Spatial and temporal variability of interhemispheric transport times

Abstract

Abstract. The seasonal and interannual variability of transport times from the northern midlatitude surface into the Southern Hemisphere is examined using simulations of three idealized "age" tracers: an ideal age tracer that yields the mean transit time from northern midlatitudes and two tracers with uniform 50-and 5-day decay. For all tracers the largest seasonal and interannual variability occurs near the surface within the tropics and is generally closely coupled to movement of the Intertropical Convergence Zone (ITCZ). There are, however, notable differences in variability between the different tracers. The largest seasonal and interannual variability in the mean age is generally confined to latitudes spanning the ITCZ, with very weak variability in the southern extratropics. In contrast, for tracers subject to spatially uniform exponential loss the peak variability tends to be south of the ITCZ, and there is a smaller contrast between tropical and extratropical variability. These differences in variability occur because the distribution of transit times from northern midlatitudes is very broad and tracers with more rapid loss are more sensitive to changes in fast transit times than the mean age tracer. These simulations suggest that the seasonalinterannual variability in the southern extratropics of trace gases with predominantly NH midlatitude sources may differ depending on the gases' chemical lifetimes.

Occurrence and seasonal variability of Dense Shelf Water Cascades along Australian continental shelves

“…As a result, differences in density, estuaries are characterized by longitudinal density gradients that may drive residual circulation with denser water flowing beneath less dense water under the influence of gravity 9 . In continental shelf regions similar processes operate and recent studies have documented this process, termed Dense Shelf Water Cascade (DSWC) from both field measurements 5,13,30,[39][40][41] and numerical model simulations 29,42,43 . These studies were limited to specific locations and did not examine the seasonal variability and its forcing mechanisms.…”

“…Coastal embayments that have inverse estuarine behaviour: Shark Bay, Spencer Gulf www.nature.com/scientificreports www.nature.com/scientificreports/ and the upper Gulf of California have values of −20, −5 and −6 × 10 -5 kgm −4 , respectively. In the mid-Atlantic Bight ρ ∂ ∂x = −2 × 10 -5 kgm −4 and was associated with winter cooling 43 . Thus although ρ…”

Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

“…This paper examined processes at a smaller scale (≈200 km) resolving finer scale features. A recent study by Wu et al [23] examined the effects of surface cooling along the mid-Atlantic Bight in the formation of DSWCs but was limited to a 2-D cross-sectional transect.…”

“…During the summer, coastal heating and evaporation result in a band of warmer, higher salinity water in the coastal boundary layer [21], which establishes a horizontal density gradient that is reinforced when cooling occurs during autumn and winter. This cross-shelf density gradient is crucial to the cross-shelf transport over the inner continental shelf [22,23]. May and June are typically when the horizontal density gradient reaches its maximum and DSWCs have been observed frequently during this time period [2,5].…”

Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell

“…The concentration of NH50 (χ 50 ) in the bottom model level is specified as a fixed mixing ratio (i.e., 100 ppbv) over the NH midlatitude region (30-50 • N, 180-180 • W). Wu et al (2018) have shown the spatial distribution of NH50, particularly its interhemispheric gradient, is strongly associated with the seasonal shift of the Intertropical Convergence Zone (ITCZ) and also likely the Southern Pacific Convergence Zone (SPCZ) over the oceans. As noted in Sect.…”