Multidecadal modulation of El Nino-Southern Oscillation (ENSO) variance by Atlantic Ocean sea surface temperatures

Observations suggest a possible link between the Atlantic Multidecadal Oscillation (AMO) and El Nino Southern Oscillation (ENSO) variability, with the warm AMO phase being related to weaker ENSO variability. A coupled ocean-atmosphere model is used to investigate this relationship and to elucidate mechanisms responsible for it. Anomalous sea surface temperatures (SSTs) associated with the positive AMO lead to change in the basic state in the tropical Pacific Ocean. This basic state change is associated with a deepened thermocline and reduced vertical stratification of the equatorial Pacific ocean, which in turn leads to weakened ENSO variability. We suggest a role for an atmospheric bridge that rapidly conveys the influence of the Atlantic Ocean to the tropical Pacific. The results suggest a non-local mechanism for changes in ENSO statistics and imply that anomalous Atlantic ocean SSTs can modulate both mean climate and climate variability over the Pacific.

The role of lateral ocean physics in the upper ocean thermal balance of a coupled ocean-atmosphere GCM

The sensitivity of the upper ocean thermal balance of an ocean-atmosphere coupled GCM to lateral ocean physics is assessed. Three 40-year simulations are performed using horizontal mixing, isopycnal mixing, and isopycnal mixing plus eddy induced advection. The thermal adjustment of the coupled system is quite different between the simulations, confirming the major role of ocean mixing on the heat balance of climate. The initial adjustment phase of the upper ocean (SST) is used to diagnose the physical mechanisms involved in each parametrisation. When the lateral ocean physics is modified, significant changes of SST are seen, mainly in the southern ocean. A heat budget of the annual mixed layer (defined as the “bowl”) shows that these changes are due to a modified heat transfer between the bowl and the ocean interior. This modified heat intake of the ocean interior is directly due to the modified lateral ocean physics. In isopycnal diffusion, this heat exchange, especially marked at mid-latitudes, is both due to an increased effective surface of diffusion and to the sign of the isopycnal gradients of temperature at the base of the bowl. As this gradient is proportional to the isopycnal gradient of salinity, this confirms the strong role of salinity in the thermal balance of the coupled system. The eddy induced advection also leads to increased exchanges between the bowl and the ocean interior. This is both due to the shape of the bowl and again to the existence of a salinity structure. The lateral ocean physics is shown to be a significant contributor to the exchanges between the diabatic and the adiabatic parts of the ocean.

Inter-hemispheric mesospheric coupling in a comprehensive middle atmosphere model

Observations of noctilucent clouds have revealed a surprising coupling between the winter stratosphere and the summer polar mesopause region. In spite of the great distance involved, this inter-hemispheric link has been suggested to be the principal reason for both the year-to-year variability and the hemispheric differences in the frequency of occurrence of these high-altitude clouds. In this study, we investigate the dynamical influence of the winter stratosphere on the summer mesosphere using simulations from the vertically extended version of the Canadian Middle Atmosphere Model (CMAM). We find that for both Northern and Southern Hemispheres, variability in the summer polar mesopause region from one year to another can be traced back to the planetary-wave flux entering the winter stratosphere. The teleconnection pattern is the same for both positive and negative wave-flux anomalies. Using a composite analysis to isolate the events, it is argued that the mechanism for interhemispheric coupling is a feedback between summer mesosphere gravity-wave drag (GWD) and zonal wind, which is induced by an anomaly in mesospheric cross-equatorial flow, the latter arising from the anomaly in winter hemisphere GWD induced by the anomaly in stratospheric conditions.

Simulated variations in atmospheric CO2 over a Wisconsin forest using a coupled ecosystem-atmosphere model

Ecosystem fluxes of energy, water, and CO2 result in spatial and temporal variations in atmospheric properties. In principle, these variations can be used to quantify the fluxes through inverse modelling of atmospheric transport, and can improve the understanding of processes and falsifiability of models. We investigated the influence of ecosystem fluxes on atmospheric CO2 in the vicinity of the WLEF-TV tower in Wisconsin using an ecophysiological model (Simple Biosphere, SiB2) coupled to an atmospheric model (Regional Atmospheric Modelling System). Model parameters were specified from satellite imagery and soil texture data. In a companion paper, simulated fluxes in the immediate tower vicinity have been compared to eddy covariance fluxes measured at the tower, with meteorology specified from tower sensors. Results were encouraging with respect to the ability of the model to capture observed diurnal cycles of fluxes. Here, the effects of fluxes in the tower footprint were also investigated by coupling SiB2 to a high-resolution atmospheric simulation, so that the model physiology could affect the meteorological environment. These experiments were successful in reproducing observed fluxes and concentration gradients during the day and at night, but revealed problems during transitions at sunrise and sunset that appear to be related to the canopy radiation parameterization in SiB2.

Influence of the Antarctic ozone hole on the polar mesopause region as simulated by the Canadian Middle Atmosphere Model

It is well established that variations in polar stratospheric winds can affect mesospheric temperatures through changes in the filtering of gravity wave fluxes, which drive a residual circulation in the mesosphere. The Canadian Middle Atmosphere Model(CMAM) is used to examine this vertical coupling mechanism in the context of the mesospheric response to the Antarctic ozone hole. It is found that the response differs significantly between late spring and early summer, because of a changing balance between the competing effects of parametrised gravity wavedrag (GWD)and changes in resolved wave drag local to the mesosphere. In late spring, the strengthened stratospheric westerlies arising from the ozone hole lead to reduced eastward GWD in the mesosphere and a warming of the polar mesosphere, just as in the well known mesospheric response to sudden stratospheric warmings, but with an opposite sign.In early summer, with easterly flow revailing over most of the polar stratosphere,the strengthened easterly wind shear within the mesosphere arising from the west ward GWD anomaly induces a positive resolved wave drag anomaly through baroclinic instability. The polar cooling induced by this process completely dominates the upper mesospheric response to the ozone hole in early summer. Consequences for the past and future evolution of noctilucent clouds are discussed

Past and future conditions for polar stratospheric cloud formation simulated by the Canadian Middle Atmosphere Model

We analyze here the polar stratospheric temperatures in an ensemble of three 150-year integrations of the Canadian Middle Atmosphere Model (CMAM), an interactive chemistry-climate model which simulates ozone depletion and recovery, as well as climate change. A key motivation is to understand possible mechanisms for the observed trend in the extent of conditions favourable for polar stratospheric cloud (PSC) formation in the Arctic winter lower stratosphere. We find that in the Antarctic winter lower stratosphere, the low temperature extremes required for PSC formation increase in the model as ozone is depleted, but remain steady through the twenty-first century as the warming from ozone recovery roughly balances the cooling from climate change. Thus, ozone depletion itself plays a major role in the Antarctic trends in low temperature extremes. The model trend in low temperature extremes in the Arctic through the latter half of the twentieth century is weaker and less statistically robust than the observed trend. It is not projected to continue into the future. Ozone depletion in the Arctic is weaker in the CMAM than in observations, which may account for the weak past trend in low temperature extremes. In the future, radiative cooling in the Arctic winter due to climate change is more than compensated by an increase in dynamically driven downwelling over the pole.

Response of the middle atmosphere to CO2 doubling: results from the Canadian Middle Atmosphere Model

The Canadian Middle Atmosphere Model (CMAM) has been used to examine the middle atmosphere response to CO2 doubling. The radiative-photochemical response induced by doubling CO2 alone and the response produced by changes in prescribed SSTs are found to be approximately additive, with the former effect dominating throughout the middle atmosphere. The paper discusses the overall response, with emphasis on the effects of SST changes, which allow a tropospheric response to the CO2 forcing. The overall response is a cooling of the middle atmosphere accompanied by significant increases in the ozone and water vapor abundances. The ozone radiative feedback occurs through both an increase in solar heating and a decrease in infrared cooling, with the latter accounting for up to 15% of the total effect. Changes in global mean water vapor cooling are negligible above ~30 hPa. Near the polar summer mesopause, the temperature response is weak and not statistically significant. The main effects of SST changes are a warmer troposphere, a warmer and higher tropopause, cell-like structures of heating and cooling at low and middlelatitudes in the middle atmosphere, warming in the summer mesosphere, water vapor increase throughout the domain, and O3 decrease in the lower tropical stratosphere. No noticeable change in upwardpropagating planetary wave activity in the extratropical winter–spring stratosphere and no significant temperature response in the polar winter–spring stratosphere have been detected. Increased upwelling in the tropical stratosphere has been found to be linked to changed wave driving at low latitudes.

Large-scale dynamics of the mesosphere and lower thermosphere: an analysis using the extended Canadian Middle Atmosphere Model

The extended Canadian Middle Atmosphere Model is used to investigate the large-scale dynamics of the mesosphere and lower thermosphere (MLT). It is shown that the 4-day wave is substantially amplified in southern polar winter in the presence of instabilities arising from strong vertical shears in the MLT zonal mean zonal winds brought about by parameterized nonorographic gravity wave drag. A weaker 4-day wave in northern polar winter is attributed to the weaker wind shears that result from weaker parameterized wave drag. The 2-day wave also exhibits a strong dependence on zonal wind shears, in agreement with previous modeling studies. In the equatorial upper mesosphere, the migrating diurnal tide provides most of the resolved westward wave forcing, which varies semiannually in conjunction with the tide itself; resolved forcing by eastward traveling disturbances is dominated by smaller scales. Nonmigrating tides and other planetary-scale waves play only a minor role in the zonal mean zonal momentum budget in the tropics at these heights. Resolved waves are shown to play a significant role in the zonal mean meridional momentum budget in the MLT, impacting significantly on gradient wind balance. Balance fails at low latitudes as a result of a strong Reynolds stress associated with the migrating diurnal tide, an effect which is most pronounced at equinox when the tide is strongest. Resolved and parameterized waves account for most of the imbalance at higher latitudes in summer. This results in the gradient wind underestimating the actual eastward wind reversal by up to 40%.

Impact of climate change on stratospheric sudden warmings as simulated by the Canadian middle atmosphere model

The dynamics of Northern Hemisphere major midwinter stratospheric sudden warmings (SSWs) are examined using transient climate change simulations from the Canadian Middle Atmosphere Model (CMAM). The simulated SSWs show good overall agreement with reanalysis data in terms of composite structure, statistics, and frequency. Using observed or model sea surface temperatures (SSTs) is found to make no significant difference to the SSWs, indicating that the use of model SSTs in the simulations extending into the future is not an issue. When SSWs are defined by the standard (wind based) definition, an absolute criterion, their frequency is found to increase by;60% by the end of this century, in conjunction with a;25% decrease in their temperature amplitude. However, when a relative criterion based on the northern annular mode index is used to define the SSWs, no future increase in frequency is found. The latter is consistent with the fact that the variance of 100-hPa daily heat flux anomalies is unaffected by climate change. The future increase in frequency of SSWs using the standard method is a result of the weakened climatological mean winds resulting from climate change, which make it easier for the SSW criterion to be met. A comparison of winters with and without SSWs reveals that the weakening of the climatological westerlies is not a result of SSWs. The Brewer–Dobson circulation is found to be stronger by ;10% during winters with SSWs, which is a value that does not change significantly in the future.

Summertime stratospheric processes at northern mid-latitudes: comparisons between MANTRA balloon measurements and the Canadian Middle Atmosphere Model

In this paper we report on a study conducted using the Middle Atmospheric Nitrogen TRend Assessment (MANTRA) balloon measurements of stratospheric constituents and temperature and the Canadian Middle Atmosphere Model (CMAM). Three different kinds of data are used to assess the inter-consistency of the combined dataset: single profiles of long-lived species from MANTRA 1998, sparse climatologies from the ozonesonde measurements during the four MANTRA campaigns and from HALOE satellite measurements, and the CMAM climatology. In doing so, we evaluate the ability of the model to reproduce the measured fields and to thereby test our ability to describe mid-latitude summertime stratospheric processes. The MANTRA campaigns were conducted at Vanscoy, Saskatchewan, Canada (52◦ N, 107◦ W)in late August and early September of 1998, 2000, 2002 and 2004. During late summer at mid-latitudes, the stratosphere is close to photochemical control, providing an ideal scenario for the study reported here. From this analysis we find that: (1) reducing the value for the vertical diffusion coefficient in CMAM to a more physically reasonable value results in the model better reproducing the measured profiles of long-lived species; (2) the existence of compact correlations among the constituents, as expected from independent measurements in the literature and from models, confirms the self-consistency of the MANTRA measurements; and (3) the 1998 measurements show structures in the chemical species profiles that can be associated with transport, adding to the growing evidence that the summertime stratosphere can be much more disturbed than anticipated. The mechanisms responsible for such disturbances need to be understood in order to assess the representativeness of the measurements and to isolate longterm trends.

Persistence and photochemical decay of springtime total ozone anomalies in the Canadian Middle Atmosphere Model

The persistence and decay of springtime total ozone anomalies over the entire extratropics (midlatitudes plus polar regions) is analysed using results from the Canadian Middle Atmosphere Model (CMAM), a comprehensive chemistry-climate model. As in the observations, interannual anomalies established through winter and spring persist with very high correlation coefficients (above 0.8) through summer until early autumn, while decaying in amplitude as a result of photochemical relaxation in the quiescent summertime stratosphere. The persistence and decay of the ozone anomalies in CMAM agrees extremely well with observations, even in the southern hemisphere when the model is run without heterogeneous chemistry (in which case there is no ozone hole and the seasonal cycle of ozone is quite different from observations). However in a version of CMAM with strong vertical diffusion, the northern hemisphere anomalies decay far too rapidly compared to observations. This shows that ozone anomaly persistence and decay does not depend on how the springtime anomalies are created or on their magnitude, but reflects the transport and photochemical decay in the model. The seasonality of the long-term trends over the entire extratropics is found to be explained by the persistence of the interannual anomalies, as in the observations, demonstrating that summertime ozone trends reflect winter/spring trends rather than any change in summertime ozone chemistry. However this mechanism fails in the northern hemisphere midlatitudes because of the relatively large impact, compared to observations, of the CMAM polar anomalies. As in the southern hemisphere, the influence of polar ozone loss in CMAM increases the midlatitude summertime loss, leading to a relatively weak seasonal dependence of ozone loss in the Northern Hemisphere compared to the observations.

Extended Canadian middle atmosphere model: zonal-mean climatology and physical parameterizations

This paper describes the energetics and zonal-mean state of the upward extension of the Canadian Middle Atmosphere Model, which extends from the ground to ~210 km. The model includes realistic parameterizations of the major physical processes from the ground up to the lower thermosphere and exhibits a broad spectrum of geophysical variability. The rationale for the extended model is to examine the nature of the physical and dynamical processes in the mesosphere/lower thermosphere (MLT) region without the artificial effects of an imposed sponge layer which can modify the circulation in an unrealistic manner. The zonal-mean distributions of temperature and zonal wind are found to be in reasonable agreement with observations in most parts of the model domain below ~150 km. Analysis of the global-average energy and momentum budgets reveals a balance between solar extreme ultraviolet heating and molecular diffusion and a thermally direct viscous meridional circulation above 130 km, with the viscosity coming from molecular diffusion and ion drag. Below 70 km, radiative equilibrium prevails in the global mean. In the MLT region between ~70 and 120 km, many processes contribute to the global energy budget. At solstice, there is a thermally indirect meridional circulation driven mainly by parameterized nonorographic gravity-wave drag. This circulation provides a net global cooling of up to 25 K d^-1.

Ozone climatology using interactive chemistry: results from the Canadian Middle Atmosphere Model

The climatology of ozone produced by the Canadian Middle Atmosphere Model (CMAM) is presented. This three-dimensional global model incorporates the radiative feedbacks of ozone and water vapor calculated on-line with a photochemical module. This module includes a comprehensive gas-phase reaction set and a limited set of heterogeneous reactions to account for processes occurring on background sulphate aerosols. While transport is global, photochemistry is solved from about 400 hPa to the top of the model at ∼95 km. This approach provides a complete and comprehensive representation of transport, emission, and photochemistry of various constituents from the surface to the mesopause region. A comparison of model results with observations indicates that the ozone distribution and variability are in agreement with observations throughout most of the model domain. Column ozone annual variation is represented to within 5–10% of the observations except in the Southern Hemisphere for springtime high latitudes. The vertical ozone distribution is generally well represented by the model up to the mesopause region. Nevertheless, in the upper stratosphere, the model generally underestimates the amount of ozone as well as the latitudinal tilting of ozone isopleths at high latitude. Ozone variability is analyzed and compared with measurements. The comparison shows that the phase and amplitude of the seasonal variation as well as shorter timescale variations are well represented by the model at various latitudes and heights. Finally, the impact of incorporating ozone radiative feedback on the model climatology is isolated. It is found that the incorporation of ozone radiative feedback results in a cooling of ∼8 K in the summer stratopause region, which corrects a warm bias that results when climatological ozone is used.