Following an El Niño event, a basin‐wide warming takes place over the tropical Indian Ocean, peaks in late boreal winter and early spring, and persists through boreal summer. Our observational analysis suggests that this Indian Ocean warming induces robust climatic anomalies in the summer Indo‐West Pacific region, prolonging the El Niño's influence after tropical East Pacific sea surface temperature has returned to normal. In response to the Indian Ocean warming, precipitation increases over most of the basin, forcing a Matsuno‐Gill pattern in the upper troposphere with a strengthened South Asian high. Near the ground, the southwest monsoon intensifies over the Arabian Sea and weakens over the South China and Philippine Seas. An anomalous anticyclonic circulation forms over the subtropical Northwest Pacific, collocated with negative precipitation anomalies. All these anomaly patterns are reproduced in a coupled model simulation initialized with a warming in the tropical Indian Ocean mixed layer, indicating that the Indian Ocean warming is not just a passive response to El Niño but important for summer climate variability in the Indo‐West Pacific region. The implications for seasonal prediction are discussed.
surface energy fluxes. Ocean surface temperature was forced by these surface energy Orbital Forcing and Ocean Feedbacks fluxes and by the flux associated with an in the Middle Holocene additional restoring term that kept the simulated SSTs close to modern-day SSTs. In the subsequent ocean model simulation for 6 ka, the heat flux associated with the temperature restoring term was added as a Simulations with a climate model that asynchronously couples the atmosphere and the prescribed flux correction. ocean showed that the increased amplitude of the seasonal cycle of insolation in the A second AGCM experiment used incom-Northern Hemisphere 6000 years ago could have increased tropical Atlantic sea surface ing solar radiation appropriate for 6 ka (the temperatures in late summer. The simulated increase in sea surface temperature and experiment is denoted by R, for radiation) but associated changes in atmospheric circulation enhanced the summer monsoon prewas otherwise identical to the first experiment cipitation of northern Africa by more than 25 percent, compared with the middle Ho-with prescribed modem SSTs. By forcing the locene simulation with prescribed modern sea surface temperatures, and provided better flux-corrected ocean with the required atmoagreement with paleorecords of enhanced monsoons.spheric variables from R, we obtained an estimate of the seasonal response of the ocean to orbital forcing (R Ocean). We then calculated the differences in monthly SSTs between Earth's present climate is considerably dif-ing (prescribed) SST. Seasonally varying the R Ocean and Control Ocean simulations ferent from that between about 12 and 5 variables from a 5-year Control simulation (ASST), added these differences to the prethousand years ago (ka), in part because of (net solar radiation, downward-directed scribed modern SST fields to obtain a first differences in Earth's orbital parameters. long-wave radiation, near-surface air temapproximation for the monthly SST field of 6Simulations with climate models have indi-perature, specific humidity, and wind and ka, and used the AGCM to simulate the cated that the orbital changes caused an wind stress) were then used to force a 500-seasonal atmospheric response to 6-ka insolaincrease in the amplitude of the seasonal year ocean simulation (Control Ocean). tion and the altered SSTs (R + ASST). We
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