Heat Insulation and Dissipation Processes in Nordic Seas in the Summer

GAO Lin, ZHAO Jinping, *, LI Shimin, FAN Xiutao, and LIU Shixuan

Heat Insulation and Dissipation Processes in Nordic Seas in the Summer

GAO Lin1), ZHAO Jinping1), *, LI Shimin2), FAN Xiutao3), and LIU Shixuan3)

1) College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao 266100, China 2) National Marine Environmental Forecasting Center, Beijing 100000, China 3) Institute of Oceanographic Instrumentation, Shandong Academy of Sciences, Qingdao 266100, China

The Nordic Seas have a significant impact on the climate system. Here 23-day air-sea heat fluxes were analyzed from anair-sea coupled buoy deployed in the Lofoten Basin from 5 August 2012 to 27 August 2012. The buoy captured two stages of strong south and north winds. The observations indicate that warm and wet air transported by the south wind can lead to decreased sensible and latent heat fluxes and net longwave radiation. The total oceanic heat loss was 50–60Wm−2. Thus, this stage was called the heat insulation process. By contrast, the heat dissipation process occurred with the north wind condition during advection of the cold and dry air. During this process, sensible and latent heat fluxes and net longwave radiation notably increased. The total oceanic heat loss during the heat dissipation process reached 240Wm−2, which was four-fold greater than that in the heat insulation process. Given that the heat insulation process is dominant in summertime, the ocean lost minimal heat but absorbed strong solar energy. Thus, a large quantity of energy was stored in the ocean. Heat was transported to the Arctic Ocean and accelerated Arctic warming. The heat dissipation process is dominant in autumn and winter when the ocean releases considerably more energy. The two processes revealed in this paper can be applied to warm-water areas in high latitudes.

Nordic Seas; radiation flux; heat flux; air-sea interactions

1 Introduction

The Nordic Seas lie between the North Atlantic Ocean and Arctic Ocean and include the Norwegian Sea, Green- land Sea, and Iceland Sea. Ridges around Jan Mayen Island divide this sea area into four basins: Greenland Basin, Norwegian Basin, Iceland Basin, and Lofoten Basin. The warm and salty water from North Atlantic Ocean across the Greenland-Scotland Ridge flows into the east of Nordic Seas as the Norwegian Atlantic Current (NAC) and flows into the Arctic Ocean through Barents Sea and Fram Strait. The cool and fresh water from the Arctic Ocean flows into the Nordic Seas through Fram Strait and flows out of the Nordic Seas through Denmark Strait. Affected by the sea floor topography, multiple separations occur from the main branch of the warm currenta process called recirculation (Mauritzen, 1996a, 1996b). The property of recirculated water is changed by mixing with colder and fresher water, a phenomenon referred to as recirculation of Atlantic water (Rudels, 2002). The upper Nordic Seas comprise three main areas: the west area that is directly influenced by cold water from the Arctic; the east area that is directly influenced by warm water from the Atlantic Ocean; the area that is influenced by recirculating water in central Nordic Seas (Swift and Aagaard, 1981; Rossby, 2009). Each area is separated by an ocean front (Aken, 1995; Kostianoy and Nihoul, 2009; He and Zhao, 2011).

The air-sea interaction in the Nordic Seas is significant due to the heat transport by the North Atlantic Current (Simonsen and Haugan, 1996). Observations indicate that the air-sea heat flux in Iceland Sea exhibits a notable relationship with the north wind brought by cold air outbreaks(CAOs) (Harden, 2015). In winter, severe CAOs that originate from Fram Strait and the icy edge of Greenland Sea may cause oceanic heat loss in Nordic Seas (Papritz and Spengler, 2017; Aemisegger and Papritz, 2018; Messori, 2018; Papritz and Sodemann, 2018; Papritz, 2019). Large heat transport from the ocean to the atmosphere in wintertime may lead to increased air temperatures and the formation of deep oceanic convection (Mei- ncke, 1997). The convected water will flow into the Atlantic Ocean in the form of an overflow (Hansen and Østerhus, 2000) and combine with the descending branch of the Atlantic meridional overturning circulation (Meincke, 1997), which is the key factor driving thermohaline circulation (Eldevik, 2005).

The air-sea interaction in each basin differs significantly given the different water sources and temperatures. The Norwegian Basin and Lofoten Basin are dominated by NAC, where the water temperature is relatively high, and the sensible heat release is the major heat resource for Europe. By contrast, Greenland Basin is the confluent area of the East Greenland Current and warm recirculation water; here, the water temperature is low, and the sensible and latent heat releases are both high (Rudels, 2005). Zhao and Drinkwater (2014) studied the differences in air-sea heat fluxes in the four basins. This study revealed that solar radiation is dominant in summer, whereas longwave radiation and sensible and latent heat fluxes released by the ocean are dominant in winter. The heat flux in Green- land Basin is unique. Specifically, solar radiation, longwave radiation, and latent and sensible heat fluxes in Greenland Basin are approximately 50%, 40%, 60%, and 4-fold higher than that in the three other basins, respectively. The correlation between heat flux and Arctic Oscillation (AO) index indicates that almost all atmospheric and oceanic processes have negative correlation with AO, whereas the ascending air flow near Iceland has a positive correlation. These processes are important factors governing the evolution of AO (Zhao, 2019).

The Nordic Seas are the key region of North Atlantic Oscillation (NAO) and/or AO (Hurrell, 1995; Thompson and Wallace, 1998) and play an important role in the climate system. NAO is significantly correlated with systemic wind anomalies, latent and sensible heat fluxes, and SST (Cayan, 1992; Kawamura, 1994) in subpolar gyres. The variation in SST, sea ice extent, and air pressure occur simultaneously (Deser and Blackmon, 1993; Slonosky, 1997). Zhao(2006) analyzed the relationship between the variation in AO and the sea level pressure (SLP) in the northern hemisphere and uncovered a special region that includes most of the Nordic Seas; this region was named the AO Core Region (AOCR). The mean SLP of AOCR is highly negatively correlated with the AO index, with a correlation coefficient of up to 0.945, indicating that the function of Nordic Seas is important to AO.

The maximum temperature difference between the westand east sides of the Nordic Seas is up to 10℃. The air-sea heat fluxes in warm and cold currents also exhibit significant differences. This phenomenon raises important questions about the following: the causes of the consistent variation between the SLP and AO index in AOCR and the homogeneity of atmospheric parameters although the ocean temperature is spatially inhomogeneous. Therefore, how the air-sea heat fluxes in Nordic Seas affect the variation in the ocean and atmosphere must be studied.

Two types of long-term continuous data are available for Nordic Seas: data from the meteorological stations in the coasts around Nordic Seas and the long-term surveillance ship located in the offshore of southern Norway (M-station, 1949–2009). Direct observations of the ocean-atmosphere coupling process in Nordic Seas are limited, and the changing process of the air-sea heat flux is not well understood.

In 2012, a set of ocean-atmosphere coupling buoys was anchored in Lofoten Basin at a location (70.0˚N, 5.0˚E) surrounded by the warm water of NAC. The first measurement stage of the buoy was conducted from 5 August to 27 August. Thereafter, satellite communication was terminated. During this period, two strong wind processes were captured, and the air-sea interaction was measured. In this study, the concept of heat insulation and dissipation processes were proposed, and the general characteristics of air-sea interactions in warm waters at high latitudes were discussed.

2 Buoy Data and Analysis Method

2.1 Buoy Data

The data used in this paper were obtained from the air-sea coupled buoy. The data parameters included the wind speed, wind direction, air temperature, and relative humidity at 3, 5, and 9m; SLP; sea surface skin temperature; the upward and downward solar radiation on the sea surface; the upward and downward longwave radiation on the sea surface; temporal resolution of 0.5h. The data length was 23 days from 5 August 2012 to 27 August 2012. To understand the regional meteorological field, we used the dataset of ERA-Interim from European Centre for Medium-Range Weather Forecasts (Dee, 2011), which includes the SLP, wind speed, and wind direction at 10m. In addition, the temporal resolution was 6h, and the spatial resolution was 0.75˚× 0.75˚.

2.2 Heat Flux Calculation

The calculation method used for sensible and latent heat fluxes was the general bulk algorithm COARE 3.0, and the formula used to calculate the turbulence fluxes is found in another work (Fairall, 1996):

whereHandHare the sensible and latent heat fluxes, respectively. A positive flux indicates the heat transported from the ocean to the atmosphere.ρis the air density,cis the specific heat at constant pressure, Lis the latent heat of evaporation,CandCare the coefficients for sensible and latent heats, respectively.is the average wind speed relative to the sea surface,Tis the sea surface skin temperature,θis the potential temperature at the height ofz,qis the specific humidity on sea surface, andqis the air specific humidity.

The formula of potential temperature is as follows:

whereTis the air temperature at the height ofz.

The air specific humidityqis calculated by relative and saturation specific humidities:

whereis the air relative humidity, andsatis the saturation specific humidity at air temperatureT. We used the average value of air temperature and relative humidities at 3, 5, 7, and 9m asTand, respectively.

The formula of saturation specific humidity is as follows:

whereeis the saturation vapor pressure. We used the empirical formula to calculate the following:

The calculation formula for sea specific humidity is as follows:

The vapor pressure is saturated on the sea surface, but the salinity of sea water causes the vapor pressure to decrease; thus, the value of saturation specific humidity for pure water multiplied by 0.98 is the saturation specific humidity on sea surface (Fairall, 1996).

2.3 Air Pressure and Wind Field During Observation

The wind field can be divided into three stages during the observation period from 5 August to 27 August.

Stage of south wind (5–13 August):Fig.1 shows the ERA-Interim 10m wind field and SLP. On 5 August 2012, the SLP on the Nordic Seas was approximately 1010hPa on average. The isobars were sparse, and the wind speed was minimal. As the high pressure extended to Scandinavian Peninsula, the SLP on Nordic Seas gradually increased in the east and decreased in the west. The whole Nordic Seas were under a strong south wind since 10 August, and the maximum wind speed reached to 9ms−1at the buoy position. Then, the south wind period gradually subsided.

Fig.1 Fields of sea level pressure (SLP) (shading) and 10 m wind vector in August 2012 based on ERA-Interim dataset. The location of the buoy is shown with a red dot.

Stage of calm wind (14–20 August): The wind weakened from 14 August to 17 August and then turned from a south wind with a speed of 1ms−1to a north wind with a speed of 5ms−1. Before 20 August, Nordic Seas were dom-inated by a high pressure. A large low-pressure area gradually formed north of Nordic Seas, whereas a strong cyclone gradually formed at the south direction.

Stage of north wind (21–25 August): From 21 August, the low pressure on Barents Sea extended rapidly to the southwest, and the north wind became stronger. From 23 August to 25 August, the low-pressure center was near 71˚N on Norwegian Sea, and the position was north and closest to the buoy. The highest wind speed on the whole Nordic Seas exceeded 15ms−1, and the wind speed at buoy position was up to 10ms−1. The low pressure gradually subsided after 25 August. The wind speed at the buoy position gradually decreased to 6ms−1to 8ms−1, and the wind direction turned southward.

Based on the changes in ERA-Interim pressure and wind field, two strong wind processes, including a strong south and a strong north wind, were observed at the position of the anchored buoy (Fig.2a), which well represented the strong wind processes in summertime on this area. The air-sea heat flux extracted from this period can represent the contribution of strong winds to heat flux and the difference in the influences of south and north winds on the ocean heat flux.

We compared the buoy data (Fig.2b) with the ERA-Interim data from the position closest to the buoy, and the changes in SLP were consistent with each other. The wind speeds and directions of the two datasets were consistent from 5 August to 14 August. The wind direction from the two datasets had a large discrepancy from 17 August to 25 August, especially in the strong north wind process from23 August to 24 August. The wind direction extracted from the buoy was southward because the buoy position was close to the low-pressure center. The air above the buoy was dominated by the cold air carried by low pressure.

Fig.2 SLP (black line), wind vector (blue vector), and wind speed (yellow line) from 5 August 2012 to 27 August 2012 at the buoy location; the time interval was 6h. Data from (a) ERA-Interim dataset and (b) the buoy.

3 Variation in the Sea Surface Heat Fluxes

The sea surface heat fluxes were all based on the buoy data. The radiation fluxes were obtained from observation data, and the turbulence fluxes were obtained using the calculation method shown in Section 2.2.

3.1 Variation in Solar Radiation

Fig.3 shows the variation in the incident, reflected, and net solar radiation on the sea surface. The solar radiation reflected from the sea surface was less than the incident solar radiation by one order of magnitude. Incident solar radiation is greatly affected by clouds, highly reflected on the cloud surface (Curry, 1996), and strongly scattered inside the cloud because of the water drops and ice crystals (Shupe, 2011). The solar radiation was 225–601Wm−2(peak at noon, the same below) from 5 August to 9 August and 327–439Wm−2from 10 August to 14 August, which is indicative of a cloudless or clear sky. A significant reduction occurred from 15 August to 19 August, and the peak of solar radiation was 121–220Wm−2, representing the state of solar radiation in a cloudy weather. Solar radiation gradually increased to 396–582Wm−2from 20 August to 22 August and decreased to 370–492Wm−2from 23 August to 27 August. The results showed that the strong wind periods were accompanied with low air pres-sure, cloudless sky, and high solar radiation. However, when the wind subsided, the air pressure increased, and the solar radiation decreased.

The results contrasted the normal conditions. In general, the southerly wind carries moist air and results in increased cloud cover. An increased (less) cloud cover is associated with low (high) pressure system. However, in this case, the solar radiation revealed that less cloud was presented in the southerly wind and low-pressure system. A possible explanation is that the high wind strengthened the boundary layer mixing and led to less cloud.

3.2 Variation in Longwave Radiation

Longwaveradiationiscontinuallyemittedfromtheocean and atmosphere. Fig.4 shows that the longwave radiation emitted from the sea surface was maintained at approximately 360Wm−2because SST exhibited minimal changes during the observation period. By contrast, the downward longwave radiation on the surface was significantly variable. The downward longwave radiation was close to the longwave radiation emitted from the sea surface before 20 August. The net longwave radiation was approximately 40–80Wm−2(peak at noon, the same below) before 13 August and 20–30Wm−2from 14 August to 20 August. After 20 August, the net longwave radiation reached the maximum value of 100Wm−2during the observation period. The downward longwave radiation was mainly relevant to the air temperature, which was higher in the south wind, resulting in high downward longwave radiation and low net longwave radiation, and.

Fig.3 Downward (blue line), upward (red line), and net solar (yellow line) radiations from the buoy observations.

Fig.4 Downward (blue line), upward (red line), and net (yellow line) longwave radiations from the buoy observations.

3.3 Variation in Sensible Heat Flux

Affected by the warm current, the air temperature of the area around the anchored buoy was influenced by the high SST in the summertime. Fig.5a shows that from 5 August to 14 August, the SST increased from 8 to 10.3℃, and air temperature increased from 6 to 10℃ because the steady south wind dominated the ocean. As the wind field turned into a north wind from 15 August to 27 August, SST decreased to 8.3℃, and the air temperature fluctuated and decreased to the minimum value of 5.5℃ on 24 August. SST started to decrease on 15 August with the wind calm.

The air-sea temperature difference is dominated by the variation of air temperature because air warms or cools faster than seawater. Fig.5b shows that the temperature difference decreased from 2℃ to −0.8℃ from 5 August to 11 August. Given that the air-sea temperature difference was minimal, the maximum of sensible heat flux reached ±10Wm−2(Fig.6). From 17 August to 19 August, the tem-perature difference was approximately 3℃, the wind speed increased to 4–6ms−1, and the sensible heat flux reached 20Wm−2. From 20 August to 27 August, with cold air trans- port, the temperature difference and wind speed reached 3.5℃ and 9ms−1, respectively, and the sensible heat flux increased to 50Wm−2. Fig.6 indicates that the sensible heat flux decreased in the south wind and increased in the north wind.

The results show that the release of sensible heat from the warm current occurred when the air-sea temperature difference and the wind speed increased simultaneously. The oceanic sensible heat release was strongly restrained by warm advection during the south wind process, and ex- cessive warming of air may have resulted in the sensible heat being transported from the atmosphere to the ocean.

3.4 Variation in Latent Heat Flux

Fig.7 shows that from 5 August to 14 August, as the warm wet air was transported by the south wind, the relative humidity on the observation area increased from 80% to 100%, the sea surface specific humidity increased from 6.5gkg−1to 7.5gkg−1, and the air specific humidity increased from 4.9gkg−1to 7gkg−1. From 15 August to 27 August, as the cold dry air was transported by the north wind, the relative humidity gradually decreased to 70%–80%, the sea surface specific humidity decreased from 7.5gkg−1to 6.5gkg−1, and the air specific humidity gradually decreased to 4.2–5gkg−1.

Fig.8a shows that from 5 August to 9 August, the latent heat flux decreased from 20–30Wm−2to the minimum be- cause the water vapor approached saturation, and the specific humidity difference decreased to zero (Fig.8b). This condition adversely affected the sea water evaporation. The latent heat flux rapidly increased to 90–100Wm−2from 20 August to 27 August during the strong north wind. The reason was attributed to the transport of cold dry air to the south as the low pressure moved from Barents Sea to north Nordic Seas; the specific humidity difference was up to 3gkg−1. Therefore, ocean evaporation was greatly promoted. The transportation of latent heat flux from the ocean to the atmosphere depended on the joint increase in the specific humidity difference and wind speed, which generally occurred in the condition similar to our observation, that is, the north wind carried a mass of cold dry air.

Fig.5 (a) Sea surface temperature (red dotted line), air temperature (red line), and wind speed (yellow line) from the buoy observations. (b) Air-sea temperature difference (red line) and wind speed (yellow line). Data were calculated by the 12h running average.

Fig.6 Sensible heat flux (red line) calculated from the buoy observations and wind speed (yellow line). Positive flux indicates the heat transported from the ocean to the atmosphere. Data were calculated by the 12h running average.

Fig.7 Sea surface specific humidity (blue dotted line), air specific humidity (blue line), and relative humidity (yellow line) from the buoy observations. Data were calculated by the 12h running average.

Fig.8 (a) Latent heat flux (blue line) calculated from the buoy observations and wind speed (yellow line) from 5 August 2012 to 27 August 2012. Positive flux indicates that heat was transported from the ocean to the atmosphere. (b) Air-sea specific humidity difference (blue line) and wind speed (yellow line). Data were calculated by the 12h running average.

4 Heat Insulation and Heat Dissipation Processes in Strong Winds

The air-sea turbulent heat fluxes depend on the air-sea temperature difference, specific humidity difference, and wind speed. Temperature and specific humidity differences depend on the cold and warm advection of the atmosphere. As the distributions of SST and air temperature are generally high in the south and low in the north, cold and warm advections are related to the north and south winds, respectively. The results in this paper are consistent with this law.

4.1 Heat Insulation Process

From 5 August to 13 August, warm wet air was continuously transported to Nordic Seas by the strong south wind. The wind speed, air temperature, SST, and air specific humidity at the buoy position all increased significantly. In addition, the air temperature was higher than the SST on 9 and 11 August. As the air rapidly warmed and became more humid, the air-sea temperature and specific humidity differences gradually decreased. The wind speed was up to 10ms−1, and the sensible and latent heats released from the ocean to the atmosphere decreased from 10 and 20Wm−2to zero, respectively. Sensible heat flux had a negative value in the decreasing period. This result indicates that the warm and wet air brought by the south wind is beneficial to the weakening of sensible and latent heat fluxes. This adverse process of oceanic heat release is referred to as the heat insulation process.

From 5 August to 9 August, clear days occurred, and the solar radiation was approximately 400–600Wm−2. From 10 August to 13 August, the increase in cloud cover led to a slight decrease in the incoming solar radiation to 320–440Wm−2. The long-term heating from solar radiation benefited the increase in air temperature and enhanced the downward longwave radiation at the surface. Meanwhile, as the air humidity was extremely high, the water vapor in air absorbed most of the longwave radiation, which also led to the increase in effective atmospheric counter radiation. The daily maximum sea surface net longwave radiation in the strong south wind period was minimal at approximately 40–80Wm−2. This condition is also one of the connotations of the heat insulation process.

Therefore, the heat insulation process, which was caused by the summertime south wind in Nordic Seas, weakened the sensible and latent heat fluxes and net longwave radiation. However, the wind speed was strong during the period in which the heat insulation process occurred. This finding indicates that the thermodynamics effect was dominant in the warm air.

4.2 Heat Dissipation Process

The cold dry air that originated from Arctic Ocean was continuously transported to Nordic Seas by the strong north wind from 21 August to 25 August. The buoy data showed that the air temperature decreased, whereas the SST showed minimal changes. The air-sea temperature difference and specific humidity difference increased significantly. The sensible heat flux transported from the ocean to the atmosphere increased from 0Wm−2to 50Wm−2, whereas the latent heat flux increased from 20Wm−2to 80–90Wm−2. The low moisture and air temperature caused by the strong north wind also led to the reduction intheeffectiveatmosphericcounterradiation.Thenetlong- wave radiation notably increased to approximately 100Wm−2. The result indicates that the cold dry air brought by the summertime north wind was beneficial to the oceanic heat release. We named this process caused by the north wind as the heat dissipation process.

Synthesizing all the analyses, the connotations of the heat dissipation process caused by the summertime north wind included the increase in sensible and latent heat fluxes and net longwave radiation. The heat dissipation process led to the rapid release of oceanic heat to the atmosphere. The total released heat was greater than 240Wm−2, which is nearly equal to the amount of solar radiation. The heat dissipation process allowed more heat transfer from the ocean to the atmosphere and cooled the ocean surface.

4.3 Comprehensive Analysis of Heat Insulation and Heat Dissipation Processes

Analysis showed that the heat release from the sea surface was greatly restrained during the heat insulation process. The heat dissipation process caused by the north wind resulted in more heat transportation from the ocean to the atmosphere at the sea surface heat fluxes greater than 240Wm−2(sensible heat, latent heat, and longwave radiation). Compared with the situation in winter, the oceanic heat release in Norwegian Basin was up to 250Wm−2in January (Zhao and Drinkwater, 2014), when solar radiation was absent. This finding indicates that the summertime oceanic heat release (solar radiation was not included) caused by the heat dissipation process was close to the wintertime oceanic heat loss.

Nordic Seas are the only area with a large volume of warm water in the high-latitude area of the north hemisphere. The heat insulation and heat dissipation processes we revealed above are specific to the warm ocean in high latitudes. Further insights into both processes occurring in this area should be sought. In summer, the heat dissipation process is the key factor that causes the oceanic heat release. The summertime fronts and cyclones may lead to a strong north wind, which causes substantial heat loss from the ocean. Fig.9 shows the summer mean structure of the SLP, 10m wind, and oceanic heat loss in the form of sensible, latent, and net longwave radiation fluxes. The summer months included July, August, and September. Icelandic Low advects warm air over the warm water of Nordic Seas. The summer mean oceanic heat loss in such area was about 80–100Wm−2. Thus, the heat insulation process was dominant over the warm water of Nordic Seas in summertime. As the cold winds had a low proportion in summer, the heat dissipation process rarely occurred in summer. However, during autumn, the heat dissipation process became dominant, the SST was higher than the air temperature in winter, and the ocean transferred a large quantity of heat to the atmosphere. Thus, the heat dissipation process was the main mechanism for oceanic heat release in autumn and winter.

However, the significance of the heat insulation process should not be underestimated. Solar radiation to the seawater may be greater than 400Wm−2in summer, but the heat insulation process restrained the heat release from the ocean. The ocean in summer lost minimal heat but obtained more solar energy. This quantity of heat was preserved in the ocean and transported farther north by the ocean current, whereas a portion was transported to Arctic Ocean. The heat insulation process benefited the increase in heat in the Arctic and amplified Arctic warming.

The heat insulation and heat dissipation processes occurred in the warm water area. As approximately one-half of the Nordic Seas is covered by cold water, we had no observation data in such an area. However, our results imply that heat insulation processes should dominate because the air temperature is frequently higher than the seawater temperature in cold-water areas. The sensible heat flux in cold water is generally negative; in addition, evaporation is very weak in summer, and longwave radiation is low (Zhao and Drinkwater, 2014). As the warm air occurred on the cold sea surface, heavy sea fog formed based on the cooling effect.

Fig.9 Summer mean (JAS) climatology of the SLP (hPa-contours); the 10m wind (ms−1-vectors); the sum of sensible, latent, and net longwave radiation (Wm−2-shading) from ERA-I 1979–2018. The thick black line represents the 25% summer mean ice concentration isocontour. The red dot denotes the buoy location.

5 Results and Discussion

In Nordic Seas, strong air-sea interaction exists between the ocean and atmosphere, which has an important influence on the climate system. An air-sea coupled buoy was anchored in Lofoten Basin in 2012, and the 23-day data from 5 August to 27 August captured a strong south wind process and a strong north wind process. Analysis of the air-sea heat flux during the strong wind period showed that the general characteristics of the air-sea interaction occurred in the warm current area of Nordic Seas in summer.

Given the NAC, the seawater temperature was higher than the air temperature in summer. The sensible and latent heats were transferred from the ocean to the atmosphere in general. Our observations indicate that warm and wet air was transported to Nordic Seas by the south wind. Given that the air temperature was close to the water temperature, a weak heat was transported from the ocean to the atmosphere. In addition, the south wind transported saturated vapor, and seawater evaporation was restrained, which reduced the oceanic latent heat release. The effective atmospheric counter radiation increased as the air temperature rose, which reduced the heat loss by net longwave radiation. Therefore, although the strong wind caused high-intensity turbulent mixing, the south wind led to a low heat release in the form of sensible heat, latent heat, and longwave radiation. The total heat loss from the ocean was 50–60Wm−2. We named the heat release condition of the south wind the heat insulation process. In the period of the heat insulation process, the ocean lost minimal heat but obtained considerable amounts of solar radiation, which resulted in minimal heat consumption for the ocean.

When the cold and dry air was transported to Nordic Seas by the north wind in summer, the increase in the air-sea temperature difference and the intense turbulent effect caused by the strong wind promoted heat transportation from the ocean to the atmosphere. The cold air benefited the rapid increase in the sensible heat release. The dry air promoted evaporation, leading to the rapid increase in ocean latent heat release. The low moisture content in the air and low air temperature decreased the atmospheric counter radiation and increased the ocean heat loss by net longwave radiation. The ocean lost its heatsensible heat, latent heat, and net longwave radiation in the north wind period. Thus, we named this phenomenon the heat dissipation process. In our buoy observation, the sea surface heat fluxes were up to 240Wm−2during the heat dissipation process and increased by four-fold compared with the heat insulation process.

As the proportion of the north wind was relatively small in summer, the heat insulation process was dominant in the warm area of Nordic Seas. In the heat insulation period, the ocean released minimal heat but obtained more solar radiation. Thus, a large quantity of heat was stored in the ocean in summer. The heat was transported to Arctic Ocean with the ocean current and warmed this body of water. The heat dissipation process was dominant in the autumn and winter when the north wind was dominant. The ocean released considerable energy to the atmosphere, and this phase was the main period of heat release from the warm water in Nordic Seas. The two processes described in this paper indicate the general situation in warm areas at high latitudes.

Acknowledgements

This research was funded by the National Natural Science Foundation of China (No. 41976022), the National Major Scientific Research Program for Global Change Re- search (No. 2015CB953900), and the Major Scientific and Technological Innovation Projectsof Shandong Province (No. 2018SDKJ0104-1).

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June 16, 2020;

August 16, 2020;

September 7, 2020

© Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

. E-mail: jpzhao@ouc.edu.cn

(Edited by Xie Jun)