Written by Sig Silber
The article tonight contains a number of regional and subregional status assessments, as well as separate reports on the hazards likely to be encountered in Spring. I don’t have such reports for all the U.S. but there are more reports than those I published. I try to cover all parts of the U.S. but that’s too much for a single article. Some areas send their reports to me directly or indirectly and those tend to be the ones I publish. I think the reader will get a good feel for the variety of situations faced by different parts of the U.S. I have also included a number of NASS reports. As usual, we include a lot of other information including an intermediate-term weather forecast. We again raise the question of the level of preparedness if we have a double-dip La Nina. I have not made much progress on that question but it is important to focus on that possibility.
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| Last Week | This Week |
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Duration of Snowpack
Pacific Northwest. This is a long report and I am only showing a few of the slides. You can access the full report here.
This is a more up-to-date map of snow coverage. It does not auto-update but you can find updates here.
Where Things Stand in terms of Spring Runoff
This is a good place to look at the California Reservoirs
Let’s look at the current drought situation.
And the week to week change
More information can be found here.
Here we look at four-time periods: one week, one month, six months, and a full year.
Drought Discussion (The Regional links are live and will take you to the drought map for that Region)
This Week’s Drought Summary
The current U.S. Drought Monitor period was highlighted by a large swath of heavy rain that started in northeast Texas and progressed northeast into the Mid-Atlantic. In this area, widespread reports of 200-400% of normal precipitation took place, with some areas of Kentucky having widespread 6-8 inch amounts. Dry conditions dominated much of the West and especially the Southwest and into the Plains. Some active weather in the Pacific Northwest and northern Rocky Mountains brought with it rain and snow, helping to boost seasonal snow totals. Temperatures during the week were cooler than normal over the West with departures of 6-9 degrees below normal widespread, while temperatures were above normal from the Plains eastward with departures of 9-12 degrees above normal over much of Alabama.
Northeast
emperatures in the region were 2-4 degrees above normal for the week, with portions of West Virginia 6-8 degrees above normal. Above-normal precipitation was recorded through much of the southern extent of the region, where 200% of normal precipitation was widespread. It was drier through much of New England and western New York, with near-normal precipitation through much of Maine. No changes to the drought status were made this week but several areas are being monitored for possible expansion or introduction of abnormally dry and drought conditions.
Southeast
Drier than normal conditions dominated much of the region, with only a few areas of southern Alabama, southern Georgia and northern Florida recording near normal or slightly above normal precipitation for the week. Temperatures were warmer than normal over the entire region, with the greatest departures (9-12 degrees above normal) in Alabama, southeast Georgia, and eastern South Carolina. Over the last 90 days, the region has continued to dry out. In southern Alabama, abnormally dry and moderate drought conditions were expanded this week and abnormally dry conditions were expanded in northern Alabama and into Georgia. Where significant rains fell over northern Alabama, areas of abnormally dry conditions were removed this week. As the region remains dry and temperatures increase, many areas will need to be monitored for degradation.
Midwest
Temperatures were mainly above normal through the region with the greatest departures over northern Wisconsin and the Ohio River Valley, where temperatures were 6-8 degrees above normal. With the above-normal temperatures and widespread rain from southern Illinois to southern Ohio, much of the remaining snowpack was diminished and the area quickly transitioned into spring-like conditions. It was dry over much of northern Missouri, Iowa, northern Minnesota and southern Wisconsin and from northern Illinois into Michigan. Some of these same areas are still holding on to drought and dryness from last fall and will continue to be monitored as we go into spring. Moderate drought was expanded over northern Minnesota this week. Areas of abnormally dry and moderate drought conditions were improved from central Illinois east into Indiana and Ohio.
High Plains
Dry conditions dominated the region, with only portions of central South Dakota, central and eastern North Dakota, portions of the High Plains in eastern Colorado and central Wyoming recording above-normal precipitation. Temperatures were above normal over most of the area, with below-normal temperatures farther west into Wyoming and Colorado. The greatest departures were in eastern South Dakota where temperatures were 6-9 degrees above normal and northwest Wyoming where temperatures were 12-15 degrees below normal for the week. Improvements were made in north central Wyoming, where areas of extreme and severe drought were showing a good snow season to allow for a reduction in drought intensity. Severe drought was expanded in far northeast North Dakota and into far northwest Minnesota.
South
There was a contrast in temperatures over the region; west Texas and Oklahoma were below normal while areas of southern Mississippi were greater than 10 degrees above normal for the week. Heavy rains fell from northeast Texas through much of southern Arkansas, northern Mississippi, and all of Tennessee this week, with areas from southern Arkansas to western Tennessee recording 400% of normal precipitation. Dry conditions dominate most of Oklahoma and the central, southern, and western portions of Texas. Improvements were made to the moderate drought over northeast Texas and southeast Oklahoma this week. A reassessment was done over southwest Oklahoma, removing the lingering extreme drought there. Exceptional drought was removed over far west Texas as the El Paso area had recorded enough precipitation recently to allow for improvement in intensity. Much of Texas saw degradations with dryness, especially over the last 4 months, dominating the indicators. Coupled with the dryness, the recent cold snap also has impacted winter wheat, with the regional agronomist stating that the drought has probably caused more loss to winter wheat across the region than the recent weather events. Early estimated losses from the recent winter storm are at least $600 million, with $230 million in damages to citrus, $228 million to livestock, and $150 million to vegetable crops in Texas. An area of extreme drought was added in the far western panhandle of Oklahoma, bridging a gap where extreme drought was being depicted in both New Mexico and Colorado.
West
Dry conditions dominated areas from California to New Mexico with just a few pockets of above-normal precipitation over central Colorado, central Wyoming, the Pacific Northwest, and northern Montana. Temperatures were cooler than normal over much of the region with much of the Great Basin and into Wyoming recording temperatures 9-12 degrees below normal. After a good month of precipitation along with the most recent precipitation in the area, many areas of Oregon saw improvements to the drought status, with long-term issues still being monitored. Much of California is enduring its second consecutive dry winter, with most areas below 75% of normal snowpack for this time of year. Many water agencies were discussing water conservation measures, with the North Marin Water District considering both voluntary and mandatory water conservation orders. Moderate drought was expanded over areas of southern California where drought is beginning to develop again after a fairly dry winter. Improvements to abnormally dry, moderate and severe drought conditions were made in Idaho while abnormally dry conditions were improved over western Montana. Eastern Montana conditions continued to deteriorate with an expansion of moderate and severe drought this week. A recent winter storm in and around the Denver metro area and areas to the west allowed for improvements to the extreme drought conditions there as snow totals for the current water year were up over 100%.
Alaska, Hawaii, and Puerto Rico
Recent rains on Molokai allowed for the removal of extreme and improvement to severe drought conditions in Hawaii. Puerto Rico continues to dry out over the short term, with abnormally dry and moderate drought conditions expanded this week and new areas of abnormally dry conditions introduced over the southern and eastern portions of the island as well as the islands of Vieques and Culebra off the eastern coast. In Alaska, areas of abnormally dry conditions were improved south of Fairbanks due to the good SNOTEL reports while abnormally dry conditions were expanded in the northern portions of the state as well as areas northeast of Fairbanks.
Pacific Islands
Dry season trade winds dominated the weather conditions across Micronesia during this USDM week (02/24/21-03/02/21). A near-equatorial surface trough persisted throughout the week south of Palau and over southern portions of Yap State, while small and weak surface troughs, trade-wind disturbances, and convergent trade-wind flow brought showers at times across parts of the Federated States of Micronesia (FSM) and Marshall Islands (RMI), and a shear line and weak trough teased the Marianas. South of the equator, the week began with the South Pacific Convergence Zone (SPCZ) bringing showers to the Samoan Islands, but a high pressure ridge quickly pushed the SPCZ south and west of the area and brought more stable conditions.
Satellite-based estimates of 7-day precipitation (QPE) showed a continuous band of precipitation (the SPCZ) extending southeast from Indonesia toward and past the Samoan Islands. The QPE indicated 0.5 to 2.0 inches of rain across American Samoa, with 4+ inches in a large band south of the Samoan Islands. There were patches of rain over Micronesia, but the Inter-Tropical Convergence Zone (ITCZ) was mostly absent. A 1-inch band of precipitation extended from Palau southeastward across Yap State, with embedded areas of 2+ inches, associated with the near-equatorial surface trough which persisted in the area throughout the week. Patches of up to half an inch were indicated across southern parts of Chuuk and Pohnpei States, and over parts of the RMI. Otherwise, little to no rainfall was detected by the QPE across the rest of Micronesia.
This was a dry week in the Marianas. With 0.04 inch of rain this week at Guam, 1.01 inches for February, and 5.12 inches for January-February, the USDM status at Guam was worsened from D0-S to D1-S. February 2021 ranked as the third driest February with a percentile of 0.05 and January-February was 8th driest (percentile 0.13). The percentiles equate to D3 for February and D1 for January-February. Reports have been received of grasses browning and regular swordgrass (sakati) fires developing across southern Guam.
Rota reported 0.16 inch this week, 0.97 inch for February, and 3.07 inches for January-February. These rank as the second driest February and driest January-February, with percentiles of 0.08 and 0.04, respectively. These are in the D3-D2 ranges; since this is the dry season, D1-S continues at Rota.
Saipan has been a little wetter, with 0.32 inch at the IAP for the week, 3.15 inches for February, and 5.45 inches for January-February. Saipan ASOS recorded 0.16 inch for the week, 2.38 inches for February, and 3.92 inches for January-February. The values at Saipan NPS station were 0.12, 1.63, and 2.73, respectively. With the last 2 months dry (each below the 4-inch monthly minimum), D0-S continues at Saipan.
Weekly rainfall totals in the FSM ranged from 0.03 inch at Woleai, 0.08 inch at Fananu, and 0.14 inch at Yap (Yap had 4 days missing this week), to 1.37 inches at Lukonor, 1.39 inches at Kapingamarangi, 1.43 inches at Ulithi, and 2.24 inches at Pohnpei. D1-S continues at Fananu where February had 1.57 inches and ranked as the driest February in the 7-year record (percentile 0.14 or in the D1 range). The National Weather Service has received reports of very low water tank levels on Fananu and all of the Hall Islands. Some crops are beginning to yellow. Nomwin island (14 miles SW of Fananu) reported one drinking water tank left. Murilo (east of Fananu) reported dry taro patches. Similar conditions are possible for the Namanuitos to the west of Fananu/Nomwin such as Onoun.
Kapingamarangi has been getting rain in recent weeks, but the monthly totals are still below the 8-inch monthly minimum needed to meet most water needs. February 2021 (with 5.86 inches) ranked 12th driest (35th percentile), January-February (11.78 inches) ranked 6th driest (20th percentile), December-February (15.55 inches) ranked 4th driest (15th percentile), and November-February (18.38 inches) ranked third driest (12th percentile). These recent percentiles are in the D1 range. But longer time scales are driest on record, including March-February (6th percentile). This long-term dryness and lingering agricultural impacts resulted in D3-SL continuing.
The rest of the stations continued at D-Nothing status, but several have been dry for the last 2 weeks and for February. Pingelap has been dry the last 2 weeks and dry for February, but the previous week was wet. Ulithi was dry for February and this week but wet the week before. Kosrae has been dry the last 2 weeks, but wet for February. These stations will be watched for possible D0-S if the dryness continues.
In the RMI, no analysis was made for Utirik due to lack of data, but the pattern suggests it has been dry there. Rainfall totals for the week included a trace at Wotje, 0.03 inch at Jaluit, 0.40 inch at Kwajalein, 0.46 inch at Ailinglaplap, 0.61 inch at Mili, and 1.02 inches at Majuro. February totals range from 0 inches at Wotje and 1.97 inches at Kwajalein to 4.40 inches at Ailinglaplap and 4.88 inches at Mili. These are all below the 8-inch monthly minimum needed to meet most water needs.
Wotje has been dry for the last several weeks, has reported no measurable rain for February, 1.82 inches for January-February, and 5.64 inches for December-February. Two other years (1990 and 1994) had zero rainfall for February, which ranks February 2021 as a tie for driest on record (8th percentile, or tied at 3 out of 38 years). January-February 2021 ranks 5th driest on record (13th percentile), and December-February ranks 9th driest (26th percentile). The National Weather Service has received reports of empty water catchments at the islands in the Wotje atoll with only one water catchment on the main island of Wotje (at the Church in the community) having water usable for drinking. Groundwater has been used for all other purposes. Vegetation on Wotje has turned yellow. Based on precipitation percentiles, D2-S continues at Wotje.
It has been dry at Kwajalein, with 1.97 inches for February, 4.31 inches for January-February, and 8.33 inches for December-February. The ranks (and percentiles) are 28th driest (41st percentile), 17th driest (25th percentile), and 8th driest (12th percentile), respectively. Kwajalein has a history of rainfall totals during the dry season being below the monthly minimum needed to meet most water needs. D2-S continues at Kwajalein.
Jaluit has been dry (below the 2-inch weekly minimum) for the last 7 consecutive weeks and below the monthly minimum for the last 2 months. The February rainfall total of 2.24 inches ranks 6th driest (16th percentile) and January-February total of 7.92 inches ranks 4th driest (11th percentile). The National Weather Service has received reports of empty water catchments on Jaluit and yellowing of some vegetation. Based on the precipitation percentiles and impacts reports, the USDM status at Jaluit was worsened from D0-S to D1-S.
Majuro has been dry for three of the last 4 weeks, February was dry (3.89 inches), and the reservoir has been below the level of concern for the last several weeks, generally decreasing throughout February and ending the month at 23.862 million gallons, which is 66% of maximum. Majuro was worsened to D0-S this week.
Ailinglaplap and Mili were both dry this week and last week, and February was dry, but it was wet the week before. D-Nothing continues for now, but they will be watched for possible D0-S if the dryness continues.
The Palau IAP and Koror COOP/ROR stations were wet this week with 2.59 and 2.45 inches, respectively. The American Samoan stations were wet or nearly wet with weekly totals of 1.19, 0.97, and 1.06 inches, respectively, at Pago Pago and the automated stations at Siufaga Ridge and Toa Ridge. All of these stations were wet for February and in previous weeks, so D-Nothing continues in Palau and Tutuila.
Virgin Islands
The weather conditions across the U.S. Virgin Islands (USVI) during this USDM week (02/24/21-03/02/21) were dominated by dry air and high pressure ridging in the mid and upper levels of the atmosphere. Patches of moisture moving in the easterly trade winds brought scattered showers, but the dry stable conditions aloft limited rainfall totals.
Radar-based estimates of rainfall (QPE) for the 7 days ending at 12z on Tuesday showed locally up to half an inch of rain across parts of St. Thomas and St. John islands, and southwest parts of St. Croix. Other parts of the islands had an estimated less than a fourth of an inch, and eastern parts of St. Croix had little to no precipitation based on this indicator.
Weekly rainfall totals on St. Croix ranged from 0.11 inch at the CoCoRaHS station at Christiansted 4.1 ESE, 0.20 inch at Christiansted 1.8 ESE, and 0.21 inch at Frederiksted 2.3 N to 0.32 inch at Frederiksted 2.3 SSE, 0.39 inch at Frederiksted 0.5 N, and 0.45 inch at Christiansted 1.6 E. East Hill recorded 0.27 inch and Rohlsen AP reported 0.37 inch. February monthly totals ranged from 0.79 inch at Frederiksted 2.3 SSE to 1.29 inches at Christiansted 1.6 E. East Hill had 0.82 inch, which was 58% of normal, and Rohlsen AP had 0.88 inch, which was 53% of normal. February 2021 was the 13th driest February in the 61-year record at Rohlsen AP, January-February the 24th driest, and December-February the 18th driest. The Standardized Precipitation Index (SPI) at Rohlsen AP was at D1 levels for the last 6 months and D0 at the 9- and 12-month levels. The USGS Adventure 28 well has steadily declined from January 1 to February 12, then rose, but has been dropping in the last 12 days. The USDM status at St. Croix was changed from D1-S to D1-SL to reflect the longer-term moisture deficits.
Weekly rainfall totals on St. Thomas ranged from 0.13 inch at the CoCoRaHS station at Charlotte Amalie 5.1 E and 0.27 inch at Anna’s Retreat 2.5 ESE to 0.40 inch at Charlotte Amalie West 4.2 WNW, 0.44 inch at King AP, and 0.56 inch at Charlotte Amalie 1.4 NE. February totals ranged from 1.13 inches at King AP (which was 79% of normal), 1.35 inches at Charlotte Amalie 5.1 E, and 1.40 inches at Anna’s Retreat 2.5 ESE, to 2.84 inches at Charlotte Amalie West 4.2 WNW and 2.90 inches at Charlotte Amalie 1.4 NE. At King AP, February 2021 was the 21st driest February in the 56-year record, January-February was 8th driest, and December-February ranked 7th driest. The SPI at King AP was slightly negative at the 1-month time scale but -1.73, or in D3 range, for the last 3 months. The USGS Grade School 3 well has been in general decline during the last 2 months and steadily declined in the last 2 weeks. D1-S continues at St. Thomas.
On St. John, weekly rainfall totals ranged from 0.22 inch at Windswept Beach to 0.41 inch at Cruz Bay 0.8 NE and 0.62 inch at Cruz Bay 1.6 E. February monthly totals included 2.01 inches at Cruz Bay 1.6 E and 2.05 inches at Cruz Bay 0.8 NE, but 1.42 inches at Windswept Beach. The USGS Susannaberg DPW 3 well has been steadily declining since January 1. D1-S continues on St. John.
Looking Ahead
Over the next 5-7 days, it is anticipated that precipitation totals will be greatest along the West Coast from central California into the Pacific Northwest. The Plains and Midwest as well as the Rocky Mountains may see a more active pattern, with the greatest amounts of precipitation expected over the northern Plains and Upper Midwest and portions of the central Plains. Dry conditions will dominate the Mid-Atlantic and into the Tennessee River Valley while the Florida peninsula may have several opportunities for precipitation.
The 6-10 day outlooks show the greatest chances of above-normal precipitation centered on the Midwest, with much of the country showing above-normal chances of above-normal precipitation. Coastal areas of the Southeast and into the peninsula of Florida are showing the greatest chances of below-normal precipitation. There are above-normal chances of above-normal temperatures for most areas east of the Rocky Mountains, with the greatest chances in the Midwest. Above-normal chances of below-normal temperatures are expected over much of the West, with coastal areas having the greatest chances.
More Regional Reports
The result of the drought is very dry soil conditions
March 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.
NASS Reports.
There are a number of them. I am covering three of them. You can find all of them here.
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
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And then Precipitation
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The Week 3-4 Discussion is always interesting.(to be added on Friday)
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.
International
This week unfortunately we do 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.
Major Sources of Information Used in this Weekly Report
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