many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

Using datasets of sea surface temperature (SST), surface heat flux, upper ocean thermal data, and climatological temperature and salinity profiles, we try to detect ‘reemergence’ areas of winter SST anomalies in the world's oceans, and describe characteristics of these areas in terms of mixed layer depth (MLD), annual mean heat flux and properties of waters formed in winter mixed layer. Eventually, seven reemergence areas are found: four in the Northern Hemisphere and three in the southern Hemisphere. All areas have a large seasonal variation of MLD, and are the regions where annual mean heat fluxes are relatively small except for two regions in the Northern Hemisphere. In the viewpoint of water properties, it is found that these areas correspond to the mode water formation regions: subtropical mode waters of the Indian Ocean, South Pacific, South Atlantic, North Pacific and North Atlantic, and North Pacific central and North Atlantic subpolar mode waters.

Long-term variations of the Aleutian Low (AL) defined by the sea level pressure (SLP) minimum within the region of [30 N-60 N, 150 E-150 W] during winter (December-February) are investigated, using atmospheric reanalysis datasets. The intensity, latitudinal position, and longitudinal position of AL reveal di¤erent temporal variations: the longitudinal shift accompanies intensity variation with an interdecadal timescale (about 20 years), and the latitudinal shift does with a decadal timescale (about 10 years). The AL intensity variation and the longitudinal shift are related to activity of the Pacific/North American teleconnection pattern: in a strengthening (weakening) phase of AL, the AL shifts eastward (westward); westerlies strengthen (weaken), and both subtropical and subpolar gyres spin-up (spin-down) simultaneously. The latitudinal shift is associated with activity of the West Pacific teleconnection pattern. It is independent of the intensity variation of AL: when the AL shifts northward (southward), the westerlies correspondingly move northward (southward). Consequently, the gyre boundary, which is defined by the zero line of the Sverdrup stream function, also shifts northward (southward).The role of AL north-south shift on the upper oceanic variations is investigated by using a wind-driven hindcast model. The oceanic Rossby wave formed as a result of the baroclinic response for the AL movement influences the sea surface temperature in the Kuroshio-Oyashio Extension region.

Variations of turbulent heat fluxes (sum of sensible and latent heat fluxes) in the North Pacific during 16 winters from December 1992/February 1993 to December 2007/February 2008 are investigated because the months from December to February correspond to the period having peak winter conditions in the atmosphere field. Turbulent heat fluxes are calculated from the bulk formula using daily variables [surface wind speed, surface air specific humidity, surface air temperature, and sea surface temperature (SST)] of the objectively analyzed air–sea flux (OAFlux) dataset and bulk coefficients based on the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm 3.0. The winter turbulent heat fluxes over the Kuroshio–Oyashio Confluence Region (KOCR; 142°–150°E, 35°–40°N) have the largest temporal variances in the North Pacific. The relative contributions among observed variables in SST, surface air temperature, and surface wind speed causing turbulent heat flux variations in the KOCR are assessed quantitatively by performing simple experiments using combinations of two types of variables: raw daily data and daily climatological data. Results show that SST is primarily responsible for the turbulent heat flux variations—a huge amount of heat is released in the state of the positive SST anomaly. Using the datasets of satellite-derived SST and sea surface height with high spatial and temporal resolutions, it is found that the SST anomalies in the KOCR are formed through activities of the anticyclonic (warm) eddies detached northward from the Kuroshio Extension; SSTs take positive (negative) anomalies when more (less) anticyclonic eddies are distributed there, associated with a more convoluted (straight) Kuroshio Extension path.

The 137°E repeat hydrographic section of the Japan Meteorological Agency across the western North Pacific was initiated in 1967 as part of the Cooperative Study of the Kuroshio and Adjacent Regions and has been continued biannually in winter and summer. The publicly available data from the section have been widely used to reveal seasonal to decadal variations and long-term changes of currents and water masses, biogeochemical and biological properties, and marine pollutants in relation to climate variability such as the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In commemoration of the 50th anniversary in 2016, this review summarizes the history and scientific achievements of the 137°E section during 1967-2016. Through the publication of more than 100 papers over this 50-year span, with the frequency and significance of the publication increasing in time, the 137°E section has demonstrated its importance for future investigations of physical-biogeochemical-biological interactions on various spatiotemporal scales, and thereby its utility in enhancing process understanding to aid projections of the impact of future climate change on ocean resources and ecosystems over the twentyfirst century.