Record high Pacific Arctic seawater temperatures and delayed sea ice advance in response to episodic atmospheric blocking

Arctic sea ice is rapidly decreasing during the recent period of global warming. One of the significant factors of the Arctic sea ice loss is oceanic heat transport from lower latitudes. For months of sea ice formation, the variations in the sea surface temperature over the Pacific Arctic region were highly correlated with the Pacific Decadal Oscillation (PDO). However, the seasonal sea surface temperatures recorded their highest values in autumn 2018 when the PDO index was neutral. It is shown that the anomalous warm seawater was a rapid ocean response to the southerly winds associated with episodic atmospheric blocking over the Bering Sea in September 2018. This warm seawater was directly observed by the R/V Mirai Arctic Expedition in November 2018 to significantly delay the southward sea ice advance. If the atmospheric blocking forms during the PDO positive phase in the future, the annual maximum Arctic sea ice extent could be dramatically reduced.

www.nature.com/scientificreports/ the exceptionally high SST in November 2018, because the PDO index was close to zero in 2018. The correlation coefficient between the PDO and SST becomes even higher if 2018 is removed (Fig. 2b), which indicates the high monthly SST in November 2018 was likely caused by other factors. AMSR satellite measurements captured rare monthly SST conditions in the Chukchi and Bering seas in 2018 (Fig. 3). The monthly SST values were almost unchanged from August to September in 2018, although the historical SST values usually decrease by several degrees during that time of the year (Fig. 3). The stable temperature was also recorded by an Argo float that was operating in the central Chukchi Sea at that time in 2018 ( Supplementary  Fig. 1). Previous studies indicated that the Pacific water takes at least three months to travel from the Bering Strait to the mouth of the Barrow canyon. 50,51 This indicates that the warm Chukchi Sea observed in November 2018 can be linked to the high seawater temperatures that were present in September 2018.    (Fig. 4). Based on the atmospheric reanalysis of ERA5 data, the southerly wind anomaly was approximately 5 m/s over the Bering Strait. The 5 m/s southerly wind speed anomaly corresponds to 0.70 Sv increase of the transport through the Bering Strait based on the balance between the wind stress and the sea bottom friction 26 . This additional wind-driven transport is considerable compared to the annual (September) mean transport of 1.03 Sv (0.99 Sv) calculated for 2003-2015 26 . In addition, the Bering Sea SST has increased as a result of wind-driven warm water advection. Therefore, if examined with the simple   Analysis of ERA5 data showed that the unusual wind pattern in September 2018 was caused by a persistent atmospheric blocking high system in the upper troposphere over the Bering Sea (Fig. 5a). At the 500 hPa pressure level, there was positive geopotential height anomaly in the central part of the blocking high (Fig. 5a). The area-averaged geopotential height near the Bering Strait in September 2018 was approximately 180 m above the September climatology calculated as the mean from 1979-2018 (Fig. 5b). The anomaly was larger than the four standard deviations above the long-term mean. The positive pressure anomaly extended to the surface and corresponded to the near-surface southerly wind pattern (Fig. 4). This region along western Alaska can be categorized as the typical place for Pacific blocking 52,53 . The frequency of Pacific blocking is, however, significantly lower in the summer months compared to the winter months 53,54 . The atmospheric blocking in September 2018 over the Bering Sea can therefore be regarded as an episodic event.
A possible relation between the atmospheric blocking high system over the Bering Sea and the PDO was also investigated. We calculated the point correlations between the monthly z500 and PDO and found the maximum value was at most 0.42 ( Supplementary Fig. 2). In addition, the spatial distribution would not explain the southerly wind pattern near the Bering Strait in September 2018. These results are consistent with the previous study that reported the Alaskan blocking is not in phase with PDO 52 . It is reasonable to think that the atmospheric blocking high system formed independently from the PDO.
With respect to the surface heat flux from the atmosphere to the ocean, the blocking high system in September 2018 caused positive (downwards) anomalies of sensible and latent heat fluxes over the Chukchi Sea (Fig. 6a). The positive anomalies were, however, mostly cancelled by the negative anomaly of latent heat flux in October (Fig. 6b). The variations in net short-wave and long-wave radiation were less significant ( Supplementary Fig. 3). The resulting net heat flux anomaly over the Chukchi Sea from August to October 2018 was on average 2.2 W/ m 2 . This corresponds to a seawater temperature increase by only 0.1 ℃ if a 40 m thick surface mixed layer was present. The minor contribution of the air-sea heat flux in 2018 indicates that the wind stress is likely the primary cause of the observed anomalous warm Chukchi Sea and the delayed sea ice freeze up in 2018. In contrast, the warmest September SST on record rapidly cooled down in October 2007 (Fig. 3). In October 2007, the relatively strong sustained easterly off-ice wind was present in the northern part of the Chukchi Sea, but such an atmospheric condition was not present in 2018.   www.nature.com/scientificreports/ R/V Mirai conducted unique daily hydrographic and atmospheric measurements along a fixed transect toward the sea ice edge during 9-21 November 2018 to observe variations of air-wave-ice-ocean conditions in the MIZ (see, the green line in Fig. 1 and Supplementary Fig. 4). The continuous measurements included near-surface air temperature and wind, sea surface temperature and salinity, and horizontal current ( Supplementary Figs. 5-7). The most notable event was that there were sustained below-freezing, off-ice northeasterly wind conditions during 17-21 November (Supplementary Fig. 5). The wind speed of the cold-air outbreak was over 10 m/s, and the air temperature was often below − 10 °C near the sea ice. Despite the ideal freezing conditions, the unusually warm water prevented sea ice advance. The freezing wind generally decreased the seawater temperature, but the temperature was still much higher than the freezing point of − 1.8 °C. As a result, the sea ice advance was minimal during this off-ice wind condition period. The slowing of sea ice advance during 17-21 November (Fig. 7a,c) is obvious if compared with the sea ice advance of 30-50 km during 9-12 November under the moderate northerly winds ( Supplementary Figs. 5b and 8).

Anomalously warm Chukchi Sea that delayed sea ice advance in
Interestingly, the SST near the MIZ on 20 November was even slightly warmer compared to 17 November (Fig. 7a,c), despite the large heat release expected during the intense freezing off-ice wind conditions. A possible surface warming mechanism could be upward heat transfer by oceanic vertical mixing caused by strong winds and waves 56 . An alternative mechanism is the horizontal advection of warm water. The ocean currents measured by the shipborne Acoustic Doppler Current Profiler (ADCP) support the possibility of horizontal warm water advection. During the MIZ transect observation period of 9-21 November 2018, the eastward currents near the MIZ gradually veered northeastward until the current eventually turned westward on 20 November (Supplementary Fig. 7). The sustained ~ 0.12 m/s eastward currents over the 10 days indicate that the integrated transport distance of the warm water corresponds to 100 km. The observed warm seawater near the MIZ was likely advected from the central Chukchi Sea where relatively high SST was detected (Fig. 1). Another support for this is that the near-surface profiles of salinity at 162°W matched with those along 163°W on 21 November, with a difference of less than 0.1 psu (Fig. 7b,d). Assuming salinity as a tracer, the agreement indicates there was northeastward horizontal advection of warm Pacific water along the MIZ transect. These facts indicate that the warm water advection likely increased the SST near the MIZ.
Satellite measurements of the sea ice concentration (SIC) provided additional information about the delay of the sea ice advance around the broader area of the Chukchi Sea. Compared to the mean during 2002-2018, SIC was lower from October 2018 and the SST was higher (Fig. 8). In 2018, SIC started increasing rapidly after 8 November, which indicates the southward sea ice advance from the central part of the Arctic to the Chukchi Sea. The advance of the sea ice, however, was significantly slowed from 13 November to 4 December 2018 (Fig. 8). SIC changed very slowly during that period, while the seawater temperature decreased significantly by approximately 3 °C by the anomalous turbulent surface heat flux ( Supplementary Fig. 3a,b). These changes indicate that the sea ice advance was prevented by the warm water over the shelf until the oceanic heat was fully released into the atmosphere.
In November 2018, the heat release from the Chukchi Sea to the atmosphere was significantly higher than the monthly mean from 1979-2018 ( Fig. 6c and Supplementary Fig. 3). This larger air-sea heat flux was caused by the unusually warm and open Chukchi Sea because the upward turbulent fluxes can be significantly increased. Since the atmospheric condition in November 2018 was more favorable for sea ice formation than the previous years, the slowing of the sea ice advance in November 2018 appears to have been primarily caused by the anomalously warm Chukchi Sea.

Discussion
The anomalous warm and open Chukchi Sea in late autumn 2018 was studied in the context of interannual variation using a variety of data. We found that seawater temperature in the Chukchi Sea in the sea ice freezing season was highly correlated with the PDO index in the last two decades. However, the highest autumn seasonal values of SST in the Chukchi Sea were recorded in 2018 when the PDO index was close to zero. The primary factor causing the anomalous warm conditions in the Chukchi Sea was found to be the episodic blocking high pressure system over the Bering Sea in September 2018. The blocking high caused an unusual near-surface southerly wind pattern near the Bering Strait. As a result, warm Pacific seawater intruded into the Chukchi Sea in September 2018 and subsequently delayed sea ice advance in November 2018.
Atmospheric blocking over the northern North Atlantic was previously reported to influence the Atlantic multidecadal ocean variability 57 . More recently, the North Atlantic atmospheric blocking was found to have an additional role in modulating sea ice export through the Fram Strait and eventually impacts the Atlantic Meridional Overturning Circulation 58 . However, an impact of the atmospheric blocking on the autumn sea ice growth over the Pacific Arctic region has never been reported. This study presented the first evidence that the atmospheric blocking over the Pacific Arctic region plays a significant role in the modulation of the Pacific Ocean inflow to the Arctic Ocean and its impact on the sea ice advance.
We further examined whether PDO has a strong effect on the Chukchi Sea SST for months other than November. The correlation coefficient between the monthly averaged Bering Sea SST and the annual PDO index was larger than 0.7 for most of the months (Supplementary Fig. 9). For the Chukchi Sea, however, the correlation was spotty as it was previously reported with the sustained in-situ measurement results at the Bering Strait 45 . The low correlation in the summer months implies the other factors are influential such as the Bering Strait volume transport and the ocean surface heat flux over the Chukchi Sea. These two factors are strongly influenced by the atmospheric condition over the Pacific Arctic region 45 . The condition can be in part described by climate indices such as the Arctic Oscillation index. The potential factors could be however more diverse as this study showed that the single atmospheric blocking event had a significant influence on the Chukchi Sea SST condition from September to November.  as (a, b) The elevated SST and delayed sea ice formation reported in this study have widespread impacts over the Pacific Arctic region and beyond. For example, a recent study showed that unusual mortality of tufted puffins occurred in the eastern Bering Sea in October 2016 through January 2017 59 . Since the shifts in the zooplankton community and forage fish distribution followed a period of elevated sea surface temperature 59 , a bottom-up shift in seabird prey availability is possible. Decreasing SIC also affects species like polar bears and seals that depend on the ice being present each year 11,12 . The longer ice-free periods also permit Pacific water to transport heat to the central part of the Arctic basin 60 A better representation of the oceanic heat transport from the Pacific to the Arctic is also important for safe ship navigation over the sea ice covered Arctic Ocean. The ships in the Arctic can encounter hazards such as collision with perennial sea ice, sea-spray icing, and high winds and waves during cyclones 63 . A recent study also showed correlations between wind speeds and increases of wave heights in the Arctic Ocean from 1979 to 2016 64 . Accurate representations of the sea ice distribution were recently found to be an important factor for making daily wave forecasts 65 , which are of interest to the Arctic shipping community. These studies indicate that the findings of this study about the factors and consequences of the Pacific heat inflow may be able to facilitate better prediction of the sea states and safe ship navigation in the Arctic.
The delayed sea ice advance over the Pacific Arctic region can also be influential even on mid-latitude atmospheric conditions. The linkage between the anomalous Arctic warming near Alaska and the cold winters in eastern North America during 2013/2014 and 2014/2015 was previously discussed 66 . More recently, it was shown that jet stream pathways can be changed by the large upward heat flux from the open Chukchi Sea in the early winter 67 . Consequently, the atmospheric thermal front moves southward and causes cold winters in Asia and North America. The small sample of years with a large sea ice loss, however, makes it difficult to clarify the causal relation between the sea ice decline and the changes in mid-latitude weather systems 68 .
This study showed that the evolution of the atmospheric blocking is independent of PDO, but the blocking high system can form during the PDO positive phase in the future. The emergence of the blocking high system in summer seasons over the Bering Sea is considered to be rare event even during the rest of the twenty-first century 54 . However, from a statistical point of view, atmospheric blocking will eventually form during the PDO positive phase in the future. The regression analysis indicates the Chukchi Sea SST increased by approximately 1 °C in 2015 when the PDO index was at a maximum (Fig. 2). The results also indicate that the Chukchi Sea SST response in November to the atmospheric blocking in September 2018 was slightly higher than 1 °C. If the atmospheric blocking forms during the PDO positive phase in the future, the SST could increase more than 2 °C and dramatically reduce the annual maximum Arctic sea ice extent. Careful future monitoring of the atmospheric conditions in late summer will be important for seasonal prediction of the sea ice extent in winter, which is concerning at both the local and global scales during this period of global warming.  The monthly averaged spatial distributions of SST and SIC were first created by taking the temporal mean without including missing data. The spatially averaged SST and SIC for the Chukchi Sea and the northern Bering Sea were further calculated as the mean over the areas 65-75˚N, 160-180˚W and 55-65˚N, 160-180˚W, respectively (see, Fig. 1b). The SST values near the sea ice covered areas and coastal areas are missing and not included when the spatial average is calculated.

Decadal variation of North Pacific SST. Decadal variation of North Pacific SST is represented by the
Pacific Decadal Oscillation (PDO) Index. PDO index is statistically defined as the leading principal component of North Pacific monthly sea surface temperature variability (poleward of 20°N). The PDO index values data were downloaded from the website of Dr. Mantua at http://resea rch.jisao .washi ngton .edu/pdo/. The annual mean values were calculated using the mean monthly values. Since the index was available up to September 2018 on the website, the annual mean value for 2018 was calculated as the mean from January to September.

Atmospheric conditions over the Pacific Arctic and North Pacific. Estimates of winds and air-sea
heat flux over the study area were made for 1979-2018 with ERA5 reanalysis data produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The total air-sea heat flux was calculated as the sum of surface latent heat flux, surface sensible heat flux, surface net solar radiation, and surface net thermal radiation. Geopotential height anomalies at 500 hPa levels were used to identify the atmospheric blocking following the study of Dole and Gordon 69 . Mean sea level pressure anomalies were also used to discuss the atmospheric blocking over the study area. The monthly mean data were obtained from the ECMWF data server (http://apps.ecmwf .int/datas ets/), with a spatial resolution of 0.25°. November 2018 to conduct observations of atmosphere, ocean, waves, and sea ice during the R/V Mirai cruise MR18-05C. This Arctic expedition was approximately three weeks and finished on 24 November 2018 by passing out of the Bering Strait. During the expedition, the near-surface atmospheric and oceanographic variables were continuously measured by the Shipboard Oceanographic and Atmospheric Radiation (SOAR) measurement system 70 . The air temperature and wind were measured at 25 m using the anemometer (Model 05106, R.M. Young), while the seawater temperature and salinity were measured at 5 m below the sea surface using an SBE-45, (Sea-Bird Electronics, Inc.). The 5-min moving averages were applied for the visualizations. The shipboard ADCP (75 kHz, RDI) was used to measure the ocean currents. The 30-min averages were applied for the visualizations. These data can be obtained from the Data and Sample Research System for Whole Cruise Information in JAMSTEC (http://www.godac .jamst ec.go.jp/darwi n/cruis e/mirai /mr18-05c/e). Temperature, conductivity, depth (CTD) measurements were conducted with an SBE9plus (S/N 09P54451-1027, Sea-Bird Electronics, Inc.). Following previous studies 56,71 , the oceanic mixed layer depth was calculated as the shallowest depth at which potential density is more than 0.1 kg/m 3 higher than the near-surface mean potential density, and the near-surface mean potential density was calculated as the average from 5 to 10 m below the sea surface.

Argo float in the central
The sea ice extent from SAR images. The Level-2 SAR data of the normalized radar cross-section from Sentinel-1A/B were used to estimate the sea ice extent near the study area. The data were linearly interpolated to the regular longitude and latitude grids with 1/500° resolution for the visualizations. The data were created by NOAA and were obtained from NOAA CoastWatch (http://coast watch .noaa.gov). The original data of the Sentinel-1A/B are provided to NOAA by the Copernicus Program.

Data availability
All new data that support the findings of this study can be provided upon a request by the time of possible publication. At the time of publication, the data can be fully accessed via the Arctic Data archive System, managed by the Research Organization of Information and Systems, Polar Environment Data Science Center, National Institute of Polar Research, Japan.

Scientific Reports
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