Monday, August 20, 2007

When Will The Atmosphere Go "Up"?

Note: The ESRL/PSD AAM plots now present the “complete” budget (updated through 17 August as of this writing), and will be referred to.

Per NOAA/CPC and NOAA/PMEL/TAO buoy data; there is not much overall change to the spatial distribution of global tropical SSTs. The equatorial Indian Ocean and particularly the west central and northwest Pacific Ocean basins (with the horseshoe into the subtropics/midlatitudes) remain above climatology. Anomaly magnitudes are ~1-1.5C with the former and up to 2c for the latter, with totals in excess of 31C along the equator near 140E. The west Pacific Ocean anomalies are deep extending to at least 400m, suggesting an anomalously warm west Pacific Ocean warm pool. Will some of this warmth strongly shift east “sometime during the future?”

Anomalies across the equatorial East Pacific cold tongue remain below normal with recent 5-day averaged values varying from ~.5C just east of the date line to greater than 2C near 110W (~Nino 3). The cool anomalies still extend to around 150-200m deep in the region of 140-160W, with values less than minus 3C at 150m/140W. However, these cool anomalies are relatively localized. It will be interesting to monitor the persistence of these anomalies going into boreal fall since the seasonal cycle favors cooling. The tropical Atlantic Ocean basin remains near average with totals ~27-28C with plus 1-2C anomalies in the areas of the Caribbean and Gulf of Mexico. Overall, again, while the global circulation is strongly La-Nina like (GSDM Stage 1), the SSTs only weakly support that response.

Enhanced tropical convective forcing continues across the Eastern Hemisphere, extending from the central Indian Ocean into portions of the Tropical Northwest Pacific Ocean (TNWP). During the past few days some consolidation of enhanced rainfall has been occurring near 5N/80E, within this larger envelop. Real-time Multivariate MJO Index (RMM) phase space plots from various operational global weather centers suggest a weak MJO signal may be developing in the region of the Indian Ocean. There is also a hint of this possibility from Hovmoller plots designed to isolate coherent modes of tropical variability. Animations of upper tropospheric daily mean vector wind anomalies indicate fairly well defined twin tropical/subtropical anticyclones ~60-90E with downstream twin cyclones west of the date line. Whether or not this evolves into any real coherent MJO dynamical signal or even a northward propagating mode is unclear. Predictions from the RMMs are inconclusive.

Global total and relative atmospheric angular momentum (AAM) are at the lowest magnitudes in at least a year, with the latter near 4 AMUs (at least 3 standard deviations) below the R1 data climatology through 17 August. In fact, the quasi-phase space plot depicting the Global Wind Oscillation (GWO) is “off the scale”. Deep zonal mean easterly wind anomalies dominate much of the tropical atmosphere, with the largest magnitudes across the Southern Hemisphere near 15S (~10 m/s at 200mb). The cause of this extremely low AAM, GSDM Stage 1 regime is unclear. I can speculate this all goes back to dynamical forcing-response feedback processes linked to AAM transports and large GWO circuits that started ~ 1 December 2006, when El-Nino was “stopped in its tracks”. In any event, the SSTs themselves do not support this global circulation. The intense zonal mean easterly wind anomalies have been broadening in latitude during the last couple of weeks. Daily monitoring of if/when these zonal mean easterlies propagate poleward off the equator are critical to determining the most probable future global circulation state.

In recent days both global surface torques has become positive, with the frictional and mountain torques ~10-15 Hadleys. In addition, global relative AAM tendency is near zero and the earth AAM is slightly positive (please see ESRL/PSD R1 data plots for zonal mean details). I think this may be linked to the Eastern Hemisphere tropical convective forcing discussed above. In other words, we should see increasing trades as a response to the Indian Ocean convection. The latter is starting to occur particularly just south of the equator hence driving up the frictional torque. Additionally, I offer speculation that there may also be downward transport of the easterly wind flow anomalies to the earth’s surface coming both from the synoptic eddies (lots of Rossby wave energy dispersion recently going on linking the tropics to the extratropics) and Hadley circulations. However, a diagnostic tool such as the baroclinic CHI problem is needed in real time to quantify this diagnosis.

My bottom line is that at least some zonal mean subtropical/midlatitude westerly flow should be added to the atmosphere during the next 1-3 weeks. However, at best this will only be reflected as a weak oscillation depicted the GWO within an overall steady state GSDM Stage 1 circulation regime. Interestingly, the seasonal transition into boreal fall will impact the outcome of regional-scale circulation anomalies tied to this global weather-climate situation. For example, a reverse phase of the Pacific-North American teleconnection is most probable during GSDM Stage 1 during the cold season. Should this low AAM regime persist through boreal winter, much of the USA focusing on the western USA into the Plains may have the odds tilted toward a cold and stormy pattern (western trough/southeast states ridge), if there is an Arctic cold air source. Careful daily weather-climate monitoring will be needed this fall on just how steady state our Stage 1 regime will be. For example, should there be 2 regions of Eastern Hemisphere tropical convective forcing with the west Pacific Ocean becoming dominate, a “completely different” and sudden evolution may occur.

Most ensemble prediction schemes from operational global weather centers are coming into general agreement for the western USA trough-southeast states ridge pattern weeks 1-2. Improving model agreement makes sense given our general stationary regime having regular Rossby wave energy dispersions linking the tropics and extratropics. However, there is still large uncertainty per issues discussed above.

There is little change from the outlook I issued nearly a week ago. Cooler and wetter is the most probable trend for particularly the Rockies into the Plains of the USA during the next 1-3 weeks. In fact, portions of the northern Rockies may see their first snowfall. The center part of the country may become increasingly stormy with severe thunderstorms and flooding rainfall (which has already occurred in some areas during the last few days). Some of this activity is likely to extend into the Ohio Valley and eastern states. The southeast is likely to continue with above normal heat and humidity. It is obviously unclear to say where any possible land falling tropical cyclone will go. However, the need to monitor this situation is here, and please see the latest statements from the NOAA/NWS/Tropical Prediction Center.

Locations from the central and eastern equatorial Indian Ocean into China and Southeast Asia are likely to continue with the poundings of intense monsoon rainfall for possibly a few more weeks. This region of intense convection may shift northward week 2. The likelihood of thunderstorm activity across the TNWP including the Philippines during weeks 1-3 is unclear. However, frontal activity coming off of East Asia interacting with the warm SSTs could quickly lead to very active weather across those regions. Equatorial North Africa into the Atlantic may have climatologically active thunderstorm activity including tropical cyclone development.


An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory component of the GSDM.

These are probabilistic statements, and work is ongoing to quantify in future posts. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. My next update will again be early next week due to travel.

Ed Berry


Jim H. said...

Fascinating stuff. It would appear the GLAAM is having it's moment of fame right now. The current readings are far below anything on record at least back to 1958. At least according to the data I'm finding on it.

I am a big La Nina fan and can't help but think this one is still going to take off. you have any links to numerical data on the Earth AAM anomalies. I am trying to do some reasearch on it and the graph data isn't as useful.

Thank you for a very interesting and insightful blog!

Ed Berry said...

Hi Jim,

Thank you for the comment. Right now there is no available ASCII file for the actual data used to plot the graphs. I hope we can make that available at some point.