The 2011 Tohoku-oki Earthquake Tsunami: Similarities and Differences to the 869 Jogan Tsunami on the Sendai Plain

A post-tsunami field survey following the 2011 Tohoku-oki Earthquake Tsunami was carried out to asses inundated area in Sendai Plain, Northeast Japan. The type of inundation was classified into two categories (major and minor) according to the amount of accumulated debris, garbage and sediment. Major and minor inundations were identified up to 4 and 5 km from the coastline, respectively. Many artificial geomorphological features, such as roadway embankments and canals, were believed to have affected the run-up process of the tsunami. The inundation area of the 2011 tsunami on the Sendai Plain is compared with that of the 869 Jogan tsunami, which was reconstructed using numerical modeling based on available historical and geological records. The inundation area of the 2011 Tohoku-oki tsunami is comparable to that of the 869 Jogan tsunami, although a direct comparison is difficult due to differences in geomorphological contexts between the paleo period and the present.     

Distribution and photo-physiological condition of phytoplankton in the tropical and subtropical North Pacific

To better understand the vertical distribution of phytoplankton in the tropical and subtropical North Pacific, we used fast repetition rate fluorometry to investigate the photo-physiological condition of the phytoplankton assemblage in this region between February and March 2007. Along 155°E, between the equator and 24°N, the peak of fluorescence (F _m), an indication of the deep chlorophyll maximum (DCM), was deeper than the top of the nitracline and occurred at the 2.4 ± 1.3 % (mean ± SD) light depth (relative to 0 m). The photochemical efficiency (F _v/F _m) and effective absorption cross-section of photosystem II (σ_PSII) were low at the surface but increased rapidly at depths between the top of the nitracline (40–138 m) and the DCM (70–158 m), an indication that the photo-physiological condition of the phytoplankton improved below the top of the nitracline. The depth of the maximal F _v/F _m [Z(F _v/F _{m max})] was 18–32 m deeper than the DCM and corresponded to the 0.8 ± 0.2 % light depth. The values of F _v/F _m at the Z(F _v/F _{m max}) were 20 % higher than those at the DCM and averaged 0.48 ± 0.01. These results suggest that the phytoplankton assemblage beneath the DCM had a high potential photosynthetic performance capacity and was growing by using the very low ambient light in this region.     

Ocean Tide Models Developed by Assimilating TOPEX/POSEIDON Altimeter Data into Hydrodynamical Model: A Global Model and a Regional Model around Japan

A global ocean tide model (NAO.99b model) representing major 16 constituents with a spatial resolution of 0.5° has been estimated by assimilating about 5 years of TOPEX/POSEIDON altimeter data into barotropic hydrodynamical model. The new solution is characterized by reduced errors in shallow waters compared to the other two models recently developed; CSR4.0 model (improved version of Eanes and Bettadpur, 1994) and GOT99.2b model (Ray, 1999), which are demonstrated in comparison with tide gauge data and collinear residual reduction test. This property mainly benefits from fine-scale along-track tidal analysis of TOPEX/POSEIDON data. A high-resolution (1/12°) regional ocean tide model around Japan (NAO.99Jb model) by assimilating both TOPEX/POSEIDON data and 219 coastal tide gauge data is also developed. A comparison with 80 independent coastal tide gauge data shows the better performance of NAO.99Jb model in the coastal region compared with the other global models. Tidal dissipation around Japan has been investigated for M₂ and K₁ constituents by using NAO.99Jb model. The result suggests that the tidal energy is mainly dissipated by bottom friction in localized area in shallow seas; the M₂ ocean tidal energy is mainly dissipated in the Yellow Sea and the East China Sea at the mean rate of 155 GW, while the K₁ energy is mainly dissipated in the Sea of Okhotsk at the mean rate of 89 GW. TOPEX/POSEIDON data, however, detects broadly distributed surface manifestation of M₂ internal tide, which observationally suggests that the tidal energy is also dissipated by the energy conversion into baroclinic tide.     

East-west distribution of POC fluxes estimated from <Superscript>234</Superscript>Th in the northern North Pacific in autumn

Observations of primary productivity, ²³⁴Th, and particulate organic carbon (POC) were made from west to east across the northern North Pacific Ocean (from station K2 to Ocean Station Papa) during September–October 2005. Primary productivities in this region varied longitudinally from approximately 236 to 444 mgC m⁻²d⁻¹ and clearly indicate the West High East Low (WHEL) trend. We estimated east-west variations in the POC flux from the surface layer (0–100 m) by using ²³⁴Th as a tracer. POC fluxes in the western region (44–53 mgC m⁻²d⁻¹) were higher than those in the eastern region (21–34 mgC m⁻²d⁻¹). However, the export ratios (e-ratios) ranged from approximately 8% to 16% and did not show the WHEL trend. Contrary to our expectation, no relation between POC flux (or e-ratio) and diatom biomass (or dominance) was apparent in autumn in the northern North Pacific.