119 research outputs found
Recommended from our members
The Storm-Track Response to Idealized SST Perturbations in an Aquaplanet GCM
The tropospheric response to midlatitude SST anomalies has been investigated through a series of
aquaplanet simulations using a high-resolution version of the Hadley Centre atmosphere model (HadAM3)
under perpetual equinox conditions.
Model integrations show that increases in the midlatitude SST gradient generally lead to stronger storm
tracks that are shifted slightly poleward, consistent with changes in the lower-tropospheric baroclinicity. The
large-scale atmospheric response is, however, highly sensitive to the position of the SST gradient anomaly
relative to that of the subtropical jet in the unperturbed atmosphere. In particular, when SST gradients are
increased very close to the subtropical jet, then the Hadley cell and subtropical jet is strengthened while the
storm track and eddy-driven jet are shifted equatorward. Conversely, if the subtropical SST gradients are
reduced and the midlatitude gradients increased, then the storm track shows a strong poleward shift and a
well-separated eddy-driven jet is produced. The sign of the SST anomaly is shown to play a secondary role
in determining the overall tropospheric response.
These findings are used to provide a new and consistent interpretation of some previous GCM studies
concerning the atmospheric response to midlatitude SST anomalies
Recommended from our members
ENSO related variation of equatorial MRG and Rossby waves and forcing from higher latitudes
The contrasting behaviour of westward-moving mixed Rossby-gravity (WMRG) and the first Rossby (R1) waves in El Niño (EN) and La Niña (LN) seasons is documented with a focus on the Northern Hemisphere winter. The eastward-moving variance in the upper troposphere is dominated by WMRG and R1 structures that appear to be Doppler-shifted by the flow and are referred to as WMRG-E and R1-E. In the East Pacific and Atlantic the years with stronger equatorial westerly winds have the stronger WMRG and WMRG- E. In the East Pacific, R1 is also a maximum in LN. However, R1-E exhibits an eastward-shift between LN and EN.
The changes with ENSO phase provide a test-bed for the understanding of these waves. In the East Pacific and Atlantic, the stronger WMRG-E and WMRG with stronger westerlies are in accord with the dispersion relation with simple Doppler-shifting by the zonal flow. The possible existence of free waves can also explain stronger R1 in EN in the Eastern Hemisphere. 1-D free wave propagation theory based on wave activity conservation is also important for R1. However, this theory is unable to explain the amplitude maxima for other waves observed in the strong equatorial westerly regions in the Western Hemisphere, and certainly not their ENSO-related variation. The forcing of equatorial waves by higher latitude wave activity and its variation with ENSO phase is therefore examined. Propagation of extratropical eastward-moving Rossby wave activity through the westerly ducts into the equatorial region where it triggers WMRG-E is favoured in the stronger westerlies, in LN in the East Pacific and EN in the Atlantic. It is also found that WMRG is forced by Southern Hemisphere westward-moving wavetrains arching into the equatorial region where they are reflected. The most significant mechanism for both R1 and R1-E appear to be lateral forcing by subtropical wavetrains
Recommended from our members
The dependence of wintertime Mediterranean precipitation on the atmospheric circulation response to climate change
Climate models indicate a future wintertime precipitation reduction in the Mediterranean region but there is large uncertainty in the amplitude of the projected change. We analyse CMIP5 climate model output to quantify the role of atmospheric circulation in the Mediterranean precipitation change. It is found that a simple circulation index, i.e. the 850 hPa zonal wind (U850) in North Africa, well describes the year to year fluctuations in the area-averaged Mediterranean precipitation, with positive (i.e. westerly) U850 anomalies in North Africa being associated with positive precipitation anomalies. Under climate change, U850 in North Africa and the Mediterranean precipitation are both projected to decrease consistently with the relationship found in the inter-annual variability. This enables us to estimate that about 85% of the CMIP5 mean precipitation response and 80% of the variance in the inter-model spread are related to changes in the atmospheric circulation. In contrast, there is no significant correlation between the mean precipitation response and the global-mean surface warming across the models. It follows that the uncertainty in cold-season Mediterranean precipitation projection will not be narrowed unless the uncertainty in the atmospheric circulation response is reduced
Recommended from our members
Improving climate change detection through optimal seasonal averaging: the case of the North Atlantic jet and European precipitation
The detection of anthropogenic climate change can be improved by recognising the seasonality in the climate change response. This is demonstrated for the North Atlantic jet (zonal wind at 850 hPa, U850) and European precipitation responses projected by the CMIP5 climate models. The U850 future response is characterised by a marked seasonality: an eastward extension of the North Atlantic jet into Europe in November-April, and a poleward shift in May-October. Under the RCP8.5 scenario, the multi-model mean response in U850 in these two extended seasonal means emerges by 2035-2040 for the lower--latitude features and by 2050-2070 for the higher--latitude features, relative to the 1960-1990 climate. This is 5-15 years earlier than when evaluated in the traditional meteorological seasons (December--February, June--August), and it results from an increase in the signal to noise ratio associated with the spatial coherence of the response within the extended seasons. The annual mean response lacks important information on the seasonality of the response without improving the signal to noise ratio. The same two extended seasons are demonstrated to capture the seasonality of the European precipitation response to climate change and to anticipate its emergence by 10-20 years. Furthermore, some of the regional responses, such as the Mediterranean precipitation decline and the U850 response in North Africa in the extended winter, are projected to emerge by 2020-2025, according to the models with a strong response. Therefore, observations might soon be useful to test aspects of the atmospheric circulation response predicted by some of the CMIP5 models
Recommended from our members
The longitudinal variation of equatorial waves due to propagation on a varying zonal flow
The general 1-D theory of waves propagating on a zonally varying flow is developed from basic wave theory, and equations are derived for the variation of wavenumber and energy along ray paths. Different categories of behaviour are found, depending on the sign of the group velocity (cg) and a wave property, B. For B positive the wave energy and the wave number vary in the same sense, with maxima in relative easterlies or westerlies, depending on the sign of cg. Also the wave accumulation of Webster and Chang (1988) occurs where cg goes to zero. However for B negative they behave in opposite senses and wave accumulation does not occur. The zonal propagation of the gravest equatorial waves is analysed in detail using the theory. For non-dispersive Kelvin waves, B reduces to 2, and analytic solution is possible. B is positive for all the waves considered, except for the westward moving mixed Rossby-gravity (WMRG) wave which can have negative as well as positive B.
Comparison is made between the observed climatologies of the individual equatorial waves and the result of pure propagation on the climatological upper tropospheric flow. The Kelvin wave distribution is in remarkable agreement, considering the approximations made. Some aspects of the WMRG and Rossby wave distributions are also in qualitative agreement. However the observed maxima in these waves in the winter westerlies in the eastern Pacific and Atlantic are not consistent with the theory. This is consistent with the importance of the sources of equatorial waves in these westerly duct regions due to higher latitude wave activity
Recommended from our members
The equivalent barotropic structure of waves in the tropical atmosphere in the Western Hemisphere
Tropical waves are generally considered to have a baroclinic structure. However, analysis of ERA-Interim and NOAA OLR data for the period 1979-2010 shows that in the equatorial and Northern Hemisphere near-equatorial regions in the tropical western hemisphere (WH), westward and eastward-moving transients, zonal wavenumber 2-10, period 2-30 days, have little tilt in the vertical, and can be said to be equivalent barotropic. The westward-moving transients in the equatorial region have large projection onto the westward mixed Rossby-Gravity (WMRG) wave and those in the near-equatorial region project onto the gravest Rossby wave and also the WMRG. The eastward-moving transients have large projections onto the Doppler shifted eastward-moving versions of these waves.
To examine how such equivalent barotropic structures are possible in the tropics, terms in the vorticity equation are analysed. It is deduced that waves must have westward intrinsic phase speed and can exist in the WH with its large westerly vertical shear. Throughout the depth, the advection of vorticity by the zonal flow and the β-term are large and nearly cancel. In the upper troposphere the zonal advection by the strong westerly flow wins and the residual is partially balanced by vortex shrinking associated with divergence above a region of ascent. Below the region of ascent the β-term wins and is partially balanced by vortex stretching associated with the convergence. An equivalent barotropic structure is therefore maintained in a similar manner to higher latitudes. The regions of ascent are usually associated with deep convection and, consistently, WH waves directly connected to tropical convection are also found to be equivalent barotropic
Recommended from our members
A simple model for interpreting temperature variability and its higher-order changes
Atmospheric temperature distributions are often identified with their variance, while the higher-order moments receive less attention. This can be especially misleading for extremes, which are associated with the tails of the Probability Density Functions (PDFs), and thus depend strongly on the higher-order moments. For example, skewness is related to the asymmetry between positive and negative anomalies, while kurtosis is indicative of the ”extremity” of the tails. Here we show that for near-surface atmospheric temperature, an approximate linear relationship exists between kurtosis and skewness squared. We present a simple
model describing this relationship, where the total PDF is written as the sum of three Gaussians, representing small deviations from the climatological mean together with the larger amplitude cold and warm temperature anomalies associated with synoptic systems. This model recovers the PDF structure in different regions of the world, as well as its projected response to climate change, giving a simple physical interpretation of the higher-order temperature variability changes. The kurtosis changes are found to be largely predicted by the skewness changes. Building a deeper understanding of what controls the higher-order moments of the temperature variability is crucial for understanding extreme temperature events and how they respond to climate
change
- …