m3 (upper coloured lines) representing
the top and base of the halocline (b) halocline thickness between isopycnals 25 kg/m3 and 27.4 kg/m3 and (c) APE for moorings A, B,
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C and D during 2003–2018. Note that the abnormal values record eddies were existent at that time.
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APE, a good integral indicator of changes in overall halocline strength in the CB, is also computed here by Eq. (4). As is shown
in Fig. 2c, the variation is similar with that of halocline thickness. Initially, APE revealed a striking difference between the
northern (moorings B and C) and southern sites (moorings A and D) around the basin. The trend of APE showed a weak
decreasing after 2008 and then recovered to some extent at the southern moorings. In contrast, APE at the northern moorings
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kept on improving until 2014 and then the growth stagnates. The difference among moorings reduced in final, and APE all
remained about 3×105 J/m2, that was the maximum value over the years. We infer the variability of halocline and APE have
a relationship with the BG spin-up and largest increasing of FWC during 2003–2007 (Giles et al., 2012; Krishfield et al., 2014;
Timmermans and Toole, 2023). And then partial variables exist stagnant in the post spin-up term during 2008–2014 (Regan et
al., 2020).
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Table 1. Trends (whthin the brackets, unit: m/yr) and mean values (outside the brackets, unit: m) of the halocline’s top, base and
thickness in three periods for moorings A, B, C and D, respectively.
Periods
Moorings
2003–2007
2008–2014
2015–2018
A
top
75.4(–2.7)
77.8(–2)
86.2(–1.19)
base
236.4(7.3)
261.1(–4.49)
278.1(7.91)
thickness
161.0(10.04)
183.3(–2.48)
191.9(9.10)
B
top
69.1(0.47)
69.8(–1.98)
66.8(–1.13)
base
184.1(4.94)
241.08(5.35)
252.62(3.57)
thickness
115.0(4.46)
171.28(7.33)
185.82(–2.44)
C
top
74.26(–5.74)
base
186.54(2.73)
thickness
112.29(8.47)
D
top
69.31(2.16)
73.58(–3.92)
81.52(3.09)
base
223.69(0.4)
239.44(–0.35)
267.62(8.89)
thickness
154.37(–1.76)
165.86(3.57)
186.1(5.8)
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3.2 Changes of meridional asymmetry
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The gyre located in the CB is marked by a pronounced asymmetry (Regan et al., 2019) with changing spatial distribution of
the freshwater and ocean dynamic height. The isopycnal slope is steeper over the southern continental slope than that in the
northern basin (Fig. 1d), almost in line with previous researches (Proshutinsky et al., 2019; Regan et al., 2019; Zhang et al.,
2023). The former observations have revealed that isopycnals have deepened with different rates among the northwestern and
northeastern parts in the basin during 2002–2016 (Zhong et al., 2019). According to section 3.1, we find the main differences
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of evolution only between northern and southern basin are obvious, which is not completely identical with previous findings.
Therefore, we next turn to the inhomogeneous gridded in situ hydrographic data from the latest CTD observation so as to get
a better understanding of overall asymmetric halocline across the basin. From the perspective of the horizontal maps in the
three periods (Fig. 3) that are determined referring to the trends of halocline variables at the moorings, the spatial patterns of
the halocline base and APE, implying the location and strength of the BG in the basin, both show evident changes. In the first
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period, maps of APE and halocline exhibit the same asymmetry and then there are an overall deepening of halocline as well
as a gradual decreasing of spatial difference. The maximum of halocline depth is in the interior of the basin. At the mean time,
APE in the latest period is much more remarkable than that in the first term along the continental slopes of Canadian Arctic
Archipelago and Northwind Ridge where isopycnal gradient and baroclinic instability are significant as well as in the abyssal
plain where the halocline base is deepest.
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Figure 3. (a-c) Horizontal distribution of depth of halocline base across the Canada Basin in 2004–2007 (before 2008), 2008–2014,
and 2015–2021 (after 2014), respectively. (d-e) As the same with (a-c), but for APE in every period (integration between the top and
base of halocline).
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In addition, the in situ hydrographic data are interpolated onto the regular grids to examine the varying vertical structures of
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halocline along the selected transect (Fig. 1a). Notably, the hydrographic structures along 150°W and 140°W sections are
similar (Timmermans and Toole, 2023). Thus, we only select a representative north–south transect mainly along 150°W to
analyze here. The vertical distribution of the isopycnal σ= 27.4 kg·m−3 surface show that it is shallowest ~ 200 m at the
margins of the BG region and up to 80 m deeper in the interior BG in the later years (Fig. 4). Among the early, median and
later years shown, the vertical structures of isopycnals especially the lower boundary of the halocline reveal apparent changes
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between the marginal and interior gyre. From transects of potential density (Fig. 4), initially there was an distinct uplift of the
halocline towards the north with the depth of halocline base in the south (~74°N) about 50 m lower than in the north (~77°
N). The difference between the north and the south was narrowed with isopycnals generally deepening from the view of the
average vertical structure during 2008–2014, and even the northern halocline is lower than southern district (the difference is
less than 10 m). In the third period (after 2014), the depth of the halocline has changed less in comparison with the previous
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periods, and the halocline is clearly meridionally symmetric shaped like a horizontal bowl, as if it has reached a state of
equilibrium. As can be seen from the spatial maps and vertical structures of the halocline and APE, the characteristic of
meridional asymmetry was gradually weakening in recent years. We infer there is possibly existing other physical process join
in the variability and we are plan to discuss below.
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Figure 4. Vertical transects of potential density using data from CTD measurements in 2004–2007 (before 2008), 2008–2014, and
2015–2021 (after 2014), respectively. The dashed (solid) lines indicate depth of σ = 25 (27.4) kg·m–3 representing the top (base) of
halocline.
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4 Spatio-temporal variability of eddy activity
As was revealed by the former research, a regime shift of the BG occurred in 2007–08, with a spin-up phase of the gyre
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occurred from 2003 to 2007 and a stabilization after 2007 (Regan et al., 2020). The depth, strength, core location of halocline
all imply the shift of the gyre and FWC variability. With BG spin-up and environmental conditions changing, mesoscale eddies
are responding to dissipate extra energy input and influence the potential energy redistribution. In this section the spatio-
temporal variability of eddies by eddy detection and EKE (a critical criterion to measure the strength of eddies) will be
discussed.
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4.1 Eddy vertical distribution
We now outline how mesoscale eddies can be detected based on moored observations. When eddies occur locally, there are
strong horizontal velocities accompanied by isopycnal displacements. As for anticyclonic (cyclonic) eddies, the isopycnals are
convex (concave). By distinguishing the anormal speeds larger than 10 cm/s and the isopycnal displacements which are both
the criterion used in the past works (Timmermans et al., 2008; Zhao et al. 2014; Zhao and Timmermans, 2015), we counted
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the annual number of eddies in the upper layer (Fig. 5a). In all, there are 37, 40, 7 and 43eddies detected above 500 m at
mooring A-D, respectively. They are mostly concentrated between the upper and lower halocline boundaries. As is the same
with previous works (e.g., Zhao et al., 2014; Zhao and Timmermans, 2015), in the majority of instances, the abnormal
temperature/salinity and convex isopycnal displacements in the eddy core are pervasive. The all cold-core eddies are accounted
for 61.4%. Most of these are anticyclones and only 3 eddies detected at mooring C are cyclones. The cold-core anticyclones
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are popular in the BG region due to oceanic stratification and large-scale dominated circulation. Furthermore, for the location
of mooring C less controlled by the BG, characteristics of eddies there is different from others. Some of eddies are cyclone
that are seldom discovered at other moorings. The existence of cyclones are related to frontal instability near 80° N that
contributes the eddy formation (Manucharyan and Timmermans, 2013; Timmermans et al., 2008).
Additionally, comparing the vertical structures of EKE profiles in three periods at the moorings, we find that EKE changed
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significantly above halocline in the three periods (Fig. 5b). The EKE below halocline is relatively weaker than that in the upper
layer, and its multiyear variation is much smaller. The vertical structures of EKE in the basin and its marginal seas can be
classified into two types. The first type is that EKE is surface-intensified up to ~ 0.01 m2/s2 at surface and it decays with depth.
The second one is bimodal with separate comparably high values at surface less than 50 m and subsurface approximately 90–
250 m between the upper and lower halocline boundaries. In the first period, EKE above the BG halocline remained at a
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relatively low level, and it has increased in the second period when the BG circulation appeared to be stabilizing (Zhang et al.,
2016). The results from three moorings (all of them except mooring C are detected after 2008) show that EKE was strengthened
to varying degrees, accompanied by a deepening of the halocline lower boundary. At the southwestern corner (A) of the basin，
EKE increased in the second period and remained stable in the third period; northwestern (B) EKE strengthened in the second
period and weakened in the third period; southeastern (D) subsurface EKE didn’t occur apparent growth until the third period.
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https://doi.org/10.5194/egusphere-2023-501
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Figure 5. (a) Hovmöller diagrams of depth against time showing annual single eddy counts in the upper layer at moorings A–D,
respectively. Blue, purple and green shadings denote the spans of three periods. (b) Vertical profiles of mean EKE for four moorings
over years in the three periods. Coloured stars indicate the depths of the halocline base in corresponding periods.
4.2 Interannual EKE patterns
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The BG region, a focal area for mesoscale phenomenon in the previous studies (Armitage et al., 2020; Regan et al., 2020; Zhao
and Timmermans, 2015; Zhao et al., 2016), mainly consists of a southern narrow continental shelf close to the Alaska coast
and a sizable deep basin. The Chukchi–Beaufort slope is the major sector for eddy generation by baroclinic instability (Spall
et al., 2008) with a surface front approximately along 300 m isobath (Timmermans and Toole, 2023), and then eddies carrying
pacific water propagate to the central BG by boundary current. Here we focus on this area to investigate the variety of EKE
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from a broad perspective used by satellite derived dynamic heights. Further, we seek for the main EKE patterns at surface in
the three periods. As is shown (Fig. 6), the high value areas of EKE is mainly located along the continental slopes of the
marginal CB especially the Alaska coast mostly between 1000 m and 3000 m isobaths. Indeed, energy is the strongest at the
southwestern shelf break of CB near the Barrow Cape which can even reach more than 5×10–3 m2/s2 while it is even less than
1×10–3 m2/s2 in the interior basin. Notably, the horizonal pattern of EKE is not identical with that of MKE obtained by annual
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mean geostrophic current (not shown here). On the whole the area where EKE is strongest is closer to the inshore shelf sea
side than the area where MKE is strongest. EKE in every term is all significantly enhanced comparing with that in the former
term, and the strong EKE gradually developed from coasts to offshore and the central basin with time. For instance, from the
interannual mean horizonal patterns the region with the strongest EKE was mostly concentrated at southern part of 72°N if
https://doi.org/10.5194/egusphere-2023-501
Preprint. Discussion started: 27 March 2023
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we on