Form and variability of northerly low-level jets
The climatology of nighttime winds (850-hPa) (Fig. 1) shows that easterly winds in the equatorial Atlantic veer to northerly directions as they reach South America, partially due to the blocking effect of the Andes Mountains. However, other physical processes and dynamical mechanisms may explain the jet’s variability on multiple scales, and those will depend on the jet’s strength and spatiotemporal characteristics21,26. To identify and categorize nighttime occurrences of northerly LLJ, we developed an algorithm that incorporates the jet structure, magnitude and spatial extensions along the eastern Andes (Supplementary Fig. 1). The method considers contiguous regions of wind speeds (CRWS) at 850-hPa greater than 10 m s−1, the magnitudes of wind speeds (850-hPa) and vertical wind shear (700–850 hPa). Using this algorithm, four main types of SALLJ are defined. In the Central type, the LLJ extends only over southeast Peru, Bolivia, northern Argentina and Paraguay (Fig. 1, regions 1,2,3). Note that in any given Central type day, the LLJ may not necessarily extend over all three regions simultaneously. Instead, the jet may extend over different combinations of regions; the algorithm identifies these situations (see Methods). The second type is defined as the Northern LLJ, which extends over northern Peru, Colombia and Venezuela (Fig.1, regions 3,4,5). The third type, denoted as Andes, is defined when low-level jets occur simultaneously in the northern and central branches. Note also that in the Andes type, the low-level jet may or may not extend along the entire eastern Andes. In the most frequent Andes type cases, the LLJ occurs simultaneously in the northern and central Andes but is absent over eastern Peru. In a subtype of the Andes type, denoted Andes-X, the LLJ is substantially intense and cover the entire extension of the eastern Andes. The fourth type, denoted as Peru, occurs much less frequently, is short-lived and covers only eastern Peru (Fig. 1, region 3). The Peru type is classified as a separate type to avoid the arbitrary decision of including it as a Central or Northern type, which would bias statistical analyses discussed next.
Fig. 1: Nighttime climatology of winds at 850-hPa during 1 November-31 March, 1979-2021.
Nighttime corresponds to 23 UTC- 09 UTC (19:00-05:00 Bolivia local time). Regions 1-5 indicate the mask used to identify LLJ days.
Figure 2 shows composites of winds at 850-hPa during Central, Northern, Andes types, and the Andes-X subtype; Supplementary Fig. 2 shows the Peru type.
Fig. 2: Northerly low-level jet types.
Composites of winds at 850-hPa during (a) Central, (b) Northern, (c) Andes types and (d) Andes-X LLJ sub-type.
To illustrate differences among the LLJ types, Fig. 3 shows the spatiotemporal evolutions of two LLJ events. In the first example (Fig. 3a), a Central type has a well-defined CRWS over southeastern Peru and Bolivia. Grid points satisfying low-level jet definitions (see Methods) are only present on the eastern part of the CRWS. In the next day (Fig. 3b), the LLJ intensified and expanded considerably, forming an Andes type with spatially disconnected low-level jets in the northern and central Andes. The LLJ subsequently ceased in the central region and remained present in eastern Peru, Colombia and Venezuela (Fig. 3c). In a contrasting evolution (Fig. 3d–f), the LLJ started as a Northern type and evolved into Andes-X several days later. Subsequently, the LLJ evolved to a Central type. The LLJ was particularly active during December 2016.
Fig. 3: Examples of LLJ transitions.
Top row shows transitions from (a) Central to (b) Andes to (c) Northern types. Bottom row shows transitions from (d) Northern to (e) Andes-X to (f) Central type. Thin black lines show contiguous regions with wind speeds equal or greater than 10 m s−1. Hatching shows grid points satisfying both wind speed and vertical wind shear criteria within contiguous regions with wind speeds equal or greater than 10 m s−1. Blue “X” shows the core of the LLJ: location of maximum wind speed (850-hPa) within the LLJ.
These examples suggest that the LLJ may exhibit distinct evolutions along the eastern Andes, i.e., the LLJ may transition differently from one type to another and, interestingly, may evolve from south to north or vice versa. To investigate this problem, conditional probabilities are calculated to express the likelihood of initiation, transition between types and termination of the LLJ (Table 1). The 1st row shows the likelihood of LLJ initiation in a given type. We note that the LLJ is more likely to initiate as Central or Northern types (1st row, 0.295*, 0.117#, respectively) rather than Peru or Andes types. If initiated as Central type, it is more likely to persist in the same type (0.554), transition to Andes type (2nd row, 0.200*) or terminate (0.181). If an Andes type, it may stay in the same type (5th row, 0.611), transition back to Central (0.148) or change to Northern type (0.181*). Likewise, if initiated as Northern type (1st row, 0.117#), it is more likely to transition to Andes type (3rd row, 0.277#) and subsequently to Central type (5th row, 0.148#). Table 1 also shows the likelihood of the LLJ to persist in the same type (main diagonal) or to terminate (1st column, rows 2-5). The results support the hypothesis that the LLJ may follow two more likely pathways: No-LLJ ⇒ Central ⇒ Andes ⇒ Northern or No-LLJ ⇒ Northern ⇒ Andes ⇒ Central, indicating south-to-north or north-to-south evolutions.
Table 1 Transition probabilities between different LLJ types: Central, Northern, Peru and Andes.Large-scale controls of northerly low-level jets
While boundary layer processes are important local mechanisms generating nocturnal low-level jets27, large-scale processes may control the variability of the LLJ. Figure 4 shows composites of meridional winds (850-hPa, V850), zonal winds (200-hPa, U200) and geopotential height (200-hPa, Z200) anomalies during Central, Northern and Andes types. In all LLJ types, V850 anomalies show wave train activity originating in the South Pacific. In the Central type (Fig. 4a), as expected, negative V850 anomalies extend from the central Andes towards the South Atlantic, whereas positive V850 anomalies are observed over the northern Andes. An opposite pattern is observed during Northern type (Fig. 4c), where cross equatorial negative V850 anomalies favor the formation of the LLJ in the northern Andes and positive V850 anomalies inhibit the formation of Central type. During Andes type (Fig. 4e), negative V850 anomalies originate in the North Atlantic, cross the equator and flow along the eastern Andes towards the South Atlantic, thus favoring the simultaneous LLJ formation in the northern and central branches of the Andes. In the upper levels, all LLJ types also show wave train activity originating in the South Pacific. In the Central type (Fig. 4b), negative and positive Z200 anomalies over southern South America straddle positive U200 anomalies, indicating an enhancement of the subtropical jet. We note also that the pattern of Z200 anomalies suggests equatorial forcing over Indonesia. In the Northern type (Fig. 4d), similar wave train activity is present, but the important feature to note is that the phase of Z200 anomalies arriving in southern South America is opposite of the Central type. A somewhat different wave train activity pattern is observed during Andes type (Fig. 4f), where Z200 anomalies propagate more zonally than in the Central and Northern types, and positive Z200 anomalies are located over the South Atlantic rather than over the continent.
Fig. 4: Composites of LLJ Central (top row), Northern (middle row) and Andes (bottom row) types.
Left column: meridional wind (850-hPa) anomalies; Right column: zonal wind (200-hPa) (colors) and geopotential height (200-hPa, Z200) (contours) anomalies. Positive (negative) Z200 anomalies are shown in solid (dashed) contours (5 m interval); zero contour omitted. Anomalies are statistically significant at 5% level.
Figure 5 shows the annual distributions of Central, Northern and Andes types as functions of El Niño, neutral and La Niña years. Figure 5a suggests that El Niño favors the occurrence of the Central type; this is especially the case during strong El Niño years (i.e., 82–83, 86–87, 97–98, 09–10, 15–15). In contrast, La Niña years appears to favor the occurrence of the Northern and Andes types (Fig. 5b, c). Supplementary Table 1 shows the percentages of LLJ types with ENSO phases. El Niño and La Niña years increase the frequencies of Central and Northern types, respectively. However, the difference between El Niño and La Niña is statistically significant (10% level) only for the Central type. The results above motivate the investigation of these two questions: 1) Is the wave train activity (Fig. 4) higher during La Niña or El Niño years? 2) Does ENSO modulate the persistence of LLJ types?
Fig. 5: Interannual variability of LLJ types.
Annual frequency distributions of LLJ types: a Central, b Northern and c Andes types. El Niño, neutral and La Niña years are indicated by red, gray and blue bars, respectively. Period: 1 November-31 March.
To investigate the first question, we calculate an empirical orthogonal function (EOF) analysis of Z200 anomalies during all days of Central type. To emphasize the southern hemisphere wave train, the domain is bounded between 10°S–80°S. The first two EOF modes are shown in Supplementary Fig. 3. They represent 10% and 8% of the total variance of Z200 anomalies. If the EOF analysis is done using the Northern type days, the EOFs are essentially the same but with opposite signs. We project all days of Z200 anomalies onto the first two EOF modes resulting in two time series PC1 and PC2, and calculate ({PC}=sqrt{{{PC}1}^{2}+{{PC}2}^{2},}). Next, we calculate the percentage of days during La Niña, neutral and El Niño years in which PC is greater than the 75th percentile of the frequency distribution. The results indicate that wave train activity is higher during La Niña (40.4%) than in El Niño (30%) years.
We calculate persistence as the number of consecutive days in each LLJ type (1 day minimum duration). We then separate the persistences for each LLJ type according to El Niño, neutral and La Niña years and compute interquartile ranges, means, 90th, 95th percentiles and maximum durations (Fig. 6). ENSO has a distinct influence on the distribution of LLJ persistences. La Niña years favor long persistences of the Northern type, whereas El Niño years favor long persistences of the Central, which is particularly noted in the 90th and 95th percentiles and maxima persistences.
Fig. 6: Persistence of LLJ types and ENSO phases.
Distributions of persistence of (a) Northern and (b) Central types. Distributions are calculated separately during La Niña, Neutral and El Niño years as indicated in the horizontal axis. Box indicates the interquartile range; the horizontal line within the box shows the mean persistence. X and O symbols indicate the 90th and 95th percentiles; maximum persistence in shown in red.
Additional understanding about interannual changes in the large-scale circulation and LLJ types is demonstrated by compositing daily meridional winds (850-hPa, V850), Z200 and OLR anomalies on Central, Northern and Andes types. Composites are stratified by La Niña, neutral and El Niño years. During Central type (Supplementary Fig. 4) and all ENSO phases (Fig. S4a, c, e), broad regions of negative V850 anomalies extend from the central Andes towards the South Atlantic, thus favoring the formation of the Central type. In contrast, positive V850 anomalies suppress the LLJ formation in the northern Andes. The V850 wave train patterns indicate remote forcing from the Pacific Ocean. During La Niña years (Fig. S4b), negative OLR anomalies are located over Indonesia and Z200 anomalies extend from the South Pacific towards South America. During neutral years (Fig. S4d), negative OLR anomalies are in the western Pacific and an enhanced Pacific South American (PSA) pattern forces Z200 anomalies propagating over South America. Negative OLR anomalies over the Central Pacific during El Niño are associated with Z200 anomalies emanating to South America (Supplementary Fig. 4f). Supplementary Figs. 5 and 6 show composites of Northern and Andes types as functions of ENSO phases. While El Niño and La Niña years change the climatological location of tropical convective activity and basic state, these results show that for the same type of LLJ, the spatial patterns and magnitudes of the wave trains in Z200 anomalies are not modified by ENSO. In contrast, ENSO influences the activity of the wave train and persistence of LLJ, such that the Central type is skewed by El Niño years, whereas the Northern type is skewed by La Niña years.
On subseasonal time scales, the MJO is a fundamental mode influencing global weather-climate variability (see also Supplementary Fig. 7). To gain further understanding on MJO and LLJ relationships, Fig. 7 shows the distributions of Central, Northern and Andes types versus MJO phases. They are constructed by counting the number of LLJ days in that type and phase, and dividing by the total number of LLJ type days. The mean frequency is removed for easier interpretation. The influence of the MJO on the Central type occurs in phases 5–6 (Fig. 7a), when the MJO convective signal is over Indonesia (Supplementary Fig. 7). In the Northern type (Fig. 7b), the highest frequencies are in phases 1 and 4–6, when enhanced convection is in the western Pacific or propagating from the Indian Ocean to Indonesia. Supplementary Fig. 8 shows composites of OLR and Z200 anomalies during Central type and MJO phases 4, 5 and 6, and show the evolution of the wave train reaching South America. Unlike in the Central type, where there is a systematic shift from negative-to-positive-to-negative anomalies in the frequencies, the Northern type is influenced by the MJO and synoptic variability unrelated to the MJO. In contrast, a systematic shift in the Andes type frequencies shows a control by the MJO when enhanced convection intensifies and propagates eastward over the Indian Ocean (Fig. 7c, Supplementary Fig. 7).
Fig. 7: Distributions of LLJ types and MJO phases.
Anomalous frequencies of LLJ occurrences as a function of MJO phases during (a) Central, (b) Northern and (c) Andes types. Anomalies were computed as [number of occurrences in a given phase – mean occurrences in all phases] / [mean occurrences in all phases].
The distributions of LLJ types as functions of MJO phases during warm and cold ENSO are shown in Supplementary Fig. 9. The MJO influences the Central type preferentially in phase 6 (convective anomalies over Indonesia) during El Niño, whereas it changes to phase 5 during La Niña years. This change is consistent with the westward retraction of mean convection during La Niña years. For Northern type, the frequency distributions are more evenly distributed with MJO phases during El Niño and La Niña years. It is difficult to judge any preferential MJO phase for Andes types during El Niño years, highlighting complex non-linear interactions between these two modes of climatic variability. In contrast, during La Niña years, the Andes type are more frequent in phase 3, when the enhanced MJO convection is over the Indian Ocean.
Northerly low-level jets and precipitation variability in South America
It is clear from the results aforementioned that northerly low-level jets along the eastern Andes may occur independently or simultaneously in the northern and central branches of the Andes. Figure 8 shows precipitation anomalies during Central, Northern and Andes types from two datasets. Consistent with previous studies10,20, when the LLJ is present only in the central Andes (Fig. 8a, d), precipitation enhancement is observed over the La Plata Basin. Moreover, positive precipitation anomalies are found in the northern Andes; negative precipitation anomalies over the Amazon and southeast Brazil indicate suppression of the South Atlantic Convergence Zone (SACZ)28. In contrast, during Northern types (Fig. 8b, e), precipitation is enhanced over the Amazon, whereas precipitation decreases over the La Plata Basin and northern Andes. Positive anomalies are particularly large over most of eastern Peru and northern Bolivia, which is related to the intense moisture transport by the LLJ and the blocking effect of the Andes. Note also that IMERG suggests an enhancement of oceanic SACZ28. Distinct precipitation anomaly patterns are found during Andes type (Fig. 8c, f). While positive precipitation anomalies are found over the La Plata Basin, they are not as strong during Central types. Positive precipitation anomalies are small or not statistically significant over a large region over the Amazon Basin. It is worth noting that negative precipitation anomalies are enhanced over northern South America over a larger area than during Northern type.
Fig. 8: Composites of precipitation anomalies (mm day−1) and winds (850-hPa) (m s−1) during Central, Northern and Andes LLJ types.
Panels (a–c) show composites based on IMERG and (d–f) on CHIRPS data. Boxes in (d) and (e) are used to calculate persistence of precipitation during LLJ types shown in Fig. 9.
The previous results show that cold and warm ENSO phases control the activity of wave trains and persistence of Northern and Central LLJ types. To investigate potential ENSO influences on the persistence of precipitation during LLJ types, we select the two domains indicated in Fig. 8d, e. In each region, daily mean precipitation is computed by spatially averaging gridpoints with precipitation above 1 mm. CHIRPS is used because there is a longer record than IMERG. Persistence is calculated as the number of consecutive days with precipitation greater than 1 mm (1 day minimum). Calculations are done separately for Northern and Central types, and El Niño, neutral and La Niña years. During Northern types (Fig. 9a), La Niña years favor precipitation events with longer durations than in El Niño years. In contrast, during Central types (Fig. 9b), El Niño years favor persistences with longer duration than in La Niña years. This ENSO influence is particularly noticeable in the tails (90th and 95th percentiles and maxima) of the frequency distributions of persistence.
Fig. 9: Persistence of precipitation during LLJ types and ENSO phases.
Distributions of persistence of precipitation during (a) Northern and (b) Central types. Distributions are calculated separately during La Niña, Neutral and El Niño years as indicated in the horizontal axis. Box indicates the interquartile range; the horizontal line within the box shows the mean persistence. X and O symbols indicate the 90th and 95th percentiles; maximum persistence in shown in red. SESA is Southeast South America.
Source link : https://www.nature.com/articles/s41612-023-00501-4
Author :
Publish date : 2023-10-31 03:00:00
Copyright for syndicated content belongs to the linked Source.
