Naoki Sato

Dynamical processes related to the appearance of quasi-stationary waves on the subtropical jet in the midsummer northern hemisphere

Statistical features of quasi-stationary planetary waves were examined on the subtropical jet in the midsummer Northern Hemisphere by using objectively analyzed data and satellite data. As a result, a quasi-stationary wave train that is highly correlated with the midsummer climate over Japan was identified. A clear phase dependency of the appearance of waves was also confirmed. An analysis of temporal evolution and wave activity flux revealed that the eastward propagation of the wave packet starts in the Middle East, passes over East Asia, and reaches North America. The anomaly pattern is strengthened through kinetic energy conversion near the entrance of the Asian jet over the Middle East. The interaction between the anomaly pattern and the basic field contributes to the appearance of the anomalous wavelike pattern. Although the wave train is correlated with the anomaly of convective activity over the western North Pacific and the Indian Ocean, it is implied that internal dynamics are important in determining the statistical features of the appearance of anomalous quasi-stationary waves on the subtropical jet.

Dynamical Processes Related to the Appearance of the Okhotsk High during Early Midsummer

Abstract The authors identified an upper-level pressure anomaly pattern corresponding to the interannual variability of the Okhotsk high in midsummer (late July and early August) as a predominant anomaly pattern in the Northern Hemisphere, by using objectively analyzed data. According to the results of empirical orthogonal function (EOF) analyses and composite analyses, a positive pressure anomaly appeared near the tropopause over eastern Siberia in years with strong Okhotsk highs. Examination of the heat budget in the lower troposphere revealed that a negative surface temperature anomaly observed in northern Japan was brought by the advection of the climatological temperature gradient from the anomalous wind associated with the upper-level anticyclonic anomaly. It was also demonstrated that the anomaly field over Siberia does not accompany predominant vorticity forcing or Rossby wave propagation from the west with a specific phase. However, positive kinetic energy conversion from the climatological basic...

Mechanism of the northward propagation of mesoscale convective systems observed on 15 June 2005 during PALAU2005

[1]xa0This paper describes the structure and evolution of northward propagating mesoscale convective systems (MCSs) observed over the tropical western Pacific on 15 June 2005. A wedge-shaped convective cloud area, consisting of three groups of MCSs, was generated near the equator to the north of New Guinea at around 0000 LST on 15 June, and one of the groups of MCSs propagated toward Palau (7°–8°N). Dropsonde analysis revealed that the MCSs developed at the northern edge of a cold air mass that had a horizontal scale of more than 400 km. Global objective analysis data from the Japan Meteorological Agency showed that the cold air mass observed by the dropsonde analysis originated in a land breeze from New Guinea. The cold air mass was initially formed by cold advection from the south and was maintained by cooling with the MCSs. When the initial MCSs were generated along the land-breeze front, an easterly wave, which was accompanied by low-level southerly wind and a low-pressure area, was located near Palau. The cold air mass associated with the land breeze between 132° and 137°E was locally extended toward the center of the low-pressure area. The results suggest that the easterly wave located around Palau helped locally promote northward cold advection from New Guinea, inducing the long-distance northward propagation of MCSs generated along the land-breeze front.

An air mass with high potential vorticity preceding the formation of the Marcus Convergence Zone

[1]xa0We examined the convective activity and dynamical field over the subtropical North Pacific by focusing on the relationship between upper cold lows (UCLs) and the formation of the Marcus Convergence Zone (MCZ). In mid-July, we detected a UCL near Marcus Island that was associated with the formation of the MCZ. The convective activity associated with the MCZ followed the UCL, implying that the UCL contributed to the MCZ formation. Analysis of the temporal evolution of the dynamic field revealed that the UCL appeared in a large-scale, high-potential vorticity (PV) air mass migrating from the east. We had already identified the high-PV region near the mid-Pacific trough in late July. It stretched westward and reached Marcus Island in mid-July, suggesting that the UCL in the high-PV air mass over the North Pacific contributed to the formation of the MCZ.

An ITCZ-like convergence zone over the Indian Ocean in boreal late autumn

[1]xa0We examined the convective activity over the tropical Indian Ocean in boreal autumn using satellite-observation data. In November and December, we detected an intertropical convergence zone-like (ITCZ-like) precipitation zone between Maldives and Somalia that is unobserved in the other periods of the year. It accompanied a high sea-surface temperature (SST) north of the equator, and a cold tongue near the equator, similar to the ITCZs over the Pacific and Atlantic Oceans. However, the formation mechanism of the SST anomalies over the Indian Ocean is not the same as that in the Pacific and Atlantic Oceans. The high-SST region forms in early November corresponding to weak sea-surface wind during the monsoon transition. The low SST along the equator appears, associated with the upwelling of the subsurface water.

Influence of mechanical mixing on a low summertime SST in the western North Pacific ITCZ region

[1]xa0A 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.

Influences of intraseasonal disturbances on the oceanic mixed layer in the western North Pacific ITCZ region

[1]xa0Using the Argo float data, changes in the temperature and salinity in the oceanic mixed layer corresponding to the passage of intraseasonal-scale disturbances were examined over the western North Pacific (WNP) intertropical convergence zone (ITCZ) region. Events where the near-surface temperature decreased by more than 1°C within a 10-day period were identified, corresponding to the intraseasonal variability (ISV) of the atmosphere. Analyzing the vertical profiles and the T-S relationships of the mixed layer revealed that the water was cooled mainly by mechanical mixing and Ekman upwelling related to the strong sea surface wind. Thermal cooling at the sea surface does not appear to be a predominant process in the abrupt cooling. Such cooling events were observed only when the mixed layer was shallow.

Meridional SST gradient in the western North Pacific warm pool associated with typhoon generation

[1]xa0We examined the sea surface temperature (SST) distribution in the western North Pacific warm pool when a typhoon is generated in northern summer. Analyzing satellite observation data revealed higher (lower) SST north (south) of the typhoon. As a result, the meridional SST gradient became especially steep, and the SST was not homogeneously high, when a typhoon is generated. In-situ observation data in the oceanic surface layer suggested that the low SST was caused by vertical mixing related to the strong southwesterly monsoon. In contrast, the hot spots north of the typhoon were associated with weak sea surface wind and vertical mixing. According to satellite observation, the maxima of surface heat flux and water vapor shifted northward corresponding to the high SST. They may produce an asymmetry of the thermodynamic structure of a typhoon.