Tuesday, February 20, 2007

MJO and Atmospheric Angular Momentum

Tropical SSTs remain roughly 0.5-1.0C above average over much of the South Indian Ocean as well as around the equatorial date line into the South Pacific. Actual SSTs are generally in the 29-30C range across these regions. Significant cool anomalies are now being observed by the TAO buoy array around 0/120W with magnitudes of about minus 1C. Values as low as minus 6C at ~50m deep have been recently observed across this region. The latter is the result of an upwelling oceanic Kelvin wave from the December 2006 MJO and may be contributing to a transition from El-Nino to La-Nina.

Full disk satellite imagery and other monitoring tools depict intense tropical convection across several regions of the globe. These include South Africa, the South Indian Ocean, the north coast of Australia into the South Pacific and much of Brasil. This is a rather complicated situation; however, a slowly evolving dynamical signal projecting onto the MJO is emerging currently centered ~10S/80E. Statistical tools such as the multivariate EOF Wheeler technique and empirical methods such as the time-filtered coherent modes Hovmollers support this notion. Furthermore, a rough phase speed computation of the convective envelope gives me about 4 m/s (3 deg long/day), consistent with a MJO.

There is also strong diagnostic support that a MJO signal is developing across the Eastern Hemisphere. Recall there was an intense flare-up of tropical convection across the warm South Pacific SSTs during about the middle of January. Through interactions with the Southern Hemisphere extratropics, a dynamical signal moved east to intensify convection across South Africa. It is this forcing that has now shifted into the South Indian Ocean. The global circulation has responded strongly to this redistribution of tropical forcing, with, for example, the tendency of global relative atmospheric angular momentum (AAM) down to about minus 50 Hadleys as of February 17th per re-analysis data! There are also other contributions to this tendency such as the current “negative phase” of a mountain-frictional torque index cycle. This is largest magnitude of a negative AAM tendency signal in at least a year and in itself represents an extreme weather-climate event. What is even more interesting is this negative AAM tendency is after the positive maximum of ~30 Hadleys around January 10th (roughly 50 days ago).

The point is westerly flow is being removed from the atmosphere by the earth and the spatial rearrangement of the tropical forcing has contributed to it. Furthermore, if one more carefully understands the time and space scales of what was first adding westerly flow to the atmosphere, early January, then the current removal of it, there is consistency with the MJO time scale. Currently zonal mean anomalous easterly flow prevails throughout the tropical and subtropical atmospheres with magnitudes of ~5-10m/s. Additionally, there are anomalous twin upper tropospheric subtropical anticyclones developing across the Indian Ocean with downstream cyclones and surface wind anomalies that are generally opposite. This means we already have a tropical baroclinic mode as a response to the developing MJO convection.

To me it is not a matter of whether there is a MJO, it is simply how strong will it be during the next several weeks. I do think the eastward movement of this MJO is going to be truncated (not get past roughly Indonesia) given recent SST trends. I also think our west central-South Pacific signal will continue with roughly 10-15 day flare-ups and assist with enhancing the climatologically strong southern USA subtropical jet.

We are currently in GSDM Stage 4 and it is probable to go into Stage 1 during the next 2-3 weeks (with at times strong subtropical jets). More models and their ensembles have become supportive of my past feelings about particularly the western and central portions of the USA going into an active regime. The ensemble means do struggle with differing solutions (along with a lot of spread) after about day 10. This is not at all a surprise to me given the abrupt shifts is circulation anomalies as measured by AAM tendency. With variations in amplitude including “lulls”, I think this regime may mature during March, especially if the tropical forcing becomes more persistent around 120E in the presence of the central and South Pacific signal.

As stated in my February 16th posting, we should several troughs first impact the west coast and then move northeast across the Plains (loosely – already starting). The western states will cool while the east warms. Arctic air may initially build up across Alaska (due to blocking west of the state) and then come into the western/central states during early March. March 2007 may be exceptionally active (above climatology) with heavy rain/severe storms across the Deep South into the east while late winter/early spring blizzards pound much of the Rockies into the Plains/Upper Mississippi Valley.

Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post at least a short update this Friday. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.

Ed Berry

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