Charlie A. Liles

National Weather Service Albuquerque NM

Introduction

In the paper APacific Decadal Oscillation and New Mexico Precipitation@  I demonstrated the strong relationship between the Pacific Decadal Oscillation (PDO) index and New Mexico=s precipitation. I also posed the question of whether the lack of precipitation during strongly-negative PDO indices is mainly related to shutting off of winter precipitation, or if there is a relationship between the Southwest Monsoon and the PDO.

The decrease in winter precipitation over New Mexico associated with La Niña has been well documented and is statistically significant. During La Niña episodes, Pacific storms are shunted to the north. Consequently, the Pacific Northwest gets the brunt of winter storm activity, and these storms tend to cross the central and northern Rocky Mountains. Further south, persistent mid to upper tropospheric ridging blocks development of winter storms. Statistically, it has been shown that chances for a dry, warm winter generally increase from northeast to southwest across New Mexico during La Niña episodes. Although it is yet to be proven, it appears the negative (cold) side of the PDO cycle leads to increased frequency of La Niña episodes, and possibly enhances the impacts or affects of La Niña. It also appears El Niño frequency may increase and be enhanced during the positive (warm) phase of the PDO.

This paper gives a quick look at the Albuquerque seasonal precipitation relationship with the PDO. A climate division by climate division look at the relationship between the PDO and seasonal precipitation would be a good topic for further research. Also, since New Mexico=s water supply is also a function of precipitation over southern Colorado, the climate divisions in the southern half of Colorado would be very appropriate for further research when studying the impacts of the PDO on the state of New Mexico.

Methodology

Using the 1931-1998 data, Albuquerque=s seasonal precipitation was calculated for years in which the average PDO (calculated from the monthly values) was more than 0.50 from zero. The same definitions used in the paper mentioned in the first paragraph were used to determine the positive (+), negative (-), strongly positive (++), and strongly negative (- -) years. Years were determined positive if the average PDO was greater than +0.50. If the PDO was greater than +1.00, the year was determined to be strongly positive. For years in which the PDO was less than -0.50, the year was determined to be negative. If the PDO was less than -1.00, that year was strongly negative. Seasons were defined as follows: Spring (March 1 through May 31), Summer (June 1 through August 31), Autumn (September 1 through November 30), and Winter (December 1 through February 28/29). Precipitation was considered to be normal if the seasonal total was +/- 10 percent of the long-term average.

Tables 1 through 5 show the number seasons for which the PDO was positive or negative, along with the total seasonal precipitation averages. Tables 4 and 5 are both for winter to cover the obvious question: Should we be looking at the winter for the year for which the PDO category began or the following winter?

Table 1.

Table 2.

Table 3.

Table 4.

* Winter season beginning in the PDO category year. Table 5 shows the results for the following winters.

Table 5.

Table 6 shows the breakdown according to season along with the annual averages for each category of the PDO. Winter1 is for winters that begin the year of the PDO category. Winter2 is for the following winter. Annual totals are shown for both cases. The winter2 cases are included in the annual totals in which precipitation is shown in parentheses. Table 7 shows the seasonal precipitation relative to normal.

Table 6. 

(Inches)

Table 7.

Percent of Normal

Discussion of Results

It=s obvious precipitation during the transitional seasons (Spring/Autumn) was favored during the positive PDO years. For Spring seasons, the average precipitation was the same for positive and strongly positive years. For Autumn, precipitation was actually greater during the positive years than the strongly positive years. Meanwhile, precipitation during the transitional seasons is lower during the negative phase of the PDO cycle, especially for the strongly negative years.

Even though spring precipitation averaged approximately 15 percent above normal during positive PDO years, it appears the probabilities of wet versus dry springs during the positive phase of the cycle are close to 50-50. Six of the positive and strongly positive years had wet springs, seven had dry springs, and four were normal. The relationship between negative PDO phases and spring precipitation is more significant. For years in which the PDO was either negative or strongly negative, three springs were wet, 12 were dry, and three were normal. Quantity of precipitation for the strongly negative years averaged only 59 percent of normal.

The relationship between the PDO and Albuquerque=s autumn precipitation is especially significant. Precipitation for the positive and strongly positive PDO autumns was approximately 140 percent of normal. Thirteen of the autumns were wet, only 1 was dry, and four were normal. For negative PDO years, average autumn precipitation was 90 percent of normal. Strongly negative autumns were even drier, with precipitation 74 percent of normal. Only two of the negative and strongly negative autumns were wet, twelve were dry, and four were normal.

It=s interesting to note winter precipitation was actually below normal for all categories of the PDO (winter1). For the following winter (winter2), precipitation was normal to slightly below normal for all except the strongly positive cases. For the winter following a strongly positive year, precipitation averaged roughly 150 percent of normal. For the 34 years in which the PDO was +/- 0.50 of zero (considered normal years), the winter precipitation averaged 1.54 inches.

For winters in which the PDO was categorized as positive or negative, there was very little difference in whether or not the PDO was positive or negative (for winter1 years). For the positive PDO winters, six were wet, 11 were dry, and none were normal. For the negative PDO winters, six were wet, 10 were dry, and one was normal.

However, for the winters following categorized PDO year, the results were a bit different. For those positive years, nine were wet, six were dry, and two were normal. For the negative winters, six were wet, 11 were dry, and two were normal. Consequently, the winters following a categorized PDO year tend toward wet for positive phases and dry for negative phases. However, with the exception of winters following strongly positive PDO years, the PDO cycle and Albuquerque=s winter precipitation don=t seem to have a very significant relationship.

Albuquerque receives approximately 40 percent of its annual precipitation during the summer, and over 30 percent of its annual precipitation during July and August. It was interesting to find summer precipitation averaged above normal for all PDO categories except strongly negative years. In fact, while the positive PDO years favored production of above-normal summer precipitation (ten wet years, six dry years and three normal), the wettest summers were actually associated with negative (but not strongly negative) PDO years. For the negative PDO years, eight summers were wet, two were dry, and one was normal. Average precipitation for the negative PDO years was 4.34 inches, 127 percent of normal, representing a surplus of nearly an inch. However, summer precipitation for strongly negative PDO years was another story. In those cases, there were no wet summers, five dry ones, and two normal ones. The average precipitation was 2.51 inches, only 74 percent of normal.

This is a topic for further study. However, I would suggest negative PDO year summers are characterized by persistent mid-upper tropospheric ridges of high pressure over the central United States with less ridge migration than in normal summers. These persistent patterns favor good influx of tropical moisture northward around the periphery of the ridge, usually providing a healthy ASouthwest Monsoon@ for New Mexico and Arizona. The patterns also favor drought in the central U.S. So, what happens during the summers for years in which the PDO is strongly negative? There was not a single wet summer in those seven years. Either the persistent high pressure ridge over the central U.S. expands westward, moving the moist influx to Arizona or southern California, or the tropical tap gets shut off from the U.S. altogether.

Since there appears to be a marked difference in summer precipitation from negative to strongly negative years, one important question would be "at what point in time do we know whether it=s a negative or a strongly negative year?" Some of the El Niño Southern Oscillation (ENSO) models seem to possess skills to determine how the cycle will evolve over the next 12 months or longer. Perhaps development of a PDO model in the future will lead to the ability to forecast the PDO index.