Written by Sig Silber

Unusual weather has impacted agriculture, mining, oil and gas, and water supplies. We are reporting findings of the IPCC International Panel on Climate Change as they are released. But our focus is on current conditions not looking into the future. We also attempt, within the limits of our resources, to consider if the observed impacts are outside of the range of expected variation. We have a lot to report tonight including a very interesting audio interview that the reader can listen to. We also report on an oddity favorably impacting the fishing industry in Japan. We have some other pretty interesting international reports including Canada, Russia, and Ukraine. As usual, we include a lot of other information including an intermediate-term weather forecast.

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The below (H/T John Hooker) relates to a very interesting radio presentation related to the Colorado River. If you click on the image or here, you will get the audio. There will be about 45 seconds of a commercial. Be patient!. To get back to this article after you have listened to as much of the interview as you want, hit the return arrow in the upper left corner of your screen.

Climate Change

This is a good place to discuss Climate Change which I prefer to call Global Warming because the warming part is fairly easy to measure and the impact on Climate is much more difficult to predict or confirm within current weather over an appropriate period of time. But there is not room in this article to address the statistical issues involved. But I came across an interesting article while attempting to figure out what is wrong with the JAMSTEC web site.

So I use a provocative title. Is CC always a negative? The research by JAMSTEC was not as philosophical as the above question. Here are the highlights from their research project which was published recently but covered a period of time that ended five years ago. I am not sure what t make of it but I am sure that I do not read Japanese and there is no chance that I can extend the data beyond that which was reported in the article. So here goes.

Now it gets interesting. Below is the observed change in the current.

And here is where it gets interesting. I have written about the Climate of the Southwest

Climate of the Southwest

Written by Sig Silber

As Presented at the Fourth Santa Fe Conference on Global and Regional Climate Change February 5-10, 2017 Santa Fe, New Mexico

But we are not going to ignore the drought or the poor prospects of an adequate Spring runoff. So that’s where we start:

Last Week	This Week

Two Weeks Ago	This Week

Recent Events.

This should be similar to the above

Here is what the raw data looks like.

And here is the raw data.

And it is always useful to look at the last seven days.

re

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)

A vigorous and slow-moving area of mid-level low pressure tracked from the Four Corners region to the Central Great Plains from March 13 to 15. Widespread precipitation (rain and snow totaling 2 to 6 inches, liquid equivalent) fell across the Central Rockies, Central Great Plains, and Lower Missouri River Basin. Snowfall amounts ranged from 2 to 4 feet from Colorado north to Wyoming. However, across North Dakota, dry weather continued. As a low pressure system tracked eastward, rainfall (locally more than 2 inches) overspread the Ohio and Tennessee Valleys along with parts of the Southeast on March 14 and 15. Mostly dry weather prevailed along the East Coast from March 9 to 15. During this 7-day period, below normal precipitation was observed throughout much of the Pacific Northwest while precipitation amounts exceeded 2 inches (liquid equivalent) for the Sierra Nevada Mountains and coastal ranges of California. A low pressure system and trailing front resulted in heavy rainfall and flooding across parts of the Hawaiian Islands during early March.

Northeast

Mostly dry weather prevailed during the past week and 30-day precipitation has averaged below normal for most of this region. Small areas of short to long-term moderate drought (D1) continue across parts of northern New York, Vermont, and New Hampshire. Abnormal dryness (D0) was introduced to southeast New England where little or no precipitation has occurred through the first half of March and short-term precipitation deficits are increasing.

Southeast

Short-term precipitation deficits continue to increase throughout the Florida Peninsula and along the northern Gulf Coast, prompting an expansion of abnormal dryness (D0) for these areas. Conversely, heavy rain that fell just prior to 12Z Tuesday, March 16 led to improvements across parts of west-central Alabama. The largest 60-day precipitation deficits (2 to 6 inches) are centered across the southern half of Alabama where 28-day streamflows are running between the 10th to 24th percentile and soil moisture is below the 30th percentile. Maximum temperatures recently warmed above 80 degrees F across southern Alabama, increasing evapotranspiration rates. According to the Alabama Forestry Commission, 261 wildfires charred about 5,700 acres between March 7th and March 13th. Despite these recent drying trends (prior to March 16), the abnormal dryness and moderate drought areas depicted across Georgia and Alabama were not expanded this week and will be reassessed next week. Below normal precipitation during the past 60 days extends northeast to the upstate of South Carolina and this area will be closely monitored in subsequent weeks.

South

As an intense mid-level low pressure system crossed the Four Corners Region, an early season severe weather outbreak occurred across the Texas Panhandle on March 13. According to the Storm Prediction Center, more than a dozen tornado reports were tallied. Along with the severe weather, locally more than 1 inch of rainfall was observed across the Texas Panhandle, resulting in drought improvement. This 1-category improvement extended east to parts of southwest Oklahoma where Oklahoma Mesonet gauges recorded 1 to 2.5 inches of rainfall this past week. Elsewhere across the southern Plains, western Gulf Coast, and lower Mississippi Valley, mostly dry weather prevailed along with above normal temperatures and periods of increased winds. Based on short-term indicators, abnormal dryness (D0) and drought (D1-D3) were expanded in coverage across south-central Oklahoma and Texas. Wheat is entering the critical hollow stem stage across south-central Oklahoma. The northeast quarter of Louisiana has received less than 4 inches of precipitation during the past 60 days which prompted a slight increase in the coverage of short-term moderate drought (D1). An increase in the coverage of D0 was made across parts of Louisiana and Mississippi, due to increasing short-term precipitation deficits along with soil moisture and 28-day streamflows below the 30th percentile.

Midwest

A major storm brought widespread heavy precipitation (1 to 3 inches, liquid equivalent) to western Iowa and Missouri. This recent precipitation resulted in a 1-category improvement for areas that received more than 1.5 inches of precipitation. A slight eastward expansion of abnormal dryness (D0) across northeast Missouri, which merged with the ongoing D0 in west-central Illinois, was supported by short-term SPI values and decreasing 28-day average streamflows. The largest 90-day precipitation deficits (2 to 4 inches) for this region exist across the northern half of Indiana and Ohio along with parts of Michigan. Based on 60 to 90 day SPIs, along with low soil moisture (below the 30th percentile), abnormal dryness (D0) was expanded to include more of eastern Indiana and bordering areas of Ohio with a slight expansion of short-term moderate drought (D1) across northern Ohio. Conditions improve farther south over Ohio where a slight reduction in abnormal dryness was warranted.

High Plains

A strong and slow-moving low pressure system brought widespread heavy precipitation (more than 2 inches, liquid equivalent) to northeast Colorado, southeast Wyoming, southwest South Dakota, northern Kansas, and much of Nebraska. Denver, Colorado received 27.1 inches of snowfall on March 13 and 14, making it the 4th largest snowfall on record. According to the storm summary from the Weather Prediction Center, other notable snowfall amounts included: 42″ near Buckhorn Mountain, CO, 27″ near Hot Springs, SD, and 19″ near Gering, NE. The highest snowfall amount reported from this storm was 52.5″ at Windy Peak, WY. The heavy snowfall brought snow water content close to average for mid-March across most of Colorado and Wyoming. This recent heavy precipitation also eliminated precipitation deficits and resulted in precipitation surpluses for the past 90 days for much of the central Plains. Therefore, a broad 1-category improvement was made for areas that received 1 inch or more of precipitation. Based on SPI values at various time scales, small areas of 2-category improvements were justified for the central Great Plains west to the central Rockies. Please note that the drought impact type was changed from SL (short and long-term) to L (long-term) drought only where the heaviest precipitation occurred, but where long-term indicators such as SPI support a continuation of D1+ long-term drought.

Based on 120-day SPI values, a 1-category improvement was made to parts of southwest Wyoming and bordering areas of southeast Idaho and northeast Utah. Farther to the north, the northern third of South Dakota and North Dakota missed out on the drought relief. During the past two weeks, widespread 1 to 2-category degradations were made and only slight changes were necessary this week. However, based on soil moisture below the 5th percentile and SPI values, D3 (extreme) drought was introduced to parts of North Dakota. If below normal precipitation persists later into the spring when temperatures warm, rapid intensification of drought conditions may occur.

West

Below normal precipitation prevailed across the Pacific Northwest during the past week. Recent dryness during the past 30 days and below normal 28-day average streamflows supported a northward expansion of abnormal dryness (D0) across eastern Washington. Near to above normal precipitation was generally observed throughout California, the Great Basin, and Desert Southwest during early to mid-March. As of March 16, snow water content is running 60 to 75 percent of average across the Sierra Nevada Mountains. No changes were made this week to much of the moderate (D1) to exceptional (D4) drought areas across the West.

Alaska, Hawaii, and Puerto Rico

Abnormal dryness continues for northern and eastern Mainland Alaska with no changes this week. The water year precipitation remains below 70 percent for northeast parts of Alaska.

Heavy rainfall continued to affect parts of Hawaii during the second week of March. A low pressure system and associated front resulted in additional heavy rainfall for Kauai, Oahu, and the Big Island. Numerous preliminary reports of 6-day totals, ending on March 13, exceeded 10 inches across parts of these islands and maximum amounts of more than 20 inches were reported. Much of this heavy rain fell outside of existing drought areas but improvements can be expected in subsequent assessments.

During the past week, above normal precipitation resulted in a decrease of abnormal dryness (D0) and short-term moderate drought (D1) across much of Puerto Rico. D0 was maintained for the islands of Vieques and Culebra.

Pacific Islands

The weather conditions across the U.S.-Affiliated Pacific Islands (USAPI) during this USDM week (03/10/21-03/16/21) consisted of dry trade winds across the Marianas and northern portions of the Marshall Islands (RMI) and Federated States of Micronesia (FSM) from Chuuk State eastward; trade-wind convergence and surface troughs within an Inter-Tropical Convergence Zone (ITCZ) over much of Chuuk State to the southern RMI; and a weak circulation and surface troughs over Yap State and Palau. Weak cold fronts/shear lines teased the Marianas and there was some coupling of surface convergence with upper-level divergence over western FSM and Palau. South of the equator, the South Pacific Convergence Zone (SPCZ) brought rain to American Samoa early in the week, then a ridge dried things out as the week progressed.

Satellite-based estimates of 7-day precipitation (QPE) showed two broken bands of precipitation — one from the Philippines eastward across Micronesia and the second stretching southeastward from Indonesia past the Samoan Islands. Areas of 2+ inches of rain, with locally 4+ inches, were evident across the FSM and southern RMI. Less than an inch was indicated in a pocket over Palau, while little to no precipitation was detected by this tool over most of the Marianas and northern RMI. The SPCZ was represented by a northwest-to-southeast broken band of 0.5 to 2.0 inches of rain across the Samoan region.

For Saipan, Rota, and Guam, conditions varied widely while the Mariana Islands were relatively dry overall. Saipan had paltry rainfall ranging from nothing to just 0.03 inches, continuing a dry stretch of weather there. As a result, conditions degraded to short-term moderate drought. On Guam, 1.19 inches of rain fell at the airport, bringing them over their needed weekly rainfall. Reports from other parts of Guam indicate substantially more rainfall fell elsewhere, allowing grasses to green and the fire risk to subside. Thus, Guam has been improved to short-term abnormal dryness. On Rota, 1.26 and 1.28 inches of rain have been recorded the last couple weeks, respectively, and conditions have improved to short-term abnormal dryness there as well.

Normal conditions continued on Palau, where rainfall of 0.99 and 1.12 was recorded this week.

In the Federated States of Micronesia, short-term moderate drought improved to short-term abnormal dryness on Fananu, while no other changes were made to the drought depiction this week. At least 1.61 inches of rain have been recorded on Yap this week, and normal conditions continued there. At least 1.34 inches of rain fell on Ulithi this week, so normal conditions continued there as well. On Woleai, only 0.40 inches of rain has been reported, though a few days of data were missing. Given that over 4 inches of rain fell there last week, normal conditions continued. On Fananu, 4.37 inches of rain fell last week, and at least 2.02 inches have fallen there this week. Given the recent rainfall, water tanks have been refilling, and conditions have been able to improve from short-term moderate drought to short-term abnormal dryness on Fananu. On Chuuk, another wet week occurred with 4.91 inches of rain being recorded, and normal conditions continued. On Lukunor, 2.82 inches of rain were measured, breaking a streak of two consecutive weeks with below weekly minimum rainfall. Thus, drought-free conditions continued. Another drier week took place on Nukuoro, with only 1.07 inches of rainfall. While this is the third consecutive week below the weekly minimum, conditions in January and February were on the wetter side, so abnormal dryness has not developed. On Kapingamarangi, 3.51 inches of rain have helped to fill water tanks, though vegetation is still struggling there, and short- and long-term extreme drought continued for another week. Pohnpei received 7.50 inches of rain this week, so wet conditions continued there. Rainfall of 3.93 inches accumulated this week on Pingelap, so normal conditions continued there. On Kosrae, 8.06 inches of rain followed 6.02 inches last week, so wet conditions continued there.

Short-term moderate drought continued on Kwajalein, where 1.19 inches of rain fell this week. Ailinglapalap received 6.72 inches of rain this week after 8.14 inches fell last week, and wet conditions continued for another week. Jaluit received 1.79 inches of rain this week, and short-term abnormal dryness continued. No depiction was made for Utirik, as the data there were unavailable. Short-term extreme dryness continued with another dry week on Wotje. On Majuro, 2.61 inches of rain fell this week, and normal conditions continued. Mili received 4.96 inches of rain this week, and normal conditions continued there too.

Normal conditions continued for American Samoa this week. Toa Ridge received 0.59 inches of rain, Pago Pago received 1.78 inches, and 2.06 inches fell at Suifaga Ridge.

Virgin Islands

Mostly dry weather continued to envelop the U.S. Virgin Islands this week, aside from an area of rainfall indicated by radar data in northwest St. Croix. As a result, groundwater levels have continued to drop on St. John, St. Thomas, and St. Croix. Short-term precipitation deficits have worsened on St. Thomas and St. John, particularly at the three-month time scale, and severe short-term drought has now developed there. Short- and long-term moderate drought continued on St. Croix.

Looking Ahead

During the next 5 days (March 18 to 22), a low pressure system is forecast to bring widespread precipitation (0.5 to 2 inches, locally more) to the Ohio and Tennessee Valleys and Mid-Atlantic. Farther to the south across the Florida Peninsula, dry weather is likely to persist. Little to no precipitation is also forecast for the Southwest and northern Great Plains. As a mid-level area of low pressure progresses inland from the East Pacific, rain and high-elevation snow are expected to overspread the Pacific Northwest, northern California, and the northern and central Rockies.

The CPC 6-10 day extended range outlook (valid from March 23 to 27) favors above normal precipitation throughout most of the central and eastern U.S. except for the Florida Peninsula, where probabilities for below normal precipitation are slightly elevated. Increased chances of above (below) normal precipitation are forecast for the Southwest (West Coast). Above normal precipitation is favored for Alaska. Above normal temperatures are likely from the Mississippi River to the East Coast, while below normal temperatures are favored for much of the western U.S. and Alaska.

More Regional Reports

The result of the drought is very dry soil conditions

March Drought Outlook

Latest Seasonal Assessment – A La Niña Advisory remains in effect, with a 60% chance that La Niña conditions will transition to ENSO-neutral during the Northern Hemisphere Spring, April-May-June (AMJ). As such, La Niña will likely influence conditions across the U.S. Although La Niña signals typically dissipate during AMJ across the CONUS, it could influence the tropics late in the period. However, its potential effects on the tropics are not considered in this month’s Seasonal Drought Outlook (SDO), given increased uncertainty three months from now related to multiple factors that can influence tropical activity.

During the past month, no major overall expansion of drought coverage was experienced nation-wide. However, varying regional changes are evident. The Pacific Northwest saw reduction in drought coverage due to above-normal seasonal precipitation resulting in above-normal snowpack and near to above normal reservoir levels. Drought removal is likely to continue in south-central Washington and north-central Oregon for these reasons. Although the Central and Southern Rockies saw some improvements, many of those areas remain in extreme (D3) to exceptional (D4) drought, with drought persistence likely. Development is likely in abnormally dry (D0) areas also, as temperatures and precipitation are expected to remain above and below-average, respectively. Dryness across the Northern Plains continued, exacerbated by below-normal winter snowfall totals and above-normal winter temperatures. North Dakota experienced its driest September to February period on record. Extreme cold across the Great Plains during February further exacerbated drought impacts, adversely affecting winter wheat and livestock. However, in the week leading up to the SDO release, a strong winter storm dumped heavy snow and rain across the Central Plains, delaying any further degradation in the short-term, but long-term drought indicators remain very dry and long-range models favor below-normal precipitation and increased chances of above-normal temperatures, hence drought persistence in the Northern Plains, with development likely in the Central Plains. A wetter-than-average AMJ is forecast for much of the Midwest and Northeast, indicating drought removal (D1) is likely for those regions. In the Southern Region, drought persistence and development are favored across Texas and Oklahoma, associated with favored above-normal temperatures and below-normal precipitation during AMJ. Farther east in Louisiana and Alabama, short-term (Week-2) wetness favors drought removal in northern Louisiana and central Alabama (Southeast Region). Conversely, increased chances for below-normal precipitation at the monthly and seasonal time scales indicate drought persistence is likely in southern Louisiana, despite short-term wet signals. Abnormal dryness in the Florida Panhandle is likely to develop into drought during the first two-thirds of the period (April and May), with increased odds of above-normal temperatures (April-June) and below-normal precipitation (April). However, uncertainty increases in June, as Florida enters a climatologically wetter period and the potential for rapid removal of any drought that develops early in the period. Development is more likely across the Florida Peninsula due to recent drying and the likelihood of below normal precipitation during the remainder of March through April.

No development is likely in Alaska, with above-normal precipitation favored over much of the western Mainland, and equal chances for above or below normal precipitation elsewhere. Heavy rain and flooding in recent weeks in Hawaii fell mainly in areas not currently depicted in drought. However, above-normal precipitation is favored for much of the state early in the period, indicating drought removal is likely. Moderate drought (D1) in northwestern Puerto Rico diminished slightly this past week with above-normal precipitation in recent weeks and improved drought indicators. However, with above-normal temperatures and only near-normal precipitation favored during AMJ, drought persistence is forecast.

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.

NASS Reports.

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.(to be added on Friday)

Week 3-4 Forecast Discussion Valid Sat Apr 03 2021-Fri Apr 16 2021

La Nina conditions are present across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) are below average from the west-central to approximately 110 W longitude in the eastern equatorial Pacific Ocean with enhanced upper level westerly wind anomalies across the central and eastern tropical Pacific. The RMM index indicates an increase in the amplitude of the intraseasonal signal over the Western Hemisphere during the last week. Dynamical models continue to favor the eastward propagation of MJO during week-1, but favor an overall decrease in amplitude by week-2. 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 La Nina and long-term trends, as well as the predicted evolution of the pattern from Week-2 forecasts.

Good agreement exists among the dynamical models regarding the anomalous 500-hPa height pattern 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 most of the CONUS. Another broad anomalous 500-hPa ridge is forecast across the North Pacific Ocean.Below normal 500-hPa heights are predicted over most of Mainland Alaska. Near normal 500-hpa height anomalies are likely over Hawaii.

Above normal temperatures are expected across much of the CONUS, under predicted above normal 500-hPa heights, except for the West Coast where near normal temperatures are indicated. The highest confidence for above-normal temperatures lies across the parts of the Central Plains and the Middle Mississippi Valley (greater than 80%) tied to the forecast position of the anomalous 500-hPa ridge axis. Predicted anomalous northerly flow results in increased probabilities of below normal temperatures for southern Mainland Alaska, while decadal trends lead to predicted above normal temperatures for northern Mainland Alaska, consistent with the manual blend tool.

The dynamical model guidance is in reasonably good agreement on the spatial pattern of anomalous precipitation during the Week 3-4 period. Below normal precipitation is favored throughout most of the CONUS, except for California, parts of southwestern Texas and southeastern New Mexico, where above normal precipitation is indicated, consistent with dynamical and statistical guidance. Anomalous troughing and several shortwave disturbances lead to enhanced probabilities of above normal precipitation over Mainland Alaska, supported by most of the dynamical tools.

Above-normal temperatures are favored over Hawaii, shifting to equal chances from west-to-east, given decreasing sea surface temperature anomalies oriented similarly. The SubX MME anomaly forecast indicates likely above median precipitation for the week 3-4 period for Hawaii.

International

First some reports on Individual countries. I show the highlights. The full reports can be obtained here.

Canada

Russia

Ukraine

Uruguay Soybeans

Guyana

This week fortunately we do 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.

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 since last week 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 next week.

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.

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