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Tomohiko Tomita and Masami Nonaka

Abstract

In the North Pacific, the wintertime sea surface temperature anomaly (SSTA), which is represented by March (SSTAMar), when the upper-ocean mixed layer depth (h Mar) reaches its maximum, is formed by the anomalous surface forcing from fall to winter (S′). As a parameter of volume, h Mar has a potential to modify the impact of S′ on SSTAMar. Introducing an upper-ocean heat budget equation, the present study identifies the physical relationship among the spatial distributions of h Mar, S′, and SSTAMar.

The long-term mean of h Mar adjusts the spatial distribution of SSTAMar. Without the adjustment, the impact of S′ on SSTAMar is overestimated where the h Mar mean is deep. Since h Mar is partially due to seawater temperature, it leads to nonlinearity between the S′ and the SSTAMar. When the SSTAMar is negative (positive), the sensitivity to S′ is impervious (responsive) with the deepening (shoaling) of the h Mar compared to the linear sensitivity. The thermal impacts from the ocean to the atmosphere might be underestimated under the assumption of the linear relationship.

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Masami Nonaka and Hideharu Sasaki

Abstract

Equatorward propagation of temperature–salinity (or spiciness) anomalies on an isopycnal surface emanating from the eastern subtropical South Pacific and their formation mechanism are investigated based on a hindcast simulation with an eddy-resolving quasi-global ocean general circulation model. Because of density-compensating meridional distributions of temperature and salinity, the meridional density gradient is weak at the sea surface in the eastern subtropical South Pacific. With these mean fields, cool sea surface temperature anomalies (SSTAs) can make the outcrop line of an isopycnal surface migrate equatorward more than 5° and induce warm and salty anomalies on the isopycnal surface. Subducted warm, salty anomalies propagate to the equatorial region over approximately 5 yr and may influence equatorial isopycnal temperature–salinity anomalies. Although the associated effects are unclear, if these anomalies could further induce warm eastern equatorial SSTAs that are positively correlated with eastern South Pacific SSTAs, opposite sign temperature–salinity anomalies would be formed in the subtropical South Pacific, and a closed cycle having a decadal time scale might be induced.

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Masami Nonaka and Kensuke Takeuchi

Abstract

While high salinity water extends to the equator in the upper thermocline of the Pacific in the Southern Hemisphere (SH), it hits the western boundary (WB) farther north of the equator in the Northern Hemisphere (NH), suggesting that no interior pathway exists to the equatorial region. By contrast, high tritium water appears on the equator in the central Pacific, apparently through a NH interior pathway within the thermocline. The mechanisms of forming these salinity and tritium distributions and the causes of their difference are investigated using a realistic ocean general circulation model (OGCM).

The OGCM reproduces the properties of tropical salinity distribution quite well and displays interior pathways in the NH. Analysis indicates that the observed salinity distribution is compatible with the existence of a NH interior pathway. Key to the hemispheric difference in thermocline salinity is the sea surface salinity (SSS) distribution in relation to the so-called WB (interior) exchange window, from which subducted water goes to the equatorial region through the WB region (interior ocean). In the NH, high SSSs are found only in the WB exchange window, and high salinity water thus appears to turn onto the WB before reaching the equator. In the SH, on the other hand, high SSSs are found in both the WB and interior exchange windows, and, as a result, high salinity water extends to the equatorial region through both the WB region and interior ocean.

The sea surface tritium field has high values near the eastern boundary within the interior exchange window in the midlatitude North Pacific. Thus, high tritium water takes the NH interior pathway to the equatorial region after the subduction. This is demonstrated by a passive tracer experiment with a sea surface distribution resembling that of tritium. This result suggests that the apparent differences between the isopycnal salinity and tritium distributions are largely due to differences in surface distribution, raising caution about interpreting ocean circulation with tracer fields alone.

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Masami Nonaka and Shang-Ping Xie

Abstract

Satellite microwave measurements are analyzed, revealing robust covariability in sea surface temperature (SST) and wind speed over the Kuroshio Extension (KE) east of Japan. Ocean hydrodynamic instabilities cause the KE to meander and result in large SST variations. Increased (reduced) wind speeds are found to be associated with warm (cold) SST anomalies. This positive SST–wind correlation in KE is confirmed by in situ buoy measurements and is consistent with a vertical shear adjustment mechanism. Namely, an increase in SST reduces the static stability of the near-surface atmosphere, intensifying the vertical turbulence mixing and bringing fast-moving air from aloft to the sea surface.

South of Japan, the Kuroshio is known to vary between nearshore and offshore paths. These paths are very persistent and can last for months to years. As the Kuroshio shifts its path, coherent wind changes are detected from satellite data. In particular, winds are high south of Tokyo when the Kuroshio takes the nearshore path while they are greatly reduced when this warm current leaves the coast in the offshore path.

The positive SST–wind correlation over the strong Kuroshio Current and its extension is opposite to the negative one often observed in regions of weak currents such as south of the Aleutian low. The latter correlation is considered to be indicative of atmosphere-to-ocean forcing.

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Hirohiko Nakamura, Masami Nonaka, and Hideharu Sasaki

Abstract

Previous observations have indicated that the Kuroshio’s path in the northern Okinawa Trough of the East China Sea is destabilized and accompanied by meanders with periods of 1–3 months during the winter–spring period. The present study investigates the mechanism responsible for this recurrent seasonally fixed phenomenon. A hypothetical mechanism is constructed based on both a simple wind-driven Ekman-pumping model, acting within the Kuroshio, and a bifurcation model of the Kuroshio path states in the northern Okinawa Trough, established in the previous study. A high-resolution ocean general circulation model is used to examine the hypothetical mechanism.

The numerical model reveals the following mechanism: the wintertime northerly wind prevailing over the Okinawa Trough blows against the Kuroshio, generating Ekman divergence, and hence upwelling within the inshore side of the Kuroshio from the sum of the earth’s rotation and the geostrophic current shear. A necessary condition for this upwelling is probably given by the exponential velocity structure of the surface Kuroshio on the inshore side of the current. This kind of wintertime upwelling acts to make the mean Kuroshio path separate from the continental slope in the northern Okinawa Trough, so that baroclinic instability destabilizes the Kuroshio path, as shown by the bifurcation model of Kuroshio path states.

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Ayako Yamamoto, Hiroaki Tatebe, and Masami Nonaka

Abstract

Despite its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is far from settled. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a recent study showed that the atmospheric stochastic forcing is sufficient. In this study, we resolve this conundrum by partitioning the multidecadal SST tendency into a part caused by surface heat fluxes and another by ocean dynamics, using a preindustrial control simulation of a state-of-the-art coupled climate model. In the model, horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies; however, when the vertical component is also considered, the ocean dynamics overall acts to cool the region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, although the associated surface heat flux anomalies are upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling, thus leading to relative warming by increasing the ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions.

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Bunmei Taguchi, Hisashi Nakamura, Masami Nonaka, and Shang-Ping Xie

Abstract

Influences of oceanic fronts in the Kuroshio and Oyashio Extension (KOE) region on the overlying atmosphere are investigated by comparing a pair of atmospheric regional model hindcast experiments for the 2003/04 cold season, one with the observed finescale frontal structures in sea surface temperature (SST) prescribed at the model lower boundary and the other with an artificially smoothed SST distribution. The comparison reveals the locally enhanced meridional gradient of turbulent fluxes of heat and moisture and surface air temperature (SAT) across the oceanic frontal zone, which favors the storm-track development both in winter and spring. Distinct seasonal dependency is found, however, in how dominantly the storm-track activity influences the time-mean distribution of the heat and moisture supply from the ocean.

In spring the mean surface sensible heat flux (SHF) is upward (downward) on the warmer (cooler) side of the subarctic SST front. This sharp cross-frontal contrast is a manifestation of intermittent heat release (cooling) induced by cool northerlies (warm southerlies) on the warmer (cooler) side of the front in association with migratory cyclones and anticyclones. The oceanic frontal zone is thus marked as both the largest variability in SHF and the cross-frontal sign reversal of the SHF skewness. The cross-frontal SHF contrasts in air–sea heat exchanges counteract poleward heat transport by those atmospheric eddies, to restore the sharp meridional gradient of SAT effectively for the recurrent development of atmospheric disturbances. Lacking this oceanic baroclinic adjustment associated with the SST front, the experiment with the smoothed SST distribution underestimates storm-track activity in the KOE region.

In winter the prevailing cold, dry continental airflow associated with the Asian winter monsoon induces a large amount of heat and moisture release even from the cooler ocean to the north of the frontal zone. The persistent advective effects of the monsoonal wind weaken the SAT gradient and smear out the sign reversal of the SHF skewness, leading to weaker influences of the oceanic fronts on the atmosphere in winter than in spring.

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Masami Nonaka, Hisashi Nakamura, Youichi Tanimoto, Takashi Kagimoto, and Hideharu Sasaki

Abstract

Output of an eddy-resolving OGCM simulation is used to investigate mechanisms for interannual-to-decadal variability in the Oyashio and its influence on the subarctic frontal zone in the western North Pacific. Lag correlation analysis reveals that positive anomalies both in basin-scale wind stress curl and in local Ekman pumping can intensify the southward Oyashio almost simultaneously via barotropic and baroclinic Rossby wave propagations, respectively. The Oyashio strength can also be influenced by anomalous Ekman pumping that is exerted in the western portion of the basin through the baroclinic wave propagation with the lag of 3 yr, which appears to arise from a periodicity in the wind field. The intensification of the Oyashio is accompanied by negative anomalies both in the sea surface temperature and height off of Hokkaido Island of Japan and is followed by their eastward development along the southern branch of the Oyashio Extension and associated subarctic frontal zone in association with a southward displacement of their axes. These changes are associated with cool sea surface temperature anomalies and low potential vorticity anomalies at the thermocline level in the frontal zone. The surface cooling, thus induced in the frontal zone by those oceanic processes, accompanies anomalous downward surface heat fluxes, indicative of ocean-to-atmosphere feedback forcing associated with the Oyashio variations.

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J. V. Ratnam, Masami Nonaka, and Swadhin K. Behera

Abstract

The machine learning technique, namely artificial neural networks (ANN), is used to predict the surface air temperature (SAT) anomalies over Japan in the winter months of December, January, and February for the period 1949/50–2019/20. The predictions are made for the four regions Hokkaido, North, Central, and West of Japan. The inputs to the ANN model are derived from the anomaly correlation coefficients among the SAT anomalies over the regions of Japan and the global SAT and sea surface temperature anomalies. The results are validated using anomaly correlation coefficient (ACC) skill scores with the observation. It is found that the ANN predictions over Hokkaido have higher ACC skill scores compared to the ACC scores over the other three regions. The ANN-predicted SAT anomalies are compared with that of ensemble mean of eight of the North American Multimodel Ensemble (NMME) models besides comparing them with the persistent anomalies. The ANN predictions over all the four regions have higher ACC skill scores compared to the NMME model skill scores in the common period of 1982/83–2018/19. The ANN-predicted SAT anomalies also have higher hit rate and lower false alarm rate compared to the NMME-predicted SAT anomalies. All these indicate that the ANN model is a promising tool for predicting the winter SAT anomalies over Japan.

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Masami Nonaka, Julian P. McCreary Jr., and Shang-Ping Xie

Abstract

The stratification of the equatorial thermocline is a key variable for tropical climate dynamics, through its influence on the temperature of the water that upwells in the eastern equatorial ocean. In this study, two types of ocean models are used, an ocean general circulation model (GCM) and a 1½-layer model, to investigate processes by which changes in the midlatitude winds affect the equatorial stratification. Specifically, the influences of anomalous mode-water formation, Ekman pumping, and entrainment in the subpolar ocean are examined. The effects of a “sponge layer” adjacent to the northern boundary of the basin are also assessed. Solutions are forced by idealized zonal winds with strong or weak midlatitude westerlies, and they are found in rectangular basins that extend from the equator to 36°N (small basin) or to 60°N (large basin). In the GCM solutions, a prominent response to reduced winds is the thinning of the mixed layer in the northwestern region of the subtropical gyre, leading to less subduction of low-potential-vorticity mode water and hence thinning of the upper thermocline in the central-to-eastern subtropics. Almost all of this thinning signal, however, recirculates within the subtropics, and does not extend to the equator. Another midlatitude response is shallowing (deepening) of the thermocline in the subtropical (subpolar) ocean in response to Ekman pumping. This, primarily, first-baroclinic-mode (n = 1) response has the most influence on the equatorial thermocline. First-baroclinic-mode Rossby waves propagate to the western boundary of the basin where they reflect as packets of coastal Kelvin and short-wavelength Rossby waves that carry the midlatitude signal to the equator. Subsequently, equatorial Kelvin waves spread it along the equator, leading to a shoaling and thinning of the equatorial thermocline. The layer-thickness field h in the 1½-layer model corresponds to thermocline depth in the GCM. Both the sponge layer and subpolar Ekman suction are important factors for the 1½-layer model solutions, requiring water upwelled in the interior ocean to be transported into the sponge layer via the western boundary layer. In the small basin, equatorial h thins in response to weakened westerlies when there is a sponge layer, but it thickens when there is not. In the large basin, equatorial h is unaffected by weakened westerlies when there is a sponge layer, but it thins when water is allowed to entrain into the layer in the subpolar gyre. It is concluded that the thinning of the equatorial thermocline in the GCM solutions is caused by the sponge layer in the small basin and by entrainment in the subpolar ocean in the large one.

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