Coastal Upwelling Front Detection off Central Chile (36.5–37°S) and Spatio-Temporal Variability of Frontal Characteristics

Vera Oerder,Joaquim Bento,Samuel Hormazabal,Carmen Morales,Oscar Pizarro

doi:10.3390/rs10050690

Abstract

In Eastern Boundary Upwelling Systems, cold coastal waters are separated from offshore by a strong cross-shore Sea Surface Temperature (SST) gradient zone. This upwelling front plays a major role for the coastal ecosystem. This paper proposes a method to automatically identify the front and define its main characteristics (position, width, and intensity) from high resolution data. The spatio-temporal variability of the front characteristics is then analyzed in a region off Central Chile (37°S), from 2003 to 2016. The front is defined on daily 1 km-resolution SST maps by isotherm T0 with T0 computed from mean SST with respect to the distance from the coast. The probability of detecting a front, as well as the front width and intensity are driven by coastal wind conditions and increased over the 2007–2016 period compared to the 2003–2006 period. The front position, highly variable, is related to the coastal jet configuration and does not depend on the atmospheric forcing. This study shows an increase by 14% in the probability of detecting a front and also an intensification by 17% of the cross-front SST difference over the last 14 years. No trend was found in the front position.

Highlights

In the main Eastern Boundary Current Systems (EBUS: California, Humboldt, Canarias, and Benguela), upwelling forced by alongshore equatorward winds usually creates a surface cold tongue in the coastal band
Wind conditions are upwelling-favorable during September–April, the imprint of this forcing on the Sea Surface Temperature (SST) is from November to June
Some arbitrary parameters may need to be adjusted, the method is general enough to be applied to different Eastern Boundary Upwelling Systems

Summary

Introduction

In the main Eastern Boundary Current Systems (EBUS: California, Humboldt, Canarias, and Benguela), upwelling forced by alongshore equatorward winds usually creates a surface cold tongue in the coastal band This cold tongue is limited offshore by a strong cross-shore density gradient zone, referred to as the upwelling front (e.g., [1,2,3,4]). Such front separates the nutrient-rich shelf waters nearshore from the lower nutrient water masses in the oceanic area or Coastal Transition Zone (CTZ) (e.g., [5,6,7]). Off Central Chile, the SST gradient intensity over the front computed from seasonal means was found to be ∼0.01 ◦C km−1, whereas weekly SST gradients can be one order of magnitude higher, ∼0.1–0.25 ◦C km−1 [7]

Objectives

Methods

Results

Discussion

Conclusion