Mi-Kyung Sung

A physical mechanism of the precipitation dipole in the western United States based on PDO‐storm track relationship

It is known that the western United States (US) precipitation displays a north-south contrast, i.e., the so-called “precipitation dipole,” during El Nino and La Nina winters. Furthermore, the Pacific Decadal Oscillation (PDO) has been known to modulate this precipitation dipole. However, the underlying physical mechanism regulating this modulation is not well understood. This study revisits previous studies and suggests a physical mechanism of precipitation dipole modulation based on the PDO-storm track relationship. We found that both jet stream and storm track tend to move northward (southward) over the North Pacific during negative (positive) PDO winters, contributing to the increase of precipitation over the northwestern (southwestern) US, respectively. This relationship is robust regardless of El Nino-Southern Oscillation (ENSO), possibly facilitating modulation of the precipitation dipole. Moreover, changes in oceanic baroclinicity associated with the PDO phase are suggested to be responsible for anchorage of storm tracks over the North Pacific.

Altered atmospheric responses to eastern Pacific and central Pacific El Niños over the North Atlantic region due to stratospheric interference

The two types of El Niño that have been identified, namely the eastern Pacific (EP) and central Pacific (CP) El Niños, are known to exert different climatic impacts on the North Atlantic region during winter. Here, we investigate the characteristics of the teleconnection of the two El Niño types with a focus on the stratosphere-troposphere coupling. During the EP El Niño, polar stratospheric warming and polar vortex weakening frequently occur with a strong tendency for downward propagation near the tropopause. Consequently, the atmospheric pattern within the troposphere over the North Atlantic sector during midwinter closely resembles the negative North Atlantic Oscillation pattern. In contrast, during CP El Niño events stratospheric warming events exhibit a much weaker downward propagation tendency. This difference in the stratospheric circulation response arises from the different seasonal evolution of the tropospheric wave response to the two El Niño types. For the EP El Niño, the Aleutian Low begins growing during December and is sustained throughout the entire winter (December to February), which provides favorable conditions for the continuous downward propagation of the stratospheric warming. We also discuss the origin of the difference in the teleconnections from the two types of El Niño associated with the distinct longitudinal position of the warm SST anomaly that determines troposphere-stratosphere coupling.

Arctic-North Pacific Coupled Impacts on the Late Autumn Cold in North America

The Pacific decadal oscillation (PDO) is known to bring an anomalously cold (warm) period to southeastern (northwestern) North America during the cold season of its positive phase through a Rossby wave linkage. This study provides evidence that the remote connection between the North Pacific and the downstream temperature over central North America is strengthened by the warm arctic conditions over the Chukchi and East Siberian Sea, especially in the late autumn season. The modulation effect of the Arctic manifests itself as an altered Rossby wave response to a transient vorticity forcing that results from an equatorward storm track shift, which is induced collaboratively by the PDO and the warm Arctic. This observational finding is supported by two independent modeling experiments: (1) an idealized coupled GCM experiment being nudged toward the warm arctic surface condition and (2) a simple stationary wave model experiment forced by transient eddy forcing.

Role of Tropical Atlantic SST Variability as a Modulator of El Nino Teleconnections

The present study suggests that the off-equatorial North Atlantic (NATL) SST warming plays a significant role in modulating El Niño teleconnection and its impact on the North Atlantic and European regions. The El Niño events accompanied by NATL SST warming exhibit south-north dipole pattern over the Western Europe to Atlantic, while the ENSO teleconnection pattern without NATL warming exhibits a Rossby wave-like pattern confined over the North Pacific and western Atlantic. Especially, the El Niño events with NATL warming show positive (negative) geopotential-height anomalies over the North Atlantic (Western Europe) which resemble the negative phase of the NAO. Consistently, it is shown using a simple statistical model that NATL SSTA in addition to the tropical Pacific SSTA leads to better prediction on regional climate variation over the North Atlantic and European regions. This role of NATL SST on ENSO teleconnection is also validated and discussed in a long term simulation of coupled global circulation model (CGCM).

An alterative effect by the tropical North Atlantic SST in intraseasonally varying El Niño teleconnection over the North Atlantic

ABSTRACT It is known that there is a distinct intraseasonal variation in wintertime atmospheric responses to El Niño over the North Atlantic, namely a positive North Atlantic Oscillation (NAO)-like response during early winter (November to December) and a negative NAO-like response during late winter (January to March). In this article, we suggest that the tropical North Atlantic (TNAL) sea surface temperatures (SSTs) significantly alter the North Atlantic atmospheric response to El Niño: the warm TNAL SST condition intensifies the negative NAO-like response and vice versa. During late El Niño winters, the TNAL SST tends to increase due to the atmospheric bridge between the tropical Pacific and the Atlantic oceans. The warm tendency in the TNAL SST can intensify the height contrast in the El Niño teleconnection between early and late winter. During early winter (when the response of the TNAL SST to El Niño has not been established yet), atmospheric circulation over the North Atlantic varies widely under various TNAL conditions. Consequently, the average influence of El Niño over the North Atlantic region becomes weaker in the early winter. By contrast, the overall warming over the TNAL region during the late El Niño winter is conducive to the formation of a negative NAO pattern. As a result, the climatic impact becomes stronger during the late winter.

Asymmetric impact of Atlantic Multidecadal Oscillation on El Niño and La Niña characteristics

The long-lasting cold surface conditions of North Atlantic, i.e., the negative phase of Atlantic Multidecadal Oscillation (AMO), can intensify the El Nino–Southern Oscillation through the enhanced air-sea coupling under the increased central-to-eastern tropical Pacific mean sea surface temperature. However, the impact of warmer mean sea surface temperature (SST) is more efficient in the intensifying El Nino than La Nina, because of the nature of the exponential growth of atmospheric convection to SST change. Moreover, the farther eastward shift of the atmospheric convection during the negative AMO leads to the stronger El Nino due to the longer delayed negative feedback by oceanic waves. Therefore, the AMO mainly influences the El Nino intensity rather than La Nina intensity.