Spatial variation of future trends in Atlantic upwelling cells from two CMIP6 models

TL;DR

This study uses CMIP6 models to analyze the spatial and vertical variations of future Atlantic upwelling cells, highlighting regional differences and potential poleward shifts under high-emission scenarios.

Contribution

It introduces a direct assessment method based on vertical velocities to study upwelling, providing new insights into depth and spatial structures of upwelling cells in climate models.

Findings

Vertical transport index aligns with wind-derived Ekman index.

Different subregions show distinct depth and distance characteristics.

Potential poleward shift of upwelling regions under future scenarios.

Abstract

Eastern Boundary Upwelling Systems (EBUS) are characterized by wind-triggered upwelling of deep waters along the coast. They are hotspots of biological productivity and diversity and therefore have a high economic, ecological and social importance. In the past, different methods using surface data have been used to estimate upwelling. Recently, the IPCC has suggested directly assessing vertical velocities as a promising method. We use this method to study the two Atlantic EBUS from CMIP6 models from the HadGEM3-GC3.1 and the CNRM6-CM6 family, for both the historical period and a high-emission future scenario with spatial resolutions in the ocean component ranging from 1{\deg}to 1/12{\deg}. The two major upwelling regions are divided in subregions depending on their seasonality. The vertical transport index shows similar values to a wind-derived Ekman index. Directly evaluating upwelling from transport processes further provides information about the depth of the upwelling, which has previously been identified as an important factor for nutrient availability. We show that depending on the subregion of the upwelling system, different cell structures can be seen in terms of depth and distance to the coast of maximum velocities. When looking at possible future changes, high interannual variability limits the significance of the trends but could indicate a poleward shift of the upwelling regions.A detailed comparison of the spatial structures and the distinction in subregions is important to explain contradictory trends in previous works.