Aleutian Low Research Articles

The Role of the Aleutian Low in the Relationship between Spring Pacific Meridional Mode and Following ENSO

Abstract The spring Pacific meridional mode (PMM) is an important precursor of El Niño–Southern Oscillation (ENSO). However, recent studies reported that only about half of the spring PMM events were followed by ENSO events. This study examines the role of internal climate variability in modulating the impact of PMM on ENSO using 100-member ensemble simulations of the Max Planck Institute Earth System Model (MPI-ESM). The relationship between spring PMM and following winter ENSO shows a large spread among the 100 members. The variation of spring Aleutian low (AL) intensity is identified to be an important factor modulating the PMM–ENSO relation. The spring AL affects the PMM–ENSO relationship by modifying PMM-generated low-level zonal wind anomalies over the tropical western Pacific. The strengthening of the spring AL is accompanied by westerly wind anomalies over the midlatitude northwestern Pacific, leading to sea surface temperature (SST) cooling there via an enhancement of upward surface heat flux. This results in increased meridional SST gradient and leads to northerly wind anomalies over the subtropical northwestern Pacific, which turn to surface westerly wind anomalies after reaching the equatorial western Pacific due to the conservation of potential vorticity. Thus, the low-level westerly (easterly) wind anomalies over the tropical western Pacific associated with the positive (negative) spring PMM were strengthened (weakened), which further contributes to an enhanced (a weakened) PMM–ENSO relation. The mechanism for the modulation of the AL on the spring PMM–ENSO relationship is verified by a set of AGCM simulations. This study suggests that the condition of the spring AL should be considered when predicting ENSO on the basis of the PMM. Significance Statement Spring Pacific meridional mode (PMM) is a predictor of ENSO, but not all spring PMM events are accompanied by the occurrence of ENSO events. This study aims to explore the influence of internal climate variability on the relationship between spring PMM and following ENSO. It is revealed that the Aleutian low exerts a crucial modulation on the spring PMM–ENSO relationship. The underlying physical mechanisms for the impact of the Aleutian low on the relationship between spring PMM and ENSO are further examined. The results of this study have important implications for improving the prediction of ENSO.

Impact of Arctic sea ice on the boreal summer intraseasonal oscillation

This study investigates the relationship between sea ice concentration (SIC) in the Arctic Ocean and the Boreal Summer Intraseasonal Oscillation (BSISO) from 1991 to 2020 and its underlying mechanism. A significantly positive (negative) correlation was found between the frequency of phase 7 (3) of BSISO1 (30–60 d) and the preceding winter SIC, which is located the north of the East Siberian-Beaufort Sea (ESBS). Compared with low-SIC years, the conditions including northeasterly vertical wind shear, an enhanced ascending branch of the anomalous Walker circulation, an eastward water vapour transport channel, and an increased humidity gradient induce active convection over the Philippine Sea in high-SIC years, which benefits (hinders) to phase 7 (3) of BSISO1. The positive SIC anomaly during the transition from winter to spring influences local temperature and pressure through anomalous local sensible heat flux. This anomaly induces wave activity flux from the ESBS, which converges over the Bering Sea, enhancing the Aleutian Low (AL). Subsequently, the AL triggers an anomalous subtropical anticyclone through wave-mean flow interaction in the North Pacific. Due to southerly wind stress and increased sea surface heat flux, positive sea surface temperature anomalies near Japan persist in the summer, heating the lower troposphere and increasing baroclinicity. Significant positive geopotential heights and anticyclone anomalies occur over Japan, accompanied by a negative vorticity anomaly. The enhanced ascending motion over the Philippine Sea, facilitated by Ekman pumping, favours convection and influences the frequency of phases 7 and 3.

Southeastward Extension of the Aleutian Low Favors Long Decaying El Niño Events

Abstract El Niño can be categorized into long and short decaying cases in terms of the lengths of decaying phases. We find that the difference between long and short decaying El Niño emerges as early as the decaying spring, rather than the decaying summer mentioned in previous studies. Observational evidence suggests that this difference corresponds to the different positions of the spring Aleutian low (AL). Further analysis using 15 Coupled Model Intercomparison Project phase 6 (CMIP6) model simulations suggests that the AL that extends southeastward into the northeastern subtropical Pacific contributes to the long decaying El Niño. The northeastern subtropical Pacific AL, with an anomalous southwesterly on the southeast edge, is associated with warming in situ, which further induces cyclonic circulation anomalies over the eastern tropical Pacific as Gill’s response. The anomalous westerly at the equator along the southern side of the cyclonic circulation anomalies is related to positive SST anomalies over the central and eastern tropical Pacific and therefore the persistence of El Niño in the decaying spring. On the other hand, the AL restricted to the North Pacific contributes to anticyclonic circulation anomalies over the central Pacific and favors the decline of El Niño through the seasonal footprint mechanism. Moreover, the models that reproduce many more long (short) decaying El Niño cases tend to simulate the AL southeastward extension (restriction over the North Pacific) during the El Niño decaying spring. The results imply that the AL position in the El Niño decaying spring could be a precursor to the length of El Niño decaying phase. Significance Statement El Niño events can be classified into long and short decaying cases with regard to the different lengths of decaying phases. We find that the difference between the two types of El Niño appears as early as the decaying spring, rather than the decaying summer noticed in previous studies. We highlight that this difference is due to the different positions of the spring Aleutian low (AL). Particularly, the AL that extends southeastward into the northeastern subtropical Pacific is associated with warming over the central and eastern tropical Pacific and contributes to the persistence of El Niño in the decaying spring. The results indicate that locations of the spring AL should be paid attention to when considering the decaying phase of El Niño.

Intensity change and zonal and meridional movements of the Aleutian Low and their associated broad-scale atmospheric-oceanic characteristics

The changes in the intensity and position of the Aleutian Low (AL) exert significant effects on the ocean-land-atmosphere interactive processes and climate over East Asia, the North Pacific, and North America. In this study, the interannual variations of the AL intensity, meridional shift, and zonal shift, as well as their associated dynamical processes are investigated. The results show that the intensity, meridional shift, and zonal shift of the AL are well characterized by the three leading EOF modes of sea level pressure over the North Pacific, and are strongly correlated with the Pacific-North American pattern, the West Pacific pattern, and the Tropical/Northern Hemisphere pattern, respectively. The intensity variation of the winter AL seems to be closely associated with the ENSO-related atmosphere-ocean processes in the previous autumn, whereas the meridional shift is not associated any significant signal in the previous months, signifying a relatively higher predictability of the former. Besides, the eastward and westward movements of the AL show asymmetric features, attributed possibly to their asymmetric relationships with ENSO.

Interannual Variability of the Mass-Weighted Isentropic Zonal Mean Meridional Circulation in the Northern Hemisphere Winter

Abstract The atmospheric general circulation diagnosed by the mass-weighted isentropic zonal mean exhibits an extratropical direct (ETD) circulation driven by eddy momentum transport. We investigated regional atmospheric modulations associated with the year-to-year variability of ETD circulation in boreal winter using a reanalysis dataset. Composite analyses showed that the interannual variability of ETD circulation accompanies a seesaw-like variation between the Aleutian low (AL) and Icelandic low (IL), which coincides with the Pacific–North American teleconnection pattern and North Atlantic dipole anomaly. A diagnosis using isentropic airmass fluxes representing the geographical Lagrangian mean motion indicated that the significant intensification of upper-tropospheric poleward warm airflow over the eastern North Pacific is responsible for the enhanced ETD circulation, together with the intensified equatorward lower cold airflow over East Asia and eastern North America. The resulting enhanced upward propagation of the stationary planetary waves corresponds to the intensification of the divergence of the Eliassen–Palm flux, which explains the modulated ETD circulation in view of the balance between the Coriolis force and eddy momentum flux convergence. These results suggest that the AL–IL seesaw promotes variations in the global general circulation, and the anomalous ETD circulation interacts with several teleconnection patterns involving the modulation of planetary waves through anomalous meridional heat transport by the basinwide-scale eddies.

Diminished Impact of the East Asian Winter Monsoon on the Maritime Continent Rainfall After the Late‐1990s Tied to Weakened Siberian High–Aleutian Low Covariation

AbstractStrong East Asian winter monsoon (EAWM) can enhance the Maritime Continent rainfall (MCR) via frequent southward penetration of cold air along the coast of East Asia. However, such impact diminished after the late‐1990s, primarily due to the weakened covariation between the Siberian high and Aleutian low (AL). During 1979–1998, the co‐variability between the Siberian high and AL facilitates a strong east‐west pressure gradient over East Asia. The resultant northerlies invade southward and facilitate the enhanced MCR. In contrast, such covariation is weak during 2000–2018, and it impedes the southward invasion of northerly winds and diminishes EAWM's impact. Further inspections indicate that the weakened covariation is primarily caused by the reduced variability of the AL. The underlying mechanism is confirmed by the historical simulation of version 6 Model for Interdisciplinary Research on Climate (MIROC6) that participates in phase 6 of the Coupled Model Intercomparison Project and numerical experiments with European Center‐Hamburg atmospheric model version 6.3 (ECHAM6).

Role of the Aleutian Low in Modulating the Connection between the Moderate El Niño and the Following Spring North Tropical Atlantic SST Anomalies

Abstract El Niño events are likely to be followed by significant positive sea surface temperature anomalies (SSTAs) over the north tropical Atlantic (NTA). We find that this is not always true for El Niño with moderate intensity. Nearly half of the moderate El Niño cases are followed by the strong NTA SSTAs, but others are followed by the weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA. The differences in the El Niño–related NTA SSTAs are due to the different extratropical teleconnection between El Niño and the NTA, manifesting as a Pacific–North American (PNA) pattern. Further analysis suggests that the deepened Aleutian low (AL) induces the negative North Pacific SSTA, which is associated with a wave train propagating eastward, and, modulates the El Niño–generated PNA pattern. Therefore, El Niños that are accompanied by the strong AL induce a strong PNA pattern that, in turn, leads to the strong NTA SSTAs. The strength of the AL coinciding with El Niño plays a crucial role in modulating the connection between the moderate El Niño and the NTA SSTAs. This observational evidence about the uncertainty in the El Niño–NTA connection and the role of the AL in modulating this connection is further supported by the model simulations that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). These results indicate that the combined effects of El Niño and the AL must be considered to fully understand the NTA SSTA variability. Significance Statement El Niño events often lead to significant warming over the north tropical Atlantic (NTA). We find that the strong El Niño almost always is accompanied by the strong NTA sea surface temperature anomalies (SSTAs), whereas the moderate El Niño can be followed by either the strong or weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA SSTAs. Further analysis demonstrates that El Niño, which is accompanied by the strong (weak) Aleutian low leads to the strong (weak) NTA SSTAs, suggesting that the role of the Aleutian low must be considered to fully understand the El Niño–NTA connection.

Enhanced impact of the Aleutian Low on increasing the Central Pacific ENSO in recent decades

In this study, we reveal a marked enhanced impact of the early-spring Aleutian Low (AL) on the following winter El Niño and Southern Oscillation (ENSO) after the late-1990s. This enhanced impact of the early-spring AL may have an important contribution to the increased emergence of the central Pacific ENSO during recent decades. After the late-1990s, decrease (increase) in the early-spring AL strength tends to induce an anomalous cyclone (anticyclone) over subtropical North Pacific via wave-mean flow interaction. The associated westerly (easterly) wind anomalies to the south side of the subtropical anomalous cyclone (anticyclone) over the tropical western Pacific contribute to occurrence of central Pacific-like El Niño (La Niña) in the following winter via tropical Bjerknes feedback. Further, the subtropical anomalous cyclone (anticyclone) leads to sea surface temperature (SST) increase (decrease) in the equatorial Pacific in the following summer via wind-evaporation-SST (WES) feedback, which further contributes to succeeding central Pacific-like El Niño (La Niña). Enhanced impact of early-spring AL on ENSO is attributable to enhancement of the mean circulation over the North Pacific, which leads to increased wave-mean flow interaction and strengthened WES feedback after the late-1990s. The results offer the potential to advance our understanding of the factors for the reduced prediction skill of ENSO since the late-1990s.

How Well Can Current Climate Models Simulate the Connection of the Early Spring Aleutian Low to the Following Winter ENSO?

Abstract A recent study revealed an impact of the intensity of early spring Aleutian low (AL) on the succeeding winter ENSO. This study examines the ability of 41 climate models that participated in CMIP6 in simulating the early spring AL–winter ENSO connection. It is shown that there exists a large diversity among the models in simulating this AL–ENSO linkage. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. Diversity of the AL–ENSO connection is related to the spread in the spatial structure of AL-related atmospheric anomalies over the North Pacific. In the models that capture the observed AL–ENSO connection, weakened AL induces an anomalous anticyclone over the northern middle and high latitudes and an anomalous cyclone over the subtropical North Pacific. The resultant westerly wind anomalies over the tropical western-central Pacific (TWCP) induce an El Niño sea surface temperature (SST) anomaly pattern in the following winter. By contrast, in the models with the AL–ENSO relation opposite to the observations, the AL-associated anomalous anticyclone over the North Pacific extends too southward. As such, the subtropical North Pacific is dominated by northeasterly wind anomalies and SST cooling. The subtropical North Pacific SST cooling induces easterly wind anomalies over the TWCP via wind–evaporation–SST feedback, and leads to a La Niña anomaly pattern in the following winter. The spread in the spatial structure of the AL-associated atmospheric anomalies over the North Pacific is partly due to the diversity in the amplitude of the climatological mean flow. Significance Statement A recent study suggested that variation of the AL intensity in early spring could exert a significant impact on the following winter ENSO. It indicated that inclusion of the early spring AL signal could improve the prediction of ENSO and to some extent help reduce the spring predictability barrier of ENSO. To employ the AL as a predictor in the ENSO prediction and forecast, the current climate model should have the ability to simulate realistically the early spring AL variation as well as the physical process linking the early spring AL with the subsequent winter ENSO. Hence, this study examines the performance of the current coupled climate models that participated in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the linkage between the early spring AL and the following winter ENSO. We show that there exists a large diversity among the CMIP6 models in simulating the early spring AL–winter ENSO connection. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. The factors leading to the spread are further examined. Results of this study would have implications in improving our understanding of the impact of extratropical atmospheric forcing on the ENSO and improving the seasonal forecasting of the ENSO.

Multidecadal Regime Shifts in North Pacific Subtropical Mode Water Formation in a Coupled Atmosphere‐Ocean‐Sea Ice Model

AbstractA regime shift in the formation mechanisms of the North Pacific subtropical mode water (NPSTMW) and its causes were investigated using a 2,000‐year‐long pre‐industrial control simulation of a fully coupled atmosphere‐ocean‐sea ice model. The volume budget analysis revealed that the air‐sea flux and ocean dynamics (OD) were the two primary driving mechanisms for NPSTMW formation, but their relative importance has periodically alternated in multidecadal timescales of approximately 50–70 years. The regime shift of the NPSTMW formation was closely related to the meridional (50 years) and zonal (70 years) movements of the Aleutian Low (AL). When AL shifted to the south or east, it induces the sea surface height anomalies propagating westward from the central North Pacific and preconditions the NPSTMW formation, thus the OD become relatively more important.

Interdecadal Change in the Relationship between the Winter Siberian High and Summer Tropical Cyclone Genesis Frequency over the Western North Pacific

Interdecadal change in the relationship between the winter Siberian high (SH) and tropical cyclone genesis frequency (TCGF) is investigated using observational and reanalysis data. Focus is on the Western North Pacific (WNP) where environmental background associated with TCGF is closely related to the winter SH. It is demonstrated that the relationship presents clear interdecadal change during 1980–2020. A significant negative correlation dominates in the last two decades of the 20th century (P1), but it clearly weakens from the early 2000s onward (P2). Observational evidence shows that such interdecadal change is related dominantly to variations in the air–sea responses associated with the winter SH over the North Pacific. During P1, when the SH is anomalously strengthened, an Aleutian Low (AL) response occurs over the downstream North Pacific, which gradually develops into a meridional dipole distribution pattern consistent with the negative North Pacific Oscillation (NPO) mode during the following spring. The northeasterly wind anomaly over the subtropical North Pacific maintains and further delivers into the tropical Pacific. It favors easterly wind anomaly enhancement over the tropical Pacific around summer and grows into an anticyclonic circulation response in the WNP, which restrains the genesis of TCs. During P2, the eastward shift of the SH anomaly favors a weak AL over the Northeast Pacific followed by a northeastward-shifted anticyclone over the subtropical central-eastern Pacific, which confines the relatively weak northeasterly wind anomalies far away from the tropical Pacific. This was concurrent with a weakening relationship between the SH and WNP TCGF, indicating weaker downstream impacts of the winter SH.

INTERACTION OF CIRCULATION OBJECTS AND THEIR CLIMATE DYNAMICS BY THE EXAMPLE OF ATMOSPHERIC ACTION CENTERS IN THE NORTHERN HEMISPHERE

The article deals with the centers of action of the atmosphere of the Northern Hemisphere - the Icelandic and Aleutian lows, the Azores and Hawaiian highs, the Arctic and winter Asian anticyclones. The change in their characteristics in the annual course is considered. As such characteristics, the geographical coordinates of each center of action and the pressure in its center are taken. It is noted that the most mobile is the Arctic anticyclone. The winter Asian anticyclone practically does not change its position from month to month. The Icelandic Low is more mobile than the Azores High. The displacements of the Aleutian cyclone and the Hawaiian anticyclone are commensurate with each other. The winter Asian anticyclone affects the intensity of the centers of action of the atmosphere in the North Atlantic and the North Pacific. Before the appearance of the Asian winter anticyclone, pressure changes in the centers of the Atlantic and Pacific AAC occur in concert. During the formation of the winter Asian anticyclone, a mismatch occurs in the course of pressure in these centers of action. The degree of influence of the winter Asian anticyclone on pressure fluctuations in the centers of baric formations was made by the method of dispersion analysis. The degree of influence of the winter Asian anticyclone on the course of pressure in the centers of action of the North Atlantic atmosphere is estimated at 87%, in the centers of action of the Pacific Ocean - at 71 %.

The Alaskan Arctic regime shift since 2017: A harbinger of years to come?

Recent fingerprints of Alaskan Arctic climate change include new weather patterns whose impacts propagate through the Alaskan marine ecosystem. Multiple lines of observational evidence draw attention to the last five years (2017–2021) as a remarkable period of change. Bering Sea winter and spring ice coverage was remarkably low in 2018 and 2019 associated with a shifted Aleutian Low (AL) into the western Bering Sea, and ridging of the overlying polar jet stream (PJS) that supported southerly winds and reduced ice growth. The climatological Beaufort High and associated Beaufort Gyre saw multiple winter-long collapses (2017 and 2020), the only two such events of the modern reanalysis era. In contrast, from 2012 to 2016 the AL and PJS were located southeastward across the Gulf of Alaska, which supported sea ice growth. The recent collocation of the exposed ocean surface and atmospheric anomalies supports the emergence of a regional atmospheric circulation response to sea-ice loss. We propose that the regional system shift after 2017 foreshadows the beginning of a period with an increasing frequency of major sea-ice loss events and associated impacts – floods, delayed spring blooms, and marine food chain disruptions – which is sooner than projected by climate models.

Atmospheric Forcing of the Pacific Meridional Mode: Tropical Pacific‐Driven Versus Internal Variability

AbstractThe Pacific Meridional Mode (PMM) impacts tropical Pacific sea surface temperature variations, which in turn affect the PMM through the excited atmospheric teleconnections. Previous studies linked this loop to the tropical Pacific‐excited North Pacific Oscillation (NPO; the second empirical mode of North Pacific sea level pressure variability), while a recent study proposed the linkage to the excited Aleutian low (AL) variability (the first empirical mode). Unraveling their relative importance for the loop is thus crucial for better understanding subtropical‐tropical interactions. Here, using tropical Pacific pacemaker experiments we show that tropical Pacific‐forced AL variability is tied to the loop, while the tropical Pacific‐forced NPO does not effectively induce PMM variability, hence not being in the loop. Our study emphasizes the role of tropical Pacific‐forced AL variability in the PMM‐tropical Pacific interaction, which should be paid more attention in future studies.

Recent weakening linkage between Arctic oscillation and Aleutian low during boreal winter and its impact on surface temperature over Eastern Eurasia

AbstractWe investigated interdecadal relationship changes between the Arctic oscillation (AO) and Aleutian low (AL) for the period 1980–2021 along with the linkage on surface air temperature (SAT) over Eastern Eurasia during the boreal winter (December–January–February). The AO–AL relationship has weakened since the mid‐1990s. To explore a possible explanation, we compared atmospheric fields for two contrasting periods, one when the AO–AL relationship is the strongest (1986–2006) and one when it is the weakest (2000–2020). In both periods, atmospheric circulation over the Arctic region for the two AO–AL in‐phase relationships (i.e., either +AO/+AL or –AO/−AL) evolves with negative AO‐like patterns, contributing to a weakening AO–AL relationship in concurrent with a cool SAT in Eastern Eurasia. We found that the correlation between AL and SAT over Eastern Eurasia decreases, while the AO and SAT correlation remains unchanged. This implies that low‐frequency atmospheric circulation changes such as those of the AO and AL should be considered when studying SAT changes in Eastern Eurasia.

Interannual to Interdecadal Variability of the Southern Yellow Sea Cold Water Mass and Establishment of “Forcing Mechanism Bridge”

The Yellow Sea cold water mass (YSCWM) occupies a wide region below the Yellow Sea (YS) thermocline in summer which is the most conservative water and may contain clearer climate signals than any other water masses in the YS. This study investigated the low-frequency variability of the southern YSCWM (SYSCWM) and established the “forcing mechanism bridge” using correlation analysis and singular value decomposition. On the interannual timescale, the southern oscillation can affect the SYSCWM through both the local winter monsoon (WM) and the sea surface net heat flux. On the decadal timescale, the Pacific decadal oscillation (PDO) can affect the SYSCWM via two “bridges”. First, the PDO affects the SYSCWM intensity by Aleutian low (AL), WM, and surface air temperature (SAT). Second, the PDO affects the SYSCWM by AL, WM, Kuroshio heat transport, and Yellow Sea warm current. The Arctic oscillation (AO) affects the SYSCWM by the Mongolian high, WM, and SAT. Before and after the 1980s, the consistent phase change of the PDO and the AO contributed to the significant decadal variability of the SYSCWM. Finally, one simple formula for predicting the decadal variability of SYSCWM intensity was established using key influencing factors.

Recent Trends in Oceanic Conditions in the Western Part of East/Japan Sea: An Analysis of Climate Regime Shift That Occurred after the Late 1990s

The western part of East/Japan Sea (WES) is an important area for understanding climate change processes and interactions between atmospheric and oceanic conditions. We analyzed the trends in recent oceanic conditions in the WES after the recent climate regime shift (CRS) that occurred in the late 1990s in the North Pacific. We explored the most important climate factors that affect oceanic conditions and determined their responses to changes in climate change. In the CRS that occurred in the late 1980s, changes in oceanic conditions in the WES were influenced by intensity changes in climate factors, and, in the late 1990s, it was by spatial changes in climate factors. The latitudinal shift of the Aleutian low (AL) pressure influences recent changes in oceanic and atmospheric conditions in the WES. The intensity of the Kuroshio Current and the sea level pressure in the Kuroshio extension region associated with the latitudinal shift of the AL pressure affects the volume of transport of the warm and saline water mass that flows into the WES and its atmospheric conditions. In addition, the fluctuations in the oceanic conditions of the WES affect various regions and depth layers differently, and these variations are evident even within the WES.

Future intensification of extreme Aleutian low events and their climate impacts

Extreme Aleutian Low (AL) events have been associated with major ecosystem reorganisations and unusual weather patterns in the Pacific region, with serious socio-economic consequences. Yet, their future evolution and impacts on atmosphere–ocean interactions remain uncertain. Here, a large ensemble of historical and future runs from the Community Earth System Model is used to investigate the evolution of AL extremes. The frequency and persistence of AL extremes are quantified and their connection with climatic variables is examined. AL extremes become more frequent and persistent under the RCP8.5 scenario, associated with changes in precipitation and air temperature patterns over North America. Future changes in AL extremes also increase the variability of the sea surface temperature and net heat fluxes in the Kuroshio Extension, the most significant heat and energy flux region of the basin. The increased frequency and persistence of future AL extremes may potentially cause substantial changes in fisheries and ecosystems of the entire Pacific region as a knock-on effect.

Changes in sediment provenance and ocean circulation on the northern slope of the Bering Sea since the last deglaciation

Changes in water circulation in the Bering Sea have profoundly impacted biological productivity, carbon cycle and global ocean circulation, especially from the last deglaciation to the Holocene. Grain size and clay mineral composition were investigated in core ARC6-B10, collected from the northern Bering slope, in order to reveal changes in circulation and sediment provenance since the late deglaciation (~13.7 ka). The results highlight changes in sea level and atmospheric circulation as the primary controls on the variations in grain size and clay mineralogy. Deglacial sea level rise deepened the water passages along the Aleutian Arc, enhanced the inflow of North Pacific water that strengthened the circulation of the Bering Sea, and receded the coastline. These changes increased the supply of smectite-enriched sediments from the Aleutian Islands to the core site. A temporary reduction in smectite and an increase in the illite and chlorite contents during the Preboreal period correspond to the opening of the Bering Strait; this is attributed to rapid sea level rise which resulted in increased sediment resuspension from the shelf. During the mid- to late Holocene, when the sea level stabilized, changes in the Aleutian Low (AL) were a major factor modulating changes in ocean circulation. A weakened AL in the mid-Holocene intensified the northward Bering Slope Current and the increased the flow through the Bering Strait, while an intensified AL, along with an expanded sea ice cover during the late Holocene may have weakened the northward flow.

Aleutian Low variability for the last 7500 years and its relation to the Westerly Jet

AbstractThe Aleutian Low (AL) is one of the major atmospheric systems that determines environmental conditions during winter in the North Pacific Ocean, with impacts that affect the climates of both Asia and North America from mid- to high latitudes. However, the multi-centennial and longer scale behavior of the AL during the Holocene is not fully understood. In this study, AL variability since 7.5 ka was examined by applying the principal component analysis technique to published δ18O data derived from sedimentary calcite, peat, ice, and speleothem from western North America. The extracted Principal Component 1 (PC1) represents a dramatic change from the mid- to late Holocene, and appears to reflect long-term intensified AL related to interactions between orbitally-driven southward shift of the Westerly Jet (WJ) over East Asia and the northwestern Pacific, and intensification of the El Niño–Southern Oscillation. In contrast, PC2 is characterized by multi-centennial to millennial-scale oscillations, with a spatial loading pattern that suggests PC2 reflects AL intensity and position shifts. These oscillations are contemporaneous with both WJ latitude and/or the meandering path shifts over East Asia and solar activity change, suggesting that a decrease/increase in solar irradiance is related to AL variability via interactions with the WJ.