A strong summer phytoplankton bloom southeast of Vietnam in 2007, a transitional year from El Niño to La Niña

Abstract

Summer upwelling occurs frequently off the southeast Vietnam coast in the western South China Sea (SCS), where summer phytoplankton blooms generally appear during June-August. In this study, we investigate inter-annual variation of Ekman pumping and offshore transport, and its modulation on summer blooms southeast of Vietnam. The results indicate that there are low intensities of summer blooms in El Niño years, under higher sea surface temperatures (SST) and weaker winds. However, a different pattern of monthly chlorophyll a (Chl-a) blooms occurred in summer of 2007, a transitional stage from El Niño to La Niña, with weak (strong) wind and high (low) SST before (after) early July. There is a weak phytoplankton bloom before July 2007 and a strong phytoplankton bloom after July 2007. The abrupt change in the wind intensity may enhance the upwelling associated with Ekman pumping and offshore Ekman transport, bringing more high-nutrient water into the upper layer from the subsurface, and thus leading to an evident Chl-a bloom in the region.

Fig 1. Location of the study area in the South China Sea.

Box A is the upwelling area southeast of Vietnam.

Fig 2. Spatial distribution of summer Chl-a concentration in the SCS.

(A) Summer Chl-a (averaged for 1 June—31 August), for 1998 A(i), 2007 A(ii) and 1998–2014 A(iii); (B) and (C) Chl-a averaged for 15 days during summer of 2007 and climatology of 1998–2014, respectively.

Fig 3

(A) The first EOF modes of SST. A(i) is the spatial field and A(ii) is the time series; (B) The first EOF modes of Chl-a. B(i) is the spatial field and B(ii) is the time series.

Fig 4. Spatial distribution of SST in the western SCS.

(A) Summer SST (averaged for 1 June—31 August), A(i):1998; A(ii): 2007;A(iii): 1998–2014; Time series of SST averaged for 15 days during summer of 2007 (B) and climatology of 1998–2014 (C), respectively.

Fig 5. Spatial distribution of Ekman pumping velocity in the western SCS.

(A) Summer Ekman pumping velocity (averaged for 1 June—31 August), A(i):1998; A(ii): 2007;A(iii): 1998–2014; (B) and (C) Ekman pumping velocity averaged for 15 days during summer of 2007 and climatology of 1998–2014, respectively.

Fig 6. Spatial distribution of precipitation in the western SCS.

(A) Summer precipitation (averaged for 1 June—31 August), A(i):1998; A(ii): 2007, A(iii): 1998–2014; (B) and (C) precipitation averaged for fifteen days during the summer of 2007 and the summer climatology of 1998–2014, respectively.

Fig 7. Time series data sample west of the SCS for summer (1 June—31 August) for the period from 1998 to 2014.

(A) SST and EPV; (B) Chl-a and EPV; (C) SST and ET; (D) Chl-a and ET.

Fig 8. Fifteen-day averaged wind filed (V for speed and Vd for direction), sea level anomaly meridional velocity (V for speed) and SST for Box A during summer in 1998, 2007 and climatology.