April 17, 2021 Weather Impacts On Economic Activity – Drought And Flooding

Written by Sig Silber

Drought to the west and flooding to the east. This pattern may continue for some time. It is not likely to be good for the economy. It does create lots of material for this article but I would much prefer to have more normal weather. I am deluged with regional reports on the drought and we cover some of them. We also present and discuss the new seasonal Drought Forecast and have our standard growing season crop progress report. We review March state rankings with respect to precipitation and temperature. See our second video in what is planned to be a weekly education program on sustainable agriculture. We have not removed the first video so that readers 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.

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Of Special Interest.

Seasonal Issue with Economic Impacts

Spring Runoff: We are actively tracking that situation

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

Last week’s storm system brought heavy rain and thunderstorms to much of the central and eastern U.S., bringing drought condition improvements to parts of the Midwest and eastern Plains. Meanwhile, warm, dry conditions persisted in the West and New England, resulting in deteriorating conditions. Parts of the Southwest are now experiencing record levels of dryness for the last 12 months. In New England, year-to-date precipitation ranks in the top 10 driest on record.

Northeast

Warm, dry weather across New England led to the expansion of moderate drought (D1). Here, daytime high temperatures over the last week ranged from 6 to 19 degrees above normal. Year-to-date precipitation at many stations ranks in the top 10 driest, down about 3 to 5 inches below normal. This has led to very low soil moisture, reduced streamflow, and declining groundwater. New Hampshire state officials urged the public to conserve water in response to the growing deficits and are preparing to adopt restrictions if the dry conditions persist. Likewise, Massachusetts residents and business are also being urged to curb water use. Widespread precipitation across the Mid-Atlantic last week helped prevent additional degradations in drought areas and alleviate any pockets of developing dryness.

Southeast

Much of the region remains free of drought, with pockets of D0 (abnormal dryness). The only exception is southern Florida, where D1 (moderate drought) remains. Showers and thunderstorms across the region held off potential degradations, but the precipitation wasn’t enough to make up remaining deficits of about 1.5 to 4 inches of rainfall.

South

Showers and thunderstorms across the South last week led to slight improvements to D0 (abnormally dry) and/or D1 (moderate drought) in Louisiana and East Texas. While the rain helped some, shortages still exist at 60 to 90 days. Having missed out on the rainfall, conditions continued to deteriorate in south-central Oklahoma and in North, South and West Texas, where another week of warm, dry weather continued to build long-term deficits and further dried out soils.

Midwest

Widespread precipitation across the Midwest led to broad improvements to drought across the region, erasing moisture deficits and improving soil moisture and streamflow. Locally heavy totals (more than 2 inches) led to a two-category improvement in western Minnesota. These short-term gains also finally chipped away at the D3 (exceptional drought) area in northwestern Iowa that had been in place since October 2020. Cooperative Extension notes that tile lines are running again and water is standing in roadside ditches. Soil moisture has also started to rebound.

High Plains

The High Plains generally saw cooler than normal temperatures and widespread precipitation over the last week. Locally heavy amounts of rainfall (more than 2 inches) helped erase long moisture deficits in eastern South Dakota and southeastern North Dakota, resulting in improvements to moderate drought. In the remainder of the region, precipitation wasn’t enough to prevent worsening conditions. Moderate drought (D1) expanded in South Dakota and extreme drought (D3) expanded in both North and South Dakota to reflect the growing moisture deficits and its effect on soil moisture. Agricultural field reports indicate planters are being idled, except in the southeastern corner of the state, and cattle are being culled.

West

Once again this week, much of the West remained dry. Where precipitation did fall, in the Pacific Northwest and the Northern Rockies, it either missed the drought-inflicted areas or wasn’t enough to overcome shortages. The only exception was in north-central Wyoming and southern Montana, where last week’s snowfall lessened precipitation deficits and improved streamflow and soil moisture resulting in a one-category improvement to drought. In eastern Washington, D0 (abnormally dryness), D1 (moderate) and D2 (severe) drought expanded as precipitation deficits continued to increase, drying out soils and lowering streamflow. Conditions once again deteriorated in Oregon this week with expansions in D2, D3 (extreme) and D4 (exceptional drought). The warm, dry winter added to deficits that had been in place for more than a year, leaving soils extremely dry and limiting runoff. Crop condition reports that counties in the D3 and D4 regions are “heading into their worst water year ever for irrigators with the potential for large amounts of acres left fallowed.” Little to no water for irrigation is expected for Upper Klamath Lake this year. Similarly, drought also expanded in Idaho where a lack of precipitation for almost two months has limited runoff, resulted in earlier than normal snowmelt and put short-term precipitation below the 10th percentile at many locations. Drought conditions also expanded in northern and central California. Another week without rainfall has continued to build upon longer deficits. Cooperative Extension notes that the majority of the season’s creeks aren’t flowing and that stock ponds are still dry. Decreases in water allocation and reduced or negligible forage are causing producers to respond by culling and selling herds. Drought conditions are also prompting Cal Fire to bring in fire crews earlier in the season. Other changes to this week’s map include an expansion of D1 in western Wyoming, and D1, D2 and D3 in eastern Montana reflect the lack of precipitation over the last two to three months and its effect on soil moisture and streamflow. In the Southwest, D4 was expanded over southeast Arizona as another week of hot, dry weather showed its impact on streamflow, soil moisture, and vegetation. It’s worth noting that high temperatures ranged from 10 to 20 degrees above normal last week and that many areas are now at record dry levels for the past 12 months.

Alaska, Hawaii, and Puerto Rico

In Alaska, continued snowy conditions once again led to improvements along the southern and western edges of the abnormally dry area (D0) in the interior. In Hawaii, trade winds kept most of the rainfall confined to the east-facing windward slopes. Isolated, brief thunderstorms produced some rainfall on the leeward slopes, but the totals weren’t enough to keep conditions from deteriorating. Increasing rainfall deficits and declining vegetation health led to the expansion of D0 and the introduction of D1 on the Big Island. Puerto Rico also saw an expansion of D0 on the southwest part of the Island in response to growing rainfall shortages and low streamflow.

Pacific Islands

Following an extended period of slow organization, Tropical Storm Surigae (02W) formed while moving northwestward between Koror (Republic of Palau) and Yap (Federated States of Micronesia), passing near Ngulu Atoll on April 14. A broad moisture field surrounding the developing tropical circulation produced locally heavy showers across the Republic of Palau and Yap State, as well as neighboring islands. Farther north, however, only light precipitation fell across large sections of the Mariana Islands, where dryness (D0-S) and moderate short-term drought (D1-S) persisted. Meanwhile, significant drought continued to affect the northern islands in the Republic of the Marshall Islands, with Wotje remaining in extreme short-term drought (D3-S) and Kwajalein experiencing deterioration to severe drought (D2-S), compared to last week’s D1-S designation. Elsewhere, rainfall was sufficient to maintain drought-free conditions in all other areas, except Kapingamarangi (southernmost atoll in the Federated States of Micronesia), which is experiencing severe drought (D2-L) while continuing to slowly recover from acute long-term precipitation deficits.

In the Commonwealth of the Northern Mariana Islands, patchy showers embedded in a mostly dry trade wind pattern prevented further deterioration but did not provide much, if any, relief. Based on data from Guam International Airport, the Keetch-Byram Drought Index (KBDI) rose to 700 by April 14, up from 651 on April 5. KBDI values above 600 can be indicative of dryness/drought and potentially erratic fire behavior, when fires occur. From April 7-13, Guam International Airport received rainfall totaling just 0.20 inch, warranting a continuation of D0-S. On the west coast of Guam, the village of Agat received 2.04 inches during the same 7-day period. D0-S also persisted on Rota, where only 0.20 inch fell. Moderate drought (D1-S) continued across Saipan, where all reporting sites received less than an inch of rain from April 7-13.

In the Federated States of Micronesia (FSM), all islands were free of dryness and drought except Kapingamarangi, where long-term severe drought (D2-L) existed. Kapingamarangi, which endured the punishing effects of exceptional drought (D4) on crops and water supplies as recently as January 2021, has experienced slow recovery. Rainfall on Kapingamarangi has totaled at least 2 inches in four of the last 5 weeks, with more than 5 inches falling from April 7-13. Elsewhere in the FSM, most of the focus was on Tropical Storm Surigae, which led to the issuance of a Tropical Storm Warning on April 14 for Ngulu Atoll and Yap Island. Weekly (April 7-13) totals reached 8.12 inches at an observation site (North Fanif) in the Yap State municipality of Fanif and 7.76 inches on Woleai Atoll.

In the Republic of the Marshall Islands (RMI), water conservation measures remain in place on some northern islands. Wotje received 2.00 inches of rain from April 7-13 and reported a 2-week total of 3.02 inches. However, a protracted dry spell on Wotje since late 2020 warrants a continuation of extreme drought. On Kwajalein, where severe drought was introduced, there have been mostly only light rain showers since March 8-10, when 3.34 inches fell. Kwajalein’s rainfall from March 11 – April 13 totaled 3.16 inches. Although Kwajalein has high-quality water from reverse osmosis units, some neighboring islands are dependent on natural sources or water transported from Kwajalein. Ebeye, the most populous island of the Kwajalein Atoll, has experienced some water shortages, along with Mejatto and Ebadon. In addition, some crops on these islands are yellowing due to drought. Meanwhile, Majuro has turned drier in recent days, with a corresponding drop in reservoir storage. After peaking at 30.473 million gallons (85 percent of capacity) on April 3, Majuro’s storage fell to 27.218 million gallons (76 percent) by April 13. Majuro remains free of dryness, but short-term trends in rainfall and reservoir storage will need to be monitored.

Elsewhere, neither dryness nor drought is present across the Republic of Palau and American Samoa. Palau was on the periphery of nascent Tropical Storm Surigae, resulting in locally heavy showers. From April 7-13, rainfall at Palau International Airport totaled 5.54 inches. Meanwhile, slightly drier weather developed across American Samoa, with weekly totals less than 1.10 inches at all observation sites. However, earlier wetness in American Samoa currently precludes any dryness-related concerns.

Virgin Islands

Following some beneficial showers across the U.S. Virgin Islands in early April, mostly dry weather returned. In addition, some unusually dry air briefly overspread the islands. On April 10, for example, the dewpoint temperature fell as low as 57 degF at Rohlsen Airport (TISX) on St. Croix, resulting in a relative humidity of 42 percent. On the same date, King Airport (TIST) on St. Thomas had a minimum dewpoint of 56 degF. Weekly (April 6-12) rainfall totaled 0.01 inch at Rohlsen Airport and 0.16 inch at King Airport.

Among volunteer (CoCoRaHS) weather stations in the U.S. Virgin Islands, the highest April 6-12 total was 0.73 inch – all of which fell in a downpour on the 7th – along the northern shoreline of St. John at VI-SJ-3. Just miles away, near Cruz Bay, 0.12 inch fell during the drought-monitoring period at VI-SJ-5. According to the U.S. Geological Survey (USGS) well on St. John, depth to water has steadily increased throughout 2021 and is approaching 15.5 feet. Given the short-term dryness and groundwater shortages, severe drought (D2-S) persisted on St. John.

In recent days, depth to water on St. Thomas has stabilized near 13.35 feet, based on information from USGS well data. Aside from a period in 2020 when information was not available, depth to water has not been greater on St. Thomas since the summer of 2017, shortly before Hurricanes Irma and Maria devastated the U.S. Virgin Islands. During the drought-monitoring period, rainfall at four volunteer (CoCoRaHS) weather stations ranged from zero to 0.07 inch. Given the return to dry weather and lack of well recovery, St. Thomas retained a severe drought (D2-SL) designation.

On St. Croix, drought-monitoring period rainfall at all airport and volunteer (CoCoRaHS) sites totaled less than one-tenth of an inch. Depth to water at the USGS well on St. Croix increased to more than 22.4 feet for the first time since December 2016, although data was missing for part of 2020. As a result, the moderate drought (D1-SL) designation was maintained for St. Croix.

Looking Ahead

The National Weather Service Weather Prediction Center forecast for the next five days (April 15-19) shows slow moving storm systems affecting large parts of the Lower 48. This storm is expected to bring cold temperatures and late season snows across the Northern Plains, Central Rockies and Northern Great Basin. In the Southwest, dry weather combined with gusty winds is expected to persist, leading to an elevated fire risk. In contrast, the Lower Mississippi Valley and eastern Gulf Coast states are expected to see heavy rainfall. Moving into next week, the Climate Prediction Center six to 10 day outlook (valid April 19-23) favors above normal temperatures across the West, Northeast and Southeast. Below normal temperatures are most likely across the Great Plains, Midwest and Mississippi Valley. Below normal precipitation is expected across much of the country with the exception of the Southern High Plains, Florida and New England.

Regional Reports

California/Nevada

Southwest

Soil Conditions

The result of the drought is very dry soil conditions

Soil Dryness in More Depth.

Surface Soil

Subsurface

April Drought Outlook

Here is the discussion released with the drought outlook. I am showing the short version as it is easier to understand.

Latest Seasonal Assessment – Drought expanded or intensified across much of Texas and the Dakotas since March due to increasing precipitation deficits along with periods of enhanced winds. Persistence is favored for most of these areas, based on elevated probabilities of below normal precipitation and above normal temperatures during May-June-July. Development is expected across west-central Texas along with parts of Oklahoma where 60-day precipitation deficits exist and below normal precipitation is likely during May, one of the wettest months of the year. Recent heavy precipitation limits the coverage of development across the central Great Plains. The forecast confidence for persistence and development across the Great Plains is low since May-June-July is their wettest time of year. Heavy rainfall, forecast during mid to late April, favors drought removal for southeast Texas and northeast Louisiana. A wet climatology and the May-June-July outlook calling for equal chances of below, near, or above normal precipitation support drought improvement or removal across eastern North Dakota and northwestern Minnesota. The drought removal forecast for the Great Lakes and eastern Corn Belt is consistent with the monthly and seasonal precipitation outlooks. The Great Plains and Corn Belt will be closely monitored in the monthly drought outlooks through the summer.

Persistence is likely for California and the Southwest where the climatology is relatively dry throughout much of the outlook period. Rainfall, associated with the North American Monsoon, typically begins across the Southwest during mid-July. Since below normal precipitation and above normal temperatures are favored across much of the Pacific Northwest and Intermountain West, drought is forecast to expand in coverage across the West by the end of July.

A wet latter half of April along with the onset of the convective season strongly support removal across the small drought areas of southern Florida. The removal forecast for the Northeast is based on elevated probabilities of above normal precipitation during May-June-July.

Alaska is forecast to remain drought-free through the end of July. Drought is favored to gradually end across Puerto Rico during the next few months. Drought development and persistence is forecast across the leeward sides of the Hawaiian Islands.

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.

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

“Meat leads CPI increases”. You can read the full story here. It is difficult to know what if any the role of weather is in this case but weather has driven up the cost of feed. So it is not unreasonable that it contributes to the cost of meat.

“Senators Ernst, Marshall push back against ‘war on meat'” is another good article. You can access it here. There are a number of challenges to meat but from a health perspective and from a Climate Change perspective. But people do not seem to realize that cattle have replaced bison and you need animals to graze to keep the grasslands healthy.

I may be adding news articles during the week. I have a number of them but I am running up against my publishing deadline.

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 now since USDA is always publishing new material.

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.

La Nina conditions continued but weakened across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) have trended closer to average across the west-central equatorial Pacific Ocean, but enhanced upper level westerly wind anomalies persist across the eastern tropical Pacific. The RMM index depicts an eastward propagation of the MJO over the West Pacific during mid-April. Dynamical models continue to favor the eastward propagation of the MJO over the Pacific and the Western Hemisphere during the remainder of April. The Week 3-4 temperature and precipitation outlooks are based primarily on dynamical model forecasts from the NCEP CFS, ECMWF, and JMA, and the Subseasonal Experiment (SubX) multi-model ensemble (MME) of experimental and operational ensemble prediction systems with additional considerations for MJO, La Nina, and long-term trends, 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. Most dynamical models feature anomalous ridging with above normal 500-hPa heights over the West Coast and southwest CONUS, while anomalous trouging is forecast across the north-central CONUS. Above normal 500-hPa heights are predicted over most of Mainland Alaska. Near normal 500-hpa heights are likely over Hawaii.

Below normal temperatures are favored over parts of the Northern Rockies, the Northern Plains and the Upper Mississippi Valley due to below normal 500-hpa heights. Above normal temperatures are more likely across the remainder of the CONUS, under predicted above normal 500-hPa heights. The highest confidence for above-normal temperatures lies across Arizona and New Mexico (greater than 60%) tied to the forecast position of the anomalous 500-hPa ridge axis. Predicted ridging and above normal 500-hPa heights lead to above normal temperatures favored over Alaska.

The dynamical model guidance is in reasonably good agreement on the spatial pattern of anomalous precipitation during the Week 3-4 period. Near to below normal precipitation is favored throughout most of the western CONUS, under the near to above normal 500-hPa heights, while predicted troughing and below normal 500-hPa heights increase the chances of above normal precipitation over the Great Lakes, Ohio Valley, and Mid-Atlantic, consistent with dynamical and statistical guidance. Anomalous ridging and above normal 500-hPa heights lead to enhanced probabilities of below normal precipitation over Alaska.

Most dynamical model guidance, including the experimental SubX models, depict enhanced chances for above normal temperatures for Hawaii. Dynamical model precipitation forecasts generally favor below normal precipitation over the southeastern Hawaiian islands.

International

This week fortunately we did have a map.

Soybean production in US versus selected countries in Latin America

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.

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 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.