Exercises: Chapter 8 Answers
1. Compare the longitude of the maximum in high frequency eddy variance (T<7 days) with the longitude of the maxima in intermediate frequency variance (7<T<30 days) in Fig. 8-­‐2a. Are your results consistent for the Atlantic and Pacific Ocean regions? Yes in both regions the intermediate frequency variance peaks downstream (East) of the high-­frequency variance. 2. The low-­‐frequency variability in the Southern Hemisphere in Fig. 8-­‐2b is centered upstream of the Palmer Peninsula that juts out from Antarctic toward South America. Can you think of any reasons why this might be the case? How do you think this might influence the variability of sea ice in the Ross, Amundsen and Bellingshausen Seas in that region? The mountains of South America and the Palmer Peninsula will generate Rossby Waves. Since topography is fixed in space, it corresponds to zero frequency, so the mountains could generate low-­frequency variability, especially since the zonal winds vary with the SAM, etc. These Rossby waves generate north-­south winds that can push the ice around and supply either warm or cold air to the sea ice. When the wind blows the sea ice offshore cold advection also happens so that the leads left by the ice leaving the coast are quickly frozen over and the sea ice expands in area coverage and northward extent. When the wind blows from the north the sea ice is pushed southward and warm air is supplied to the ice area, so the ice extent contracts. 3. The Northern Hemisphere has two wintertime storm tracks, one over the Pacific Ocean and one over the Atlantic Ocean, but the Southern Hemisphere has only one long one centered in the southern Indian Ocean. Why do you think this is the case? The shape of continent of Antarctica, the placement of continents, and the SST all give a strong meridional gradient and weak zonal gradients in that area, so it is perfect for a storm track. Between the Pacific and Atlantic the Andes and Palmer Peninsula break up the storm track with mountains. 4. Fig. 8-­‐3 indicates that the spatial scale of disturbances get larger as the time scale gets longer. Do you know why? This has to do with Rossby wave dynamics and is a more advanced dynamical subject. Briefly, long waves have a stronger effect of planetary vorticity advection compared to relative vorticity advection. Since planetary vorticity advection causes the waves to propagate westward, while the winds advect vorticity toward the east, longer waves can remain stationary in stronger winds than short waves, which tend to be blown downstream with the flow. 5. Fig. 8-­‐4 indicates that a north-­‐south migration of the Southern Hemisphere extratropical jet explains more of the variance of zonal wind than the sharpening or broadening of the jet. Can you explain why? This is an advanced dynamical meteorology subject. The jet is sharpened by the propagation of waves away from the jet. When the jet gets too sharp, then the waves can’t propagate very far and the feedback between eddy propagation and jet strength is weakened. If the jet keeps the same shape, but just shifts longitude, then the wave propagation away from the westerly wind anomalies can reinforce the initial anomaly. So jet shifts without change of shape are helped by dynamical feedbacks, whereas the width of the jet is set by dynamical feedbacks that tend to return the jet width to its optimal value, a negative feedback if climatology is near this optimal value. 6. Fig. 8-­‐6 shows that the OLR anomalies associated with the Madden-­‐Julian Oscillation are strongest in the tropical Indian and Western Pacific regions, but the wind anomalies extend to other longitudes and into the extratropics. Can you explain why? The OLR anomalies are associated with tropical convection that drives vertical motion that excites waves, both Rossby waves and Kelvin waves. These waves propagate away from their sources in the convective region and influence the winds far away from the heat source. 7. Looking at Fig. 8-­‐7 it is apparent that a strong temperature gradient exists from east to west below the surface. If warm temperature is associated with less dense water, what would you expect the pressure gradient to be 150meters below the surface? What direction would the water at 150 meters deep move if there was no wind stress being applied at the surface? Since cold water is denser, you would think that the pressure would be higher in the East Pacific. Since the pressure is higher in the east, that would tend to push the water toward the west. Probably then the water at the surface would have to move east to compensate. The slope of the thermocline would then decrease. 8. Compare and contrast the OLR anomalies associated with ENSO in Fig. 8-­‐10 with those associated with the MJO in Fig. 8-­‐6. It has been argued that a big MJO event can be a precursor to a warm ENSO event. Can you explain why? A big MJO event can cause the OLR anomalies to move eastward, just as they do in a warm ENSO event (El Niño). This would be expected to weaken the trades in the West Pacific, which might decrease upwelling there, which might cause the SST to warm, which is like an incipient El Niño. Since a big MJO starts the feedbacks that lead to warm ENSO, then maybe a big MJO could set the timing of the start of El Niño. 9. Theory says that tropical heating anomalies generate Rossby waves near the equator that follow great circle routes as they propagate into the extratropics during winter. Can you connect the centers of the anomalies in Fig. 8-­‐11 and trace out the paths along which eddy energy must be propagating? Nov-March
May-Sept
Magenta arrows show direction of Rossby Wave Propagation. 10. Assemble a list of arguments for and against the following statement. “The PDO is just the low-­‐frequency, extratropical signature of ENSO, nothing more.” For: 1) It has the same centers of action in the East Pacific and North Pacific and its shape looks a lot like like the ENSO signal, except that the midlatitude part is relatively bigger. 2) We know the atmospheric bridge effect imposes the midlatitude signal in the North Pacific, so the cool SST there are caused by warm enso events. And warm in N. Pacific by La Niña. 3) We know that the dynamics of oceanic Rossby waves are slower in the midlatitudes than the equator, so there is a physical reason why the midlatitude part would persist longer. Against: 1) The time scale is different, with the PDO lasting decades in one phase and ENSO only 3 or 4 years. 2) Extratropical parts of PDO persist even when ENSO shifts from warm to cold phase.