A Lagrangian Analysis of the Northern Hemisphere Subtropical Jet

Abstract

The North Atlantic jet stream during winter 2010 was unusually zonal, so the typically separated Atlantic and African jets were merged into one zonal jet. Moreover, the latitude-height structure and temporal variability of the North Atlantic jet during this winter were more characteristic of the North Pacific. This work examines the possibility of a flow regime change from an eddy-driven to a mixed eddy-thermally driven jet. A monthly jet zonality index is defined, which shows that a persistent merged jet state has occurred in the past, both at the end of the 1960s and during a few sporadic months. The anomalously zonal jet is found to be associated with anomalous tropical Pacific diabatic heating and eddy anomalies similar to those found during a negative North Atlantic Oscillation (NAO) state. A Lagrangian back-trajectory diagnosis of eight winters suggests the tropical Pacific is a source of momentum to the Atlantic and African jets and that this source was stronger during the winter of 2010. The results suggest that the combination of weak eddy variance and fluxes in the North Atlantic, along with strong tropical heating, act to push the jet toward a merged eddy-thermally driven state. The authors also find significant SST anomalies in the North Atlantic, which reinforce the anomalous zonal winds, particularly in the eastern Atlantic.

Suppressed eddy driving during southward excursions of the North Atlantic jet on synoptic to seasonal time scales

“…The tropical Pacific iT anomalies are stronger for the most persistent S-jet events (Figure 5b), and quite stable overall, but slightly weaker at leads of 30 days ( Figure S3). This enhanced convection upstream in the tropical Pacific has been proposed to act as a source of momentum for the North Atlantic jet downstream (Harnik et al, 2014;Martius, 2014). Note that iT signals in the tropical Atlantic are quite weak ( Figure 5), suggesting that changes in "local" thermal driving are not instrumental for the S-jet configuration (Li and Wettstein, 2012), although they do seem to affect the zonal extent of the African jet (e.g., weaker Atlantic iT is associated with a retracted African jet in Figure 4a, black contours; see also Li and Wettstein (2012), their Figure 2).…”

Recent Trends in the Waviness of the Northern Hemisphere Wintertime Polar and Subtropical Jets

“…Equally unknown in the wake of the present analysis is the role of the tropics in forcing the observed increased waviness of the subtropical jet. A recent analysis of the subtropical jet by Martius (2014) considered the interaction of the tropics and extratropics with the jet from the perspective of trajectory analysis. She showed that air parcels that ended up in the jet over Africa (East Asia/western Pacific) ascended over South America (Indian Ocean and the Maritime Continent) before following an anticyclonic path toward the jet.…”

Quantifying the dynamics of the interannual variabilities of the wintertime East Asian Jet Core

“…13a). Thus, the convective heating over the Bay of Bengal-South China Sea, in addition to the climatology of the East Asian jet (Martius 2014), is also crucial for the interannual variability of the jet strength. Our finding shows that, in addition to the tropical heating induced Rossby wave train as suggested in Zheng et al (2013) and Yang et al (2010), heating over the Bay of Bengal to the equatorial Western Pacific can affect the winter climate of East Asia more directly through a thermally driven local Hadley Cell.…”