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Since August 2017, the large meander (LM) path of the Kuroshio south of Japan has continued to date for more than five years, with a local Kuroshio recirculation gyre off Shikoku (RGOS). The formation and temporal evolution of Subtropical Mode Water (STMW) in the isolated RGOS were examined by using Argo profiling float and shipboard observation data. Since the late winter of 2018, which was the first winter of the current LM event, multiple STMW layers with different densities overlapping with each other have been observed for more than three years inside the RGOS. The deeper STMW layer of 18.0 °C, which was formed in the late winter of 2017 before the current LM event began, has survived at least until September 2021 while its thickness and horizontal extent has decreased gradually. We may have succeeded in discovering the longest survived STMW ever observed due to a suitable environment for STMW conservation inside the RGOS. On the other hand, the shallower STMW layer(s) of 19.0–19.8 °C, which was originally formed over the deeper STMW layer in the late winter of 2018, has been repeatedly renewed or dissipated in the following years. The oxygen utilization rate at the core of the deeper STMW at a depth of 500 dbar was estimated to be 4.6 μmol kg−1 yr−1, which is believed to be explained solely by oxygen consumption due to remineralization.

Since August 2017, the Kuroshio has taken a large-meander (LM) path, which has forced the Kuroshio extension (KE) to be in its stable state against its wind-forced decadal variability. How such current conditions have impacted the formation and advection of North Pacific subtropical mode water (STMW) over its distribution region was examined using Argo float data during 2005–2020. Out of the whole STMW defined as a low-potential vorticity layer of 16–19.5 ºC, a relatively cold variety of 16–18 ºC, which was formed south of the KE and advected westward and southward, occupied more than 80% of the total volume. The formation rate of the 16–18 ºC variety was low during 2006–2009 in an unstable-KE period and high during 2010–2015 in a stable-KE period, and then dropped drastically in 2016 despite the KE still being in the stable state. After a short unstable-KE period in 2016–2017, the LM-forced, stable-KE period began, but the formation rate of the 16–18 ºC variety has not restored, possibly due to stronger background stratification propagated from the central North Pacific. In addition, the 16–18 ºC variety has had to make a southern detour around the LM, and its westward advection from the formation region south of the KE to the region south of Japan has been significantly decreased, possibly because it is dissipated more strongly over a southern part of the Izu–Ogasawara Ridge. Due to such decline in the formation and advection, the volume of the 16–18 ºC variety and hence that of the whole STMW have gradually decreased since 2016.

The interbasin exchange between the Sea of Okhotsk and the North Pacific governs the intermediate water ventilation and fertilization of the nutrient-rich subpolar Pacific, and thus has an enormous influence on the North Pacific. However, the mechanism of this exchange is puzzling; current studies have not explained how the western boundary current (WBC) of the subarctic North Pacific intrudes only partially into the Sea of Okhotsk. High-resolution models often exhibit unrealistically small exchanges, as the WBC overshoots passing by deep straits and does not induce exchange flows. Therefore, partial intrusion cannot be solely explained by large-scale, wind-driven circulation. Here, we demonstrate that tidal forcing is the missing mechanism that drives the exchange by steering the WBC pathway. Upstream of the deep straits, tidally-generated topographically trapped waves over a bank lead to cross-slope upwelling. This upwelling enhances bottom pressure, thereby steering the WBC pathway toward the deep straits. The upwelling is identified as the source of joint-effect-of-baroclinicity-and-relief (JEBAR) in the potential vorticity equation, which is caused by tidal oscillation instead of tidally-enhanced vertical mixing. The WBC then hits the island chain and induces exchange flows. This tidal control of WBC pathways is applicable on subpolar and polar regions globally.

We demonstrate that assimilation of water temperature measurements by sea turtles into an operational ocean nowcast/forecast system improves representation of mesoscale eddies and front variations in the Kuroshio-Oyashio Confluence region. For a period from August to December in 2009, vertical ranges of the measurements by six turtles covered from surface to maximum 150 m depth, and the number of the measurements in the region was comparable to that of usual sampling by research vehicles and profiling floats. Comparison of the turtle measurements and products of an operational ocean nowcast/forecast system JCOPE2M elucidated low-temperature bias in JCOPE2M due to assimilation of high-resolution satellite sea surface temperature data. Additional assimilation of the turtle data into a modified JCOPE2M system with correction of the low-temperature bias led to reasonable modification of horizontal temperature/salinity gradient associated with northward moving warm core rings separating from the Kuroshio Extension and southward moving the Oyashio water intrusion branches. Effects of the turtle data assimilation propagated to far east of the Kuroshio-Oyashio confluence region along the subarctic boundary and front through advection. Another experiment assimilating the in situ profile data excluding the traditional temperature/salinity profiles data but including only the turtle data showed that the turtle measurements captured the warm core rings better than the Oyashio intrusion branches, suggesting possible optimum design of future ocean observing systems composed of different kinds of animals and/or autonomous underwater vehicles.

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.

Simultaneous launches of radiosondes were conducted from three research vessels aligned meridionally across a sea surface temperature (SST) front on the flank of the Kuroshio Extension. The soundings carried out every 2 h over 5 days in early July 2012 provided a unique opportunity in capturing unambiguous data on anomalous easterly winds derived from a pronounced meridional SST gradient. The data indicate that a meridional contrast in surface heat fluxes from the underlying ocean enhanced the air temperature anomaly across the SST front, which was observed from the surface up to 300-m altitude. Correspondingly, high and low pressure anomalies that reached 800-m altitude formed on the north and south sides of the SST front, respectively. These temperature and pressure anomalies were maintained even during the passage of synoptic-scale disturbances. Although the free-tropospheric winds are overall westerly, winds below the 1000-m level were easterly due to geostrophic anomalies driven by the northward pressure gradient near the surface. During periods of the northerlies at the surface, especially over the warmer side of the SST front, the wind direction changed in a clockwise direction from 1500 m to the surface, in the opposite sense to the Ekman spiral. The vertical wind shear is apparently in the thermal wind balance ascribed to the meridional contrast in air temperature derived from the SST anomaly.

Between 1950 and 2012, boreal summer (rainy season) rainfall in the Sahel changed from a multi-decadal decreasing trend to an increasing trend (positive trend shift) in the mid-1980s. We found that this trend shift was synchronous with similar trend shifts in global oceanic evaporation and in land precipitation on all continents except the Americas. The trend shift in oceanic evaporation occurred mainly in the Southern Hemisphere (SH) and the subtropical oceans of the Northern Hemisphere (NH). Because increased oceanic evaporation strengthens the atmospheric moisture transport toward land areas, the synchrony of oceanic evaporation and land precipitation is reasonable. Surface scalar winds over the SH oceans also displayed a positive trend shift. Sea surface temperature (SST) displayed a trend shift in the mid-1980s that was negative (increasing, then decreasing) in the SH and positive in the NH. Although SST had opposite trend shifts in both hemispheres, the trend shift in evaporation was positive in both hemispheres. We infer that because strong winds promote evaporative cooling, the trend shift in SH winds strengthened the trend shifts in both SST and evaporation in the SH. Because high SST promotes evaporation, the trend shift in NH SST strengthened the NH trend shift in evaporation. Thus differing oceanic roles in the SH and NH generated the positive trend shift in evaporation; however, the details of moisture transport toward the Sahel are still unclear, or perhaps there is no single determining influence.

Two mesoscale convective events in the baiu frontal zone (BFZ) were documented, based on intensive atmospheric soundings and oceanic castings in the East China Sea during May 2011, in addition to continuous surface meteorological observations, satellite products, and objective analyses. These events occurred while the BFZ was nearly stagnant and a mesolow was deepening in the zone. Near-surface southerlies associated with the low-level jet transported a warm, humid air mass from south of the BFZ. Enhanced evaporation, which was mainly attributable to the high sea surface temperature of the Kuroshio, augmented the moisture content of the air mass and helped maintain a convectively unstable stratification in the lower troposphere around the BFZ.

Radiosondes were launched in August 2006 from a research vessel above a spot around the Kuril Islands between the North Pacific and the Okhotsk Sea characterized by an extremely cold sea surface temperature (SST) and a large horizontal temperature gradient caused by strong tidal mixing. Because this cold SST was caused solely by this oceanic process, the observed data chiefly reflect the oceanic influence on the atmosphere. In this area, where the summertime SST is typically lower than in other seas in the same latitude belt, a minimum SST of 2°C and a horizontal gradient of 7°C/10 km were observed in August 2006. During the cruise, the area was covered by a layer of fog up to about 1000 m thick. The following observations reveal the presence of an atmospheric mesoscale anticyclone and a cold dome caused by ocean cooling of the atmosphere. The wind direction over the cold spot and the strong gradient region was northeasterly below the altitude of about 500 m, in contrast to the southwesterly geostrophic wind, and the area of northeasterly wind was characterized by a cold air mass and a high-pressure anomaly. This ageostrophic wind distribution indicates mesoscale anticyclonic circulation below 500 m. The fog layer was also thinner in the northeasterly wind area than in other areas, suggesting vertical dry air advection (i.e., a subsidence) associated with the mesoscale anticyclone. In addition, our observational results suggest that the pressure adjustment mechanism can explain this mesoscale circulation.

To confirm whether surface winds strengthen above warm waters around oceanic fronts using in situ data, a field measurement was conducted using both expendable bathythermographs and Global Positioning System sondes released concurrently across the Kuroshio front in the East China Sea in December 2010. In contrast to previous studies mainly based on satellite observations, the finding of the present field survey is the local weakening of surface winds at the northern flank of the Kuroshio front. From the above field observation in conjunction with a regional numerical model experiment, it is suggested that the northwesterly winds crossing the Kuroshio front from the cooler side first weaken at the northern flank of the front because of the onset of upward transfer of the “nonslip” condition at the sea surface. Thereafter, as the atmospheric mixed layer with warm and humid air mass develops gradually downwind over the Kuroshio region, the surface winds are gradually accelerated by the momentum mixing with strong winds aloft. The surface winds remain strong over the cool East China Sea shelf, and it is thus considered that the surface winds only weaken at the northern flank of the Kuroshio front. However, numerical modeling indicates that this local weakening of the surface winds occurs as a transient state with a short duration and such a structure has thus rarely been detected in the long-term averaged wind fields observed by satellites.

Sea surface temperature (SST) fronts in mid- to high-latitude oceans have significant impacts on extratropical atmospheric circulations and climate. In the western subarctic Pacific, sharp SST fronts form between the cold subarctic water and the recently found quasi-stationary jets that advect warm waters originating in the Kuroshio northeastward. Here we present a new mechanism of the jet formation paying attention to the propagation of baroclinic Rossby waves that is deflected by eddy-driven barotropic flows over bottom rises, although their height is low (~500 m) compared with the depth of the North Pacific Ocean (~6000 m). Steered by the barotropic flows, Rossby waves bring a thicker upper layer from the subtropical gyre and a thinner upper layer from the subarctic gyre, thereby creating a thickness jump, hence a surface jet, where they converge. This study reveals an overlooked role of low-rise bottom topography in regulating SST anomalies in subpolar oceans., Sea surface temperature fronts in mid-and-high latitudes give significant impacts on atmospheric circulations and climate. Here, the authors uncover a new mechanism on the sea surface front genesis in the subpolar oceans in which small-amplitude bottom topography is surprisingly effective.