The summer monsoon, sometimes brings severe droughts and intense flooding in many parts of the world including South Asian countries like Pakistan. Human life and economy in Pakistan considerably depends on the summer monsoon activities. So, an essential question arises "how can we better manage the water resources (i.e. drought and flood management) of the region for the societal needs and benefits". Therefore, to address the concern as a hydrologist, we need to develop a basis of the scientific understanding of the different contrast of the climatology during extremely dry and heavy rainfall events over Pakistan region. However, compared to other regional studies i.e. Indian Summer Monsoon (ISM) and South-East Asian Monsoon (SEAM), the basis of the thermodynamical structure and the processes associated with upper tropospheric conditions during the climatological mean Pakistan Summer Monsoon (PSM) and its extreme events have not been addressed deeply yet and need to be investigated. Thus, comprehensive understanding of the thermodynamical structure and the associated processes during climatological mean PSM and its extreme events is immensely vital for the hydrologist as a first step to develop the basis of scientific understanding. By data analysis, an attempt has been made to accomplish this objective in this study.

Firstly, the climatological tropospheric conditions and the associated processes from pre-monsoon phase to the PSM mature phase are investigated, with a special focus on the vorticity and heat budgets on 10-day mean time scale. During pre-monsoon phase (last dekad of June), the stronger upper level convergence around Pakistan accompanies with the upper level vortex stretching, mid-upper level subsidence, radiative cooling and adiabatic warming, which suppresses the convective activities. During the PSM onset and mature phases (early and mid-late July), the basic structure of atmosphere significantly differs from the pre-monsoon phase. However, the PSM mature phase (second and last dekads of July) have following salient features compared to the pre-monsoon phase: (1) the mid-upper tropospheric warming, which causes the reversal of meridional temperature gradient (MTG), gets strengthened and expands; (2) the Asian Jet (AJ) together with the AJ ridge located around Caspian Sea (CS), slightly shifts northwestward; (3) the upper level convergence around Pakistan observed in pre-monsoon, shifts and reallocates to the northwest of Pakistan during mature phase; (4) the low level trough around Pakistan deepens and becomes stronger enough for the maturity of PSM during second dekad of July; and (5) the moisture flux associated with the low level deflected southeasterly and southwesterly monsoon winds respectively from the Bay of Bengal (BoB) and Arabian Sea (AS) increases and intrudes over Pakistan. Therefore during the PSM mature phase, the atmospheric structure (i.e. the upper level vortex stretching and the associated upper level convergence, mid-upper level subsidence and adiabatic warming) slightly moves northwestward of Pakistan which represents the weakening of the upper level convergence around Pakistan observed in pre-monsoon phase. In addition, there is associated rapid decrease (increase) of upper (mid) level adiabatic warming (diabatic heating) around Pakistan. Further in the lower troposphere, the monsoon trough around Pakistan deepens together with the deflected southeasterly and southwesterly moisture flux respectively from BoB and AS increases and intrudes over Pakistan.

Secondly, comparing the PSM mature phase climatological mean characteristics, the characteristics of the extremely dry (wet) PSM events are investigated by focusing their common anomalous tropospheric conditions observed during the period 1979-2008. The extremely dry (wet) PSM events are found to be closely related with the persistence of: (1) the anomalous mid-upper tropospheric cooling (warming) around Pakistan and to its north/northwest, which represents weakening (strengthening) of the reversal of MTG; (2) the climatological mean AJ ridge located around CS contracts (expands) out longitudinally and also shifts southward (northward) together with the AJ, which is associated with the anomalous upper tropospheric cyclonic (anticyclonic) circulation around northwest of Pakistan; (3) the anomalous upper level convergence (divergence) located around Pakistan is associated with the deceleration (acceleration) of air mass over the AJ north of Pakistan; (4) the PSM low level trough anomalously weakens (strengthens) the climatological mean PSM trough to suppress (enhance) the PSM activities; and (5) the anomalous northwesterly (southeasterly) low level monsoon winds moving along the southern slope of the Tibetan Plateau (TP) towards BoB (Pakistan), which anomalously decreases (increases) the climatological mean deflected southeasterly and southwesterly moisture flux respectively from the BoB and AS to intrude over Pakistan. The associated processes of the PSM dry (wet) events are investigated and the author found that associated with the anomalous cyclonic (anticyclonic) circulation is the shifting of AJ southward (northward) together with the CS ridge contraction (expansion) longitudinally. And associated with the anomalous cyclonic (anticyclonic) circulation, is the anomalous upper level negative (positive) advection of absolute vorticity i.e. NAVA (PAVA) around Pakistan. In addition to NAVA (PAVA), negative (positive) vertical advection of relative vorticity i.e. NVVA (PVVA) is balanced by vortex stretching (shrinking). Further, the upper level anomalous vortex stretching (shrinking) has to accompany with the upper level anomalous convergence (divergence) and the associated anomalous subsidence (updraft) below to suppress (enhance) PSM activities. The anomalous upper level convergence (divergence) is associated with the anomalous extratropical (subtropical) northerly (southerly) ageostrophic wind related with the deceleration (acceleration) of air mass over the AJ north of Pakistan. In addition, the mid tropospheric anomalous diabatic cooling (heating) and the cold (warm) horizontal advection balanced with the anomalous warm (cold) vertical advection. The wind glides (does not glide) down the sloping isentropes to partly enhance subsidence (updraft), which is associated with the anomalous diabatic cooling (heating) over Pakistan. Further, the mid-upper tropospheric cold (warm) temperature anomaly around Pakistan and to its north represents anomalous weakening (strengthening) of the positive reversal of the MTG, which is consistent with the anomalous lower-upper level westerly (easterly) around Pakistan. Associated with the anomalous mid-upper level downdraft (updraft) is the suppressed (enhanced) PSM convective activity through anomalous weakening (deepening) of the low level trough and the anomalous decrease (increase) of the deflected southeasterly and southwesterly moisture flux from the BoB and AS to intrude over Pakistan. So, all theses processes occur simultaneously.

However, hydrologists need to improve their scientific understanding of the triggering mechanism for this kind of the knowledge for the improvement of Integrated Water Resources Management (IWRM). If hydrologists get a signal in advance, IWRM will be much more effective. However, for the anomalous states following precursors are found:

1.Circulation anomaly northwest of Pakistan triggers the PSM convection anomaly.

2.Circulation anomaly northwest of Pakistan might be associated with some upstream event around midlatitude or the anomalous state of Atlantic Ocean Oscillation (NAO) through the midlatitude wave train (i.e. pressure pattern anomaly).

3.Anomalous state of El-Nino (La-Nina) might bring PSM suppressed (enhanced) convection anomaly.

4.Northward propagating convective perturbation triggers the PSM convection anomaly. However further research is needed to clarify, what triggers the convective perturbation around equator and how this convective perturbation propagate northward around Pakistan region?

Therefore, the above mentioned Sr.1-4 researches on predictability, especially, should be strengthened. Further, the persistence of the cyclonic (anticyclonic) circulation northwest of Pakistan are presented for further study i.e. (1) the persistence of extratropical upper tropospheric alternate pressure pattern anomaly (negative-positive and positive-negative) contrast; and (2) the suppressed (enhanced) monsoon activities around the tropical monsoon region i.e. AS and western India. Therefore, it is speculated that the extremely dry (wet) events over the PSM region might be triggered by the disturbances from the tropics and/or extratropics which act at the same time or act individually, which needs further investigations to reach a definite conclusion.

各地域の異常寡雨と異常多雨のそれぞれのパターンの特徴、持続性、メカニズムを理解することは、統合的水資源管理を実現するための水文学的検討の第一段階として重要な課題である。本論文は、アジアモンスーン地域の北西縁に位置するパキスタンでの水資源管理に大きな影響を与える異常寡雨、異常多雨の発生パターンの理解を目的として、1979年~2008年に発生したそれぞれ4回のイベントを対象として、夏季モンスーンの開始直前から最盛期(6月下旬~7月下旬)の大気循環場の気候学的な特徴およびその熱力学的な構造の解明を目的としている。

本論文では、対流圏上層の再現性が大幅に改善された長期再解析データ(JRA25)から得られる10日平均の気候値を用いて、渦度収支解析、および熱エネルギー収支解析により、まず、パキスタンにおける夏季モンスーンの開始直前から最盛期に至る大気循環場の季節進行の気候学的特性を次の通り明らかにしている。

(1)モンスーン開始直前には、パキスタン上空の対流圏では、上層に強い収束が生じ、高気圧性渦度が強化され、対流圏中層~上層に下降流が生じ、それに伴う断熱加熱と放射冷却により、対流活動が抑制される。

(2)モンスーンの開始から最盛期には、対流圏中層~上層の加熱が進み、緯度方向の温位勾配の逆転が強化され、亜熱帯ジェットの軸がパキスタン北部からカスピ海上空へと北西に移動し、対流圏上層の収束域も北西に移動する。

(3)この変化とともに、対流圏下層にトラフが形成、強化され、湿潤な南東、南西風がベンガル湾とアラビア海からパキスタン域に蛇行しながら流入し、モンスーンの最盛期を迎える。

次に、上記の気候的な平均場と比較して、夏季モンスーンの開始直前から最盛期の大気のエネルギー収支の季節進行が、異常寡雨、異常多雨のそれぞれ4回のイベント時では、共通して対比的に異なること見出し、そのコントラストを明瞭に示している。さらに、モンスーンの最盛期として7月を対象として、気候値と比較して、異常寡雨(異常多雨)時の4年の共通した大気構造を次の通り明らかにしている。

(1)パキスタンから北西部にかけての対流圏中層~上層の温位が気候値に比べて低下(上昇)し、緯度方向の温位勾配の逆転が弱化(強化)する。

(2)カスピ海上空の亜熱帯ジェットの峰が気候値に比べ弱化(強化)し、対流圏上層に現れる低圧性(高圧性)の偏差とともに、南方(北方)に 移動する。

(3)パキスタン対流圏上層の収束(発散)が気候値に比べ強くなり、同時にパキスタン北部の亜熱帯ジェットが弱まる(強まる)。

(4)パキスタン付近の対流圏下層のトラフが気候値に比べ弱まり(強まり)、モンスーン活動は抑制(強化)される。

(5)チベット高原からベンガル湾に向かう北西風(南東風)に偏差が現れ、ベンガル湾やアラビア海からパキスタンへの水蒸気の移流が減少(増加)する。

本論文ではさらに、これらの7月の異常寡雨、異常多雨時に、大西洋からパキスタンにわたる対流圏上層に、各4年間で類似の気圧偏差の波列パターンが持続的に現れること、パキスタン上空での外向き長波放射の変動に対して、10日遅れで対流圏上層に低圧性の偏差が現れることなどを示しており、異常寡雨、異常多雨の予測の可能性に関する重要な知見を提示している。その上で、北大西洋や熱帯太平洋における海面温度の偏差と対流圏上層の波列パターンの関連性に関する解析的研究や数値モデルを用いた感度分析、あるいはパキスタン付近を囲む低緯度帯から高緯度帯への偏差の伝播に関する理論的、モデル研究の必要性を提起している。

以上、本研究は、パキスタンにおける水資源管理に大きな影響を与える異常寡雨、異常多雨について、過去のそれぞれ4回のイベントに共通する気候学的な特徴およびその熱力学的な構造を明示し、水資源管理上有用な知見を与えている。さらに本研究では、パキスタンにおけるこれらの異常現象と全地球規模での海面温度や気圧場の偏差との関係や、前兆現象の理解につながる大気偏差の時間発展についても言及しており、これらの指摘は、今後の大気力学分野における研究によって予測可能性の研究へと発展する可能性示唆している。よって、本研究の成果は、水資源管理はもとより、農業、生態系などの社会的利益分野にも貢献するところが大きく、工学的、社会的有用性に富む独創的な研究成果と評価できる。よって本論文は博士(工学)の学位請求論文として合格と認められる。