Major Writings
- Journal of Meteorological Society of Japan, 2009, Vol.87, pp807-819
Characteristics of the CMIP3 Models Simulating Realistic Response of Tropical Western Pacific Precipitation to Nino3 SST Variability (Tomoaki OSE and Osamu ARAKAWA)
The 20th century simulations from the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset are examined statistically using numerical skills to capture the characteristics of the models which realistically simulate responses of the tropical western Pacific (TWP) precipitation to sea surface temperature (SST) variability over the Niño3 region [150°-90°W, 5°S-5°N].
The simultaneous correlation of precipitation anomaly over the TWP region of [90°-170°E, 20°S-20°N] with Niño3 SST variability is successfully reproduced with relatively high skills for June to August (JJA) as compared with those for December to February (DJF) and March to May. The high skill models have common characteristics of realistically simulating the observed largest precipitation response to Niño3 SST variability over the equatorial central Pacific east of the dateline. Furthermore, the realistic simulation of the climatological mean equatorial precipitation west of the dateline seems to be responsible for the realistic response of the TWP precipitation to Niño3 SST variability.
Characteristics of High-Skill Models in DJF （Precipitation Variability and Climate）
A few of the models successfully simulate the delayed response of the JJA precipitation over the TWP region of [90°-170°E, 10°S-30°N] to the preceding DJF Niño3 SST variability with high skills. Those models reproduce the statistically observed features of subtropical northwestern Pacific SST anomaly, precipitation and SST anomalies over the Indian Ocean in JJA following DJF Niño3 SST variability. Another distinctive characteristic of those models is to reproduce almost null correlation of the equatorial central Pacific SST anomaly in JJA with the preceding DJF season Niño3 SST variability.
<< To Simulate a Realistic Response of Tropical Western Pacific Precipitation to Nino3 SST Anomaly >>
- Journal of Meteorological Society of Japan, 2000, Vol.78, pp93-99
A Biennially Oscillating Sea Surface Temperature and the Western Pacific Pattern (Tomoaki OSE)
It is found that the appearance of the western Pacific (WP) pattern and the Pacific/North American (PNA) pattern in the northern winter is statistically related to a biennially oscillating sea surface temperature (SST) in the South China Sea (SCS) and associated precipitation variability over the tropical western Pacific in the following way;
(1) When precipitation is strongly suppressed (enhanced) east of the Philippines at the warm (cold) phase of NINO4 SST, the WP (the inverse WP) pattern tends to appear. This is qualitatively consistent with a local Hadley circulation theory.
(2) When negative (positive) precipitation anomalies are small east of the Philippines or when those are shifted westward at the warm (cold) phase of NINO4 SST, the PNA (the inverse PNA) pattern tends to appear rather than the WP (the inverse WP) pattern.
Figure Caption : (a) 500hPa height anomaly composite for inactive precipitation case to east of the Phillipines during Northern Winter's El Nino. (b) The same as (a) except for the other cases during Northern Winter's El Nino. Colors are for 95% significance.
(3) The above variability of the precipitation in the tropical western Pacific corresponds to that responsible for the biennial oscillation in the SCS SST and the neighboring SST.
<< Two types of Northern Winter Atmospheric Response to El Nino >>
- Journal of Meteorological Society of Japan, 1998, Vol.76, pp1045-1063
Seasonal Change of Asian Summer Monsoon Circulation and Its Heat Source (Tomoaki OSE)
The Asian summer monsoon circulation, especially its climatological seasonal change, was studied as the model response to the prescribed zonal mean field and the prescribed diabatic heat source from the observation. The obtained results are summarized as follows.
(1) During the Asian summer monsoon season, the prescribed deep heat sources in the southern part of Asia form the Tibetan High, the monsoon trough, the low-level circulation over South Asia, and furthermore, the downward motion in the western part of the Eurasian Continent. The heat sources near the surface over central Asia also induce downward motions aloft.
(2) In early summer (June), the deep heat sources in the southern part of Asia tend to form southwesterly low-level flows and upward motion southeast of Japan. Those are considered to be the background for the Baiu formation in East Asia as well as heat lows produced in the southern part of Asia. The mid-latitude heat sources associated with the Baiu precipitation produce a low-level jet south of that.
(3) Climatological seasonal change from early summer (June) to mid-summer (July) is characterized by an air temperature increase in the whole Northern Hemisphere and a northward shift of a weakened westerly jet. When in the model a zonal mean field in June is replaced by that in July, the major characteristics of the seasonal change are obtained qualitatively; low-level jets and upward motion areas in South Asia and East Asia shift from the ocean side of the coasts toward the land side. This change of vertical motion is consistent with the seasonal change of deep heat sources from June to July.
(4) The climatological seasonal change from mid-summer (July) to late summer (August) is characterized by enhanced convective activity in the extended area of the subtropical western Pacific. When deep heat sources in July are replaced by those in August over the western Pacific only, the major characteristics of the seasonal change over the Pacific and the Indian Ocean are obtained. The expansion of the Tibetan High at the upper-level and the Pacific High at low-level over Japan is also simulated by the seasonal change of the western Pacific heat sources only.
(5) The model simulation with the combination of the diabatic heat source for August and the zonal mean field for June is compared with the climatological August simulation. It is indicated that the zonal mean field delayed from its seasonal migration could be related to weak monsoon circulation and the associated precipitation anomalies in the mid-latitudes and the subtropics.
<< Seasonal March of Northern Summer Asia Monsoon in view of Atmospheric Response to Heating Source >>
- Journal of Meteorological Society of Japan, 1997, Vol.75, pp1091-1107
Sea Surface Temperature in the South China Sea -an Index for the Asian Monsoon and ENSO System- (Tomoaki OSE, Yukuan SONG and Akio KITOH)
Interannual variability of the sea surface temperature anomalies (SSTA) over the South China Sea (SCS) is recognized as an index for the Asian monsoon and ENSO system because of its special geographical location for that system. The following results are obtained by the statistical analysis of the observational data.
(1) In the northern winter, the SCS SSTA are quite sensitive to the longitudinal shift of global wind anomalies associated with the equatorial Pacific SSTA. This fact is related to that the SCS SSTA and the neighbor SSTA have strong biennial oscillation.
(2) When the global wind anomalies are shifted eastward in the winter (BO-type years), the tropical eastern Pacific SSTA tend to change in the following spring. On the other hand, when those wind anomalies are shifted westward (LF-type years), the eastern Pacific SSTA tend to be maintained through the year. The associated differences between the BO and LF-type years are found in the seasonal change of the low-level tropical wind anomalies from the preceding summer through winter.
Figure Caption : (a) Precipitation anomaly composite for inactive precipitation cases to east of the Phillipines during Northern Winter's El Nino. (b) Except for the other cases during Northern Winter's El Nino. Colors are for 95% significance.
(3) The northern summer SCS SSTA seem to be controlled by in-situ low-level wind anomalies. Furthermore, easterly anomalies over South Asia and the tropical western Pacific and westerly anomalies over East Asia are found in the lower atmosphere for the positive SCS SSTA. It is also shown that the summer SCS SSTA have a statistical relationship with the equatorial central Pacific SSTA in the preceding winter. This fact suggests a relationship between the summer Asian monsoon and the winter phase of ENSO.
<< Two-type Responses of Tropical Western Pacific Precipitation to Northern Winter's El Nino and the following Summer >>
- Journal of Meteorological Society of Japan, 1996, Vol.74, pp845-866
The Comparison of the Simulated Response to the Regional Snow Mass Anomalies over Tibet, Eastern Europe and Siberia (Tomoaki OSE)
The atmospheric response to the regional snow mass anomalies in early spring over Tibet, Eastern Europe, and Siberia is compared by general circulation model ensemble experiments. The positive snow mass anomalies over Tibet produce the largest cooling anomalies in the atmosphere from spring to early summer. Almost no significant forcing anomalies are systematically formed by the snow mass anomalies over Eastern Europe and Siberia.
The model experiments show that the cooling source over Tibet works significantly to delay the seasonal transition from spring to summer in the Northern Hemisphere. It is confirmed by the simulated weak Asian monsoon; a weak lower tropospheric monsoon jet in South Asia, a weak large-scale divergence center at the upper troposphere in Southeast Asia, negative lower-layer geopotential height anomalies in the North Pacific and the North Atlantic, and the weak Walker circulation in the equatorial Pacific are simulated. The above noticeable response of the atmosphere to the snow mass anomalies over Tibet is associated with the following characteristics of Tibet as compared with Eastern Europe and Siberia;
(1) The small snow melt speed over Tibet, probably due to its high elevation, maintains the early spring additional snow mass until almost the end of the climatic snow-melt season.
(2) The snow albedo is effective because of large solar incidence and relatively small cloudiness over Tibet.
(3) The existence of the anomalous snow cover works to cut the upward sensible heat flux rather than the evaporation over Tibet because of its dry ground condition.
(4) The dry ground over Tibet makes it possible that all the anomalous snow-melt water is not drained as runoff, but a part of it is stored in the ground.
(5) As pointed out by many researchers, the Tibetan Plateau plays an important rôle in the establishment of the Asian monsoon.
The above conditions (1)-(5) for the effective snow mass anomalies are found in the model Tibet, but these conditions would be satisfied by the real regions where the snow mass anomalies affect the atmosphere efficiently. The extended atmospheric responses are found in May for the East European case and in August for the Siberian case. These are accompanied by significant ground condition anomalies in northern Eurasia. Those ground condition anomalies are not directly related to the successive ground condition anomalies followed by the anomalous snow mass melt, and seem to be created after the water and heat of the initial snow mass anomalies are supplied into the atmosphere.
<< Necessary conditions for Eurasian Spring Snowmass Anomaly to be Influential in Seasonal Evolution >>
Major Presentations
- WCRP Workshop on Seasonal Prediction, Barcelona Spain June 4-7, 2007
[Page for Seasonal Forecast Research Laboratory of Climate Research Division]