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

Abstract

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 quasilinear 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.

Oxygen Variability During ENSO in the Tropical South Eastern Pacific

“…This flux reduction was partly driven by the reduction of the vertical velocity (Figure 7d), which counteracted the impact of the subsurface oxygen increase (Figure 7c). Note that the modeled vertical velocity decrease (∼40% at 50 m depth, Figure 7d) cannot be fully attributed to a decreased Ekman pumping during EN (e.g., Halpern, 2002;Chamorro et al, 2018, for the 1997-1998, as it was only ∼5% in our simulation (Figure not shown).…”

“…During EN, when the interannual wind variability is suppressed (in Wclim), a relatively weak subsurface oxygen increase is observed. This can be explained as follows: as the alongshore wind increases during EN (see Figure 9 in Enfield, 1981;Espinoza-Morriberón et al, 2017;Chamorro et al, 2018), the coastal upwelling intensifies. This partly compensates the ZO2 deepening, bringing deep oxygen-poor waters in this depth range.…”

Forcings and Evolution of the 2017 Coastal El Niño Off Northern Peru and Ecuador

“…A comparable wind stress curl anomaly developed during the 1997-1998 El Niño off southern Peru near Pisco (15 • S), enhancing the downwelling primarily driven by the very intense downwelling coastal trapped waves (Halpern, 2002). The dynamical processes responsible for the 1997-1998 nearshore wind increase were recently analyzed by Chamorro et al (2018). Using a regional atmospheric model, the authors showed that the increase of nearshore wind was driven by the SST alongshore gradient, associated with the accumulation of warm waters along the northern shores of Peru.…”

“…During February and March, the Ekman pumping negative anomaly associated with the wind stress curl (Figure 2E) may have deepened the thermocline, and thus the cooling due to enhanced upwelling at the coast may have been mitigated by the upward flux of anomalously warm source waters. The atmospheric response to the SST gradient, described by Chamorro et al (2018) for the 1997-1998 El Niño, may thus feedback positively on the warming initiated in January. The ocean-atmosphere coupled processes occurring during this event would need to be more fully investigated using a regional coupled ocean-atmosphere model (Oerder et al, 2016(Oerder et al, , 2018.…”

Valley-mountain circulation associated with precipitation formation in the tropical Andes (Rio Santa Basin)

“…A.2), both identified in previous in the latitude of 15°S (e.g. Belmadani et al, 2014;Chamorro et al, 2018) as part of the Pacific coastal temperature inversion. In this way, the easterly flow at higher levels does not have a direct influence on coastal circulation.…”