Interannual variability of the boreal summer tropical UTLS in observations and CCMVal-2 simulations

Abstract

Abstract. During boreal summer the upper troposphere/lower stratosphere (UTLS) in the Northern Hemisphere shows a distinct maximum in water vapour (H2O) mixing ratios and a coincident minimum in ozone (O3) mixing ratios, both confined within the Asian monsoon anticyclone (AMA). This well-known feature has been related to transport processes emerging above the convective systems during the Asian summer monsoon (ASM), further modified by the dynamics of the AMA. We compare the ability of chemistry–climate models (CCMs) to reproduce the climatological characteristics and variability of H2O, O3, and temperature in the UTLS during the boreal summer with MIPAS satellite observations and ERA-Interim reanalyses. By using a multiple linear regression model the main driving factors, the strength of the ASM, the quasi-biennial oscillation (QBO), and the El Niño–Southern Oscillation (ENSO), are separated. The regression patterns related to ENSO show a coherent, zonally asymmetric signal for temperatures and H2O mixing ratios for ERA-Interim and the CCMs and suggest a weakening of the ASM during ENSO warm events. The QBO modulation of the lower-stratospheric temperature near the Equator is well represented as a zonally symmetric pattern in the CCMs. Changes in H2O and O3 mixing ratios are consistent with the QBO-induced temperature and circulation anomalies but less zonally symmetric than the temperature pattern. Regarding the ASM, the results of the regression analysis show for ERA-Interim and the CCMs enhanced H2O and reduced O3 mixing ratios within the AMA for stronger ASM seasons. The CCM results can further confirm earlier studies which emphasize the importance of the Tibetan Plateau/southern slope of the Himalayas as the main source region for H2O in the AMA. The results suggest that H2O is transported towards higher latitudes at the north-eastern edge of the AMA rather than towards low equatorial latitudes to be fed into the tropical pipe.

A comprehensive overview of the climatological composition of the Asian summer monsoon anticyclone based on 10 years of Aura Microwave Limb Sounder measurements

“…These results are consistent with those of Park et al [] and Park et al [], who found the maxima in MLS H 2 O and CO, respectively, during the 2005 ASM season to be nearly coincident with the region of deep convection at 215 hPa but centered within the anticyclone and distant from the deepest convection at 100 hPa. A similar shift to the northwest between the H 2 O maximum at 360 K and that at 370 and 380 K has also been seen in a multiyear July–August climatology based on Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data [ Kunze et al , ]. Although the stratospheric tracers are primarily discussed in section 4.5.3, we note here that MIPAS ozone also showed this difference [ Kunze et al , ], as do the distributions of the MLS stratospheric tracers (Figures and S5–S7), with the westward shift in the location of the minima between 350 and 370 K especially evident in HNO 3 .…”

“…A similar shift to the northwest between the H 2 O maximum at 360 K and that at 370 and 380 K has also been seen in a multiyear July–August climatology based on Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data [ Kunze et al , ]. Although the stratospheric tracers are primarily discussed in section 4.5.3, we note here that MIPAS ozone also showed this difference [ Kunze et al , ], as do the distributions of the MLS stratospheric tracers (Figures and S5–S7), with the westward shift in the location of the minima between 350 and 370 K especially evident in HNO 3 .…”

“…Indeed, horizontal in‐mixing of ozone‐rich extratropical air driven by the ASM circulation has been shown to be the dominant factor producing the observed maximum in O 3 in the upper part of the tropical tropopause layer (∼360–420 K) in northern summer [ Konopka et al , , ; Ploeger et al , ], with the contribution from in‐mixing peaking sharply just above the tropical tropopause around 390 K [ Abalos et al , ], consistent with the relative amplitudes of the signatures in the maps of MLS data on the different isentropic surfaces. A ring of high ozone around the anticyclone formed by tongues of air along its eastern flank has also been reported as evidence of in‐mixing in MIPAS data at 360–380 K [ Kunze et al , ].…”

“…Satellite measurements of cloud top height distributions [ Devasthale and Fueglistaler , ] and precipitation features [ Qie et al , ] indicate that deep convection in the region occurs most frequently in July and August but penetrates to higher altitudes in the UTLS in May and June, during the early stages of the monsoon season. The attendant rapid vertical transport strongly influences constituent distributions in the UTLS, episodically bringing up relatively moist, ozone‐poor air from near the surface, leading to climatologically high values of water vapor and low values of ozone over the region from June through September [e.g., Rosenlof et al , ; Jackson et al , ; Dethof et al , ; Randel et al , ; Gettelman et al , ; Randel and Park , ; Park et al , ; Das et al , ; Uma et al , ; Randel et al , ; Kunze et al , ]. The intense convection also rapidly lofts insoluble gas‐phase boundary layer pollutants, as well as aerosols and/or their gaseous precursors, into the UTLS, where air masses remain largely confined within the closed circulation of the anticyclone for days to weeks [ Randel and Park , ; Ploeger et al.…”

Simulating the atmospheric response to the 11-year solar cycle forcing with the UM-UKCA model: the role of detection method and natural variability

“…1) The second method is a multiple linear regression (MLR) technique. The MLR code is the same as that used and described in SPARC (2010), with a similar method also used in Bodeker et al (1998) and Kunze et al (2016). In this case, the time evolution of a variable, y(t), can be defined as:…”