April 10, 2021 Weather Impacts On Economic Activity – Crop Reporting Starts In Earnest

Written by Sig Silber

Tonight’s article is jam-packed. There is an ENSO Update, an early forecast on the Hurricane Season, an energy status report, a report on sorghum production in Argentina, and the first full crop progress report of the growing season. I have added some additional slides (mostly on sorghum) to the two Executive Briefings we introduced last week and we added a new Executive Briefing on Citrus. We have our first video in what is planned to be a weekly education program on sustainable agriculture. This article contains all of our regular features and as usual, we include an intermediate-term weather forecast.

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ENSO Update

The Southern Oscillation Index or SOI being above 7 confirms that the Atmosphere is in tune with the Sea Surface Temperatures in the Pacific along the Equator. At least temporarily the SOI Index is in neutral territory and is not confirming the La Nina. This is another graphic that we have not frozen.

Preliminary Hurricane Report Just Out

Spring Runoff: We are actively tracking that situation

Here is the pretty version of the snow depth map..

This one is a bit more up-to-date…..but not by much.

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

A cold front moving across the eastern half of the country last week brought showers and thunderstorms and left record-breaking cold temperatures in its wake. Meanwhile, dry conditions and warmer than normal temperatures continued in the West with many locations setting daily record high temperatures. The overall effect was a general deterioration of conditions across the Lower 48 as moisture deficits continued to build in the West and in locations in the eastern half of the country that missed out on the heaviest rainfall. Improvements were minimal and limited to parts of the Midwest and Southern Plains.

Northeast

Widespread precipitation occurred across much of the region, limiting the expansion of drought and abnormal dryness. In western New York, D1 (moderate drought) expanded again this week where precipitation deficits are close to 3 inches over the last 90 days and are reflected in low streamflow values and declining groundwater. Farther east, D0 (abnormal dryness) increased in eastern New York and Massachusetts. D1 remains in New Hampshire and Vermont. Last week, New Hampshire officials warned the public that water shortages may occur if the drought persists since conditions haven’t fully recovered from the drought of 2020.

Southeast

A strong cold front swung across the region last week, bringing showers and thunderstorms. Much of the region remains free of drought with pockets of D0 (abnormal dryness). Last week’s rain held off most potential degradations. The only exception is in South Florida, which missed out on most of the rain. Here, D0 and D1 (moderate drought) expanded in response to rainfall totals ranging from 5% to 50% of normal, extremely low streamflow, rapidly falling groundwater levels and increased fire danger.

South

Showers and thunderstorms impacted the Lower Mississippi and Tennessee valleys last week, resulting in improvements to abnormally dry (D0) areas in Mississippi. Having missed out on the heaviest rainfall, drought and abnormal dryness generally expanded in the western part of the region. In Texas, degradations occurred throughout the state in response to rainfall deficits, increased evaporative demand and vegetation health. Most notable is an expansion of D3 (extreme) and D4 (exceptional) drought in the long-term drought area in the western part of the state. In Oklahoma, this week’s map shows broad expansions of D0 and D1 (moderate drought). Warm, dry weather combined with gusty winds increased evaporative demand, drying out soils and vegetation. The only improvements this week occurred in the Texas Panhandle and western Oklahoma. Despite the lack of rain last week, a reassessment of indicators shows that conditions have started to recover.

Midwest

The most notable weather in the Midwest last week was the development of subfreezing temperatures. Locations in central Ohio, southern Indiana, southern Illinois and Kentucky set daily low records. For the most part, precipitation that fell generally missed the drought areas in the northern part of the region. The cooler than normal temperatures generally kept conditions from deteriorating despite drier than normal weather. Changes to this week’s map include an expansion of moderate drought (D1) in northeast Illinois and southeast Wisconsin in response to rainfall shortages of about 2 to 5 inches over the last 90 days and declining soil moisture.

High Plains

High temperatures in Nebraska and the Dakotas reached the upper 70s to mid-80s last week. With values of about 20 to 30 degrees above normal, many locations set daily records. These warm temperatures combined with low relative humidity and gusty winds to increase fire danger across the region. North and South Dakota declared fire emergencies due to ongoing drought conditions and increased wildfire activity. Severe (D2) and extreme (D3) drought expanded in both states as precipitation deficits continued to grow and increased evaporative demand dried out soils and stressed vegetation. USDA reports that, as of April 4, 92% of North Dakota’s topsoil and 68% of South Dakota’s topsoil was rated short to very short, indicating that soil moisture supplies are significantly less than what is required for normal crop growth development. In North Dakota, county Extension agents report that producers are starting to de-stock livestock herds by culling cows and grain farmers are very concerned about the lack of moisture. Photos show soil drift due to the dry conditions and high winds.

West

As the wet season begins to wind down in the West, widespread extreme (D3) to exceptional (D4) drought continues across much of the Southwest. Another week of warm, dry weather didn’t help. High temperatures ranged from 4 degrees above normal in the Northwest to 15 degrees above normal in the Southwest while little to no precipitation fell across much of the region. Where exceptions occurred, in the Pacific Northwest and the Northern Rockies, totals generally weren’t enough to overcome shortages. In eastern Washington, abnormal dryness (D0) and moderate drought (D1) expanded as precipitation deficits continued to increase, drying out soils. Conditions also deteriorated in Oregon. Most notably, D3 expanded and D4 was introduced in south central Oregon in response to record low water-year-to-date total precipitation, streamflow and soil moisture. It’s worth noting that since the U.S. Drought Monitor began in 2000, this is only the third time that D4 has occurred in the state, and only the second time that more than one percentage point of D4 has occurred. (In 2015, 0.1% of the state experienced D4 for two weeks in April. In 2003, there were 25 weeks of drought that included D4.) In California, the April 1 snow survey showed that snow water content in the Sierra Nevada Mountains was at 59% of average and the state, as a whole, received about 50% of its average precipitation for the water year. Two consecutive dry years have left reservoirs about half full. These precipitation deficits, combined with high temperatures, have reduced streamflow, dried out soils, and stressed vegetation. Changes to this week’s map include an expansion of D3 in northern California and western Nevada and an expansion of D2 (severe drought) and D3 in southern California. D1 expanded in northwest Wyoming and southeast Montana to reflect below normal precipitation over the water year and its effect on soil moisture and streamflow. Eastern Montana also saw deteriorating conditions with an expansion of D2 and D3. Here, the lack of precipitation over the last 2 to 3 months has dried out soils and stressed vegetation. USDA reports that, as of April 4, 76% of the state’s topsoil was rated short to very short indicating that soil moisture supplies are significantly less than what is required for normal crop growth development.

Alaska, Hawaii, and Puerto Rico

In Alaska, April has started off snowy in parts of the Alaskan interior. With only one week into the month, it is already the third-wettest April on record. For the most part, much of this snowfall either missed the abnormally dry (D0) areas of the northern and far-eastern interior or wasn’t enough to make up for the season-long deficits. Improvements were limited to northwest Alaska and the southern and western edge of the D0 area in the interior. Hawaii also saw improvements this week. A solid trade wind pattern combined with a wet March resulted in the removal of D0 on Maui and Molokai. D0 remains on the Big Island where rainfall totals continue to be below average. Puerto Rico saw both improvements and degradations this week. On the northwest part of the island, slight decreases were made to D0 and D1 (moderate drought) conditions improved in response to above normal rainfall totals over the last 90 days.

Pacific Islands

During the drought-monitoring week of March 31 – April 6, there was little overall change in the weather pattern across the U.S.-Affiliated Pacific Islands. As a result, there were no map changes in the depiction of dryness and drought. For the Commonwealth of the Northern Mariana Islands, mostly drier-than-normal weather continued. The NWS office on Guam reported that recent showers have resulted in a slight lowering of the Keetch-Byram Drought Index (KBDI) to 651 by April 5. However, KBDI values above 600 can still be indicative of dryness/drought and potentially erratic fire behavior, when fires occur. Guam International Airport received rainfall totaling 1.05 inch from March 31 – April 6, warranting a continuation of D0-S. Dryness (D0-S) also continued across Rota. Less than 2 inches of rain fell during March at all reporting sites on Saipan, where moderate drought (D1-S) persisted. All Saipan stations received less than an inch of rain from March 31 – April 6.

In the Federated States of Micronesia (FSM), all islands were free of dryness and drought except Kapingamarangi, where long-term severe drought (D2-L) persisted, owing to lingering impacts such as poor water quality from wells and stressed vegetation and crops. The weather observer on Kapingamarangi recently reported that water storage tanks are more than half full and that vegetation is slowly recovering, with green taro sprouts noted. Well water on Kapingamarangi is still not potable, but is being used for other purposes. Elsewhere in the FSM, a few islands have experienced drier weather in recent days. For example, Yap had rainfall totaling just 0.44 inch from April 1-6 but since March 1 has received 9.07 inches (142 percent of normal).

In the Republic of the Marshall Islands (RMI), water conservation measures remain in place on some northern islands. Wotje received more than an inch of rain (1.02 inches) during a drought-monitoring period for the first time since late 2020 — but remains in extreme drought (D3-S). On Kwajalein, where moderate drought (D1-S) persists, only light rain has fallen over the last month, with the last significant event occurring when 3.34 inches fell from March 8-10. Most other islands in the RMI continue to receive abundant rainfall; March totals included 24.40 inches on Ailinglapalap; 21.26 inches on Mili; and 15.00 inches on Majuro. Reservoir storage on Majuro exceeded 30 million gallons on March 30, peaking at 30.473 million gallons (85% of capacity) on April 3.

Weekly rainfall totals were above the minimum needed to meet most water requirements in the Republic of Palau, where the international airport measured 1.66 inches. The airport reported 8.84 inches of rain in March. As a result, D-nothing continued in the Republic of Palau. Similarly, plenty of rain fell in American Samoa, where D-nothing continued as the international airport collected 9.68 inches during March and 2.55 inches during the March 31 – April 6 drought-monitoring period.

Virgin Islands

During the drought-monitoring period ending April 6, portions of the U.S. Virgin Islands received beneficial showers as a frontal boundary stalled to the northwest. Some of the most significant rain fell across St. Thomas in early April, when showers and thunderstorms developed in a deep layer of tropical moisture south of the boundary. King Airport on St. Thomas received 1.35 inches of rain on April 2-3. During all of March, rainfall at King Airport totaled just 0.61 inch (43% of normal). Elsewhere on St. Thomas, volunteer (CoCoRaHS) observer VI-ST-5 also received more than an inch of rain, with 1.07 inches falling in early April. Despite the rain, the D2-SL designation was retained on St. Thomas, partly due to lingering impacts from earlier dryness and the lack of groundwater response, as shown by the U.S. Geological Survey well on the island. In addition, recent heat on St. Thomas has aggravated the effects of a drier-than-normal spring. On the 30th, King Airport reported a 90-degree reading in March for the first time since 1996.

Late-March and early-April showers were generally lighter and more scattered across St. John (unchanged at D2-S) and St. Croix (also unchanged at D1-SL), although a few spots noted more than an inch of rain. The observer at Windswept Beach on the north shore of St. John received less than 2 inches of rain in each of the first 3 months of the year for only the third time in the last 38 years, along with 2011 and 2019. For the 7-day period ending April 6, Windswept Beach received 0.55 inch, while volunteer observer VI-SJ-5 collected 0.53 inch. Farther south, a very dry March on St. Croix was followed by scattered showers late in the month and in early April. March rainfall at Rohlsen Airport on St. Croix totaled 0.55 inch (35% of normal), followed by 0.55 inch from April 1-6. Rainfall for the week ending April 6 was a bit heavier at volunteer (CoCoRaHS) observation site VI-SC-8, where 1.01 inches fell, and VI-SC-10, which received 1.05 inches.

Looking Ahead

The National Weather Service Weather Prediction Center forecast for the next 5 days (April 8 through the 12) forecasts heavy rain and the potential for thunderstorms for the central U.S. As the storm system pushes eastward, chances increase for heavy rain and thunderstorms across the Upper Midwest, south-central, and southeastern U.S. In the Northwest, a storm moving in from the Pacific will bring colder than normal temperatures with snow likely falling in the Cascades and Northern Rockies and rain at lower elevations. In the Southwest and southern High Plains, warm, dry weather combined with gusty winds is expected to persist, leading to the potential continuation of dangerous fire weather conditions. Moving into next week, the Climate Prediction Center six- to 10-day outlook (valid April 12 through April 16) favors above normal temperatures across the West, Northeast and Southeast, with the largest probabilities centered over the Great Basin and New England. Below normal temperatures are most likely across the Great Plains, Mississippi Valley and Alaska. The greatest probabilities of above normal precipitation are across the Southern Plains, Southeast and Mid-Atlantic states.

More Regional Reports

The result of the drought is very dry soil conditions

Soil Dryness in More Depth.

Surface Soil

Subsurface

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 Conditions

State Agriculture Summary Reports.

Crop Progress Reports

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

Cargill: “Consumers would alter food choices to curb climate change”

By Susan Kelly on 4/9/2021 Published in meatingplace.com. The original article can be found here.

A large majority (80%) of consumers reported a willingness to make a change in the type of food they purchase or eat if it would help reduce climate change, according to a new survey from Cargill.

Almost half (47%) said they are willing to pay a premium for a product promising a low-carbon footprint. And 42% said they believe livestock can reduce methane emission.

Among participants in Cargill’s Feed4Thought survey, those who indicated climate change is important to them also rated livestock and agriculture lowest in negative impact compared with other industries generally regarded as significant contributors, the company said in a news release.

The survey, conducted in January, included responses from 2,510 consumers in the U.S., France, South Korea and Brazil. Participants ranked transportation and deforestation as the greatest contributors to climate change.

Regarding who is most responsible for accelerating change, 59% said federal and national governments bear the highest responsibility for addressing climate change, while 57% saw companies involved in beef production and 50% saw cattle farmers as responsible for reducing the impact of livestock.

About 14.5% of global human-originated emissions can be attributed to agriculture, and about 3% of U.S. emissions are attributed to methane emitted as a result of the digestive process in ruminant animals, Cargill said.

Of those surveyed, one-fourth said they would purchase more beef if cattle were fed an additive or used other technology to reduce methane emissions.

USDA Global Market Analysis.

USDA Executive Briefings and other NASS Reports.

We reported on this last week and I have added some slides to the reports.

There were two Executive Briefings that were released last week. One was on Hogs and Pigs and the other was the first report on Principle Crops. It was not possible for us to present those reports due to lack of time. What we are doing is presenting a key graphic from each and a link to the full report and we may do a more extensive report next week. I think one of the reports is 58 pages. That is a lot of snipping and uploading etc. It is the first report of the new season.

First I will start with the new Citrus Executive Briefing.

Hogs and Pigs. You can access the full report here.

Grain Stocks, Prospective Plantings and Rice Stocks. This is a long report covering a number of topics basically two: Prospective Plantings and the carryover inventory of grain and rice. The full report can be found here. We may or may not present the full report next week. USDA can produce information faster than I can publish it. It is a large organization and I am one person. The report is definitely worth reading but the first report of the season may reflect a lot more intentions than what the ultimate reality will be. They say they published this on March 31 and sometimes it shows up a little later than the stated publishing date and March 31 is the day that NOAA published their updated forecast for April. So I was busy with that. But you do not need to wait for me as I have provided you with the link to get the PDF of this report. I will now show you what I consider to be the key graphics in this very long report.

Focus on Sorghum

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.

Week 3-4 Forecast Discussion Valid Sat Apr 24 2021-Fri May 07 2021

As La Nina looks to be on the way out, a robust Madden-Julian Oscillation (MJO) event is coming into focus over the West Pacific. The latter phenomena is associated with a robust westerly wind burst that appears likely to initiate an upwelling Kelvin wave that would help usher out the bulk of what cold water volume remains over the Central Pacific. The typical response to an active MJO in Phase 6 during boreal Spring is somewhat weak, marked most prominently by anomalous troughing over northeastern North America early in the period. Given the aforementioned setup, the present outlook is primarily rooted in dynamical model guidance with some deference to long-term trends.

The forecast circulations are reasonably consistent among model guidance. The Weeks 3-4 mean perspective features anomalous 500-hPa ridges centered over the Bering Sea and Great Basin with anomalous troughing over Hudson Bay that extends to varied degrees southward toward the CONUS. This troughing is likely the models’ representation of the canonical MJO response noted in the prior paragraph, with the feature retreating northward over the course of the forecast period in most model guidance.

The highest confidence among temperature forecasts is across portions of the Four Corners region tied to the anticipated proximity to the anomalous ridge axis. Above-normal temperatures are otherwise favored across the CONUS for most areas west of the Mississippi River from this same feature. There is some extension of increased above-normal temperature odds eastward through the Carolinas. Equal chances from the Upper Mississippi Valley eastward through New England are tied to potential for early period cold from the anomalous troughing before temperatures moderate as the trough likely lifts northward. Equal chances for the Southeast are tied to the potential for below-normal temperatures to linger from the Week-2 period that would recover as ridging builds. Above-normal temperatures are also favored for the Aleutians and western mainland Alaska (proximity to anomalous ridging) and North Slope (long-term trends). Mixed signals lead to equal chances across the remainder of Alaska.

With ridging forecast near the Pacific Coast, a fairly dry pattern appears likely for the CONUS. The greatest confidence for below-median precipitation is across the West given the proximity to the ridge. Equal chances are carved out for parts of the Desert Southwest (climatologically arid) and a region from the Great Lakes through Mid-Atlantic and New England (possible mean frontal zone lifting northward through the period). Ridging forecast over the Bering Sea results in elevated odds for below-median precipitation for the Aleutians and western mainland Alaska.

Observed sea surface temperature anomalies are near zero surrounding the Big Island and shift toward increasingly above-normal values while approaching Ni’ihau. This favors above-normal temperatures in the western islands that decrease to equal chances for Hilo. Weak precipitation signals result in equal chances across the entire chain.

International

This week unfortunately we did not have a map.

In the box are shown the major resources we use. We will not be using them all each week but the reader is welcome to refer to these resources.

Short-Term Energy Analysis

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 Nina persistence. The first is the preceding El Nino amplitude. From a linear perspective, the discharging amount is proportional to the El Nino amplitude. With strong discharging, significant SSTA cooling could last more than 2 years. The preceding El Nino 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 Nina decaying phase. This initiates a La Nina 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 Nino 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 Nino preceding a La Nina 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 Nina. When a La Nina 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 Nina persistence, it is reasonable to consider both the preceding El Nino amplitude and recharging/discharging efficiency by anomalous SST/wind-stress patterns as the La Nina evolves. In this study, we investigated how the evolution of single- and multi-year La Nina 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 Nina 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 (Nino-4 > Nino-3) El Nino tends to exist one year prior to a single-year La Nina event, whereas a strong EP-type (Nino-3 > Nino-4) El Nino tends to exist prior to a multi-year La Nina event.

2. For single-year La Nina events, negative WSC, associated with CP-type El Nino and confined to the western Pacific, induces weak discharge that leads to a weak La Nina. In contrast, a widely extended negative WSC of a multi-year La Nina in the Pacific basin causes efficient discharge, resulting in a strong La Nina.

3. From the decay of an El Nino to a La Nina developing period, SSTA cooling is initiated by anomalous easterly wind in the eastern Pacific for single-year La Nina. However, for multi-year La Nina events, SSTA cooling is triggered by a mid-latitudinal influence (i.e., PMM/NPO). Such a difference in initiation of a La Nina explains why the meridional width of multi-year La Nina events exceeds that of single-year La Nina events.

4. During a La Nina peak period, single-year La Nina events have an equatorially confined SST cooling structure; however, multi-year La Nina events have intensified SSTA cooling and meridionally expanded easterly wind anomalies. In addition, the overall structure of multi-year La Nina events is shifted westward. Comprehensively, the SST structure of multi-year La Nina events is similar to Mega-ENSO. Thus, the recharging process of a singleyear La Nina is strong due to equatorially confined SST cooling/strong WSC, whereas that of a multi-year La Nina is relatively weak due to meridionally expanded SST cooling/weak WSC.

5. During a La Nina decaying period, the SSTA cooling of a single-year La Nina disappears with strong recharging, resulting in the termination of the La Nina. For multi-year La Nina, 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 Nina in boreal winter (Chen et al. 2016).

6. The iteration of processes 3 through 5 for a multiyear La Nina may lead to the development of another La Nina in the subsequent winter.

In summary, by analysing the statistically significant differences in precursory signals between single- and multi-year La Nina 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 Nina persistence. This indicates that there is similar dynamics operating during mega-ENSO and multi-year La Nina 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 Nina events.

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