Patterns of co-variability between physical and biological parameters in the Arabian Sea

Abstract

The relationship between physical forcing and biological response observed in the Arabian Sea for the years 1978–1986 were examined. Spatial and temporal patterns of variability in a climatological time-series of three possible physical forcing parameters and CZCS-derived phytoplankton pigment concentration during the annual cycle were quantified using single and joint empirical orthogonal function (EOF) and singular-value decomposition (SVD) analyses. Monthly composites of the NASA regional pigment data were interpolated to fill data voids and binned corresponding to the physical flux data. Nearly all the spatial-temporal analyses consistently partitioned a large portion of the variability using only 1 or 2 dominant modes and indicated a lag in the timing of the peak pigment concentration behind the maxima in physical forcing. In all cases, major modes of variability resembled the Southwest Monsoon pattern, with the Northeast Monsoon contributing very little to the total variance and covariance. The Joint EOF and SVD analyses incorporated subtle features surrounding the peak Southwest Monsoon phenomena. Correlation maps of the joint EOF analysis depicted differences in spatial variability of pigment concentration associated with stress and curl, showing areas of curl-driven upwelling distinct from coastal upwelling, with possible off-shore advection of the curl-induced high pigment waters.

Introduction

The Arabian Sea undergoes regular oscillations of strong climatic forcing in the form of semi-annual monsoons, making it unique in comparison to other ocean basins. Here, dramatic variability in atmospheric forcing drives basin-scale changes in circulation patterns and in nutrient supplies (Wyrtki, 1971; Krey and Babenerd, 1976; Conkright et al., 1994) through changes in upwelling or entrainment patterns (Currie et al., 1973; Bauer et al., 1991), cycling from a normally oligotrophic system to a eutrophic system (Smith et al., 1998). Relationships among biological, physical and chemical parameters in the Arabian Sea were the focus of the US Joint Global Ocean Flux Study (JGOFS) and Office of Naval Research (ONR) Forced Upper Ocean Dynamics program. Several authors have noted qualitative relationships between wind forcing and chlorophyll a (Currie et al., 1973; Banse and McClain, 1986; Brock et al., 1991; Brock and McClain, 1992; Bauer et al., 1991; Banse and English, 1993; McCreary et al., 1996), while others have quantified these relationships (Yentsch and Phinney, 1992; Brock et al., 1998). This study quantitatively investigates the relationship between physical processes and surface phytoplankton biomass distribution observed in the Arabian Sea basin. Our objective is to document the patterns in time and space over which chlorophyll a covaries with physical-forcing mechanisms presumed to drive primary productivity, thereby delineating the relative importance of different mechanisms in driving pigment variability. Using climatological, monthly mean composites of near-surface phytoplankton pigment concentrations derived from the Coastal Zone Color Scanner (CZCS), and wind stress, wind stress curl, and net surface heat flux for the CZCS-observing period taken from the analysis of Jones et al. (1995), patterns of variability are quantified using empirical orthogonal function (EOF) and singular-value decomposition (SVD) analyses. The EOF and SVD analyses provide a compact description of the spatial and temporal variability and covariability of the data in terms of a reduced set of orthogonal statistical modes. The focus is on the large-scale seasonal cycle, so that small-scale spatial and temporal variability will be smoothed in our analysis.

The atmospheric conditions driving the seasonal variability in the Arabian Sea are related to the onset of monsoons occurring semiannually in the basin. During the Southwest Monsoon (June–September) winds blow from the southwest Indian Ocean and are steered by the East African highlands to form a strong low level atmospheric jet, referred to as the Findlater Jet (Findlater, 1966, Findlater, 1971; Saha, 1973). During the Northeast Monsoon (December–February) winds reverse to blow from the northeast but lack the intensification found during the Southwest Monsoon. Circulation in the Arabian Sea undergoes a corresponding cyclic reversal. During the Southwest Monsoon a strong western boundary current, the Somali Current, develops along the coast of Somalia and flows northeastward (Wyrtki, 1971, Wyrtki, 1973; Bruce, 1973, Bruce, 1979, Bruce, 1983; Schott, 1983; Swallow et al., 1983). The East Arabian Current (Tomczak and Godfrey, 1994) flows to the northeast along the coasts of Yemen and Oman. Strong upwelling and eddying motions are found in the Somali Current and in the East Arabian Current, with some eddies recurring from one year to another (Cagle and Whritner, 1980; Evans and Brown, 1981; Swallow and Fieux, 1982; Simmons et al., 1988). During the Northeast Monsoon, the Somali Current reverses and flows southward across the equator. Flow in the interior of the Arabian Sea north of the equator is almost entirely westward and circulation is shallow (Wyrtki, 1973; Schott, 1983; Knox, 1987; Pickard and Emery, 1990). The cold, dry, continental winds drive surface cooling and convective overturning (Shetye et al., 1991).

The circulation affects mixed-layer dynamics and nutrient injection into the surface layers, resulting in seasonal changes in phytoplankton concentration. The different mechanisms responsible for nutrient injection (Fig. 1) include: coastal upwelling and upward Ekman pumping, which lift the nutricline into the photic zone; wind stirring and convective overturning, which cause turbulent mixing and deepen the mixed layer to entrain the nutricline; and advection, which introduces nutrients laterally. During the Southwest Monsoon, nutrient injection is a response to a combination of coastal upwelling, open-ocean upwelling, and turbulent mixing. Along the coast of Oman there is a band of coastal upwelling, which combines with a band of open-ocean upwelling found on the cyclonic side of the Findlater Jet to produce a wide upwelling band approximately 400 km off-shore, paralleling the Arabian coast for nearly 1000 km (Smith and Bottero, 1977; Bruce, 1983; Bauer et al., 1991; Brock et al., 1991, Brock et al., 1992). High wind stress during the Southwest Monsoon contributes to increase turbulent mixing (Molinari et al., 1986; Rao et al., 1989). Downwelling, caused by downward Ekman pumping resulting from convergence in the Ekman layer, occurs on the anticyclonic side of the Jet and is reflected in deeper thermocline depths (up to 100 m, versus <50 m on the near-shore side, Krey and Babenerd, 1976; Hastenrath and Grieschar, 1983). In the northern Arabian Sea, during the Northeast Monsoon, nutrients enter the photic zone primarily as a result of a deepening of the mixed layer due to convective overturning of surface waters, which results in a weaker winter bloom (Banse and McClain, 1986; McCreary and Kundu, 1989; Banse and English, 1993; McCreary et al., 1993).

The resulting surface nutrient concentrations, documented in atlases of nutrient (phosphate, nitrate, silicate) concentrations (Wyrtki, 1971; Krey and Babenerd, 1976; Conkright et al., 1994), show seasonality with respect to Monsoon and Intermonsoon periods. Generally, higher nutrients are located near the coast and concentrations decrease offshore (Wyrtki, 1971, Wyrtki, 1973; D'Souza and Sastry, 1975; Sen Gupta et al., 1976; Sen Gupta and Naqvii, 1984).

Research prior to the 1995 studies found that the distribution and seasonality of phytoplankton pigments and productivity coincided with spatial variability in circulation and mixed layer depth associated with monsoon periods both from in situ data (Kabanova, 1968; Aruga, 1973; Kimor, 1973; Krey, 1973; Krey and Babenerd, 1976; Kuz'menko, 1977; Brock et al., 1988) and from remotely sensed data (Banse and McClain, 1986; Brock et al., 1991; Brock and McClain, 1992; Yentsch and Phinney, 1992; Banse and English, 1993). Generally, the Southwest Monsoon exhibits highest rates of primary production, up to 6.0 g C m⁻² d⁻¹ along the coasts of Oman and northern Somalia and approximately 0.5 g C m⁻² d⁻¹ in the central Arabian Sea. During the Northeast Monsoon, primary production was lower than in the Southwest Monsoon and appeared to be located in the extreme northern basin (Kabanova, 1968; Aruga, 1973; Cushing, 1973; Krey, 1973; Krey and Babenerd, 1976; Kuz'menko, 1977; Owens et al., 1993). The 1995 studies revealed much less contrast in primary production between monsoon seasons.