Tropical convective forcing continues to reorganize across the Eastern Hemisphere. The MJO signal has weakened significantly. The intense thunderstorm activity currently along the South Pacific Convergence Zone (SPCZ) was initiated by the downstream troughs associated with the MJO and not a coherent eastward propagating convectively coupled mode. In fact, the western portion of this tropical forcing is shifting west as a convectively coupled Rossby mode.
While there was a strong signal of the MJO during much of December into early this month, several complex forcing-response-feedback circulation variations occurred. To save space, what I hope is most relevant to the readers is discussed here (comments welcomed). First, there was a strong trade wind surge downstream of the MJO that led to significant cooling of the warm central Pacific SSTs around the date line (anomaly and actual SST tendencies ~negative 1C). In consideration of other monitoring tools such as relative atmospheric angular momentum (AAM) transports, my feeling is that the warm ENSO peaked during December and is now decaying. The recent anomalous low level westerly winds along the SPCZ (weekly mean anomalies ~15m/s centered on the date line around 10S) I feel is transient and not the beginning of a “coupled warm ENSO response” (which is different than what I would have thought about a month ago). Secondly, as was discussed in my posting dated December 15th, a regime transition to cold/wet pattern for much of the USA occurred starting about a week ago (more said below). In fact, a negative phase of the PNA teleconnection index evolved which is consistent with the GSDM Stage 1 response that occurred (and not with the composite warm ENSO signal).
Per ESRL/PSD reanalysis data plots, as of 3 days ago the global tendency of relative AAM was ~ plus 25 Hadleys. As the MJO shifted east, zonal mean westerly flow anomalies initially developed along the equator and then propagated poleward, contributing to this positive tendency. These westerlies are currently in the subtropical atmospheres of both hemispheres with anomalies ~5-10m/s at 200mb (contributing to the current subtropical jet across the southern USA). Hence a nice evolution from GSDM Stage 1-2 has occurred. However, unlike my thinking from about a month ago, it now appears unlikely that a coupled ocean-atmosphere response involving the tropical forcing east of the date line will occur, typical of a warm ENSO (leading to GSDM Stage 3). Instead, this process may happen ~10S/160E during the next couple of weeks and then shift to perhaps 120-140E during February. As I type tropical forcing is once again increasing across the Indian Ocean where SST tendencies during the past week were ~plus 1C, and twin upper tropospheric anticyclones are already appearing.
My outlooks for weeks 1-3 remain unchanged from our January 12th posting. It remains most probable for the trough-ridge-trough pattern from the west/central Pacific to the central USA to persist, with the usual synoptic variations of amplitude. The subtropical jet should continue to undercut the east Pacific ridge leading to split flow along/off the west coast and a cold/wet regime for much of the country particularly east of the Continental Divide. If my notions of the centroid tropical convective forcing shifting back to ~10S/120-140E during February are correct, the above circulation pattern would be expected to do the same allowing locations along the USA west coast to get precipitation especially California (loosely GSDM Stage 4-1).
We are planning on issuing another discussion this Friday, along with a parallel posting on the HMT Blog. The MWR WB paper on the GSDM is expected to appear in the February 2007 issuance.