Impacts of Arctic sea-ice loss on California’s precipitation

Figure 1 and Supplementary Fig. 1 illustrate some of the differences in Arctic sea-ice cover between control and “low Arctic ice” simulations. During August and September, monthly mean Arctic sea-ice area for the “low Arctic ice” ensemble mean falls below the 1 million km2 threshold used to define a nearly ice-free Arctic19. By comparison, CMIP-5 simulations of twenty-first century climate and extrapolations of observed changes in ice volume place the time horizon for a nearly ice-free Arctic roughly between 2020 and 204019. The smallest and largest differences between the “low Arctic ice” and control Arctic sea-ice area (Supplementary Fig. 1) occur in March and September, respectively, in accord with the seasonality of the observed Arctic sea-ice loss since the beginning of the satellite era.

Fig. 1 Impact of sea-ice physics parameter perturbations on Arctic and Antarctic sea-ice cover. a Arctic sea-ice concentrations, September. b Antarctic sea-ice concentrations, March. Shown are monthly mean sea-ice concentrations. Areas contained within contour lines have sea-ice fractions larger than 15%. Thick black lines denote control ensemble means, thick purple lines show “low Arctic ice” in a and “low Antarctic ice” ensemble means in b. Thin purple lines indicate individual “low Arctic ice” (“low Antarctic ice”) simulations. The colored shading indicates the average observed September (a) and March (b) sea-ice concentrations over the period 1992–2001 (from the Center for Satellite Exploitation and Research; http://nsidc.org/data/sipn/data-sets.html#seaice-conc-ext). A brief comparison of Arctic vs. Antarctic sea-ice changes is provided in Supplementary Note 1 Full size image

While the temperature impacts of sea-ice loss are most pronounced in the northern high- to mid-latitudes (Fig. 2a), precipitation anomalies show a more global response (Fig. 2b, c). We focus our analysis on the December–February season, because these months yield the largest impact of sea-ice changes on Californian precipitation in our model simulations. The most striking feature of the precipitation response to Arctic sea-ice loss is the reorganization of tropical rainfall and an apparent northward precipitation shift. The Arctic sea-ice decline also results in significantly less precipitation over California—a consequence of a geopotential ridge in the North Pacific that steers the wet winter air masses northward into Alaska and Canada, away from California (Fig. 2e).

Fig. 2 Atmospheric impacts of Arctic sea-ice loss. Shown are the ensemble mean differences (“low Arctic ice” minus control) for December–February (DJF) season. Stippling indicates anomalies that are statistically significant at the 90% confidence level. a Surface temperature anomalies. b Precipitation anomalies (absolute). c Precipitation anomalies (relative). d Outgoing longwave radiation (OLR) anomalies (shading) and the high cloud cover anomalies (contours). e 500 hPa geopotential distribution changes. f Stream function changes. Negative cloud cover anomalies in d are indicated with dashed lines Full size image

These findings are consistent with previous claims of a sea-ice driven component of Californian precipitation changes15, 29, 30. We suggest, however, that the persistent geopotential height anomaly in the North Pacific shown in Fig. 2e is not just a direct consequence of sea-ice-induced surface warming and associated high-latitude planetary wave perturbations (as hypothesized previously), but rather is forced by sea-ice-induced convection changes over the tropical Pacific. We find that the key components of this two-step teleconnection are changes in the location and intensity of tropical Pacific convection and the northward-propagating Rossby wavetrain they initiate.

Equatorward propagation

In the first step of this teleconnection, sea-ice loss alters the high-latitude energy budget (see annual mean flux changes shown in Supplementary Figs. 2 and 3a). These high-latitude energy flux changes are dominated by a decrease in the net top-of-atmosphere (TOA) radiation to space (Supplementary Fig. 2a). Sea-ice loss leads to an increase in the net downward TOA shortwave flux (Supplementary Fig. 2b) that is only partly compensated by an increase in net TOA upward longwave flux. This yields an increased net TOA heat flux into the atmosphere, and thus less radiation to space. Over the high northern latitudes, the TOA net flux changes are on average larger than the net surface flux changes (Supplementary Figs. 2a, c and 3a), leading to an increased heat flux into the atmospheric column (Supplementary Fig. 2d). With the exception of very high northern latitudes, sea-ice induced decrease in the net upward shortwave flux at the surface is largely compensated by an increase in the latent and sensible heat fluxes (Supplementary Fig. 2e, f, g). Several previous studies30, 31 support our finding of larger high-latitude TOA flux anomalies (relative to surface flux anomalies) in response to sea-ice changes. Experimental configurations that impose artificial surface energy fluxes in order to remove sea-ice cover necessarily yield a substantially larger surface energy imbalance than the one described here.

As a consequence of the increased high-latitude heat flux into the atmospheric column, the atmospheric heat transport from mid-latitudes into the high northern latitudes decreases (see the large white arrow in Supplementary Fig. 3a). The high-latitude energy surplus is compensated for at lower latitudes, with most of the energy emitted to space through the TOA flux changes over the tropical Pacific between 20°S and 20°N (Supplementary Fig. 3a, b). Increase in the tropical net TOA flux to space is associated with changes in high cloud cover. As seen from Supplementary Fig. 3b, regions of increased TOA flux to space coincide with the regions of decreased high cloud cover. An increase in the net TOA flux to space over the regions of decreased high cloud cover is a consequence of an increase in the outgoing longwave radiation (Supplementary Fig. 3c) due to decreased radiation height, that is only partially compensated by an increase in the net downward shortwave flux.

Tropical compensation of high-latitude energy budget perturbations has been noted in other studies employing AGCMs coupled to a slab ocean model21, 23. Such compensation is also evident in the short-term response to sea-ice changes in simulations employing fully coupled AOGCMs30. Specifically, previous work suggests that reorganization of atmospheric convection over the tropical Pacific (and resulting adjustment of the amount of energy emitted to space) can provide a compensation for energy budget perturbations elsewhere21, 23. This is not dissimilar to the observed changes in tropical TOA fluxes during ENSO-related convection changes that are also capable of affecting the global energy budget32.

The mechanism by which high-latitude changes propagate into the tropics and induce tropical convection responses involves two processes: (i) a southward advection of the initial high-latitude temperature perturbation over the mid-latitudes through transport and mixing; and (ii) a wind/evaporation/sea-surface temperature feedback in the region of the northeasterly trades. The tropical response to high-latitude sea-ice loss in our model simulations is consistent with these findings (see Supplementary Fig. 4). Once advected into the mid-latitudes, the sea-surface temperature (SST) anomalies propagate into the tropics by giving rise to anomalous meridional surface pressure gradients that in turn affect the strengths of easterly winds. These wind strength changes then further drive the evaporative flux changes21,22,23. Specifically, over the northern tropics, in the region of the northeasterly trades, the progression of cold (warm) SST anomalies will lead to an increase (decrease) in the surface wind strengths. This results in increased (decreased) evaporative cooling, thus allowing for further propagation of cold (warm) anomalies toward the equator. Over the southern tropics, the response is the opposite, due to the background southeasterly flow. Negative (positive) wind strength and latent heat flux anomalies over the northern (southern) tropics in response to Arctic sea-ice loss are shown in Supplementary Fig. 4b, c.

The progression of these relatively small SST changes into the tropics, in combination with tropospheric temperature changes aloft, can drive convection changes in the tropical Pacific21,22,23. Areas of large zonal and meridional temperature gradients adjacent to deep convection regions in the tropical Pacific represent so called “convective margins”23, 33, 34. These are areas where small changes in atmospheric conditions can initiate or halt deep convection. Even weak sea-surface temperature anomalies (Supplementary Fig. 4a), in combination with small tropospheric temperature changes (Supplementary Fig. 4d), are sufficient to trigger convection changes over these regions (Fig. 2d).

Poleward propagation

We suggest that the tropical Pacific convection changes described above play a key role in driving the North Pacific circulation changes. Linkages between the equatorial Pacific convection anomalies and extratropical (North Pacific) atmospheric circulation changes are found in both observational and modeling studies13, 35, 36 and have been invoked to explain North Pacific circulation differences during different phases of the ENSO cycle. These studies find that enhanced winter convection (and corresponding enhanced upper-level divergence) in the equatorial Pacific initiates a northward-propagating Rossby wavetrain. The characteristics of this wavetrain are positive stream function and geopotential anomalies immediately outside the region of increased convection (anticyclonic flow), and negative anomalies in the extratropical Pacific (cyclonic flow). We find that this mechanism also operates in our simulations.

During the DJF season, the area of strongest decrease in high cloud cover is located over the central tropical Pacific (see contour lines in Fig. 2d). This region of decreased tropical deep convection coincides with the area of the largest outgoing longwave radiation (OLR) increase (see shading in Fig. 2d). In the second teleconnection step, decreased deep convection in the tropical Pacific (Fig. 2d) and the associated decrease in upper-level divergence force a northward-propagating Rossby wavetrain with negative (cyclonic) anomalies in the northern tropical Pacific and positive (anticyclonic) anomalies in the extratropical North Pacific sector. This is evident from the 250 mb stream function changes (Fig. 2f), which indicate anomalous cyclonic flow in the tropics and anomalous anticyclonic flow in the North Pacific. The 250 mb North Pacific anticyclonic flow is the upper-level manifestation of the geopotential ridge in Fig. 2e—a factor responsible for steering the wet winter air masses away from California. The atmospheric anomalies in the North Pacific exhibit an equivalent barotropic response (Supplementary Fig. 5a).

The convection changes described above are consistent with the simulated upper-level vertical velocity anomalies and Hadley circulation changes. From a climatological perspective, the descending branch of the winter Hadley cell is positioned over (and to the west of) California (Fig. 3a and Supplementary Fig. 6). Arctic sea-ice loss markedly amplifies this subsidence over California, while also affecting the uplift and subsidence regions across the tropical Pacific (Fig. 3b). The tropical Hadley circulation plays a key role in the atmospheric transport of heat between the tropics and extratropics21, 37, 38. Changes to the Hadley circulation are an expected and known dynamical response to the energy budget perturbations shown in Supplementary Fig. 3 21,22,23, 39.

Fig. 3 Changes to 250 mb vertical velocity (hPa/s). a Control simulation climatology. b The response to Arctic sea-ice loss (“low Arctic ice” minus control ensemble mean). Positive sign denotes subsidence. Stippling in b indicates anomalies that are statistically significant at the 90% confidence level Full size image

The schematic in Fig. 4 summarizes the hypothesized two-step teleconnection, involving sea-ice loss, tropical convection reorganization, a northward-propagating Rossby wavetrain, and a decrease in precipitation over California. Several previous studies have suggested that tropical and subtropical Pacific convection changes affect precipitation over California by inducing geopotential anomalies in the North Pacific6, 8, 13. While such tropical and subtropical convection changes could be a consequence of natural variability, our analysis indicates that they can also be forced from the high latitudes by substantial sea-ice loss. Our simulations imply that two hitherto separate hypotheses—that Californian rainfall is primarily influenced by tropical convection changes (“tropical hypothesis”)6, 7, 11 and that Arctic sea-ice loss can drive precipitation changes over California (“Arctic sea-ice loss hypothesis”)15, 29, 30—are not easily separable. Our study demonstrates that the impacts of Arctic sea-ice changes require understanding complex atmospheric teleconnections that can propagate back and forth between the extratropics and the tropics.

Fig. 4 Schematics of the two-step teleconnection. In step 1 (equatorward propagation), Arctic sea-ice loss induced high-latitude changes propagate into tropics, triggering tropical circulation and convection response. Decreased convection and decreased upper-level divergence in the tropical Pacific in turn drive a northward-propagating Rossby wavetrain with anticyclonic flow forming in the North Pacific. This ridge is responsible for steering the wet tropical air masses away from California Full size image

Atmospheric impacts of Antarctic sea-ice loss

To further investigate the proposed two-step teleconnection, and additionally verify that sea-ice-induced tropical convection changes are forcing the circulation changes in the North Pacific, we explore the impacts of another source of remote forcing: Antarctic sea-ice loss. Antarctic sea-ice decline will result in a warming at mid- to high latitudes of the southern hemisphere. One consequence of this warming is a southward shift of tropical precipitation, with convection increase in the southern and central tropical Pacific40. This is the opposite to the precipitation response accompanying Arctic sea-ice loss. In accord with previous studies13, 35, 36 and the mechanism advanced here, increased convection should in turn drive a northward-propagating Rossby wavetrain consisting of an anticyclonic anomaly in the tropical Pacific and a cyclonic anomaly in the North Pacific. This cyclonic flow is associated with negative geopotential height anomalies over the North Pacific (geopotential low) leading to wetter conditions over California. Simulations in which the sea-ice physics parameter perturbations are prescribed in the Southern hemisphere only (“low Antarctic ice” simulations, Fig. 1b and Supplementary Fig. 1b, d) show a response that is consistent with these expectations. Tropical precipitation shifts southward (Fig. 5a), anticyclonic flow is established over the northern tropical Pacific, and cyclonic flow is formed over the North Pacific (Fig. 5c). The geopotential ridge in the North Pacific is replaced with a geopotential trough, favoring the propagation of tropical storms across California (Fig. 5b). As a result, precipitation over California increases (Fig. 5a).

Fig. 5 Atmospheric impacts of Antarctic sea-ice loss. a Precipitation anomalies. b 500 hPa geopotential distribution changes. c Stream function changes. Shown are the ensemble mean differences (“low Arctic ice” minus control) for December–February (DJF) season. Stippling indicates anomalies that are statistically significant at the 90% confidence level Full size image

The “low Antarctic ice” simulations show that the proposed mechanism can be triggered by sea-ice changes in either hemisphere. Since Antarctic sea-ice loss involves northward propagation in both teleconnection steps (i.e., Antarctic sea-ice affecting the tropical Pacific, which in turn affects the North Pacific) and no high northern latitude changes, it provides additional support for our conjecture that the sea-ice changes can influence North Pacific geopotential height through tropical convection changes. More generally, any high-latitude perturbation (northern or southern hemispheric warming or cooling) that impacts the position of the tropical Pacific ITCZ, will have an impact on California’s rainfall.