The Influence of the Indonesian Throughflow on the Eastern Pacific Biogeochimical Conditions

Fig.1 The last year of the two runs is used to force offline the biogeochimical model PISCES [Aumont et al., 2001] for 20 years. PISCES is a 2 phytoplankton, 2 zooplankton compartment model (Fig.1). There are 5 colimitations to the growth rate of the 2 phytoplanktons: nitrate, ammonium, phosphate, silicate and iron.

Most tropical models used to document the biogeochimical conditions of the Pacific basin run with closed boundaries [Chail et al, 1996, Christian et al, 2002]. But this approach neglects the effect of boundaries, and in particular the Indonesian Throughflow (IT). In this study, our objective is to document and understand the impacts of closing the IT on the mean state of a few major biogeochimical fields.

Fig.2 In the panel above, the first column shows climatological Levitus data [Boyer & Levitus, 1997] and NODC chlorophyl data, the second column presents the model result in the « IT open » configuration. The validation in sea surface temperature (SST) shows that the warm-pool is warmer in our model than in the data. For nitrate, the model shows lower concentration. The latter bias is mainly related to the known weakness of the NCEP winds which result in a too deep thermocline and nutricline. The vertical profile of temperature at the equator shows that the thermocline is also too diffuse in the model a common bias in ocean model.

Fig.3 The difference in SST shows a warming up to 0.5°C on the equator and up to 1°C near the South-American coast, when the IT is closed mainly due to the deepening of the thermocline (Fig.4). This feature is well documented [e.g Lee et al., 2001]. The associated nitracline deepening explains the weakening in surface nitrate in the eastern Pacific. There is also a strong increase in nitrate concentration near the South American coast between 8S and 15S that remains to be understood but is related to the model biased vertical structure (Fig. 4). The depletion of iron in the central to eastern Pacific follows the same mechanisms but is more strongly modulated by the surface ecosystem. To the west, iron increases but chlorophyll is not significantly changed or decreased (Figure 4) while in the east, chlorophyll decreases more strongly. Further studies will assess the dynamical-biological balances and nutrient co-limitations that control this . The low chlorophyll at the nitrate region border could be related to the shift of the nitrate-rich regions.

Fig.5 The comparison in term of nitrate of the water coming respectively from the mindanao, NGUC and the EUC shows a significant bias in the model. The nitrate concentration of model NGUC is too weak (2 to 4 times weaker than levitus) in the range of density of the EUC core. The characteristics in nitrate of mindanao and NGUC current are similar, unlike in observations. In the control run, the nitrate concentration at 165°E in the EUC which should be close to the nitrate concentration in the NGUC (Levitus) is actually close to the nitrate concentrations of the Mindanao waters. Therefore, the nitrate alimentation of the EUC is not correct. In the closed IT, no major differences in term of nitrate input in the EUC are observed even though the north to south transport ratios are inversed. This again points out to the previous deficiency.

Gorgues, T; Menkes, C; Rodgers, K; Aumont, O; Madec, G; Ludicone, D; Vialard, J; Dandonneau, Y; Murray, J.W.

Fig.4 Vertical equatorial sections for the experiments. Nitrate display a strong bias near South America that does not appear in Iron and requires further investigating. In the west, there is also a decoupling between the nitrate and iron behavior in the upper layer that is not related to the biology. Actually in our model, rivers are source of iron, but not of nitrate. Deeper, around 300m, there is a dipole in nitrate and in iron. These patterns are related to the modification of the coastal NGUC and Mindanao current systems.

Closing the IT, which transports 11Sv in our model, has two major effects. First an equatorial thermocline and nutricline deepening through wave adjustment [Hirst and Godfrey, 1993, Lee et al., 2001] within a year. Second, a changing of the ratio between the water coming from the northern hemisphere (NH) and from the southern hemisphere (SH) into the equatorial undercurrent on longer time scales. With the opened IT, almost 2/3 of the waters come from the SH and with the closed IT, 2/3 of the waters come from the NH. Our model gives for IT open: 13. Sv SH, 8.2 NH and for IT close: 8. Sv SH, 12.7 NH.

The OGCM used in this study is a global configuration of the OPA model [Madec et al., 1998]. The simulation was forced by climatological fluxes taken from the NCEP reanalysis [Kalnay et al, 1996]. The dynamical model was run for 100 years for both control and closed IT.

Black:NUGCRed:EUC, 165EBlue:Mindanao

Closing the IT induces a significant depletion of nitrate, iron and chlorophyll in the upwelling region. This is mainly induced by deepening due to wave adustment. Second, the alimentation of the EUC is changed which should change the biochemical concentrations in the model but does not in our case. Obviously, the model is poor in setting the biogeochemical conditions of the EUC in the west. Understanding the model deficiencies will help understanding the role of the western Pacific sources for the eastern Pacific conditions.

LODYC, Paris, France, e-mail:tglod@lodyc.jussieu.frEGU 2004