Friday, September 21, 2007

La-Nina in Command, Nemesis Lurks

The spatial distribution of global tropical SSTs has become much more consistent with what may be evolving into a basin-wide cold event. Per TAO buoy data and CPC analyses, negative equatorial SST anomalies currently extend from about 160-165E to South America. Cold anomalies are greater than minus 2C ~130W extending to at least a depth of 200m near 160W. Given the seasonal cycle, these anomalies are not trivial.

The Indian and particularly the west central Pacific Ocean basins remain warmer than climatology. Anomalies are greater than 2C with totals in excess of 30C for the latter, with a warm horseshoe spatial pattern extending into the extratropics. The warmth across the west central Pacific has shifted slightly to the south during the past week. Slightly warmer than average SSTs remain from the Caribbean into the north tropical Atlantic Ocean.

My own feeling is during the past 4-6 weeks we have been observing the equatorial SSTs respond to the atmosphere (discussed below), going back to subseasonal variations late boreal fall 2006. There is now observational evidence that ocean-atmosphere coupling has been starting to occur during the last few weeks. However, the problem of 2 regions of tropical convective forcing: west central Pacific and Indian Ocean basins, remains. I also remain concerned this La-Nina will mature before the heart of boreal winter.

Tropical convective forcing continues strong across the Eastern Hemisphere, centered ~10-15N/120E while extending from the central Indian into the northwest and west central Pacific Ocean basins. This represents a loose consolidation between the Indian and west central Pacific Oceans tropical forcing since early this year. As a response the easterly trades along the equator have strengthened during the past several weeks, with wind speed anomalies in excess of 5m/s at times, leading to the recent SST cooling discussed above.

Four subseasonal variations consisting of northeastward propagation of tropical convection within this stationary signal have occurred since ~1 June 2007. These have had periods of ~30 days, and I do not consider these to be MJOs. These events have had strong interactions with the extratropics including ~15 day variations of the global mountain torque. Event number 3 occurred prior to 1 September and number 4 is in progress as I type, currently enhancing convection around the Philippines. In fact, from the full disk satellite imagery and OLR/A Hovmollers, I can still easily identify 2 regions of enhanced tropical rainfall, the north central Indian Ocean and from the west central into the northwest Pacific. The point is these subseasonal variations may lead us back to “nemesis” by boreal winter.

The last subseasonal event had a decent zonal mean anomalous poleward AAM transport signal in the 30-60N band around mid August, followed the ~30 Hadley positive global mountain torque just before the start of this month. As of 19 September per ESRL/PSD R1 AAM plots, once again there is a strong poleward AAM transport signal in the 30-60N band (~0.5-1 Hadleys), and another decent positive global mountain torque event may be probable. These types of behaviors have been contributing to the poleward propagation zonal mean easterly wind flow anomalies from the equatorial into the subtropical atmospheres of both hemispheres. In fact, at 200mb while zonal mean easterly wind anomalies of ~5-10m/s are present around 25-30N, there are weak zonal mean westerly wind flow anomalies along the equator. The latter are tied to twin upper tropospheric tropical cyclones ~150W. The Global Wind Oscillation (GWO) quasi-phase space plot updated through 18 September shows at least 2 circuits around GSDM Stage 1 since mid August due to the subseasonal events and meridional propagation of the anomalous zonal mean equatorial easterlies.

Global relative AAM remains low, ~ 2 AMUs below the R1 data climatology, while the GWO is still minus 2 standard deviations at GSDM Stage 1 (typical of La-Nina). The global frictional torque is only slightly positive but increasing. I think the global mountain torque will increase forcing the AAM tendency upward during the next 1-2 weeks. Anomalous zonal mean westerly flow, roughly 10m/s at 200mb, continues poleward of the easterly wind anomalies, ~50N and 40S. This translates to anomalously strong and poleward shifted ridges for both hemispheres.

Animations of upper tropospheric daily mean vector wind anomalies give a relatively simple and stationary picture (at least in a weekly mean) of twin tropical/subtropical anticyclones ~120E with downstream cyclones ~150W. Rossby wave energy disperions arcing from this tropical forced response into the extratropics link up nicely to central Pacific Ocean ridges and western continental troughs across the Americas. This whole pattern is roughly a 30 degree westward shift from about 10 days ago. In fact, poleward amplification of midlatitude ridges across the Southern Hemisphere into the high latitudes may lead to a rare sudden stratospheric warming (SSW) in the Antarctica region (currently austral spring). Per Tokyo Climate Center E-P fluxes have been strongly directed upward at 60S and 10hPa temperatures have risen at least 35C since early this month.

I think we are now observing a coupled response involving the tropical SSTs, enhanced Eastern Hemisphere tropical convection and the global circulation. To me, this whole evolution started late boreal fall 2006 first with the Indian Ocean tropical forcing dominating, then La-Nina like atmospheric base state commencing ~ 1 December 2006, and finally the SSTs catching up. Given that the anomalous zonal mean easterlies are propagating off the equator; I remain concerned this La-Nina may mature before 2008. In any case, the evolution toward a coupled La-Nina (meaning not just the atmosphere) is already impacting the synoptic weather variability not only across North America, but globally.

Most week 1-2 ensemble means from the global operational centers show the general west coast-Rockies USA trough and eastern states ridge. Model agreement should be better now given possible coupling. This means an active flow regime for much of the western 2/3rds of the country, and temperature and precipitation anomalies should be understood by now. Particular attention will need to be paid to the Pacific Northwest as strong baroclinic storms containing high winds and heavy rain periodically move ashore. I have a thought for a brief GSDM Stage 2 response after an intense baroclinic storm moves across the Plains next weekend. I tie this to the possibility of a mountain-frictional torque variation linked to subseasonal #4 discussed above.

As boreal winter approaches, western USA troughs should become stronger and more persistent, leading to an active southwest flow storm track across the Plains. This type of pattern can be favorable for Arctic outbreaks combined with intense precipitation. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008. On the latter, careful daily monitoring of tropical forcing across the very warm west central Pacific is a must starting now.

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Per reasons discussed in my 14 September posting, I think AWB has put the kibosh on the Atlantic tropical cyclone season, at least for now. Yes, there is another “Gabrielle” festering as TD#10.

The west central into the northwest Pacific Ocean is probable to continue with a tropical cyclone hazard through week 2. Impact areas include the Philippines possibly north into Japan. Another area of concern is the Bay of Bengal including coastal sections. In general, locations from India into Southeast Asia are probable to continue with rounds of intense/severe thunderstorms. Much of Europe is probable to have active weather for at least the next couple of weeks.


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 stochastically forced component of the GSDM.

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). 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. I will try to post another discussion ~ 28 September.

Ed Berry

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