Written by Sig Silber
Edited at 5:17 pm EDT April 23, 2021 to add the Week 3 – 4 forecast which suggests a continuation of the current pattern of west to east progression of storms that will not break the current drought.
Scroll down into the International Section of the Article to read the rather comprehensive assessment of crops around the world. For many purposes, it is useful to look at the situation worldwide. We have a detailed report on U.S. crops and U.S. Livestock. The article this week begins with the USDA/NRCS Water Supply Forecast for the Western United States. Because sustainable agriculture will be increasingly important as world population grows and Climate Change may have negative impacts, we plan to have an ongoing section of this weekly article devoted to sustainable agriculture.
See our third video in what is planned to be a weekly education program on sustainable agriculture. We have not yet removed the first two videos so that readers who missed viewing them, can start from the beginning on this very important topic. This article contains all of our regular features and as usual, we include an intermediate-term weather forecast. We will see how long crops can continue to defy drought. It is easier to do when the temperature is cool to moderate.
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Of Special Interest.
Highlights
Although the 2020-21 La Niña continues to decay, impacts on the Western winter wet season remained apparent in mid-April snowpack and season-to-date precipitation totals. In early April, the Climate Prediction Center of the National Weather Service indicated that “the trend in the coupled oceanatmosphere system is consistent with a weakening La Niña.”
Characteristic of La Niña, the most favorable snowpack and watersupply prospects existed across the northern tier of the western U.S. In contrast, below-average snowpack was noted from California to the southern Rockies.
By mid-April, the average water equivalency of the high-elevation Sierra Nevada snowpack stood at 10 inches, according to the California Department of Water Resources, down from a late-March seasonal peak of about 18 inches (just under two-thirds of normal). Premature melting of snowpack in the Sierra Nevada and elsewhere in the nation’s southwestern quadrant was further evidence of mounting watersupply concerns for summer, as many reservoirs will receive little replenishment while competing demands for agricultural, municipal, environmental, recreational, and industrial water seasonally spike.
According to the U.S. Drought Monitor, 78 percent of the 11-state Western region was experiencing drought on April 13; that number has varied little since October 2020, ranging from 75 to 80 percent. On the same date, regional coverage of extreme to exceptional drought (D3 to D4) stood at 42 percent, down from a January peak of 47 percent. D3 to D4 covered more than two thirds of Utah (90 percent), Arizona (87 percent), New Mexico (80 percent), and Nevada (74 percent).
Snowpack and Precipitation
Spring storms provided a favorable boost in mountain snowpack in the central Rockies, while parts of the Northwest maintained favorable snowpack due to earlier storminess and cool weather during much of March and early April. By April 19, 2021, near- or above-average snowpack values were common across the eastern slopes of the northern and central Rockies, as well as the Pacific Northwest. Meanwhile, significant snowpack deficiencies were observed in the Sierra Nevada and large sections of the Great Basin, Intermountain West, and Southwest. The snowpack situation was especially dire in the Southwest, where premature melting has occurred – and little snow remained by mid-April. By April 19, snowpack was less than one-third of normal for this time of year in several river basins across Arizona, New Mexico, and southwestern Utah (figure 1).
Season-to-date precipitation (October 1, 2020 – April 19, 2021) was near to slightly above normal in many basins from the Pacific Northwest to the northern and central Rockies, while below-normal totals were common across the southern two-thirds of the West (figure 2).
Seasonal precipitation was disappointingly low in much of California, the Great Basin, and the Intermountain West, with totals less than two-thirds of normal in several basins. The Southwest has fared even worse, with a few watersheds in Arizona and New Mexico reporting seasonal precipitation less than one-half of normal.
Spring and Summer
Streamflow Forecasts By April 1, 2021, projections for spring and summer streamflow were indicating the likelihood of mostly favorable runoff prospects from the Pacific Northwest to the northern Rockies, courtesy of a La Niña dominated storm track. In contrast, runoff and water-supply concerns were widespread from California to the southern Rockies. In some drought-stricken areas of the Southwest, dry soils beneath already melting snowpack will lead to poor reservoir recharge, as much of the remaining snow could be soaked up before reaching rivers and lakes.
Reservoir Storage
On April 1, statewide reservoir storage as a percent of average for the date was 44 percent in New Mexico (figure 3). California’s storage stood at 74 percent of average. Below-average storage was also noted in Arizona, Colorado, Nevada, Oregon, and Washington. Meanwhile, near- or above-average storage was reported in several states, including Idaho, Montana, Utah, and Wyoming.
For More Information
The National Water and Climate Center homepage provides the latest available snowpack and water supply information. Please visit: http://www.wcc.nrcs.usda.gov
Severe Weather
Spring Runoff: We are actively tracking that situation
We will update the above map (or maps) weekly (or more often when the situation is changing rapidly) but more frequent updates can be obtained here.
Here is what the SNOTEL data is telling us about Spring runoff.
And here is the water year.
And it is always useful to look at the last seven days.
Reservoir Situation
Let’s look at the current drought situation.
And the week to week change
Here we look at four time periods: one week, one month, six months, and a full year.
Drought Discussion (The Regional links are live and will take you to the drought map for that Region)
This Week’s Drought Summary
The upper-level circulation over the contiguous U.S. (CONUS) during this U.S. Drought Monitor (USDM) week consisted of a ridge over the West Coast and trough over the north central states. Strong closed lows rotated within this upper-level flow, bounding from the Southwest to the Northeast in their trek across the country. The ridge blocked Pacific moisture from entering the country and kept the West Coast warmer than normal for the week. The trough funneled cold and dry Canadian air into the Plains. The cold fronts moved south and east, bringing colder-than-normal air to much of the CONUS from the Great Basin to the Appalachians and beyond to the Mid-Atlantic Coast. Only parts of the Southeast and Northeast averaged warmer than normal for the week. The fronts and their surface lows brought rain and snow to the northern and central Rockies as they bumped up against the mountains, with precipitation amounts meeting or exceeding weekly normals in parts of the northern to central Rockies and High Plains. The fronts and surface lows spread above-normal rainfall across Oklahoma and northern Texas to Florida, with amounts along the northern Gulf Coast exceeding 5 inches in places. They also brought rain to the Northeast, with some areas nearing or exceeding weekly normals. But much of the West, northern Plains to Ohio Valley, Southeast, and Maine were drier than normal for the week. Streamflow was well below normal all along the West Coast and in the interior West, in North Dakota and northwest South Dakota, parts of southern Texas, the southern and eastern Great Lakes, and parts of the Northeast. Soils continued to dry out in the West and Southeast, according to U.S. Department of Agriculture (USDA) reports. Satellite and station reports, as well as modeled indices, showed dry soils across the West, northern and southern Plains, the southern and eastern Great Lakes, and into the Northeast, as well as parts of the Southeast. Groundwater observations from wells and estimated from satellite measurements revealed dry conditions across the West, northern and southern Plains, Northeast, and parts of the Southeast. The cumulative effect of lack of precipitation and drying soils has stressed vegetation, as seen in such indices as the Vegetative Health Index and VegDRI, as well as field reports.
Northeast
Half an inch or more of rain fell across much of the Northeast, from New York to Maine, with up to 2 inches locally across parts of New England. Half an inch was also noted across southern parts of West Virginia, Maryland, and Delaware. Less than half an inch fell in between across Pennsylvania, southern New Jersey, and northern Maryland and Delaware. The rain was enough to prevent further expansion of the moderate drought (D1) and abnormal dryness (D0) in the region, but not enough to warrant contraction. The rain did improve soil moisture conditions, dropping the percentage values of soils short to very short of moisture down to 65% in New Hampshire and in the 20s and 30s in other New England states. Still, local authorities in New Hampshire were monitoring water supplies and preparing to adopt water restrictions if dry conditions persist, while Massachusetts residents continued to conserve water.
Southeast
The extreme northern and southern ends of the Southeast received rain this week, with very little precipitation in between. Half an inch of precipitation was noted in Virginia with half an inch to 2 inches, and locally more, over Florida and extreme southern Alabama. The Florida rain contracted D0 in the central and southern parts of the state, while D1 in the south was reduced to better reflect improving drought indicators. D0 expanded in Alabama and Georgia and spread into the Carolinas as dryness this week added to deficits that have been growing for the last 3 months. Several in situ (ground) based and satellite-based indicators revealed drying soils, and field reports from South Carolina included cattlemen in Aiken having to feed hay now due to forage growth being limited, farmers having to begin irrigating crops (which is rare to non-existent for this time of year), and other farmers having to stop planting corn due to lack of soil moisture. According to USDA reports, the percent of the soils short or very short of moisture doubled in South Carolina this week to 55% of the state having dry soils, while the value in Georgia increased to 26%.
South
Heavy rains fell across frontal zones over Louisiana and southern Mississippi where up to 5 inches was reported for the week. Rainfall totals of 2 inches or more covered most of Louisiana into northeast Texas and parts of Oklahoma. Half an inch or more was widespread across Arkansas, eastern to central Oklahoma, and northern, eastern, and extreme southern Texas. Meanwhile, most of the Rio Grande Valley, west Texas, and western Oklahoma received little to no precipitation. Drought and abnormal dryness contracted in parts of Louisiana, Oklahoma, and Texas, while extreme (D3) to exceptional (D4) drought expanded in parts of west Texas. The rains improved soils across parts of Texas and Oklahoma, but USDA reports still have 60% of Texas and 26% of Oklahoma with topsoil moisture short or very short.
Midwest
Precipitation brushed the edges of the Midwest region, with half an inch or more falling over parts of western Missouri, southwest Iowa, northern Minnesota, and Upper Michigan. Most of the region received less than 0.25 inch of precipitation for the week. D1 expanded in northeast Illinois, northern Ohio, and across much of Michigan. D0 expanded in parts of Illinois, Indiana, Michigan, and Ohio. But D0 and D1 contracted in northern Minnesota where recent precipitation has reduced drought indicators. USDA reports of short or very short topsoil moisture ranged from 9% in Illinois to 29% in Iowa. County extension field reports in east central Indiana included drought impacts on planting, pasture growth, and pond levels.
High Plains
Cooler temperatures and snow spread across parts of the northern Plains this week. Western and southern parts of the High Plains region received 0.5-1.5 inches of precipitation this week, while the Dakotas were mostly dry with less than 0.25 inch. Precipitation in Wyoming in recent weeks has improved several drought indicators, especially the Standardized Precipitation Index (SPI), resulting in significant contraction of the D3 area in the central part of the state. D0-D2 shrank in other parts, but D1-D2 expanded in western Wyoming. Colorado saw contraction of D1-D2 in the north. Half an inch to over an inch of precipitation in southwest South Dakota shrank D2, while D2 was removed from southeast South Dakota due to improving indicators. Even though much of Montana received welcome snow this week, in most areas it amounted to below-normal precipitation, so D1-D3 expanded. Abnormal dryness contracted in south central Montana where precipitation has been above normal. In North Dakota, the snow was enough to prevent further deterioration but not enough to reduce deficits. Parts of the state remain in a burn ban and are experiencing dry soils, poor pasture conditions, and drying ponds and dugouts, some of which were testing high in TDS and sulfates. Producers are selling or making plans to cull more livestock. Dust storms have been reported recently in North Dakota and Montana. USDA reports show 78% of North Dakota, 61% of Montana, 58% of South Dakota, 56% of Wyoming, and 49% of Colorado with topsoil moisture short or very short.
West
Just the higher mountains of northern Utah, northeast Nevada, and the Sierra Nevada in California received any precipitation this week, and that was only 0.25-1.5 inches, which was barely normal for this time of year. Most of the rest of the West received no precipitation. Low streamflow, dry soils, and precipitation deficits over the last 3 months or longer prompted expansion of abnormal dryness and drought all along the West Coast. In Washington, D0 expanded to the coast and D1 spread northeastward, with only above-normal snow water content (SWE) and water-year-to-date (WYTD) precipitation keeping the Olympic and Cascade Mountains free of abnormal dryness. The D1, and some D0, spread across northern Idaho and into northwest Montana. D1-D2 expanded in western and northeastern Oregon. D1-D3 expanded in California to better reflect the soil moisture, streamflow, and SPI indicators. Agricultural impacts along the southern California coast were especially severe. The sparse timing of rain that has occurred this season (end of December and end of January) has contributed to especially poor growth of the annual grasses that are needed for livestock feed. According to reports, the amount of forage on rangelands is low, with producers in Ventura County already shipping whole herds of cattle out of county because there is almost no forage. As the USDM week ended, California Governor Newsom declared a drought emergency in Mendocino and Sonoma counties. Other drought impacts around the region: Historically low water levels caused the closure of some launch ramps on Lake Powell. The Klamath Project will receive 33,000 acre-feet of water in 2021, about one-tenth the average amount, for the lowest allocation in the project’s history, due to drought and low inflows into Upper Klamath Lake. Dust storms have been reported recently in Oregon and blowing dust in eastern Washington. According to USDA reports, the percentage of topsoil moisture short or very short jumped this week to 65% of California and Oregon and 60% of Washington. The percentage increased slightly to 87% of New Mexico. April 21 reports of mountain SWE in California included 32% of normal in the North, 37% in the Central, and 16% in the South.
Alaska, Hawaii, and Puerto Rico
No change was made to the depiction in Alaska or Hawaii. The week was drier than normal across both states, and warmer than normal in Alaska but cooler than normal in Hawaii, and streamflow was still normal in Hawaii. D0 was expanded along the southern coast of Puerto Rico and into eastern sections due to below-normal precipitation for the last week to 4 months, low streamflows, and drying soils.
Pacific Islands
Palau had over 12 inches of rain in just one week, surpassing the monthly threshold of 8 inches to meet most water needs. This resulted in continued drought free conditions for Palau.
Dry conditions were present across the Marianas this week, with Guam, Rota, and Saipan reporting less than half an inch of rain. Monthly precipitations for the last few months were also below the threshold of 4 inches to meet most water needs for all three locations. According to the NWS Tiyan, Guam forecast, the KBDI (an index used to determine forest fire potential) was 718. The maximum value for KBDI is 800, which represents very dry conditions. Due to the dry weather, the NWS doesn’t recommend outdoor burning since spot fires will ignite easily. Given this, Guam’s drought classification was degraded by one category this week. Rota’s (D0-S) and Saipan’s (D1-S) drought remained unchanged.
Across the Federated States of Micronesia, Yap, Kosrae, and Lukunor had the highest rainfall totals this week, with rainfall totals above 5 inches. Pohnpei, Pingelap, Nukuoro, Ulithi, and Woleai also had a wet week with rainfall totals surpassing the weekly threshold of 2 inches of rain. Fananu had a little less than 2 inches of rain this week. However, all of these locations continued to be drought free. Chuuk was the only location with little rain this week, receiving only 0.32 inch of rain. However, drought free conditions were unchanged this week since last month rainfall totals were over 18 inches. Kapingamarangi also had over 5 inches of rain this week, resulting in a month-to-date rainfall of 15.05 inches. According to the drought information statement produced by the NWS Tiyan, Guam, catchments are full and vegetation is starting to recover across the region. For this reason, Kapingamarangi’s drought classification was improved by one category (from D2-L to D1-L).
This was a wet week for most of the Marshall Islands, with most locations receiving over 2 inches of rain. Ailinglaplap had the most rainfall at 5.61 inches. Meanwhile, Wotje had the least rainfall at 0.90 inch of rain. Wotje’s drought classification of extreme drought continued since the last few months have been very dry. Even though Kwajalein had a very wet week (3.90 inches), its drought classification of severe drought was unchanged due to local reports on yellowing vegetation and water shortages. There were no drought concerns in the other locations.
Drought free conditions persisted in Tutuila since rainfall totals across the three locations were close to or over 2 inches of rain.
Virgin Islands
Dry and very warm conditions continued to affect the USVI this week. St. Croix had no rain at the Henry E. Rohlsen airport and the Christainset 4.1 ESE CoCoRaHs station this week, while only 0.11 inch of rain was reported at the Christiansted 4.11.6 E station. The dry conditions were accompanied by very warm temperatures, with daily maximum temperatures of 87 deg F or higher. On April 14, the maximum temperature of 89 deg F at the airport tied the daily record last set in 1993. Groundwater levels at the Adventure 28 well in St. Croix dropped to 22.63 feet below land surface as of April 17. This was the greatest depth since November 2016. Meanwhile, the month-to-date rainfall total at the airport of 0.56 inch was only 50% of normal precipitation. Similarly, the airport’s year-to-date rainfall total was 3.85 inches, which is only 59.4% of normal. SPI values at the 1 (-0.53) and 3 (-0.53) months are consistent with abnormally dry conditions, while the 6 (-1.28) and 12 (-1.08) month periods are indicative of moderate drought. Due to the little rain and groundwater impacts, the drought classification was changed to severe drought.
St. Thomas and St. John also continued in severe drought this week as they received no rain. The month-to-date rainfall total at St. Thomas airport was 1.52 inches or 105.6% of normal rainfall. However, the year-to-date rainfall of 4.01 inches was 60.1% of normal. SPI values for the 1 (0.03) and 6-month (-0.37) period was indicative of drought free conditions. On the other hand, SPI values for 3 (-0.69) and 12 month (-0.79) periods were indicative of abnormally dry conditions. The Grade School 3 Well had groundwater at 13.51 feet below land surface (as of April 17), which is also the greatest depth since August 2017. St. John’s month-to-date rainfall total at the Cruz Bay 1.6 E CoCoRaHs station was 0.57 inch and 1.18 inches at the Windswept Beach. SPI values for the 3 (-0.68), 6 (-0.55), and 12 (-0.62) months were indicative of abnormally dry conditions. Groundwater at the Susannaberg DPW 3 Well was 15.79 feet below surface.
Looking Ahead
A weather system moved across the Northeast as this USDM week ended and the new week began. During April 22-27, the western ridge will break down, allowing a couple weather systems to move across the CONUS. One will move across the South and Southeast at mid-week, while another moves into the West as the USDM week ends. Much of the CONUS is expected to receive 0.10-0.25 inch of precipitation, with 1.0-2.0 inches in the mountains of the Coastal, Cascade, and Sierra Nevada ranges and northern Rockies. But the heaviest precipitation will be from eastern Texas to South Carolina, and southeast Kansas to southern Illinois, where 1.0-3.0 inches is forecast to fall. An inch or more is expected across a broad area from eastern portions of Texas, Oklahoma, and Kansas, across the Ohio Valley, to the Mid-Atlantic coast; south to the Gulf of Mexico coast; and across New England. Temperatures are expected to be cooler than normal in the east as the Canadian air masses exit the CONUS, then moderate to near to warmer than normal CONUS-wide. The outlook for April 28-May 1 warmer than normal in the East and cooler than normal in the Pacific Northwest. Odds favor wetter-than-normal conditions in the Pacific Northwest; along the Mississippi, Ohio, and Tennessee Valleys; and in the Great Lakes; with below-normal precipitation in most of the southern Plains. In Alaska, odds favor below-normal precipitation across the state except the panhandle, warmer-than-normal temperatures in the west, and cooler-than-normal temperatures in the east.
Regional Reports
Soil Conditions
The result of the drought is very dry soil conditions
Soil Dryness in More Depth.
April Drought Outlook
Since we are entering fire season it is time to start showing the wildland fire potential outlook. These will update automatically shortly after the month changes.
Crop Progress Reports A more detailed report by crop within each state can be accessed here.
Crop | Crop Progress | Crop Condition |
Corn | Planted an Emerged OK or better | NA |
Soybeans | Planted OK but very early | NA |
Winter Wheat | Headed a bit slow | Not Great |
Spring Wheat | Planted and looking good | NA |
Cotton | Planted looking good | NA |
Sorghum | Planted Just OK | NA |
Rice | Planted and Emerged OK but below 5-Year Ave. | NA |
Oats | Planted and Emerged and looking good | NA |
Barley | Planted and looking good | NA |
Peanuts | Planted and too soon to tell | NA |
Sugar Beets | Planted and looking good | NA |
Sunflowers | NA | NA |
Executive Briefing on Citrus
USDA and NASS Executive Briefings and other reports.
We may revisit the recent three Executive Briefings, but I have removed that partial analysis of those three Executive Briefs to keep this article to a manageable size. We may revisit those topics or not since USDA is always publishing new material.
Selected NASS Reports.
Now we look at the monthly report. The annual report is history. The monthly report is recent history. You can access the full report here.
Sustainable Agriculture Weekly Video. Source: Sare.Org but we may include other related videos as they become available….You want to watch these! They are short but very informative.
News
Nothing to report
Intermediate-Term Weather Forecast
Showing from left to right, Days 1- 5, 6 – 10, 8 – 14, and Weeks 3 – 4 You can click on these maps to have them enlarge. Also, the discussions that go with these forecast maps can be found here (first two weeks) and here (Weeks 3 and 4).
First Temperature
And then Precipitation
The Week 3-4 Discussion is always interesting. Similar discussions for the 6 -10 Day and 8 – 14 Day Forecasts are available here.
Week 3-4 Forecast Discussion Valid Sat May 08 2021-Fri May 21 2021
Since early March, a robust Madden-Julian Oscillation (MJO) has continued to influence global tropical rainfall and tropical cyclone development. The RMM index depicts a slower eastward propagation of the MJO during mid-April, but this may be related to Super Typhoon Surigae that tracked westward over the West Pacific. The 200-hPa velocity potential anomalies feature strong upper-level convergence centered across the Indian Ocean with upper-level divergence, of smaller anomalies, overspreading the Western Hemisphere. Although the recent period of anomalous cold across the east-central U.S is consistent with the MJO evolution this month, the MJO is expected to play less of a role in the mid-latitude circulation pattern later during May. With the transition from La Nina to ENSO-neutral conditions also well underway, the week 3-4 temperature and precipitation outlooks are based primarily on dynamical model guidance including the GEFS, CFS, Canadian, ECMWF, JMA, and SubX multi-model ensemble (MME). Decadal climate trends and soil moisture conditions are factors in the temperature outlook.
Dynamical model solutions are in good agreement, for this time of year, with a mean 500-hPa ridge over the Northeast Pacific. However, downstream over the CONUS, there are large model differences on the placement and amplitude of the longwave trough. The CFS model depicts this trough digging into the Pacific Northwest and Intermountain West, while the JMA model has a broad trough extending from the north-central CONUS to the East Coast. The ECMWF model offers a reasonable compromise between these two model solutions and features a 500-hPa trough of modest amplitude centered over the Great Plains. This predicted longwave pattern favors above normal temperatures for the western CONUS along with the southern Great Plains. Soil moisture remains below the 5th percentile across the Rio Grande Valley and Southwest which supports increasing probabilities of above normal temperatures for these areas. Probabilities of below normal temperatures are slightly elevated for the eastern Dakotas and Upper Mississippi Valley based on the expectation of anomalous troughing with below normal 500-hPa heights. Downstream of this predicted trough axis, above normal temperatures are more likely for the Northeast and Mid-Atlantic. Forecast confidence in the temperature outlook is lowest across the Middle to Lower Mississippi Valley. Heavy rainfall during the remainder of April is likely to maintain saturated soils for these areas which could provide a cooling effect to surface temperatures. Given the model consensus for above normal 500-hPa heights, above normal temperatures are only slightly favored for the Lower Mississippi Valley.
The predicted mean 500-hPa ridging near the West Coast supports increased probabilities of below normal precipitation for the western CONUS, except for southern California and the desert Southwest which are climatologically dry during mid-May. The proximity of a 500-hPa trough, expected surface temperature gradient, and the weighted probabilities from the CFS, ECMWF, and JMA models elevate probabilities of above normal precipitation from the Great Lakes southwest to parts of the central Great Plains. A broader spatial area of equal chances forecast to the north and south of this favored wet area is due to varying precipitation tools. Based on above normal 500-hPa heights and good agreement among precipitation tools, below normal precipitation is more likely along the East Coast, especially for the Northeast.
The dynamical models depict an amplified 500-hPa ridge extending from the Northeast Pacific northward to Alaska or the Bering Sea. This predicted longwave pattern favors above normal temperatures and below normal precipitation throughout Alaska. The largest probabilities (above 70 percent) of above normal temperatures forecast across eastern Mainland Alaska are supported by the calibrated SubX MME.
Equal chances of below or above normal temperatures are forecast for Hawaii, based on near average sea surface temperatures along with little to no signal among temperature tools. If the MJO continues to propagate eastward to the Indian Ocean and western Maritime Continent (phases 2, 3, and 4) during early to mid-May, that evolution would tend to suppress rainfall across Hawaii during weeks 3 and 4. Therefore, below normal precipitation is favored for Hawaii.
International
This week fortunately we did have a map.
WADSE Report
Energy Analysis
No report this week.
Major Sources of Information Used in this Weekly Report. We will not be using them all each week but the reader is welcome to refer to these resources.
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Science Feature
Double Dip La Nina Continued
I want to attempt to present a paper that explains why we have more La Ninas than El Ninos. I think you can read the full paper here. I am not positive of that because I am registered with ResearchGate but they did not ask me to log in so I think you can access the full article but hopefully I will present enough for readers to understand the concept.
I have not made any progress on this but I am leaving it in the article for those who may want to think about the issue and I will try to expand the discussion below at some point.
The rest of the discussion is later on in the article. And consider this an introduction because some of my readers may be familiar with this but for me, it is uncharted territory. So I need to do a lot more research. But what caught my interest was the part of the discussion not shown here but in the slides that are presented later in this article which suggests that the Pacific Decadal Oscillation or PDO does not explain as much about weather cycles (what I call low-frequency weather cycles) as I have assumed and that there are other factors now that need to be included in the analysis. One of the two is the Pacific Meridional Model or PMM mentioned in the Abstract. If that is the case, and I can come across some other references to this it is very important – let me just say it that way.
I have moved the other slides I made to somewhere close to the end of this article. I will be coming back to this topic very soon. I am not sure yet if this theory if you want to call it that explains why we may have a two-year La Nina this time around. It is not clear that we will but some of the models suggest it. Not all La Nina’s are dry for the Southwest but most are. So if the ratio of two-year La Ninas to one-year La Ninas is increasing, that is very important. I could have held back all this information until was read to fully discuss it but that did not seem to be proper. Some of my readers may know more about this and can leave me messages in the comment section that follows every article I publish.
I also see the possible connection of what is discussed in this paper with the storm we just had. In the article I posted last night on the NOAA four-season forecast I provided some snapshots of the Equatorial Pacific which showed a lack of movement of the cool and warm anomalies. I showed four snapshots which basically covered three months but I could have gone back many more months and it would have looked similar. You can see that set of graphics here. The importance of that is this La Nina is not decaying rapidly. So it could last into next winter or fade for a short period of time and later regain La Nina Status. I am not making any predictions here but I am raising the question of has something fundamentally changed?
I am certainly familiar with the deliveries to Elephant Butte the major reservoir on the Rio Grande and I am familiar with the deliveries to the three beneficiaries of that project namely an irrigation district in New Mexico, a similar irritation district in El Paso County Texas, and Mexico. Shortly after the MegaNino of 1997 and the subsequent La Nina of 1998 deliveries declined to Elephant Butte Reservoir and to the three (there is actually a fourth in good years when there is surplus water available to Hudspeth County Tx under a Warren Act Contract). I concluded that this was a normal change of phase of the PDO but perhaps there is more to it. So that explains my interest in the topic and although I frame the issue in a New Mexico context it impacts essentially all of the U.S.
Just what is the Pacific Meridional Mode?
This may help a bit. It is a second reference to the issue and this time an NOAA article. BTW, the fish are never wrong. We create indices on observed fish behavior. If their behavior deviates from what our indices suggest their behavior should be, it is not because the fish are confused it is because our indices or models no longer are accurate.
This graphic also might help
4 | DISCUSSION AND SUMMARY
From the present study, it is necessary to discuss two possible factors responsible for modulating La Niña persistence. The first is the preceding El Niño amplitude. From a linear perspective, the discharging amount is proportional to the El Niño amplitude. With strong discharging, significant SSTA cooling could last more than 2 years. The preceding El Niño amplitude also affects the intensity of the interbasin SSTA gradient by influencing the adjustment time of the Indian and tropical Atlantic Oceans to the Pacific SSTA (Wu et al., 2019). More specifically, the delayed response of SST cooling over the Indian and tropical Atlantic Oceans, in comparison with the Pacific cooling, allows the easterly wind anomaly over the western Pacific to be dominated even after the La Niña decaying phase. This initiates a La Niña that tends to persist up to 2 years. However, the role of the Indian Ocean capacitor effect on western Pacific wind anomalies has been questioned by Chen et al. (2016). Using data analysis and numerical experiment, the authors claim that the Indian Ocean basin warming during mature El Niño wintertime events has little effect on the easterly anomalies in the equatorial western Pacific, and the local process over the western Pacific has more influence over the Indian Ocean capacitor effect. Further numerical experiments may provide quantitative insight in to the importance of these factors, which is beyond the scope of this study. The second factor is a recharging/discharging efficiency with respect to anomalous wind-stress/WSC relevant to SSTA patterns, which contributes to ENSO phase transition. This was the focus of this study. An EP-type El Niño preceding a La Niña leads to strong discharge by positive WSC (0 – 15N) that is widely spread over the Pacific basin, which induces a strong and long-lasting La Niña. When a La Niña develops through the mid-latitude connection (PMM or NPO), SSTA cooling and a tropical easterly wind anomaly expand to the North Pacific. In this case, an anomalous negative WSC is weak and, thus, the recharging efficiency by equatorial Rossby waves is low. Consequently, to understand La Niña persistence, it is reasonable to consider both the preceding El Niño amplitude and recharging/discharging efficiency by anomalous SST/wind-stress patterns as the La Niña evolves. In this study, we investigated how the evolution of single- and multi-year La Niña events differs from their onset phase by analysing both observational and climate model data sets. Our analyses showed that there was a significant difference in the evolution between the two types of La Niña events, even from their initial development associated with a mid-latitude connection. These differences are described as follows (Figure 10).
1. A weak CP-type (Niño-4 > Niño-3) El Niño tends to exist one year prior to a single-year La Niña event, whereas a strong EP-type (Niño-3 > Niño-4) El Niño tends to exist prior to a multi-year La Niña event.
2. For single-year La Niña events, negative WSC, associated with CP-type El Niño and confined to the western Pacific, induces weak discharge that leads to a weak La Niña. In contrast, a widely extended negative WSC of a multi-year La Niña in the Pacific basin causes efficient discharge, resulting in a strong La Niña.
3. From the decay of an El Niño to a La Niña developing period, SSTA cooling is initiated by anomalous easterly wind in the eastern Pacific for single-year La Niña. However, for multi-year La Niña events, SSTA cooling is triggered by a mid-latitudinal influence (i.e., PMM/NPO). Such a difference in initiation of a La Niña explains why the meridional width of multi-year La Niña events exceeds that of single-year La Niña events.
4. During a La Niña peak period, single-year La Niña events have an equatorially confined SST cooling structure; however, multi-year La Niña events have intensified SSTA cooling and meridionally expanded easterly wind anomalies. In addition, the overall structure of multi-year La Niña events is shifted westward. Comprehensively, the SST structure of multi-year La Niña events is similar to Mega-ENSO. Thus, the recharging process of a singleyear La Niña is strong due to equatorially confined SST cooling/strong WSC, whereas that of a multi-year La Niña is relatively weak due to meridionally expanded SST cooling/weak WSC.
5. During a La Niña decaying period, the SSTA cooling of a single-year La Niña disappears with strong recharging, resulting in the termination of the La Niña. For multi-year La Niña, the SSTA cooling persists with a weak recharge. Because of the insufficient recharge, the surface cooling in the equatorial Pacific persists until the following summer. Bjerknes feedback is then initiated, resulting in the development of a second La Niña in boreal winter (Chen et al. 2016).
6. The iteration of processes 3 through 5 for a multiyear La Niña may lead to the development of another La Niña in the subsequent winter.
In summary, by analysing the statistically significant differences in precursory signals between single- and multi-year La Niña events, we observed that a midlatitude connection associated with the PMM/NPO, such as the meridional expansion of anomalous SST and wind stress fields, is critical for La Niña persistence. This indicates that there is similar dynamics operating during mega-ENSO and multi-year La Niña events. These results have important implications for improved ENSO prediction and provide an appropriate alternative interpretation to the state-of-the-art climate models in predicting multiyear La Niña events.