Mechanisms of the intensification of the upwelling-favorable winds during El Nino 1997-1998 in the Peruvian upwelling system

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Chamorro, A
Colas, F
Echevin, V
Oerder, V
Quispe-Ccalluari, C
Tam, J
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Springer Nature
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The physical processes driving the wind intensification in a coastal band of ~ 100 km off Peru during the intense 1997–1998 El Niño (EN) event were studied using a regional atmospheric model. A simulation performed for the period 1994–2000 reproduced the coastal wind response to local sea surface temperature (SST) forcing and large scale atmospheric conditions. The model, evaluated with satellite data, represented well the intensity, seasonal and interannual variability of alongshore (i.e. NW–SE) winds. An alongshore momentum budget showed that the pressure gradient was the dominant force driving the surface wind acceleration. The pressure gradient tended to accelerate the coastal wind, while turbulent vertical mixing decelerated it. A quasi-linear relation between surface wind and pressure gradient anomalies was found. Alongshore pressure gradient anomalies were caused by a greater increase in near-surface air temperature off the northern coast than off the southern coast, associated with the inhomogeneous SST warming. Vertical profiles of wind, mixing coefficient, and momentum trends showed that the surface wind intensification was not caused by the increase of turbulence in the planetary boundary layer. Moreover, the temperature inversion in the vertical mitigated the development of pressure gradient due to air convection during part of the event. Sensitivity experiments allowed to isolate the respective impacts of the local SST forcing and large scale condition on the coastal wind intensification. It was primarily driven by the local SST forcing whereas large scale variability associated with the South Pacific Anticyclone modulated its effects. Examination of other EN events using reanalysis data confirmed that intensifications of alongshore wind off Peru were associated with SST alongshore gradient anomalies, as during the 1997–1998 event.
This research is part of A. Chamorro’s PhD thesis, funded by the fellowship from CIENCIACTIVA/CONCYTEC-PERU at the University Pierre and Marie CURIE of Paris, France. It is also part of the IDB project PE-G1001/PE-T1297 (Adaptation to Climate Change of the Fishery Sector and Marine-Coastal Ecosystem of Peru), and it is a contribution to the cooperative agreement between the Instituto del Mar del Perú (IMARPE) and the Institut de Recherche pour le Developpement (IRD), the LMI DISCOH and JEAI EMACEP. The simulations were performed on the supercomputer Curie from the GENCI at the CEA (projects x2014011140, x2015011140 and x2016011140). Francois Pinsard and Sebastien Masson are acknowledged for their help in the making of lateral boundary forcing for the regional atmospheric model. Francis Codron, Clémentine Junquas, and lab colleagues from LMOECC at IMARPE are acknowledged for useful discussions.
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Peruvian upwelling system