Reduction in meridional heat export contributes to recent Indian Ocean warming

Since 2000, the Indian Ocean has warmed more rapidly than the Atlantic or Pacific. Air-sea fluxes alone cannot explain the rapid Indian Ocean warming, which has so far been linked to an increase in temperature transport into the basin through the Indonesian Throughflow (ITF). Here, we investigate the role that the heat transport out of the basin at 36°S plays in the warming. Adding the heat transport out of the basin to the ITF temperature transport into the basin, we calculate the decadal mean Indian Ocean heat budget over the 2010s. We find that heat convergence increased within the Indian Ocean over 2000-2019. The heat convergence over the 2010s is the same order as the warming rate, and thus the net air-sea fluxes are near zero. This is a significant change from previous analyses using trans-basin hydrographic sections from 1987, 2002, and 2009, which all found divergences of heat. A two year time series shows that seasonal aliasing is not responsible for the decadal change. The anomalous ocean heat convergence over the 2010s compared to previous estimates is due to changes in ocean currents at both the southern boundary (33%) and the ITF (67%). We hypothesize that the changes at the southern boundary are linked to an observed broadening of the Agulhas Current, implying that temperature and velocity data at the western boundary are crucial to constrain heat budget changes.

Extreme IOD induced tropical Indian Ocean warming in 2020

The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.

Historic Yangtze flooding of 2020 tied to extreme Indian Ocean conditions

Heavy monsoon rainfall ravaged a large swath of East Asia in summer 2020. Severe flooding of the Yangtze River displaced millions of residents in the midst of a historic public health crisis. This extreme rainy season was not anticipated from El Niño conditions. Using observations and model experiments, we show that the record strong Indian Ocean Dipole event in 2019 is an important contributor to the extreme Yangtze flooding of 2020. This Indian Ocean mode and a weak El Niño in the Pacific excite downwelling oceanic Rossby waves that propagate slowly westward south of the equator. At a mooring in the Southwest Indian Ocean, the thermocline deepens by a record 70 m in late 2019. The deepened thermocline helps sustain the Indian Ocean warming through the 2020 summer. The Indian Ocean warming forces an anomalous anticyclone in the lower troposphere over the Indo-Northwest Pacific region and intensifies the upper-level westerly jet over East Asia, leading to heavy summer rainfall in the Yangtze Basin. These coupled ocean-atmosphere processes beyond the equatorial Pacific provide predictability. Indeed, dynamic models initialized with observed ocean state predicted the heavy summer rainfall in the Yangtze Basin as early as April 2020.

Warming of the Indian Ocean weakens the Atlantic Niño - El Niño Southern Oscillation connection

<p>The tropical Indian Ocean has warmed by 1 degree Celsius since the mid-twentieth century. This warming is likely to continue as the atmospheric carbon dioxide levels keep rising. Here, we discuss how the warming trend could influence the El Niño Southern Oscillation (ENSO) via interaction with the Pacific and the Atlantic Ocean mean state and variability. The warming trend leads to the strengthening of easterlies in the western equatorial Pacific, subsequent downwelling and increase of the mixed later depth in the west, and an increase in the subsurface temperature gradient across the equatorial Pacific. In the eastern equatorial Pacific, the response of upwelling ocean currents to surface wind stress decreases, resulting in a weakening of ENSO amplitude. The Indian Ocean warming influences ENSO via the Atlantic Ocean as well. There, it is associated with the strengthening of equatorial easterly winds, and anomalous warming in the west and upwelling induced cooling in the east, especially in austral winter, during the peak of the Atlantic Niño. Consequently, this results in a decrease of the amplitude of Atlantic Niño events and weakening of the Atlantic Niño-ENSO teleconnection, thereby hindering the transition of El Niño events to La Niña events. Thus, the Indian Ocean warming trend is found to modulate tropical Pacific and Atlantic mean state and variability, with implications for ENSO predictability under a warming climate.</p>

Delayed impact of Indian Ocean warming on the East Asian surface temperature variation in boreal summer

A significant negative relationship is found between the summer mean North Indian Ocean sea surface temperature (SST) and East Asian surface temperature anomalies. However, the relationship is distinctively different for each month and shows a time-lagged relation rather than a simultaneous one. The North Indian Ocean warming in June is responsible for significant cold anomalies over the Korea-Japan region that peak in July, exhibiting a 1-month leading role. The SST increase is closely associated with enhanced convective activity in the region in June, but the relationship between SST and resultant precipitation is substantially weakened afterward. This dependency of the precipitation sensitivity to SST anomaly is related to the climatological evolution of SST. The relatively low background SST due to the strengthening of southwesterly monsoons in the late summer can weaken the sensitivity of the precipitation to SST anomaly, yet the background SST in June is strong enough to maintain an increased sensitivity of precipitation. Thus, the Indian Ocean warming in June effectively drives atmospheric Kelvin waves that propagate into the equatorial western Pacific. In the western North Pacific (WNP), the resultant Kelvin wave-induced Ekman divergence triggers suppressed convection and anticyclonic anomalies. The WNP suppressed convection and anticyclonic anomalies move slowly northeastward until they are located near 20°N through the local air-sea interaction, and act as a source of the Pacific-Japan teleconnection. This teleconnection pathway brings clod surface anomalies to the Korea-Japan region due to the cyclonic circulation that causes the radiative and horizontal advection.

Interdecadal Change in the Relationship Between the Bay of Bengal Summer Monsoon and South China Sea Summer Monsoon Onset

The interdecadal change of the BOBSM–SCSSM relationship around the late 1970s is investigated in this paper. We found that the correlation between the BOBSM and SCSSM is 0.22 in 1958–1979, while it is 0.66 in 1980–2018. Further analyses showed that the strength of the South Asian High (SAH) at upper troposphere circulation experiences an interdecadal enhancement around the late 1970s; meanwhile its meridional shift exhibits a wider range in the second subperiod. Both the interdecadal change of the strength and meridional shift of the SAH contribute to a closer relationship of the BOBSM and SCSSM through modulating the divergent field at upper troposphere. As for the external forcing, the basin warming of the Indian Ocean after the late 1970s may serve as a relatively primary factor, which could induce a consistent background flow that may favor a closer BOBSM–SCSSM relationship in the second period. It is noted that the Indian Ocean warming is related to high pressure anomaly widely lying to the south of 20°N at upper troposphere, accompanied by the low pressure anomaly center to the north of 20°N. And this kind of upper-level circulation may result in strong westerly anomaly at the domain where the pressure gradient is large and then modulate the onset of the BOBSM and SCSSM in 1980–2018 through changing the upper-level divergent field. Besides, the low troposphere circulation associated with Indian Ocean warming is featured by the zonal-elongated high pressure anomaly spanning from the BOB and SCS to the northwest Pacific. The above coupling of the upper and lower troposphere, as a larger-scale consistent background flow controls the BOB and SCS, can modulate the interannual variation of the BOBSM and SCSSM synchronously and contributes to the closer relationship of the BOBSM and SCSSM in the second subperiod.