Time series of upwelling index in Northern South China Sea (1967-2013)

Published: 17-08-2016| Version 3 | DOI: 10.17632/jhcpn74p8y.3
Contributor:
Anthony banyouko Ndah

Description

Monthly and hourly upwelling indices have been derived for four locations on the northern/north-eastern coastline of the South China Sea (SCS) (1967-2013) using the standard NOAA-PFEL method. The study locations include: off S.W Taiwan, Off Hanoi, off Fujian, Off Guangdong. Moreover, annual averages and annual anomalies have been calculated for the entire 47-year period for all four locations. This is the first comprehensive and lengthy time-series of upwelling indices in the South China Sea which can enhance understanding of the changes in phenomenon over inter-annual to decadal and multi-decadal time scales, as well as allowing for a comparison of the occurrence of the patterns and magnitude of the process across multiple coastal locations. Acknowledgement: Special appreciation on the development of this dataset goes to Ms. Lynn Dewitt, "Environmental Research Division, Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA".

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The Pacific Fisheries Environmental Laboratory (PFEL) of NOAA’s Pacific Fisheries Environmental Group (PFEG) 1-degree Upwelling Index method is used to calculate a time series (monthly and 6-hourly) of upwelling indices in four coastal zones of Northern South China Sea (NSCS). In the first step, synoptic pressure data are obtained from FNMOC (Fleet Numerical Meteorology and Oceanography Center), from which geostrophic winds and sea surface circulation are approximated. Sea surface wind-stress (Tx) is calculated from geostrophic winds as follows: Tx = Pa .Cd .√(u^2+v)2. u Tx = Pa .Cd .√(u^2+v)2. v Where: ρa is the air density u and v are the wind speed components near the sea surface Cd is the empirical drag coefficient Next, the direction of ocean water flow is estimated from the wind-stress component, the coriolis factor and the water density as follows: Qx = ty/(f.p).103 Qy = (-tx)/(f.p).103 Where: Qx, Qy are the ocean-ward flows in the West-East and North-South directions respectively tx, ty are the wind stress components estimated from the wind speed f is the Coriolis factor ρ is the sea density Ekman transport is pre-calculated from the atmospheric pressure data provided by FNMOC (Fleet Numerical Meteorology and Oceanography Center). Ekman Transport, M is calculated as follows: M = 1/f τ x k Where: k is the unit vector directed vertically upward The upwelling index is calculated from the offshore component of Ekman transport on a 1-degree grid as follows: Mx = Ty/f Where: x is normal and y parallel to the local coastal orientation; these are then reversed to reflect negative (offshore) Ekman transport which leads to positive vertical transport (upwelling) and positive (onshore) Ekman transport which results in negative vertical transport (down-welling). Finally, Upwelling index is calculated from the offshore component of Ekman transport. The following is the part of the Ferret script used for the calculation of the upwelling index: let coast_angle = some_angle_in_degrees (where coast angle is defined as: let pi = 3.1415927 let degtorad = pi/180. let alpha = (360-coast_angle)*degtorad let s1 = cos(alpha) let t1 = sin(alpha) let s2 = -1*t1 let t2 = s1 let perp = s1*ektrx+t1*ektry let para = s2*ektrx+t2*ektry Where: ektrx and ektry are the x- and y- components of Ekman transport perp and para are the rotated components perpendicular and parallel respectively to the coastline Upwelling index [m^3s/100 m of coastline] will therefore be equivalent to: perp/10