Modeling the Frozen-In Anticyclone in the 2005 Arctic summer stratosphere

Abstract

Immediately following the breakup of the 2005 Arctic spring stratospheric vortex, a tropical air mass, characterized by low potential vorticity (PV) and high nitrous oxide (N<sub>2</sub>O), was advected poleward and became trapped in the easterly summer polar vortex. This feature, known as a "Frozen-In Anticyclone (FrIAC)", was observed in Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) data to span the potential temperature range from ~580 to 1100 K (~25 to 40 km altitude) and to persist from late March to late August 2005. This study compares MLS N<sub>2</sub>O observations with simulations from the Global Modeling Initiative (GMI) chemistry and transport model, the GEOS-5/MERRA Replay model, and the Van Leer Icosahedral Triangular Advection (VITA) isentropic transport model to elucidate the processes involved in the lifecycle of the FrIAC, which is here divided into three distinct phases. During the "spin-up phase" (March to early April), strong poleward flow resulted in a tight isolated anticyclonic vortex at ~70–90° N, marked with elevated N<sub>2</sub>O. GMI, Replay, and VITA all reliably simulated the spin-up of the FrIAC, although the GMI and Replay peak N<sub>2</sub>O values were too low. The FrIAC became trapped in the developing summer easterly flow and circulated around the polar region during the "anticyclonic phase" (early April to the end of May). During this phase, the FrIAC crossed directly over the pole between the 7 and 14 April. The VITA and Replay simulations transported the N<sub>2</sub>O anomaly intact during this crossing, in agreement with MLS, but unrealistic dispersion of the anomaly occurred in the GMI simulation due to excessive numerical mixing of the polar cap. The vortex associated with the FrIAC was apparently resistant to the weak vertical shear during the anticyclonic phase, and it thereby protected the embedded N<sub>2</sub>O anomaly from stretching. The vortex decayed in late May due to diabatic processes, leaving the N<sub>2</sub>O anomaly exposed to horizontal and vertical wind shears during the "shearing phase" (June to August). The observed lifetime of the FrIAC during this phase is consistent with timescales calculated from the ambient horizontal and vertical wind shear. Replay maintained the horizontal structure of the N<sub>2</sub>O anomaly similar to MLS well into August. The VITA simulation also captured the horizontal structure of the FrIAC during this phase, but VITA eventually developed fine-scale N<sub>2</sub>O structure not observed in MLS data

Tracer transport during the Arctic stratospheric final warming based on a 33‐year (1979‐2011) tracer equivalent latitude simulation

“…The high N 2 O near the pole persists into August, four months after the SFW. This persistence of high‐N 2 O is similar to the 2005 FrIAC, which was also observed in MLS data [ Allen et al , 2011], but the 2005 N 2 O anomaly was considerably smaller.…”

“…The vortices decay over about a month, but the accompanying tracer anomalies persist much longer. FrIACs have been identified using trace gas observations in 2003, 2005, and 2007 [ Allen et al , 2011; Lahoz et al , 2007; Manney et al , 2006; Thiéblemont et al , 2011]. Manney et al [2006]also found FrIAC‐like signatures in analyzed potential vorticity in 1982, 1994, 1997, and 2002, but there are no accompanying observations of long‐lived chemical tracers during those years Thiéblemont et al [2011]…”

The contributions of chemistry and transport to low arctic ozone in March 2011 derived from Aura MLS observations

“…High horizontal resolution was chosen to reduce the area of polar cap averaging used by the advection scheme (by a factor of 4 compared to a 2 o x 2.5 o simulation). This improvement is crucial for the accurate representation of descent over the pole; see Allen et al [2011] for an illustration of this effect. In a second GMI integration ("No Het"), the rates for heterogeneous reactions involving Cl and Br were set to zero, turning off processing by PSCs, binary and ternary solutions, and ice.…”

Poleward transport variability in the Northern Hemisphere during final stratospheric warmings simulated by CESM(WACCM)

“…This date, hereafter referred to as “turnover,” is defined as the day when the zonal mean zonal wind at 10 hPa and 60°N becomes irreversibly easterly. Although several definitions of the spring dynamical transition have been proposed in the literature [e.g., Wei et al ., ], the one selected here is particularly relevant for FrIAC detection since their persistence is modulated by the background wind and favored by summer easterlies [e.g., Allen et al ., ].…”

“…Observed FrIACs generally occur under strong and abrupt SFWs which are associated to a strong enhancement of planetary wave activity [e.g., Thiéblemont et al ., ]. The detailed modeling studies of the 2005 [ Allen et al ., ] and 2011 [ Allen et al ., ] FrIACs provided a thorough description of the FrIAC life cycle. This can be divided into three phases: (1) a “spin‐up” phase in which the FrIAC develops and is advected to the polar region, (2) an “anticyclonic” phase in which the FrIAC is trapped in the developing summer easterlies and resists the weak vertical wind shear (April–May), and finally (3) a “shearing” phase in which the anticyclonic anomaly decays due to diabatic processes and the tracer anomaly is sheared out by the background wind (beginning by the end of May).…”