Abstract Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.

The transition zone of the Canary Current upwelling region

The north Indian Ocean accounts for 6% of the global tropical cyclones annually. Despite the small fraction of cyclones, some of the most devastating cyclones have formed in this basin, causing extensive damage to the life and property in the north Indian Ocean rim countries. In this review article, we highlight the advancement in research in terms of ocean-atmosphere interaction during cyclones in the north Indian Ocean and identify the gap areas where our understanding is still lacking.

A review of the ocean-atmosphere interactions during tropical cyclones in the north Indian Ocean

The Kelantan River Basin, situated in the northeastern Malaysian Peninsula, suffers serious flood/inundation damage, related to the northeast monsoon season (November–January), every few years. In this river basin, rainfall observation systems have been progressively developed since 1948, and long-term time-series data at distributed rainfall stations have been accumulated. This study firstly investigated the homogeneity of the accumulated time-series data for the purpose of constructing a reliable database for various hydrologic analyses. The homogeneity of rainfall time-series data was established using four absolute homogeneity tests: the Pettitt test, standard normal homogeneity test, Buishand range test, and von Neumann ratio test. It was found that among 50 rainfall stations within the river basin, 9 were flagged by the tests. Of these, inhomogeneous time-series data from four stations were omitted from further analysis. Secondly, using the homogenous time-series rainfall data, a trend analy...

Homogeneity and trends in long-term rainfall data, Kelantan River Basin, Malaysia

Tropical cyclones (TCs) are strong natural hazards that are important for local and global air–sea interactions. This manuscript briefly reviews the knowledge about the upper ocean responses to TCs, including the current, surface wave, temperature, salinity and biological responses. TCs usually cause upper ocean near-inertial currents, increase strong surface waves, cool the surface ocean, warm subsurface ocean, increase sea surface salinity and decrease subsurface salinity, causing plankton blooms. The upper ocean response to TCs is controlled by TC-induced mixing, advection and surface flux, which usually bias to the right (left) side of the TC track in the Northern (Southern) Hemisphere. The upper ocean response usually recovers in several days to several weeks. The characteristics of the upper ocean response mainly depend on the TC parameters (e.g. TC intensity, translation speed and size) and environmental parameters (e.g. ocean stratification and eddies). In recent decades, our knowledge of the upper ocean response to TCs has improved because of the development of observation methods and numerical models. More processes of the upper ocean response to TCs can be studied by researchers in the future.

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Upper ocean response to tropical cyclones: a review

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A review of ocean-atmosphere interactions during tropical cyclones in the north Indian Ocean

Underwater gliders are used to investigate the variations on the ocean surface and subsurface during the 14 typhoons that passed over the Kuroshio region near Taiwan in 2010–2013. Typhoon-induced subsurface layer warming, which was formed on the basis of the heat pump effect of the vertical mixing process, is observed in this study. In addition, the gliders observe the variations in salinity during the passage of the typhoons. Typhoons, accompanied by heavy rainfall, introduce a considerable amount of fresh water into the upper ocean, diluting the surface salinity. The diluted salinity accompanied deepening of the mixed layer which moved downward to the subsurface by vertical mixing, supplying fresh water to both the surface and subsurface layers. Additionally, because of vertical mixing, maximum temperature variations occur at the bottom of the mixed layer or at a level deeper than the mixed layer. Changes of the heat content show that a series of vertical mixing events occurs in the upper ocean. Pairs of salinity profiles indicate comprehensive vertical mixing in the upper ocean and yield schematics to compare typhoons with or without heavy rainfall. Higher sustained wind speeds may contribute to a considerable drop in sea surface temperature, but possibly not to the magnitude of the subsurface warming. The upper ocean thermocline gradient before a typhoon is an important factor to determine the magnitude of subsurface warming. This study concludes a glider observational result that typhoons cause subsurface layer warming and freshening in the Kuroshio region near Taiwan.

Typhoon-induced ocean subsurface variations from glider data in the Kuroshio region adjacent to Taiwan

Investigation of the island-induced ocean vortex train of the Kuroshio Current using satellite imagery

Satellite remote-sensing data and glider data are used to study the Kuroshio meander and surface properties, east of Taiwan. The Kuroshio meandered eastward 13 times between 1993 and 2013 because of cold eddies propagating from the western Pacific. The maximum duration of the meanders was 80 days. The farthest eastward shift of the Kuroshio axis was approximately 270 km from its original position, depending on the size of the cold eddy. Cold eddies reduce the current speed at the Kuroshio axis to 84% of its seasonal average, which is approximately 0.75 m/s. According to glider data, isopycnal uplifting is produced when cold eddies impinge on the Kuroshio, and satellite observations show that the sea surface temperature (SST) drops $1{-}3\;^\circ{\text{C}}$ and that the chlorophyll-a (chl-a) concentration increases up to $0.54\text{ mg}/{\text{m}}^{3}$ .

Effects of Cold Eddy on Kuroshio Meander and Its Surface Properties, East of Taiwan

Tide-Induced Periodic Sea Surface Temperature Drops in the Coral Reef Area of Nanwan Bay, Southern Taiwan

The HadISST (Hadley Centre Sea Ice and Sea Surface Temperature) dataset is used to define the years of El Nino, El Nino Modoki, and La Nina events and to find out the impacts of these events on typhoon activity. The results show that the formation positions of typhoon are farther eastward moving in El Nino years than in La Nina years and much further eastward in El Nino Modoki years. The lifetime and the distance of movement are longer, and the intensity of typhoons is stronger in El Nino and in El Nino Modoki years than in La Nina years. The Accumulated Cyclone Energy of typhoon is highly correlated with the Oceanic Nino Index with a correlation coefficient of 0.79. We also find that the typhoons anomalously decrease during El Nino years but increase during El Nino Modoki years. Besides, there are two types of El Nino Modoki, I and II. The intensity of typhoon in El Nino Modoki I years is stronger than in El Nino Modoki II years. Furthermore, the centroid position of the Western Pacific Warm Pool is strongly related to the area of typhoon formation with a correlation coefficient of 0.95.

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Po-Chun Hsu

Papers

Typhoon-induced ocean subsurface variations from glider data in the Kuroshio region adjacent to Taiwan

Investigation of the island-induced ocean vortex train of the Kuroshio Current using satellite imagery

Effects of Cold Eddy on Kuroshio Meander and Its Surface Properties, East of Taiwan

Tide-Induced Periodic Sea Surface Temperature Drops in the Coral Reef Area of Nanwan Bay, Southern Taiwan

Impacts of Two Types of El Niño and La Niña Events on Typhoon Activity