The Gulf of Aden and the Summer Monsoon

The most prominent climatic feature of the Indian Ocean is the monsoon. Winds blow from the south-west during the boreal summer season and from the north-east during the boreal winter season. The monsoon is strongest on the western part of the Indian Ocean and the Arabian Sea.

Core TY93-929 (T29) shows that PP increases generally with precession cycle (Fig. 3). Most of the models and data from cores north of this position show a reversed pattern (Prell, 1984; Clemens et al., 1991; Anderson and Prell, 1993; Emeis et al., 1995; Reichart et al., 1998; Schulz et al., 1998; Schulte et al., 1999; Altabet et al., 2002). Florisphaera profunda is not the only productivity proxy that shows that trend: Corg and alkenone concentration vary in parallel to the PP reconstituted in the same core (Rostek et al., 1994). That latter work suggests that PP dynamics observed is related to the winter monsoon. However, the summer monsoon has a dominant effect on PP. A supposed lower PP related to the lower LGM summer monsoon (Prell et al., 1980) could not have been compensated by the small increase in PP due to a more intense winter monsoon. More likely, PP dynamics observed in core T29 is related to the precession influence of the migration of the low-levels Finlater jets (Beaufort, 1996). These middle-altitudes winds, closely related to the monsoon, migrate from the south to north during the onset of the summer Indian monsoon (Wyrtki, 1973; Hastenrath et al., 1993). Their path could be modified by seasonal insolation and deflected to the south east along the equator during maximum precession (Beaufort, 1996). This is confirmed by results of ocean/atmosphere-coupled models that indicate that during maximum precession, monsoon winds and rains were located to the south in the Indian Ocean (De Noblet

TY93-929

TY93-929

Wyrtki Jet

Figure 3: Variations of primary production estimates in Core TY93-939 (solid line) compared with precession (Laskar, 1990) (doted line). Vertical lines represent Termination I and II.

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Figure 3: Variations of primary production estimates in Core TY93-939 (solid line) compared with precession (Laskar, 1990) (doted line). Vertical lines represent Termination I and II.

et al., 1996). That could be also described in terms of stronger Indian Ocean Dipole (Webster and Palmer, 1997; Saji et al., 1999) and this is described in Section 4.2. The T29 PP record exhibits the same variability as those established from cores located to the south, close to Soccotra Island (Caulet et al., 1992; Venec-Peyre et al., 1995; Beaufort et al., 2001).

For some peaks, the relation between PP and precession is not perfect and a shift of up to 5 kyr can be observed (Fig. 3). This can be attributed to (1) stratigraphic uncertainty, or (2) the existence of other processes influencing PP or (3) a phase modulation of the precession cycle by a glacial/ interglacial condition (Beaufort et al., 2001, 2003).

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