Marie-Hélène Radenac

Variation of the western equatorial Pacific Ocean, 1986–1988

Twenty-one oceanographic sections made along 165°E during 1984–1988 provide a unique picture of the 1986–1987 El Nino and the subsequent La Nina in the western equatorial Pacific. The mean of six cruises from January 1984 through June 1986, a relatively normal period, provides a reference with which the later sections are compared. The net warm water transport across 165°E within 10° of the equator was small in this mean reference section: 7 × 106 m3 s−1 to the east. In December 1986, strong westerly winds at and to the west of 165°E increased the net eastward transport of warm water to 88 × 106 m3 s−1, and the 1986–1987 El Nino was underway. During the following 2 years the net transport varied widely and rapidly; the extrema were 56 × 106 m3 s−1 to the east and 58 × 106 m3 s−1 to the west. Changes in the stratification along 165°E were correspondingly large, reflecting both the geostrophic balance of the strong zonal currents and the changes in the volume of warm water in the western equatorial Pacific. The anomaly of warm water volume corresponded closely to the time integral of the warm water transport across 165°E. Local wind forcing and remotely forced waves were both important causes of the transport fluctuations. Winds, precipitation, and currents were all important factors determining the depth of the surface mixed layer and the thickness of the underlying barrier layer. The way in which these factors interact is a strong function of latitude.

The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.

Modeled and observed impacts of the 1997–1998 El Niño on nitrate and new production in the equatorial Pacific

The impact of the strong 1997-1998 E1Nifio event on nitrate distribution and new production in the equatorial Pacific is investigated, using a combination of satellite and in situ observations, and an ocean circulation-biogeochemical model. The general circulation model is forced with realistic wind stresses deduced from ERS-1 and ERS-2 scatterometers over the 1993-1998 period. Its outputs are used to drive a biogeochemical model where biology is parameterized as a nitrate sink. We first show that the models capture the essential circulation and biogeochemical equatorial features along with their temporal evolution during the 1997-1998 event, although the modeled variability seems underestimated. In particular, the model fails to reproduce unusual bloom conditions. This is attributed to the simplicity of the biological model. An analysis of the physical mechanisms responsible for the dramatic decrease of the biological equatorial production during E1 Nifio is then proposed. During the growth phase (November 1996 through June 1997), nitrate-poor waters of the western Pacific are advected eastward, and the vertical supply of nitrate is reduced due to nitracline deepening. These processes result in the invasion of the equatorial Pacific by nitrate-poor waters during the mature phase (November 1997 through January 1998). At that time, the central Pacific is nitrate limited and experiences warm pool oligotrophic conditions. As a result, the modeled new production over the equatorial Pacific drops by 40% compared to the mean 1993-1996 values. Then, while E1Nifio conditions are still present at the surface, the nitracline shallows over most of the basin in early 1998. Therefore the strengthening of the trade winds in May 1998 efficiently switches on the nitrate vertical supply over a large part of the equatorial Pacific, leading to a rapid return of high biological production conditions. Strong La Nifia conditions then develop, resulting in a biologically rich tongue extending as far west as 160oE for several months.

Picophytoplankton dynamics in the equatorial Pacific: Growth and grazing rates from cytometric counts

During a 7 day time series in the central equatorial Pacific (0°, 150°W, October 1994) flow cytometry measurements were performed four times per day throughout the surface layer. Cell abundance of the major algal groups, Prochlorococcus, picoeukaryotes, and Synechococcus, exhibited a well-marked diel rhythm within the mixed layer (50–60 m deep) whereas the signal became unclear below. Cell numbers were minimum at the midday or dusk stations and maximum in the middle of the night. The amplitude of the diel variations in the mixed layer, as observed, was of the order of 40% of the daily minimum and varied significantly during the time series. For each cell group the abundance variations implied that each day, cell division was rather tightly synchronized and that grazing was efficiently competing growth. Assuming that abundance variations were only due to these two processes, a simple model was designed to estimate their rates. The division rates averaged 0.53 (±0.18) d−1 for Prochlorococcus, 0.42 (±0.13) d−1 for the picoeukaryotes, and 0.56 (±0.21) d−1 for Synechococcus in the mixed layer and decreased rapidly below. The rates varied significantly along the time series for all groups whereas growth and grazing closely balanced at the day scale. The estimates compared well with those obtained using various methods during the time series and previously in the equatorial Pacific. Primary production was tentatively predicted from the growth rates. Prochlorococcus, the picoeukaryotes, and Synechococcus contributed 57%, 33%, and 10% of the picoplankton total, and the predictions were consistent with the 14C measurements during the time series.

Rossby wave and ocean color: The cells uplifting hypothesis in the South Atlantic Subtropical Convergence Zone

[1] Rossbywavessignaturesonfilteredoceancolordataare detected in the subtropical convergence zone of the South Atlantic ocean. We investigate whether these chlorophyll anomalies can be accounted for by the uplifting mechanism of phytoplanktonic cells associated with the passage of a Rossby wave. We consider vertical chlorophyll profiles exhibiting a subsurface chlorophyll maximum typical of the South Atlantic Subtropical Convergence Province. Chlorophyll remotely-sensed concentrations resulting from an academic uplifting of the three chlorophyll profiles are reconstructed with a radiative transfer model. Amplitude of the chlorophyll enrichments found with this hypothesis (0.06 mg chl.m 3 ) compares well with propagative ocean color anomalies detected in the SeaWiFS data. However, other processes such as the Rototiller effect [Siegel, 2001] or advection of chlorophyll gradients could also play a non negligible role. INDEX TERMS: 4560 Oceanography: Physical:Surface wavesandtides (1255);9325Information Related to Geographic Region: Atlantic Ocean; 4552 Oceanography:

Biophysical responses near equatorial islands in the Western Pacific Ocean during El Niño/La Niña transitions

The biological response in the western equatorial Pacific Ocean during El Nino/La Nina transitions and the underlying physical mechanisms were investigated. A chlorophyll a bloom was observed near the Gilbert Islands during the 2010 El Nino/La Nina transition, whereas no bloom was observed during the 2007 El Nino/La Nina transition. Compared to the previously observed bloom during the 1998 El Nino/La Nina transition, the 2010 bloom was weaker, lagged by 1-2 months, and was displaced eastward by similar to 200 km. Analysis suggested that the occurrence, magnitude, timing, and spatial pattern of the blooms were controlled by two factors: easterly winds in the western equatorial Pacific during the transition to La Nina and the associated island mass effect that enhanced vertical processes (upwelling and vertical mixing), and the preconditioning of the thermocline depth and barrier layer thickness by the preceding El Nino that regulated the efficiency of the vertical processes. Despite the similar strength of easterly winds in the western equatorial Pacific during the 1998 and 2010 transitions to La Nina, the 20092010 El Nino prompted a deeper thermocline and thicker barrier layer than the 1997-1998 El Nino that hampered the efficiency of the vertical processes in supplying nutrients from the thermocline to the euphotic zone, resulting in a weaker bloom.