Subarctic Frontal Zone Research Articles

Wintertime ocean–atmosphere interaction processes associated with the SST variability in the North Pacific subarctic frontal zone

Recent research indicates that the midlatitude oceanic frontal zones are the key regions of ocean–atmosphere interaction. The thermal condition of midlatitude ocean in frontal zones can affect the atmosphere efficiently through both diabatic heating and transient eddy feedback. In this study, the wintertime SST variability in the subarctic frontal zone (SAFZ) of the North Pacific and the associated ocean–atmosphere interaction mechanism are examined based on observational and theoretical analyses. It is found that the SAFZ-related SST anomaly is characterized as a large-scale interannual mode that can persist during the whole winter, and that its evolution is accompanied with local ocean–atmosphere interaction processes. The initial anticyclonic surface wind anomaly associated with the weakened Aleutian Low forces a large-scale warm SST anomaly in midlatitude North Pacific by driving northward Ekman flow and downward heat flux. With the increase of SST anomaly, the air-sea heat flux exchange reverses, indicating that the ocean starts to heat the atmosphere. In addition to increasing the diabatic heating, the warm SST anomaly strengthens the SST gradient in the north part of SAFZ. The low-level atmospheric baroclinicity is adjusted to synchronize with the SAFZ correspondingly due to oceanic thermal influence, causing change of transient eddy activities. Though all the ocean-induced diabatic heating, transient eddy heating and transient eddy vorticity forcing are enhanced over SAFZ, the last physical process plays the most important role in shifting and maintaining the equivalent barotropic atmospheric circulation anomalies. Therefore, the ocean–atmosphere interaction provides a mechanism for the development and maintenance of SAFZ-related anomalies of the North Pacific ocean–atmosphere system throughout the winter.

Siderophore production and utilization by marine bacteria in the North Pacific Ocean

AbstractSiderophores are strong iron‐binding molecules produced and utilized by microbes to acquire the limiting nutrient iron (Fe) from their surroundings. Despite their importance as a component of the iron‐binding ligand pool in seawater, data on the distribution of siderophores and the microbes that use them are limited. Here, we measured the concentrations and types of dissolved siderophores during two cruises in April 2016 and June 2017 that transited from the iron‐replete, low‐macronutrient North Pacific Subtropical Gyre through the North Pacific Transition Zone (NPTZ) to the iron‐deplete, high‐macronutrient North Pacific Subarctic Frontal Zone (SAFZ). Surface siderophore concentrations in 2017 were higher in the NPTZ (4.0–13.9 pM) than the SAFZ (1.2–5.1 pM), which may be partly attributed to stimulated siderophore production by environmental factors such as dust‐derived iron concentrations (up to 0.51 nM). Multiple types of siderophores were identified on both cruises, including ferrioxamines, amphibactins, and iron‐free forms of photoreactive siderophores, which suggest active production and use of diverse siderophores across latitude and depth. Siderophore biosynthesis and uptake genes and transcripts were widespread across latitude, and higher abundances of these genes and transcripts at higher latitudes may reflect active siderophore‐mediated iron uptake by the local bacterial community across the North Pacific. The variability in the taxonomic composition of bacterial communities that transcribe putative ferrioxamine, amphibactin, and salmochelin transporter genes at different latitudes further suggests that the microbial groups involved in active siderophore production and usage change depending on local conditions.

Summer Habitat and Fishing Ground of <i>Ommastrephes bartramii</i> Related with the North Pacific Subarctic Frontal Zone Using Long-term Field Research Data

The neon flying squid, Ommastrephes bartramii, is an economically important oceanic squid species that has been harvested commercially in the North Pacific by Japan, Korea, China, and Taiwan. The Fisheries Research Agency (FRA) of Japan conducted long-term research for this species from 1980 to 2009 using commercial vessels to clarify the resource ecology and it obtained a total dataset of approximately 9,000 days, where following from this and to better clarify the relationship between habitat preference and oceanographic conditions, a generalized additive model was applied and mapped onto the Simple Ocean Data Assimilation ocean/sea ice reanalysis (SODA) data. The resultant habitat map of the squid showed an area of high abundance at the Subarctic Frontal Zone (SAFZ), with abundance areas increasing the squid abundance from April to August. The SAFZ extends from 40°N to 43°N separating the cold, low-salinity, subarctic water to the north from the waters of the North Pacific Transition Zone to the south. This high abundance area intersects between subarctic area and temperate area. It has characteristic nutrient regimes, productivity cycles, and nektonic faunal compositions. This paper suggests that the SAFZ thus plays an important role in North Pacific ecosystems by providing an optimal balance between environmental temperature and food density for the neon flying squid.

A comparative analysis of the mesoscale thermohaline features across subarctic frontal zones in the Northern Hemisphere

Mesoscale eddies and their thermohaline features across the two subarctic frontal (SAF) zones, the Oyashio Extension (OE) and the Gulf Stream (GS), in the Northern Hemisphere are investigated and compared using both satellite-based observations and Argo float data. Our results show that the thermohaline properties (temperature, salinity and associated density) induced by mesoscale eddies have significant differences between the south and north sides of each SAF zone. The relationships between the sea surface height anomaly (SSHA) and the sea surface temperature anomaly (SSTA), called the H-T amplitude relationship, are examined and used to depict the mesoscale sea surface characteristics. In both the OE and GS regions, SSTAs on the north sides of the SAFs are more sensitive to the SSHA than that on their southern sides, and this sensitivity is stronger in the GS than in the OE. It is found that in the OE, the difference in the H-T amplitude relationships between the north and south sides is mainly due to the thermal expansion coefficient, which is contingent on the ambient sea surface temperature. In the GS, the difference between the north and south sides in the H-T amplitude relationships is mainly due to the change of the vertical profiles of mesoscale eddies, which are related to the ambient meridional density front.

Double Acceleration Effects of Closely Spaced Pairs of Ocean Fronts on High‐Wind Occurrence Frequency During Boreal Winter

AbstractPrevious studies have found that sea surface temperature (SST) fronts have significant impacts on high‐wind frequency. Strong winds over the mid‐latitude North Pacific and North Atlantic Oceans occur frequently downstream of SST fronts in boreal winter. An SST front can significantly increase the instability of the marine atmospheric boundary layer, which accelerates the wind on the warmer flank. This study uses satellite observations to examine the “double acceleration” effects of closely spaced paired fronts (with interfrontal distances of 300–1,000 km) on high‐wind occurrence frequency in the Pacific subarctic frontal zone east of Japan and over the Atlantic Ocean east of Newfoundland. During boreal winter each year, mean westerly winds frequently cross two or more closely spaced SST fronts, with probabilities of 85% over the North Pacific and 87% over the North Atlantic. Over the warmer flank of the first front, the average high‐wind occurrence frequency (wind speed) increases due to the vertical‐mixing mechanism. Downstream of the second front, average high‐wind occurrence frequency (wind speed) reaches maxima of ∼9% (11.5 m s−1) over the Pacific and ∼11.5% (12 m s−1) over the Atlantic. Stronger westerly winds lead to greater high‐wind frequency. The first front contributes to the observed high winds downstream of the second front due to little friction and suppression of upward vertical momentum transfer between paired fronts.

Investigating air-sea interactions in the North Pacific on interannual timescales during boreal winter

We study the air-sea interaction in a perspective of interannual timescale with wintertime observed sea surface temperature (SST), turbulent heat flux (THF) and sea level pressure (SLP) based on lead-lag correlation and regression. Being forced by the atmosphere, the ocean also has a response to the atmosphere, especially in the subarctic frontal zone (SAFZ) and the eastern North Pacific. The air-sea interactions in two regions are quite similar but associated with different patterns of atmospheric variability when SST leading SLP for one month. Since the SST changes in the eastern North Pacific and the equatorial east-central Pacific are synchronous in the dominant mode (EOF1) of the SST anomalies, the SLP responses to SST in the eastern North Pacific and central Pacific (CP) ENSO both show Pacific-North American (PNA) teleconnection pattern. The CP ENSO dominates the tropical influence on the air-sea interaction in the eastern North Pacific during November. Specifically, the response of Aleutian low to CP El Niño events leads to heat transport from ocean to atmosphere in the eastern North Pacific. In CP La Niña years, the air-sea heat exchange is affected by the combination of Aleutian low and lower troposphere westerlies.

Surface water pathways in the subtropical–subarctic frontal zone of the western North Pacific

A peculiar water mass forms as a result of stirring and mixing between water originating from the Kuroshio and Oyashio currents in the Transition Domain, which is a boundary region between the subtropical and subarctic gyres extending zonally at approximately 40°N in the western North Pacific west of the dateline. The domain is important not only for oceanographic processes such as the inter-gyre exchange of water masses but also for mid-latitude climate and biological processes. We investigated how water from the Kuroshio and Oyashio currents is transported to the Transition Domain and then mixed, especially in relation to the geostrophic velocity fields, bottom topographic features, and the local eddy activity in the upstream region of the domain. We have clarified the pathways of the water from the Kuroshio and Oyashio currents from a Lagrangian point of view by conducting drifting buoy observations in 2015 and 2017, as well as by particle tracking using geostrophic surface flow fields. Herein, we visualize how the water pathways within and around the Transition Domain are closely tied to low-rise bottom topographic features. Time-variation of the geostrophic field in the upstream region of the Transition Domain also likely plays an important role in supplying water from the Kuroshio region to the Transition Domain; if a steady climatological flow field is used, the Transition Domain will be occupied solely by water from the Oyashio current. Once the water from the Kuroshio region enters a quasi-stationary jet in the western North Pacific (the so-called western Isoguchi Jet; J1) due to the eddies at the Oyashio Second Branch in addition to eddies at another region of the J1′s meander, it is further transported northeastward in the subarctic gyre via the Transition Domain. The subtropical–subarctic exchange through the Transition Domain could essentially be driven by the eddies in these local regions.

Projected climate change in the western North Pacific at the end of the 21st century from ensemble simulations with a high-resolution regional ocean model

Future climate change in the western North Pacific at the end of the 21st century (2081–2100) was examined using a high-resolution regional ocean model (10-km mesh) under the RCP2.6 and RCP8.5 scenarios. The range of uncertainty in future projections was estimated from ensemble simulations. Projected results indicated no significant change in the Kuroshio net transport and the latitude of the Kuroshio Extension under both RCP scenarios; the changes were within the range of variability associated with the present climate. Projected sea surface temperature (SST) increased by as much as several degrees Celsius, especially in SST fronts, including the subarctic frontal zone. The significant increase of SST east of Japan was attributed to the northward expansion of the northern part of the subtropical gyre in response to basin-scale atmospheric changes. The projected area of sea ice in the Sea of Okhotsk decreased in both RCP scenarios. The projected offshore sea-level rise was larger in the subtropical gyre and smaller in the subpolar gyre. The sea-level rise along the coast of Japan, in contrast, showed no significant spatial variation. The mean sea-level rise along the coast of Japan was mostly comparable to the global mean sea-level rise.

Spatial Habitat Shifts of Oceanic Cephalopod (Ommastrephes bartramii) in Oscillating Climate

Short- and long-term climate oscillations impact seascapes, and hence, marine ecosystem structure and dynamics. Here, we explored the spatio-temporal patterns of potential squid habitat in the western and central North Pacific across inter-decadal climate transitions, coincident with periods of persistent warming and cooling. Potential habitat distributions of Ommastrephes bartramii were derived from the outputs of multi-ensemble species distribution models, developed using the most influential environmental factors to squid distribution and occurrence data. Our analyses captured the underlying temporal trends in potential squid habitat in response to environmental changes transpiring at each climatic transition, regulated by phase shifts in Pacific decadal oscillation (PDO) from 1999–2013. The spatial differences in environmental conditions were apparent across transitions and presumably modulate the local changes in suitable squid habitat over time. Specifically, during a cold to warm PDO shift, decreases in the summer potential habitat (mean rate ± standard deviation: −0.04 ± 0.02 habitat suitability index (HSI)/yr) were observed along the southern edge of the subarctic frontal zone (162°E–172°W). Coincidentally, this area also exhibits a warming trend (mean temporal trend: 0.06 ± 0.21 °C/yr), accompanied with the prevalence of cold-core mesoscale eddies, west of the dateline (mean temporal trend in sea surface height: −0.19 ± 1.05 cm/yr). These conditions potentially generate less favorable foraging habitat for squid. However, a warm-to-cold PDO transition underpins a northward shift of suitable habitat and an eastward shift of regions exhibiting the highest rate of potential squid habitat loss (170–160°W; mean temporal trend: −0.05 ± 0.03 HSI/yr). Nonetheless, the emergence of the areas with increasingly suitable habitat regardless of climate transitions suggests the ecological importance of these regions as potential squid habitat hotspots and climatic refugia.

Impacts of North Pacific Subtropical and Subarctic Oceanic Frontal Zones on the Wintertime Atmospheric Large-Scale Circulations

AbstractAn oceanic frontal zone is a confluent region of warm and cool ocean currents, characterized by a strong meridional gradient of sea surface temperature (SST). High-resolution SST observations show that the wintertime North Pacific exhibits a unique double-oceanic-front structure, with a subtropical frontal zone (STFZ) and a subarctic frontal zone (SAFZ), whose impacts on the weather and climate over the East Asia–North Pacific–North American region need further investigation. In this study, we conduct groups of multiyear and ensemble simulations using a WRF high-resolution regional climate model, through which the different impacts of the STFZ and SAFZ on the wintertime atmospheric circulations are identified and compared. Our multiyear simulations show that the STFZ, although with weaker intensity, exerts evident and consistent impacts on the storm track and westerly jet in the North Pacific by enhancing and elongating the eddy activity, zonal wind, and Aleutian low. The SAFZ exhibits coherent impacts on the low-level atmospheric baroclinicity and storm track; however, its impacts on the upper-level storm track and atmospheric circulations are divergent, exhibiting strong year-by-year difference. Our study suggests that the SAFZ’s impacts on the atmospheric circulations strongly depend on the background mean state, which contributes to the divergent impacts of the SAFZ. Furthermore, our results highlight the role of diabatic heating for the above different impacts of the STFZ and SAFZ on the atmosphere. We argue that the much deeper diabatic heating induced by the STFZ, via affecting the baroclinicity through the whole troposphere, can exert consistent influence on eddy activities and atmospheric circulations.

Midlatitude atmospheric transient eddy feedbacks influenced ENSO-associated wintertime Pacific teleconnection patterns in two PDO phases

The El Nino-Southern Oscillation (ENSO)-associated wintertime atmospheric teleconnection patterns in two Pacific decadal oscillation (PDO) phases are investigated using ERA-20C reanalysis data for 1950–2010. A strengthened ENSO-associated Pacific-North American (PNA) teleconnection pattern presents in PDO positive phase, while a West Pacific (WP) pattern over Northwestern Pacific and a squeezed PNA pattern coexist when ENSO occurs in PDO negative phase. The dynamical role of atmospheric transient eddy feedbacks to the teleconnection patterns are highlighted in the present study. When ENSO occurs in PDO positive phase, the uniform strengthened westerly jet anomalies downstream of the climatological main body of jet accompany with energetic transient eddy anomalies over Northeastern Pacific. The transient eddy feedbacks largely enhance and favor the strengthened PNA pattern. When ENSO occurs in PDO negative phase, the strengthened westerly jet anomalies appear to separate into two parts, one locating north of the climatological main body of jet and the other at the downstream. The accompanied transient eddy anomalies also split into two parts. Under such conditions, the transient eddy feedbacks are limited over Northeastern Pacific and favor a weak PNA pattern. However, the transient eddy anomalies over Northwestern Pacific strengthen, and the feedbacks also strengthen and largely contribute to the WP pattern. Moreover, the transient eddy anomalies over Northwestern Pacific seem to be anchored along the anomalously poleward strengthened oceanic subarctic frontal zone (SAFZ) in PDO negative phase. The enhanced atmospheric baroclinicity anomalies, coupled with the strengthened SAFZ, energize atmospheric transient eddy anomalies, and work as the potential maintenance in shaping the WP pattern.

Sperm whale (Physeter macrocephalus) acoustic ecology at Ocean Station PAPA in the Gulf of Alaska – Part 2: Oceanographic drivers of interannual variability

Understanding top predator responses to environmental variability is key to assessing potential impacts of global warming on marine ecosystems. However, tracking environmental changes and their effects across multiple trophic levels up to predators can be difficult. Here, we related the interannual (2007–2012) acoustic occurrence of sperm whales (Physeter macrocephalus) at Ocean Station PAPA (OSP), in the offshore Gulf of Alaska (GOA), to environmental drivers within an explicit time-series modeling framework. In a model based on a combination of in-situ and remotely sensed variables, ocean heat content (HC), meridional current (V), eddy kinetic energy (EKE), strength of the permanent pycnocline (maxBVF), sea surface temperature (SST) and SST standard deviation (SSTsd) explained 51% of the variability in sperm whale presence, indicating a positive relationship between sperm whale occurrence and increased ocean heating, vertical stratification, and circulation. Sperm whale detections were anomalously high in summer 2010 and winter 2011, and anomalously low at the end of summer and winter 2008, and spring 2011. Results suggest these strongly anomalous detection events reflect a response to El Niño–Southern Oscillation (ENSO) events, with an 8-month lag. In the study area, El Niño leads to a stronger permanent pycnocline, a weakening of the Alaska Gyre, a northward expansion of the North Pacific Subarctic Frontal Zone, and a prevalence of offshore mesoscale eddies, likely favoring the abundance of squid and their predator, the sperm whale. The overall results are consistent with a scenario of increasing sperm whale occurrence at high latitudes under increased ocean warming.

Seasonal variations of the relationship between the North Pacific storm track and the meridional shifts of the subarctic frontal zone

This study investigates the seasonally meridional shifts of the subarctic frontal zone (SAFZ) and its relationship with North Pacific storm track (NPST). It is found that the SAFZ is at its southernmost in winter but northernmost in summer, and the maximum standard deviations of the SAFZ meridional position is between 153° E and 163° E, with the maximum amplitude of about 2.5 latitudes. When the SAFZ shifts northward (southward), the NPST moves poleward (equatorward) with the distinct seasonally anomalous patterns, i.e., the NPST meridional displacement is larger in winter and autumn, followed by that in spring, and it is smallest in summer. It is also revealed that the near-surface baroclinicity and baroclinic energy conversion (BCEC) may be responsible for the seasonal variations of the relationship between the NPST and the SAFZ shifts. Accompanied by the northward (southward) shift of the SAFZ, the near-surface baroclinicity zone moves northward (southward) significantly, and more mean available potential energy converts to eddy available potential energy and further transferred to eddy kinetic energy in the northern (southern) part of the NPST, resulting in more pronounced northward (southward) movement of the NPST in winter and autumn. However, the NPST anomalous patterns are relatively weak when the near-surface baroclinicity and BCEC anomalies are small in spring and summer.

Interannual to Decadal Variability of the Upper-Ocean Heat Content in the Western North Pacific and Its Relationship to Oceanic and Atmospheric Variability

Three-dimensional oceanic thermal structures and variability in the western North Pacific (NP) are examined on the interannual to decadal time scales and their relationship to oceanic and atmospheric variability is discussed by analyzing observation and reanalysis data for 45 years (1964–2008), which is much longer than the satellite-altimetry period. It is shown that the meridional shift of the Kuroshio Extension (KE) and subarctic frontal zone (SAFZ) is associated with the overall cooling/warming over the KE and SAFZ region (KE–SAFZ mode). It appears, however, that changes in KE strength induce different signs of thermal anomalies to the south and north of the KE, not extended to the SAFZ (KE mode), possibly contributing to noncoherent variability between the KE and SAFZ. Thus, the KE and SAFZ are dependent on each other in the context of the KE–SAFZ mode, while the KE is independent of the SAFZ in terms of the KE mode. This intricate relationship is associated with different linkages to atmospheric variability; the KE–SAFZ mode exhibits a relatively fast response to the large-scale wind stress curl forcing in the NP, whereas the KE mode is related to a delayed response to the atmospheric forcing via jet-trapped baroclinic Rossby wave propagation. It is suggested that further knowledge of the underlying mechanisms of the two modes would contribute to understanding ocean–atmosphere feedback as well as potential predictability over the western boundary current region in the NP.

Seasonal variation of the North Pacific storm-track relationship with the Subarctic frontal zone intensity

The relationship between the North Pacific storm track and the intensity of the subarctic frontal zone (SAFZ) in each season is investigated in this study. Associated with the strengthened SAFZ, the storm track in winter characterize a distinct downstream intensification, and a comparable enhancement appears in spring in the northwestern part of the storm track; while the storm-track anomalies are much weaker in summer and autumn. It is found that the seasonal variation of the storm-track anomalous patterns may be attributed to the near-surface baroclinicity and the baroclinic energy conversion (BCEC) anomalies. When the SAFZ intensifies in winter (spring), accompanied by the increased near-surface baroclinicity in the central and eastern (northwestern) part of the storm track, more energy is converted form the mean available potential energy to the eddy available potential energy and then transferred to the eddy kinetic energy, which favors the robust downstream (northwestward) enhancement of the storm track in winter (spring). While the weaker storm-track anomalous patterns occur in summer and autumn, due to small near-surface baroclinicity and BCEC anomalies.

Impacts of the subarctic frontal zone on the North Pacific storm track in the cold season: an observational study

ABSTRACTIn this study, potential impacts of the North Pacific subarctic frontal zone (SAFZ) variation, including its intensity variation and meridional shift, upon the subseasonally varying North Pacific storm track are investigated by using the 100‐year reanalysis data sets. Regression analysis indicates that the changes in the SAFZ intensity and meridional position have significant influence on the North Pacific storm track, which intensifies with the strengthening of the SAFZ and moves northwards following the northwards shift of the SAFZ. However, the storm‐track response pattern exhibits distinct differences from one calendar month to another. Specifically, the storm‐track response to the SAFZ intensity variation is strongest in February and March; while its response to the SAFZ meridional shift is most pronounced in November and December. However, the storm‐track response is relatively weak in January. Further analysis shows that the intensified (or northwards shifted) SAFZ would result in changes in the near‐surface baroclinicity and hence affects the storm track, while the weak storm‐track response in January is not the result of the anomalous near‐surface baroclinicity. The investigation of the local energetics reveals that changes in the baroclinic energy conversion (BCEC) associated with the SAFZ variation are consistent with the storm‐track anomalies, indicating that the BCEC plays a crucial role in modulating the subseasonal changes in the storm‐track response. In January, the weakened BCEC contributes to the reduced storm‐track response to the SAFZ variation.

Mechanisms for the Maintenance of the Wintertime Basin-Scale Atmospheric Response to Decadal SST Variability in the North Pacific Subarctic Frontal Zone

Abstract Mechanisms for the maintenance of a large-scale wintertime atmospheric response to warm sea surface temperature (SST) anomalies associated with decadal-scale poleward displacement of the North Pacific subarctic frontal zone (SAFZ) are investigated through the following two ensemble experiments with an atmospheric general circulation model (AGCM): one with climatological-mean SST and the other with positive SST anomalies along the SAFZ prescribed on top of the climatological-mean SST. As actually observed, the simulated January ensemble response over the North Pacific is anticyclonic throughout the depth of the troposphere, although its amplitude is smaller. This response is maintained through energy conversion from the ensemble climatological-mean circulation realized under the climatological SST as well as feedback from anomalous transient eddy activity, suggesting that the response may have characteristics as a preferred mode of variability (or “dynamical mode”). Conversions of both available potential energy and kinetic energy from the climatological-mean state are important for the observed anomaly, while the latter is less pronounced for the model response. Net transient feedback forcing is also important for both the observed anomaly and simulated response. These results imply that a moderate-resolution (~1°) AGCM may be able to simulate a basin-scale atmospheric response to the SAFZ SST anomaly through synoptic- and basin-scale dynamical processes. Weaker PNA-like internal variability in the model may lead to the weaker response, suggesting that misrepresentation of intrinsic atmospheric variability can affect the model response to the SST anomaly.

Two typical modes in the variabilities of wintertime North Pacific basin‐scale oceanic fronts and associated atmospheric eddy‐driven jet

AbstractThe role of oceanic fronts in the midlatitude air–sea interaction remains unclear. This study defines new indexes to quantify the intensity and location of two basin‐scale oceanic frontal zones in the wintertime North Pacific, i.e. the subtropical and subarctic frontal zones (STFZ, SAFZ). With these indexes, two typical modes, which are closely related to two large‐scale sea surface temperature (SST) anomaly patterns resembling Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), respectively, are found in the oceanic front variabilities as well as in their associations with the midlatitude atmospheric eddy‐driven jet. Corresponding to an PDO‐like SST anomaly pattern, an enhanced STFZ occurs with a southward shifted SAFZ, which is associated with enhanced overlying atmospheric front, baroclinicity and transient eddy vorticity forcing, thus with an intensification of the westerly jet; and vice versa. On the other hand, corresponding to an NPGO‐like SST pattern, an enhanced SAFZ occurs with a northward shifted STFZ, which is associated with a northward shift of the atmospheric front, baroclinicity, transient eddy vorticity forcing, and westerly jet; and vice versa. These results suggest that the basin‐scale oceanic frontal zone is a key region for the midlatitude air–sea interaction in which the atmospheric transient eddy's dynamical forcing is a key player in such an interaction.

Decadal Variability of Upper-Ocean Heat Content Associated with Meridional Shifts of Western Boundary Current Extensions in the North Pacific

Generation and propagation processes of upper-ocean heat content (OHC) in the North Pacific are investigated using oceanic subsurface observations and an eddy-resolving ocean general circulation model hindcast simulation. OHC anomalies are decomposed into physically distinct dynamical components (OHC ρ) due to temperature anomalies that are associated with density anomalies and spiciness components (OHC χ) due to temperature anomalies that are density compensating with salinity. Analysis of the observational and model data consistently shows that both dynamical and spiciness components contribute to interannual–decadal OHC variability, with the former (latter) component dominating in the subtropical (subpolar) North Pacific. OHC ρ variability represents heaving of thermocline, propagates westward, and intensifies along the Kuroshio Extension, consistent with jet-trapped Rossby waves, while OHC χ variability propagates eastward along the subarctic frontal zone, suggesting advection by mean eastward currents. OHC χ variability tightly corresponds in space to horizontal mean spiciness gradients. Meanwhile, area-averaged OHC χ anomalies in the western subarctic frontal zone closely correspond in time to meridional shifts of the subarctic frontal zone. Regression coefficient of the OHC χ time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, and suggest that OHC χ is generated via frontal variability in the subarctic frontal zone.

Recent climate changes of thermohaline structure in the North-West Pacific

Variability of temperature and salinity in the 0-1000-m layer of the North-West Pacific is analyzed on the data of shipboard oceanographic observations obtained in 1950-2014. Significant decadal changes of these parameters are revealed at the depth of 300-600 m that exceeds the depth of seasonal fluctuations, with the highest rate in the Subarctic frontal zone. The heat content was the highest in the 1960-1974, then decreased in the 1975-1999, increased again in the 2000-2014, and the next phase of thermal «discharge» may begin supposedly in 5-6 years. Mechanisms of such climate changes are discussed. Similar changes are observed in different regions of the World Ocean that means that it is a global-scale phenomenon. These climate changes of water temperature in the subsurface layer of the ocean and air temperature over the continent have opposite directions that denotes the auto-oscillation in the open non-linear system «ocean-atmosphere-continent» as their main mechanism. The previous period of the water thermal discharge was conditioned by active winter center of low atmospheric pressure - Aleutian Low. It was accompanied by strengthening of the East-Kamchatka Current and Oyashio Current and high abundance of japanese sardine. The modern period of warming from the beginning of the 21 st century is distinguished by negative values of PDO index, observed since 1998. However, PDO is positive again since January 2014, though stability of this change is not clear yet. On the other hand, winter SST in the spawning grounds of japanese sardine, both in the Japan Sea and North-West Pacific, gradually decreased after the maximum in the 1998-2000 and reached negative anomalies in the areas at Japan in 2011-2015 that was favorable for its reproduction. The sardine biomass was gradually increased, too, in particularly in the 2010-2012, and its migration to the Russian EEZ became more active, up to 0.7 million tons in the South-Kuril area in 2015.