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Questions asked at beginning of lecture Tropical Meteorology I (MET 5533) Spring 2010 Selected Questions:
Zonally Symmetric and Asymmetric TropicsMonsoons and Indian Ocean VariabilityGill AtmosphereInterannual variations of the monsoonsIntraseasonal VariabilityMonsoon In a Changing ClimateITCZENSOTropical Atlantic Variability
Zonally Symmetric and Asymmetric TropicsThere should be some jet because of the thermal wind relationship (The thermal wind is a vertical shear in the geostrophic wind caused by a horizontal temperature gradient. Its name is a misnomer, because the thermal wind is not actually a wind, but rather a wind gradient.) There is a level where the gradient goes to zero, the tropopause, this is where the maximum winds occur. Upper level easterlies are weaker in the winter so vertical shear increases in JJA. The moisture gradient is strong and the maximum is centered in the tropics. The mass continuity equation. advection by the Hadley cell occurs from the winter to summer hemisphere near the surface and from summer to winter at the top of the troposphere. Sensible heat is therefore transported from the winter to summer hemisphere. However, potential energy is transported from summer to winter at a greater magnitude making enthalpy net transport toward the winter hemisphere. The Hadley cell, standing eddies, and transient eddies. The Hadley Cell. Eddy momentum flux and total heat flux. A retrospective data assimilation method. To determine the impacts of model bias and sparse data. The model bias in the reanalysis. The subsidence region of the Hadley cell was not as well defined in the in-situ data; the southern hemisphere Hadley cell does not extend to 200mb; and the Farrell cell is very broad in the southern hemisphere in-situ data. By comparing in-situ data to sub-sampled reanalysis the effects of model bias are seen. The Hadley cell was seen to be 20%-30% stronger in the model. Based on SSTs, the Pacific Ocean is the most asymmetric. Atlantic is the most zonally symmetric in the tropics. In the boreal summer there is the Tibetan High, Mexican High, and West african high. In boreal winter there is the West Pacific High, Bolivian High (counterpart of the Tibetan High), and Siberian High. in both summers there is a mid-oceanic trough. These asymmetries are caused by land-ocean contrasts and within land, arid/monsoon contrasts. Near the surface there is the southwesterly, cross-equatorial monsoon. The Somali jet, subtropical highs, and pacific trades are also seen here. Saturation vapor pressure related to the temperature of the air mass. Specific humidity is lower over the ocean than land in summer because the land temperatures are higher. Show EquationPrecipitation is all asymmetric. Major features include the monsoons, ITCZ, and other convergence zones. The short answer is that there is more land mass in the Northern hemisphere. The long answer is that the thermal inertia of water is greater offsetting the seasonality of solar radiation making the thermal equator to the north. Net solar radiation absorbed at TOA. Latent heat of condensation (from monsoons). No, because there is a transport of heat via ocean currents. This is the location of the sinking branch of the Hadley cell. Radiative cooling would be the most-guessed wrong answer. Baroclinic eddies organized through storm tracks and quasi-stationary waves transport energy pole ward, thus compensating for the subsidence warming from the descending branch of the Hadley cell. Monsoons and Indian Ocean VariabilityThe transport of energy per unit area per unit time. 1027kg/m^3 Sea water: 3994 J/kgK which is less than water: 4185 J/kgK About 1.5 petawatts 1.5 petawatts - it balances! Ekman transport. First, we need to stop being parochial and only concern ourselves with local events. Second, the Hadley and Walker cells are nothing more than monsoons. Furthermore, the onset of the monsoon is so spectacular it effects the entire globe. Siberian High because both are low-level pressure systems (850mb). The Tibetan High as both are 200mb pressure systems. The gradient changes from negative to positive. Upwelling and evaporation. At the lower level. When they are parallel, the divergent (irrotational) circulations will transfer energy to the non-divergent circulations. The spectrum is dominated by wave numbers 0 and 1. The orientation of the divergent component of the wind and diabatic differential heating. The horizontal scale is much larger than the vertical scale. (gh)^1/2 -Beta/(k^2 + l^2) If it takes two months for a Kelvin wave to propagate across the ocean it will take 6 months for the Rossby wave. About 1600km: y_c = (2c/Beta)^1/2 The Kelvin waves propagate east and Rossby waves west. Parabolic cylinder. Spherical harmonics a.k.a Legendre polynomials in the y direction and Fourier transform in the x. The Kelvin wave moves 3 times faster so the response is wider. 1 steady state atmosphere; 2 free surface and fixed bottom; 3 the forced motion is sufficiently weak that it can be treated by linear dynamics; 4 solutions are for long waves not gravity waves; 5 sinusoidal heating profile; 6 the thermal damping is Newtonian cooling; 7 the friction is Rayleigh damping; 8 the thermal and frictional damping rates are assumed to have the same time scale everywhere; 9 Beta plane approximation. Homogenizes the atmospheric motion in vertical, and a strong vertical uniform wind is found below the heat source; adding any momentum damping removes an surface wind response to the elevated heat source. Localizes the heat source. MC: shallower, moves slower, dissipates faster Up, due to the lateral transport of energy from the monsoons and subsidence that occurs. 40% 800mb Horizontal advection, because the thermal gradient is weak. Because there is asymmetric heating in the monsoon, there is only a Rossby wave response. Equivalent potential temp surface slopes toward the monsoon region producing adiabatic warming. Interannual variations of the monsoonsIt is the predictability of the second kind meaning that the predictability is a function of the boundary conditions not on initial values. Chaos theory; that forecasts are an initial value problem. The forecast prediction is sensitive to conditional sensitivity; a small perturbation in the initial conditions can lead to a large change in the forecast. Weather: Predictability whose source is knowledge of the initial conditions and which is limited by the flow itself. Climate: Predictability whose source is based on knowledge of the boundary conditions. This is false! Predictability of the second kind is higher in the tropics and lower in the midlats because the temperature gradients are weak, instabilities are far less, and changes in saturation vapor pressure are smaller in the tropics. The positive ENSO was counter acted by the Indian Ocean Dipole (IOD) There is a seasonal persistent component which was assumed external. The correlations of daily rainfall vary somewhat from day to day which indicates internal forcing. The matter is still under debate. However, the argument that IOD is a projection of ENSO seems more convincing. When the monsoon is weak, the southwesterlies are weak and the Ekman drift is weak so there is less heat transport. When the monsoon is strong, there is more Ekman transport. More latent heat is released in a strong monsoon year and therefore the transport compensates for this. It lowers the variance because it is the model noise which causes the variance to be too high particularly in the western equatorial Pacific. The average of noise tends to zero. It is an ensemble atmospheric model coupled to an ocean model where the mean value of the ensemble AGCM forces the OGCM and the OGCM forces each member of the AGCM ensemble. TBO is the advancement of the monsoon in the north-south direction; modified TBO includes ENSO and IOD. * The correlation between Nino3.4 SST and Indian Ocean increased (became closer to observations than standard coupled COLA model). The correlations were negative between rainfall and SST. With rainfall leading SST is when the correlations were largest. That would suggest that the SST evolution (especially in the summer time is govern by the atmospheric forcing) Intraseasonal VariabilityAnything below 90 days. 20: northwest; 45: northeast Strong or weak year cases are a function of the non-oscillatory eigenmodes, a.k.a. interannual, (3,4,5) not the 20- and 45- day osciilations (6,7) and (1,2) respectively. No, there is not a shift in the PDF of intraseasonal activity on interannual scales therefore there will not be more storms in a strong monsoon year. Historical evidence also shows there is not a difference between weak and strong years and the number of storms produced. In the mid-latitudes they are dynamically driven (barotropic). In the tropics they are convectively driven (baroclinic). 45-day; the correlations with the 20-day were very weak. East Pacific anomalies In the subtropics; in the deep tropics (between 10S-10N) the SST is forcing the atmosphere. You need SSTs with intraseasonal variations (coupled air-sea interactions). If you have high frequency SSTs in the forcing then you can reproduce MJO. The model is perfect and has no biases. To see how well the prediction is with good initial conditions. How long will the anomalies that existed at the initial time persist and provide skill over climatological monthly mean SST. It is mostly over the western pacific, there is a jump from eastern Indian Ocean to the western Indian Ocean. The Rossby waves were travelling northwestward and mixed Rossby-gravity waves were travelling northeastward. Higher vertical resolution leads to better convection in the model. This was because the melting layer was being resolved; this 600-500mb layer has increased stability after ice falls through it which will suppress the convection allowing moist static energy to build up until the next active phase of the MJO. The suppression of convection also leads to detrainment of moisture below that layer (trimodal distribution of clouds – shallow, mid-level, and deep convective). Before then it was either shallow or deep convection. The variance of the zonal wind at 200mb averaged between 10S-10N Monsoon In a Changing Climate1 The Walker circulation shifted. The zonal mean flow has linearly increased in the recent decades at the expense of the MJO 2 snowpack is less over Siberia in a warming environment (global warming) because the ground temperatures are rising. Less snow means the land is warmer and land-sea contrast has increased so the monsoon is dictated less by external forcing. The monsoon fails when there is an eastern Pacific warm event because the sinking branch of the Walker cell is right over India. During a central Pacific warm event the maximum upper-level convergence is more westward. Because the strong events are becoming more common but the weak to moderate events are less common. There is an interdecadal oscillation of the walker circulation that modulates the teleconnection. By first finding the component of wind stress in the pacific caused by monsoon, they prescribed a wind stress anomaly and found the effects it had on the phases of ENSO. The impact is systematically weaker for the cold phase. It weakens the event; easterly anomaly will act against the warm phase. Strengthens and lengthens it; the westerly anomaly is a positive feedback allowing the warm event to continue longer than usual. ITCZNo. Deszoeke (2008) illustrates that the zonally averaged winds in the eastern Pacific are southeasterly all year. Rain is at a maximum in the southern branch during MAM but is far less than the rain seen in the northern limb. The rain diminishes compared to the rest of the year but remains as strong as the southern limb. An incipient disturbance causes Ekman convergence in the boundary layer that fuels the convection which in turn fuels more convergence. There is a positive feedback for Ekman frictional convergence. Coriolis and Friction. The stronger coriolis is the more frictional convergence is favored. Coriolis is at a maximum at the poles. There is no moisture at the poles to promote convection! there can be no positive feedback even if there is an incipient disturbance. The high level of moisture near the equator pulls the ITCZ back to the equator. (Sumi 1992) preformed an aqua planet experiment with uniform SSTs and it produced a single ITCZ at the equator due to inertial stability - not because of increased moisture at the equator. Rotation (f) not only feeds CISK but also has inertia stability which resists convection. This can be shown from the vorticity equation. So the reduced rotation at the equator allows convection to occur more freely. Note: the value of f is not what is important but the beta effect – without a gradient of Coriolis there is no preference for the ITCZ to reside at the equator. Aqua plant with uniform SST over the globe, Constant solar angle (boreal summer), constant radiative cooling rates. There is more radiative cooling outside of the ITCZ due to less humidity. So the downward subsidence motion has to become stronger to compensate for this cooling. Exactly the opposite happens within the ITCZ. The Hadley cell becomes stronger which means stronger surface winds and the role of CISK comes into play resulting in ITCZ moving away from the equator. -- Because you have intensified the convection at the ITCZ and enhanced the subsidence, surface winds have increased, convergence must increase. Frictional (Ekman) convergence has to become larger (f must be larger) and so CISK is greater. Effects of CISK are nullified because there is no positive feedback in response to the convection. The continental bulge of the West African landmass heats up faster than the ocean and the temperature gradient causes the convection to be stronger in the north moving the ITCZ north. The angle of the coast of Peru causes stronger upwelling than in the north. The colder waters in the south versus the north move the ITCZ north. Warm SST and lower troposphere stability lead to more clouds. ENSOFar eastern south equatorial Pacific. Western and central equatorial Pacific. Three month running mean over .5 degree C for 5 months. It cannot be used as a prediction tool. Strong response to warm events and poor response to cold events. Also has the poorest correlation with SOI. Strong response to cold events. temperatures and currents to a depth of 500 meters Ekman Divergence. Stronger easterlies and shallow thermocline in the east. North Equatorial counter current, North and South equatorial current. N. Equatorial counter current - 50 cm/s N. Equatorial current - 40 cm/s S. Equatorial current - 10 cm/s Equatorial under current - 120 cm/s Look at the mean SST to see the warm pool in the west and cold tongue in the east of the equatorial Pacific. Look at the slope of the thermocline depth gradient; should be deep in the west and shallow in the east. Mean precipitation should show modulation of Walker circulation. Seasonal cycle of SSTs in the eastern Pacific should be robust. They are warmest in boreal spring and coldest in boreal fall. Boreal spring Boreal fall SST variability peak in boreal winter and diminishes in boreal spring with relatively weak variability in the boreal summer and early fall. SST gradient is very strong in SON, so phase locking of ENSO peaks at that time too. It shows the spatial pattern of ENSO SST variability. Most models on SST pattern correlation with Nino3 SST extending west dateline. Tilt mode which explains ~50% of the variance and the warm water volume mode which explains ~30%. The first is associated with the phase of ENSO, the second is the recharge-discharge of warm water in the equatorial Pacific. Positive wind stress curl (westerlies at the equator) = pole ward transport of water. Negative wind stress curl (easterlies at the equator) = equator ward transport of water. Show Equation The nodal line is around 10S instead of south of 20S; the meridional extent of the mode is too small. The ENSO timescale is too short because the recharge-discharge is occurring too quickly. The autocorrelation will drop down the e folding scale at the typical length of an event. It shows the duration of events, the periodicity of ENSO, and zonal extent of the signal. Models commonly extend to far west and occur to regularly and frequently. It shows the eastward propagation of upwelling/downwelling Kelvin wave. It takes about 4 months for Kelvin wave cross the Pacific Ocean. We are looking for teleconnection with Indian ocean SST and northern tropical Atlantic SST. Winter Indian Ocean covaries with ENSO. The meridional width is a reflection of the periodicity of ENSO in the model (narrow = shorter). Also, look to see positive relationship; if not the mechanism of variability is not ENSO. They show the anomalous Walker circulation versus the Hadley circulation. The 200mb height anomalies forced by the heat source of ENSO. It is a stationary Rossby wave. It is a coupled air-sea interactions lead to the growth of the anomalies. The problem is that is requires an anomaly before it can grow. When the trades weaken, it allows the SST to rise because there is less upwelling and less evaporative cooling. The increases SSTs reduce the temperature gradient slowing the winds further. From the westerly wind stress anomaly a down welling Kelvin wave is generated which causes warm SSTs in the east. Eventually the corresponding Rossby wave reflects as an upwelling Kelvin wave which acts to restore conditions to normal. The period of ENSO is too short. It is just another name for the Delayed Oscillator theory. This theory requires that wind stress to be in phase with thermocline depth anomalies. It was found wind stress is not in equilibrium with thermocline depth in the observations as this theory predicted. SSTA was in equilibrium with thermocline depth anomalies. His theory suggested that it takes multiple reflections of waves to change the phase of ENSO. The problem with this is the multiple waves are not observed. Off-equatorial Rossby waves are much slower than equatorial Rossby waves so they take longer to reach the boundary. The Kelvin waves reelected by these seem to cancel out with the equatorial Kelvin waves. As a result the upwelling Kelvin wave has a reduced amplitude and is less efficient at removing the SST anomalies and so more waves are required (ENSO period is lengthened). There is an inversion at the cloud top and no convection in the column. SST gradients are stronger due to reflection, which gives rise to stronger easterlies at the surface, which induces a La Nina effect (the Walker circulation has moved more to the west). It is due to the ocean thermostatic properties. Increasing the warming over the ocean leads to an increased temperature gradient. This means increases winds and more upwelling of cold water in the east. The walker circulation weakens because the moisture content has gone up. The precipitation does not increase at the same rate as moisture therefore the mass flux must decrease. Tropical Atlantic VariabilityJJA OND and is located off the southwest coast of Africa. MAM The Pacific is confined to the east; the Atlantic is over the whole basin. Also, the cold season is shorter in the Atlantic than the Pacific. The wind stress is strong mainly in the West Atlantic and in the East Pacific. The warm anomaly in Nino3.4 SST heats the troposphere and spreads quickly in the atmosphere from Kelvin and Rossby waves around the globe. The increase tropospheric temperatures lead to a more stable environment which reduces the convection and circulation which reduces the evaporative cooling at the surface. This is how the SSTs warm along the equator around the globe. OLR also reduces. There is uncorrelated wind stress variability. When the wind stress is stronger westerly over the West Atlantic there is a strong teleconnection with ENSO. When there are easterlies, there is strong upwelling in the Atlantic and so no teleconnection with ENSO It is the anomalous pressure difference between Icelandic polar low and the Azores subtropical high. It is both interannual and interdecadal. Inter-hemispheric is also known as Meridional mode and tropical Atlantic variability. It is a dipole of warming of SSTs between the north and south tropical Atlantic. It is caused by wind induced surface heat fluxes either from ENSO (weakened trades; warmer in the north) or WISHE. |
