|Hiroki Tokinaga||Last modified date：2021.06.16|
Professor / Division of Earth Environment Dynamics / Research Institute for Applied Mechanics
|Hiroki Tokinaga||Last modified date：2021.06.16|
|1.||Ingo Richter, Hiroki Tokinaga, An overview of the performance of CMIP6 models in the tropical Atlantic: mean state, variability, and remote impacts, Climate Dynamics, 10.1007/s00382-020-05409-w, 55, 2579-2601, 2020.08, General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic and its linkage to the tropical Pacific. While, on average, mean state biases have improved little, relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement..|
|2.||N. C. Johnson, D. J. Amaya, Q. Ding, Y. Kosaka, H. Tokinaga, S.-P. Xie, Multidecadal modulations of key metrics of global climate change, Global and Planetary Change, doi:10.1016/j.gloplacha.2020.103149, 2020.05.|
|3.||J. Ma, L. Zhou, G.R. Foltz, X. Qu, J. Ying, H. Tokinaga, C. R. Mechoso, J. Li, X. Gu, Hydrological cycle changes under global warming and their effects on multiscale climate variability, Annals of New York Academy of Sciences, doi:10.1111/nyas.14335, 2020.03.|
|4.||Thomas Kilpatrick, Shang‐Ping Xie, Hiroki Tokinaga, David Long, Nolan Hutchings, Systematic scatterometer wind errors near coastal mountains, Earth and Space Science, 10.1029/2019EA000757, 6, 10, 1900-1914, 2019.09.|
|5.||H. L. Tanaka, Hiroki Tokinaga, Baroclinic instability in high latitudes induced by polar vortex
A connection to the arctic oscillation, Journal of the Atmospheric Sciences, 10.1175/1520-0469(2002)059<0069:BIIHLI>2.0.CO;2, 59, 1, 69-82, 2002.01, In this study, baroclinic instability of the northern winter atmosphere is investigated in the context of the dynamical interpretation of the Arctic oscillation. The unstable solutions, obtained by a method of 3D normal mode expansion, are compared for observed zonal basic states with strong and weak polar vortices in reference to the Arctic oscillation index. As a result of the eigenvalue problem of the linear stability analysis, a characteristic unstable solution is obtained that dominates in high latitudes when the polar vortex is strong. The mode is called a monopole Charney mode M1, which is similar to an ordinary Charney mode Mc in midlatitudes. In order to understand the origin of the M1 mode, a hypothetical zonal basic state that has only the polar jet with no subtropical jet is analyzed. It is found that the M1 mode in high latitudes is excited by the baroclinicity associated with the polar vortex. The Ml mode in high latitudes is dynamically the same Charney mode as Mc but is excited by the baroclinicity of the polar jet instead of the subtropical jet. As the Mc mode intensifies the subtropical jet by the eddy momentum transfer, the M1 mode transfer eddy momentum to high latitudes to intensify the polar jet. Since M1 mode appears during the strong polar jet and feeds the westerly momentum to the polar jet, there is a positive feedback between the Ml mode and the polar vortex. This positive feedback would produce a persistent strong polar jet, which may in tern result in the occurrence of the annular mode of the Arctic oscillation..
|6.||Shang Ping Xie, Jan Hafner, Youichi Tanimoto, W. Timothy Liu, Hiroki Tokinaga, Hairming Xu, Bathymetric effect on the winter sea surface temperature and climate of the yellow and East China seas, Geophysical Research Letters, 29, 24, 81, 2002.12, Whether and how the atmosphere reacts to changes in extratropical sea surface temperature (SST) is under intense debate and this lack of understanding has been a major obstacle in the study of non-El Nino climate variability. Using new satellite measurements, we detect clear ocean-to-atmospheric feedback in the Yellow and East China (YEC) Seas that is triggered by the submerged ocean bottom topography. Under intense surface cooling in winter, water properties are well mixed up to 100 m deep. Ocean depth thus has a strong influence on SST of the continental shelf, leading to a remarkable collocation of warm tongues and deep channels. High winds and increased cloudiness are found over these warm tongues; one such band of ocean-atmospheric co-variation meanders through the basin, following a deep channel for an amazing distance of 1000 km. In addition to these climatic effects, the Kuroshio Front-where the warm current meets the much colder shelf water-strengthens the growth of storms..|
|7.||Hiroki Tokinaga, Youichi Tanimoto, Seasonal transition of SST anomalies in the tropical Indian Ocean during El Niño and Indian Ocean dipole years, Journal of the Meteorological Society of Japan, 10.2151/jmsj.2004.1007, 82, 4, 1007-1018, 2004.08, 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 Niño 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 Niño 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..|
|8.||Shang Ping Xie, Masami Nonaka, Youichi Tanimoto, Hiroki Tokinaga, Haiming Xu, William S. Kessler, R. Justin Small, W. Timothy Liu, Jan Hafner, A fine view from space, Bulletin of the American Meteorological Society, 85, 8, 1060-1062, 2004.08, New satellite observations are revealing rich structures of oceans and the overlying atmosphere in more detail. This paper discusses a few recent examples to illustrate the great potential satellite data offers for studying air-sea interaction and climate. The examples include air-sea phenomena near ocean fronts and those triggered by terrain, features narrow in space and poorly resolved by traditional datasets..|
|9.||Hiroki Tokinaga, Youichi Tanimoto, Shang Ping Xie, SST-induced surface wind variations over the Brazil-Malvinas confluence
Satellite and in situ observations, Journal of Climate, 10.1175/JCLI3485.1, 18, 17, 3470-3482, 2005.09, The confluence of the Brazil-Malvinas Currents maintains strong sea surface temperature (SST) fronts in the midlatitude southwestern Atlantic year-round. SST effects on near-surface stability and surface wind variations are examined in this region using satellite and in situ datasets. Satellite observations show strong (weak) surface wind speeds over the warm Brazil (cold Malvinas) Current. A novel feature of this study is the construction of a high-resolution surface meteorological dataset that is based on historical ship observations. Analysis of this new in situ dataset reveals an increased (reduced) sea-air temperature difference over the Brazil (Malvinas) Current, indicating destabilization (stabilization) in the atmospheric boundary layer. These results are consistent with the SST-induced vertical mixing mechanism for wind adjustment. The SST effect on the near-surface atmosphere is observed both in the climatology and on interannual time scales in the Brazil-Malvinas confluence. Along a zonal SST front at 49°S northeast of the Malvinas/Falkland Islands, there is a collocated line of surface wind divergence, with moderate convergence to the north. Vertical mixing does not explain this divergence pattern because the prevailing surface winds are westerly, blowing in parallel with the front. An additional mechanism is proposed for boundary layer wind adjustment..
|10.||Hiroki Tokinaga, Youichi Tanimoto, Masami Nonaka, Bunmei Taguchi, Tomohiro Fukamachi, Shang Ping Xie, Hisashi Nakamura, Tomowo Watanabe, Ichiro Yasuda, Atmospheric sounding over the winter Kuroshio Extension
Effect of surface stability on atmospheric boundary layer structure, Geophysical Research Letters, 10.1029/2005GL025102, 33, 4, 2006.02, Shipboard radiosonde surveys were conducted during the 2003-04 winter east of Japan to study atmospheric boundary layer (ABL) structure over the Kuroshio Extension. ABL displayed large variations in vertical structure, most of which are attributable to changes in atmospheric surface stability. Where the surface atmosphere was unstable (neutral) as measured by the sea-air temperature difference, surface turbulent heat flux increased (decreased) and a mixed-layer developed (undeveloped) with weakened (intensified) vertical wind shear. A linear regression analysis indicates that ABL height tends to increase by 1km as the sea-air temperature difference increases by 7°C or surface turbulent heat flux by 500 Wm-2. While meridional thermal advection by weather disturbances seems to cause much of atmospheric stability variability during the 43-day surveys, the strong sensitivity of vertical mixing and wind shear to stability is consistent with the observed in-phase co-variability of SST and surface wind from satellite on monthly and longer timescales..
|11.||N. Sato, Hiroki Tokinaga, R. Shirooka, N. Suginohara, Influence of mechanical mixing on a low summertime SST in the western North Pacific ITCZ region, Geophysical Research Letters, 10.1029/2006GL025997, 33, 14, 2006.07, A region with a low sea-surface temperature (SST) was identified in the western North Pacific (WNP) Intertropical Convergence Zone (ITCZ) or warm water pool region in boreal summer. The SST decreases by up to 0.5°C from May to the June-July-August (JJA) season just east of Mindanao Island. Analyses of the observed data indicated that a northeastward surface current constantly exists throughout the year, supplying cold subsurface water through the Molucca Strait. As a result, the subsurface water is colder by several degrees compared with that in the surrounding regions. The sea-surface wind is strong over this region in the JJA season. Examination of Argo float data demonstrated that the mixed layer becomes deeper from May to the JJA season. It is suggested that the strong wind causes vertical mixing between the surface and subsurface layers, resulting in the low summertime SST..|
|12.||Shang Ping Xie, Kaiming Hu, Jan Hafner, Hiroki Tokinaga, Yan Du, Gang Huang, Takeaki Sampe, Indian Ocean capacitor effect on Indo-Western pacific climate during the summer following El Niño, Journal of Climate, 10.1175/2008JCLI2544.1, 22, 3, 730-747, 2009.02, Significant climate anomalies persist through the summer (June-August) after El Niño dissipates in spring over the equatorial Pacific. They include the tropical Indian Ocean (TIO) sea surface temperature (SST) warming, increased tropical tropospheric temperature, an anomalous anticyclone over the subtropical northwest Pacific, and increased mei-yu-baiu rainfall over East Asia. The cause of these lingering El Niño effects during summer is investigated using observations and an atmospheric general circulation model (GCM). The results herein indicate that the TIO warming acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific Oceans. It causes tropospheric temperature to increase by a moistadiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific. In the northwest Pacific, this equatorial Kelvin wave induces northeasterly surface wind anomalies, and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone. The GCM results support this Kelvin wave-induced Ekman divergence mechanism. In response to a prescribed SST increase over the TIO, the model simulates the Kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest Pacific. An additional experiment further indicates that the north Indian Ocean warming is most important for the Kelvin wave and northwest Pacific anticyclone, a result corroborated by observations. These results have important implications for the predictability of Indo-western Pacific summer climate: the spatial distribution and magnitude of the TIO warming, rather than simply whether there is an El Niño in the preceding winter, affect summer climate anomalies over the Indo-western Pacific and East Asia..|
|13.||Youichi Tanimoto, Shang Ping Xie, Kohei Kai, Hideki Okajima, Hiroki Tokinaga, Toshiyuki Murayama, Masami Nonaka, Hisashi Nakamura, Observations of marine atmospheric boundary layer transitions across the summer Kuroshio extension, Journal of Climate, 10.1175/2008JCLI2420.1, 22, 6, 1360-1374, 2009.04, The baiu and Kuroshio Extension (KE) fronts, both zonally oriented and nearly collocated east of Japan, are the dominant summertime features of the atmosphere and ocean, respectively, over the midlatitude northwest Pacific. An atmospheric sounding campaign was conducted on board the R/V Roger Revelle during the 2005 summer. Transects of soundings across the KE front are analyzed to study its effects on the atmosphere, along with continuous surface meteorological and ceilometer cloud-base observations. While the KE front remained nearly stationary during the cruise, the baiu front displayed large meridional displacements that changed wind direction across the KE front. The presence of sharp sea surface temperature (SST) gradients anchored by the KE enhanced the thermal and moisture advection, causing substantial changes in the marine atmospheric boundary layer (MABL) structure. When the baiu front was displaced north of the KE front, southwesterly winds advected warm, humid air from the subtropics over the cold water, producing a surface inversion favorable to fog formation.When the baiu front was to the south, on the other hand, northerly winds across the KE front destabilized the MABL, leading to the formation of a solid low-cloud deck beneath a strong capping inversion. The wind changes with the meridional displacement of the baiu front thus caused large variations in near-surface atmospheric stability and surface turbulent heat fluxes, with potential feedback on deep convection and fog/low-cloud formation around the front..|
|14.||Hiroki Tokinaga, Shang Ping Xie, Ocean tidal cooling effect on summer sea fog over the Okhotsk Sea, Journal of Geophysical Research Atmospheres, 10.1029/2008JD011477, 114, 14, 2009.07, Ocean tide is a periodic phenomenon of the rise and fall of the sea level that depends on the bathymetry and positions of the moon and sun relative to the earth. In the Okhotsk Sea, the ocean tide causes strong vertical mixing in the upper ocean, generating rich structures in summer SST with extraordinarily cold patches on Kashevarov Bank, along the Kuril Islands, northeast of the Terpeniya Peninsula, and inside Shelikhov Bay. The present study analyzes and synthesizes a suite of ship-board and satellite observations, and numerical simulations to investigate the ocean tidal cooling effect on summer sea fog over the Okhotsk Sea. Over the tide-induced cold patches, atmospheric stability increases, and surface wind weakens markedly possibly through suppressed atmospheric vertical mixing. Our analysis of ship-board observations reveals frequent sea-fog occurrence over the cold patches with a maximum exceeding 70% along the Kuril Islands in July-August. Numerical simulations with the Weather Research and Forecasting (WRF) model support that the cold patches cool air temperature, decelerate surface wind, and form a strong surface inversion layer up to 0.5 km high, creating favorable conditions for sea-fog occurrence. The WRF simulation without the tidal cooling effect reduces sea-fog occurrence by more than 20% over cold patches, illustrating the importance of SST products for climate models..|
|15.||Hiroki Tokinaga, Youichi Tanimoto, Shang Ping Xie, Takeaki Sampe, Hiroyuki Tomita, Hiroshi Ichikawa, Ocean frontal effects on the vertical development of clouds over the western North Pacific
In situ and satellite observations, Journal of Climate, 10.1175/2009JCLI2763.1, 22, 16, 4241-4260, 2009.08, A suite of shipboard and satellite observations are analyzed and synthesized to investigate the three-dimensional structure of clouds and influences from sea surface temperature fronts over the western North Pacific. Sharp transitions are observed across the Kuroshio Extension (KE) front in the marine atmospheric boundary layer (MABL) and its clouds. The ocean's influence appears to extend beyond the MABL, with higher cloud tops in altitude along the KE front than the surroundings. In winter, intense turbulent heat release from the ocean takes place on the southern flank of the KE front, where the cloud top penetrates above the MABL and reaches the midtroposphere. In this band of high cloud tops, frequent lightning activity is observed. The results of this study suggest a sea level pressure mechanism for which the temperature gradient in the MABL induces strong surface wind convergence on the southern flank of the KE front, deepening the clouds there. In early summer, sea fog frequently occurs on the northern flank of the subtropical KE and subarctic fronts under southerly warm advection that suppresses surface heat flux and stabilizes the surface atmosphere. Sea fog is infrequently observed over the KE front even under southerly conditions, as the warm ocean current weakens atmospheric stratification and promotes vertical mixing. The KE front produces a narrow band of surface wind convergence, helping support a broad band of upward motion at 700 hPa that is associated with the eastward extension of the baiu rainband from Japan in June-July..
|16.||Akira Nagano, Kaoru Ichikawa, Hiroshi Ichikawa, Hiroyuki Tomita, Hiroki Tokinaga, Masanori Konda, Stable volume and heat transports of the North Pacific subtropical gyre revealed by identifying the Kuroshio in synoptic hydrography south of Japan, Journal of Geophysical Research: Oceans, 10.1029/2009JC005747, 115, 9, 2010.01, We conducted synoptic hydrographic surveys three times under distinctly different conditions of the Kuroshio in the region off the southern coast of Japan; the large-meander (October 2004) and non-large-meander (September 2005 and September 2006) states. As a result of the water mass analysis, we could separate the volume and heat transports of the North Pacific subtropical gyre from the local recirculation gyre and mesoscale eddies. Despite the different flow conditions of the Kuroshio, the volume transport and the volume transport-averaged temperature of the subtropical gyre fluctuate within the ranges of 23.3-29.0 × 106 m3 s-1 and 17.3-17.6°C, respectively, which are quite stable with respect to those for the Kuroshio south of Japan estimated in the past studies. Taking into account the decrease in the volume transport-averaged temperature in the North Pacific interior region, the net heat transport of the subtropical gyre across the latitude of 30°N was estimated to be between 0.19 and 0.22 × 10 15 W..|
|17.||Shang Ping Xie, Yan Du, Gang Huang, Xiao Tong Zheng, Hiroki Tokinaga, Kaiming Hu, Qinyu Liu, Decadal shift in El Niño influences on Indo-western Pacific and East Asian climate in the 1970s, Journal of Climate, 10.1175/2010JCLI3429.1, 23, 12, 3352-3368, 2010.06, El Niño's influence on the subtropical northwest (NW) Pacific climate increased after the climate regime shift of the 1970s. This is manifested in well-organized atmospheric anomalies of suppressed convection and a surface anticyclone during the summer (June-August) of the El Niño decay year [JJA(1)], a season when equatorial Pacific sea surface temperature (SST) anomalies have dissipated. In situ observations and ocean- atmospheric reanalyses are used to investigate mechanisms for the interdecadal change. During JJA(1), the influence of the El Niño-Southern Oscillation (ENSO) on the NW Pacific is indirect, being mediated by SST conditions over the tropical Indian Ocean (TIO). The results here show that interdecadal change in this influence is due to changes in the TIO response to ENSO. During the postregime shift epoch, the El Niño teleconnection excites downwelling Rossby waves in the south TIO by anticyclonic wind curls. These Rossby waves propagate slowly westward, causing persistent SST warming over the thermocline ridge in the southwest TIO. The ocean warming induces an antisymmetric wind pattern across the equator, and the anomalous northeasterlies cause the north Indian Ocean to warm through JJA(1) by reducing the southwesterly monsoon winds. The TIO warming excites a warm Kelvin wave in tropospheric temperature, resulting in robust atmospheric anomalies over the NW Pacific that include the surface anticyclone. During the preregime shift epoch, ENSO is significantly weaker in variance and decays earlier than during the recent epoch. Compared to the epoch after the mid-1970s, SST and wind anomalies over the TIO are similar during the developing and mature phases of ENSO but are very weak during the decay phase. Specifically, the southern TIO Rossby waves are weaker, so are the antisymmetric wind pattern and the North Indian Ocean warming during JJA(1). Without the anchor in the TIO warming, atmospheric anomalies over the NW Pacific fail to develop during JJA(1) prior to the mid-1970s. The relationship of the interdecadal change to global warming and implications for the East Asian summer monsoon are discussed..|
|18.||Shoshiro Minobe, Masato Miyashita, Akira Kuwano-Yoshida, Hiroki Tokinaga, Shang Ping Xie, Atmospheric response to the Gulf Stream
Seasonal variations, Journal of Climate, 10.1175/2010JCLI3359.1, 23, 13, 3699-3719, 2010.07, The atmospheric response to the Gulf Stream front in sea surface temperature is investigated using high-resolution data from satellite observations and operational analysis and forecast. Two types of atmospheric response are observed with different seasonality and spatial distribution. In winter, surface wind convergence is strong over the Gulf Stream proper between Cape Hatteras and the Great Banks, consistent with atmospheric pressure adjustments to sea surface temperature gradients. The surface convergence is accompanied by enhanced precipitation and the frequent occurrence of midlevel clouds. Local evaporation and precipitation are roughly in balance over the Florida Current and the western Gulf Stream proper. In summer, strong precipitation, enhanced high clouds, and increased lightning flash rate are observed over the Florida Current and the western Gulf Stream proper, without seasonal surface convergence enhancement. For the precipitation maximum over the Florida Current, local evaporation supplies about half of the water vapor, and additional moisture is transported from the south on the west flank of the North Atlantic subtropical high. Atmospheric heating estimated by a Japanese reanalysis reveals distinct seasonal variations. In winter, a shallow-heating mode dominates the Gulf Stream proper, with strong sensible heating in the marine atmospheric boundary layer and latent heating in the lower troposphere. In summer, a deep-heating mode is pronounced over the Florida Current and the western Gulf Stream proper, characterized by latent heating in the middle and upper troposphere due to deep convection. Possible occurrences of these heating modes in other regions are discussed..
|19.||Haiming Xu, Hiroki Tokinaga, Shang Ping Xie, Atmospheric effects of the Kuroshio large meander during 2004-05, Journal of Climate, 10.1175/2010JCLI3267.1, 23, 17, 4704-4715, 2010.09, In the summer of 2004, the Kuroshio took a large meander path south of Japan for the first time since 1991, and this large meander event persisted until the next summer. Satellite observations and numerical model simulations are used to study the effect of this large meander event on the atmosphere. The large meander leaves a cool water pool between the Kuroshio and Japanese coast. Sea surface temperature (SST) in the cool water pool is about 2°-3°C colder than the surroundings during winter and spring, whereas the SST signature substantially weakens in summer. A local reduction of wind speed is found over the cool water pool, and the positive SST-wind speed correlation is indicative of ocean forcing of the atmosphere. Cloud liquid water (CLW) content and precipitation also decrease over the cool SST pool. A regional atmospheric model successfully simulates atmospheric response to the Kuroshio large meander. The model experiments suggest that the reduced surface wind speed and precipitation are due to the large meander-induced SST cooling. Analysis of the surface perturbation momentum budgets shows the importance of the pressure adjustment mechanism in surface wind response to the cold SST anomalies..|
|20.||Hiroki Tokinaga, Shang Ping Xie, Wave- and Anemometer-based sea surface wind (WASWind) for climate change analysis, Journal of Climate, 10.1175/2010JCLI3789.1, 24, 1, 267-285, 2011.01, Ship-based measurements of sea surface wind speed display a spurious upward trend due to increases in anemometer height. To correct this bias, the authors constructed a new sea surface wind dataset from ship observations of wind speed and wind wave height archived in the International Comprehensive Ocean- Atmosphere Data Set (ICOADS). The Wave- and Anemometer-based Sea surface Wind (WASWind) dataset is available for wind velocity and scalar speed at monthly resolution on a 4° × 4° longitude-latitude grid from 1950 to 2008. It substantially reduces the upward trend in wind speed through height correction for anemometer-measured winds, rejection of spurious Beaufort winds, and use of estimated winds from wind wave height. The reduced global upward trend is smallest among the existing global datasets of in situ observations and comparable with those of reanalysis products. Despite the significant reduction of globally averaged wind speed trend, WASWind features rich spatial structures in trend pattern, making it a valuable dataset for studies of climate changes on regional scales. Not only does the combination of ship winds and wind wave height successfully reproduce major modes of seasonal-to-decadal variability; its trend patterns are also physically consistent with sea level pressure (SLP) measurements. WASWind is in close agreement with wind changes in satellite measurements by the Special Sensor Microwave Imagers (SSM/Is) for the recent two decades. The agreement in trend pattern with such independent observations illustrates the utility of WASWind for climate trend analysis. An application to the South Asian summer monsoon is presented..|
|21.||Hiroki Tokinaga, Shang Ping Xie, Weakening of the equatorial Atlantic cold tongue over the past six decades, Nature Geoscience, 10.1038/ngeo1078, 4, 4, 222-226, 2011.04, Seasonal and interannual variations of the equatorial cold tongue are defining features of the tropical Atlantic Ocean, with significant climatic and biogeochemical effects. However, its long-term changes are poorly understood owing to biases in observations and climate models. Here we use a suite of bias-corrected observations, and find that cold-tongue variability has weakened during the past six decades. We find that sea surface temperature has increased across the basin, with a local enhancement over the eastern equatorial Atlantic. This warming pattern of the sea surface is most pronounced during boreal summer, reducing the annual cycle through a positive ocean-atmosphere feedback. Specifically, the eastward-intensified warming leads to enhanced atmospheric convection in the equatorial eastern Atlantic region, as well as to less vigorous trade winds. These in turn deepen the thermocline in the east, and reinforce the sea surface warming pattern. The flattened thermocline and reduced thermocline feedback weaken interannual variability of equatorial sea surface temperatures and Guinea coast precipitation associated with the Atlantic Niño. We suggest that the observed changes could be associated with cooling by anthropogenic aerosols, an effect that is stronger in the Northern than in the Southern Hemisphere. If the aerosol emissions decrease in the next decades, the tropical Atlantic may experience yet another shift as the greenhouse gas forcing increases..|
|22.||Youichi Tanimoto, Tomohisa Kanenari, Hiroki Tokinaga, Shang Ping Xie, Sea level pressure minimum along the Kuroshio and its extension, Journal of Climate, 10.1175/2011JCLI4062.1, 24, 16, 4419-4434, 2011.08, Atmospheric effects of sea surface temperature (SST) fronts along the Kuroshio and Kuroshio Extension (K-KE) are investigated by examining spatial characteristics of the climatological sea level pressure (SLP), surface winds and surface heat flux (Q) fields based on an in situ observation dataset. A hydrostatic effect of the SST front is observed during the northwesterly monsoon characterized by a westward-extending low-SLP wedge (trough) slightly south of the peak in Q along the K-KE. Ageostrophic surface westerlies crossing SLP isobars toward a trough center are found north of the low-SLP wedge apparently because of the eastward acceleration by the vertical mixing effect. This feature of the ageostrophic winds is less pronounced south of it, a north-south asymmetry arising as vertical mixing accelerates (decelerates) eastward ageostrophic winds north (south) of it. While the SLP trough near the SST front is found nearly year-round except for summer, its meridional location seasonally migrates probably due to the change in the mean surface flow. Regional atmospheric model experiments are conducted to examine the effect of the SST front on the overlying atmosphere. The observed features in winter are adequately simulated when high-resolution SST is prescribed as the boundary condition. The strong Q along the K-KE and resultant SLP trough and ageostrophic surface winds are absent when the SST boundary condition is smoothed in space to weaken the SST front. These results illustrate that the cross-frontal change inQis of great importance to leave the SST imprint on the overlying atmosphere via hydrostatic and vertical mixing adjustments..|
|23.||Xuhua Cheng, Shang Ping Xie, Hiroki Tokinaga, Yan Du, Interannual variability of high-wind occurrence over the North Atlantic, Journal of Climate, 10.1175/2011JCLI4147.1, 24, 24, 6515-6527, 2011.12, Interannual variability of high-wind occurrence over the North Atlantic is investigated based on observations from the satellite-borne Special Sensor Microwave Imager (SSM/I). Despite no wind direction being included, SSM/I data capture major features of high-wind frequency (HWF) quite well. Climatology maps show that HWF is highest in winter and is close to zero in summer. Remarkable interannual variability of HWF is found in the vicinity of the Gulf Stream, over open sea south of Iceland, and off Cape Farewell, Greenland. On interannual scales, HWF south of Iceland has a significant positive correlation with the North Atlantic Oscillation (NAO). An increase in the mean westerlies and storm-track intensity during a positive NAO event cause HWF to increase in this region. In the vicinity of the Gulf Stream, HWF is significantly correlated with the difference between sea surface temperature and surface air temperature (SST 2 SAT), indicative of the importance of atmospheric instability. Cross-frontal wind and an SST gradient are important for the instability of the marine atmospheric boundary layer on the warm flank of the SST front. Off Cape Farewell, highwind occurs in both westerly and easterly tip jets. Quick Scatterometer (QuikSCAT) data show that variability in westerly (easterly) HWF off Cape Farewell is positively (negatively) correlated with the NAO..|
|24.||Hiroki Tokinaga, Shang Ping Xie, Axel Timmermann, Shayne McGregor, Tomomichi Ogata, Hisayuki Kubota, Yuko M. Okumura, Regional patterns of tropical indo-pacific climate change
Evidence of the walker circulation weakening, Journal of Climate, 10.1175/JCLI-D-11-00263.1, 25, 5, 1689-1710, 2012.03, Regional patterns of tropical Indo-Pacific climate change are investigated over the last six decades based on a synthesis of in situ observations and ocean model simulations, with a focus on physical consistency among sea surface temperature (SST), cloud, sea level pressure (SLP), surface wind, and subsurface ocean temperature. A newly developed bias-corrected surface wind dataset displays westerly trends over the western tropical Pacific and easterly trends over the tropical Indian Ocean, indicative of a slowdown of the Walker circulation. This pattern of wind change is consistent with that of observed SLP change showing positive trends over the Maritime Continent and negative trends over the central equatorial Pacific. Suppressed moisture convergence over the Maritime Continent is largely due to surface wind changes, contributing to observed decreases in marine cloudiness and land precipitation there. Furthermore, observed ocean mixed layer temperatures indicate a reduction in zonal contrast in the tropical Indo-Pacific characterized by larger warming in the tropical eastern Pacific and western Indian Ocean than in the tropical western Pacific and eastern Indian Ocean. Similar changes are successfully simulated by an ocean general circulation model forced with the bias-corrected wind stress. Whereas results from major SST reconstructions show no significant change in zonal gradient in the tropical Indo-Pacific, both bucket-sampled SSTs and nighttime marine air temperatures (NMAT) show a weakening of the zonal gradient consistent with the subsurface temperature changes. All these findings fromindependent observations provide robust evidence for ocean-atmosphere coupling associatedwith the reduction in theWalker circulation over the last six decades..
|25.||J. S. Chowdary, Shang Ping Xie, Hiroki Tokinaga, Yuko M. Okumura, Hisayuki Kubota, Nat Johnson, Xiao Tong Zheng, Interdecadal variations in ENSO teleconnection to the Indo-Western Pacific for 1870-2007, Journal of Climate, 10.1175/JCLI-D-11-00070.1, 25, 5, 1722-1744, 2012.03, Slow modulation of interannual variability and its relationship to El Niño-Southern Oscillation (ENSO) is investigated for the period of 1870-2007 using shipboard surface meteorological observations along a frequently traveled track across the north Indian Ocean (NIO; from the Gulf of Aden through Malacca Strait) and the South China Sea (to Luzon Strait). During the decades in the late nineteenth-early twentieth century and in the late twentieth century, the El Niño-induced NIO warming persists longer than during the 1910s-mid-1970s, well into the summer following the peak of El Niño. During the epochs of the prolonged NIO warming, rainfall drops and sea level pressure rises over the tropical northwest Pacific in summer following El Niño. Conversely, during the period when the NIO warming dissipates earlier, these atmospheric anomalies are not well developed. This supports the Indian Ocean capacitor concept as a mechanism prolonging El Niño influence into summer through the persistent Indian Ocean warming after El Niño itself has dissipated. The above centennial modulation of ENSO teleconnection to the Indo-northwest Pacific region is reproduced in an atmospheric general circulation model forced by observed SST. The modulation is correlated not with the Pacific decadal oscillation but rather with the ENSO variance itself. When ENSO is strong, its effect in the Indo-northwest Pacific strengthens and vice versa. The fact that enhanced ENSO teleconnections occurred 100 years ago during the late nineteenth-early twentieth century indicates that the recent strengthening of the ENSO correlation over the Indo-western Pacific may not entirely be due to global warming but reflect natural variability..|
|26.||Hiroki Tokinaga, Shang Ping Xie, Clara Deser, Yu Kosaka, Yuko M. Okumura, Slowdown of the Walker circulation driven by tropical Indo-Pacific warming, Nature, 10.1038/nature11576, 491, 7424, 439-443, 2012.11, Global mean sea surface temperature (SST) has risen steadily over the past century, but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation. Observations suggest a slowdown of the zonal atmospheric overturning circulation above the tropical Pacific Ocean (the Walker circulation) over the twentieth century. Although this change has been attributed to a muted hydrological cycle forced by global warming, the effect of SST warming patterns has not been explored and quantified. Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950-2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations. Model experiments with this trend pattern robustly simulate the observed changes, including the Walker circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker circulation is presumably driven by oceanic rather than atmospheric processes..|
|27.||Hiroyuki Tomita, Shang Ping Xie, Hiroki Tokinaga, Yoshimi Kawai, Cloud response to the meandering Kuroshio Extension front, Journal of Climate, 10.1175/JCLI-D-13-00133.1, 26, 23, 9393-9398, 2013.12, A unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere..|
|28.||Yali Yang, Shang Ping Xie, Yan Du, Hiroki Tokinaga, Interdecadal difference of interannual variability characteristics of South China Sea SSTs associated with ENSO, Journal of Climate, 10.1175/JCLI-D-15-0057.1, 28, 18, 7145-7160, 2015.09, The correlation between sea surface temperature (SST) and El Niño-Southern Oscillation (ENSO) persists into post-ENSO September over the South China Sea (SCS), the longest correlation in the World Ocean. Slow modulations of this correlation are analyzed by using the International Comprehensive Ocean-Atmosphere Dataset (ICOADS). ENSO's influence on SCS SST has experienced significant interdecadal changes over the past 138 years (1870-2007), with a double-peak structure correlation after the 1960s compared to a single-peak before the 1940s. According to the ENSO correlation character, the analysis period is divided into four epochs. In epoch 3, 1960-83, the SST warming and enhanced precipitation over the southeastern tropical Indian Ocean, rather than the Indian Ocean basinwide warming, induce easterly wind anomalies and warm up the SCS in the summer following El Niño. Besides the Indian Ocean effect, during epochs 2 (1930-40) and 4 (1984-2007), the Pacific-Japan (PJ) pattern of atmospheric circulation anomalies helps sustain the SCS SST warming through summer (June-August) with easterly wind anomalies. The associated increase in shortwave radiation and decrease in upward latent heat flux cause the SCS SST warming to persist into the summer. Meanwhile, the rainfall response around the SCS to ENSO shows interdecadal variability, with stronger variability after the 1980s. The results suggest that both the remote forcing from the tropical Indian Ocean and the PJ pattern are important for the ENSO teleconnection to the SCS and its interdecadal modulations..|
|29.||Megumi O. Chikamoto, Axel Timmermann, Yoshimitsu Chikamoto, Hiroki Tokinaga, Naomi Harada, Mechanisms and predictability of multiyear ecosystem variability in the North Pacific, Global Biogeochemical Cycles, 10.1002/2015GB005096, 29, 11, 2001-2019, 2015.11, Aleutian Low variations provide vorticity, buoyancy, and heat-flux forcing to the North Pacific Ocean, which in turn cause changes in ocean circulation, mixed layer characteristics and sea ice coverage. In this process the white noise atmospheric characteristics are integrated dynamically and thermodynamically to generate red noise ocean spectra. Using the Community Earth System Model (version 1.0.3) we study the resulting biogeochemical and ecosystem responses in the North Pacific. We find that ocean dynamical variables have an impact on the tendencies of key nutrients and biological production, which leads to a further reddening of biogeochemical spectra resulting in potential predictability on time scales of 2-4 years. However, this low-pass filtering does not apply to all biogeochemical variables and is regionally dependent. It is shown that phytoplankton biomass in the Central North Pacific adjusts to the much shorter-term variability associated with changes in mixed layer depth, light availability, and zooplankton grazing, thus limiting the predictability of phytoplankton anomalies to about 1 year. In the eastern North Pacific the slow advection of anomalous nutrient concentrations leads to longer persistence of phytoplankton variability and increased potential predictability of up to 3 years..|
|30.||Hai Wang, Shang Ping Xie, Hiroki Tokinaga, Qinyu Liu, Yu Kosaka, Detecting cross-equatorial wind change as a fingerprint of climate response to anthropogenic aerosol forcing, Geophysical Research Letters, 10.1002/2016GL068521, 43, 7, 3444-3450, 2016.04, Anthropogenic aerosols are a major driver of the twetieth century climate change. In climate models, the aerosol forcing, larger in the Northern than Southern Hemispheres, induces an interhemispheric Hadley circulation. In support of the model result, we detected a robust change in the zonal mean cross-equatorial wind over the past 60 years from ship observations and reanalyses, accompanied by physically consistent changes in atmospheric pressure and marine cloud cover. Single-forcing experiments indicate that the observed change in cross-equatorial wind is a fingerprint of aerosol forcing. This zonal mean mode follows the evolution of global aerosol forcing that is distinct from regional changes in the Atlantic sector. Atmospheric simulations successfully reproduce this interhemispheric mode, indicating the importance of sea surface temperature mediation in response to anthropogenic aerosol forcing. As societies awaken to reduce aerosol emissions, a phase reversal of this interhemispheric mode is expected in the 21st century..|
|31.||Hiroki Tokinaga, Shang Ping Xie, Hitoshi Mukougawa, Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability, Proceedings of the National Academy of Sciences of the United States of America, 10.1073/pnas.1615880114, 114, 24, 6227-6232, 2017.06, With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20thcentury Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-Atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-To-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region..|
|32.||Shang Ping Xie, Qihua Peng, Youichi Kamae, Xiao Tong Zheng, Hiroki Tokinaga, Dongxiao Wang, Eastern pacific ITCZ dipole and ENSO diversity, Journal of Climate, 10.1175/JCLI-D-17-0905.1, 31, 11, 4449-4462, 2018.06, The eastern tropical Pacific features strong climatic asymmetry across the equator, with the intertropical convergence zone (ITCZ) displaced north of the equator most of time. In February-April (FMA), the seasonal warming in the Southern Hemisphere and cooling in the Northern Hemisphere weaken the climatic asymmetry, and a double ITCZ appears with a zonal rainband on either side of the equator. Results from an analysis of precipitation variability reveal that the relative strength between the northern and southern ITCZ varies from one year to another and this meridional seesaw results from ocean-atmosphere coupling. Surprisingly this meridional seesaw is triggered by an El Niño-Southern Oscillation (ENSO) of moderate amplitudes. Although ENSO is originally symmetric about the equator, the asymmetry in the mean climate in the preceding season introduces asymmetric perturbations, which are then preferentially amplified by coupled ocean-atmosphere feedback in FMA when deep convection is sensitive to small changes in cross-equatorial gradient of sea surface temperature. This study shows that moderate ENSO follows a distinct decay trajectory in FMA and southeasterly cross-equatorial wind anomalies cause moderate El Niño to dissipate rapidly as southeasterly cross-equatorial wind anomalies intensify ocean upwelling south of the equator. In contrast, extreme El Niño remains strong through FMA as enhanced deep convection causes westerly wind anomalies to intrude and suppress ocean upwelling in the eastern equatorial Pacific..|
|33.||Hiroki Tokinaga, Ingo Richter, Yu Kosaka, ENSO influence on the Atlantic Niño, revisited: Multi-year versus single-year ENSO events, Journal of Climate, 10.1175/JCLI-D-18-0683.1, 32, 14, 4585-4600, 2019.06, The influence of El Niño-Southern Oscillation (ENSO) on the Atlantic Niño over the past 113 years is investigated by comparing multi-year and single-year ENSO events. Multi-year ENSO events sustain an anomalous zonal gradient of sea surface temperature (SST) in the equatorial western to central Pacific even during boreal spring and summer. This SST gradient is coupled with an anomalous Walker circulation and atmospheric deep convection through the Bjerknes feedback. During multi-year La Niñas, for example, a strengthened Pacific Walker circulation extends into the tropical Atlantic in boreal spring, a season when both the Pacific and Atlantic intertropical convergence zones become more symmetric about the equator. As a result, surface westerly wind anomalies appear over the equatorial Atlantic, triggering an Atlantic Niño. By contrast, such a teleconnection is not found in the spring following the peak of single-year ENSO events. A Pacific pacemaker model experiment reproduces the observed atmospheric response and its impact on the Atlantic Niño, further supporting the importance of prolonged ENSO forcing. The contrasting influence of multi-year and single-year events explains the fragile relationship between ENSO and the Atlantic Niño. An empirical orthogonal function (EOF) analysis shows that the leading EOF mode (EOF-1) for the spring tropical western to central Pacific SST anomalies captures the characteristics of multi-year ENSO events. EOF-1 is highly correlated with the summer Atlantic Niño over the past 113 years while the Niño-3 SST is not. These correlations indicate that ocean-atmosphere coupling in the equatorial western to central Pacific plays a major role in shaping ENSO teleconnections in boreal spring..|