Indian Ocean climate histories and prognoses

The unique seasonality of the Indian Ocean and the subregional monsoonal patterns are affected by the differences between ter restrial and ocean temperatures. The double monsoon pattern reflects quite different seasonal hydrological regimes. The northeast monsoon which spans December through March is primarily driven by high sea surface temperatures and colder winter continental temperatures. This results in intense tropical storms and an area of high humidity ranging northward from the ITCZ, the position of which varies strongly from year to year (from 10° to 15°S latitude in its southernmost position).

The southwest monsoon, which dominates the ocean current system and climate off eastern Africa from June through September (but April through May from Thailand eastward), with peak seasonal rainfalls in June and July, is driven by terrestrial summer heating. The inter-monsoon periods, April-May and October-November, are relatively calm, with characteristic equatorial westerlies.

Ocean currents and oceanographic properties are functions of the seasonal wind changes. The northern Indian Ocean coastal areas are most affected, as reflected in its ocean dynamics. The Somalia Current at the western boundary of the Indian Ocean is very responsive to seasonal changes. The thermal structure, as measured by shifts in the thermocline depth, varies from 50 to over 200 m during the course of weeks to months.

Tropical tuna habitat in the Indian Ocean extends from the northern coastal boundaries to about 25° to 30°S latitude, with the greatest southerly extension occurring in the southern summer months. The seasonal thermocline varies dramatically from region to region, and in the the western region (west of 80°E), it is "shoalest" (most close to the sea surface) in the April-May inter-monsoon and deepest, and therefore less constraining to the fish during August-September, at the end of the southwest monsoon period.

Early prospection by the French vessel Yves de Kerguellen showed strong seasonal changes in the distributions and relative abundances of the two major tuna species, skipjack and yellowfin, as the initial fishing year progressed. As more vessels joined the fishery, a more opportunistic approach to fishing ensued and catch rates tended to even out, as evident from the statistics in Table 16.3.

There remains, however, considerable year-to-year catch variability. This is to be expected, particularly considering the histor ical perspectives that are provided by long-term climate summaries records such as those in the comprehensive ocean and atmosphere data set (COADS), which provides options for exploring historical observation information from the nineteenth to the twentieth century.

The dates cited in Fig. 16.2 correspond to major climatic changes and ecological shifts that are well recorded in the global fisheries literature. For example, the Pacific sardine off Japan and eastern Asia, and California, began to bloom in about the mid-19208, peaking in the late 1930s. The 1940s period marked the beginning of the decline in the Pacific Basin sardine abundance, as well as the Gulf of Alaska Pacific halibut fisheries. The 195565 cool period marked the great harvests of the anchovy off the western coast of South America, while the recent epoch of Pacific Basin sardine and north Pacific halibut expansion and abundance started in the late 1960s. Basin-wide warming trends began in the late 1960s, and the sardine populations bloomed throughout the Pacific Ocean. Following the 1982-83 El Nino-Southern Oscillation (ENSO) event, ocean cooling, particularly off South America, has been accompanied by declining adult sardine abundance, primarily because of decreasing recruitment and increasing exploitation. Recruitment failure is inevitable, particularly if ocean changes continue, for example, leading to further cooling or intensified upwelling, as part of longer-term climatic cycles, or if warming and cooling cycles increase in frequency.

Similar but shorter time series - 1947 to present - for the western boundary of the Indian Ocean are shown in Fig. 16.3. These data were also extracted from the COADS, available through the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado at Boulder (USA).

Tropical tunas are very responsive to upper ocean thermal and oxygen gradients, as a function of their foraging behavior as well as for physiological reasons. It is important to consider the effects of any unusual or systematic climate-driven ocean changes on their fisheries.

Figure 16.4 provides the MOODS file climatology for monthly one degree square SST and 90 m temperature information needed to understand the seasonal dynamics of tuna fisheries around the globe. The months of March and September are shown in Fig. 16.4, as examples. The data are arrayed in a fashion that will allow

SCALAR WIND DEPARTURES WESTERN INDIAN OCEAN (meters per second)

Equator to 26 N, 34E to 60E

SCALAR WIND DEPARTURES WESTERN INDIAN OCEAN (meters per second)

Equator to 26 N, 34E to 60E

Equator to 16 S, 30E to 60E

All Seasons

16S to 32S, 30E to 60E

All Seasons

Fig. 16.3 The western coast of the Indian Ocean was divided into three climate-ocean regimes for the instrumental record period from 1947 to present, in which we have the most confidence. The four seasons are portrayed for the northernmost sector, and only the "All seasons" composites are shown for the other two.

casual browsing of the climatological mean one degree latitude-longitude, values of SST, and six levels, at 30 m depth intervals for the world ocean, as well as salinities, as devised.*

* The complete annual data are available from the NOAA Center for Ocean Analysis and Prediction, Monterey, California 93943-5005, to anyone with access to a Macintosh II computer, and the spreadsheet software package, WingZ. The data were compiled by Margaret Robinson and Roger Bauer, of Compass Systems, Inc., San Diego, California. The Macintosh software and formats for portraying these data were designed by G.D. Sharp and implemented by Mark Sutton, APEIRON, Inc., Dallas, Texas.

Fig. 16.4 The temperature distributions from the monthly one degree Bauer and Robinson Ocean Atlas for March and September at 30 m and 90 m for the Indian Ocean outline the seasonal features that define the vulnerability of the tropical tunas. The sea surface temperature information is of limited value in this regard, as the subsurface dynamics provide the best insights into the tuna's habitat variability in response to the strong seasonal forcing in this region. The lighter gray locations at 30 m near the equatorial countercurrent in March, and their counterparts in the western Arabian Sea in September are prime fishing features, as documented by Marsac & Hallier (1991). The broader light grey areas at 90 m in both months identify the gross regions where the tropical tuna habitat is limited to the upper ocean and, therefore, vulnerable to seine gear.

Fig. 16.4 The temperature distributions from the monthly one degree Bauer and Robinson Ocean Atlas for March and September at 30 m and 90 m for the Indian Ocean outline the seasonal features that define the vulnerability of the tropical tunas. The sea surface temperature information is of limited value in this regard, as the subsurface dynamics provide the best insights into the tuna's habitat variability in response to the strong seasonal forcing in this region. The lighter gray locations at 30 m near the equatorial countercurrent in March, and their counterparts in the western Arabian Sea in September are prime fishing features, as documented by Marsac & Hallier (1991). The broader light grey areas at 90 m in both months identify the gross regions where the tropical tuna habitat is limited to the upper ocean and, therefore, vulnerable to seine gear.

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