Feature ArticleDeep NINJA Collects Profiles Down to 4,000 Meters
By Taiyo Kobayashi • Kazuhiro Watanabe • Michihiko Tachikawa
A schematic comparison between present Argo float and deep-float observations.
However, such estimates of the deep ocean include larger errors than other components of the global climate system and thus are recognized as bottlenecks in understanding the anthropogenic global change and predicting its future accurately. The largest cause of these errors is the sparseness of deep-ocean observations. Therefore, more deep-ocean observations are required for not only scientific reasons, but for the environmental security of human society as well. The efforts to increase the number of deep-ocean observations, however, are almost limited since a majority of the observations are conducted by research vessels. Thus, establishing a monitoring network with numerous floats in the deep ocean, called Deep Argo, was proposed to the international community of ocean science.
Floats have several advantages over existing devices for ocean observations. They can operate in severe conditions, e.g., at high latitudes during winter, and as a result, obtain data without seasonal limitations. After deployment, the float can continue to perform observations for long periods automatically, which provides oceanic data in inaccessible regions, such as the Southern Ocean. The higher cost-performance is another advantage of float observations.
The development of Deep Argo, however, has not started yet because most floats cannot reach depths greater than 2,000 meters. Therefore, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Tsurumi Seiki Co. Ltd. (Yokohama, Japan), began to develop the deep-ocean float in 2009.
The First Deep Float: Deep NINJA
The first deep-float prototype was assembled in March 2011, and its control module was tested in coastal waters. Two comprehensive field tests were conducted with JAMSTEC research vessels in Sagami Bay in August 2011 and the Sea of Japan in May. Then, in the summer of 2012, a field test was carried out in the Pacific Ocean. Deep profiles from the depth of 4,000 meters were obtained by the floats. The successfully developed deep float was named Deep NINJA.
Deep NINJA can measure oceanic profiles from the sea surface to a depth of 4,000 meters. It can obtain measurements throughout the global ocean, from the tropics to the high-latitude waters seasonally covered with sea ice.
This suggests that approximately 90 percent of the ocean's volume is measurable by Deep NINJA, whereas existing floats, which can only submerge to 2,000 meters, can measure about 50 percent of the ocean's volume. It has a height of 210 centimeters (including the antenna) and a weight of approximately 50 kilograms in air.
The pressure hull is made of aluminum-alloy and has a diameter of 20 centimeters for the cylindrical components and 25 centimeters for the central bulge.
The conventional model has a Sea-Bird Electronics Inc. (Bellevue, Washington) SBE41CP CTD sensor on top. It is designed to have enough capacity to load additional sensors, and there are now some models with additional sensors (i.e., dissolved oxygen sensors) under development.
Observed data are transmitted to land-based stations by the Iridium Short Burst Data service. This also enables two-way communication so that the at-sea operations of the deep float can be changed by operators via e-mail. Float locations at the sea surface are fixed by GPS. The Deep NINJA float is powered by lithium batteries.
Deep-Sea Field Test Results
Deep NINJA's deep-sea field test was conducted east of Japan in the Pacific Ocean for about six weeks from August to October. In August, two prototypes were deployed from the Wakataka-maru, a fisheries research vessel of the Tohoku National Fisheries Research Institute in Japan. To continue this article please click here.
Taiyo Kobayashi is a senior research scientist in physical oceanography at JAMSTEC, with expertise in float observations. He has worked with the Argo program for more than 10 years, since its beginning. He is also involved in float development.
Kazuhiro Watanabe is a senior mechanical engineer at Tsurumi Seiki Co. Ltd. He has worked on the development of oceanographic instruments for almost 40 years and has been engaged in the development of Deep NINJA since the beginning of its feasibility studies.
Michihiko Tachikawa is the president of Tsurumi Seiki Co. Ltd., a manufacturer of oceanographic instruments and systems.