Robust Diagnostic Model Research Articles

Study of seasonal transport variations in the Indonesian seas

Seasonal transport variations between the Pacific and Indian Oceans via the Indonesian seas were studied by the Euler‐Lagrangian method. The velocity field was calculated with a fairly high resolution robust diagnostic model. The model well reproduces the features of seasonal variations in the Indonesian seas. The total volume transport of the Indonesian throughflow is 20±3 Sv (1 Sv = 106 m3 s−1), the maximum being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulation models with a wide Indonesian passage despite resolution of the presence of the many small islands in the Indonesian seas. Although a large portion of the net transport is contained in the upper layer, deep transport below 1000‐m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard of the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calculated velocity field clarified the sources and pathways of the Indonesian throughflow. The major route is a western one through both the Makassar and Lombok Straits. Most of the North Pacific water supplied from the Mindanao Current passes along this route, entering the Indian Ocean within several months with almost no loss of its properties (intense vertical mixing around the Lombok sill reported by observations could not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequently mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original properties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial circulation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal spring to summer in association with the linkage between the current flowing through the Makassar Strait and the Mindanao Current. In other seasons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the Pacific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spring, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi‐steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.

Seasonal variation of circulations in the East China Sea and the Yellow Sea

Seasonal variation of the water circulations in the East China Sea and the Yellow Sea is investigated with use of a robust diagnostic numerical model. Water circulations in four season are calculated diagnostically from the observed water temperature and salinity data from JODC (Japan Oceanographic Data Center) and wind data from COADS (Comprehensive Ocean-Atmosphere Data Set). Counter-clockwise circulations are developed at the upper and middle layers and a clockwise one at the lower layer in the central part of he Yellow Sea in summer. On the other hand, a clockwise circualtion is developed from the surface to the bottom in the Yellow Sea and a counter-clockwise one in, the northern part of the East China Sea in winter.

World ocean circulation diagnostically derived from hydrographic and wind stress fields: 1. The velocity field

Using a robust diagnostic model, the steady circulation in the world ocean is determined from climatological annual mean fields of hydrographic data and wind stress. The upper circulation reflects the surface geopotential anomaly of the input hydrographic data. In addition, the “warm water route” is prominent: the water passing through the Indonesian seas flows westward across the Indian Ocean to return to the North Atlantic. In the deep Atlantic Ocean, the North Atlantic Deep Water is transported southward by the dominant deep western boundary current in the depth range from about 1500 m to the bottom, whereas the Weddell Sea Deep Water does not extend northward from the Argentine Basin. An important result is a westward current appearing near 25°N in the middepth Atlantic Ocean. Such a current has been inferred from tracer distributions but hardly detected with objective methods. The velocity maximum at middepth suggests that the current is not a lower part of a wind gyre but is a baroclinic flow caused by a thermohaline effect. In the Indian Ocean a swift middepth current appears along the western boundary, but it may be questionable because the resulting heat transport does not agree with earlier estimates. The deep water flowing into the southern basins from the Antarctic Circumpolar Current does not flow further northward even in the Madagascar Basin. The cause of these unsuccessful results is probably that the present model does not take into account large seasonal variations in the Indian Ocean.

Diagnostic calculation for circulation and water mass movement in the deep Pacific

The steady circulation of the deep Pacific is estimated with a robust diagnostic model, which is internally constrained by hydrographic data. It is shown that the input data should be modified to fit the model in inverse proportion to the Coriolis parameter because a density field inconsistent with the model generates unrealistic geostrophic flows. The model reproduces most of the deep currents previously reported, such as the deep western boundary current east of New Zealand. In addition, as a new feature, the present model diagnoses an anticyclonic circulation around the East Pacific Rise. This circulation is discovered to be associated with a rise of isopycnals at middepth. Tracking of many particles in the diagnosed velocity field reveals that two water masses enter the Southwest Pacific Basin. One is the deep water of the South Indian Basin which enters through a gap to the south of New Zealand. The other is the upper water of the Antarctic Circumpolar Current; this water becomes dense near the Ross Sea and sinks into the deep Southeastern Pacific Basin. The anticyclonic circulation around the East Pacific Rise transports it to the Southwest Pacific Basin. These waters supply comparable volumes to the Southwest Pacific Basin; the residence time is estimated to be 86 years. The deep water in the Southwest Pacific Basin is brought northward rapidly by the deep western boundary current east of New Zealand; it takes only a few decades to move from the east of New Zealand to the North Pacific.

An ocean transport model for the North Atlantic

Three‐dimensional solutions are obtained for the circulation of the North Atlantic using a robust diagnostic model. In contrast to previous diagnostic models the robust diagnostic model incorporates the conservation of the large‐scale fields of heat and salinity as well as momentum. An approximate fit to observed fields of temperature and salinity is obtained by a closure condition. The method is robust in the sense that it does not have the extreme sensitivity to the density input fields of the classical diagnostic method. Equilibrium solutions are obtained by numerical integration of the time‐dependent equations. Error estimates for the velocity field can be obtained indirectly from the numerical solutions. Temperature observations used as input have an effective resolution of 3°×3° of latitude and longitude and a sampling error of ± 0.15°C. The equivalent vertically integrated velocity error is estimated to be ±0.5–1.0 cm/s depending on bottom topography. The suitability of the model for geochemical work is judged by comparison with heat and salinity balance estimates. Best results are obtained for the case in which the model has a minimum observational constraint below the surface.