Why are we doing a research cruise in the Indian Ocean?

Author: Denis Volkov

Our technological ability to monitor the state of the ocean has greatly advanced over the last few decades. If in earlier days taking measurements from ships was the only available option, nowadays oceanographers are well equipped with Earth-orbiting satellites and autonomous devices. While ship-based measurements are very limited in space and time, satellites can see the entire Earth surface in several days, and today numerous profiling floats are sampling most parts of the ocean. So one may reasonably ask why oceanographers still go at sea if they can get plenty of information about the ocean remotely, literally sitting in front of a computer?

Indeed, there is an enormous amount of data gathered by satellites and autonomous devices, but these modern observing systems also have limitations. For example, satellites can see only the surface of the ocean. Floats are usually separated by distances of hundreds of kilometers. Autonomous devices can dive, but most of them, like Argo floats, sample only the upper 2000 m water column. Although deep Argo floats capable of reaching 6000 m depth already exist, their quantity is still very limited to provide global and sustained observations of the deep ocean. In addition, not all ocean variables can be measured by satellites and autonomous devices, and small sensors on autonomous instruments are usually less accurate. Therefore, ship-based hydrography still remains the only method for obtaining high-quality, high spatial and vertical resolution measurements of a full suite of essential physical, chemical, and biological variables over the full-depth water column. And in the end, speaking about autonomous devices, somebody still has to go at sea and deploy them in a preplanned location. This is something most research cruises do as a supplemental duty (“piggyback” projects), and during our cruise we will also deploy Argo floats and drifters.

This will be the first scientific occupation of line IO7N section since 1995. The scientists are eager to learn how the state of the Western Indian Ocean has changed over the last 23 years. Has the deep ocean warmed? Have the regional concentrations of dissolved oxygen, carbon dioxide, nutrients changed? Has the Western Indian Ocean become more acidic? These and many more questions will be addressed by scientists after the completion of the cruise.

One of the most climatically significant variables is the amount of heat that is stored in the ocean. Because the heat capacity and density of seawater are much larger than those of air (water can absorb more than 4000 times as much heat as air per unit volume), changes in oceanic heat content have profound and long-lasting effects on global and regional climate. Existing observations from different platforms show that the upper-ocean heat content for the World Ocean has been steadily increasing since 1970s. The Indian Ocean is the warmest ocean on our planet and its upper 2000 m heat content has also been increasing. But has the excess heat penetrated deeper than 2000 meters in the Western Indian Ocean? We do not know, but we are going to find out during the cruise.

Figure 1. The rate of sea level rise from 1995 to 2015 in mm per year calculated from satellite altimetry measurements.

As seawater warms, it expands, and the sea level rises, which is among the most challenging consequences of ocean warming. Since the early 1990s, the global mean sea level has been steadily rising at a mean rate of 3.3±0.4 mm per year. About one third of the present-day sea level rise is due to the thermal expansion of seawater and the remaining two thirds are due to melting ice sheets and glaciers. The latter contribution is an indirect effect of ocean warming on the sea level rise, because the warmer ocean may enhance basal melting and thinning of ice shelves and marine-terminating glaciers. As evidenced by altimetry satellites, the sea level rise is above the global average almost in all parts of the Indian Ocean (Figure 1). River deltas and small island states in the Indian Ocean are particularly vulnerable to sea-level rise. For example, Bangladesh, located in the Ganges-Brahmaputra delta with its low elevation and severe tropical storms, is among the most affected countries. The Maldives, which is the lowest country in the world, may in fact become uninhabitable by 2100 if the current rates of sea level rise remain the same.

Figure 2. Distribution of oxygen at 26.9 kg/m3 neutral density surface. From the WOCE Indian Ocean Atlas (http://whp-atlas.ucsd.edu/indian_index.html).

It is worth noting that our cruise is heading towards an oxygen minimum zone (OMZ) in the Arabian Sea (Figure 2), which is the thickest of the three oceanic OMZ, and it is of global biogeochemical significance. The oxygen deficient waters of the OMZ are important because in extremely low oxygen environments, denitrification is a prominent respiratory process that converts nitrate (NO3), which is a form of nitrogen readily available to most plants, into free nitrogen gas (N2), which most plants cannot use. The OMZ appear to be increasing substantially and posing a threat to the marine ecosystems and fisheries. In addition, carbon-dioxide, phosphate, and nitrate all increase substantially to the north throughout much of the water column in the Arabian Sea. These variables include contributors to ocean acidification and important nutrients for phytoplankton growth in the ocean, which are important in this distinct biogeochemical province.