The sudden ocean warming and its potential influences on early-frozen landfast ice in the Prydz Bay, East Antarctica

The sudden ocean warming and its potential influences on early-frozen landfast ice in the Prydz Bay, East Antarctica

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Haihan Hu¹^,², Jiechen Zhao³^,⁴^,^*, Jingkai Ma⁵, Igor Bashmachnikov⁶^,⁷, Natalia Gnatiuk⁶^,⁷, Bo Xu⁸, Fengming Hui¹^,²^,^*

Acta Oceanologica Sinica | 2024, 43(5) : 65 - 77

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Acta Oceanologica Sinica | 2024, 43(5): 65-77

• Physical Oceanography, Marine Meteorology and Marine Physics •

The sudden ocean warming and its potential influences on early-frozen landfast ice in the Prydz Bay, East Antarctica

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Haihan Hu¹^,², Jiechen Zhao³^,⁴^,^*, Jingkai Ma⁵, Igor Bashmachnikov⁶^,⁷, Natalia Gnatiuk⁶^,⁷, Bo Xu⁸, Fengming Hui¹^,²^,^*

Affiliations

¹ School of Geospatial Engineering and Science, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China

² Key Laboratory of Comprehensive Observation of Polar Environment (Sun Yat-sen University), Ministry of Education, Zhuhai 519082, China

³ Qingdao Innovation and Development Base (Centre) of Harbin Engineering University, Qingdao 266500, China

⁴ Key Laboratory for Polar Acoustics and Application of Ministry of Education (Harbin Engineering University), Harbin 150001, China

⁵ Key Laboratory of Research on Marine Hazards Forecasting, National Marine Environmental Forecasting Centre, Beijing 100081, China

⁶ Nansen International Environmental and Remote Sensing Centre, St. Petersburg 199034, Russia

⁷ Saint Petersburg State University, St. Petersburg 199034, Russia

⁸ Antarctic Great Wall Ecology National Observation and Research Station, Polar Research Institute of China, Shanghai 201209, China

Published: 2024-05-25 doi: 10.1007/s13131-024-2326-7

Outline

Abstract

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The ocean conditions beneath the ice cover play a key role in understanding the sea ice mass balance in the polar regions. An integrated high-frequency ice-ocean observation system, including Acoustic Doppler Velocimeter, Conductivity-Temperature-Depth Sensor, and Sea Ice Mass Balance Array (SIMBA), was deployed in the landfast ice region close to the Chinese Zhongshan Station in Antarctica. A sudden ocean warming of 0.14℃ (p < 0.01) was observed beneath early-frozen landfast ice, from (−1.60 ± 0.03)℃ during April 16–19 to (−1.46 ± 0.07)℃ during April 20–23, 2021, which is the only significant warming event in the nearly 8-month records. The sudden ocean warming brought a double rise in oceanic heat flux, from (21.7 ± 11.1) W/m² during April 16–19 to (44.8 ± 21.3) W/m² during April 20–23, 2021, which shifted the original growth phase at the ice bottom, leading to a 2 cm melting, as shown from SIMBA and borehole observations. Simultaneously, the slowdown of ice bottom freezing decreased salt rejection, and the daily trend of observed ocean salinity changed from +0.02 d⁻¹ during April 16–19, 2021 to +0.003 d⁻¹ during April 20–23, 2021. The potential reasons are increased air temperature due to the transit cyclones and the weakened vertical ocean mixing due to the tide phase transformation from semi-diurnal to diurnal. The high-frequency observations within the ice-ocean boundary layer enhance the comprehensive investigation of the ocean’s influence on ice evolution at a daily scale.

Key words

sudden ocean warming / oceanic heat flux / landfast ice / Zhongshan Station / in-situ observation

Cite this Article

Haihan Hu, Jiechen Zhao, Jingkai Ma, Igor Bashmachnikov, Natalia Gnatiuk, Bo Xu, Fengming Hui. The sudden ocean warming and its potential influences on early-frozen landfast ice in the Prydz Bay, East Antarctica[J]. Acta Oceanologica Sinica, 2024 , 43 (5) : 65 -77 . DOI: 10.1007/s13131-024-2326-7

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1 Introduction

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Sea ice is a fundamentally important component of the Antarctic climate system (Parkinson and Cavalieri, 2012; Stammerjohn et al., 2012; Singh et al., 2021). Its seasonal growth and retreat affect almost every aspect of the Antarctic system, from atmospheric stability and ocean dynamics to ice sheet mass balance and biological productivity (Maykut and Untersteiner, 1971; McMinn et al., 2000; Heil, 2006; Massom et al., 2009, 2010; Miles et al., 2017; Moreau et al., 2020). Correspondingly, the seasonal change of sea ice is also affected by the seasonal thermodynamic ice-ocean feedbacks (Himmich et al., 2023; Purich and Doddridge, 2023). Decadal hindcasts showed that the Southern Ocean subsurface warming has persisted and gradually destabilised the ocean, becoming one of the influential factors in the dramatic changes observed in Antarctic sea ice (Meehl et al., 2019; Zhang et al., 2022). Oceanic heat flux is an essential parameter in determining the thermodynamical thickness of sea ice (Maykut and Untersteiner, 1971; Ebert et al., 1995; Zhao et al., 2017a; Guo et al., 2019). Oceanic heat flux is related to the heat storage and turbulent mixing in the boundary layer (McPhee, 1992). Sea ice growth and melt are determined by the heat balance between the oceanic heat flux and the conductive heat transfer through the overlying ice cover (Maykut, 1986). Accurately quantifying the changes in heat flux caused by ocean warming is essential for understanding the process of sea ice growth.

Landfast ice near Zhongshan Station, East Antarctica, usually attached to the coastline, shelf edge, and grounded icebergs. In contrast with pack ice, landfast ice has a longer annual duration and is generally thicker, and its growth is mainly influenced by thermodynamic processes (Heil et al., 1996; Heil, 2006; Lei et al., 2010; Zhao et al., 2017a, 2019; Li et al., 2023). The ice growth or melt rate at the ice-ocean interface is determined by the heat flux balance between that transmitted through the ice layer and that supplied to the ice bottom by the ocean (Lytle et al., 2000; Lei et al., 2022). Previous studies in the coastal regions of the Prydz Bay suggest that oceanic heat flux has a seasonal cycle with the maximum flux in early-frozen stages (Allison, 1981; Heil et al., 1996; Lei et al., 2010; Yang et al., 2016; Zhao et al., 2019; Hu et al., 2023). These studies have mainly focused on seasonal variations in oceanic heat flux. At the same time, because it is difficult to observe the oceanic heat flux directly, most studies used the heat balance equation based on the sea ice bottom to calculate the oceanic heat flux (residual method).

The ice growth rate is relatively high in the early-frozen stages due to cold air forcing. The rapid rejection of brine by sea ice makes the thermohaline environment of sea water beneath sea ice change rapidly, which makes the change of oceanic heat flux more intense. To further explore the variations of ocean temperature, salinity, velocity and oceanic heat flux on the growth of landfast ice, a set of high-resolution ice-ocean observation equipment was deployed on the landfast ice about 1 km away from Zhongshan Station, including an Acoustic Doppler Velocimeter (ADV), Conductivity-Temperature-Depth (CTD) Sensors, and a Sea Ice Mass Balance Array (SIMBA).

Previous researchers have studied oceanic heat flux under landfast ice near the Zhongshan Station, based on the residual method. Li et al. (2023) analysed observations from IMB buoys near Zhongshan Station during 2013 to 2018 and found that during the early-frozen stages, the rapid growth rate and salt rejection process resulted in relatively large oceanic heat flux in June (about 22 W/m²). Lei et al. (2010) obtained an oceanic heat flux of approximately 20 W/m² in April 2006 based on field measurements and Yang et al. (2016) reported 25 W/m² based on model estimates. Zhao et al. (2019) found that the oceanic heat flux remained at 20–40 W/m² during the initial ice growth period in 2012. Compared to the residual method, the direct high- frequency measurements of ocean parameters in the mixed layer can provide more accurate estimates of oceanic heat flux by the bulk parameterisation method, as the previous studies conducted in the Arctic and Antarctic (McPhee, 1992; Maykut and McPhee, 1995; McPhee et al., 1996, 2008; Sirevaag, 2009; Sirevaag and Fer, 2009; Kirillov et al., 2015; Peterson et al., 2017; Lei et al., 2022; Hu et al., 2023).

Based on the high-frequency observations during 2021 near Zhongshan Station, Hu et al. (2023) revealed the seasonal variations of oceanic heat flux beneath the landfast ice and further determined that the high-frequency time series of oceanic heat flux exhibited larger perturbations than the residual methods, and more important, higher peaks were found during sudden ocean warming event. To further investigate the atmosphere-snow-ice-ocean interactions during those unusual sudden warming, this study chose the significant event that occurred in April as an example and focused on the synoptic scale, a period of 7-day, to analyse the sudden ocean warming during the early-frozen season.

Therefore, this study is an extent to Hu et al. (2023), and the data used focuses on the time series in April. By analysing the variations in ocean temperature, salinity, and current, along with their impact on oceanic heat flux, this study also tries to discuss the potential influence of the tide and cyclones on the sea ice evolution on a synoptic scale.

The details of observations and methods are presented in Section 2. The observations were deeply analysed and fully described in Section 3. The relationship between air-ice-ocean is discussed in Sections 4 and 5. The conclusions are presented in Section 6. This paper will help better understand the oceanic effects on the diurnal scale of landfast ice growth in the Prydz Bay, Antarctica.

2 Data and methods

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2.1 Field observations

Chinese Zhongshan Station is located in the Prydz Bay, East Antarctica (69°22'S, 76°22'E) (Fig. 1a). It was established in February 1989 and operated year-round from then on. The extent of landfast ice may reach 60–100 km north of Zhongshan Station in the cold season and break up due to the mechanical forcing in the austral summer (Li et al., 2020; Zhao et al., 2020). In 2021, an integrated high-frequency ice-ocean observation system was established by the wintering team of Zhongshan Station from April on the coastal landfast ice about 1 km off Zhongshan Station (Fig. 1b). A cable-type CTD (model: ALEC ACTD-DF) was deployed 2 m beneath the ice surface, which records ocean temperature, conductivity (salinity) and depth. The measurement frequency was set to 30 s for the salinity (conductivity) and temperature sensors, with accuracy of ±0.02 mS/cm (±0.03) for conductivity (salinity) and ±0.02℃ for temperature. An Acoustic Doppler Velocimeter (model: SonTek Argonaut-ADV) was deployed 5 m below the ice surface, with a 3-D ocean velocity sensor and one temperature sensor. The velocity and temperature observations frequency was 40 s, and the velocity sensor accuracy was ±0.001 m/s. A Sea Ice Mass Balance Array (model: SRSL SIMBA) was deployed north of the CTD. The SIMBA contained 240 temperature sensors mounted on the thermistor string with 2 cm intervals to measure the vertical temperature profile across the air-snow-ice-ocean column every 6 h. The SIMBA temperature sensors had a resolution of ±0.0625°C. As per the instrument deployment, the study numbered the temperature sensors from the lowest to the highest. According to the instrument records, the sensor labelled 180 was positioned on the surface of the sea ice, suggesting that around 1.2 m of the temperature chain remained in the atmosphere above the ice surface. Also, the sea ice thickness was under 0.5 m throughout the week, signifying that roughly 3 m of the temperature chain lay beneath the bottom of the sea ice. Snow depth and ice thickness were measured manually around the observation site by the wintering team members.

A data quality control was applied to the original time series to eliminate the anomalous values. All the time series were integrated into 2-min intervals for the convenience of the inter-comparisons. The study used observational data during April 16–23, 2021, during which all instruments were operational. The analysis was confined to a brief period due to technical issues with the equipment.

2.2 Reanalysis products

In this study, the reanalysis data was utilized to investigate the potential impact of polar cyclones on local atmosphere and ice cover conditions. The European Centre for Medium-Range Weather Forecasts (ECMWF) provides a European Reanalysis dataset (ERA). ERA5 is the fifth generation ECMWF reanalysis for the global climate and weather for the past eight decades. Reanalysis combines model data with observations from worldwide into a globally complete and consistent dataset using the laws of physics (Hersbach et al., 2020). It provides hourly estimates for atmospheric and horizontal grid resolutions of 0.25° × 0.25°. The ERA5 dataset is freely accessible at https://cds.climate.copernicus.eu. This study obtained hourly mean sea level pressure over the Prydz Bay in Antarctica during April 16–24, 2021 in the ERA5 reanalysis dataset.

2.3 Methods of ocean-to-ice heat flux calculation

Ocean-to-ice heat flux, also known as oceanic heat flux underneath the ice, can be parameterised by many formulas according to the available parameters we observed in the ocean-ice-atmosphere boundary layer (McPhee, 1979, 1992; McPhee and Untersteiner, 1982; McPhee et al., 2008). This study adopted two parameterisations, residual method and bulk parameterisation, as described below.

2.3.1 Residual method

The residual method used here to calculate oceanic heat flux is a modified version of the classical Stefan Law. It estimates oceanic heat fluxes by analysing ice vertical temperature profiles and measuring ice bottom growth or ablation. This method has been widely used in previous studies (McPhee and Untersteiner, 1982; Lytle et al., 2000; Perovich and Elder, 2002; Purdie et al., 2006; Lei et al., 2010; Zhao et al., 2019). At the ice bottom, the heat flux balance could be expressed as follows:

(1)

$ {F}_{\mathrm{w}}={F}_{\mathrm{c}}+{F}_{\mathrm{l}}+{F}_{\mathrm{s}}, $

where F_w and F_c are the oceanic and conductive heat flux, respectively; F_l is the latent heat flux caused by the freezing or melting of the sea ice; F_s is the specific heat flux generated by the change in ice temperature. In Eq. (1), the signs of melting, heating, and upward heat flow are positive; otherwise, they are negative.

The heat flux terms and freezing point

$ {T}_{\mathrm{f}} $

can be further expressed as follows (Semtner, 1976; Millero, 1978; Lei et al., 2014):

(2)

$ {F}_{\mathrm{c}}={k}_{\mathrm{i}}\frac{{T}_{0}-{T}_{\mathrm{f}}}{h}, $

(3)

$ {F}_{\mathrm{l}}=-{\rho }_{\mathrm{i}}{L}_{\mathrm{i}}\frac{\mathrm{d}H}{\mathrm{d}t}, $

(4)

$ {F}_{\mathrm{s}}={\rho }_{\mathrm{i}}{c}_{\mathrm{i}}h\frac{\mathrm{d}T}{\mathrm{d}t}, $

(5)

$ -{T}_{\mathrm{f}}=0.057\;5{S}_{\mathrm{w}}-1.710\;523\times {10}^{-3}{S}_{\mathrm{w}}^{3/2}+2.154\;996\times {10}^{-4}{S}_{\mathrm{w}}^{2}, $

where k_i is the ice thermal conductivity; T₀ is the ice temperature in the reference layer (details are provided in Section 4.1); h is the ice thickness of the reference layer and was set to 0.2 m, according to McPhee and Untersteiner (1982); ρ_i is the landfast ice density; L_i and c_i are the latent and specific heat capacity of the sea ice; H is ice thickness and dH/dt is the ice growth rate; dT/dt is the temporal gradient of ice temperature (Untersteiner, 1961; Millero, 1978; McPhee and Untersteiner, 1982; Lei et al., 2010, 2014; Zhao et al., 2022). T₀ was measured by the SIMBA buoy, and

$ {S}_{\mathrm{w}} $

was ocean salinity measured by CTD in this study. The ice thickness (H) were estimated from SIMBA buoy, according to the method of Zhao et al. (2017b).

2.3.2 Bulk parameterisation

The bulk parameterisation of oceanic heat flux relies on the direct measurements of the high-frequency current velocity, temperature, and salinity in the ice-ocean boundary layer beneath the ice cover, which is called the bulk parameterisation method (McPhee, 1992; McPhee et al., 2008). F_w could be calculated by the following formula (Guo et al., 2015):

(6)

$ {F}_{\mathrm{w}}={\rho }_{\mathrm{w}}{c}_{\mathrm{w}}\langle{w}'{T}'\rangle, $

where ρ_w is the ocean density, c_w is the specific heat capacity of the ocean;

$ \langle{w}'{T}'\rangle $

is the turbulent heat flux.

The ocean density (

$ {\rho }_{\mathrm{w}} $

) is a function of temperature and salinity. Millero and Poisson (1981) proposed an internationally recognised atmospheric equation for the state of the ocean, which is calculated as follows:

(7)

$ {\rho }_{\mathrm{w}}-{\rho}_{0}=A{S}_{\mathrm{w}}+B{S}_{\mathrm{w}}^{3/2}+C{S}_{\mathrm{w}}^{2}, $

where,

$ \rho_0 $

(unit in kg/m³) is the absolute densities, and could be calculated using the value for pure water from Bigg (1967):

(8)

$\begin{split} {\rho }_{0}=\ & 999.842\;594+6.793\;952\times {10}^{-2}{T}_{\mathrm{w}}-\\&9.095\;290\times {10}^{-3}{T}_{{\mathrm{w}}}^{2}+1.001\;685\times {10}^{-4}{T}_{{\mathrm{w}}}^{3}-\\&1.120\;083\times{10}^{-6}{T}_{{\mathrm{w}}}^{4}+6.536\;332\times {10}^{-9}{T}_{{\mathrm{w}}}^{5}.\end{split} $

The coefficients A, B, and C could be calculated as below (Millero and Poisson, 1981):

(9)

$\begin{split} A=\ &8.244\;93\times {10}^{-1}-4.089\;9\times {10}^{-3}{T}_{\mathrm{w}}+7.643\;8\times\\& {10}^{-5}{T}_{{\mathrm{w}}}^{2}- 8.246\;7\times {10}^{-7}{T}_{{\mathrm{w}}}^{3}+5.387\;5\times {10}^{-9}{T}_{{\mathrm{w}}}^{4}, \end{split}$

(10)

$ B=-5.724\;66\times {10}^{-3}+1.022\;7\times {10}^{-4}{T}_{\mathrm{w}}-1.654\;6\times {10}^{-6}{T}_{{\mathrm{w}}}^{2}, $

(11)

$ C=4.831\;4\times {10}^{-4}, $

where

$ {S}_{\mathrm{w}} $

and

$ {T}_{\mathrm{w}} $

are ocean salinity and temperature, which CTD measured in this study.

The heat transferred from the ocean to the ice depends on both the turbulent stress at the ice-ocean interface (characterised by frictional velocity

$ {u}_{0}^{\mathrm{*}} $

as the square root of the kinetic stress at the interface) and the effective ocean heat content in the turbulent boundary layer, which is roughly proportional to the height of the ocean temperature above freezing point (McPhee, 1992; McPhee et al., 1999; Kirillov et al., 2015). The turbulent heat flux could be parameterised as:

(12)

$ \langle{w}'{T}'\rangle={c}_{{\mathrm{H}}}{u}_{0}^{\mathrm{*}}\Delta T, $

where c_H is the Stanton number of heat exchange efficiency;

$ \mathrm{\Delta }T $

is usually expressed as the difference between the ocean temperature and the freezing point; and

$ {u}_{0}^{\mathrm{*}} $

is the friction velocity at the interface. The exchange coefficient c_H are constant of 0.0057 (McPhee, 2002). Therefore, Eq. (6) could be expressed as:

(13)

$ F_{\mathrm{w}}=\rho_{\mathrm{w}}c_{\mathrm{w}}c_{\mathrm{H}}u_0^{\mathrm{*}}\Delta T. $

Based on the methods mentioned in the previous study (McPhee, 1979), the friction velocity

$ {u}_{0}^{\mathrm{*}} $

can be expressed as follow:

(14)

$ {u}_{0}^{\mathrm{*}}=\sqrt{0.010\;4{\mathit{V}}^{1.78}}, $

where V is the absolute velocity relative to the motionless landfast ice, which the ADV observed in this study.

3 Ocean features beneath ice during the sudden warming

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3.1 Sudden warming and increased variability of ocean temperature

The CTD sensor measured ocean temperature and salinity at 2 m beneath the landfast ice surface. During April 16–23, 2021, the upper ocean layer beneath the ice showed an average temperature of (−1.53 ± 0.08)℃, while the ice temperature at the bottom (approximately 0.1 m above the ice bottom) was (−3.12 ± 0.71)℃. Consequently, heat flux transfer from warm water to ice is due to the temperature difference. A jump in ocean temperature was observed on April 20, 2021(Fig. 2). During April 16–19, 2021, the mean ocean temperature was (−1.60 ± 0.03)℃ and the maximum was −1.50℃, while the mean significantly increased to (−1.46 ± 0.07)℃ and the maximum rose to −1.29℃ during April 20–23, 2021. The sudden increase was significant, with a confidence level of 99%. The mean standard deviation also showed a significant increase from 0.03℃ to 0.07℃. The largest daily deviation was 0.35℃ on April 21, 2021, while only 0.12℃ on April 18, 2021.

The warmer ocean may allow more heat flux into the ice bottom, balancing the conductive heat flux and inhibiting sea ice growth. This is confirmed by the observed basal melt of 2 cm by SIMBA, as shown in Section 4.2.

3.2 Gradual increase of ocean salinity and density

Ocean salinity remained at 33.35 ± 0.03; the most significant daily deviation was 0.13 on April 16, 2021, and the smallest was 0.06 on April 19, 2021 (Fig. 3). The mean increasing trend was significant (0.02 d^–1) during April 16–19 and then small (0.003 d^–1) during April 20–23, 2021, which might be related to the slowdown of the ice bottom freezing due to the increase of the ocean temperature, which paused the salt rejection to the ocean.

The ocean density exhibited a similar increasing trend during April 16–23, 2021 (Fig. 4). The mean ocean density was (1026.84 ± 0.02) kg/m³. The mean increasing trend was 0.01 (kg·m⁻³)/d during April 16–19, while largely reduced to 0.003 (kg·m⁻³)/d during April 20–23, 2021. The daily deviation of ocean density varied between 0.05 kg/m³ and 0.11 kg/m³.

3.3 Periodic oscillation of ocean current

The 3-D velocity of ocean current at 5 m beneath the ice surface was obtained by ADV. Figure 5 shows the velocity in the meridional (U-component), zonal (V-component), vertical (W-component), and horizontal speed (V_h, the vector sum of U-component and V-component). During April 16−23, 2021, ocean currents showed periodic oscillation, with a mean V_h of about 3 cm/s and a maximum of up to 10 cm/s. The 2-min mean time series showed that 98% of the U-component and 95% of the V-component varied within ±5 cm/s, and 98% of the W-component varied within ±1 cm/s.

To further analyse the time series, the empirical mode decomposition (EMD) was used to retain the high-frequency component (Huang et al., 1998). Then, the periodogram method was used to detect the periodicity of the time series (Welch, 1967). The power spectrum analysis shows that a significant half-day period appeared in the U-component, V-component and V_h but not in the W-component. The domain oscillation periods of ocean currents transferred from half-day during April 16−19 to quarter-day, half-day, and one day during April 20−23, 2021 (Fig. 6).

The rose diagram of the 2-min mean V_h is shown in Fig. 7. The domain direction (34%) was southeast (120°–150°), and 70% of the velocity was within 2−8 cm/s. In the study area, the southeast direction means onshore current, which brings warm water in the deep sea to the shallow shore, leading to a jump in ocean temperature.

4 Influence of ocean-to-ice heat flux on ice evolution

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4.1 Sudden increase of ocean-to-ice heat flux

To further investigate the effect of sudden warming events on the growth of landfast ice, ocean-to-ice heat flux was calculated using two methods, the residual method and bulk method, similar to Hu et al. (2023).

In the residual method, the vertical gradient of ice temperature is important when calculating the conductive heat flux (F_c). In the cold and snow-free conditions, the vertical temperature profile was approximately linear, while the vertical temperature profile in the thick snow or warm cases was not linear (Lei et al., 2010; Zhao et al., 2019). Therefore, a reference layer (close to the bottom of the ice) is usually used to calculate the conductive heat flux. McPhee and Untersteiner (1982) chose a reference layer of 0.4 m above the ice bottom. Perovich and Elder (2002) used several reference layers between 0.4–0.8 m above the ice bottom for different ice thickness conditions. Lei et al. (2014) set the reference layer at 0.4–0.7 m above the ice bottom. In this study, the observed ice thickness was around 0.4 m. Therefore, the reference layer (h) was chosen 0.2 m above the ice bottom. The vertical temperature gradient in F_c was calculated using the SIMBA’s 2 cm interval temperature profile. The freezing points were estimated by Eq. (5) based on the ocean salinity observations recorded by the CTD. There is a possible difference between ice bottom salinity and 2 m depth salinity, but there are no detailed observations in this region.

Figure 8 shows the heat fluxes in the residual method. The variation of the latent heat flux (F_l) was strongly correlated with the growth and ablation of sea ice. During April 16–19, 2021, the sea ice bottom was freezing, and F_l was negative. During April 20-23, 2021, a 2-cm melting occurred at the ice bottom due to sudden ocean warming, and F_l was positive. F_s was relatively smaller throughout the investigation period, varying around 0. The mean oceanic heat flux was (35.1 ± 18.2) W/m² during the whole study period and showed a significant increase and reached the maximum on April 21, 2021 ((69.4 ± 4.3) W/m²). The mean oceanic heat flux was (24.2 ± 7.6) W/m² during April 16–19, 2021 and nearly doubled ((44.8 ± 19.4) W/m²) during April 20–23, 2021, which led to a 2 cm ice bottom melting.

In contrast to the residual method, bulk parameterisation methods were developed when ocean observations were available (McPhee, 1979, 1992; Sirevaag, 2009; Kirillov et al., 2015).

During April 16−23, 2021, the mean freezing point was −1.828℃, gradually decreasing to the minimum of −1.834℃ on April 21, along with increased ocean salinity. The height of ocean temperature above the freezing point (ΔT) increased gradually, from (0.23 ± 0.03)℃ during April 16–19 to (0.36 ± 0.08)℃ during April 20–23, 2021.

In the bulk parameterisation method, the mean oceanic heat flux was (32.3 ± 20.2) W/m² during April 16−23, 2021, increasing from (21.7 ± 11.1) W/m² during April 16−19 to a nearly double value of (44.8 ± 21.3) W/m² during April 20−23, 2021 (Table 1).

The diurnal variations of oceanic heat flux produced from the two methods were similar (Fig. 9b), which were relatively small before April 20, 2021 and increased significantly afterwards. Compared with the residual method, the daily variance of the bulk parameterisation method was more significant, which may relate to the high sampling frequency of ocean observations.

The F_w results in this study by residual method (15–75 W/m²) and bulk parameterisation (10–90 W/m²) are similar to those of previous observations, but the peak value of oceanic heat flux is larger. Moreover, the bulk parameterisation method based on high-frequency observational data provides a more detailed characterisation of the variability in oceanic heat flux.

4.2 The observed melt in the ice bottom

During the sudden ocean warming event, the ice bottom also changed significantly. SIMBA recorded the vertical temperature profiles through the atmosphere-snow-sea ice-ocean column. As Fig. 10a shows, the mean air temperature during April 16–23, 2021 was about (−10.85 ± 3.82)℃, and the daily variation of the mean air temperature was significant, up to 6.46℃ between two adjacent days. The ice temperature showed a significant gradient between 0.11℃/cm and 0.24℃/cm. The ocean temperature maintained −1.70℃ to −1.90℃, close to the freezing point. The ice thickness was estimated through visual interpretation of the temperature profile of the SIMBA temperature chain according to the method of Zhao et al. (2017b).

The results showed an increase of 2 cm from April 16 to 21, 2021, consistent with the increase of ice thickness from 0.44 m to 0.46 m recorded by the borehole drilling in the field. Soon afterwards, a 2-cm ice bottom melting was detected by SIMBA during April 21–23, 2021, along with the sudden rise of ocean temperature and corresponding ocean-to-ice heat flux. The borehole drilling also confirmed this ice bottom melt. During the study period, air temperature and ice surface temperature increased gradually. The gradual increase in sea ice temperature reduced the temperature gradient within the sea ice (Fig. 10c). Therefore, warm air also contributed to the 2-cm ice bottom melt during April 20–23, 2021.

5 Discussion

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5.1 Potential influences of the tide on sudden ocean warming

The current recorded by ADV indicated diurnal back-and-forth ocean movements under the landfast ice cover in the influences of tides, and the results of the further analysis indicated a dominant direction of the southeast (34% of total records). The tidal oscillations were reconstructed using the harmonic analysis method (Pan et al., 2018), and the harmonic constants come from E et al. (2013). The hourly tidal level experienced a decrease from around 1.3 m on April 16 to around 0.5 m on April 23, 2021 (Fig. 11), which indicates that the vertical mixing induced by the tides decreased. Meanwhile, the tide period gradually changed from 0.5 d to 1 d after April 21, 2021, indicating that the tide pattern changed from semi-diurnal to diurnal.

The comparison of current direction and ocean temperature measured by ADV indicates that the southeast (120°–150°) current was the dominant component during the study period (Fig. 12a). Figure 12b shows the ocean temperature and salinity evolution observed by CTD. The ocean mass changed from low temperature and low salt ((−1.59 ± 0.02)℃ and 33.29 ± 0.03 on April 16, 2021) to high temperature and high salt ((−1.45 ± 0.07)℃ and 33.38 ± 0.02 on April 23, 2021). These phenomena may be related to the weakening of tide-induced vertical mixing during this period.

5.2 Potential influence of polar cyclone on air warming

In this study, the cyclone activities in the Prydz Bay (65°–70°S, 70°–80°E) and the outer Southern Ocean (50°–70°S, 50°–100°E) were statistically analysed. The hourly mean sea level pressure in the hourly ERA5 dataset was averaged to obtain the daily mean sea level pressure during April 16–24, 2021 (Fig. 13). Meanwhile, the changes in mean sea level pressure, 2-m air temperature and 10-m wind speed in the Prydz Bay were calculated (Fig. 14) to analyse the influence of polar cyclone activities on the air temperature and sea ice concentration.

Two cyclones traversed the Prydz Bay from west to east during the study period. On April 17, a cyclone structure (cyclone A) formed on the western side of the Prydz Bay (Fig. 13b) and gradually progressed eastward along the Antarctic continent when the 2-m air temperature was −18℃ in the Prydz Bay and sea ice concentration was 75%. By April 19, 2021, cyclone A reached the outer edge of the Prydz Bay, exhibiting a minimum central pressure of 960 hPa (Fig. 13c). The intensification of cyclone A brought an influx of warm air, causing a rapid increase in the average 2-m air temperature within the Prydz Bay to –11.7℃, with an average maximum wind speed of approximately 14 m/s. As cyclone A continued its eastward progression on April 19, 2021, mean sea level pressure within the Prydz Bay partially recovered to 973 hPa, wind speeds weakened, and temperatures slightly decreased. Concurrently, a new cyclone (Fig. 13d, cyclone B) formed on the western side of the Prydz Bay, exhibiting a lower central pressure (948 hPa) and longer duration than cyclone A. During April 20–21, 2021, the mean air temperature was –10.4℃, and the mean wind speed was 11 m/s. The successive passage of two cyclones increased sea ice concentration to 90% in the Prydz Bay (Fig. 10c). Therefore, the cyclones caused a significant rise in air temperature (about 10℃ increment) and the sea ice concentration (from 75% to 90%), which were both favours of the sudden warm of ocean temperature.

5.3 Sensitivity experiments of oceanic heat flux

Sensitivity experiments were conducted to estimate the response of ice growth rate to the different oceanic heat flux (Fig. 15). Ice growth rate was firstly calculated based on Eq. (1), with oceanic heat flux from the bulk method as shown in Fig. 9a. This can be considered as the control run and named as Exp. A. Two sets of sensitivity experiments were defined: Exp. B assuming no sudden ocean warming and maintained the mean ocean temperature (–1.60℃) during April 16–19 to the subsequent period of April 20–23, 2021; Exp. C assuming no oceanic heat flux in the subsequent period of April 20–23, 2021.

In Exp. A, ice growth rate during April 16–19, 2021 was 0.6 cm/d, leading to an ice thickness increase of 2.3 cm; while during April 20–23, 2021, ice growth rate was –0.5 cm/d, and ice thickness decreased by 1.9 cm. The calculated increase and decrease were in line with the measurement results. Exp. B assumed that no ocean temperature rise occurred after April 20, 2021. Ocean temperature remained constant at –1.60℃ (mean during April 16–19, 2021) to subsequent period of April 20–23, 2021, and the corresponding mean oceanic heat flux was (29.0 ± 12.5) W/m², 35% smaller than the actual conditions. The ice growth rate during April 20–23, 2021 was 0.1 cm/d, and ice thickness would increase 0.3 cm. If oceanic heat flux was ignored in Exp. C, landfast ice showed a larger rate of 1.3 cm/d, and the ice thickness would increase by 4.5 cm during April 20–23, 2021.

6 Conclusions

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The ice-ocean boundary layer beneath sea ice cover plays a vital role in the growth and melting of sea ice. An integrated high-frequency ice-ocean observation system, including Acoustic Doppler Velocimeter, Conductivity-Temperature-Depth Sensor, and Sea Ice Mass Balance Array, was deployed in the landfast ice region close to the Chinese Zhongshan Station in Antarctica. The observed minute-resolution ocean temperature, salinity, density, current, and heat flux were analysed and a sudden ocean warming event occurred during April 16–23, 2021 and was further investigated in this study.

The sudden ocean warming shifted the growth phase to melt phase at the ice bottom. The CTD at 5 m beneath the landfast ice surface recorded a jump increase of ocean temperature (p < 0.01) from (−1.60 ± 0.03)℃ during April 16–19 to (−1.46 ± 0.07)℃ during April 20–23, 2021. Meanwhile, the SIMBA temperature chain recorded that landfast ice froze by 2 cm at the bottom during April 16–20 while melting by 2 cm during April 21–23, 2021.

The salt rejection at the ice bottom slow down and the growth rate of ocean salinity and density decreased. The mean ocean salinity and density were 33.53 ± 0.03 and (1026.84 ± 0.02) kg/m³ during the study period. The ocean current recorded by ADV showed that 34% of the current direction was southeast (120°–150°), and the mean velocity was (3.1 ± 1.7) cm/s, with a maximum up to 10.6 cm/s for 2 min time series and 9.0 cm/s for hourly mean time series.

The sudden ocean warming lead to increasing of oceanic heat fluxes. The estimated values became double after April 20, 2021, which was related to both the increase in ocean current velocity (from 2.69 cm/s to 3.68 cm/s) and the rise of ocean temperature (from −1.60℃ to −1.46℃). The increase in oceanic heat flux was the direct trigger on 2 cm basal melt of the landfast ice thickness.

The tide transformation from semi-diurnal to diurnal during the study period was possibly attributed to the significant rise in ocean temperature. During April 16−23, 2021, the tidal amplitude weakened, and the period of current variation became longer. Two cyclones passed across the Prydz Bay during the study period, which brought warm air and increased the air temperature, as the observations show. The sensitivity experiments with different oceanic heat fluxes showed that in the absence of sudden ocean warming events, the ice bottom would grow by about 0.3 cm during April 20−23, 2021. Therefore, the ocean currents influenced by the coastal tide and the local atmosphere conditions played essential roles in the ocean temperature and further affected the oceanic heat flux and the ice growth.

In this study, the high frequency of oceanic measurements provides a chance to investigate the influences of oceanic regimes beneath landfast ice on the diurnal scale. The current analysis timeframe is limited. Additional years of field observations will be undertaken to acquire an extended time series of data and to delve deeper into the intricacies of air-ice-ocean interactions.

Acknowledgements

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We are grateful to Antarctic Zhongshan Ice and Space Environment National Observation and Research Station of Polar Research Institute of China for providing the experimental sites and relevant supports.

Funding

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The National Natural Science Foundation of China under contract Nos 42276251, 42211530033, and 41876212; the Taishan Scholars Program.

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Year 2024 volume 43 Issue 5

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doi: 10.1007/s13131-024-2326-7

Receive Date：2023-12-20
Online Date：2025-11-18
Published：2024-05-25

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Received：2023-12-20
Accepted：2024-03-20

Funding

The National Natural Science Foundation of China under contract Nos 42276251, 42211530033, and 41876212; the Taishan Scholars Program.

Affiliations