Saturday, February 25, 2006

Linear as She (La-Nina) Goes

SSTAs remain ~minus .5-1C below normal across most of the central and eastern equatorial Pacific, from about 160E-120W. At depth, these anomalies are as low as -3C near 100m, with below normal values to at least 200m (indicative of a shallower than normal oceanic thermocline). Slightly above average SST conditions continue from the west Pacific into the subtropics east of the date line (the familiar "horse-shoe"), with values ~+2 at 150M at 165E near the equator. Typical of a cold event, there is a steeper than normal oceanic thermocline from the equatorial east Pacific into the west Pacific warm pool. See

for additional SST information.

As discussed previously, the divergent signal (in terms of near equatorial velocity potential) of the tropical convective forcing has been coming back around into the eastern hemisphere (EH). That can be seen from the following:

Please remember that negative (positive) values mean divergence (convergence) at 200mb suggestive of upward (downward) vertical motion. Indeed, while tropical convection has weakened significantly across the western hemisphere (WH), a rapid increase has occurred across the SIO into Indonesia. In fact, what is now at least a category 1 hurricane (Carina; see link below) has developed in the central SIO. Deep moist convection also continues to "pop up" across the warm SST waters from the South Pacific back northwest into the warm pool.

The convection just south of the equator across the IO (~80-100E) does project onto at least a weak moist phase of the MJO. Hovmoller diagrams employing a time-filtered technique to isolate coherent convectively coupled modes support that assertion. Other statistical techniques such as the Wheeler phase diagram also lend reasonable support to defend the observation that a moist phase of the MJO has emerged in the IO (see links below).

An astute reader may want to spend some time reading on how some of these monitoring tools are derived to gain a better appreciation on interpreting them in concert with an on-going weather-climate situation. Information from these and other tools suggest to me there could be an intense convective flare-up ~120E (Indonesia) within the next couple of weeks.

The point is that the moist tropical convective forcing is now in the EH, and the circulation has responded accordingly, as defined by SDM Stage 1. For example, while zonal mean westerly anomalies have propagated into the subtropical and lower midlatitude atmospheres, easterlies are replacing them (particularly the EH). Additionally, AAM is about 1.5 standard deviations below the 1979-1998 climatology, with a contribution from the northern high latitudes in addition to the equatorial and subtropical regions. The high latitude easterlies are related to blocking activity, including the retrograding Atlantic anticyclonic gyre. Finally, we have split flow across both ocean basins with a retracted EAJ.

So, not only is the atmospheric circulation behaving as would be expected during a cold event, there is also subseasonal modulation re-enforcing it. This type of base state can cause problems with the numerical models, especially when trying to resolve the split flow patterns across the oceans. This kind of base state can/does exhibit zonal-meriodional oscillations of anomalies which are also difficult to predict beyond a few days in advance.

Most models agree on the current east Pacific trough getting kicked inland over the next few days, followed by a stronger trough later next week. There is also agreement that the Atlantic block will retrograde into Canada (not unusual for March). The latter would lead to a split flow pattern over North America (feedback from the Arctic), and depress the storm track farther south over the CONUS. This seems reasonable. After that, lots of uncertainty.

A probable option would be strongly meridional flow across CONUS by around next weekend, with perhaps closed low development across the southwest/southern Rockies. A ridge would be expected across the central and southeast states with a deep low just off the coast of the Northeast. It would then make sense to see a cyclonic storm system roll east and northeast across the rest of the country ("bowling ball") south of the ridge over Canada. Even with the high latitude ridge moving west, other similar events may follow. All of this suggests continued active periods for the west coast, welcome rainfall for parts of the southern states, and wintry precipitation for portions of the northern Plains into the Northeast.

Is there any hope for decent precipitation by around next weekend into week 2 for southwest Kansas given all the above? Short answer is to monitor. A moist STJ (along with Gulf of Mexico moisture) with a slow moving closed low going just to our south would be favorable. However, there are still too many negative factors that do not allow me to be optimistic. This upcoming week looks to be very warm (overall) and dry, with periods of strong wind. Next weekend into the following week is unclear to me in regard to precipitation. Locations from the Front Range into the Upper Mississippi Valley may get all forms of precipitation with thunderstorms, while heavy rainfall and even severe local storms would be a concern from the south central states into the Tennessee Valley.

Ed Berry

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