La Nina and the Seasonal Transition into Spring

Nothing new to report in regard to the SSTs. Anomalies vary from ~minus .5-2C along the equatorial cold tongue with the coolest water around 125W having a temperature of about 24C. Anomalously cool water with readings as low as minus 5C extend to a depth of roughly 200m across the tropical east Pacific while anomalous warmth prevails at depth west of the date line. Typical of a cold event, the thermocline remains steeper than normal across the equatorial Pacific Ocean. Finally, SSTs remain above average from the South Indian Ocean (SIO) to the South Pacific (see http://www.pmel.noaa.gov/tao/jsdisplay/ and http://www.cdc.noaa.gov/map/clim/sst.shtml).

The trades remain ~5-10 m/s stronger than normal from the central into the west Pacific while comparable westerly anomalies persist from the SIO to north of Australia. The latter suggests enhanced lower tropospheric convergence around Indonesia, where intense convection has become persistent.

The MJO signal continues weak. The relatively coherent eastward movement of tropical convective forcing seen for the past few weeks has essentially stalled centered on the equator over Indonesia ~140E. Tropical thunderstorm activity remains strong from South Africa into the SIO, and there are flare-ups across the warm South Pacific SSTs. Please see the following links for details:

http://www.cdc.noaa.gov/map/clim/olr_modes/

http://www.ssec.wisc.edu/data/geo/mtsat/ (The user can navigate to other imagery)

As discussed above, the most intense tropical convective forcing is in the eastern hemisphere (EH). Over the past several months we have had nothing short of a constant interplay of tremendously complex interactions involving tropical SSTs forcing the atmosphere via convection, and the atmosphere forcing back via little understood non-linear feedbacks including those from the extratropics and even the stratosphere. Some attention to these matters has been given in previous Blogs and in our recent February 15th weather-climate discussion at http://www.cdc.noaa.gov/people/klaus.weickmann/disc021506/weather_climate_discussion_10Feb06.html
Additional research on understanding these issues is planned, including on extending the SDM to account for some of these behaviors.

The point of is that finally, during the past 3 weeks or so, the circulation did respond as generally would be expected with the tropical forcing shifting into the EH. At this time, anomalous twin subtropical anticylones dominate Africa and the IO with downstream twin subtropical lows around the date line. The flow is strongly split across the North Pacific with an anomalous ridge across the north central Pacific and a projection onto the negative phase of the PNA. All of this represents a circulation reversal from about early-mid February.

However, there continue to be behaviors that linear thinking will not handle. Another retrogressive high latitude anticyclonic circulation gyre is once again moving into Canada and likely headed for Alaska (which caused the NAO to be negative), and the central Pacific subtropical cyclones are displaced toward the northern hemisphere. In fact, these anomalous lows have ramped up the eastern Pacific westerly flow so much that tropospheric relative AAM is nearly 1 standard deviation above the 1968-1997 climatology (see link below).

http://www.cdc.noaa.gov/map/images/reanalysis/aam_total/glaam.sig1-21.90day.gif

For the 200mb level, this translates to at least 5-10m/s above average zonal mean westerly flow across the subtropical atmospheres of both hemispheres. Thus Stage 1 of the SDM only "loosely" represents the current-weather climate situation. In addition to feedbacks from La-Nina, the seasonal transition into boreal spring is making the atmosphere "more complicated".

In short I think we may be going into at least a quasi-persistent situation consisting of EH tropical convective forcing centered on Indonesia (near 140E) a ~SDM Stage 1-2 circulation across the PNA sector. About a year ago there was ocean-atmosphere coupling with central Pacific tropical convective forcing due to El-Nino (see http://www.cdc.noaa.gov/people/klaus.weickmann/disc022205/Draft_of_WX_CLIMATE_Disco_22FEB05.html

http://www.cdc.noaa.gov/people/klaus.weickmann/disc041505/Draft_of_WX_CLIMATE_Disco_18APR05.html for details). At that time we had SDM Stage 3, which would be expected. It is possible that a La-Nina verion of a coupling situation is now occurring. If that is the case, the lifetime is unclear. At least for week 1, the above mentioned retrogression may link up with the central Pacific ridge, leading to an even stronger projection onto a reverse PNA.

The weather for the CONUS for at least the next 7-10 days looks to be very active, with possibly several intense mobile synoptic troughs digging into the western part of the country. While most models have a reasonable handle, timing and other details are unclear after day 3 and particularly after day 5.

This means cold/wet weather is probable for much of the west coast particularly the Pacific Northwest, with an active SW-NE storm track across the Plains. While portions of the central and southern Plains may be at risk for heavy rain and severe local storms, locations from the Front Range to the Upper Mississippi Valley may be at risk for all forms of winter precipitation. Intense elevated thunderstorm activity may accompany the winter weather especially across the Upper Mississippi Valley. Finally, the Ohio and Tennessee Valley regions will also be at risk for above average rainfall and possibly severe storms. Summer-like warmth is likely for the deep south with above average temperatures for most of the east.

My suspicions is that later into week 2 (days 10-14) mobile troughs will deepen farther east toward the Rockies and Plains. That would bring colder air (there is still a source) eastward into at least the central portions of the CONUS, while the weather remains quite active. I would expect at least 1 late season winter storm across the central USA during this period.

Southwest Kansas is going to be on the storm track through at least next weekend. Most precipitation will continue to our east and north. However, there should be opportunities for at least a few rounds of showers and storms, including possibly severe. High wind, including blowing dust and fire danger, is also a concern. Opportunities for some additional precipitation should continue for week 2.

Ed Berry