The influence of sea surface temperature characteristics on downstream atmospheric blocking from a numerical model perspective

Accurately predicting blocked weather regimes in the Euro-Atlantic region remains challenging despite their relevance for extreme weather events. Previous studies have suggested a connection between the misrepresentation of blocking events in numerical weather prediction models and sea surface temperature (SST) biases in the Gulf Stream region. However, the pathway linking SST in the Gulf Stream region to the downstream upper-level flow remains not fully understood.

Here, we employ numerical sensitivity experiments with varying SST conditions for an atmospheric blocking event associated with a winter heat wave to enhance our physical understanding of the relationship between Gulf Stream SST and downstream atmospheric blocking. The blocking event was preceded and accompanied by several extratropical cyclones, whose associated rapidly ascending air streams, so-called warm conveyor belts (WCBs), played a key role in the development and amplification of the downstream upper-level ridge.

Our sensitivity experiments, which include idealized and weakened SST gradients in the Gulf Stream region, show that the SST gradient influences moisture availability in the WCB inflow region through modified air-sea interactions and, consequently, WCB ascent. For example, in our case study a stronger SST gradient leads to increased specific humidity in the lower troposphere, resulting in more pronounced WCB ascent, increased cross-isentropic ascent as well as increased WCB outflow heights, which subsequently strengthens the downstream ridge. We conclude that variations in SST representation can affect the large-scale atmospheric flow via the WCB airstream as mechanistic link between lower and upper troposphere. In particular, low-level moisture availability - influenced by SST characteristics - affects subsequent WCB ascent and outflow characteristics, which in turn modulate the downstream upper-level ridge. Moreover, available low-level moisture consistently modifies WCB characteristics throughout the inflow, ascent, and outflow phases.