In a post last June I mentioned that the atmosphere wasn't responding to warming Pacific equatorial SSTs. At that time normalized relative AAM values had dropped to around -2 sigma. Although El Nino-like SST conditions developed in the equatorial Pacific in July and persisted in August, the atmospheric component (AAM) did not follow suit. The average AAM for the period July-September was less than any other El Nino year going back to 1950. (See chart below.)
Following a relative rapid 2 sigma increase over a period of six months, the July/August value of the Multivariate ENSO Index (MEI) dropped from 1.139 to 0.579, and its rank for the season dropped from just below a 'strong' El Nino to just below a 'weak' El Nino. Over the last month SST Anomaly in Nino Region 3.4 has dropped 0.6°C. A review of the ONI record reveals that a drop in the index, which is based on a 3 month running mean of Nino 3.4 anomaly, has not occurred before the Sep-Oct-Nov season during a warm episode.
As of mid September most models were still forecasting development of weak El Nino conditions. The IRI/CPC Plume-based ENSO Forecast puts the probability of El Nino conditions in the Sep-Oct-Nov season at a little over 80%! The probability of Neutral conditions is pegged at a little under 20%. The chance of returning to La Nina conditions is considered virtually nil. Going back to 1950, year two La Nina conditions almost always transition either back to La Nina conditions or to El Nino conditions. Depending on the climatology used there is either one (1985-86) or no cases of a transition to Neutral conditions from a second year La Nina.
Many dynamical model MJO forecasts are predicting a developing MJO signal in the Western Pacific, and that appears to be occurring. Velocity potential loops and Pacific Basin stitched satellite imagery show enhanced convection west of the date line, and today's MJO phase space plot from CAWCR/BOM shows a signal beginning to emerge. On the wind side, Mountain torques are over +4 sigma and Coriolis torque is at -2 sigma. Whether the (apparently) emerging MJO will help reboot our fading El Nino remains to be seen.
Update Friday, October 5, 2012. Eastward-propagating MJO-like signal didn't evolve as forecast by GFS (and several other models). Here's today's MJO phase space plot from CAWCR/BOM. Large swings in the magnitude of Mountain and Coriolis torques have continued. Relative AAM remains at about -1 sigma. The Early October CPC/IRI Consensus Probabilistic ENSO Forecast indicates an increasing chance of Neutral conditions developing over the next several months, but still gives an edge to El Nino conditions developing before the end of 2012.
Following is a chart comparing 2012-13 to warm ENSO episodes that have occurred since 1950. The warm episodes are based on the revised Oceanic Niño Index (ONI) based on multiple-centered 30 year base periods and are those specified in the CPC's tabulation of Cold & Warm Episodes by Season. A description of the parameters follows the chart. With the exception of years prior to 1957, a GWO phase space plot is included for each warm episode.
|
Year
|
Jul-Sep
AAM
|
Nov-Mar
AAM
|
Peak MEI
|
Peak MEI Season
|
Peak
ONI
|
Peak ONI Season
|
L.A. Rain
|
GWO
Phase Plot
|
| 1951-521 |
-- |
-- |
0.853 |
JULAUG |
1.2 |
SON |
26.21 |
-- |
| 1952-533 |
-- |
-- |
0.840 |
APRMAY |
0.7 |
MAM |
9.46 |
-- |
| 1953-543 |
-- |
-- |
0.522 |
AUGSEP |
0.8 |
ASO,SON,OND |
11.99 |
-- |
| 1957-58 |
-- |
0.773 |
1.473 |
DECJAN, JANFEB |
1.8 |
NDJ, DJF |
21.13 |
|
| 1958-592 |
-0.919 |
-0.206 |
0.803 |
JANFEB |
0.6 |
NDJ, DJF,JFM |
5.58 |
|
| 1963-64 |
0.005 |
0.046 |
0.857 |
OCTNOV, DECJAN |
1.4 |
OND |
7.93 |
|
| 1965-66 |
-0.826 |
-0.748 |
1.483 |
JULAUG |
1.9 |
SON,OND |
20.44 |
|
| 1968-694 |
0.130 |
0.513 |
0.868 |
JANFEB |
1.1 |
DJF, JFM |
27.47 |
|
| 1969-704 |
0.358 |
0.413 |
0.644 |
OCTNOV |
0.9 |
SON,OND |
7.77 |
|
| 1972-73 |
-0.096 |
-0.239 |
1.886 |
JUNJUL, JULAUG |
2.1 |
OND,NDJ |
21.26 |
|
| 1976-77 |
0.284 |
-0.828 |
1.027 |
AUGSEP |
0.8 |
OND,NDJ |
12.31 |
|
| 1977-78 |
-0.646 |
1.008 |
1.007 |
SEPOCT, OCTNOV |
0.8 |
OND, NDJ |
33.44 |
|
| 1982-83 |
0.938 |
2.337 |
3.037 |
FEBMAR |
2.2 |
NDJ, DJF |
31.25 |
|
| 1986-875 |
0.232 |
0.019 |
2.122 |
APRMAY |
1.3 |
JFM |
7.66 |
|
| 1987-885 |
1.153 |
1.000 |
1.951 |
JULAUG |
1.6 |
JAS, ASO |
12.48 |
|
| 1991-92 |
-0.008 |
0.808 |
2.271 |
MARAPR |
1.6 |
DJF |
21.00 |
|
| 1994-95 |
-0.422 |
0.764 |
1.434 |
SEPOCT |
1.2 |
NDJ |
24.35 |
|
| 1997-98 |
1.811 |
1.481 |
3.001 |
JULAUG,AUGSEP |
2.4 |
OND |
31.01 |
|
| 2002-03 |
0.047 |
0.324 |
1.184 |
DECJAN |
1.3 |
OND,NDJ |
16.49 |
|
| 2004-05 |
-0.020 |
0.747 |
1.018 |
FEBMAR |
0.7 |
JAS-NDJ |
37.96 |
|
| 2006-07 |
0.143 |
-0.322 |
1.289 |
OCTNOV |
1 |
OND, NDJ |
3.21 |
|
| 2009-10 |
-0.103 |
0.303 |
1.520 |
JANFEB |
1.6 |
NDJ,DJF |
16.36 |
|
| 2012-136 |
-0.951 |
0.296 |
1.139 |
JUNJUL |
0.6 |
SON |
5.85 |
|
|
1. AAM and AAM tendency anomaly data for 1951-52 not available.
2. AAM anomaly is average for Jan-Mar 1958.
3. Continuous warm episode from DJF 1952/53 to JFM 1954.
4. Continuous warm episode from JAS 1968 to DJF 1969/70.
5. Continuous warm episode from JAS 1986 to JFM 1988.
6. Data as of May 8, 2013.
|
Jul-Sep AAM & Nov-Mar AAM: The mean of the global relative atmospheric angular momentum anomaly for the periods July 1 to September 30 amd November 1 to March 31 of the following year. Data is from the GWO phase space data file linked on the Global Synoptic Dynamic Model page of the PSD Map Room Climate Products. Reference Weickmann and Berry, 2008.
Peak MEI: The peak seasonal value of the Multivariate ENSO Index (MEI). Reference Wolter and Timlin, 1993, 1998. MEI values are normalized and may change as new data is added.
Peak MEI Season: The peak bi-monthly season(s) for which the MEI is computed.
Peak ONI: The peak Oceanic Niño Index (ONI) based on SST anomalies in the Niño 3.4 region. Reference Climate Prediction Center Cold & Warm Episodes by Season (Multiple centered 30-year base periods.)
Peak ONI Season: The peak tri-monthly season(s) for which the ONI is computed.
L.A. Rain: The water year precipitation total in inches for Downtown Los Angeles (USC). Reference NWS Los Angeles/Oxnard Downtown Los Angeles Climate Page, 1921-2006 Water Year Rainfall.
GWO Phase Space Plot: Plot of global relative atmospheric angular momentum anomaly vs. global relative atmospheric angular momentum tendency anomaly for the period November 1 to March 31 of the following year. Data is from the GWO phase space data file linked on the Global Synoptic Dynamic Model page of the PSD Map Room Climate Products. Reference Weickmann and Berry, 2008.