Understanding the Weakening Relationship of the Pacific Decadal Oscillation and Indian Ocean Basin Mode During Boreal Winter

While it is known that the Pacific Decadal Oscillation (PDO) leads the Indian Ocean Basin Mode (IOBM) with the same phase via the atmospheric bridge, we found that the relationship of PDO-IOBM during boreal winter is not stationary. Here, we investigated the PDO-IOBM relationship changes on low-frequency timescales by analyzing the observations, a long-term simulation of climate model with its large ensembles as well as the pacemaker experiments. A long-term simulation of climate model with its large ensemble simulations indicated that the non-stationary relationship of PDO-IOBM is intrinsic in a climate system and it could be at least partly due to internal climate variability. In details, we compared the PDO structures during the entire period with those during the period when the PDO-IOBM relationship was weak (i.e., 1976-2006). We found that the structures of sea surface temperature (SST) as well as its associated tropical Pacific convective forcing during the negative phase of PDO for 1976-2006 are far away from the typical structures of the negative PDO phase during the entire period, which were responsible for the weakening relationship of the PDO-IOBM in the observation. The results of the two pacemaker experiments support that a non-stationary relationship of PDO-IOBM is primarily due to the SST forcing in the Pacific.

Marine Heatwaves in the Arabian Sea

Marine heatwaves (MHWs) are prolonged warm sea condition events that cause a destructive impact on marine ecosystems. The documentation of MHWs and assessment of their impacts are largely confined to a few regional seas or in global mean studies. The Indian Ocean received almost no attention in this regard despite the fact that this ocean basin, particularly the Arabian Sea, is warming at the most rapid pace among the other tropical basins in recent decades. This study shows the characteristics MHWs for the Arabian Sea during 1982–2019. Our analysis shows that the duration of MHWs exhibit a rapidly increasing trend of ~20 days/decade (1.5–2 count/decade) in the northern Arabian Sea and in the southeastern Arabian Sea close to the west coast of India; which is more than 15 fold increase in the MHW days from the early 80s'. At the same time increase in MHW frequency is ~1.5–2 count/decade i.e an increase of ~6 fold, indicating more frequent and much longer heatwave events in the recent decade. Notably, since the beginning of the satellite record, the year 2010 and 2016 saw the maximum number of heatwave days with more than 75 % of days of the pre-monsoon and summer monsoon season experienced heatwaves. The accelerated trend of the heatwave days is found to be driven by the rapid rise of the mean SST of the Arabian Sea in the recent decade. Moreover, longer heatwave days are also associated with the dominant climate modes and among them, Indian Ocean Basin mode via the decaying phase of the El-Niño is found to be the most influencing mode contributing in more than 70–80 % of observed heatwave days in this basin. Mixed layer heat budget analysis suggests significant heterogeneity in the dominant processes across the years; however, weakening of latent heat loss is in general one of the key mechanism in the genesis of most of the MHWs.

Interdecadal Strengthening in the Independent Relationship Between the East Asian Summer Monsoon and the Indian Ocean Basin Mode around the Early 1990s

This study conducts correlation and regression analyses of the JRA-55 reanalysis data and observational rainfall datasets from China’s National Climate Center. The analyses reveal that interdecadal enhancement in the relationship between the East Asian summer monsoon (EASM) and the Indian Ocean Basin mode (IOBM) after the early 1990s, and the diminished correlation between the EASM and the Niño-3 index. The analyses also reveal that the relationship between EASM-related rainfall/circulation with IOBM also experienced an interdecadal shift at the same time. During the first epoch (1977–1989), EASM-related rainfall was correlated significantly with the Niño-3 index, and accompanied by a Pacific–Japan-like anomaly pattern of horizontal winds. In a subsequent epoch (1994–2014), EASM-related rainfall was correlated significantly with IOBM, and accompanied by a meridional dipole pattern in the horizontal winds. After the 1990s, IOBM exerted influence on EASM through land–sea thermal contrast, and the critical land area was the region 33°–47°N, 110°–140°E. The interdecadal strengthening in the EASM–IOBM linkage around the early 1990s may be attributable to a faster rate of decay of El Niño after the 1990s.

Trends in mean and extreme rainfall over Mainland Southeast Asia associated with warming-driven trends in evaporation

<p>Rain-gauge datasets indicate strong increases in both annual mean and extreme precipitation over large parts of the Mainland Southeast Asia (MSEA) including Vietnam and the northwestern part of the peninsula over the last 40 years. Increasing precipitation is associated with increased monsoon intensity in southeast Asia and a northward shift of the monsoon activity centre towards MSEA. Warming-driven evaporation increases over the three main oceanic moisture sources - the Arabian Sea, the Bay of Bengal, and the South China Sea- may partially explain increasing precipitation in large parts of MSEA. Changes in the patterns of the two main modes of natural variability in the tropical Indian Ocean – the Indian Ocean Basin Mode (IOBM) and the Indian Ocean Dipole (IOD) – contribute to surface warming in these oceanic moisture source regions supplying precipitation to MSEA. Climate model projections show robust wide-spread trends in wet season precipitation with increasing frequency and intensity of extreme precipitation events throughout MSEA over the 21<sup>st</sup> century. Similar to observations, the projected precipitation trends are associated with strong warming-driven increases in evaporation in all major oceanic moisture sources supplying precipitation to MSEA.</p>

The Interdecadal Reverse of the Relationship and Feedback Mechanism between Sea Surface Temperature and Evaporation over the Indian Ocean during Boreal Autumn

The linkage between sea surface temperature (SST) and evaporation (EVP) plays an important role in air–sea interactions. In this study, the interaction mechanism of SST and EVP during boreal autumn was studied using correlation analysis, composite analysis, the EVP decomposition method, and singular value decomposition. The results showed that the correlation between SST and EVP in the Indian Ocean was reversed from positive to negative in the late 1990s. The significant positive SST–EVP relationship was attributed to the Indian Ocean basin mode forcing upon EVP during 1980–90. The decrease in wind speed–induced EVP and SST warming led to a significant negative SST–EVP relationship during 2005–15. Moreover, the negative SST–EVP correlation occurred when the Indian Ocean dipole (IOD) and subtropical Indian Ocean dipole (SIOD) exhibited inverse phases. The low-level moisture–EVP–SST feedback dominated the negative SST–EVP correlation in the negative IOD and positive SIOD (nIOD–pSIOD) pattern, whereas the wind–EVP–SST feedback played the leading role in the positive IOD and negative SIOD (pIOD–nSIOD) pattern. The EVP anomalies induced by the low-level anomalous anticyclone and cyclone were the main causes of the SST anomalies with inverse phases of the IOD and SIOD. The correlation between the SST and EVP reversal from positive to negative implies that the effect of the atmosphere on the ocean is as important as the external forcing of the ocean on the atmosphere.

Seasonal Covariability of Dryness or Wetness in China and Global Sea Surface Temperature

Seasonal covariability of the dryness/wetness in China and global sea surface temperature (SST) is investigated by using the monthly self-calibrated Palmer drought severity index (PDSI) data and other data from 1950 to 2014. The singular value decomposition (SVD) analysis shows two recurring PDSI–SST coupled modes. The first SVD mode of PDSI is associated with the warm phases of the eastern Pacific–type El Niño–Southern Oscillation (ENSO), the interdecadal Pacific oscillation (IPO) or Pacific decadal oscillation (PDO), the Indian Ocean basin mode (IOBM) in the autumn and winter, and the cold phase of the IOBM in the spring. Meanwhile, the Atlantic multidecadal oscillation (AMO) pattern appears in every season except the autumn. The second SVD mode of PDSI is accompanied by a central Pacific–type El Niño developing from the winter to autumn over the tropical Pacific and a positive phase of IPO or PDO from the winter to summer. Moreover, an AMO pattern is observed in all seasons except the summer, whereas the SST over the tropical Indian Ocean shows negligible variations. The further analyses suggest that AMO remote forcing may be a primary factor influencing interdecadal variability of PDSI in China, and interannual to interdecadal variability of PDSI seems to be closely associated with the ENSO-related events. Meanwhile, the IOBM may be a crucial factor in interannual variability of PDSI during its mature phase in the spring. In general, the SST-related dryness/wetness anomalies can be explained by the associated atmospheric circulation changes.

Linkages between the South and East Asian Monsoon Water Vapor Transport during Boreal Summer

This study provides a water vapor transport (WVT) perspective on the linkages between the South Asian and East Asian summer monsoons (SASM and EASM) and indicates two robustly coupled modes of the vertical integrated WVT (VIWVT) over the two monsoons that accounts for above 90% of the total squared covariance fraction. The first coupled mode [singular value decomposition mode 1 (SVD1 mode)] depicts a meridional linkage between the meridional dipole VIWVT anomalies over both the SASM and EASM, while the second coupled mode (SVD2 mode) illustrates a zonal connection of an anomalous cyclonic/anticyclonic VIWVT over the SASM and a zonal wavelike VIWVT over the EASM. The SVD1 mode is linked through the anomalous subtropical high over the western North Pacific (WNPSH) and is primarily associated with the transition phase of El Niño/La Niña (ENSO) and simultaneous Indian Ocean basin mode (IOBM) SST warming/cooling. The meridional connection of the VIWVT in the SVD1 mode experienced a clear intensification since the late 1970s that may be attributed to the strengthened impacts of the ENSO/IOBM on the EASM and SASM after the late 1970s. The SVD2 mode is connected by the circumglobal teleconnection (CGT) pattern and related to the developing phase of ENSO and summer North Atlantic tripole (NAT) SST anomalies. The zonal VIWVT connection in SVD2 mode is strongly modulated by the SASM–CGT connections and reveals significant weakening since the late 1970s but reintensifies after the early 1990s. This may be associated with the weakened ENSO–SASM relationship after the late 1970s and interdecadal decreasing of the all Indian summer rainfall since the early 1990s.