Strengthening Western North Pacific High in a Warmer Environment

Abstract

The geographical response of western North Pacific subtropical high (SH) to environmental conditions such as the El Niño-Southern Oscillation (ENSO) and global warming has been one of the main concerns with respect to extreme events induced by tropical convections. By considering observed outgoing longwave radiation (OLR) as the strength of subtropical high, this study attempts to further understand the geographical response of SH strength to ENSO and global warming. Here, “SH strength” is defined as the inhibition of regional convections under SH environment. A meridional seesaw pattern among SH strength anomalies is found at 130°–175° E. In addition, the La Niña environment with weaker convections at lower latitudes is characterized by farther westward expansion of SH but with a weaker strength. Conversely, the El Niño environment with stronger convections at lower latitudes leads to shrunken SH but with a greater strength. The influence of the seesaw mechanism appears to be modulated by global warming. The western North Pacific subtropical high strengthens overall under warming in both the La Niña and El Niño environments. This suggests that the weakening effect by drier tropics is largely offset by anomalous highs induced by a warming atmosphere. It is most remarkable that the highest SH strengths appear in a warmer El Niño environment. The finding implies that every new El Niño environment may experience the driest atmosphere ever in the subtropics under global warming. The value of this study lies in the fact that OLR effectively illustrates how the ENSO variation and global warming bring the zonally undulating strength of boreal-summer SH.

1. Introduction

Subtropical high (SH) provides valuable insights into extreme weather induced by tropical convections in the western North Pacific (WNP) [1,2]. This semipermanent high is controlled by global circulation and dominates the atmospheric pattern over the ocean basin during boreal summer [3]. Convective activity over East Asia during boreal summer greatly varies depending on how SH in the region changes [4,5,6,7]. SH in the North Pacific is centered on a large subsidence zone near Hawaii. It is known by various names such as the Hawaiian High and North Pacific High. During boreal summer, it expands to its seasonal maximum with basin-wide influence. Its response to environmental conditions such as the El Niño-Southern Oscillation (ENSO) and global warming has been one of the major concerns in WNP.

ENSO is a coupled atmosphere–ocean phenomenon in the tropical Pacific with a two-to-seven-year periodicity, though its impacts reach globally [8]. This phenomenon exhibits fluctuations between two phases of El Niño and La Niña. El Niño, the warm phase of the ENSO cycle, is characterized by anomalously high sea-surface temperatures (SSTs) in the eastern tropical Pacific, which weakens the zonal atmospheric circulation, i.e., the Walker circulation [9]. This weakening further reduces the zonal SST gradient, thus enhancing the El Niño amplitude. On the other hand, the La Niña environment is associated with a steeper zonal SST gradient along with a stronger atmospheric circulation. The evolution of ENSO status has been explained through various conceptual models that are being continuously refined [3,10,11,12,13,14,15]. Generally, SST anomalies are considered to initiate ENSO phases through westerly wind bursts in the atmosphere [16], which propagate equatorial Kelvin waves eastward [17] and form Rossby waves north and south of the equator [18]. In an El Niño environment, SH shows a weaker westward expansion [19], leading to more intense tropical cyclones (TCs) in the southeastern quadrant of WNP [20,21]. Conversely, in a La Niña environment, an increased SST anomaly in the western Pacific with a stronger zonal SST gradient enhances the Walker circulation, which results in a westward expansion of SH [22,23] and reduction in the TC activities in the southeastern quadrant of WNP [24]. Whereas ENSO is a natural variability, global warming is considered an external forcing on the global climate system [8]. A warmer environment, which exhibits a La Niña-like atmospheric pattern, is accompanied by expanding SH [25,26,27,28] and induces anomalous highs in WNP [29]. Variable SH also modifies the geographical background of the atmospheric patterns for tropical convections. Yun et al. [30] investigated the atmospheric patterns in WNP under variable environmental conditions in combination with ENSO and global warming [30]. By comparing the different modes of variability, they found that SH expands farthest to the west when La Niña coincides with a warmer environment. However, the westward expansion of SH may not necessarily mean strengthening of the high anomalies all over the region.

As a sequel to the previous study on the expansion of climatological SH [30], the present study aims to investigate the accompanying patterns of regional SH, implying the patterns of tropical convections. For this, outgoing longwave radiation (OLR) is exploited as an effective indicator of SH and tropical convections at the same time. OLR shows the terrestrial radiation energy observed by satellites. While OLR is useful for monitoring tropical convections [31,32,33,34,35,36], it uses as a complementary indicator of convective inhibition when analyzed together with geopotential height [37,38,39]. So, OLR also can be understood as the regional SH strength. In this study, “SH strength” is defined as the inhibition of regional convections under SH environment. Then, the larger OLR is interpreted as indicating stronger inhibition of tropical convection, whereas smaller OLR suggests weaker inhibition of tropical convection. Using OLR, we investigate how the regional SH strength varies according to the environmental conditions. The environmental conditions can be described by combining ENSO and the global ocean warmth [29]. By modeling OLR using ENSO and the global ocean warmth, we attempt to better understand the geographical pattern of SH and tropical convections.

This paper is structured as follows. Section 2 describes the data and methods used in this study. Section 3 verifies how the regional SH strength changes relative to environmental conditions and analyzes its effect on tropical convections. Finally, Section 4 provides the summary and discussion.

2. Data and Methods

2.1. Data Descriptions

Interpolated OLR from the National Oceanic and Atmospheric Administration (NOAA)/National Centers for Environmental Prediction (NCEP) reanalysis (https://psl.noaa.gov/data/gridded, accessed on 4 October 2023) is used to indicate SH and tropical convections. The Southern Oscillation Index (SOI) from the National Oceanic and Atmospheric Administration (NOAA)/Climate Prediction Center (http://cpc.ncep.noaa.gov/data/indices/soi, accessed on 4 October 2023) is used for the ENSO status. The global ocean warmth is indicated by global mean SST (GMSST) obtained from Extended Reconstructed SST version 5 [40] of the NOAA/NCEP reanalysis (https://psl.noaa.gov/data/gridded, accessed on 4 October 2023).

The data period in this study is 36 years from 1985 to 2020, chosen for consistency with the previous study [30]. As the sun crosses the equinox during September, the heating spot of the sun migrates over to the Southern Hemisphere, causing more complicated atmospheric structure. Thus, clearly finding the geographical relationship among the environmental factors in the North Pacific is made difficult. To focus on the factors in the North Pacific, the experimental period is set during the boreal summer from June to August (JJA). Then, the 36-year (1985–2020) JJA values are analyzed.

2.2. Definition of SH Strength

This study defines the level of regional SH as “strength”, which represents the inhibition of regional convections within its environment. A larger strength simultaneously implies both divergent drier air masses and weaker tropical convections in a specific location. The climatological distribution of geopotential height and convective precipitation amount can be considered a reflection of the regional SH strength. To investigate the climate features during summer, the regional SH strength must be quantified.

2.3. Availability of OLR

The current study uses OLR as an indicator of SH strength. From a climatological perspective, this study considers OLR as the fingerprint of an environmental background as well as its resultant tropical convection. Because OLR indicates regional precipitation, which reflects variation of tropical convection in the area, variation of SH strength, which indicates the inhibition of regional convection, can be represented by OLR. OLR is a measure of terrestrial radiative energy obtained from satellite observations. OLR values are continuously distributed over a horizontal space and can be utilized to understand atmospheric patterns (see Supplementary Figure S1).

We note that climatological SH during boreal summer lies along a zonal axis between 20° and 30° N. The slope of OLR indicating SH strength is observed between 160° E and 160° W. Therefore, the western boundary of the main body of the North Pacific high is observed to be around 160° E, and OLR in WNP appears to have rather different origins from those in the eastern North Pacific. This study focuses on the geographical relationship between the regional convections and WNPSH, which varies according to environmental conditions such as ENSO and global warming. To examine the temporal variation of OLR, two latitudinal areas are respectively taken at 20°–30° N and 5°–20° N (see Figure 1).

Figure 1 shows the Hovmöller diagrams of the zonally distributed OLR. Here, the values are meridionally averaged over 20°–30° N and 5°–20° N, respectively, for SH and the tropical convections. Considering the physics of OLR, which exhibits larger radiation over the region with less cloud formation, the larger OLR values at higher latitudes confirm a stronger SH strength, whereas the smaller OLR values at lower latitudes indicate more active tropical convections. Both the higher and lower latitudes show semiperiodic fluctuations, which can be associated with ENSO. On the other hand, the SH strengths in both latitudinal areas appear to be stronger over time. The response of the SH strength to global warming also requires investigation.

This study investigates the geographical response of the regional SH strength to ENSO by modeling OLR using SOI. Although the SOI does not capture the diversity of ENSO phases, it contributes to models’ stability since it exhibits significantly low collinearity with GMSST [24]. In addition, the geographical response of the regional SH strength to global warming is also explored by adding GMSST as another explanatory variable. Here, the ordinary least squares (OLS) method is used to model OLR, so equations are directly written in the form of regression models between OLR and relevant explanatory variables. Now, the WNPSH pattern and its westward expansion under variable environmental conditions are expected to be better understood. Interpretation of the results can include perception of how the warmer environment influences WNPSH by modulating the SH strength over the region.

3. Results

3.1. Response of SH Strength to ENSO Variation

The geographical distribution of the SH strength varies with environmental variability. The SH strength can be modeled using the ENSO variation. Here, OLR is regressed on SOI. The model can be expressed as

$$\mathrm{OLR}\sim \mathrm{SOI},$$

(1)

where ∼ denotes the relationship between the explanatory and response variables. In the model, the positive (negative) sign of standardized SOI indicates the level of La Niña (El Niño) status. Figure 2 shows that the La Niña environment leads to the slight westward expansion of WNPSH and narrowing region for tropical convections. An opposite case occurs in the El Niño environment [30]. The Walker circulation, in conjunction with the zonally steeper SST gradient, illustrates how the variable ENSO environment influences the WNPSH expansion [41]. On the other hand, this statistical model reveals that the regional SH strength in WNP becomes weaker during the expansion in the La Niña environment. The inverse mechanism is definitely apparent in the El Niño environment.

The feature reveals that the zonally distributed anomalies follow a meridional seesaw pattern between the tropical convections and SH strength especially at 130°–175° E, bounded by the two vertical lines (see Figure S2). Under the La Niña environment, tropical convection is inhibited at 130°–175° E, as indicated by larger OLR in Figure 2b. This weakening of convection is accompanied by a decrease in SH strength in the subtropics, shown by smaller OLR in Figure 2a. In contrast, the El Niño environment features strengthening of convection in the tropics (smaller OLR; Figure 2b), which coincides with enhanced SH strength in the subtropics (larger OLR; Figure 2a). A seesaw pattern simply means an out-of-phase relationship of the OLR distribution between subtropics and tropics. The fact that the seesaw pattern does not appear in the eastern North Pacific also proves that the meridional connection is a unique feature of WNP, which is controlled by ENSO-induced convective anomalies. The result can be summarized as ENSO variation introduces zonally undulating SH strength in WNP. Here, “undulating” means a wavy pattern on the zonal SH slope. Consequently, the La Niña environment is characterized by the farther westward expansion of WNPSH but with a weaker strength in the middle, while the El Niño environment exhibits reduced WNPSH but with more strength.

3.2. Response of SH Strength to Global Warming

Whereas ENSO describes the internal variability of the SH strength, global warming functions as an external forcing on it. First of all, the meridional out-of-phase of OLR shown in Figure 3 (i.e., seesaw pattern) can be described as follows:

$$\mathrm{Seesaw}\mathrm{index}={\displaystyle \frac{{\mathrm{OLR}}_{\mathrm{subtropics}}-{\mu }_{{\mathrm{OLR}}_{\mathrm{subtropics}}}}{{\sigma }_{{\mathrm{OLR}}_{\mathrm{subtropics}}}}}-{\displaystyle \frac{{\mathrm{OLR}}_{\mathrm{tropics}}-{\mu }_{{\mathrm{OLR}}_{\mathrm{tropics}}}}{{\sigma }_{{\mathrm{OLR}}_{\mathrm{tropics}}}}},$$

(2)

where $\mu$ is the mean of OLR and $\sigma$ is the standard deviation. The seesaw index reflects the degree of convective inhibition associated with variation in tropical convection.

A high (low) index indicates enhanced (suppressed) convections over the tropics and thereby suppressed (enhanced) convections in the subtropics, implying a stronger (weaker) SH strength.

To better capture the combined influence of ENSO and global warming on the SH strength, OLR is modeled as a linear function. Equation (3) models this relationship as a function of global ocean warmth and ENSO phase, where global ocean warmth is represented by GMSST.

$$\mathrm{OLR}\sim \mathrm{SOI}+\mathrm{GMSST}.$$

(3)

This model allows us to disentangle the distinct contributions of ENSO variability and global ocean warming. Although SOI primarily represents the phase of ENSO, GMSST reflects broader thermodynamic changes associated with external forcing. Compared with the response of SH strength to ENSO, as described in Equation (1), the results highlight the distinct role of global ocean warmth in modulating SH variability. To isolate the effect of global warming, we compare the predicted OLR under two cases: (1) 1.5 standardized SOI and 1.5 standardized GMSST, and (2) 1.5 standardized SOI. Here, Equations (1) and (3) use standardized values of −1.5 and 1.5 for explanatory variables to highlight their respective response [30]. The difference between the two cases highlights the warming-induced variation of SH strength beyond what is expected under ENSO alone.

Figure 4 shows where and how much the modulation of the meridional connection occurs in the warmer environment of La Niña and El Niño. As addressed, larger index values represent stronger tropical convections and stronger SH strength at the same time. This paper focuses on 130°–175° E, where the meridional concurrency has been confirmed (see Figure S2). Modeled OLRs in La Niña and El Niño environments exhibit different distributions over the longitudinal ranges. Under warmer conditions, both the La Niña and El Niño environments experience increasing OLRs in the tropics, implying less tropical convections (blue- and orange-shaded areas, adjacent to the thick lines). As the meridional seasaw mechanism is defined by the difference between standardized OLRs, we assume that OLR anomalies in the subtropics exhibit standardized magnitudes comparable to those in the tropics (blue- and orange-shaded areas, adjacent to the thin lines). The potential distribution of counteractive OLRs in the subtropics quantitatively explains the further weakening SH strength in the warmer La Niña environment and the less strengthening SH strength in the warmer El Niño environment.

However, it is noteworthy that the modeled SH strength in a warmer environment reveals overall increases in both the La Niña and El Niño environments (green-shaded areas, adjacent to the thin lines). This suggests that the weakening SH strength by drier tropics is largely offset by anomalous high induced by warming atmosphere [30]. Conclusively, the strengthening SH under global warming is understood as overcoming the more favorable conditions for convections allowed by the seasaw mechanism. Therefore, global warming makes overall drier ENSO conditions in the subtropics as well as the tropics, while less dry in the subtropics.

This study suggests that the final expression of SH strength results from the combined effects of the meridional seesaw mechanism under ENSO conditions and the overall warming atmosphere. Among all environmental conditions, it is most remarkable that the highest SH strengths appear in a warmer El Niño environment. The finding implies that every new El Niño environment may experience the driest atmosphere ever in the subtropics under global warming.

4. Summary and Discussion

SH plays an important role in providing climatological background of extreme events induced by tropical convections. Its geographical response to environmental conditions such as ENSO and global warming has been one of the major concerns in WNP. The westward expansion of WNPSH is the farthest when La Niña coincides with a warmer environment [30]. However, the westward expansion itself does not prove the strengthening of SH all over the region. This study aims to further understand the geographical response of SH to environmental conditions by modeling its zonal distribution. This study defines the level of regional SH as “strength” , which represents the inhibition of regional convections under its environment. Subsequently, OLR is used to quantify the SH strength. OLR is considered as the fingerprint of an environmental background and its resultant tropical convections.

First, the geographical response of the regional SH strength to ENSO is investigated by modeling OLR using SOI. The statistical model clearly shows a meridional seesaw pattern of OLR anomalies especially at 130°–175° E. The results show that the La Niña environment with weaker convections at lower latitudes is characterized by the farther westward expansion of WNPSH but with weaker strength in the middle. In contrast, the El Niño environment with stronger convections at lower latitudes results in shrunken WNPSH but with more strength. The seesaw pattern does not appear in the eastern North Pacific, and the zonally undulating SH strength is a unique feature of WNP.

Second, the response of the regional SH strength to global warming is explored by adding GMSST as another explanatory variable. Here, OLR is modeled using the global ocean warmth as well as the level of ENSO status. Anomalous high induced by global warming is found to simultaneously inhibit tropical convections at lower latitudes and increase the SH strength at higher latitudes. These patterns are consistent with previous studies showing that convection over the tropical western North Pacific weakens under global warming, particularly depending on the ENSO status [42,43,44]. This suggests the possibility that convective inhibition in the western North Pacific is linked not only to local processes but also to remote teleconnections. We note that the OLR increase at higher latitudes is smaller than that at lower latitudes. This result suggests that convective activities associated with the seesaw mechanism are largely suppressed by the warming atmosphere in the subtropics. The smaller increase in OLR in the subtropics compared to the tropics could be explained by out-of-phase contributions.

As a result, global warming makes overall drier ENSO conditions in the subtropics as well as the tropics, though the subtropics remain relatively less dry. Notably, the strongest SH strength appears under the warmer El Niño environment. This finding implies that future El Niño events may bring unprecedentedly dry subtropical conditions under global warming.

The importance of this study lies in the fact that the statistical model of OLR effectively shows how the ENSO variation and global warming bring a zonally undulating strength of boreal-summer WNPSH. Future research may consider a hybrid approach that involves numerical models as dynamical input. In addition, a nonlinear approach to the ENSO–OLR relationship could be applied to climate prediction models or seasonal forecast models for disaster risk management. In this work, the results are expected to be applied in estimating the seasonal pattern of WNPSH and tropical convections when the predicted values of SOI and GMSST are given.