Eric R. Nash

An objective determination of the polar vortex using Ertel's potential vorticity

We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertels potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s -1 . We use 1992-1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.

Persistence of the lower stratospheric polar vortices

The persistence of the Arctic and Antarctic lower stratospheric vortices is examined over the period from 1958 to 1999. Three different vortex-following diagnostics (two using potential vorticity and one based solely on the zonal winds) are compared and are shown to give very similar results for the breakup date. The variability in the timing of the breakup of both vortices is qualitatively the same: There are large interannual variations together with smaller decadal-scale variations and there is a significant increase in the persistence since the mid-1980s (all variations are larger for the Arctic vortex). Also, in both hemispheres, there is a high correlation between the persistence and the strength and coldness of the spring vortex, with all quantities having the same interannual and decadal variability. However, there is no such correlation between the persistence and the characteristics of the midwinter vortex. In the Northern Hemisphere, there is also a high correlation between the vortex persistence and the upper tropospheric/lower stratospheric eddy heat flux averaged over the 2 months prior to the breakup. This indicates that the variability in the wave activity entering the stratosphere over late winter to early spring plays a key role in the variability of the Arctic vortex persistence (and spring polar temperatures) on both interannual and decadal timescales. However, the extreme values of Arctic vortex coldness and persistence during the 1990s are not echoed as a similar extreme in the eddy heat flux. This suggests that the recent increase in vortex persistence is not solely due to changes in the wave activity entering the stratosphere.

Meteorology of the polar vortex: Spring 1997

The 1996–1997 northern hemisphere spring polar vortex was very strong, cold, and symmetric, somewhat similar to those found in the Antarctic spring vortex. The vortex did not form until late in December and was very symmetric from February into late April. The spring vortex was characterized by record low temperatures, record low ozone amounts as measured from the Total Ozone Mapping Spectrometer (TOMS) instruments, and a wide band of strong winds in the lower stratosphere. Spring wave activity was greatly reduced, with 100 hPa February–March eddy heat fluxes that were lower by a factor of 2 from any previously observed values over the last 18 years.

When will the Antarctic ozone hole recover

The Antarctic ozone hole develops each year and culminates by early spring (late September - early October). Antarctic ozone values have been monitored since 1979 using satellite observations from the TOMS instrument. The severity of the hole has been assessed from TOMS using the minimum total ozone value from the October monthly mean (depth of the hole) and by calculating the average area coverage during this September-October period. Ozone is mainly destroyed by halogen (chlorine and bromine) catalytic cycles, and these losses are modulated by temperature variations in the collar of the polar lower stratospheric vortex. In this talk, I will show the relationships of halogens and temperature to both the size and depth of the hole. Because atmospheric halogen levels are responding to international agreements that limit or phase out production, the amount of halogens in the stratosphere should decrease over the next few decades. Using projections of halogen levels combined with age-of-air estimates, we find that the ozone hole is recovering at an extremely slow rate and that large ozone holes will regularly recur over the next 2 decades. The ozone hole will begin to show first signs of recovery in about 2023, and the hole will fully recover to pre-1980 levels in approximately 2070. This 2070 recovery is 20 years later than recent projections. I will also discuss current assessments of mid-latitude ozone recovery.

The Unusual Southern Hemisphere Stratosphere Winter of 2002

AbstractThe Southern Hemisphere (SH) stratospheric winter of 2002 was the most unusual winter yet observed in the SH climate record. Temperatures near the edge of the Antarctic polar vortex were considerably warmer than normal over the entire course of the winter. The polar night jet was considerably weaker than normal and was displaced more poleward than has been observed in previous winters. These record high temperatures and weak jet resulted from a series of wave events that took place over the course of the winter. The propagation of these wave events from the troposphere is diagnosed from time series of Eliassen–Palm flux vectors and autoregression time series. Strong levels of planetary waves were observed in the midlatitude lower troposphere. The combinations of strong tropospheric waves with a low index of refraction at the tropopause resulted in the large stratospheric wave forcing. The wave events tended to occur irregularly over the course of the winter, and the cumulative effect of these wave...

Quantifying the wave driving of the stratosphere

The zonal-mean eddy heat flux is directly proportional to the wave activity that propagates from the troposphere into the stratosphere. This quantity is a simple eddy diagnostic which is calculated from conventional meteorological analyses. Because this “wave driving” of the stratosphere has a strong impact on the stratospheric temperature, it is necessary to compare the impact of the flux with respect to stratospheric radiative changes caused by greenhouse gas changes. Hence we must understand the precision and accuracy of the heat flux derived from our global meteorological analyses. Herein we quantify the stratospheric heat flux using five different meteorological analyses and show that there are 15% differences, on average, between these analyses during the disturbed conditions of the Northern Hemisphere winter. Such large differences result from the planetary differences in the stationary temperature and meridional wind fields. In contrast, planetary transient waves show excellent agreement among these five analyses, and this transient heat flux appears to have a long-term downward trend.

Stratospheric meteorological conditions in the arctic polar vortex, 1991 to 1992.

Stratospheric meteorological conditions during the Airborne Arctic Stratospheric Expedition II (AASE II) presented excellent observational opportunities from Bangor, Maine, because the polar vortex was located over southeastern Canada for significant periods during the 1991-1992 winter. Temperature analyses showed that nitric acid trihydrates (NAT temperatures below 195 k) should have formed over small regions in early December. The temperatures in the polar vortex warmed beyond NAT temperatures by late January (earlier than normal). Perturbed chemistry was found to be associated with these cold temperatures.

Impact of interannual variability (1979-1986) of transport and temperature on ozone as computed using a two-dimensional photochemical model

Applied Research Corporation, Landover, Maryland Eight years of NMC (National Meteorological Center) temperature and SBUV (solar backscattered ultraviolet) ozone data were used to calculate the monthly mean heating rates and residual circulation for use in a two-dimensional photochemical model in order to examine the interannual variability of modeled ozone. Fairly good correlations were found in the interannual behavior of modeled and measured SBUV ozone in the upper stratosphere at middle to low latitudes, where temperature dependent photochemistry is thought to dominate ozone behavior. The calculated total ozone is found to be more sensitive to the interannual residual circulation changes than to the interannual temperature changes. The magnitude of the modeled ozone variability is similar to the observed variability, but the observed and modeled year to year deviations are mostly uncorrelated. The large component of the observed total ozone variability at low latitudes due to the quasi-biennial oscillation (QBO) is not seen in the modeled total ozone, as only a small QBO signal is present in the heating rates, temperatures, and monthly mean residual circulation. Large interannual changes in tropospheric dynamics are believed to influence the interannual variability in the total ozone, especially at middle and high latitudes. Since these tropospheric changes and most of the QBO forcing are not included in our model formulation, it is not surprising that the interannual variability in total ozone is not well represented in our model computations.

Residual circulations calculated from satellite data: Their relations to observed temperature and ozone distributions

Abstract Monthly mean residual circulations were calculated from eight years of satellite data. The diabatic circulation is usually found to give a good approximation to the residual circulation, but this is not always the case. In particular, an example is shown at 60°S and 30 mb where the diabatic and residual circulations show very different annual variations. Correlations between the vertical component of the residual circulation and temperature and ozone were computed. They indicate that yearly variations of temperatures in the tropics are under dynamical control while at higher latitudes they are under radiative control, except during stratospheric warmings. Interannual variations in seasonal mean temperatures are shown to be under dynamical control everywhere. Correlations between the interannual variations in the seasonal means of the vertical component of the residual circulation and ozone mixing ratios are consistent with what would be expected from the ozone variations being due to differences ...

Reply to “Comments on ‘The Unusual Southern Hemisphere Winter of 2002’”

In Newman and Nash (2005, hereafter NN2005), the evolution of the unusual Southern Hemisphere (SH) 2002 stratospheric winter was discussed. This SH winter had a very large planetary wave-2 event in late September that resulted in an SH major stratospheric sudden warming. In particular, we stated that ‘‘This large wave event resulted in the first ever observed major stratospheric warming in the SH and split the Antarctic ozone hole.’’ Sehra (2014) has asked for an explanation as to why Sehra (1975, hereafter S1975) was not referenced in NN2005. Specifically, S1975 was not cited because a major stratospheric sudden warming is not evident in those results or in the discussion. S1975 documented temperature fluctuations over Molodezhnaya station (688S, 468E) in the 30–80-km altitude range using 60 rocketsondes during 1972 (16 observations of winds using falling chaff, 55–90-km altitude range). The S1975 observations provide an early documentation of in situ upperstratospheric and mesospheric temperatures and winds. For our NN2005 paper, there are two issues with respect to S1975: