Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: Earth knowledge Bureau (ID:diqiuzhishiju).

We have talked about the relationship between El Nino and La Nina and the abnormal climate in China, and their atmospheric effects, collectively known as the ENSO cycle, can affect whether China is flooded in the south and droughts in the north or in the south from 20000 kilometers away.

After three years of La Nina, according to the National Climate Center, this year is likely to be the year of El Ni ñ o.

The average Nino3.4 index from January to March this year is-0.4 ℃.

Higher than-0.5 ℃ La Nina threshold

So triple La Nina ends ▼ by the spring of 2023

If we compare October last year, January and April this year,

You will find that the equatorial eastern Pacific is indeed shifting from La Nina to El Ni ñ o ▼.

As a very large model, the transition of ENSO from one state to another is very complex, and the current transition phase is an excellent case where we can realize the details and complexity. Today we will talk in detail about this year's El Ni ñ o.

The impact of El Ni ñ o on China is well known. "El Ni ñ o" means that the surface water temperature of the equatorial Middle East Pacific Ocean is continuous and significantly warmer, while "La Nina" is continuous and significantly cold.

The sea water in the eastern Pacific Ocean is significantly warmer than the annual average, that is, El Nino phenomenon.

(figure: NOAA Climate.gov) ▼

The temperature of the sea water in the eastern Pacific Ocean is significantly lower than the annual average, that is, La Nina phenomenon.

(figure: NOAA Climate.gov) ▼

Whether it is warm or cold requires a benchmark, that is, the "annual average", usually using data from 1981-2010 or 1991-2020. The average sea surface temperature in the range of 5 °S-5 °N, 170 °W-120 °W is selected as the standard for detecting cold and warm, which is also known as the Nino3.4 area, which is very important and will be mentioned later.

Location of Nino3.4 area indicates ▼

The so-called "persistent warmth" is often maintained for 6-24 months.

For example, El Ni ñ o grows in the northern hemisphere in the summer of the first year, reaches its peak in winter, and then decays, ending in the spring and summer of the second year, but it is also possible to last for 2-3 years.

If you look at the situation this year, in early May, the World Meteorological Organization said that La Nina had ended after three years, and the tropical Pacific was in a neutral state. It is expected that the possibility of ENSO changing from neutral to El Nino will continue to increase in May-August, from 60% to 80%, which is a high probability event.

As soon as La Nina left, El Nino came non-stop.

(figure: WMO) ▼

La Nina is retreating.

(sea surface temperature in the tropical Pacific from early January 2023 to early March 2023)

(figure: climate.gov) ▼

In the "neutral state", the prevailing trade winds on both sides of the equator will blow the surface warm water from the eastern Pacific to the western Pacific, thus forming a warm pool in the west and a cold tongue in the east. in order to compensate for the lack of surface water in the east, the cold water in the depths will be replenished upward, thus forming a situation of warm in the west and cold in the east.

This cold and warm situation leads to the rise of the atmosphere in the western Pacific and the sinking of the atmosphere in the eastern Pacific, forming a circulation of ocean-atmosphere interaction.

Circulation model in neutral state

(figure: UNEP) ▼

But if, for some reason, the trade wind, the driving force, suddenly and continuously weakens and slows down, it will cause less warm water to be transported westward, the warm water will be concentrated in the Middle East and Pacific Ocean, and the whole range of circulation will shrink eastward, causing the trade wind to weaken further and form positive feedback, resulting in the formation of El Ni ñ o.

Circulation model under El Nino

(figure: UNEP) ▼

As the most significant SST anomaly signal in the world, El Nino will gradually affect the tropical sea area and global climate within several years, and the impact on different regions may have a long time difference.

The Philippines, Indonesia and Peru are among the regions most affected by El Ni ñ o. China is a little farther away, but El Ni ñ o will have a profound impact on the East Asian monsoon, thus changing the temperature of sunshine and rain in China.

Precipitation Law ▼ under the influence of horizontal screen and El Nino

For example, in the summer when El Nino develops and grows, the rain bands of the West Pacific subtropical high and the summer monsoon will be relatively southward, and the northeast vortex will be more active, which leads to more rainfall in the south and northeast of China, while less rainfall in North China and Central Plains.

Summer-type El Nino often represents more precipitation in South China and Northeast China.

Less precipitation in North and Central China (▼)

Summer temperature anomaly ▼ during El Nino development period

In the peak period of El Nino in winter, it usually leads to the weakness of the East Asian trough and the cold air in the northwest, and warmer in most parts of China except the Qinghai-Tibet Plateau. However, there is little correlation between El Nino and warm winter, so it can not be completely said that "El Ni ñ o year must be a warm winter".

Winter temperature anomaly ▼ during the peak of El Nino event

By the following summer, El Ni ñ o had gradually declined, but at this time the impact on China was the fiercest. Although the tropical Pacific was not so hot, the tropical Indian Ocean was affected by it, and the water temperature was still relatively high.

Judging from the map, the tropical Indian Ocean is actually closer to southern China than the tropical Pacific Ocean. The high water temperature directly leads to more evaporation and more water vapor transported to China, and at the same time, it will stimulate an unusually stable and southward subtropical high through several paths, which will continuously guide the water vapor to the Yangtze River basin. in the Sichuan Basin to the Yangtze River line to form a long-term precipitation, and even cause floods.

The El Ni ñ o at the end of its strength should not be underestimated.

(horizontal screen, reference: Zhang Renhe, 2017) ▼

The decay period of El Nino events in summer

Percentage of land precipitation anomaly ▼ in China and its surrounding areas

Since the founding of the people's Republic of China, the three largest floods in the Yangtze River basin: the severe flood in 1954, the severe flood in 1998 and the severe flood in 2020, all occurred in the summer of the following year, which really cannot be prevented.

The particularity of El Nino this year, but we don't have to worry too much about it. El Ni ñ o is different every time, and this time, we will find that it is not developing so smoothly.

This is the global sea surface "anomaly temperature" in mid-May, yellow and red is warm and blue is cold.

At a glance, it is obviously warmer in the north and east of the Pacific Ocean.

(figure: NOAA) ▼

If you zoom in, this area (green box below) is the "equatorial Middle East Pacific". The coast of South America in the eastern Pacific has turned yellow and looks like a proper El Ni ñ o, but more importantly, the warming trend in this area (orange box), that is, Nino3.4, is not obvious, indicating that El Ni ñ o has not yet developed and has encountered "some kind of blocking" in the Central Pacific Ocean.

This year's Nino3.4 district seems to be a little far away from El Ni ñ o, ▼.

This central Pacific Ocean is very important and is the key battlefield for the full development of El Ni ñ o.

This picture shows a vertical profile of the sea surface temperature in the equatorial Pacific Ocean in mid-May, with an east-west span equivalent to that from Cologne to Solomon Islands.

The depth-longitude profile of the current SST anomaly in the equatorial Pacific is shown above.

The next is the average temperature distance.

(figure: NOAA) ▼

Part of the warm water layer of the western Pacific has been transferred to the east, and the significant deepening of the thermocline directly prevents the compensation of the cold water below and accelerates the warming of the eastern Pacific. The temperature is already 3-5 ℃ higher than the average.

But at the same time, we will find that the temperature line around 160 °W suddenly becomes steeper, which means that the smaller the temperature difference between the east and the west, the stronger the El Ni ñ o and the weaker the El Ni ñ o, so the Central Pacific Ocean is obviously not warm enough, indicating that the main body of warm water has not moved further eastward, which directly hinders the further development of the El Ni ñ o event.

Around 160 °W, the eastward movement of the warm water layer seems to be underpowered.

(figure: NOAA) ▼

The main reason behind this is still the driving force-the trade wind, which weakens will lead to warming in the eastern Pacific, but the trade wind in the central Pacific is unexpectedly strengthened, and the wind stress is transmitted to the thermocline, hindering the process of the transfer of the warm water layer to the east. it is equivalent to that the workers in the east have already gone on strike, and the workers in the middle are still working overtime to send warmth to the west.

Even the trade wind is rolling up, and the main thing is to send warm to the east and to the west.

(15:00, May 15, near the ground) ▼

Why did this happen? We look north to the west coast of North America, where the equatorial eastern Pacific is warming, but the west coast of North America is getting colder and extends westward to the central equatorial Pacific.

On the one hand, it cooled down here, on the other hand, it formed a local high pressure. Originally, the trade wind here was going to strike, but it was also spurred by this North American factor, and began to clock in to send warmth to work. Sure enough, all the responsibility lay with North America.

This North American factor is ▼, which is somewhat anti-bone in the body.

Therefore, judging from the ocean subsurface thermal and dynamic point of view, although the equatorial eastern Pacific Ocean warms in May, it has little impact on the Central Pacific Ocean; from June to July, due to the hindrance of the Central Pacific Ocean, the development speed of El Nino events may slow down significantly.

After all, in order to grow into a complete El Ni ñ o, in addition to the high surface water temperature, it also requires two important signs: "trade wind significantly weakened", "thermocline and Walker Ring flow eastward shift", which are not available yet. so this El Ni ñ o should not be so smooth-although it will form in the summer, it should develop more slowly than expected.

Based on the prediction of the NMME climate model

The Nino3.4 SST is likely to climb above the El Ni ñ o threshold in a few months.

The specific situation remains to be observed.

(figure: NOAA) ▼

Is this going to be the hottest year? If we summarize and compare the El Nino events in the last 50 years, we will find that there are two types of El Nino events in spring and summer. The former is fully developed in spring, while the latter is much later. This year, it obviously belongs to the type of late development and eastward location.

(a) 1997, (b) summer developmental El Nino events,

(C) 2015 and (d) March-April of this year (2023)

Outward radiation of long wave (color filling) and 200hPa velocity potential (isoline)

Contrast ▼ with 850hPa wind field (vector) anomaly

For this late-to-east El Ni ñ o, it is easy to cause the "West Pacific Subtropical High" to be weaker in the east. under its influence, there will be a trend of "flood in the south and drought in the north" in the early part of this summer, with more and longer precipitation in the south and a high temperature heat wave in the north. But in the later stage, the rain belt will still extend northward to North China and the south of Northeast China.

Influence model of eastern and central El Nino events

Sea surface temperature anomalies of early and late types of El Nino development (color filling)

And 700hPa wind field anomaly (vector)

Average comparison of ▼ between June-September and June-September

However, the El Nino event is only one of the factors that affect the meteorology of our country. If we want to make a comprehensive analysis, we have to consider a lot of problems, such as polar sea ice, sea temperature in middle and high latitudes, snow cover on the Qinghai-Tibet Plateau, and so on.

Meteorological changes are numerous and complicated, which requires a comprehensive analysis of multiple factors.

(figure: nsidc) ▼

Citing the World Meteorological Organization report, many media said that, driven by El Ni ñ o, 2023 may be the hottest year on record. Is that true?

First of all, it should be pointed out that within a few years, the impact of El Nino-La Nina on the global average temperature is about 0.1-0.2 ℃, but in more than a hundred years, the global temperature actually shows a "two steps forward and one step backward", constantly superimposed warming pace, standing on the basis of the previous generation's high temperature, if this El Nino grows stronger, it is indeed possible that one generation will be hotter than the next generation. Refresh the highest average temperature since 1880.

Monthly change chart of global average temperature since 1950

It can be seen that the El Ni ñ o event does contribute to a short-term rise in global average temperature.

(figure: NOAA) ▼

But specific to different regions, different periods of time, the difference is still quite big.

In China's Yangtze River Basin, for example, the probability of recreating the extremely high temperature during last year's Sanfeng La Nina is extremely low, but the focus of this year's south is flood control. North China and Northwest China need to guard against high temperature heat waves. And these predictions will have to be combined with Pacific observations a few months later to be more accurate.

Last year Sanfeng La Nina caused the sea surface temperature in the western Pacific to be on the high side.

The high temperature and drought brought to the Yangtze River area is still fresh in many people's memory.

(figure: NASA) ▼

Today, although we have a better understanding of the ocean-atmosphere interaction related to ENSO, ENSO is only a sub-item in the face of the wider sea and sky, and other weather systems are equally important, they bite and interact with each other, forming the underlying geographical logic of our human living environment.

Reference:

Zhang Renhe, Min Qingye, Su Jingzhi. 2017. Effects of El Nino on the interannual variability of atmospheric circulation in East Asia and precipitation in China: the role of abnormal anticyclones in the northwest Pacific. Chinese Science: Geoscience, 47VUR 544-553, doi: 10.1360 / N072016-00268

Yuan Yuan, Yang Hui, Li Chongyin. 2012. El Ni ñ o events with different Distribution patterns and their effects on the following Summer in China

The possible influence of seasonal precipitation. Journal of Meteorology, 70: 467-478

Li Chunhui, Li Xia, Liu Yan, et al. 2016. Effects of spring and summer explosive ENSO events on summer precipitation in China [J]. Climate and Environmental Studies, 21 (3): 258 − 268. Li Chunhui, Li Xia, Liu Yan, et al. 2016. Impact of spring and summer onset type ENSO on summer precipitation in China [J]. Climatic and Environmental Research (in Chinese), 21

(3) 258 − 268, doi: 10.3878/j.issn.1006-9585.2015.15133

Wen, N., Liu, Z., & Li, L. (2019) Direct ENSO impact on East Asian summer precipitation in the developing summer. Climate Dynamics, 52, 6799-6815

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