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Yuanlong Li
,
Weiqing Han
,
Fan Wang
,
Lei Zhang
, and
Jing Duan

Abstract

Multi-time-scale variabilities of the Indian Ocean (IO) temperature over 0–700 m are revisited from the perspective of vertical structure. Analysis of historical data for 1955–2018 identifies two dominant types of vertical structures that account for respectively 70.5% and 21.2% of the total variance on interannual-to-interdecadal time scales with the linear trend and seasonal cycle removed. The leading type manifests as vertically coherent warming/cooling with the maximal amplitude at ~100 m and exhibits evident interdecadal variations. The second type shows a vertical dipole structure between the surface (0–60 m) and subsurface (60–400 m) layers and interannual-to-decadal fluctuations. Ocean model experiments were performed to gain insights into underlying processes. The vertically coherent, basinwide warming/cooling of the IO on an interdecadal time scale is caused by changes of the Indonesian Throughflow (ITF) controlled by Pacific climate and anomalous surface heat fluxes partly originating from external forcing. Enhanced changes in the subtropical southern IO arise from positive air–sea feedback among sea surface temperature, winds, turbulent heat flux, cloud cover, and shortwave radiation. Regarding dipole-type variability, the basinwide surface warming is induced by surface heat flux forcing, and the subsurface cooling occurs only in the eastern IO. The cooling in the southeast IO is generated by the weakened ITF, whereas that in the northeast IO is caused by equatorial easterly winds through upwelling oceanic waves. Both El Niño–Southern Oscillation (ENSO) and IO dipole (IOD) events are favorable for the generation of such vertical dipole anomalies.

Free access
Lun Dai
,
Tat Fan Cheng
,
Bin Wang
, and
Mengqian Lu

Abstract

The Indian monsoon is of utmost concern to agriculture, the economy, and the livelihoods of billions in South Asia. However, little attention has been paid to the possibility of distinct subseasonal episodes phase-locked in the Indian monsoon annual cycle. This study addresses this gap by utilizing the self-organizing map (SOM) method to objectively classify six distinct subseasonal stages based on the 850-hPa wind fields. Each subseasonal stage ranges from 23 to 90 days. The Indian summer monsoon (ISM) consists of three substages, the ISM-onset, ISM-peak, and ISM-withdrawal, altogether contributing to 82% of the annual precipitation. The three substages signify the rapid northward advance, dominance, and gradual southward retreat of southwesterlies from mid-May to early October. The winter monsoon also comprises three substages (fall, winter, and spring), distinguishable by the latitude of the Arabian Sea high pressure ridge and hydrological conditions. This study proposes two compact indices based on zonal winds in the northern and southern Arabian Sea to measure the winter and summer monsoons, respectively. These indices capture the development and turnabouts of the six SOM-derived stages and can be used for subseasonal monsoon monitoring and forecasts. The spring and the ISM-onset episodes are highly susceptible to compound hazards of droughts and heatwaves, while the greatest flood risk occurs during the ISM-peak stage. The fall stage heralds the peak season for tropical storms over the Arabian Sea and the Bay of Bengal. The annual start and end dates of the ISM-peak are highly correlated (0.6–0.8) with the criteria-based dates proposed previously, supporting the delineation of the Indian monsoon subseasonal features.

Significance Statement

This research explores the existence of subseasonal features in the Indian monsoon annual cycle. Through the use of machine learning, we discover that the Indian summer monsoon and winter monsoon each consist of three substages. These substages’ evolution can be measured by two compact indices proposed herein, which can aid in subseasonal monsoon monitoring and forecasts in South Asia. Pertaining to hazard adaptations, this work pinpoints the subseasonal episodes most susceptible to droughts, heatwaves, floods, and tropical storms. High correlations are obtained when validating the substages’ yearly start and end dates against those documented in the existing literature, offering credibility to the subseasonal features of the Indian monsoon.

Open access
Jing Duan
,
Yuanlong Li
,
Lijing Cheng
,
Pengfei Lin
, and
Fan Wang

Abstract

The heat content in the Indian Ocean has been increasing owing to anthropogenic greenhouse warming. Yet, where and how the anthropogenic heat is stored in the Indian Ocean have not been comprehended. Analysis of various observational and model-based datasets since the 1950s reveals a robust spatial pattern of the 0–700 m ocean heat content trend (ΔOHC), with enhanced warming in the subtropical southern Indian Ocean (SIO) but weak to minimal warming in the tropical Indian Ocean (TIO). The meridional temperature gradient between the TIO and SIO declined by 16.4% ± 7.5% during 1958–2014. The heat redistribution driven by time-varying surface winds plays a crucial role in shaping this ΔOHC pattern. Sensitivity experiments using a simplified ocean dynamical model suggest that changes in surface winds over the Indian Ocean, particularly those of the SIO, caused a convergence trend in the upper SIO and a divergence trend in the upper TIO. These wind changes primarily include the enhancements of westerlies in the Southern Ocean and the subtropical anticyclone in the SIO. Albeit with systematic biases, the ΔOHC pattern and surface wind changes simulated by phase 6 of the Coupled Model Intercomparison Project (CMIP6) models broadly resemble the observation and highlight the essence of external forcing in causing these changes. This heat storage pattern is projected to persist in the model-projected future, potentially impacting future climate.

Restricted access
Qiuping Ren
,
Yuanlong Li
,
Fan Wang
,
Jing Duan
,
Shijian Hu
, and
Fujun Wang

Abstract

Historical observations have documented prominent changes of the Mindanao Current (MC) during El Niño events, yet a systematic understanding of how El Niño modulates the MC is still lacking. Mooring observations during December 2010–August 2014 revealed evident year-to-year variations of the MC in the upper 400 m that were well reproduced by the Hybrid Coordinate Ocean Model (HYCOM). Composite analysis was conducted for 10 El Niño events during 1980–2015 using five model-based datasets (HYCOM, OFES, GEOS-ODA, SODA2.2.4, and SODA3.3.1). A consensus is reached in suggesting that a developing (decaying) El Niño strengthens (weakens) the MC, albeit with quantitative differences among events and datasets. HYCOM experiments demonstrate that the MC variability is mainly a first baroclinic mode response to surface wind forcing of the tropical Pacific, but the specific mechanism varies with latitude. The upstream part of the MC north of 7.5°N is controlled by wind forcing between 6° and 9°N through Ekman pumping, whereas its downstream part south of 7.5°N is greatly affected by equatorial winds. Prevailing westerly winds and Ekman upwelling in the developing stage cause cyclonic anomalous circulation in the northwest tropical Pacific that strengthens the MC, and the opposite surface wind forcing effect in the decaying stage weakens the MC. Although ocean models show difficulties in realistically representing the northward-flowing Mindanao Undercurrent (MUC) beneath the MC and its seasonal and interannual variations, all five products suggest an enhancement of the MUC during the decaying stage of El Niño.

Open access
Chuanyu Liu
,
Xiaowei Wang
,
Zhiyu Liu
,
Armin Köhl
,
William D. Smyth
, and
Fan Wang

Abstract

The origins of an observed weakly sheared nonturbulent (laminar) layer (WSL), and a strongly sheared turbulent layer above the Equatorial Undercurrent core (UCL) in the eastern equatorial Pacific are studied, based mainly on the data from the Tropical Atmosphere and Ocean mooring array. Multiple-time-scale (from 3 to 25 days) equatorial waves were manifested primarily as zonal velocity oscillations with the maximum amplitudes (from 10 to 30 cm s−1) occurring at different depths (from the surface to 85-m depths) above the seasonal thermocline. The subsurface-intensified waves led to vertically out-of-phase shear variations in the upper thermocline via destructive interference with the seasonal zonal flow, opposing the tendency for shear instability. These waves were also associated with depth-dependent, multiple-vertical-scale stratification variations, with phase lags of π/2 or π, further altering stability of the zonal current system to vertical shear. The WSL and UCL were consequently formed by coupling of multiple equatorial waves with differing phases, particularly of the previously identified equatorial mode and subsurface mode tropical instability waves (with central period of 17 and 20 days, respectively, in this study), and subsurface-intensified waves with central periods of 6, 5, and 12 days and velocity maxima at 45-, 87-, and 40-m depths, respectively. In addition, a wave-like feature with periods of 50–90 days enhanced the shear throughout the entire UCL. WSLs and UCLs seem to emerge without a preference for particular tropical instability wave phases. The generation mechanisms of the equatorial waves and their joint impacts on thermocline mixing remain to be elucidated.

Open access
Chuanfeng Zhao
,
Liping Liu
,
Qianqian Wang
,
Yanmei Qiu
,
Wei Wang
,
Yang Wang
, and
Tianyi Fan

Abstract

This study describes the microphysical properties of high ice clouds (with bases above 5 km) using ground-based millimeter cloud radar cirrus-mode observations over the Naqu site of the Tibetan Plateau (TP) during a short period from 6 to 31 July 2014. Empirical regression equations are applied for the cloud retrievals in which the parameters are given on the basis of a review of existing literature. The results show a unimodal distribution for the cloud ice effective radius r e and ice water content with maximum frequencies around 36 μm and 0.001 g m−3, respectively. Analysis shows that clouds with high ice r e are more likely to occur at times from late afternoon until nighttime. The clouds with large (small) r e mainly occur at low (high) heights and are likely orographic cumulus or stratocumulus (thin cirrus). Further analysis indicates that ice r e decreases with increasing height and shows strong positive relationships between ice r e (μm) and depth h (m), with a regression equation of r e = 35.45 + 0.0023h + (1.7 × 10−7)h 2. A good relationship between ice r e and temperature T (°C) is found, r e = 44.65 + 0.1438T, which could serve as a baseline for retrieval of characteristic ice r e properties over the TP.

Full access
Hongli Wang
,
Juanzhen Sun
,
Shuiyong Fan
, and
Xiang-Yu Huang

Abstract

An indirect radar reflectivity assimilation scheme has been developed within the Weather Research and Forecasting model three-dimensional data assimilation system (WRF 3D-Var). This scheme, instead of assimilating radar reflectivity directly, assimilates retrieved rainwater and estimated in-cloud water vapor. An analysis is provided to show that the assimilation of the retrieved rainwater avoids the linearization error of the Zqr (reflectivity–rainwater) equation. A new observation operator is introduced to assimilate the estimated in-cloud water vapor. The performance of the scheme is demonstrated by assimilating reflectivity observations into the Rapid Update Cycle data assimilation and forecast system operating at Beijing Meteorology Bureau. Four heavy-rain-producing convective cases that occurred during summer 2009 in Beijing, China, are studied using the newly developed system. Results show that on average the assimilation of reflectivity significantly improves the short-term precipitation forecast skill up to 7 h. A diagnosis of the analysis fields of one case shows that the assimilation of reflectivity increases humidity, rainwater, and convective available potential energy in the convective region. As a result, the analysis successfully promotes the developments of the convective system and thus improves the subsequent prediction of the location and intensity of precipitation for this case.

Full access
Damao Zhang
,
Zhien Wang
,
Andrew Heymsfield
,
Jiwen Fan
, and
Tao Luo

Abstract

Measurements of ice number concentration in clouds are important but still pose problems. The pattern of ice development in stratiform mixed-phase clouds (SMCs) offers an opportunity to use cloud radar reflectivity (Z e ) measurements and other cloud properties to retrieve ice number concentrations. To quantify the strong temperature dependencies of ice crystal habits and growth rates, a one-dimensional (1D) ice growth model has been developed to calculate ice diffusional growth and riming growth along ice particle fallout trajectories in SMCs. The radar reflectivity and fallout velocity profiles of ice crystals calculated from the 1D ice growth model are evaluated with the Atmospheric Radiation Measurements (ARM) Climate Research Facility (ACRF) ground-based high-vertical-resolution radar measurements. A method has been developed to retrieve ice number concentrations in SMCs at a specific cloud-top temperature (CTT) and liquid water path (LWP) by combining Z e measurements and 1D ice growth model simulations. The retrieved ice number concentrations in SMCs are evaluated using integrated airborne in situ and remote sensing measurements and three-dimensional cloud-resolving model simulations with a bin microphysical scheme. The statistical evaluations show that the retrieved ice number concentrations in the SMCs are within an uncertainty of a factor of 2.

Full access
Li-Li Fan
,
Bin Wang
, and
Xian-Qing Lv

Abstract

Harmonic analysis of 10 yr of Ocean Topography Experiment (TOPEX)/Poseidon (T/P) along-track altimetry is performed to derive the semidiurnal and diurnal tides (M 2, S 2, N 2, K 2, K 1, O 1, P 1, and Q 1) near Hawaii. The T/P solutions are evaluated through intercomparison for crossover points of the ascending and descending tracks and comparison with the data of tidal stations, which show that the T/P solutions in the study area are reliable. By using a suitable order polynomial to fit the T/P solutions along every track, the harmonic constants of any point on T/P tracks are acquired. A new fitting method, which is characterized by applying the harmonics from T/P tracks to produce directly empirical cotidal charts, is developed. The harmonic constants derived by this fitting method show good agreement with the data of tidal stations, the results of National Astronomical Observatory 99b (NAO.99b), TOPEX/Poseidon 7.2 (TPXO7.2), and Finite Element Solutions 2004 (FES2004) models, which suggests that the fitting method is reasonable, and the highly accurate cotidal chart could be directly acquired from T/P altimetry data by this fitting method.

Full access
Hanjie Fan
,
Song Yang
,
Chunzai Wang
,
Yuting Wu
, and
Guangli Zhang

Abstract

The Pacific meridional mode (PMM) has been suggested to play an important role in modulating the development of El Niño–Southern Oscillation (ENSO). In this study, we examine the projected changes in the PMM and its impact on ENSO under greenhouse gas forcing using the models of phase 6 of the Coupled Model Intercomparison Project. These models can properly reproduce the characteristics of PMM patterns but reveal discrepant PMM–ENSO relationships owing to different wind–evaporation–sea surface temperature (SST) (WES) feedback efficiency and different magnitude of atmospheric convection response to SST anomalies. We select the models that show good performance in simulating the PMM and its impact on ENSO for investigation of future projections. Results show potential increases in both PMM amplitude and its impact on ENSO under the shared socioeconomic pathway (SSP) 585 (SSP585) warming scenario with great intermodel consensus. Diagnosis of the WES feedback indicates increasing sensitivity of latent heat flux to zonal wind speed in a warming climate, which seems to be the main reason for the projected strengthening PMM and its impact on ENSO. In addition, a slightly intensified response of atmospheric convection to SST anomalies in the subtropical Pacific may partially contribute to a stronger PMM–ENSO relationship. The results from this study highlight the increasing importance of the PMM for ENSO development, which calls for more attention to be paid to the PMM for ENSO prediction.

Significance Statement

Variability of the sea surface temperature in the equatorial Pacific related to El Niño–Southern Oscillation (ENSO) can exert a great impact on global climate. The development of ENSO is partially modulated by the dominant mode of ocean–atmospheric variation in the subtropical North Pacific, namely, the Pacific meridional mode (PMM). This study is aimed to understand the change in the amplitude of the PMM and its impact on ENSO due to climate change. Multimodel projections suggest that the PMM will likely become stronger and exert a greater impact on ENSO since the future warmer climate is favorable for the growth of the PMM. These results call for more attention to be paid to the PMM for ENSO prediction.

Open access