El Nino Responds

No, El Nino didn't technically go anywhere, but it took some vicious easterly shots for about a month, got up off the mat, and is currently on the offensive. A scroll through these various links from CPC (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml) reveals exactly what the easterly wind burst did to the overall ocean-atmosphere link. Despite SST anomalies generally remaining near 1 C between 160 W and 90 W, slightly cooler SSTs were pulled westward along the South American coast. It is likely this put some dent in the overall warming, especially from 105 W to the South American coast. Furthermore, the deep reservoir of warmer than normal subsurface water that was evident for much of June and July has been significantly depleted. If this warm water below the surface cannot be replenished, this El Nino episode may not last very long.

Running five-day means of SSTs shows that the 29 C isotherm remains near 175 W, which is approximately its location a couple of weeks ago. SST anomalies show weak warming continues across the equatorial Pacific (between 0.5 and 1 C), but only a small pocket of greater than 1 C anomalies is present between 138 W and 118 W. Contributing to this remains stronger than normal trade winds along and north and south of the equator. In the wind speed graphic, note the strong convergence taking place near 170 E; we'll discuss this shortly. The depth and structure of the thermocline shows a weak El Nino signature, but strong warming has taken place in the eastern equatorial Pacific about 50 meters below the surface. If the stronger than normal trades can relax in this area, it's possible this warmer water may rise closer to the surface. This will be something to watch in the coming months as it could have major implications on the location of the warmest anomalies in the equatorial Pacific. The location of the warmest anomalies has been shown to have major variations in precipitation patterns across the U.S. (Hoerling and Kumar 2002).




Global wind signals show that the easterly wind burst has subsided, and in fact, a renewed westerly wind burst began earlier this week with the downturn noted in the SOI. The 30 day average peaked above 10 on July 21 but has now since dropped back to about 1; expect this to go negative again in the next couple of weeks. Updated through July 27, frictional torque was near climatology but is likely stronger, especially with the addition of strong westerly winds in the equatorial western Pacific and in conjunction with the strong Aleutian low. Mountain torque has leveled off about 1 deviation above climatology with positive contributions comings from Eastern Asia and the tropics. This provides further evidence that the Asian monsoon and tropical forcing have been responsible for the global pattern observed in the last week. Relative AAM was near climatology but has likely since risen above climatology, and the tendency of relative AAM spiked to over 3 standard deviations above climatology. The combination of these two reveals that phase space 4 of the GWO was where the global atmosphere was earlier this week. With recent increases in frictional torque and relative AAM, it's likely that the current phase space projection would likely be 5. However, the current global pattern is best depicted by GWO phase 6.






The resurgence of El Nino can be attributed to the strong western Pacific tropical convection. For nearly a month, competing tropical signals were evident across the western Pacific and in association with the Indian monsoon. The later is beginning to shut down as the strongest forcing is taking place in the western Pacific basin, and in fact, satellite and OLR anomalies suggest the convection is centered near 150 E. The energy associated with this tropical convection has been dispersed into the mid-latitudes and will continue to do so for the time being. What does this mean? Any numerical models that don't handle heat (e.g. GFS), have been struggling and will continue to struggle until the tropical forcing is realized.




Synoptic impacts of the tropical forcing for week 1 will be a strengthening of the north Pacific jet by the end of the week, which will result in the development of a western trough, central states ridge, and New England trough. This will allow temperatures in the Pacific Northwest and New England to cool near and slightly below normal while the heat will return to the Southern Plains. Recent rainfall in the Southern Plains will ensure that this heat will be nothing like that experienced in the first half of July. Severe storms will also return to the Northern and Central Plains where the strong westerlies will be located. For our area, temperatures will likely be near to slightly above normal and rainfall will be below normal. For the first half of week 2, this pattern should remain in place with the heat centered slightly further east so this means continued above normal temperatures and below normal precipitation. The flow should flatten out for the later half of week two with temperatures likely backing down to near normal for our area.