Wednesday, March 08, 2006

The Atmosphere is Happy

Tropical Pacific and Indian Ocean SSTs have changed relatively little since the last Blog posting on March 4th. As shown by the following links, La-Nina conditions remain (SSTs for the Atlantic and other ocean basins can also be obtained from navigation):

Tropical convection continues over SSTs of 29C and greater, with the most intense centered on Indonesia near the equator from ~110-150E. During the past couple of days, this convection has drifted northward while extending east-southeast along the SPCZ toward the Polynesian Islands. Strong convection is also present from South Africa toward Madagascar as well as much of northern South America. Considering variations such as convectively coupled Kelvin and Rossby modes and even the diurnal cycle, the above described pattern of tropical forcing has been nearly stationary for at least the past week-10 days. The MJO signal has been very weak. As mentioned in the last posting, in terms of tropical convection, the forcing is believed to stationary and there may even be some coupling between the atmosphere and the SSTs. Stationary forcing would be probable during a La-Nina (as well as El-Nino, but with an essentially reversed distribution of tropical convection anomalies).

The circulation response is loosely consistent with what would be expected (in a composite sense) with the tropical forcing across the eastern hemisphere (EH). Interacting with synoptic variability, twin anomalous upper tropospheric subtropical anticyclones have appeared from Africa into the geographical areas of the IO and Indonesia, while twin subtropical cyclones have been present in the region of the date line. In the lower levels, while the trades have been strong east of Indonesia, actual westerlies were occuring north of Australia, all as part of a possible SST forcing atmosphere via convection positive feedback mechanism.

Linked mostly to the twin cyclones, zonal mean upper tropospheric westerly flow has been ~5-10m/s above average across the tropical and subtropical atmospheres for at least the past couple of weeks. Much of that has been in the western hemisphere (WH), including the anomalously moist STJ that has contributed to the heavy rainfall across Hawaii and the USA west coast for the past week. Recently zonal mean anomalous easterly flow has developed across subtropics particularly south of the equator, with the westerlies shifting poleward. This observation is consistent with the notion of La-Nina coupling, and AAM should be currently decreasing. In fact, the operational plot at is supportive (uses 1979-1998 climatology verses the 1968-1997 period of the NCEP/NCAR reanalysis plots, latter which lag at least 3 days from current). Finally, also connsistent with EH tropical forcing and La-Nina, split jet stream flows have been observed across the central and eastern ccean basins of the northern extratropics. In general SDM Stages 1-2 best characterize our weather-climate situation, and it is probable that will continue for at least the next 2 weeks.

Putting into perspective for the PNA region, there has been a general east Asian trough-central Pacific ridge (with the subtropical low west of Hawaii as part of the split flow)-eastern Pacific/western USA trough pattern for at least the past week-10 days. This structure has projected onto the negative phase of the PNA teleconnection index. A Rossby wave energy dispersion (RWD) on ~March 1st, linked to the subtropical lows as part of the base state discussed above, lead to the first in a series of deep troughs for the western USA last weekend. Several more troughs are following as the parent baroclinic wave packet moves through the WH. Most models show the last in this particular series of troughs to dig deeply into the western states by the end of this upcoming weekend and then move northeast into the Plains. That is reasonable, understanding uncertainty with the details.

What I think the models have been struggling with the last few days is what happens after this weekend in terms of the circulation. From monitoring I can see that the above mentioned wave packet is rapidly coming back around into the EH subtropics. In fact, the recent blow-up of thunderstorm activity across India is being forced by the trough at the leading edge of this packet. Initially, as this subtropical wave packet interacts with the Indonesian convection (and possibly intensifies it), another RWD may occur leading to another trough digging into the east Pacific just off the USA west coast by ~ middle of next week. Similar to what has already occurred, the southern portion may break anticyclonically and re-enforce the subtropical cyclone west of Hawaii. Also from monitoring there has been~10-20 amplification periodicity forced by the east Asian topography. Trying to understand the interaction of this faster mode within the La-Nina stationary base state in anything but trivial; however, I do think ALL that may translate to more at least seasonably deep troughs for the west coast/western CONUS ~next weekend into the week of March 20th.

In my last posting I went into some length about the "weather consequences" of the above discussed reverse PNA pattern for the USA, and will not repeat here. While the west coast, particularly the Pacific Northwest gets a break from the stormy weather by early next week, at least a typical March event of baroclinic cyclogenesis is probable across the middle of the country Monday-Wednesday. In its wake, colder air will spread east and southeast from the western states across much of the country.

By next weekend into the following week, another period of active weather may return from the west coast into the Plains and Ohio Valley. Heavy precipitation would again be a concern for those areas, including severe storms from the central and southern Plains into the Ohio Valley, and winter precipitation with elevated (possibly intense) thunderstorms from the Rockies into the Upper Mississippi Valley.

Now comes the story of Southwest Kansas. Even with all the above discussed active weather, can we at least get one decent precipitation event? In my last posting I tried to be optimistic. I am afraid I will have to back off on that. Simply put, it is still too early in the year for deep layered tropical moisture to get transported this far west to interact with the intense troughs/jet streaks moving out of the Rockies. As soon as the moisture makes a run, here comes a trough, southwestly momentum is quickly transferred downward given our elevated terrain, and we mix out. In some sense this is just another rain shawdowing effect of the Rockies. As we get later into spring, at least a couple of factors become more favorable. First, deep Gulf of Mexico tropical moisture may already be in place across Texas and Oklahoma; and secondly, the magnitude of the westerly flow weakens with the seasonal cycle. So, perhaps by ~mid May and afterwards, with "this type of pattern", our precipitation chances should be much better. For now, we will leave it at that.

Other than the "nickel and dime stuff" of high-based scattered showers and storms that may occur as each trough goes by, I see little opportunity of the decent welcome precipitation that everyone wants for at least next two weeks. In fact, what may become a concern is both increased fire danger and perhaps a dust storm. Nevertheless, we need to monitor, and hopefully I will eat crow. Temperatures should trend toward below normal by early next week, only to warm to near or above by late next week.

Ed Berry

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