Contributions of Internal Variability and External Forcing to the Recent Trends in the Southeastern Pacific and Peru–Chile Upwelling System

Abstract

In a warming world context, Sea Surface Temperature (SST) off central-south Peru, northern Chile and further offshore increases at a slower rate than the global average since several decades, i.e. cools, relative to the global average. This tendency is synchronous with an Interdecadal Pacific Oscillation (IPO) negative trend since ∼1980, which has a cooling signature in the southeastern Pacific. Here, we use a large ensemble of historical coupled model simulations to investigate the relative roles of internal variability (and in particular the IPO) and external forcing in driving this relative regional cooling, and the associated mechanisms. The ensemble-mean reproduces the relative cooling, in response to an externally-forced southerly wind anomaly, which strengthens the upwelling off Chile in recent decades. This southerly wind anomaly results from the poleward expansion of the Southern Hemisphere Hadley Cell. Attribution experiments reveal that this poleward expansion and the resulting enhanced upwelling mostly occur in response to increasing greenhouse gases and stratospheric ozone depletion since ∼1980. An oceanic heat budget confirms that the wind-forced upwelling enhancement dominates the relative cooling near the coast. In contrast, a wind-forced deepening of the mixed layer drives the offshore cooling. While internal variability contributes to the spread of tendencies, the ensemble-mean relative cooling in the southeastern Pacific is consistent with observations and occurs irrespectively of the IPO phase, hence indicating the preeminent role of external forcing.

Causes of the long-term variability of southwestern South America precipitation in the IPSL-CM6A-LR model

“…Simulated forced changes in these two variability modes act on SWSA precipitation in the same direction and respond to the same components of external forcings. Both modes react more strongly to external forcing in DJF than in other seasons principally in response to the stratospheric ozone depletion and, in second place, to the GHGs, in agreement with previous studies (e.g., Polvani et al, 2011;Kim et al, 2017;Jebri et al, 2020). In the rest of the seasons, the ozone depletion effect is weaker and the GHG forcing prevails.…”

“…During the positive (negative) phase, the IPO is characterized by an ENSO-like pattern of warm (cold) SST anomalies across the tropical Pacific, which extends in the subtropics over the eastern boundaries of the Pacific Ocean (Trenberth and Hurrell, 1994;Meehl et al, 2009). The IPO has experienced a trend from a positive (i.e., El Niño-like) to a negative (La Niña-like) phase over the 1980-2014 period associated with an anomalous southward shift and spin-up of the southeastern Pacific anticyclone (Jebri et al, 2020) and of the mid-latitudinal storm-tracks over SWSA during the rainy season (Quintana and Aceituno, 2012;Boisier et al, 2016Boisier et al, , 2018.Thus, the concurrent IPO shift from positive to negative phase might have contributed to the current prevailing SWSA dry conditions (Masiokas et al, 2010;Quintana and Aceituno, 2012;Boisier et al, 2016). However, during DJF and MAM seasons, rainfall interannual-to-decadal fluctuations are also significantly influenced by the dominant mode of atmospheric circulation variability in the mid-latitudes of the SH, the Southern Annual Mode (SAM), also known as the Antarctic Oscillation (Gong and Wang, 1999;.…”

“…Observed southward shifts of the subtropical drying regions and mid-latitudes baroclinic eddies during late austral spring and summer have been associated with the SAM positive trend and the expansion of the HC in recent decades (e.g., Gillett and Thompson, 2003;Previdi and Liepert, 2007;Quintana and Aceituno, 2012;Boisier et al, 2018). The HC expansion and SAM positive trends have both been attributed to increasing atmospheric greenhouse gases (GHGs) concentration and stratospheric ozone depletion (e.g., Polvani et al, 2011;Kim et al, 2017;Jebri et al, 2020). However, the tendency toward more La Niña events in relation to the recent IPO trend may also contribute to the HC poleward shift (Nguyen et al, 2013;Allen and Kovilakam, 2017) and to more frequent positive SAM phases (Carvalho et al, 2005;Fogt et al, 2012).…”

Projection of upwelling-favorable winds in the Peruvian upwelling system under the RCP8.5 scenario using a high-resolution regional model

“…strengthening of the alongshore SST gradient) could have forced a coastal wind intensification, which in turn would force a coastal cooling. So, this may imply a negative feedback on the SST warming off the coast (Echevin et al, 2020) and a positive feedback on coastal upwelling under climate change. However the strength of these feedbacks remain to be investigated.…”

“…Using the ensemble mean of a large ensemble of coupled model simulations (e.g. 30) reduces the uncertainty caused by the low-frequency variability and highlights the impact of external greenhouse gas forcing (Jebri et al, 2020), even in the case of a relatively short integration period (e.g. ten years), regardless of the selected control period (Kawase et al, 2009).…”