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更多 发布于:2014-02-24 22:25
An El Niño Coming in 2014?
We are seeing increasing evidence of an upcoming change in the Pacific Ocean base state that favors the development of a moderate-to-strong El Niño event this Spring/Summer. To begin, here is a snap shot of global sea-surface temperature (SST) anomalies (the departure of temperature from average) 30 days ago:


Note the relatively weak look to the SST anomaly pattern in the equatorial Pacific, with warm anomalies in the western half of the basin and mixed warm and cold anomalies in the eastern half of the basin. The mixed warm/cold signals in the equatorial eastern Pacific are the result of instability “easterly” waves in the ocean, which are associated with short distances between “warm” and “cold” phases. Now let’s compare to what the SST anomaly map looks this week:


We’ve observed strong cooling in the eastern half of the Pacific Basin, giving a spatial map that strongly resembles “La Niña”--the opposite of El Niño. So why I am pushing the idea that El Niño might be right around the corner if the map looks like La Niña!?

It comes down to ocean dynamics. There are other types of waves that are deep in the Pacific Ocean. One such wave is called an “Oceanic Kelvin Wave”. Oceanic Kelvin waves travel only from West to East at extremely slow speeds (2-3 m/s). These waves have been alluded to as the facilitators of El Niño. There two phases of an Oceanic Kelvin wave, the “Upwelling” phase and the “Downwelling” phase. The Upwelling phase of an Oceanic Kelvin wave pushes colder water from the sub-surface towards the surface, resulting in cooling at the surface. The Downwelling phase of an Oceanic Kelvin wave is the opposite, where warmer waters at the surface of the West Pacific warm pool are forced to sink, resulting a deepening of the thermocline and net warming in the sub-surface. To try to illustrate this, imagine someone holding a blanket. They rapidly lift the blanket up (Step 2) and then push it back down (Step 3).
它涉及到海洋动力学。这是太平洋深处的另一种波。是称为“海洋开尔文波”的这样一个波。海洋开尔文波只以极慢的速度(2 - 3米/秒) 从西向东行进。作为厄尔尼诺现象的促进者,这些波已经有所暗示。海洋开尔文波有两个阶段,“上涌”阶段和“下沉”阶段。海洋开尔文波的上涌阶段将次表层的冷水翻升到表面,导致表面冷却。海洋开尔文波的下沉阶段是相反的,在西太平洋暖池表面温暖的海域被迫下沉,导致温跃层向下加深和次表层得到加温。为了通俗易懂地说明这一点,设想有人拿着毯子,他们迅速提升毯子(第二步),然后把它向下降(第三步)。


You will get a wave in the blanket, where it travels from the source region (you) towards the opposite direction. This same exact thing happens during Oceanic Kelvin wave events, where the general wave pattern will propagate from west to east. Since Oceanic Kelvin waves travel only from west to the east, you can expect when one phase is located over a region (i.e., today an “Upwelling” phase is in the East Pacific), the opposite phase will soon to follow (i.e., the “Downwelling” phase will be in the East Pacific in 1-2 months). It may be appropriate to illustrate the evolution of the 1997 Super El Niño event to shed some more light on the similarities of the pattern then when compared to now.
你会在毯子中获得一个波,它从源头地区(你方)向对面方向行进。海洋开尔文波活动期间所发生的与这是相同的事情。一般情况下,波的传播模式将是从西向东。因为海洋开尔文波只从西向东行进,你可以预料,当某一地区处于一个阶段(即今天东太平洋处于“上涌”阶段),很快就会跟随着相反的阶段(即1 - 2个月里东太平洋将处于“下沉”阶段)。它可能是对1997年超级厄尔尼诺事件演变的合理解释,然后与现状相比,揭示模式有更多的相似之处。



Three fields are shown above in the panel of time-longitude plots. The left most figure is of the departure of the west-to-east winds (the "zonal wind anomalies') in the lower troposphere, averaged about the Equator. The figure in the middle is essentially a field to monitor the fluctuations of the thermocline in the Pacific Ocean, or can be thought of as a crude way to isolate the depth of the SST gradient. The right most figure is SST anomalies. In February and March of 1997, we observed strong westerly winds on the surface of the western Pacific (known as “westerly wind bursts”). Strong westerly wind bursts in the lower atmosphere can initiate an oceanic Kelvin wave. We observed a number of oceanic Kelvin waves that Spring, both having upwelling and downwelling phases. With each oceanic Kelvin wave event, the warm pool kept “sloshing” around until the final Kelvin wave event, which reconstructed the base state of the thermocline, and the Super El Niño was born beginning in May 1997, and lasting through April 1998. It was the strongest El Niño event ever recorded.

Let’s take a look at what happens in the sub-surface during one of these “Oceanic Kelvin waves”. Here’s an example from 2003. During the onset of an oceanic Kelvin wave, you will see strong sub-surface warming in the western Pacific, and strong cooling near the surface in the Pacific. With time, the warming pushes eastward in the sub-surface, eventually eroding the cold anomalies in the East Pacific surface, which switch to warm anomalies thereafter.



Some Oceanic Kelvin waves are not strong enough to reconstruct the Pacific Ocean base state, and will only result in minor adjustments. One way to see this is by using the current 20°C isotherm depth and anomaly Hovmoeller plots from February 2014 below. The black solid line represents the center of the “Downwelling” phase (or warming in sub-surface phase); the black-dashed line represents the center of the “Upwelling” phase (or cooling at surface phase).



There have been a series of 3 strong Kelvin waves over the past 7 months. Each Kelvin wave pushed some water from the West Pacific Warm Pool towards the East Pacific, but the subsequent upwelling phase resulted in cooling. This week, a very strong “Upwelling” phase of a Kelvin wave is pushing across the Eastern Pacific, forcing colder waters from the subsurface towards the surface, cooling the Eastern Pacific surface waters. This cooling has put the index we use to track El Niño--the so-called "ENSO 3.4 index"--at -0.5°C, right at the border of La Niña conditions. This is happening while warm surface water is being pushed down into the sub-surface, resulting in massive warming below the surface. Here is the evolution of the sub-surface SST structure beginning in January and ending this week:
过去的7个月,曾有3个连续强劲的开尔文波。每个开尔文波把一些水从西太平洋暖池推向东太平洋,但随后的上涌阶段导致了变冷。本周,一个非常强劲的开尔文波 “上涌”阶段推向对面的东太平洋,迫使冷水从次表层涌向表面水域,东太平洋表层水变冷。这种冷却已将我们使用的厄尔尼诺轨迹指数——所谓的“ENSO 3.4指数”——-在-0.5°C,恰好在拉尼娜现象的边界。这是发生在表层的暖水被下推到次表层的时候,致使表层以下深厚的变暖。这儿是次表层SST在1月开始构筑并在本周结束的演变图:



Note that there has been strong cooling in the East Pacific, associated with the strong “Upwelling” phase of the Kelvin wave and strong sub-surface warming in the western-central Pacific associated with its “Downwelling” phase. This strong Oceanic Kelvin wave is the reason why today’s SST anomaly snap-shot map has the appearance of a strong La Niña, but is just the result of processes occurring in the Pacific Ocean at sub-seasonal time scales.
注:东太平洋已获得的强烈变冷与开尔文波强烈的“上涌”阶段相关,西-中太平洋次表层强烈的变暖与其“下沉” 阶段相关联。为什么今天随拍的海温距平图出现强大的拉尼娜现象,这是强劲的海洋开尔文波的原因。但仅在太平洋次表层发生过程上的时间尺度。

The current Kelvin wave in the Pacific Ocean has achieved the same strength as the one that preceded the 1997 Super El Niño event. This is an extremely rare feat but there still has to be a number of things to happen before we can say we are headed towards a strong El Niño. We need to see the continuation of strong westerly winds near the Equator over the Central Pacific to keep the momentum forward.

What caused the development of the strong Oceanic Kelvin wave? A strong westerly wind burst (WWB1) was observed during January 19 - 30 over the equatorial West Pacific:
是什么导致海洋开尔文波的强大发展? 是1月19日- 30日期间,在赤道西太平洋观察到的强劲西风爆发(WWB1):


Note from the time-longitude plot of Madden Julian Oscillation (MJO) filtered velocity potential anomalies at 200 mb (VP200), the convectively active phase of the MJO passed across the Western Pacific in late December through early January. Therefore, WWB1 on January 19 - 30 was likely driven by other features in the atmosphere other than the MJO:
注:马登朱利安振荡(MJO)的 时间-经度图,其200 mb(VP200)上的滤波流势距平,从中可看出MJO的对流活跃阶段在12月底-1月初传送通过西太平洋。因此,1月19日- 30日的强劲西风爆发可能是由大气中除MJO以外的其他因素驱动:



Looking more into the finer details, there was a pair of strong gyres (or “low pressure systems”) mirroring each other about the Equator. One gyre set up shop over the Philippines, and caused major flooding. The gyre south of the Equator set up over northern Australia, and also produced a great deal of precipitation and thunderstorm activity.


Since the flow in the Northern Hemisphere around a low pressure system (or gyre) is counterclockwise, it produces low-level westerly flow to the south of the gyre. Vice versa, flow around a low pressure system in the Southern Hemisphere is clockwise. Thus, to the north of the low, there is also accelerated westerly flow. The net combination of these gyres over the equatorial West Pacific aided in a significant westerly wind burst in late January, as these gyres persisted for nearly 1-2 weeks.
因为北半球的低压系统(或涡流)是逆时针方向,它低层产生的西风流向涡流的南方。同理,南半球低压系统的环流顺时针方向。因此,低压北部的西风也加速流动。随着这些环流持续近1 - 2周的时间,赤道西太平洋的这些环流得到组合,助长了1月下旬重大的西风爆发。

Note that the GFS forecast calls for another amplification of mean westerly flow about the Equator west of the Date Line February 18 - 28 (WWB2). Looking at the 1000mb standardized geopotential height anomaly map, the forecast calls for *two* sets of twin cyclones mirroring each other about the Equator, with one of the Southern Hemisphere gyres forming into a tropical cyclone near the Date Line!
注意:GFS预报称2月18 – 28日日界线以西的赤道附近西风流平均值又一次增强(WWB2)。参看1000 mb标准的位势高度距平地图,预测称赤道附近有* 2 *套双气旋彼此映衬,其中南半球的一个涡流在日期变更线附近形成热带气旋!



The pair of twin cyclones will likely amplify westerly flow in the medium range, and would likely keep the forward momentum of the Oceanic Kelvin wave, providing more evidence in a possible change of the base state in the Pacific. The kicker for a full blown 1997-like Super El Niño to develop would likely be some additional assistance from the development of early-season Pacific tropical cyclones near the Equator, as the GFS model is starting to hint at south of the Equator. Note that the 1997 Super El Niño event had the help from Category 5 Super Typhoon Isa during early April, which developed close enough to the Equator over the Central Pacific to produce another significant westerly wind burst there, and continue to push the West Pacific Warm Pool eastward.
这一对双胞胎气旋可能会增强中程的偏西气流,并将可能维持海洋开尔文波的前进动力,提供太平洋基态可能变化的更多证据。像1997 -超强厄尔尼诺现象充分发展的优势一样,可能从赤道附近季初发展的太平洋热带气旋那里获得一些额外的援助, GFS模型开始暗示赤道以南有气旋形成。注意:1997年超强厄尔尼诺事件是从4月初的5级超级台风Isa那里得到援助,其中接近赤道太平洋中部所产生的另一个重大西风爆发与之有密切联系,并继续推动西太平洋暖池向东前进。


So as it stands now, the ocean has geared in towards another big eastward push of the West Pacific Warm pool towards the East Pacific. We do need to see more westerly winds develop across the Central Pacific to completely swap the base state this Spring. This piece of the puzzle is difficult to predict at such long time-scales, but there are some indications for this to occur at least in the medium-range. Both the CFSv2 and European seasonal model forecasts are extremely aggressive with this transition to El Niño idea, as shown in the time series plots below:




What does this mean for the U.S. this summer? Well, why it’s still a bit early to be certain, typical conditions over the U.S. during strong El Niño’s favor a ridge over the West and a trough over the East. Therefore, you typically see warmer than average summers over the West Coast, and colder than average temperatures over eastern two thirds of the nation, as shown by a simple surface temperature correlation map with the ENSO 3.4 index:
今年夏天,对美国来说意味着什么?好,为什么确定它仍然还有点过早,强厄尔尼诺现象期间,美国的典型的条件是西部一个脊,东部一个槽。因此,你通常看到西海岸夏季比平均温暖,东部三分之二的州比平均温冷,由一幅与ENSO 3.4指数相关的表面温度简图所示



BOTTOM LINE: The Pacific Ocean is now in a state that could reconstruct the base state of the Pacific, favoring an El Niño to develop later this Spring. That being said, it’s not a locked in solution yet as we need to monitor the atmosphere for future westerly wind bursts to help push the Western Pacific Warm Pool along. Thanks go to NOAA for providing the majority of the images used in this post.

[040813云娜于2014-02-24 23:11编辑了帖子]
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