SIBER SPIS sept 2011.pdf - IMBER

SIBER
Science Plan and Implementation Strategy
In t r o d u c t i o n
Gen e r a l ba c k g r o u n d
The Indian Ocean (IO) is a remarkable place. Even a cursory glance at a global map reveals
some striking aspects that clearly distinguish it from other major ocean basins (Fig. 1). Unlike the
Pacific and Atlantic, it has a low latitude land boundary to the north and the Indian subcontinent
partitions the northern basin. Much of the Eurasian landmass to the north is extremely arid (i.e.
the Thar Desert of NW India, northern Africa and the Arabian Peninsula) and/or dominated
by high mountainous terrain (e.g. the ranges of Afghanistan, Pakistan, India, Nepal and
southwestern China). The northern IO extends only a few degrees beyond the Tropic of
Cancer and thus has no subtropical or temperate zones. As a result, high-latitude densification
of surface waters and subsequent ventilation of intermediate and deepwater masses does not
occur. A second striking feature of the IO is the low latitude exchange between the Indian and
the Pacific Oceans, known as the Indonesian Throughflow (ITF). Thus, in comparison to the
Atlantic and Pacific, the IO is highly asymmetric, both zonally (with deep water, high latitude
exchange happening only to the south) and meridionally (with shallow water exchange along
the eastern rim).
As a result of the proximity of the Eurasian landmass and the heating and cooling of air masses
over it, the IO is subjected to strong monsoonal wind forcing that reverses seasonally, i.e.
the South West Monsoon (SWM) blows from the SW towards the NE in the boreal summer
(June-September) and the North East Monsoon (NEM) blows in the opposite direction in the
boreal winter (December-March) (for a review see Schott and McCreary, 2001). These winds
profoundly impact both the Arabian Sea (AS) and the Bay of Bengal (BoB), and their effects are
clearly apparent down to ~10ºS. The IO is also biogeochemically unique, having one of three
major open ocean oxygen minimum zones (OMZs) in the north (the others are in the eastern
tropical Pacific, one on either side of the equator). Oxygen concentrations in the intermediate
water (~100-800m) decline to nearly zero (e.g. Morrison et al., 1999), which has profound
biogeochemical impacts.
Another important aspect of the IO is the large dust and aerosol inputs that occur year round.
The various dust source regions around the northern IO include the Arabian Peninsula, the
African continent (Somalia) and Asia (Pakistan/India) (Leon and Legrand, 2003; Pease et al.,
1998). The southern IO also has significant dust regions that source from southern Africa in
the west and from Australia’s Great Western Desert in the east (McGowan et al., 2000; Piketh
et al., 2000). Anthropogenically-derived inputs are also prevalent, particularly the brown haze
from industrial pollution and biomass burning on surrounding continents that lingers over the
AS, the BoB and the southern tropical IO (Lelieveld et al., 2001; Ramanathan et al., 2007).
Finally, the IO is ecologically unique in a variety of ways. One striking ecological feature is the
presence of the largest mesopelagic fish (myctophids) stocks in the world in the AS (Gjøsaeter,
1984). These fish are specially adapted to the intense OMZ where they reside during the day
to escape predation.
Perhaps the most important consideration is that more than 16% of the world’s population
lives in the coastal and interior regions of the northern IO and they are directly impacted by the
vagaries of the monsoons and associated rains. Many other IO processes, such as seasonal
variations in oceanic circulation and the biogeochemical and ecological responses associated
5