Written by Sig Silber
Crops are still doing well but there is no immediate relief from the drought in sight except for the Gulf States. Tonight we provide an energy update report. See our fourth video in our weekly education program on sustainable agriculture. We have not removed the first video so that readers can see the introduction to this very important topic. We also provide an interesting video on the Rio Grande River. This article contains all of our regular features and as usual, we include an intermediate-term weather forecast.
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Of Special Interest.
Severe Weather
Spring Runoff: We are actively tracking that situation
This is a good place to show the situation for the California Reservoirs
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.
Here is the same information presented differently.
And here is the water year.
And it is always useful to look at the last seven days.
Here is a special report from New Mexico and the Rio Grande River. Laura Paskus interviews Gary Esslinger of the Elephant Butte Irrigation District and Dagmar Llewellyn of the US Bureau of Reclamation.
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
Several upper-level troughs and closed lows moved in the westerly jet stream flow across the contiguous U.S. (CONUS) during this U.S. Drought Monitor (USDM) week. They dragged surface fronts and low pressure systems along with them. The weather systems moved across the West then generally followed two storm tracks after crossing the Rockies – one track was along the Canadian border, while the other track was along the Gulf Coast states. As a result, above-normal precipitation fell across parts of the West, much of the Gulf Coast from eastern Texas and Oklahoma to Georgia, parts of the Mid-Mississippi Valley, and parts of the northern Great Lakes and Maine. Precipitation amounts were generally 0.5-2.0 inches in the West where precipitation fell, but exceeded 2 inches across a large part of the Gulf Coast states. The rest of the CONUS had below-normal precipitation for the week. A swath from New Mexico to Lower Michigan, and parts of the northern Plains and Far West, had little to no precipitation. With upper-level troughs and surface cold fronts dominating the circulation pattern, weekly temperatures averaged below normal across most of the CONUS. Drought or abnormal dryness contracted in parts of the southern Plains to Southeast, where heavy rains fell, and in parts of the northern Plains and Northeast, where overall conditions warranted improvement. But drought or abnormal dryness expanded or intensified across parts of the West, Midwest, other parts of the Plains, and the Carolinas, as well as much of Hawaii and Puerto Rico. In these areas, precipitation deficits grew and soils continued to dry. These changing conditions were reflected in several drought indices and indicators, including the Standardized Precipitation Index (SPI), streamflow and groundwater levels, soil moisture measurements on the ground and from satellite as well as soil moisture models, vegetation-based indices such as VegDRI, and mountain snow water content (SWE) in the West.
Northeast
Precipitation fell across most of the Northeast this week, but amounts varied considerably. An inch or more was observed across parts of northern New England, and there were patchy areas of 0.5-1.0 inch, with many other areas receiving less than 0.5 inch. Topsoil improved in some areas and worsened in others, increasing to 61% of Maine with topsoil moisture short or very short (dry or very dry) and 25% of New Jersey, according to U.S. Department of Agriculture (USDA/NASS) statistics. Dry topsoil percentages improved slightly but were still high in New Hampshire (55%), Connecticut (28%), and Massachusetts (26%). D1 was pulled back slightly in western and northeastern New York, while D0 expanded across southwest Pennsylvania into West Virginia. In Massachusetts, 2021 Phase II water restrictions were now in effect in the town of Mansfield and other communities, while in New Hampshire, “concerningly low levels” of aquifers prompted the city of Dover to urge voluntary water conservation.
Southeast
Rain soaked Alabama and Georgia and adjacent parts of South Carolina, where 2+ inches covered large parts of the states and streaks of 5+ inches occurred in southern portions. Parts of central Florida and the panhandle received 1 to locally 2+ inches, while 0.5-1.0 inch fell across North Carolina and Virginia. Dry areas included eastern North Carolina and adjacent parts of Virginia and South Carolina, as well as parts of Florida, where a half inch or less of rain was noted. Abnormal dryness contracted in Alabama, Florida, and Georgia, but expanded in eastern North Carolina. Moderate drought expanded in far southwest Florida where 1- to 3-month precipitation deficits were significant. USDA statistics revealed 46% of South Carolina’s topsoil was short or very short, while the value was 24% for Florida.
South
In the southern Plains, the western half of Texas and Oklahoma were dry this week, with little to no rain falling. But a half inch or more of rain was widespread across the rest of the South, with 2+ inches in streaks across eastern Oklahoma and Texas, Arkansas and Louisiana, and southern Mississippi. D0-D3 contracted in eastern Texas, D0-D2 contracted in parts of Oklahoma, D1 disappeared and D0 contracted in Louisiana. But further west, D0-D4 expanded in southern and southwest Texas and D0 expanded in western Oklahoma. Dry or very dry soils grew in area to 67% of Texas and 29% of Oklahoma, according to USDA statistics, and 43% of the winter wheat crop in Texas was in poor to very poor condition. In the Midland-Odessa, Texas, area, 2020-2021 had the driest 13-month April-April period on record, and the fourth driest any 13-month period ever (behind 2011 which had the top 3 driest 13-month periods). Falling levels of the Edwards Aquifer triggered water restrictions in several Texas communities, including San Antonio, New Braunfels, and San Marcos.
Midwest
In the Midwest, a half inch of precipitation fell along the Ohio and Mid-Mississippi Rivers, parts of northern Ohio and Indiana, and central Minnesota to Upper Michigan. Parts of far northern Wisconsin had an inch or more. But much of the Ohio Valley states, and Iowa to Lower Michigan, had little to no precipitation this week. Streamflow, soil moisture, and 1- to 2-month SPI indicators prompted expansion of D0 in Illinois, Indiana, and Ohio, while D1 expanded in northern parts of Indiana and Illinois, across southern Wisconsin, into southeast Minnesota and northeast Iowa where deficits were mounting as seen in 1- to 4-month SPI values. D0 also expanded in the latter 3 states. D1 was expanded across Lower Michigan where precipitation deficits over the last 90 days have been building, streams are low, and soil moisture continues to dry. USDA statistics show 30% of the state is experiencing topsoil moisture that is short to very short (dry to very dry), an increase of 17% over the last week. The statistics increased to 34% in Iowa, 25% in Minnesota, and 21% in Wisconsin.
High Plains
There were patches of 0.5-1.0 inch of precipitation in southeast Kansas, the Dakotas, Wyoming, Colorado, and the western half of Montana this week. But most of the region was dry, with less than 0.25 inch of precipitation falling. With improving conditions in the short-term (last 1-6 months), D0-D2 were pulled back in parts of South Dakota and northeastern Wyoming. But D4 expanded in northwest Colorado, and D3 expanded in eastern North Dakota where this week was dry and 6-9-month SPI are D3-D4, deeper soils are bone dry, streams are low, and stock ponds are dry or almost empty. D1-D3 expanded in northern Montana, D0-D2 expanded in western Wyoming, and D0 expanded in southwest Kansas. In the Montana Highline, reports were received from Blaine and Hill Counties of low soil moisture and stockwater ponds failing to fill. Three dozen reports were received from North Dakota, through the Condition Monitoring Observer system, of drying ponds and creeks, hay and feed shortages, and fire risk. USDA statistics rate more than half of the topsoil short or very short of moisture in Colorado (57%), Montana (57%), and Wyoming (55%), and nearly two-thirds or more so rated in South Dakota (62%) and North Dakota (80%); 32% of the winter wheat crop in Colorado was rated in poor to very poor condition. Several Colorado communities were asked to voluntarily conserve water.
West
Precipitation amounts from the Pacific weather systems that moved across the West this week were patchy and not enough to erase months of deficits. Half an inch to locally 2 inches fell across coastal areas and coastal ranges of Oregon, Washington, and northern California; in the Sierra Nevada; northern and central Rockies; and Great Basin ranges. Rainshadow areas and valleys, and much of the Southwest, received little to no precipitation. There were minor contractions of D4 in New Mexico and D0 in Idaho, but worsening dry conditions prompted expansion of drought and abnormal dryness in several western states. D1-D3 expanded along the northern California coast, D0-D2 expanded in the Pacific Northwest, and D3 expanded in western New Mexico. In California, the Marin Municipal Water District board unanimously approved mandatory restrictions for its 195,000 customers in south and central Marin County which would take effect on May 1. The goal was to curb overall district-wide water use by 40%. The potential for water shortages in Marin County was so high that the Marin Municipal Water District was talking with East Bay officials about building a pipeline across the Richmond-San Rafael Bridge to bring water into the county if the upcoming winter is similarly dry as the past two. Such a measure was used during the 1976-77 drought. Record low reservoir levels contributed to California Governor Gavin Newsom proclaiming a regional drought emergency for the Russian River watershed in Sonoma and Mendocino counties. A super bloom of lupine covered parts of the Folsom Lake lakebed in the Beeks Bight nature area in Granite Bay as historically low rainfall left the lake unusually low. Folsom Lake is at 37% of capacity and 50% of the historical average for April 27, according to the California Department of Water Resources Data Exchange Center. Shallow wells were going dry in the San Joaquin Valley, while Tulare County ranchers were culling cattle for sale months earlier than usual and were considering idling row crops to leave water for higher-value permanent crops like nut trees. The prolonged drought is causing significant die-offs of juniper trees in large areas of central and northern Arizona, according to Forest Service officials. Between 50,000 acres and 100,000 acres of junipers were affected in areas of the Prescott and Kaibab national forests between Paulden and Ash Fork and north of Williams. The cause of death appeared to be water stress, although some insects were observed. Die-offs ranged from 5% to 30% of the tree population, with some pockets of dead junipers up to 15 acres. USDA statistics showed increases in the percentage topsoil moisture short or very short – up to 89% in New Mexico, 70% in California, 69% in Oregon, 61% in Washington, 50% in Utah, and 32% in Idaho. According to media reports, the Biden-Harris administration announced the formation of an Interagency Working Group to address worsening drought conditions in the West and support farmers, Tribes, and communities impacted by ongoing water shortages. The Working Group will be co-chaired by the Departments of the Interior and Agriculture to build upon existing resources to help coordinate across the federal government, working in partnership with state, local, and Tribal governments to address the needs of communities suffering from drought-related impacts. The Working Group will work to identify immediate financial and technical assistance for impacted irrigators and Tribes.
Alaska, Hawaii, and Puerto Rico
The week was dry and warmer than normal across Alaska, but since this is the dry season and snowpack in most areas (except the north) was near or higher than normal, no change was made to the depiction in the state. D1 was added in eastern Puerto Rico and Vieques and expanded in the southwest, and D0 expanded almost island-wide, due to below-normal precipitation, rapidly dropping stream levels, low groundwater levels, and expanding dry soils. Reports have been received of grass fires and a decreasing trend in reservoirs. This week continued a dry trend in Hawaii, which has resulted in plummeting streamflow levels on many islands (except Kauai) and deteriorating pasture conditions on the Big Island. D0 expanded across Maui County, Oahu, and the Big Island, with D1 pushing up the slope to Waimea town on the Big Island.
Pacific Islands
Koror, Palau had 1.43 inches of rain this week, resulting in a month-to-date rainfall total of 20.95 inches. Drought free conditions remain since the month-to-date total exceeds the monthly 8 inches threshold to meet most water needs.
Dry conditions persisted across parts of the Marianas this week, with Saipan’s manual gauge (0.23 inch) and Rota (0.45 inch) receiving less than half an inch of rain. Guam and Saipan’s NPS had the most rain at 0.85 inch and 1.35 inches, respectively. Since this week marked Rota’s third consecutive week with rainfall totals less than half an inch, Rota’s drought classification was changed to moderate short-term drought, while Guam and Saipan continued in moderate short-term drought.
Across the Federated States of Micronesia, Pohnpei, Kosrae, Lukunoch, Kapingamarangi, and Nukuoro had a wet week with rainfall totals surpassing 4 inches. Although Kapingamarangi has been receiving beneficial rains the last three weeks, it continued in moderate long-term drought as vegetation in the region continues to recover. Meanwhile, the other locations continued in drought free conditions. Chuuk Lagoon had over 1 inch of rain this week, while Yap, Fananu, Pingelap, Ulithi, and Woleai had less than 1 inch of rain. Although this was a dry week for these locations, drought is not a concern at this time.
This was a dry week for the Marshall Islands with little to no rain in some locations. Kwajalein had the most rain at 1.39 inches, while Jaluit and Wotje had no rain. Wotje’s lack of rain secured another week of extreme short-term drought, while Kwajalein continued in severe drought. The other locations continued to be drought free.
Pago Pago had less than 1 inch of rain, while Siufaga Ridge and Toa Ridge had 1.43 and 1.45 inches, respectively. Drought free conditions continued across Tutuila as the month-to-date rainfall totals surpassed the threshold of 4 inches to meet most water needs.
Virgin Islands
Rainfall was meager across the USVI this week, with rainfall totals ranging between 0.07 – 0.20 inch across St. Thomas and St. John. The Henry Rohlsen airport at St. Croix had only 0.04 inch of rain, while the CoCoRaHS stations had a bit more at 0.51-0.73 inch. Month-to-date rainfall totals at Cyril E. King Airport at St. Thomas was at 81.8% of normal rainfall, while year-to-date was at 57.9% of normal. Henry Rohlsen airport (St. Croix) month-to-date rainfall total was 34.5% of normal, while year-to-date was 54.9% of normal. Groundwater levels have continued to decrease, reaching levels last seen in 2016. SPI values for Cyril E. King airport at the 3, 9, and 12-month periods were indicative of abnormally dry conditions. The Henry E. Rohlsen airport’s SPI values at all levels were indicative of abnormally dry conditions to moderate drought. The Windswept Beach, St. John’s SPI values were indicative of abnormally dry conditions to moderate drought at the 3,6, and 12 month periods. Severe short and long-term drought continued to affect St. Thomas and St. Croix, while St. John remained in severe short-term drought.
Looking Ahead
A large low pressure trough moving out of the Southwest triggered areas of rain across parts of the southern and central Plains into the Midwest, and another system spread precipitation across the Northeast, as the new USDM week began. During April 29-May 3, another Pacific system will move into the country. These weather systems are forecast to spread an inch or more of precipitation across the southern Plains, Lower to Mid-Mississippi Valley, Tennessee and Ohio Valleys, southern Great Lakes, and Northeast. Bands of heavy precipitation – 3 inches or more – are expected across Texas to Arkansas and along the Ohio River. An inch or more of precipitation is projected to fall across parts of northeast Colorado, Wyoming, and northwest Washington. Half an inch or less of precipitation should fall across the Southeast from Florida to Virginia, New Mexico to the central Plains, the rest of the Great Lakes, and central to northern Rockies. No precipitation is forecast to fall over much of the northern Plains and most of the rest of the West. Temperatures are predicted to be near to above normal for the week across the CONUS. The outlook for May 4-8 shows drier-than-normal weather is favored for much of the West to Great Plains with wetter-than-normal conditions for most of the Mississippi River to East Coast region. Warmer-than-normal weather is likely across most of the western, southern, and East Coast states, except for the Mid- to Upper-Mississippi Valley, Great Lakes, and New England. Odds favor wetter-than-normal weather for southern Alaska and cooler-than normal weather for most of the state.
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
Crop | Crop Progress | Crop Condition |
Corn | Planted an Emerged OK | NA |
Soybeans | Planted OK but still early | NA |
Winter Wheat | Headed a bit slow | Not Great |
Spring Wheat | Planted and looking good | NA |
Cotton | Planted OK | NA |
Sorghum | Planted just OK | NA |
Rice | Planted and Emerged OK | NA |
Oats | Planted and Emerged and looking good | NA |
Barley | Planted and looking good | NA |
Peanuts | Planted OK | NA |
Sugar Beets | Planted and looking good | NA |
Sunflowers | NA | NA |
USDA and NASS Executive Briefings and other reports.
There are available NASS Reports which you can access 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.
Introduction
Episode 4 Social Sustainability
News
Nothing to report tonight.
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 so we include it in the article. Similar discussions for the 6 -10 Day and 8 – 14 Day Forecasts are available here.
Week 3-4 Forecast Discussion Valid Sat May 15 2021-Fri May 28 2021
La Nina conditions continued but weakened across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) are below average from the west-central to eastern Pacific Ocean, as enhanced upper level westerly wind anomalies persist across most of the equatorial Pacific Ocean. The RMM index depicts an eastward propagation of the MJO from mid-March through early April, when the MJO became less coherent during mid-to-late April. The GEFS and ECMWF are in good agreement that the MJO resumes its eastward propagation over the Western Hemisphere during week-1, but uncertainty on its evolution increases later in week-2. The Week 3-4 temperature and precipitation outlooks are based primarily on dynamical guidance including the GEFS, CFS, Canadian, ECMWF, JMA, the Subseasonal Experiment (SubX) multi-model ensemble (MME) of experimental and operational ensemble prediction systems, with additional considerations for MJO, La Nina, decadal climate trends, and soil moisture conditions, as well as the predicted evolution of the pattern from Week-2 forecasts.
The forecast circulations are reasonably consistent among model guidance regarding the anomalous 500-hPa height patterns over the week 3-4 period. Dynamical model 500hPa height anomaly forecasts during week 3-4 show a fairly consistent evolution from the forecast state during Week-2. The CFS and ECMWF models feature near to above normal 500-hPa heights over the CONUS, while the JMA model favors above normal 500-hPa heights over the southern CONUS and near to below normal 500-hPa heights across the northern CONUS. The CFS and ECMWF models indicate anomalous ridging with above normal 500-hPa heights over Mainland Alaska, the Alaska Panhandle and the Aleutians. Near normal 500-hPa heights are likely over Hawaii.
Below normal temperatures are favored over parts of the Northern Plains, consistent with the CFS/ECWMF/JMA Equal Weighted and Manual Blend tools. Above normal temperatures are more likely across the remainder of the CONUS, under predicted near to above normal 500-hPa heights. The highest confidence for above-normal temperatures lies across the Southwest (greater than 70%), supported by most dynamical tools. Predicted ridging and above normal 500-hPa heights lead to predicted above normal temperatures over Alaska.
The dynamical model guidance is in reasonably good agreement on the spatial pattern of anomalous precipitation during the Week 3-4 period. Above normal precipitation is favored over parts of the Upper and Lower Great Lakes as well as the Ohio Valley, consistent with the CFS/ECWMF/JMA and CFS/ECWMF Equal Weighted as well as Manual Blend tools. Near to below normal precipitation is favored over the remainder of the CONUS, under the near to above normal 500-hPa heights, consistent with dynamical guidance. Anomalous ridging and above normal 500-hPa heights lead to enhanced probabilities of below normal precipitation over southern Mainland Alaska, the Alaska Panhandle, and the Aleutians.
Most dynamical model guidance, including the experimental SubX models, depict enhanced chances for above normal temperatures over the northwestern Hawaiian islands (Lihue and Honolulu). Equal chances of below or above normal precipitation are forecast for Hawaii, based on little to no signal among precipitation tools.
International
And this week it is shown on a map.
Energy Analysis
We provide selected information from the monthly energy report. The full report can be accessed here. The data gets updated slowly so This is the April report and the new data is from January and is preliminary. But the historical tables are very interesting. We will provide certain data this month and it will be a question of figuring out what is most useful.
Now we will focus on production.
And now we focus on consumption
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.