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A careful isolation of the externally forced component (EFC) and the internal climate variability (ICV) embedded in the observed records as well as the climate simulations is critical to investigate an actual response to the external radiative forcing and/or background dynamics in the ICV. Employing three different methods, we evaluate the EFCs contained in the observed sea surface temperature (SST) and sea level pressure (SLP) fields in the Indo-Pacific region. After removing these EFCs, we obtain the respective ICVs as the remaining anomalies. The remaining SST and SLP anomalies are then evaluated on decadal time scales in a combined empirical orthogonal function (EOF) analysis of different spatial portions: the tropical Pacific, the Indian Ocean and the whole Indo-Pacific region. After making statistical intercomparisons of the spatial patterns and associated time series of the EOF analyses, we found that the EFCs of the individual grid point values (GPVs) were appropriately estimated by regressing onto the multi-model ensemble global mean surface temperature (GMST(MME)) and were less well approximated by the conventional linear trend and the multi-model ensemble mean of the simulated GPVs. The regressed SST anomalies of the individual historical simulations onto the GMST(MME) were much larger than the observed anomalies, illustrating that the ICV-to-EFC variance ratio is a performance-improvement indicator of climate models.

In this study, salinity variations in the Kuroshio-Oyashio confluence region (KOCR) are examined through analyses of observational datasets and an ocean reanalysis product, and their potential impacts on sea surface temperature are assessed by sensitivity experiments using a one-dimensional mixed layer model (1-D ML model). We have detected prominent covariations in near surface temperature and salinity in the KOCR during the boreal winter to spring. Further investigation revealed that such covariations are closely related to the dynamical stability of the Kuroshio Extension (KE), and anomalous warming and salinification (cooling and freshening) are observed in the KOCR when the upstream of the KE is in an unstable (a stable) state. It is found that modulation heat and freshwater transport by mesoscale eddies and large-scale current anomalies are closely related to such observed variation. Then, we have quantitatively estimated the impacts of these salinity variations on local density by a detailed decomposition of total anomaly fields. Although the total density anomalies are dominated by contributions from temperature, the salinity contribution has sizable magnitude especially in the northern part of the KOCR, where the background temperature is low and the dependence of density on temperature variations is weak. To further quantify the impact of salinity anomalies, we conducted a series of sensitivity experiments utilizing the 1-D ML model. The results from these experiments revealed that salinity anomalies significantly alter the strength of vertical mixing and eventually lead to differences in sea surface temperature of approximately 1.0℃.

Sea surface temperature and salinity (SST/SSS) are critical to understanding the ocean's role in the climate system. Because seawater density is controlled by temperature and salinity, characterizing the relationship between these two variables is particularly important for oceanography, climate science and other related fields. By analyzing observational data, we provide the first global picture of covariability between SST and SSS and associated physical processes at interannual timescales. A notable feature of such SST-SSS covariability is its distinct spatial-scale dependence; at small spatial scales (<1,000 km), their covariability is attributed to fluctuations in ocean currents and the associated heat and salt transport, whereas large-scale covariability result from combinations of changes in local atmospheric conditions and large-scale ocean circulations. It is found that the current generation of climate models underestimate such distinct SST-SSS covariations, suggesting that physical processes involving in variations of these variables are not faithfully represented as observed.

In this study, salinity variations in the Kuroshio-Oyashio confluence region (KOCR) are examined through analyses of observational datasets and an ocean reanalysis product, and their potential impacts on sea surface temperature are assessed by sensitivity experiments using a one-dimensional mixed layer model (1-D ML model). We have detected prominent covariations in near surface temperature and salinity in the KOCR during the boreal winter to spring. Further investigation revealed that such covariations are closely related to the dynamical stability of the Kuroshio Extension (KE), and anomalous warming and salinification (cooling and freshening) are observed in the KOCR when the upstream of the KE is in an unstable (a stable) state. It is found that modulation heat and freshwater transport by mesoscale eddies and large-scale current anomalies are closely related to such observed variation. Then, we have quantitatively estimated the impacts of these salinity variations on local density by a detailed decomposition of total anomaly fields. Although the total density anomalies are dominated by contributions from temperature, the salinity contribution has sizable magnitude especially in the northern part of the KOCR, where the background temperature is low and the dependence of density on temperature variations is weak. To further quantify the impact of salinity anomalies, we conducted a series of sensitivity experiments utilizing the 1-D ML model. The results from these experiments revealed that salinity anomalies significantly alter the strength of vertical mixing and eventually lead to differences in sea surface temperature of approximately 1.0℃.

Sea surface temperature and salinity (SST/SSS) are critical to understanding the ocean's role in the climate system. Because seawater density is controlled by temperature and salinity, characterizing the relationship between these two variables is particularly important for oceanography, climate science and other related fields. By analyzing observational data, we provide the first global picture of covariability between SST and SSS and associated physical processes at interannual timescales. A notable feature of such SST-SSS covariability is its distinct spatial-scale dependence; at small spatial scales (<1,000 km), their covariability is attributed to fluctuations in ocean currents and the associated heat and salt transport, whereas large-scale covariability result from combinations of changes in local atmospheric conditions and large-scale ocean circulations. It is found that the current generation of climate models underestimate such distinct SST-SSS covariations, suggesting that physical processes involving in variations of these variables are not faithfully represented as observed.

The present study examined ENSO-related wind forcing contribution to off-equatorial Rossby wave formations in the eastern tropical regions of the North and South Pacific using satellite altimeter data and atmospheric reanalysis data during the period of 1993–2013. After mature phases of ENSO events, the sea surface height anomaly fields showed that off-equatorial Rossby waves propagated westward along 11°N and 8°S from the eastern Pacific. Starting longitudes of the westward propagation were distant from the eastern coast, especially for weak El Niño events in the 2000s, in contrast to the strong 1997/98 El Niño event in which the propagations started from the coast. Based on observational data, it was hypothesized that the Rossby waves could be formed by off-equatorial zonal belts of wind stress curl anomalies (WSCAs) in 135°–90°W rather than by wave emissions from the eastern coast. A numerical model forced only by WSCAs, that is, without wave emissions from the coast, successfully reproduced observed features of the Rossby waves in 180°–120°W, supporting the study’s hypothesis. During mature phases of El Niño events, equatorially symmetric negative sea level pressure anomalies (SLPAs) resulting from hydrostatic adjustment to the underlying warm sea surface temperature anomalies dominated over the eastern tropical Pacific. Anomalous surface easterlies blowing around the negative SLPA area as geostrophic winds were a major contributor in forming the anticyclonic WSCAs. The polarity of the anomalies is reversed during La Niña events. Therefore, spatial patterns of the SLPAs associated with the ENSO events are necessary to understand the Rossby wave formations.

This paper describes a new initiative in which in situ observations of the marine atmospheric boundary layer (MABL) are made by a helicopter shuttle connecting six islands south of Tokyo. This observation method aims to make frequent measurements of temperature and moisture in the MABL across an ocean front, where direct measurements of the MABL have been limited. An onboard observation system to meet flight regulations was developed. Observed temperature and moisture as a function of pressure at 1-s intervals provided vertical profiles up to the 900-hPa level above each of the islands, from 24 December 2010 to 6 April 2011, with the exception of an accidental power down in mid-February 2011. The observed values are validated by intercomparison with surface measurements from weather stations, atmospheric soundings, and mesoscale weather analysis provided by the Japan Meteorological Agency. Temperature and moisture values obtained using the system described here at the surface are significantly correlated with those from the weather station. The meridional changes revealed by the observed vertical profiles depict rich MABL structures, such as a cold-air intrusion and a strong near-surface inversion, that are not captured by the mesoscale weather analysis. However, this discrepancy is probably due to insufficient treatment in the mesoscale numerical model rather than observational errors. Additional intercomparisons indicate no influence from either artificial mixing by the helicopter rotor or by dynamic pressure caused by the fast-moving helicopter when obtaining the vertical profiles. Following these validations, the continuation of the initiative will allow for examining the influence of the ocean front on the overlying MABL on a synoptic time scale.

This paper describes a new initiative in which in situ observations of the marine atmospheric boundary layer (MABL) are made by a helicopter shuttle connecting six islands south of Tokyo. This observation method aims to make frequent measurements of temperature and moisture in the MABL across an ocean front, where direct measurements of the MABL have been limited. An onboard observation system to meet flight regulations was developed. Observed temperature and moisture as a function of pressure at 1-s intervals provided vertical profiles up to the 900-hPa level above each of the islands, from 24 December 2010 to 6 April 2011, with the exception of an accidental power down in mid-February 2011. The observed values are validated by intercomparison with surface measurements from weather stations, atmospheric soundings, and mesoscale weather analysis provided by the Japan Meteorological Agency. Temperature and moisture values obtained using the system described here at the surface are significantly correlated with those from the weather station. The meridional changes revealed by the observed vertical profiles depict rich MABL structures, such as a cold-air intrusion and a strong near-surface inversion, that are not captured by the mesoscale weather analysis. However, this discrepancy is probably due to insufficient treatment in the mesoscale numerical model rather than observational errors. Additional intercomparisons indicate no influence from either artificial mixing by the helicopter rotor or by dynamic pressure caused by the fast-moving helicopter when obtaining the vertical profiles. Following these validations, the continuation of the initiative will allow for examining the influence of the ocean front on the overlying MABL on a synoptic time scale.

This study investigated the eastern Pacific Intertropical Convergence Zone (ITCZ) as an atmospheric forcing to the ocean by using various observed and reanalysis data sets over 29 years. Climatologically, a zonal band of positive wind stress curl (WSC) with a 10° meridional width was exhibited along the ITCZ. A southward shift of the positive WSC band during the El Niño phase induced a negative (positive) WSC anomaly along the northern (southern) portion of the ITCZ, and vice versa during the La Niña phase. This meridional dipole accounted for more than 25 % of interannual variances of the WSC anomalies (WSCAs), based on analysis of the period 1993–2008. The negative (positive) WSCA in the northern portion of the ITCZ during the El Niño (La Niña) phase was collocated with a positive (negative) sea surface height anomaly (SSHA) that propagated westward as a Rossby wave all the way to the western North Pacific. This finding indicates that this off-equatorial Rossby wave is induced by the WSCA around the ITCZ. Our analysis of a 1.5-layer reduced gravity model revealed that the Rossby waves are mostly explained by wind stress forcing, rather than by reflection of an equatorial Kelvin wave on the eastern coastal boundary. The off-equatorial Rossby wave had the same SSHA polarity as the equatorial Kelvin wave, and generation of a phase-preserving Rossby wave without the Kelvin wave reflection was explained by meridional movement of the ITCZ. Thus, the ITCZ acts as an atmospheric bridge that connects the equatorial and off-equatorial oceanic waves.

The present study investigates meridional heat transport induced by oceanic mesoscale variability in the World Ocean using a 1/10°global ocean general circulation model (OGCM) running on the Earth Simulator. The results indicate prominent poleward eddy heat transport around the western boundary currents and the Antarctic Circumpolar Current, and equatorward eddy heat transport in the equatorial region, consistent with the previous studies using coarse-resolution OGCMs. Such poleward eddy heat transport in midlatitude oceans suggests that the eddies act to reduce meridional background temperature gradients across the currents, as would be expected based on baroclinic instability. Interestingly, however, along the southern flanks of the eastward jets of the Kuroshio Extension and the Gulf Stream, southward eddy heat transport occurs in subsurface layers. This is likely due to the southward migration of warm water cores originating from southern areas adjacent to these currents. Southward movement of these cores is caused by interactions with unsteady meanders and cold eddies detaching from the meanders. The potential impact on biological production in the subtropical surface layers of these southward-traveling warm water cores is also discussed.

Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropical North and South Pacific associated with the ENSO warm (cold) events are examined using reanalysis and in-situ observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over the subtropical North Pacific in the February-March period and over the subtropical South Pacific after April, and vice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, the negative air humidity anomaly and anomalous downward motion appear at the same location in either the Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending branch of local Hadley circulation is observed during the January-February period in the Northern hemisphere and after March in the Southern hemisphere, which coincides with the seasonal transition of the climatological local Hadley circulation from the Northern to Southern hemisphere. Our linear decomposition analysis of surface heat flux anomalies indicates that the negative air humidity anomaly, as well as anomalies in wind speed, contributes to the formation of the subtropical negative SSTAs through the enhanced latent heat flux induced by the anomalous air-sea humidity difference. These results suggest that the anomalous downward motion associated with the changes in local Hadley circulation can induce the subtropical negative SSTAs through the surface humidity variability. A possible mechanism for the subtropical air-sea interaction associated with the local Hadley circulation is discussed.

Reproducibility of the Pacific Decadal Oscillation (PDO) is evaluated in the sea surface temperature (SST) anomaly field in "the 20th century climate in coupled models" (20C3M) simulations of the 24 CMIP3 models. In this evaluation, we examine how well patterns of the PDO match between the observations and simulations by calculating a metric of the patterns that is a function of their spatial correlation and their standard deviation. Among the CMIP3 models, the models with the high PDO metric reproduce the decadal SST variability with opposing polarities between the central North Pacific and the tropical Pacific. As observed, temporal correlation between the PDO and decadal-ENSO indices in those simulations are negatively correlated at the statistically-significant level. The sea level pressure and outgoing longwave radiation anomalies onto the decadal-ENSO index in those simulations are realistic both in the tropical Pacific and North Pacific, indicating that this tropics-extratropics linkage in the SST anomaly field is induced by atmospheric teleconnection. This notion is consistent with the previous studies for the natural climate variability. In contrast, the models with the low PDO metric fail to reproduce those characteristics. In the simulations under a middle-range IPCC greenhouse gas emissions scenario (A1B), the PDO indices during the 21st century still represent SST variations on the decadal timescales with superimposition on a linear warming trend. Several models which reproduce the observed PDO pattern in the 20th century record tend to simulate a similar pattern over the 21st century. This indicates that the models with the high PDO metric have their own properties that tend to simulate the natural climate variations with the observed pattern under the global warming condition.

The Kuroshio Extension system links to North Pacific climate through its role in subtropical-subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1). The northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

Intraseasonal variability (10-60 days) of sea surface temperature (SST) over the north Indian Ocean and its influence on regional precipitation variability over the Indian subcontinent are examined. SST, cloud liquid water and precipitation over the Indian Ocean of the Tropical Rainfall Measuring Mission (TRMM), precipitation of Climate Prediction Center Merged Analysis of Precipitation (CMAP), and low-level atmospheric parameters of National Center for Environmental Prediction (NCEP) II reanalysis are utilized for this study. Western Ghats (WG) in the southwest and the Ganges-Mahanadi Basin (GB) in the northeast of the Indian subcontinent are observed to be the regions of maximum precipitation with large standard deviations of the intraseasonal variability. Active (break) phases of precipitation occur in these regions by the northward propagation of positive (negative) precipitation anomalies over the Arabian Sea and the Bay of Bengal. Latitude-time plots during the active phase of the WG region shows that the positive SST anomalies over the Arabian Sea formed by suppressed surface latent heat flux and increased downward shortwave radiation flux lead the positive precipitation anomalies. Surface air temperature anomalies follow the SST anomalies and then destabilize the lower atmosphere between 1000 hPa and 700 hPa. These results indicate that, in the northward propagating dynamical surface convergence, underlying SST anomalies tend to form a favorable condition for convective activity and may sustain enhanced precipitation over the convergence region. This results in enhanced precipitation anomalies over the WG region that move further northeastward and merge with the northward propagating precipitation anomalies from the Bay of Bengal, enhancing the active phase of the GB region.

We investigated seasonal transition of dominant modes of sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean, analyzing the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis products (NCEP/NCAR reanalyses), the Global sea-Ice and SST dataset (GISST2.3b), and the Simple Ocean Data Assimilation (SODA). During the coincidence years when the Indian Ocean Dipole (IOD) is followed by the major El Nino during boreal autumn-winter season, surface dipole structure in the tropical Indian Ocean tends to turn into the basinwide warm pattern in the November-December period. In contrast, the subsurface dipole keeps its structure from boreal autumn to winter. Such a surface-confined transition of SSTA is induced by latent heat flux anomalies in the eastern Indian Ocean. These latent heat flux anomalies are associated with changes in scalar wind speed anomalies. The zonal direction of climatological surface winds changes from easterly into westerly over the eastern Indian Ocean in November-December, while the anomalous Walker circulation during the El Nino induces easterly surface wind anomalies to persist there. As a result, deceleration of scalar wind speed takes place during boreal winter, and leads to warming of SST through suppressed evaporation. In addition to these latent heat flux anomalies, incoming solar radiation anomalies contribute to the net surface warming during this period. Furthermore, we discuss the role of the ocean dynamics for keeping the warm SSTAs in the western Indian Ocean.

Temporal and spatial structures of turbulent latent and sensible heat flux anomalies are examined in relation to dominant patterns of sea surface temperature anomalies (SSTA) observed over the North Pacific. Relative importance among observed anomalies in SST, surface air temperature, and wind speed in determining the anomalous turbulent heat fluxes is assessed through linearizing the observed flux anomalies. Over the central basin of the North Pacific, changes in the atmospheric variables, including air temperature and wind speed, are primarily responsible for the generation of local SST variations by changing turbulent heat flux, which supports a conventional view of extratropical air-sea interaction. In the region where ocean dynamics is very important in forming SSTAs, in contrast, SSTAs that have been formed in early winter play the primary role in determining mid- and late-winter turbulent heat flux anomalies, indicative of the SST forcing upon the overlying atmosphere. Specifically, both decadal scale SSTAs in the western Pacific subarctic frontal zone and El Niño related SSTAs south of Japan are found to be engaged actively in such forcing on the atmosphere. The atmospheric response to this forcing appears to include the anomalous storm track activity. The observed atmospheric anomalies, which may be, in part, forced by the preexisting SSTAs in those two regions, act to force SSTAs in other portions of the basin, leading to the time evolution of SSTAs as observed in the course of the winter season.