The importance of the transient variability for the warm-season Gulf Stream convergence zone

The Gulf Stream anchors a notable time-mean convergence zone during warm seasons, which produces strong precipitation and huge latent heat release in the free atmosphere. How the Gulf Stream front drives transient winds resulting in the convergence zone remains intensely debated. The oceanic-front-induced convergences are masked by strong synoptic disturbances. By synthesizing the 10-years high-frequency samplings by QuikSCAT, this study develops a novel procedure to isolate the subtle but climatically important convergence response to the oceanic front from the energetic synoptic disturbances. The binned convergence exhibits an evident cosine dependence on the angles between sea surface temperature (SST) gradients and transient winds and reaches its maximum when the winds deceleratingly blow down SST gradients, consistent with the so-called vertical mixing mechanism. The transient surface winds accelerate (decelerate) due to the enhanced (reduced) downward momentum transfer over the mesoscale warm (cold) waters. Thus, stronger down-SST-gradient winds cause stronger convergences over shaper SST gradients. The linear relationship between convergence and wind advection over SST gradients significantly contributes to the time-mean Gulf Stream convergence zone. The increased boundary-layer moisture acts to strengthen the SST-front-induced convergence and shifts convergence towards the Gulf Stream axis. As the synoptic disturbances frequent over the Gulf Stream, this study provides a potential pathway that the mesoscale SST gradients modulate synoptic disturbances through driving the surface wind convergences.