Evolution of Arctic Ocean surface circulation from 1958 to 2017

Highlights

• Proved an obvious periodic change and sudden change of the Arctic Ocean surface circulation.

• Studied the variation of Arctic ocean surface circulation paths related to Arctic Oscillation index.

• Explored the reasons for the changes in the surface water of the Arctic Ocean in past 60 years.

Abstract

Arctic Ocean surface circulation plays an important role in the global ocean circulation system. In recent years, its position and velocity have been changing constantly, attracting increasing attention. The cause of this change, however, is still a matter of disagreement. In this paper, we use empirical orthogonal function analysis (EOF) to analyze the spatial and temporal distributions of sea level pressure (SLP), sea surface height (SSH) and sea surface temperature (SST) in the Arctic Ocean. We use the wavelet transform to obtain the periodic variation law governing Arctic Ocean surface circulation as well as to explore the relationship between the Arctic Oscillation (AO) and Arctic Ocean surface circulation. Our results show that the SSH of the Arctic Ocean exhibit a periodic variation with AO, in particular the sudden change seen from 1987 to 1992 coinciding with the trend of the AO index for the same period. When the AO index is positive, the Beaufort Gyre (BG) weakens and the source of transpolar drift (TPD) moves eastward, to the East Siberian Sea; when the AO index is negative, the BG accelerates and expands and the source of TPD moves to the New Siberian Islands. As a result, the direction and amplitude of the circulation change.

Introduction

In recent years, a growing focus on climate change has drawn increasing attention to the Arctic Ocean, whose circulation is an important part of the global ocean circulation and is governed by wind stress, temperature, salt, and the atmosphere, among other factors. Although the Arctic Ocean is small and land-locked, it plays an important role in the circulation of other oceans and even the global ocean circulation in this pattern of seawater exchange. Climate models indicate that increased freshwater outflow from the Arctic may reduce convection in the North Atlantic deep water formation regions, which in turn may affects the Atlantic meridional overturning circulation (Wadley and Bigg, 2002). The Arctic is most remarkable for its perennial sea ice, which historically covered about half the Arctic Ocean, although the latest research shows that the Arctic sea ice has decreased by 2.22 million km² in the past 40 years (Simmonds and Li, 2021). Declining sea ice has profound implications for global climate change (Nghiem et al., 2007). The research of Luo et al. (2018a) on high-latitude European blocking and Ural blocking events shows that the cooling in Europe and central-eastern Asia are both related to the decrease of sea ice concentration. In recent years, under the influence of sea surface temperature (SST), atmosphere, and the Arctic Oscillation and other factors, the path and speed of Arctic circulation have changed significantly, affecting both the environment and the climate around the Arctic and playing an important role in the study of global ocean cycle and climate change.

Surface circulation in the Arctic Ocean comprises mainly transpolar drift (TPD) and the Beaufort Gyre (BG). TPD originates with river runoff in Siberia, with the surface water in the Arctic moving eastward with a westerly wind and flowing into the Atlantic Ocean along the east coast of Greenland. The BG, an anticyclonic circulation in the Beaufort Sea, plays an important role in the changes seen in the upper ocean, rotating the surface water in the Canadian Basin and causing the Arctic Sea ice to rotate with it (Zhong et al., 2015). Additionally, the BG exhibits clear seasonal and interannual variations.

A hydrographic survey of the Arctic Ocean in 1993 shows that the position of the Arctic circulation had changed since the early 1990s, with the intersection of the Pacific and Atlantic waters having moved from Lomonosov Ridge to Alpha-Mendeleyev Ridge, calling for a change to the upper ocean circulation model (Morison et al., 1998). Proshutinsky and Johnson (2001) identified two regimes of wind-forced circulation in the Arctic Ocean that may help explain recently observed significant basin-scale changes in the Arctic's temperature and salinity structure as well as the variability of ice conditions in the Arctic Ocean. Karcher et al. (2012) found that Ekman pumping in the Canadian Basin was extremely strong in the late 2000s, with the high freshwater content in the BG having restored some of the surface circulation to the level seen before the 1990s. After 2004, strong Ekman pumping in the Canadian Basin promoted the development of the BG and a new circulation mechanism appeared. Freshwater volume in the BG changes significantly on a seasonal and interannual scale, peaking from June to July and from November to January, with the freshwater center having shown a trend of southeast movement from 2003 to 2007. These changes are thought to be related to changes in wind stress curl, which affect circulation in the surface layer of the ocean through Ekman transport and cause the vertical movement of seawater. Recent studies have shown that the enhanced surface stress of the anticyclone over the BG is related to the deepening of the halocline and the accumulation of freshwater content (Ma et al., 2017). Krishfield et al. (2014) analyzed the perennial sea ice time series covering it from 2003 to 2012 and noted that sea ice content is decreasing. The study showed a decreasing trend of sea ice in the BG region, with older sea ice gradually melting and disappearing from the BG, which in turn led to changes in the freshwater content of the region. It will also impact on freshwater input and output at the same time. Zhong et al. (2015) believed that the BG had accelerated from 2003 to 2012, with upwelling dominant in most areas of the basin and northward Ekman transport released freshwater into the basin for many years indeed the total amount of Pacific winter water increased by 18% from 2002 to 2006 to 2011–2016. Changes in the BG are increasingly influencing changes to the upper ocean and climate (Zhong et al., 2019). The southeastern BG accelerated from late 2007 until 2011 and decreased to speeds near those seen in 2003–2006 (Armitage et al., 2018). Timokhov et al. (2018) showed that compared with 1949–1993, the pole crossing drift seen in 2007–2013 had become two branches: the first starting in the northern part of the Chukchi Sea and the second in the East Siberian Sea. The current bifurcation point was closer to the Canadian islands, with the flow rate increased by 1.5–2 times.

Arctic Oscillation (AO) is usually defined as the leading-mode of EOF of sea level pressure (SLP), and the main characteristic is the dipole structure with the direction of the south north along the latitude (Thompson and Wallace, 1998). Its interannual variations can significantly impact the surface air temperature anomalies over the Arctic, especially in the spatial pattern of temperature change from 1980s to 1990s (Rigor et al., 2002). In recent years, the influence of variations in the AO on the Arctic Ocean surface circulation has been widely discussed. Steele et al. (2008) analyzed ocean temperature profile data and satellite data and found that with decreases in the AO index, some parts of the Arctic Ocean cooled by about 0.5 °C every ten years from 1930 to 1965, whereas with increases in the AO index, these areas warmed during the period 1965–1995, a change particularly evident after 2000. In summer 2007, the SST anomaly reached as high as 5 °C, and it was found that when the AO index is high, the water in the Bering Sea passes directly through the Arctic Ocean by means of cross-stage drift, whereas when the AO index is low, the water in the Bering Sea is brought into the Beaufort circulation (Steele, 2004). Polyakov and Johnson (2000) showed that the SSTs of the Barents Sea and the Greenland–Iceland–Norway Sea in the Eastern Arctic were higher than average level in the 1950s but showed a downward trend until the 1960s, then returned to an upward trend in the following decades—a change consistent with low-frequency changes to the AO. Morison et al. (2012) showed that increased freshwater content in the Arctic Ocean was caused by forced atmospheric circulation accompanied by increases in the AO index. They also confirmed that runoff was an important factor affecting the Arctic Ocean, with spatiotemporal variations in its path affected by the AO rather than by wind-driven BG.

Atmosphere plays an intermediary role in the response of the Arctic Ocean to the AO, the troposphere and stratosphere as a medium make the loss of sea ice force AO to change (Screen et al., 2018; Sun et al., 2015). Nakamura et al. (2015) proved that the increase or decrease of Arctic sea ice area in November leads to more positive (negative) phases of AO in winter. This kind of connection is robust on the interannual and decadal time scales, so the weakening of AO and the corresponding mid-latitude climate change in recent years may be related to the sea ice loss in the Barents-Kara Sea (Yang et al., 2016). Changes in sea ice in the surface circulation can affect climate under sea-air-ice interactions. Chen et al. (2019); Luo et al. (2017); Luo et al. (2018b) revealed the influence of Ural blocking on the winter sea ice variability in the Barents-Kara Sea, and proposed that the combination of North Atlantic Oscillation -Ural blocking played a leading role in the reduction of BKS sea ice, and proposed the concept of “potential corticity barrier” to pointed out that the meridional potential vorticity gradient can better reflect the change of blocking duration related to Arctic warming. The abnormal circulation in the Arctic may cause extreme weather in the middle latitudes (Rudeva and Simmonds, 2021). Recent studies have shown that the East Asian cold anomaly is related to the sea ice loss in the Barents-Kara Sea and the atmospheric teleconnection patterns (Li et al., 2020). Decreases of sea ice from Barents-Kara Sea causes the tropospheric westerlies shift southward to significantly affect the winter climate and weather conditions in East Asia (Xu et al., 2021).

In this paper, we discuss changes to Arctic Ocean surface circulation from 1958 to 2017. The special geographical location and climatic conditions of the Arctic Ocean make exploration of the various characteristics of Arctic Ocean surface circulation essential to further understanding of global climate change.