Forcing factors affecting sea level changes at the coast

We review the characteristics of sea level variability at the coast focussing on how it differs from the variability in the nearby deep ocean. Sea level variability occurs on all timescales, with processes at higher frequencies tending to have a larger magnitude at the coast due to resonance and other dynamics. In the case of some processes, such as the tides, the presence of the coast and the shallow waters of the shelves results in the processes being considerably more complex than offshore. However, ‘coastal variability’ should not always be considered as ‘short spatial scale variability’ but can be the result of signals transmitted along the coast from 1000s km away. Fortunately, thanks to tide gauges being necessarily located at the coast, many aspects of coastal sea level variability can be claimed to be better understood than those in the deep ocean. Nevertheless, certain aspects of coastal variability remain under-researched, including how changes in some processes (e.g., wave setup, river runoff) may have contributed to the historical mean sea level records obtained from tide gauges which are now used routinely in large-scale climate research

Climatological mean features and interannual to decadal variability of ring formations in the Kuroshio Extension region

This study examines the climatological mean features of oceanic rings shed from the Kuroshio Extension (KE) jet and their interannual to decadal variability using satellite altimeter observations from October 1992 to December 2010. To objectively capture ring shedding from the KE jet, a new method that consists of the detection of the jet length changes and the tracking of a ring is proposed. Spatial distribution of the ring formations in the KE region indicates that cyclonic (cold-core) rings were most frequently formed in the upstream region between 143A degrees and 147A degrees E around the steady meander of the KE jet. In contrast, most of anticyclonic (warm-core) rings were formed in the downstream region west of the Shatsky Rise. These pinched-off rings in both the upstream and downstream regions generally propagated westward, but about two-thirds of the rings were reabsorbed by the jet. Nevertheless, about one-fourth of the meridional eddy heat transport at the latitude of the KE resulted from the rings that are not reabsorbed by the jet. The number of ring formations showed substantial interannual to decadal variability. In the upstream and downstream KE region, decadal and interannual variability was dominant, respectively. These ring formation fluctuations were negatively correlated with the strength of the KE jet. It is also revealed that the ring formation variations play an important role in sea surface temperature changes north of the KE jet

Atmospheric response to interannual variability of sea surface temperature front in the East China Sea in early summer

The atmospheric response, especially the response of the meiyu-baiu rainband, to interannual variability of the sea surface temperature (SST) front associated with the Kuroshio in the East China Sea in early summer is examined by using reanalysis, satellite, and rain-gauge datasets from 1982 to 2010. It is revealed that the strong (weak) SST front in the East China Sea is accompanied by the heavy (weak) precipitation over the central East China Sea and the southern Japan. Because the strong SST front largely results from the negative SST anomaly over the continental shelf, the local evaporation change in the East China Sea is not balanced by this enhanced precipitation. The moisture for this enhanced precipitation is supplied by interannual variability of horizontal wind convergence over the central East China Sea. In addition to the precipitation change, the strong SST front is also accompanied by the intensification of weather disturbances in the lower troposphere over the East China Sea. This is probably because the negative SST anomaly over the continental shelf enhances the baroclinicity in the lower troposphere. This intensification of the weather disturbances over the East China Sea can explain the enhanced precipitation over the central East China Sea in response to the interannual variability of the SST front. Because the SST anomaly over the continental shelf, which primarily determines the interannual variability of the SST front, persists for a couple of months, these results imply the predictability of the precipitation associated with the meiyu-baiu rainband

Decadal Response of the Kuroshio Extension Jet to Rossby Waves: Observation and Thin-Jet Theory

This study examines interannual to decadal variability of the Kuroshio Extension (KE) jet using satellite altimeter observations from 1993 to 2010. The leading empirical orthogonal function (EOF) mode of sea level variability in the KE region represents the meridional shift of the KE jet, followed by its strength changes with a few month lag. This shift of the KE jet lags atmospheric fluctuations over the eastern North Pacific by about three years. Broad sea level anomalies (SLAs) emerge in the eastern North Pacific 3-4 years before the upstream KB jet shift, and propagate westward along the KE jet axis. In the course of the propagation, the meridional scale of the SLAs gradually narrows, and their amplitude increases. This westward propagation of SLAs with a speed of about 5 cm s(-1) is attributed to the westward propagation of the meridional shift of the jet, consistent with the thin-jet theory, whose importance has been suggested by previous numerical studies. In addition, the westward-propagating signals tend to conserve their quasigeostrophic potential vorticity anomaly, which may explain the characteristic changes of SLAs during the propagation. After the westward-propagating signals of positive (negative) SLAs reach at the east coast of Japan, the upstream KB jet strengthens (weakens) associated with the strength changes of the northern and southern recirculation gyres. Interestingly, this strength change of the KE jet propagates eastward with a speed of about 6 cm s(-1), suggesting an importance of advection by the current

Sea Level Variability around Japan during the Twentieth Century Simulated by a Regional Ocean Model

Sea level variability around Japan from 1906 to 2010 is examined using a regional ocean model, along with observational data and the CMIP5 historical simulations. The regional model reproduces observed interdecadal sea level variability, for example, high sea level around 1950, low sea level in the 1970s, and sea level rise during the most recent three decades, along the Japanese coast. Sensitivity runs reveal that the high sea level around 1950 was induced by the wind stress curl changes over the North Pacific, characterized by a weakening of the Aleutian low. In contrast, the recent sea level rise is primarily caused by heat and freshwater flux forcings. That the wind-induced sea level rise along the Japanese coast around 1950 is as large as the recent sea level rise highlights the importance of natural variability in understanding regional sea level change on interdecadal time scales

Rapid Warming of Sea Surface Temperature along the Kuroshio and the China Coast in the East China Sea during the Twentieth Century

It has been reported that the sea surface temperature (SST) trend of the East China Sea during the twentieth century was a couple of times larger than the global mean SST trend. However, the detailed spatial structure of the SST trend in the East China Sea and its mechanism have not been understood. The present study examines the SST trend in the East China Sea from 1901 to 2010 using observational data and a Regional Ocean Modeling System (ROMS) with an eddy-resolving horizontal resolution. A comparison among two observational datasets and the model output reveals that enhanced SST warming occurred along the Kuroshio and along the coast of China over the continental shelf. In both regions, the SST trends were the largest in winter. The heat budget analysis using the model output indicates that the upper-layer temperature rises in both regions were induced by the trend of ocean advection, which was balanced in relation to the increase of surface net heat release. In addition, the rapid SST warming along the Kuroshio was induced by the acceleration of the Kuroshio. Sensitivity experiments revealed that this acceleration was likely caused by the negative wind stress curl anomalies over the North Pacific. In contrast, the enhanced SST warming along the China coast resulted from the ocean circulation change over the continental shelf by local atmospheric forcing

Dynamical downscaling of future sea level change in the western North Pacific using ROMS

The future regional sea level (RSL) rise in the western North Pacific is investigated by dynamical downscaling with the Regional Ocean Modeling System (ROMS) with an eddy-permitting resolution based on three global climate models—MIROC-ESM, CSIRO-Mk3.6.0, and GFDL-CM3—under the highest greenhouse-gas emission scenario. The historical run is forced by the air-sea fluxes calculated from Coordinated Ocean Reference Experiment version 2 (COREv2) data. Three future runs—ROMSMIROC, ROMS-CSIRO, and ROMS-GFDL—are forced with an atmospheric field constructed by adding the difference between the climate model parameters for the twentyfirst and twentieth century to fields in the historical run. In all downscaling, the RSL rise along the eastern coast of Japan is generally half or less of the RSL rise maxima off the eastern coast. The projected regional (total) sea level rises along the Honshu coast during 2081–2100 relative to 1981–2000 are 19–25 (98–104), 6–15 (71–80), and 8–14 (80–86) cm in ROMS-MIROC, ROMS-CSIRO, and ROMS-GFDL, respectively. The discrepancies of the RSL rise along the Honshu coast between the climate models and downscaling are less than 10 cm. The RSL changes in the Kuroshio Extension (KE) region in all downscaling simulations are related to the changes of KE (northward shift or intensification) with climate change