Study On The Upper Layer Circulation And Its Variation In The Bay Of Bengal

The Bay of Bengal (BOB), a marginal sea located in the northeastern Indian Ocean, is an important passage for northward vapor transport derived from the Indian Ocean, and also is an alternative exportation of the future southwest energy-channel of China. Therefore, oceanography researches on the BOB are vital. As the data coverage for the BOB is sparse, moreover, most of researches are based on numerical model or a single survey. Thus, till now, the circulation of the BOB and its variations are poorly described. Especially, in China, few studies have been conducted.In view of this, the aims of the thesis are: (i)to describe the hydrographic characteristics of the BOB;(ii) to discuss the character- istics and rules of seasonal and interannual variations of upper ocean circulation in the BOB mainly based on altimeter data; (iii) and then to investigate the mechanism of upper ocean circulation in the BOB under the effects of monsoon and oceanic oscillations such as El Ni?o-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD); (iv) to analyze the seasonal evolution of Wyrtki Jet with a new set of data from surface velocity program (SVP) drifters combined with altimeter measurements; (v) to analyze the characteristics of mesoscale phenomena in the BOB.It is suggested that variations of the BOB upper ocean circulation have multi-timescale, including semiannual, annual and interannual (2 and 3-7 yr). Variations at annual timescale are dominated by the bay wind, whereas variations at semiannual and interannual timescale are predominantly caused by the equatorial wind forcing, which generates coastal Kelvin waves that propagate into the bay. Under the dominance of the local wind and the remote forcing of semiannual variations of sea level in the equatorial Indian Ocean induced by the Wyrtki jet, the annual variations of the BOB upper ocean circulation have a three-stage evolution. However, the interannual variations of the BOB are predominantly caused by variations of equatorial sea level. ENSO and IOD are two of the most important factors. Except for coastal Kelvin waves, ENSO and IOD can also impact the BOB by induced variations of local wind and Indian monsoon current, respectively. The western bay is the region of strongest seasonal and interannual variations of currents, and also is the region where activities of eddies are the most frequent. The main results are as follow:1. The annual variations of the BOB upper ocean circulation could be divided into three stages with distinctive patterns. During the late northeast monsoon (from January through April), the circulation is mainly controlled by a basin-scale strong anticyclonic gyre. During southwest monsoon (from May to September), the circulation appears as three-eddy structure. During the early northeast monsoon (from October to December), the circulation is dominated by a basin-scale weak cyclonic pattern. It is suggested that three-stage variations of the BOB circulation are mainly driven by wind stress curl in the interior. The effect of remote forcings on the BOB circulation are from the Indian monsoon current and semiannual fluctuations of sea level in the equatorial Indian Ocean induced by the Wyrtki Jet. Especially, the upwelling coastal Kelvin wave induced by the decay of the fall Wyrtki jet during December to January may be the critical factor for turnabouts of the BOB circulation.2. The interannual variations of SLA in the Bay are concentrated in two bands: 2 and 3-7 yr. The interannual variations of SLA are predominantly caused by equatorial large-scale wind field, which generates coastal Kelvin waves that propagate into the bay. The main two modes of interannual SLA variations are induced by ENSO and IOD, respectively. The effect of ENSO on the SLA variations is more intense than that induced by IOD. ENSO impacts the BOB by two ways: one is atmosphere (local wind field) and the other is ocean (the equatorial Indian Ocean). IOD seems to impact the BOB only through oceanic way (by the coastal Kelvin wave and the Indian monsoon current), but its influence is only limited to the bay mouth.3. From the drifters'data, it is found that the fall Wyrtki Jet is stronger and wider with the core deflected slightly more southward than its counter part of spring. The Jets are developed firstly in the equatorial Indian Ocean between 75°E and 80°E and then propagate westward along the equator. This westward propagation of the Jets is also observed by altimeter. The propagation observed by altimeter appears in the western equatorial Indian ocean between 55oE and 75oE at speeds around 0.6m/s. Further analysis suggests that westward propagation of the Jets is mainly driven by equatorial zonal wind stress, but its propagation velocity is slightly slower than the latter. However, in the eastern and western boundary, the correlation between local zonal wind and zonal current is weak. Especially, in the eastern equatorial Indian Ocean, propagation of the jet signals is ambiguous, although the zonal wind pattern is observed moving east coherently. This suggests that the role of zonal wind isn't dominant.4. The highest eddy kinetic energy (EKE) is found in the western bay, where the high-frequency and low-frequency EKE have comparable amplitude. The low-frequency EKE is related to seasonal and interannual variations of the western boundary current and the Indian monsoon current. The high-frequency EKE is related to activities of mesoscale eddies. Such as to east of Sri Lanka Island, the strong high-frequency EKE is just caused by the Sri Lanka cold eddy. The cold eddy appears during June through October. Southwest monsoon impinges on the Sri Lanka Island, resulting in a mesoscale cyclonic wind stress curl inducing upward Ekman pumping that is vital for its generation. The location of the eddy is 1～2oN north to cyclonic wind stress curl, which is attributed to the eddy's northwestward movement. Its movement can be explained by the simple theories for vortex propagation on aβ-plane.5. Coastal Kelvin waves are the primary passage for the propagation of fluctuations from the equatorial Indian Ocean to the BOB. Except the annual fluctuations, the semiannual and quasi-biennial fluctuations are predominantly caused by variations of sea level in equatorial Indian Ocean, which generate coastal Kelvin waves that propagate into the bay at the phase speed of the first and third baroclinic mode, respectively.